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Stable isotopes as an exploration tool : tracking cryptic alteration surrounding the Iscaycruz Zn (Pb-Cu-Ag)… Cantor, Samuel Frank 2020

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  Stable Isotopes as an Exploration Tool: Tracking Cryptic Alteration Surrounding the Iscaycruz Zn (Pb-Cu-Ag) Skarn-CRD Deposit, Central Peru   by   Samuel Frank Cantor   BA, University of Colorado, Boulder, 2009    A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE  in  THE FACULTY OF GRADUATE AND POSTDOCTORAL STUDIES  (Geological Sciences)    THE UNIVERSITY OF BRITISH COLUMBIA  (Vancouver)  January 2020   ©Samuel Frank Cantor, 2020 ii  The following individuals certify that they have read, and recommend to the Faculty of Graduate and Postdoctoral Studies for acceptance, a thesis entitled: Stable Isotopes as an Exploration Tool: Tracking Cryptic Alteration Surrounding the Iscaycruz Zn (Pb-Cu-Ag) Skarn-CRD Deposit, Central Peru     submitted by Samuel Cantor in partial fulfillment of the requirements for  the degree of  Master of Science   in  Geological Sciences     Examining Committee Dr. Craig Hart Supervisor  Dr. James Mortensen Supervisory Committee Member  Dr. Gregory Dipple Supervisory Committee Member  Dr. Maya Kopylova Additional Examiner       iii  Abstract Proximal stable isotope haloes have been identified surrounding central Peruvian deposits including the Antamina Zn-Cu skarn and the Uchucchacua Ag-base metal veins. Studies analyzing stable isotopes around mineral deposits observed consistent alteration haloes of carbonate rocks with relatively depleted δ18O values proximal to the center of mineralization and areas of increased fluid:rock interaction.  The Iscaycruz Zn-(Pb-Cu-Ag) skarn-carbonate replacement deposit is located within a sequence of Early Cretaceous, layered carbonate and siliciclastic rocks in central Peru. A primary goal of this study was to integrate visible and cryptic alteration around a known ore body and utilize the identified relationships as an exploration tool for regional targets. The stable isotopes of carbon and oxygen were used to identify asymmetric haloes of isotopic alteration surrounding fluid flow pathways, primarily lithologic contacts, east-west faults, and east-northeast faults. The extent of isotopic alteration perpendicular to bedding was 5-20 meters. In contrast, isotopic alteration was detectable along strike for 1-4 kilometers with local variation.  Prior work at Iscaycruz utilized comparative mineralogy and fluid inclusions to infer that Iscaycruz deposits formed from a centralized mineralizing system below the Tinyag deposit. Stable isotopes of carbon, oxygen, and lead were used to provide additional lines of evidence that the deposits shared a single fluid source.  A secondary goal is to evaluate sampling strategies to inform industry best practices. As a result, the isotopic alteration patterns were compared to visible alteration, UV luminescent response, and vein generation. Red fluorescent calcite was 100% correlated with extremely depleted δ18O values and show promise as a reliable cryptic alteration indicator for ore-related fluids.   The sampling and analysis strategy employed in the thesis suggests that a simpler, more cost-effective strategy could be implemented to identify most of the patterns. For Cordilleran settings, a finite number of samples would be better utilized in many short transects across the contact instead of fewer, wider transects. These transects would collect samples perpendicular to the feature for only several meters in most cases, collecting more samples at a subset of transects to ensure that the limits of any pervasive alteration haloes may be detected as well.  iv  Lay Summary  Metals used in modern society are mined from locations near the Earth's surface called mineral deposits. When these deposits form, they sometimes change the nearby rocks and leave a specific pattern of altered rocks. These patterns can indicate to geologists how close they are to the deposit, as well as what type of metal(s) they might find. The research in this study focuses on a zinc mine in Peru where visible patterns in the limestone near the deposit are limited. The goal is to identify the carbon and oxygen patterns around the known deposit and see if any of the rocks around the mine have similar patterns. Areas with a similar pattern could be hiding another deposit. During this research process, different sampling methods and techniques were tested, with the most effective ones highlighted to aid future research and commercial use of the technique.    v  Preface  This thesis is part of the Mineral Deposit Research Unit’s (MDRU) Carbonate Alteration Footprint Initiative. The initial project design and objectives were produced by Dr. Craig Hart from the MDRU at the University of British Columbia (UBC) in concert with Dr. Abraham Escalante of Empresa Minera Los Quenuales S.A. (EMQSA), a subsidiary of Glencore. The objectives were subsequently changed and improved upon by the author, Prof. James Mortensen, and Prof. Gregory Dipple. Chapters 6 and 8 represent additional objectives outside the initial project design.   Contributions to this work include the EMQSA geologists; Dr. Escalante, Ronald Vasquez, Gonzalo Quiroz, and Christian Vela, along with EMQSA technician Henry Rios.   The author is responsible for the collection of all field data and samples, and the preparation, microdrilling, and analysis of all carbon and oxygen isotope samples. The author is not responsible for the production of geological surface maps.  Samples were prepared and subsequently analyzed using ultra-violet fluorescence and stable carbon and oxygen isotope analysis at MDRU, UBC by the author. Geological cross-sections and modelling were produced using Leapfrog 3D Geo. Exploratory data analysis and interpretation were conducted in ioGAS. Geologic maps were prepared and published in ArcGIS10. Study data was provided and is the property of EMQSA.  The author is not responsible for the accuracy of mapped lithology and structural data provided by EMQSA, nor drill core geochemistry, fluid inclusion data and interpretations, or historical geochronological work. The author is not responsible for alteration mineralogy, ore zone mineralogy, and lithology data other than from drill holes investigated in this study. Select drill holes studied contained historical observations from core that had been entirely sampled, and therefore relevant observations could not be confirmed by the author.   This research was presented in poster form at the Association for Mineral Exploration (AME) Round-Up conferences from January 2016 to January 2018, at the Society for Economic Geologists (SEG) conferences in China (2018) and Colorado (2019), and the Prospectors and Developer’s Association of Canada (PDAC) vi  convention in Toronto in 2018. Technical presentations of this research were presented at the Reflex Tech Conference in Vancouver, Canada in 2018 and the Resources for Future Generations Conference in Vancouver in 2018. The content of Chapter 8 – Luminescence in Carbonate Rocks and Minerals was presented as a short course module as part of the Think Zinc! AME Round-Up Short Course and the MDRU’s SC100 Short Course in January and July 2018, respectively.   vii  Table of Contents Abstract ........................................................................................................................................................................ iii Lay Summary ............................................................................................................................................................... iv Preface ............................................................................................................................................................................v Table of Contents ........................................................................................................................................................ vii List of Tables ..................................................................................................................................................................x List of Figures ............................................................................................................................................................... xi List of Abbreviations ............................................................................................................................................... xviii Acknowledgements ................................................................................................................................................... xix Chapter 1: Introduction ...................................................................................................................................... 1 1.1 - Project Rationale ......................................................................................................................................... 1 1.2 - Objectives .................................................................................................................................................... 2 1.3 - Thesis structure ........................................................................................................................................... 3 1.4 – Overview of Mina Chupa and Santa Este ................................................................................................. 3 1.5 - Characteristics of Skarn Deposits .............................................................................................................. 6 1.6 – Characteristics of Carbonate Replacement Deposits .............................................................................. 7 1.7 - Pb Isotope Systematics ............................................................................................................................... 8 1.8 Luminescence .............................................................................................................................................. 10 Chapter 2: Tectonic and Geologic Setting of Mina Iscaycruz ........................................................................ 17 2.1 - Introduction .............................................................................................................................................. 17 2.2 – Geologic Setting and Tectonic History of Central Peru ....................................................................... 19 2.3 – Geologic Setting of Mina Iscaycruz and Surrounding Areas ............................................................... 22 Chapter 3: Methodology ................................................................................................................................... 29 3.1 - Carbon and Oxygen Isotope Analysis ..................................................................................................... 29 3.2 - Establishment of Background Values Thresholds for Carbon and Oxygen Isotopes ........................ 32 3.3 Pb Isotopes ................................................................................................................................................... 45 3.4 U-Pb Dating ................................................................................................................................................. 46 3.5 Luminescence .............................................................................................................................................. 47 3.6 Data Display ................................................................................................................................................. 51 3.7 Isotopic Results Nomenclature .................................................................................................................. 52 viii  Chapter 4: Santa Este ......................................................................................................................................... 53 4.1 – Introduction.............................................................................................................................................. 53 4.2 – Santa Este Background and Location ..................................................................................................... 53 4.3 – Sample Collection .................................................................................................................................... 59 4.4 – Analysis and Results................................................................................................................................. 71 4.5 –Discussion .................................................................................................................................................. 91 4.6 – Exploration Implications ....................................................................................................................... 100 4.7 - Conclusions ............................................................................................................................................. 102 Chapter 5: Mina Chupa ................................................................................................................................... 103 5.1 – Introduction, Purpose, and Previous Studies ...................................................................................... 103 5.2 – Mina Chupa Background and Location ............................................................................................... 104 5.3 – Sample Collection .................................................................................................................................. 107 5.4 – Analysis and Results............................................................................................................................... 110 5.5 –Discussion ................................................................................................................................................ 116 5.6 – Exploration Implications and Conclusions ......................................................................................... 129 Chapter 6: Pb Isotope Classification of Mineralizing Events ....................................................................... 131 6.1 – Introduction and Purpose ..................................................................................................................... 131 6.2 - Sampling Strategy and Selection ............................................................................................................ 133 6.3 - Results ...................................................................................................................................................... 135 6.4 – Discussion and Interpretation .............................................................................................................. 147 6.5 – Conclusions ............................................................................................................................................ 155 Chapter 7: Geochronology .............................................................................................................................. 156 7.1 – Introduction and Purpose ..................................................................................................................... 156 7.2 - Previous Work ........................................................................................................................................ 157 7.3 – Sample Selection, Methodology, and Analytical Techniques ............................................................ 158 7.4 – Results ..................................................................................................................................................... 159 7.5 – Discussion and Conclusion ................................................................................................................... 161 Chapter 8: Luminescence in Carbonate Rocks and Minerals ...................................................................... 162 8.1 – Introduction and Purpose ..................................................................................................................... 162 8.2 – Evaluation of Luminescence in Study Samples ................................................................................... 162 8.3 – Results for Luminescent Rocks and Minerals Around Iscaycruz ...................................................... 162 ix 8.4 – Discussion and Interpretations ............................................................................................................. 169 Chapter 9: Discussion and Conclusions ........................................................................................................ 174 9.1 – Discussion ............................................................................................................................................... 174 9.2 - Stable Isotope Alteration Patterns ......................................................................................................... 174 9.3 – Pattern Consistency ............................................................................................................................... 175 9.4 - Luminescent Rocks and Minerals ......................................................................................................... 176 9.5 - Fluid Pathways ........................................................................................................................................ 176 9.6 – Mina Iscaycruz as a Single Mineralizing System ................................................................................. 177 9.7 – Final Conclusions ................................................................................................................................... 178 9.8 – Exploration Implications and Recommendations for Future Work ................................................. 179 References.................................................................................................................................................................. 183 Appendices ................................................................................................................................................................ 189 Appendix A1 – Surface Isotope Sample Data ........................................................................................................ 190 Appendix A2 – Subsurface Isotope Sample Data ..................................................................................................205 Appendix A3 – Santa Este Transects and Results .................................................................................................. 222 Appendix A4 – Mina Chupa Transects and Results ............................................................................................. 262 Appendix A5 - Geochronology ............................................................................................................................... 302 Appendix A5.1 – Zircon Sample Prep ........................................................................................................... 302 Appendix A5.2 – SFCPBLP6 Data ................................................................................................................. 304 Appendix A6 – Pb Isotopes ..................................................................................................................................... 321 Appendix A6.1 – Analytical Procedure and Raw Data ................................................................................ 321 Appendix A6.2 – Pb isotope data used in comparative study ..................................................................... 323 Appendix A6.3 – Comparison Graphs .......................................................................................................... 346 x  List of Tables Table 3.1 - A comparison between Valanginian carbonates and Late Aptian-Early Albian carbonates ............ 36 Table 3.2 - SFCRS005 Analytical Results – Santa Limestone ................................................................................. 39 Table 3.3 - δ13C and δ18O Thresholds for Pariahuanca and Santa Formation Rocks .......................................... 45 Table 4.1 - Summary of Santa Este North surface transects ................................................................................... 63 Table 4.2 – SE Subsurface Transects ......................................................................................................................... 68 Table 4.3 – Spot Type categories and the associated qualitative definition for each group ................................ 72 Table 5.1 – Mina Chupa Transects ......................................................................................................................... 107 Table 5.2 – Summary of Mina Chupa drillholes .................................................................................................... 109 Table 5.3 - Summary of Carbonate Wall Rock and Vein Pair δ18O composition from Mina Chupa ............. 129 Table 7.1 - Prior Geochronological Work on Palpas Tonalite ............................................................................. 158 Table 7.2 - Iscaycruz and Palpas Area Geochronological Work .......................................................................... 160     xi  List of Figures Figure 1.1: Mina Iscaycruz deposits with 2015 and 2016 surface transects ............................................................ 5 Figure 1.2: Relative abundances of U, Th, and Pb isotopes at the formation of the Earth (left) and the present day (right); adapted from Gulson 1986. Quantities of 206Pb, 207Pb, and 208Pb have increased relative to their primordial abundances, whereas 204Pb has not. ...................................................................................................... 9 Figure 2.1: Map of Central Peru highlighting prominent tectonic regions (Figure 2.2, dashed box), location of Mina Iscaycruz, and A-A’ cross section (Figure 2.2.1) ........................................................................................... 18 Figure 2.2: Tectonic and depositional setting of central Peru in the Early Cretaceous. Iscaycruz host rocks (Santa and Pariahuanca formations) formed in the back-arc portion of the Western Peruvian Trough – Adapted from Scherrenberg 2012 ............................................................................................................................. 19 Figure 2.3: Generalized cross section of the Western Cordillera showcasing the Quechua phase folds (adapted from Pfiffner and Gonzalez 2013) ............................................................................................................................. 20 Figure 2.4: Schematic of the evolution of arc magmatism in Central Peru during the Cenozoic (adapted from Bissig 2005). Each section represents the same region of Peru at a different time period. .................................. 22 Figure 2.5: Stratigraphic column of rocks at Mina Iscaycruz and surrounding areas with corresponding ages. Column displays thickness (m), width conveys the unit’s resistance to erosion and rocks are colored according to their unit’s color code throughout the thesis. Sources for index fossils and ages: (1) Benavides-Ciceres 1956, (2) Wilson 1963, (3) Von Hillebrandt 1970, (4) Romani 1982 ...................................................... 23 Figure 2.6: Location of intrusive rocks in the Iscaycruz area. Palpas Tonalite to the west was sampled for this study and yielded an age that is between the two ages determined for the Quellacocha Lake Dikes. ................ 27 Figure 3.1: Schematic diagram illustrating three example surface transect types centered on a lithologic contact, a fault contact, and a mineralization contact. ............................................................................................ 29 Figure 3.2: Diagram demonstrating how subsurface transects were sampled. Samples were collected at specific distances from the feature being examined. The distances measured are perpendicular to the orientation of the feature. ................................................................................................................................................................... 30 Figure 3.3: Location of background samples relative to Iscaycruz deposits and nearby mines .......................... 34 Figure 3.4: δ18O vs δ13C scatterplot of all background and reference samples. Colored fields denote observed range of background compositions. Inca reference samples assigned δ18O value of 33‰ for plotting purposes ...................................................................................................................................................................................... 37 Figure 3.5: δ18O Plot for Pariahuanca background samples displaying the lower values for SFCRS016 compared to the other samples analyzed. ................................................................................................................ 41 Figure 3.6: δ18O vs δ13C scatterplot of background, reference, and all study results. ....................................... 43 Figure 3.7: δ18O vs δ13C scatterplot of background, reference, and all study results. ....................................... 44 Figure 3.8: Diagram illustrating organization and domaining of luminescent samples. Fluorescence intensity ranges from 0.5-3F, and phosphorescent intensity ranges from 0.5-4P. The color fill indicates the four major color categories observed ........................................................................................................................................... 50 Figure 3.9: Sample FS142, calcite displaying 3F white intensity UV fluorescence (left) and 4P green UV phosphorescence (right). ........................................................................................................................................... 51 xii  Figure 3.10: FS90 – Sample with 3F white, blue, and yellow fluorescent calcite, 3P green and 4P blue phosphorescent calcite, and 3P phosphorescent, Santa limestone wall rock. ....................................................... 51 Figure 4.1: Santa Este deposit showing the nearby surface transects and drill holes included in the study. ..... 54 Figure 4.2: Iscaycruz area looking NW. Open pit outlines are yellow dashed lines, boundary of Santa Formation is shown as black dashed lines ................................................................................................................ 55 Figure 4.3: Iscaycruz geologic model highlighting the location of the Santa Este deposit. 2015 and 2016 surface isotope transects are rendered as small white circles. ............................................................................................. 56 Figure 4.5: Sample EXGR00008108 from SE-L4-1 displaying a stylolite with localized alteration and recrystallized calcite nodules with intense UV fluorescence and phosphorescence. ........................................... 58 Figure 4.6: Santa Este deposit, Santa Este transects, and Santa Este North transects........................................... 60 Figure 4.7: Overhead view of Santa Este pit showing locations of SE-L1-2, SE-L4-1, and SE-L5-1 transects. .. 61 Figure 4.8: SE-L4-1 transect, centered on gossan contact (red dotted line) and a secondary transect centered on fault contact (yellow dotted line) ......................................................................................................................... 62 Figure 4.9: Stereonet of the 18 lithologic contacts from Santa Este North ............................................................ 64 Figure 4.10: Stereonet of the 17 faults sampled from Santa Este North ................................................................ 65 Figure 4.11: Location of SEN transects near a large structural intersection with 500m sinistral displacement. Gossanous transects were mostly associated with bedding parallel and stockwork structures. ......................... 66 Figure 4.12: Sample photos from the SEN-L2-2 transect centered on a brecciated, skarn altered outcrop with a 2cm calcite vein perpendicular to bedding. ............................................................................................................. 67 Figure 4.13: Santa Este drill holes sampled during the 2015 and 2016 field seasons ........................................... 68 Figure 4.14: Comparison of background Santa Formation rock composition and range of calcite in equilibrium with magmatic fluid (Bowman et. al 1998). Adapted from Escalante 2008. .................................... 72 Figure 4.15: δ13C (left) and δ18O (right) results for all Santa Este surface results ............................................. 73 Figure 4.16: δ13C vs δ18O results from Santa samples by surface vs subsurface. Subsurface samples were located proximal to the Santa Este deposit and yielded lower average δ13C and δ18O values than surface samples. ....................................................................................................................................................................... 74 Figure 4.17: Surface sample results by broad spot type. Calcite vein samples yielded the lowest average values for both δ13C and δ18O. Dolomitized limestone displayed the largest contrast between δ18O and δ13C results. .......................................................................................................................................................................... 75 Figure 4.18: δ13C vs δ18O results by their UV fluorescent color. Non-fluorescent samples are not shown. Generalized compositional fields are drawn around samples of the same color. Lines indicate threshold values, dashed = background -> depleted, dotted = extremely depleted threshold ............................................. 76 Figure 4.19: δ13C vs δ18O results by transect feature (Surface samples only). Mineralized transects, gossans, lithologic contacts, and faults yielded the lowest results for δ18O. ....................................................................... 77 Figure 4.20: Tukey Plots for isotopic results for all Santa Este surface transects showing δ13C and δ18O (‰) against sample distance from transect center .......................................................................................................... 78 Figure 4.21: Tukey Plots for isotopic results for Santa Este surface gossan transects showing δ13C and δ18O (‰) against sample distance from transect center. ................................................................................................. 78 xiii  Figure 4.22: δ13C vs δ18O results in the southern portion of Santa Este North. The three highlighted regions contain multiple transects with extremely depleted results. ................................................................................... 79 Figure 4.23: δ13C vs δ18O results in the northern portion of Santa Este North. The highlighted regions contain multiple transects with extremely depleted results. ................................................................................... 80 Figure 4.24: Zone 1 highlighting smaller scale displacement in Santa limestone from east-west faults. Transects in dashed boxes had isotopic depletions mostly in limestones, whereas samples in the solid box were depleted mostly in calcite veins. ....................................................................................................................... 81 Figure 4.25: Zone 2 highlighting depletions in δ18O adjacent to E-W structures and depletions in δ13C at lithologic contacts. Pink and red UV fluorescent calcite veins were depleted in δ18O in E-W oriented, late-stage calcite veins. ....................................................................................................................................................... 82 Figure 4.26: Zone 3 displaying δ13C (left) and δ18O (right) results and locations of pink and red UV fluorescent calcite veins. Depletions in δ18O are focused around E-W faults whereas δ13C depletions are localized near lithologic contacts south of the Mancacuta Fault............................................................................ 83 Figure 4.27: Zone 4 transects displaying results for δ13C (left) and δ18O (right). Lithologic contact and dolomitized contact samples on the north and south sides of the E-W fault yielded extremely depleted values. ...................................................................................................................................................................................... 84 Figure 4.28:  Parallel downhole plots for SE-1-15-01 and SE-14-05 showing δ13C and δ18O (‰), alongside sample spot type, background thresholds, and key features................................................................................... 85 Figure 4.29: Parallel downhole plots for SE-14-07 and SE-14-11 showing δ13C and δ18O (‰), alongside sample spot type, background thresholds, and key features................................................................................... 86 Figure 4.30: Parallel downhole plots for SE-14-08 and SE-14-1 4 showing δ13C and δ18O (‰), alongside sample spot type, background thresholds, and key features................................................................................... 87 Figure 4.31: Parallel downhole plots for SE-14-16 and SE-13-06 showing δ13C and δ18O (‰), alongside sample spot type, background thresholds, and key features................................................................................... 88 Figure 4.32: Parallel downhole plots for SE-13-07 and SE-7130-04 showing δ13C and δ18O (‰), alongside sample spot type, background thresholds, and key features................................................................................... 89 Figure 4.33: Parallel downhole plots for SE-13-07 and SE-7130-04 showing δ13C and δ18O (‰), alongside sample spot type, background thresholds, and key features................................................................................... 90 Figure 4.34: Subsurface Santa results by spot type. Lines indicate threshold values, dashed = background -> depleted, dotted = extremely depleted threshold .................................................................................................... 91 Figure 4.35: Tukey Plots for calcite and dolomite vein isotopic results for all Santa Este surface transects showing δ13C and δ18O (‰) against sample distance from transect center. Distances beyond 15m contained a total of seven samples and are not shown.............................................................................................................. 92 Figure 4.36: δ13C vs δ18O results by their simplified spot ID. Detailed paragenesis at individual sample locations can vary, but simpler cross-cutting relationships between middle and late calcite veins was more reliable. ........................................................................................................................................................................ 94 Figure 4.37: δ13C vs δ18O results by their distance to the Santa Este deposit ..................................................... 96 xiv  Figure 4.38: δ13C vs δ18O results for Santa surface and subsurface samples. Colors displayed are a point density cloud with colder colors (blue to green) indicating low to moderate point density, and warmer colors (yellow to red) indicating high point density. .......................................................................................................... 99 Figure 4.39: δ13C vs δ18O results for Santa surface samples highlighting the two main fluid evolution paths. Transects were attributed with the fluid evolution path that best represented their sample distribution. ........ 99 Figure 4.40: Locations of transects with gossans, oxidized samples, or skarn alteration in Santa Este North. Gossans and skarn samples were only observed south of the Mancacuta Fault. ................................................ 101 Figure 5.1: Iscaycruz Mine in Central Peru with locations of Mina Chupa and associated drill holes (black labels) relative to other deposits and prospects in the mine area. ........................................................................ 104 Figure 5.2: Leapfrog model of study area. Image is looking to the S, showing location of Iscaycruz deposits (colored circles). Drill hole collars (black circles) and surface transects (blue circles) are also displayed. ...... 105 Figure 5.3: Overview of Mina Chupa area lithologies, looking  NW atop ridge above mine entrance ............ 106 Figure 5.4: Mina Chupa South and North Regions with locations of surface transects (white labels) and drill holes from study (black labels). ............................................................................................................................... 108 Figure 5.5: Illustration of background and depleted values for Pariahuanca rocks. Modified from Escalante 2008. ........................................................................................................................................................................... 111 Figure 5.6: Isotopic results for all Mina Chupa surface transects showing δ13C and δ18O (‰) against sample distance from transect center. ................................................................................................................................. 112 Figure 5.7: δ13C vs δ18O results for Mina Chupa surface samples colored and shape coded by spot type. ... 112 Figure 5.8: Tukey Plots for isotopic results for all Mina Chupa South surface transects showing δ13C and δ18O (‰) against sample distance from transect center ..................................................................................... 113 Figure 5.9: Parallel downhole plots for CHS-7-14-01 and CHS-7-14-02 showing δ13C and δ18O (‰), alongside sample spot type, background thresholds, and key features. .............................................................. 114 Figure 5.10: Parallel downhole plots for PA-12-02 and CHN-13-02 showing δ13C and δ18O (‰), alongside sample spot type, background thresholds, and key features................................................................................. 115 Figure 5.11: Parallel downhole plot for CH-8-12-05  showing δ13C and δ18O (‰), alongside sample spot type, background thresholds, and key features. ..................................................................................................... 116 Figure 5.12: Tukey Plots for isotopic results for all Mina Chupa surface transects showing δ13C and δ18O (‰) for samples within 5 meters from transect center. Calcareous sandstone samples show significantly lower δ13C values than other lithologies. ......................................................................................................................... 117 Figure 5.13: δ18O results (‰) and threshold values within PA-12-02, highlighting the wall rock results surrounding  the fault zone in Transect 2 .............................................................................................................. 118 Figure 5.14: Fence section across Mina Chupa area with MCS and MCN transect locations. .......................... 121 Figure 5.15: Fence section of the Mina Chupa area with all surface transects displaying the δ18O value for the first sample in each transect. .................................................................................................................................... 122 Figure 5.16: Fence section of the Mina Chupa area with all surface transects displaying the average δ18O value for sample results from 0-10 meters from the transect center. .................................................................. 122 Figure 5.17: Fence section of the Mina Chupa area with all surface transects displaying the average δ18O value for sample results from >10 meters from the transect center. .................................................................... 123 xv  Figure 5.18: δ18O results (‰) and threshold values within CH-8-12-05. Alteration haloes surround both the mineralized skarn and the mineralized fault .......................................................................................................... 125 Figure 5.19: δ13C vs δ18O results for all Mina Chupa samples. Fluid evolution paths for Pariahuanca rocks in and around Mina Chupa are shown. Surface samples are closer to background values than subsurface samples, and calcareous sandstone samples are more depleted on average.  Both Mina Chupa and Santa Este ore zones yielded a similar field of isotopic composition. .................................................................................... 127 Figure 5.20: δ13C vs δ18O results for all Mina Chupa samples. Composition of Mina Chupa ore zone, ore zone calcite, and igneous calcite in equilibrium with magmatic intrusions from skarn deposits (Bowman 1998). Meteoric fluids interacting with ore zone rocks either during or after mineralization can significantly the reduce δ18O composition of host carbonates. ................................................................................................ 128 Figure 6.1: Iscaycruz District showcasing location of Pb isotope samples and Palpas tonalite intrusion in the west and A-A’ cross section location ...................................................................................................................... 134 Figure 6.2: Pb isotopic results from Bissig and Tosdal 2009 and this study. Results from Iscaycruz are displayed in the three blue fields:  primary economic mineralization at all Iscaycruz deposits (dotted), results from a late stage vein-controlled galena sample (solid), and results from four samples from EMQSA exploration projects between Iscaycruz and Palpas (dashed). The mineralization intersected in the drill hole (solid) is derived from a localized, late stage, post-mineral north/south vein-controlled event (EMQSA pers. comm)........................................................................................................................................................................ 136 Figure 6.3: (207Pb/204Pb vs 206Pb/204Pb) and (208Pb/204Pb vs 206Pb/204Pb) fields for host and country rocks and host lithology for ore samples. Results from the combined studies for host lithology and country rocks were shape coded by their interpreted Pb source and color coded by their host lithology ..................... 138 Figure 6.4: Pb isotopic results for syngenetic and exhalative deposits are displayed in fields according to the region in Peru they are derived from. Results from the combined studies were color coded by their region and shape coded by their deposit type. Results are from ore mineral samples only and not from host rock or background samples. ................................................................................................................................................ 139 Figure 6.5: Pb isotopic results for skarn and carbonate replacement deposits are displayed in fields according to the region in Peru they are derived from. Results from the combined studies were color coded by their region and shape coded by their deposit type. Results are from ore mineral samples only and not from host rock or background samples. ................................................................................................................................... 140 Figure 6.6: (207Pb/204Pb vs 206Pb/204Pb) and (208Pb/204Pb vs 206Pb/204Pb) – Cenozoic isotopic composition fields showing the Early Oligocene (solid), Middle Miocene (dashed/dots), Late Miocene (dashed), and Early Pliocene (dots). ....................................................................................................................... 142 Figure 6.7: (207Pb/204Pb vs 206Pb/204Pb) and (208Pb/204Pb vs 206Pb/204Pb) – Paleozoic to Mesozoic isotopic composition fields showing data from the Pennsylvanian and Permian-Triassic-Jurassic (solid), Early Cretaceous (dashed), and Rhaetian (dots). ............................................................................................................ 143 Figure 6.8: (207Pb/204Pb vs 206Pb/204Pb) and (208Pb/204Pb vs 206Pb/204Pb) – Iscaycruz isotopic composition fields showing data from the Limpe Centro West and Nava (solid), the main Iscaycruz trend (dashed/dots), and the late stage post-mineral event (dots). ................................................................................ 144 Figure 6.9: (207Pb/204Pb vs 206Pb/204Pb) and (208Pb/204Pb vs 206Pb/204Pb) Fields for ore samples from  syngenetic deposits separated by age. ..................................................................................................................... 146 xvi  Figure 6.10: (207Pb/204Pb vs 206Pb/204Pb) and (208Pb/204Pb vs 206Pb/204Pb) – Bissig and Tosdal 2009 Samples by Group Number, Shaped by Mineral Sampled. Three samples classified as Group 2 were also classified as possible transitional examples and are displayed in a purple field (Bissig and Tosdal 2009)....... 148 Figure 6.11: (207Pb/204Pb vs 206Pb/204Pb) and (208Pb/204Pb vs 206Pb/204Pb) – Bissig and Tosdal 2009 samples by group number and samples from this study, shaped by mineral sampled. The transitional composition field contains multiple samples from this study, however the Group 2 field does not. The black circle represents three samples from this study that could have been used in Bissig and Tosdal 2009. ........... 148 Figure 6.12: Tukey box plot Pb isotopic results from Bissig and Tosdal 2009 and this study by ore mineral sampled. Both studies showed that pyrite and sphalerite samples had higher average isotopic compositions than samples taken from galena. ............................................................................................................................. 150 Figure 6.13: (207Pb/204Pb vs 206Pb/204Pb) and (208Pb/204Pb vs 206Pb/204Pb) – Iscaycruz Pb Isotopes show fields for the main Iscaycruz trend distal (dashed line), the late stage mineralization (solid line), and the Iscaycruz regional samples (dotted line). ............................................................................................................... 151 Figure 6.14: (207Pb/204Pb vs 206Pb/204Pb) and (208Pb/204Pb vs 206Pb/204Pb) –Iscaycruz Pb Isotopes show fields for proximal (solid line), medial (dotted line), and distal (dashed line) ore samples. Proximal samples are located from Mina Chupa, medial samples are from Limpe Centro and Santa Este, and distal samples are from Anelcocha, Antapampa, and Limpe Norte. ............................................................................. 151 Figure 7.1: Iscaycruz District showcasing location of Pb isotope samples and Palpas tonalite intrusion in the west. Sample SFCPBLP6 is shown on the map. ..................................................................................................... 156 Figure 7.2: SFCPBLP6 sample location alongside the five historical samples. ................................................... 159 Figure 7.3: Concordia diagram for Palpas tonalite sample SFCPBLP6. .............................................................. 160 Figure 7.4: Weighted average of 206PB/238U ages for Palpas tonalite sample SFCPBLP6. .............................. 161 Figure 8.1: δ13C vs δ18O – UVP Intensity in Santa and Pariahuanca Rocks Around Iscaycruz. Calcite veins are not displayed. UVP Pariahuanca limestone samples varied in δ18O but did not display significant depletions in δ13C. The vast majority of UVP Santa carbonates were above background however individual samples from drill holes within the Santa Este pit (dashed line) yielded extremely depleted values for δ18O and a mix of background to extremely depleted for δ13C. .................................................................................. 164 Figure 8.2: δ13C vs δ18O – UVF Color and Intensity in Pariahuanca Rocks Around Mina Chupa. Bright red calcite veins yielded only extremely depleted values in δ18O. ............................................................................. 165 Figure 8.3: 3F red fluorescent calcite from Mina Chupa drill hole CHS-7-14-02 .............................................. 165 Figure 8.4: δ13C vs δ18O – Santa UVP – Carbonate rock samples only. The dashed line field highlights the UVP samples located in Santa Este drill holes ....................................................................................................... 166 Figure 8.5: δ13C vs δ18O – Santa UVP – Carbonate vein samples only. ........................................................... 167 Figure 8.6: δ13C vs δ18O – UVF Color Families in Santa Formation Rocks. Large contrasts in average composition are present between the color families. ............................................................................................ 168 Figure 8.7: δ13C vs δ18O – UVF Color Families in Santa Carbonate Rocks and Veins. Tukey Box and Whiskey Plots............................................................................................................................................................ 168 Figure 8.8: Location of UVP colors in carbonate veins. UVP veins were rarely encountered proximal to the deposits. Blue UVP calcite was correlated to structures south of the Mancacuta Fault..................................... 171 xvii  Figure 8.9: Location of red and pink family UVF samples from Mina Chupa and Santa Este. Three surface samples (one in MCN and two in SEN) displayed red fluorescence, all others were in deposit-proximal drill holes. .......................................................................................................................................................................... 172 Figure 9.1: δ13C vs δ18O results for all samples. Fluid evolution paths for Pariahuanca and Santa Formation rocks are shown in red and black dashed lines respectively. For background and altered samples, isotopic results for Pariahuanca rocks show less variation than Santa rocks. Both Mina Chupa and Santa Este ore zones yielded a similar field of isotopic composition. ..................................................................................................... 178 Figure 9.2: Simplified conceptual diagram demonstrating that a higher number of short transects targeting suspected fluid pathways is the optimal sampling strategy to begin a project. If a pervasive alteration halo is detected, transect widths can always be increased in response, or a lower number of wider transects can be periodically collected and only analyzed in the case of a pervasive halo. ............................................................ 181   xviii  List of Abbreviations  CRD: Carbonate replacement deposit EMQSA: Empresa Minera Los Quenuales S.A. LGR - Los Gatos Research MDRU: Mineral Deposit Research Unit MIA - Mineral Isotope Analyzer NS - Not Sampled: This denotes a sample that was collected but not microdrilled and analyzed on the MIA OA-ICOS: Off-Axis Integrated Cavity Output Spectroscopy SE: Santa Este SEN: Santa Este North SW: Short Wave (type of fluorescent excitation) UBC: University of British Columbia UV: Ultraviolet UVF: Ultraviolet fluorescence UVP: Ultraviolet phosphorescence VPDB - Vee Pee Dee Belemnite VSMOW - Vienna Standard Mean Ocean Water   xix  Acknowledgements  I want to express the utmost gratitude to my wonderful and endlessly patient wife, Kara Zucker. This thesis would not exist without you. Your continued motivation and attention to the completion of this work was a consistent and driving force.   Thank you to Craig Hart (supervisor) for initiation of the project and guidance throughout, it was always helpful to have you pull me out of the weeds when I got too deep. Thank you to Greg Dipple for all of the MIA attention and McGyvering, as well as your critical guidance on isotopic data interpretation. Additionally, thank you for letting me put hundreds of hours of lamp time with your Superbright 3 lamp, its superior brightness to the old lamps spawned an entire chapter of research that sourced a short course module. A tremendous thanks to Jim Mortensen for providing me with interesting coursework that turned into an entire chapter, as well as your timely and comprehensive thesis edits. Thank you to Abraham Escalante for providing project guidance, continuous assistance in field work coordination, ensuring safe and complete sample delivery, and your continued interest throughout the project. You have been a great person to brainstorm ideas with.  Ronald Vasquez, Gonzalo Quiroz, and Christian Vela are thanked for their invaluable help with geologic mapping and sampling, and for their assistance in organizing and coordinating the core retrieval and preparation. I would also like to thank Henry Rios for handling the labeling and processing of over 800 samples in a mere three weeks.   To my cadre of MDRU and EOAS friends, thank you for the good times, the kvetching, the edits, and the practice runs. A big thank you to Niki, Fabien, Chris, Dave, Kaleb, and Raja for thesis edits and practice. The biggest and brightest thank you goes to Ally Brown for the tremendously helpful and productive full thesis review.  To our wonderful little geode arriving in March, this thesis is for you.  1  Chapters Chapter 1: Introduction 1.1 - Project Rationale Successful mineral exploration efforts are dependent on the prudent use of modern pathfinders and tools to reduce the cost of finding new deposits to support the global economy. Many mineral deposit types have characteristic visible alteration patterns in the surrounding rocks that can act as a vector for exploration geologists, commonly with kilometer-scale haloes of well-studied mineral zonation patterns (e.g., porphyry copper deposits; Sillitoe, 2010). Alteration patterns associated with hydrothermal ore deposits result from extensive interaction between the surrounding rocks and the relatively hot hydrothermal fluids (Hedenquist and Lowenstern, 1994, Galley et al. 2007, Sillitoe 2010, Meinert 2013). The geometry of the host rocks, temperature of fluids, volume of fluids, depth of formation, initial isotopic composition, and deposit type can all affect the specific characteristics of an alteration pattern, and explorers must understand the dynamics of these factors to be able to best utilize them for exploration predictions (Barker and Dipple 2019).  Carbonate replacement deposits and skarns have highly reactive carbonate host rocks that can neutralize acid present in the hydrothermal fluid, which can lead to relatively narrow visible alteration patterns (typically tens of meters), and thus present an additional challenge to exploration efforts (Friehauf & Pareja 1998, Megaw 1998, Meinert 2013, Barker and Dipple 2019). As early as the 1950’s, researchers discovered that the use of the stable isotopes of carbon and oxygen, δ13C and δ18O, could be used to explore for these and other deposit types (Engel et al., 1958). The cost per sample, analytical turnaround time, and unproven track record of the application of stable isotopes prevented significant industry adoption of the technique. Despite these factors, stable isotopes have been applied in studies of carbonate-hosted ore systems (Shimazaki et al. 1986, Shimazaki & Kusakabe 1990, Ghazban et al. 1991, Naito et al. 1995, Vazquez et al. 1998, Marie and Kesler 2000, Velasco et al. 2003, Escalante 2008) which mainly examined the relationship between fluid:rock interaction and changes in isotopic composition of host rocks and veins.  Traditionally, gas-source isotope mass spectrometry techniques have been too time consuming and expensive for the application to industry problems (Green and Taheri 1991, Escalante 2008). For application in 2  hydrothermal ore systems, the difference in isotopic composition of background rocks vs hydrothermally altered carbonates is commonly single to double-digit integer differences, whereas prior analytical techniques provided isotopic results to two or three decimal places (Green and Taheri 1991, Escalante 2008). The development of Off-Axis Integrated Cavity Output Spectroscopy (OA-ICOS) technology in recent years has provided the industry a cheaper and faster alternative to conventional stable isotope ratio mass spectrometry (Barker et al. 2011, Barker et al. 2013). This technology uses infrared absorption to measure the isotopic composition of different gas species that resulted from the dissolution of carbonate samples. Studies using this technology have been able to research hydrothermal systems via the analysis of 1,000’s of samples instead of dozens (e.g., Lepore 2013, Cook 2016, Herron 2018, Beinlich et al. 2019). The University of British Columbia (UBC) has an OA-ICOS instrument, the Mineral Isotope Analyzer (MIA), and studies using the MIA have been conducted through the Mineral Deposit Research Unit (MDRU).  The application of stable isotopes in conjunction with other investigative methods (fluid inclusions, mineralogy, Pb isotopes) can provide a comprehensive understanding of the evolution of a hydrothermal ore system and provide practical exploration constraints by highlighting which ore-forming factors are key to identifying potential new areas of mineralization (Rye 1993). This thesis focuses on defining the isotopic alteration footprints of the Mina Chupa skarn deposit and Santa Este carbonate replacement deposit, both deposits of the Iscaycruz system in central Peru, primarily through the analysis of C and O isotopes. A secondary research component within this thesis is to test the hypothesis that Iscaycruz formed from a single, centralized mineralizing system (Escalante and Hart 2011, 2012) using Pb, C and O isotopes to investigate the homogeneity of the mineralizing fluids. This study aims to develop more effective exploration tools to identify alteration halos and improve the methodology for the discovery of skarn and carbonate replacement deposits, which include both quantitative metrics (C and O isotopes) as well as qualitative methods like the luminescent response of carbonate rocks and minerals.   1.2 - Objectives The following research objectives have been defined for this study: 1. Measure the extent and orientation of stable isotope alteration patterns within host and country rocks at the Mina Chupa and Santa Este deposits at both local and regional scales 3  2. Delineate the differences and relationships between identified alteration patterns of the two deposits  3. Utilize Pb isotope and stable isotope data to confirm or reject the existing hypothesis that Iscaycruz formed from a single mineralizing system 4. Investigate the relationship between luminescent properties of carbonate rocks at Mina Chupa and Santa Este with stable isotope, mineralization, or alteration data. 5. Evaluate the efficacy of stable isotopes and luminescence as exploration tools and provide practical recommendations for industry to reliably identify alteration patterns applicable to exploration of skarn and carbonate replacement deposits.  1.3 - Thesis structure Chapter 1 provides an overview into the project objectives and research questions and a brief background in skarn and carbonate replacement deposits,  together with a summary of Mina Iscaycruz, and background on stable isotopes, Pb isotopes, and the basics of luminescence in rocks and minerals. Chapter 2 details the regional tectonic and geologic setting of Mina Iscaycruz and supplies some detail on the regional factors that contribute to polymetallic ore deposits in Central Peru. Chapter 3 details the methodology utilized for sample collection, analysis, and display for the thesis, and focuses on the isotopic background of Iscaycruz area rocks, as well as a comparison to the global isotopic background for rocks and seawater of the same age. Chapter 4 focuses on the Santa Este deposit and the surface exploration efforts in the 9-km long region north of the mine, designated Santa Este North. Chapter 5 focuses on Mina Chupa and the surrounding surface rocks and serves as a lithologic and deposit type contrast with results from Chapter 4. Chapter 6 highlights the Pb isotopic composition of sulfides from Iscaycruz deposits and compares them with results of other Pb isotope studies of Peruvian deposits. Chapter 7 discusses the geochronology of a tonalite intrusion to the west of Iscaycruz and provides a comparison to previous geochronological work by Escalante and Hart. Chapter 8 details the study into the luminescent properties of Iscaycruz carbonate rocks and veins and how luminescence can complement isotopic studies and exploration efforts. Chapter 9 integrates Chapters 2-8 and presents the overall conclusions of the thesis and recommendations for future work.  1.4 – Overview of Mina Chupa and Santa Este 1.4.1 - Mina Chupa background and mineralization Mina Chupa is a Cu-Zn skarn deposit hosted in the carbonate and siliciclastic rocks of the Pariahuanca and Farrat formations (Flores 1990, Escalante and Hart 2011, 2012, Chapter 2). The deposit is the only zone of 4  mineralization within Iscaycruz that is hosted in Pariahuanca limestone, and the skarn characteristics provide a deposit-type contrast to the carbonate replacement style deposits of Santa Este, Limpe Centro, and others at Iscaycruz (described by Flores 1990, Escalante and Hart 2011, Escalante and Hart 2012). The Mina Chupa deposit is located west of the Iscaycruz anticline to the south of a topographic depression (Figure 1.1). The orebody at Mina Chupa has a semi-cylindrical shape with dimensions approximately 600m x 200m (Escalante and Hart 2011). Mina Tinyag is the nearest deposit, located ~400m to the northeast of Mina Chupa. The main structural trends in the Mina Chupa area consist of the larger, N-N300W trending, bedding-parallel structure which forms the central Iscaycruz anticline, and an E-NE trend of primarily strike-slip structures with some apparent vertical displacement (Escalante and Hart 2011). At Mina Chupa, mineralization occurs primarily at the intersection of these two controlling trends, with mineralization bounded by a subvertical fault (Fault 1) in the north and a low angle fault (Fault 2) in the south (Escalante and Hart 2011). Mineralization at Mina Chupa is primarily stratabound within the carbonate and siliciclastic rocks of the Pariahuanca Formation along the contact with the sandstones and quartzites of the Farrat Formation. Ore minerals consist of coarse-grained sphalerite and massive chalcopyrite in centimeter to meter scale layers and veins with minor to trace pyrrhotite and pyrite filling the interstices in tremolite, actinolite, and ilvaite rich layers (Escalante and Hart 2011).  The surface above the underground mine contains scattered Fe and Cu oxide showings that occur sub-parallel to bedding (Escalante and Hart 2011). Layers of ore mineralization are interbedded with limestone, chloritized calcareous sandstone, dolomitized limestone, and grey, brown, and green marble (Escalante and Hart 2011). Observable skarn alteration of host rocks is not pervasive within the host units and occurs near ore minerals and mineralizing veins. Textural relationships indicate that an initial phase of pyrite deposition was followed by pyrrhotite + chalcopyrite + sphalerite, followed by the second phase of pyrite (Escalante and Hart 2011). The primary alteration assemblage at Mina Chupa includes andradite, Mg-calcite, Mg-chlorite, clinochlore, epidote, muscovite, phlogopite, rutile, and talc, whereas lower levels of the mine contained andradite + magnetite + titanite + ilvaite (Escalante and Hart 2011). Retrograde alteration at Mina Chupa partially replaced garnet minerals in an initial phase of epidote + clinochlore + phlogopite + johannsonite, followed by a second phase of actinolite + stilpnomelane + calcite (Escalante and Hart 2011). Dickite, illite, followed by kaolinite, occur as millimeter scale haloes around ore minerals (Escalante and Hart 2011).   5   Figure 1.1: Mina Iscaycruz deposits with 2015 and 2016 surface transects  1.4.2 - Santa Este background and Mineralization The Santa Este deposit at Iscaycruz is a distal, Zn-rich carbonate replacement deposit hosted in carbonate and siliciclastic rocks of the Santa, Chimu, and Carhuaz formations (Flores 1990, Escalante and Hart 2011, 2012). 6  The deposit is located on the eastern limb of the Iscaycruz anticline (Figure 1.4.1) and is similar in mineralization style to Limpe Centro; however, it has fluid characteristics indicating slightly lower temperature and salinity (Escalante and Hart 2012). The deposit was discovered from the presence of goethite-rich gossans at the surface at the contact between the carbonate rocks of the Santa formation and quartzites and siliciclastic rocks of the Chimu formation (Escalante and Hart 2012, Chapter 2). Mineralized ore bodies at Santa Este are observed to be cut by staircase-geometry listric faults that dip to the east (Empresa Minera Quenuales S.A. geologists). Bedding-parallel faulting within the Santa/Chimu contact is observed as high-angle, west dipping faults that resulted in a vertical displacement of lithologic contacts. Mineralization at Santa Este is primarily stratabound and proximal to the contact between Santa Formation limestones to the east and Chimu Formation sandstones and quartzites to the west (Flores 1990, Escalante and Hart 2012). Prior studies have indicated up to eight phases of sulfide/oxide mineral deposition at Santa Este: Magnetite → pyrite I ± chalcopyrite I → pyrite II → pyrrhotite → sphalerite I ± chalcopyrite → chalcopyrite II + sphalerite II + pyrite III → bornite → galena →covellite + hematite + digenite + specularite (Escalante and Hart 2012). Decarbonization, dolomitization, and chloritization alteration patterns have been noted in prior studies and observed in both surface and subsurface samples (Escalante and Hart 2012). Alteration patterns at Santa Este are consistent with those observed at Limpe Centro and other deposits west of the Iscaycruz anticline (Escalante and Hart 2012). Oxidized gossans are present on the surface, and shallow portions of drill holes are characterized by goethite, Al-goethite, chlorite-serpentine, sphalerite, phosgenite and lepidocrocite (Escalante and Hart 2012).   1.5 - Characteristics of Skarn Deposits Skarns are a diverse class of mineral deposits which share the core characteristics of an assemblage of calc-silicate minerals, primarily garnets and pyroxenes (Meinert 1992). Skarns vary from barren to rich in base and precious metals (Au, Cu, Fe, Pb, Sn, W, Zn) and each commodity can be tied to different formation characteristics (Einaudi 1981, Cox and Theodore 1986, Meinert 1992). Skarns form primarily from contact metamorphism between intrusive bodies and carbonate rocks and can form in a wide variety of environments and host lithology types (Meinert 1992). Skarns usually share a similar formation process that is subdivided into three phases; the isochemical (mainly thermal) phase of contact metamorphism (first), followed by a prograde skarn metasomatism phase associated with fluid infiltration, dissolution of host rocks, and 7  precipitation of calc-silicate and garnet minerals (second), and finally, by a retrograde alteration phase which alters the earlier phases as it cools and deposits ore minerals (Meinert 1992). The skarn formation process commonly produces a deposit that is mineralogically zoned from the intrusion contact and along fluid pathways and/or lithologic contacts (Meinert 1992). Exoskarn is the term applied to portions of the deposit which form in the host rock, whereas endoskarn is reserved for skarn formation within the causative intrusion (Meinert 2013). Skarn classification is dependent on geologic setting, primary commodity, mineralogy, and carbonate protolith; however, most skarns are usually simply classified by the dominant economic metal present (Einaudi and Burt 1982).  1.6 – Characteristics of Carbonate Replacement Deposits Carbonate replacement deposits (CRDs) are intrusion-related hydrothermal ore deposits which typically form in lenses, mantos, or chimneys after mineralizing fluids have dissolved and replaced the carbonate protolith with sulfide-dominant Pb-Zn-Ag-Cu-Au mineralization (Megaw 1998). CRDs can vary widely in grade and tonnage and are typically found in the distal portions of larger porphyry and skarn systems (Megaw 1998, Sillitoe 2010). In contrast to porphyry and skarn systems, CRDs typically have limited visible hydrothermal alteration outside of the mineralized zones, with sharp country rock contacts (Megaw 1998). The individual characteristics of CRDs are largely dependent on host rock composition and characteristics, structural controls, and distance from intrusion source (Titley 1993). Characteristics common to most CRDs include a sulfide-dominant ore mineral assemblage of galena, sphalerite, and chalcopyrite for base metals and a variety of sulfides and sulfosalts for precious metals (Titley 1993, Megaw 1998). Limestones, dolostones, and dolomitized limestones are the major host rocks, and temperature of formation typically ranges from 250-500oC with acidic, saline, and reduced ore fluids (Titley 1993, Megaw 1998). Deposit geometry is host lithology-dependent with pods, lenses, pipes, and mantos representing the majority of CRD shapes (Megaw 1998). Mineralization commonly occurs throughout the majority of a single stratigraphic sequence with minor mineralization occurring in adjacent, less favorable beds (Megaw 1998). Isotopic studies of sulfur, carbon, oxygen, and lead have indicated that intrusion-proximal CRD ore fluids are magmatic in origin, with distal CRDs yielding greater meteoric and brine components as a function of distance (Megaw et al. 1988, Haynes and Kesler 1988, Beaty et al. 1990, Thompson and Beaty 1990, James and Henry 1993, Barton et al. 1995, Megaw et al. 1996). The causative intrusions are typically related to later, evolved phases of multi-phase felsic intrusions, and as such 8  CRDs tend to have a large degree of overlap in location with porphyry deposits, epithermal vein deposits, and other carbonate-related deposits (Titley 1993, Megaw 1998).  1.7 - Pb Isotope Systematics   Lead has four naturally occurring stable isotopes: 206Pb, 207Pb, 208Pb and 204Pb.  The first three of these form as a result of radioactive decay of 238U, 235U and 232Th, respectively, and their concentration in natural systems increases over time.  The fourth isotope, 204Pb, is non-radiogenic; hence its concentration remains fixed.  Lead isotopic compositions are typically measured as ratios of one of the radiogenic Pb isotopes to the “common” 204Pb, and Pb isotopic systematics are discussed in terms of the evolution of 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb ratios, which increase with time (Villa 2009; Figure 1.2).  The exception to this relationship occurs in sulfide minerals and igneous feldspars, due to the general absence of significant amounts of uranium and thorium in these minerals. Without these two radiogenic parent elements adding to the amount of radiogenic daughter products, the Pb isotopic composition present at the time of sulfide formation is locked and represents both a snapshot in time and an identifiable fingerprint of the specific ore fluid. The Pb compositions of intrusions at the time of emplacement, and therefore the composition of potentially mineralizing magmatic fluids, can be determined by measuring the composition of igneous feldspars. The isotopic composition of sulfide minerals in skarns and carbonate replacement deposits can be influenced by multiple factors, including the isotopic composition of the parent fluid and the composition of country rock reacted with to form the sulfides. These relationships are important because they not only provide context regarding the fluid evolution of an ore system, but they can also be used to differentiate between separate ore forming events.  The Pb isotopic composition of a magmatic body will change over time due to radioactive decay; however, this effect is negligible for relatively young systems such as Iscaycruz (Miocene; Escalante and Hart 2012).  9   Figure 1.2: Relative abundances of U, Th, and Pb isotopes at the formation of the Earth (left) and the present day (right); adapted from Gulson 1986. Quantities of 206Pb, 207Pb, and 208Pb have increased relative to their primordial abundances, whereas 204Pb has not.  When investigating the isotopic composition of a mineralized system it is important to distinguish between Pb isotopic measurements obtained from galena (PbS) and other Pb-rich sulfides and sulfosalts, as well as other low-Pb sulfide minerals (pyrite, sphalerite, etc.) and/or host rocks. Isotopic compositions can be determined for each of these types of samples; however, the role of ore fluids is somewhat different between these media, and their results provide different types of information. Lead isotopic compositions can be most precisely determined for Pb-rich minerals like galena, as opposed to trace Pb analyses from low-Pb sulfides and host rocks (Gunnesch 1990, Tosdal et al. 1999).  In addition, galena and other Pb-rich minerals represent very concentrated samples of Pb, whose isotopic composition is unlikely to be significantly altered by superimposed effects such as metamorphism, hydrothermal, or surface alteration.  Mineralization that contains a significant amount of Pb typically forms from fluids with relatively high Pb concentrations, and Pb isotopic compositions of such mineralization is likely to closely reflect the isotopic composition of the source fluids (Tosdal et al. 1999).  Mineralization with low Pb concentrations, however, generally form from mineralizing fluids with low Pb contents, whose isotopic compositions can be readily changed by admixtures of Pb from other sources, or 10  from host rocks (Gunnesch 1990).  In such cases the measured Pb isotopic compositions of the mineralization may not directly reflect that of the source fluid.  The texture and style of mineralization can also influence the final isotopic composition of the mineralization, especially when a significant amount of fluid rock interaction has taken place (Gunnesch 1990, Tosdal et al. 1999). Fissure vein style mineralization commonly shows less fluid rock interaction than carbonate replacement style mineralization (Tosdal et al. 1999). Host rocks may contain a significant detrital component derived from much older rocks that will commonly have significantly more radiogenic Pb isotopic compositions (Gunnesch 1990).  Lead isotopic compositions for sulfide mineralization that are hosted in such rocks may therefore show a mixed composition between the ore fluid composition and the more evolved, radiogenic composition of the entrained sediments (Gunnesch 1990, Macfarlane 1990). Detailed paragenesis is a critical accompaniment to sample selection and interpretations. For this study, only galena, pyrite, feldspar, and sphalerite were used, and a description of the source and paragenesis is provided as part of the interpretation of results.   Lead isotopic compositions are conventionally discussed in terms of variations in the 206Pb/204Pb, 207Pb/204Pb and 208Pb/204 ratios.  The 204Pb, however, generally only represents ~1.4% of the Pb contained in a sample, and ratios to 204Pb therefore generally have relatively large errors (the “204 error”).  In addition, most studies depict Pb isotopic data graphically on two diagrams; a 207Pb/204Pb versus 206Pb/204Pb (“uranogenic”) plot, and a 208Pb/204Pb versus 206Pb/204Pb (“thorogenic”) plot. Because U and Th behave somewhat differently in hydrothermal systems, these two plots may lead to somewhat different conclusions.  A preferred strategy is to present the data in terms of 207Pb/206Pb versus 208Pb/206Pb, both because these ratios are more precisely determined, and variations in the uranogenic and thorogenic systems can be evaluated simultaneously (Tosdal et al. 1999).  Unfortunately, many older Pb isotopic studies do not report ratios to 206Pb, and this preferred approach cannot always be employed.  1.8 Luminescence    1.8.1 – Introduction to Luminescence Luminescence is the natural phenomenon wherein non-thermal visible light is emitted from a substance (Rakovan and Waychunas 1996). Mineral luminescence encompasses five subtypes: fluorescence, phosphorescence, cathodoluminescence, triboluminescence, and thermoluminescence. Individual localities 11  will often have minerals that cross multiple types. Relevant types and causes of luminescence in minerals commonly found in carbonate related deposits are presented in this section.  An understanding of the factors involved in mineral luminescence is critical to understanding when and if luminescent minerals can provide relevant exploration insight.   1.8.2 –Ultraviolet Fluorescence and Phosphorescence Ultraviolet fluorescence and phosphorescence are both forms of photoluminescence, where ultraviolet light temporarily excites electrons to an elevated state, followed by a return to the ground state where an emission of a photon of visible light with a specific energy occurs (Marfunin 1979). Ultraviolet fluorescence (UVF) occurs when inner shell electrons carrying lower amounts of energy are excited by UV light, causing them to jump to a higher energy cell temporarily (Mukherjee 2012). This temporary position is referred to as “singlet state” and is followed by a return to the original inner shell location, the “ground state”. The entire process usually occurs within approximately ten nanoseconds.   Ultraviolet phosphorescence (UVP) involves excitation to a triplet state where the electron is trapped with forbidden energy transitions to the lower singlet state, resulting in a time delayed release of energy (Marfunin 1979). Generally, this manifests as luminescence that has a duration longer than 1x10-3 seconds, however this delayed emission of energy can last up to minutes or days in some substances.  1.8.3 – Other Types of Luminescence The remaining three main types of mineral luminescence are cathodoluminescence, triboluminescence, and thermoluminescence. Cathodoluminescence involves the bombardment of a luminescent material with high energy electrons thereby inducing the emission of photons (Marfunin 1979). Triboluminescence is a type of luminescence that is caused by a physical shock to the luminescing material. A physical breaking or ripping of chemical bonds occurs which can release photons of visible light (Marfunin 1979). Finally, thermoluminescence occurs when the luminescing material previously absorbed ionizing or electromagnetic radiation and is then heated, resulting in an emission of visible light (Aitken 1985). The luminescence in this study was restricted to UVF and UVP.  12  1.8.4 – Activators, Quenchers, and Sensitizers in Minerals The presence of luminescence in minerals is contingent on a specific mix of three factors called activators, quenchers, and sensitizers and their interaction with a host mineral lattice capable of forming emission centers (Gaft et. al. 2015). Activators are elemental impurities and/or defects in the crystal lattice and are concentration and temperature dependent. When a particular activator is present in the right quantity and ambient temperature, specific mineral species will luminesce when exposed to ultraviolet light. Multiple activators can be present within a mineral, and each of the activators is present as emission lines on the electromagnetic spectrum (Marfunin 1979).   Sensitizers are impurities that can either strengthen or weaken the intensity of the fluorescence of an activator. Sensitizers involve the transfer of band energy to another activator when both ions are present in the correct concentration range (Marfunin 1979). A wide variety of scenarios can occur when two or more ions are present in the crystal lattice. Certain combinations of sensitizers and activators can result in intensified fluorescence, suppressed fluorescence, conditional fluorescence (only occurring when a specific activator/sensitizer combination is present), cascading energy transfers (a series of rapid energy transfers from ions of similar type), and other more complicated interactions (Marfunin 1979.)  Quenchers are similar to activators; however they inhibit a mineral from luminescing when they are present by blocking the emission spectra from one or more activators. The absorption bands for quenching materials lie within the UV range, however the tail ends of the spectra can overlap some or all of the visible range, blocking the wavelengths of activators (Marfunin 1979). Quenching is dependent on the concentration of the quenching ions, and the degree of quenching varies by ion combination and presence of sensitizers.   Luminescence in minerals functions as a balance of activators, sensitizers, and quenchers present. A non-luminescing mineral does not necessarily mean that it does not contain certain activating characteristics, merely that the net balance was such that its luminescing potential was not met (Marfunin 1979).   13  1.8.5 – Luminescence in Limestones and Carbonate Rocks Limestones and dolomites can be fluorescent and/or phosphorescent; however, detailed studies of their UVF and UVP properties in these rocks are scarce (Wang 1997, Besouska 1998) as compared to individual minerals as described below.  Prior studies have observed similarities between the role of organic carbon and thermal maturation in petrochemicals and their relation to the fluorescence of limestones (Wang, 1997, Besouska 1998). These studies compared both acid digested whole rock samples of limestone and untreated limestone rocks of similar composition and observed that the limestones mirrored luminescence trends of the maturation of crude oil. The observed fluorescent intensity of the limestones among a variety of tested excitation wavelengths demonstrated that fluorescence in limestones behaved nearly identically to fluorescence in crude oils (Besouska 1998). As limestones and crude oils become more diagenetically transformed, intensity of fluorescence decreases, indicating that fluorescence in some limestones is due to the presence of organic carbon (Wang 1997, Besouska 1998). It was indicated that UVP is also linked to organic carbon as diagenetic processes could remobilize bitumen into strongly UVF/UVP stylolites. It should be noted that as with all luminescence, the presence or absence of luminescence in limestones should not be taken as a lone indicator of diagenetic maturity, as the presence of quenchers and sensitizers could potentially mask, diminish, or intensify the presence of organic carbon.   1.8.6 – Luminescence in Carbonate Minerals  Carbonate minerals are a diverse group of minerals that all contain the ion CO32-. Carbonate replacement deposits, skarns, and other carbonate hosted deposits contain a broad variety of carbonate minerals, some of which are primary minerals related to the host rocks and others which form as a byproduct of ore genesis. Carbonate minerals from various deposit types are listed below along with information about their luminescent properties. Luminescent colors for minerals described below are from shortwave (SW = 254nm) UV excitation unless otherwise stated. The presence of Fe, Ni, and/or Cu have a strong quenching effect on the carbonate minerals listed below (Gies 1975).  Calcite Calcite (CaCO3, trigonal) is one of the most common carbonate minerals and can be found in both the host rocks and as a product of ore genesis in carbonate related deposits. Luminescence in calcite has been extensively 14  studied and a variety of ionic activators have been identified including: Mn2+, Pb2+, Ce3+, (UO2)2+, Dy3+, Sm3+, Tb3+, Tm3+, Nd3+, and Eu3+ (Marfunin 1979; Waychunas 1989; Gorobets and Rogojine 2001; Nasdala et al. 2004; Gaft et al. 2005.). REE3+ (primarily Ce3+) and Mn2+ are the most common activators for calcite as the ions substitute for the Ca in the crystal lattice, however Pb2+ and UO22+ are also recorded as key activators in fluorescing calcites (Gaft et al 2015, Gies 1975.). From a mineral exploration perspective, the Mn2+ activator is one of the most important as it has the potential to be correlated with mineralization near skarn, nonsulfide zinc, Carlin-type Au, and CRD deposits and is commonly found at elevated concentrations (Palache 1937, Escalante 2008, Herron 2018, Megaw 2018.) Studies at Antamina and Uchuchaccua have identified a correlation between proximity to the mineralization and strong orange-red fluorescence of calcite veins, as well as their use as an identifier for vein paragenesis (Escalante 2008.) Studies at the El Mochito skarn in Honduras and the Cinco de Mayo CRD deposit in Mexico have also observed spatial correlation between strong red fluorescent calcite veins and proximity to mineralization (Megaw 2018.) The Mn2+ substitution causes a pink to red fluorescence, characterized by a single peak at 615nm, which intensifies with increasing concentration up to 800 ppm (Ali et al. 1993) This pink to red fluorescence is intensified to a strong red (or orange-red) fluorescence by the accompanying presence of Pb2+ or Ce3+, which act as sensitizers and transfer their excitation energy to the Mn2+ center (Marfunin 1979).  The Mn2+ energy transfer follows complicated but measurable cascade transfers depending on the ions present, with four main types in calcite veins: (1) Pb2+ → Ce3+, (2) Pb2+ → Mn2+, (3) Ce3+ → Mn2+ and (4) Pb2+ → Ce3+ → Mn2+ (Sidike, Aierken, et al. 2006). These cascading energy transfers are important to bear in mind when attempting to determine the actual cause of luminescence in minerals, as a bright red fluorescing calcite indicates a sensitized Mn2+ center, the cause of which could potentially be any combination of the above cascade transfers. Prior research indicated a relationship between Ce3+ sensitization and a higher intensity long wave fluorescence in calcite, as well as a shift from orange-red to red with higher Mg2+ content in short wave excitation (Gaft et al. 2008.) The exact energy transfer route taking place during fluorescence cannot be determined empirically with only a UV illumination source, and more advanced laboratory instruments are required. For calcite and other carbonates, the presence of Fe2+/Fe3+, Ni3+, and Co2+ in higher concentrations has a strong quenching effect (Marfunin 1979). 15  Aragonite Aragonite (CaCO3, orthorhombic) is a polymorph of calcite with different symmetry and crystal structure. Fewer studies on aragonite luminescence were found, however studies identified many of the same ionic activators such as Mn2+, Ce3+, (UO2)2+ and Eu3+ (Gaft et al. 2015.) Many of the calcite color/activator/sensitizer combinations have been observed for aragonite, and much like calcite, it can be fluorescent in almost any color. Early studies were limited by sample size but observed a potential correlation between the presence of Zn2+ and phosphorescence intensity in aragonites (Gies, 1975.)  Dolomite Dolomite (CaMg(CO3)2, trigonal) is another common mineral present in carbonate-related ore deposits either existing prior to ore genesis or as a byproduct of the process. Dolomite is not commonly fluorescent as either a carbonate rock or a mineral, however multiple colors of fluorescence in dolomite have been observed in nonsulfide zinc, CRD, and skarn deposits around the world as well as in this study (Ali et. al. 1993, Gorobets et. al. 2002, Gaft et. al. 2015).  Mn2+ remains an important activator in dolomites with the associated red UVF in short wave activation, however the presence of two potential crystallographic sites for the cation results in an uneven partitioning that favors the Mg2+ site (Ali et al. 1992.) Similar studies indicated that the Mn2+ ion is about four times more likely to favor the Mg2+ site over the Ca site in sedimentary dolomites (Wildeman 1970.) This ratio decreased systematically in dolomites formed from high grade contact skarns, however other hydrothermal and crystallographic interactions during skarn development may factor into this partition (Angus et al. 1984.) UV fluorescent dolomite is commonly white to yellowish white, red when activated by Mn2+, and occasionally blue or green (Gorobets and Rogojine 2002.)   Smithsonite Smithsonite (ZnCO3, Trigonal) is a zinc carbonate mineral present in nonsulfide zinc deposits, CRDs, and can be present in weathered and oxidized zones of zinc deposits as a secondary product of sphalerite. Smithsonite displays a variety of fluorescent and phosphorescent colors and intensities depending on the mix of activators, sensitizers, and quenchers (Gies 1975.) Prior detailed studies on smithsonite identified a suite of potential cation substitutions for the Zn2+ location, primarily Ni, Pb, Co, Ca, Ce, Cd, and Cr (Garcia-Guinea et al 2009.) Mn2+ can produce a red color similar to other carbonates, however blue UVF is also a common color for 16  smithsonite, with REE3+ ions as proposed activators (Gies 1975). As with other carbonates, the presence of Fe, Ni, and Co in higher concentrations has a strong quenching effect, however REE activated fluorescence appears to be less sensitive to common quenchers (Gies 1975.)   17  Chapter 2:  Tectonic and Geologic Setting of Mina Iscaycruz 2.1 - Introduction Mina Iscaycruz is located in central Peru, approximately 140 km north of Lima (Figure 2.1). This portion of Peru contains the central Andes mountain range, a region covering central Peru to northern Chile. The central Peruvian portion of the Andes contains two major mountain ranges, the Western and Eastern cordilleras. Iscaycruz is located within the Western Cordillera which trends WNW for 725 km from near Yauricocha in the south to Cajamarca in the northwest.   The Western Cordillera contains the Coastal Belt and Marañon Fold-and-Thrust Belt (MFTB) in central Peru, the latter of which includes the Western Peruvian Trough, the Marañon High, and the Eastern Peruvian Trough (Scherrenberg 2012). Comprehensive studies of the geology of central Peru were conducted by Wilson (1964), Megard (1984), Jaillard (1987), Benavides (1999), Scherrenberg (2012), and Pfiffner and Gonzalez (2013), and much of the descriptive information in this chapter is summarized from those studies. The deposit and mine scale geology in the Iscaycruz area is largely based on mapping and analysis by Glencore and EMQSA geologists, together with observations and interpretations from this study.  18    Figure 2.1: Map of Central Peru highlighting prominent tectonic regions (Figure 2.2, dashed box), location of Mina Iscaycruz, and A-A’ cross section (Figure 2.2.1) 19  2.2 – Geologic Setting and Tectonic History of Central Peru Peru is located on the west side of South America in the Andes, a region that has experienced consistent subduction style tectonism since the breakup of Pangea in the Triassic (Coira et al., 1982). During this period of tectonism, the Nazca plate has been descending beneath the western margin of the South American continental plate, creating magmatic arcs, intra-arc basins, and back-arc basins (Pfiffner and Gonzalez 2013). The back-arc basin depositional setting of the sedimentary host rocks at Iscaycruz are a direct result of this tectonic environment, as the main host rock formations formed in the Western Peruvian Trough of the MFTB (Figure 2.2) (Scherrenberg 2012).    Figure 2.2: Tectonic and depositional setting of central Peru in the Early Cretaceous. Iscaycruz host rocks (Santa and Pariahuanca formations) formed in the back-arc portion of the Western Peruvian Trough – Adapted from Scherrenberg 2012    2.2.1 – Andean Orogeny and Key Tectonic Intervals The structural evolution of the Peruvian Andes includes several regional phases of deformation that developed as fronts that migrated eastward from the Pacific Coast to the southern portion of the Andes (Pfiffner and Gonzalez 2013). In the Western Cordillera, the Mochica phase represented a period of shortening and is now 20  recorded by Early Cretaceous plutons of the Coastal Batholith which intruded folded Jurassic to Early Cretaceous volcaniclastics (Pfiffner and Gonzalez 2013). Cenozoic deformation in central Peru, specifically the late Paleocene to Eocene Incaic orogenic phase, created a thrust and fold belt that represents the foundation of the Western Cordillera (Bissig et al. 2008). Cenozoic deformation folded the rocks into northwest-trending anticlines and synclines which were cut by parallel thrust faults. The Incaic I phase (59-55 Ma) of the Late Paleocene formed a fold-and-thrust belt with the most intense deformation present between the rigid block of the coastal batholith and the Marañon arch (Bissig et al. 2005). The Incaic phase is bound by an unconformity between the Mid-Eocene to Early Miocene Calipuy Group volcanics, across which upright folds in the MFTB have been truncated (Pfiffner and Gonzalez 2013). Around 42 Ma, a subsequent Incaic II phase overprinted the previous belt and formed the foundation for the modern-day Eastern Cordillera (Bissig et al. 2005). Lithologic units and structural fabrics in the district have a NNW trend which serves as the primary control for ore mineralization and alteration for mineral deposits in the region (Flores 1990, Bissig et al. 2005). The Paleogene and Neogene periods consisted of five separate, brief pulses of deformation that were punctuated by periods of relative calm. These five events (Inca III and Quechua I, II, III, and IV) occurred at 26, 17, 10, 7, and 2 Ma respectively and are believed to be associated with the economically important magmatism in the Middle Miocene (Figure 2.3, Bissig et al. 2005).  Figure 2.3: Generalized cross section of the Western Cordillera showcasing the Quechua phase folds (adapted from Pfiffner and Gonzalez 2013)  21  2.2.2 – Cenozoic Magmatism Cenozoic magmatism in central Peru corresponded primarily to changes in the geometry of the subduction zone, resulting in multiple pulses of magmatism interspersed between quieter periods of inactivity (Bissig 2005). In central Peru, the deposition of Late Eocene Calipuy Group volcanics occurred after the termination of the final Incaic Phase tectonic events and were followed by the emplacement of Early Oligocene intrusive rocks (Pfiffner and Gonzalez 2013). Previous studies indicate that the active magmatism throughout central Peru paused during the Middle Oligocene from ~29.3-25.5 Ma and resumed to the east of Uchuchaccua at around 25 Ma (Bissig et al. 2005). Rates and directions of plate convergence have changed since the Early Cretaceous as the convergence was aligned north-south with a relatively fast pulse of convergence which slowed in the Late Cretaceous (Pilger, 1981, Pardo-Casas and Molnar, 1987). A shift in convergence direction to a northeast-southwest orientation took place in the Paleocene with two periods of faster pulses in the Eocene between 55-40 Ma and 25-10 Ma (Pfiffner and Gonzalez 2013). The renewal of magmatic activity east of Uchuchaccua may have been caused by a change in the subduction direction from a northeast to an eastern direction, together with the increased rate of convergence (Pilger, 1981, Pardo-Casas and Molnar, 1987).  Active magmatic activity in the Early Miocene resumed to the south of Domo de Yauli around 18.5 Ma which possibly corresponded to partial melting of crustal material that preceded the return of arc magmatism (Bissig et al. 2005). Following this period, magmatism migrated west towards the Western Cordillera around 17 Ma and resumed in the north near Cerro de Pasco, Mina Iscaycruz, and across most of the central Andes around 15 Ma (Bissig and Tosdal 2008). This transition into a period of widespread magmatism is attributed to a change from a flatter subduction regime into a steeper setting (Bissig et al. 2005).  The resurgence of magmatism in the Middle Miocene is associated with a series of mineral deposits in central Peru, primarily from calc-alkaline igneous rocks of granodioritic composition. Magmatism near Cerro de Pasco, Colquijirca, and most likely Mina Iscaycruz from ~14.1-11 Ma represents some of the largest mineralized districts in Central Peru during that time (Figure 2.4).  22   Figure 2.4: Schematic of the evolution of arc magmatism in Central Peru during the Cenozoic (adapted from Bissig 2005). Each section represents the same region of Peru at a different time period.   2.3 – Geologic Setting of Mina Iscaycruz and Surrounding Areas Mina Iscaycruz is located in the Western Cordillera, and the surrounding area contains the majority of the Goyllarizquisqa Group stratigraphic section from west to east (Figure 2.5). This section of Early Cretaceous rocks is approximately 2,000 meters thick in the Iscaycruz area and sedimentary and siliciclastic rocks from this group are subvertical and strike to the north-northwest. The sedimentary formations of the district are categorized into an upper calcareous group and a lower clastic group (Flores 1990). The upper calcareous group consists of the Casapalca, Celendin, Jumasha, Pariatambo, Chulec, and Pariahuanca formations, with the lower clastic group consisting of the Farrat, Carhuaz, Santa, Chimu, and Oyon formations (Flores 1990). The orientation of the beds results in the youngest unit  (Jumasha) being exposed mainly in the west and the oldest (Chimu) in  the east. A steeply dipping, east-facing anticline duplicates the stratigraphy to the east of the mine area where the Santa Este deposit is located. 23    Figure 2.5: Stratigraphic column of rocks at Mina Iscaycruz and surrounding areas with corresponding ages. Column displays thickness (m), width conveys the unit’s resistance to erosion and rocks are colored according to their unit’s color code throughout the thesis. Sources for index fossils and ages: (1) Benavides-Ciceres 1956, (2) Wilson 1963, (3) Von Hillebrandt 1970, (4) Romani 1982 The Oyon Formation is the basal unit within the Goyllarizquisqa Group and consists of an alternating sequence of dark grey to black carbonaceous shale and thinly bedded sandstone with an approximate thickness of over 375 meters (Scherrenberg 2012). The Oyon Formation does not outcrop significantly at Iscaycruz because it is mostly buried within the core of the Iscaycruz anticline.   24  The Chimu Formation is ~600 to 900 meters thick and consists primarily of sandstone and quartzite which overlies the Oyon Formation. The Chimu Formation serves as a rheological contrast to the Santa Formation along high angle lithologic contacts and fault contacts. This unit contains a lower sequence of medium to coarse grained sandstone layers interspersed with shale and coal horizons. Above that sequence is a series of >1m thick beds of white orthoquartzite (Scherrenberg 2012). The orthoquartzite forms the majority of the current geomorphology of the Iscaycruz anticline. Although the Santa Formation contains most of the mineralization, several zones around the mine have mineralized intervals contained within the Chimu Formation and its contact with the Santa Formation.  The Santa Formation overlies the upper quartzite dominated portion of the Chimu Formation. The Santa Formation can be up to 260 meters thick in parts of Peru; however, in the Iscaycruz area it generally is only 60 to 70 meters in total thickness (Escalante and Hart 2011, Scherrenberg 2012). The formation is a gray to dark gray, interbedded sequence of micritic to microgranular fossiliferous limestone, shales, marls, siltstone, sandstone, and calcarenites that commonly display frequent facies variations over small sections (Escalante and Hart 2011 , Scherrenberg 2012). Fossil content appears to vary widely across the study area, with some drill holes containing only a single 2-meter section of fossiliferous limestone.  Previous studies have also identified facies-centric alteration relationship with siliciclastic-rich sections hosting chloritization and impure carbonate units yielding the most intense dolomitization (Escalante and Hart 2011). Dolomitization of Santa limestone was a precursor to primary mineralization at Iscaycruz where it considerably enhanced the porosity and permeability of the host rocks (Escalante and Hart 2011).   Overlying the Santa Formation is the Carhuaz Formation, a 500 to 800 meters thick alternating sequence of fine to medium grained sandstone and fossiliferous shale and marl with thin limestone beds (Flores 1990, Scherrenberg 2012). The Carhuaz Formation is poorly represented in outcrop in the Iscaycruz area due to erosion.   Above the Carhuaz Formation is the Farrat Formation, a ~100 meters thick unit composed primarily of a lower siliceous sandstone and an upper calcareous sandstone, with sections of red quartzitic sandstone (Flores 1990). As with the Chimu Formation, the relative difference in rheology between the Farrat Formation sandstones and 25  quartzites and the neighboring Pariahuanca Formation rocks allowed bedding parallel faults to develop (Flores 1990, Escalante and Hart 2011). Additionally, the contact zone between the two units is commonly calcareous sandstone, which may have aided calc-silicate skarn formation at the lower depths of the Chupa deposit. Samples from this transition zone are represented in the majority of the Pariahuanca surface transects detailed in Chapter 5.   The Pariahuanca Formation is the other host rock unit, and like the Santa Formation it is relatively thin (80 to 100 meters) and crops out on the west end of the main Iscaycruz valley with varying degrees of surface exposure. The composition of the unit ranges from massive limestone to thin layers of fine sandstone in a calcareous matrix and is typically gray to dark gray to brown in color with occasional fossiliferous zones. The Mina Chupa deposit is hosted primarily in Pariahuanca and is the only deposit at Iscaycruz to be hosted in the unit.   The Chulec and Pariatambo formations sequentially overlie the Pariahuanca and crop out west of the Pariahuanca on the Iscaycruz property in a heavily eroded valley between the resistant ridges of Farrat to the east and Jumasha to the west. The Chulec Formation is a 200 meters thick unit of a fossiliferous, gray to dark gray limestone and marl mixture. The Pariatambo Formation is ~150 meters thick and ranges from gray to black in color, and from marl to fossiliferous limestone in composition, with abundant fossils throughout the package.   Bounding the Iscaycruz area to the west is a folded ridge of the Jumasha Formation in a similar orientation to the Iscaycruz anticline. This unit is up to 2,000 meters thick in the Iscaycruz district (Flores 1990), and consists primarily of thick limestone beds and thin limestone beds intercalated with black marl and chert. Although Jumasha is the host rock to several deposits elsewhere in Peru (Antamina, Yauricocha, Uchuchaccua), mineralization has not been identified in this unit around Iscaycruz. Consequently, 2015-2016 sampling campaigns at Iscaycruz did not sample Jumasha limestone other than regional background samples. Earlier isotopic studies of Iscaycruz also included a limited number of samples from this formation.   26  2.3.1 – Tectonic and Depositional Setting During the Cretaceous During the Early Cretaceous, layered carbonate and siliciclastic rocks of the Goyllarisquizga Group were deposited in the Iscaycruz area while subduction processes beneath central Peru resulted in periods of shallow marine sedimentation, back-arc extension, crustal thickening, and uplift (Flores 1990, Benavides 1999). These carbonate units host many central Peruvian mineral deposits including Antamina (Jumasha/Celendin), Mina Iscaycruz (Santa, Pariahuanca), Huanzala (Santa), Yauricocha (Jumasha/Celendin(?), Pallca (Santa/Pariahuanca), and Uchuchaccua (Jumasha) (Gomi 1998, Bissig et al. 2005, Escalante 2008, Bissig et al. 2008).  This depositional environment persisted from the Early Cretaceous until the Turonian in the Western Peruvian Trough. Limestone and siliciclastic rocks dominated during periods of extensional back-arc environments which have given a minimum age of 115.3 + 0.6 Ma (Bissig et al. 2008.) Rocks from the Oyon Formation have local coal lenses which have been interpreted to be swamp deposits from the start of the Cretaceous based on dated plant remains (Scherrenberg 2012). The depositional conditions for the Chimu Formation are interpreted to represent a fluvio-deltaic system with an eolian sandstone source (Benavides-Cáceres, 1956). The original depositional environment of the Santa Formation has been interpreted as a brackish, near shore, transitional environment with oolitic tidal facies, deposited during a maximum transgressive period in the Valanginian (Benavides-Cáceres, 1956). Surface and subsurface samples collected from this study demonstrate a high degree of variability in ultraviolet phosphorescent response within the Santa Formation carbonates which could be indicative of varying degrees of organic carbon and diagenesis temperatures (Wang et al 1997). Fossils present in the Carhuaz Formation indicate that the original environment was coastal to coastal-marsh, brackish, and indicative of a tidal flat region coinciding with the end of the Valanginian transgression (Scherrenberg 2012). Other studies have interpreted Farrat to represent a fluvial-deltaic environment due to rare plant remains and sandy marl horizons containing ripple marks (Scherrenberg 2012). The Pariahuanca Formation is typical of foreshore environments, and in other areas of Peru it appears to change into a continental facies (Palacios et al., 1995). The Chulec Formation is interpreted to represent several transgressions (Scherrenberg 2012). The Pariatambo Formation is interpreted to represent anoxic conditions that are required to produce the bituminous content in some of the rocks (Scherrenberg 2012). Other studies have identified key layers of hydrocarbon-rich rocks without fossils and interpreted the Jumasha Formation to represent wide variations in eustatic sea-level during transgressive periods (Scherrenberg 2012). 27   2.3.2 – Intrusive Rocks Intrusive rocks are exposed in three locations in the Iscaycruz district (Figure 2.6). The Quellaycocha Lake dikes (QLD) are intermediate in composition, weakly propylitized, and crop out west of Limpe Norte near Quellaycocha Lake (Escalante and Hart 2012). The QLDs were not observed to directly cut mineralization; however, they are controlled by east trending faults that displaced mineralized zones and are thus interpreted to be post-mineral (Escalante and Hart 2012). Previous geochronology work (Escalante and Hart 2012) utilized U-Pb CA TIMS on zircons recovered from the QLDs, and yielded ages of 13.48 ± 0.12 Ma and 12.64 ± 0.25 Ma that correspond to the end of the Quechua I event (Escalante and Hart 2012). The Escondida intermediate dikes crop out in the hinge of the Iscaycruz anticline. These dikes have been largely argillized by acidic fluids during a later event, and now contain abundant high temperature clay minerals. Unaltered samples of these dikes were also dated using U-Pb TIMS on zircons and yielded an age of 41.08 + 0.07 Ma, which is contemporaneous with the end of the Incaic II tectonic phase (Escalante and Hart 2012)  Figure 2.6: Location of intrusive rocks in the Iscaycruz area. Palpas Tonalite to the west was sampled for this study and yielded an age that is between the two ages determined for the Quellacocha Lake Dikes. 28  A large tonalite intrusion crops out approximately 10km west of Iscaycruz near the town of San Pedro de Palpas (Figure 2.6) and contains proximal endoskarn and exoskarn as well as a contact metamorphic aureole of extensive hornfels and marble. The age and significance of this intrusion, called the Palpas tonalite, is discussed in more detail in Chapters 6 and 7.  2.3.3 – Local Tectonic Setting and Structural Controls to Mineralization The successive tectonic phases of the Incaic orogenic period folded the Early Cretaceous rocks of the Goyllarizquisqa Group (Flores 1990, Escalante and Hart 2011). The regions closest to the Cordillera experienced the most intense deformation, uplift, and erosion (Bissig et al. 2008), and this tectonism provided critical structural controls for many of the Paleogene and Neogene mineral deposits in the region.  Later Quechua-period tectonic phases also influenced the orientation of the rocks and in some areas were associated with Early and Middle Miocene igneous activity that drove mineralization (Bissig et al. 2008). These intrusions and their associated fluids were aided by the high-angle bedding contacts in rock units in the Iscaycruz area, and the strong rheological contrasts between softer and more reactive carbonate units and more resistive and less permeable sandstone and quartzite units. Two factors which focused mineralizing fluids into primarily the Santa and Pariahuanca formations (Flores 1990). This contrast allowed bedding parallel faults to develop, and also for the mineralizing fluids to concentrate on the more reactive Santa Formation rocks and minimize the overall size of each of the deposits, leading to higher grade Zn mineralization in Santa hosted deposits (Flores 1990). A precursor event of dolomitization that is observed across the Iscaycruz area prepared the host rocks for mineralization by increasing permeability (Escalante and Hart 2012). Previous studies have utilized fluid inclusion data, mineralogy, and structural observations to interpret that the center of the system is located below the Tinyag deposit (Escalante and Hart 2011, 2012). One of the two faults is north-trending and appears to be related to the Incaic phases, and this fault is intersected by sets of ENE-trending faults that converge below Tinyag (Escalante and Hart 2012). Sulfide deposition was controlled by the intersection of NNW and ENE-trending faults that created cataclastic breccia zones and extensional fractures (Escalante and Hart 2011). The presence of the ENE-trending faults helped to both localize sulfide mineralization as well as displace the sulfide bodies up to 100 meters during later reactivation stages (Escalante and Hart 2011). More detailed summaries of structural controls to mineralization at Mina Chupa and Santa Este are found in Chapters 4 and 5.    29  Chapter 3:  Methodology 3.1 - Carbon and Oxygen Isotope Analysis 3.1.1 - Sample Selection and Preparation  Surface Transects Surface and subsurface samples for carbon and oxygen isotope analysis were collected either as individual samples or in specified transects. For purposes of this study, a transect is defined as “a series of samples that span outwards parallel to the orientation of a particular feature of interest, in increments measured perpendicular to the orientation of the feature” (Figure 3.1). A feature of interest is any geologic feature that could potentially serve as an indicator of fluid flow, such as a fault, vein, visible mineralization, alteration, or change in lithology. The orientation of a feature of interest consists of the strike and dip of the feature utilizing the right-hand rule. The default protocol for surface transects involved a sample centered on the feature of interest as the start of the transect, with an accompanying geochemical sample collected from the same material. Subsequent samples were collected outwards in one direction at the following specific distance intervals where possible: 1m, 2m, 5m, 10m, 15m, 20m, 25m, and 30m. Local conditions dictated specific intervals and the majority of transects did not reach 30m due to access or safety restrictions. Several select transects exceeded the 30m default where justification and access were present. Upon the completion of the first direction of the transect, a series of samples was collected at the same intervals in the opposite direction.  Figure 3.1: Schematic diagram illustrating three example surface transect types centered on a lithologic contact, a fault contact, and a mineralization contact. 30  For surveying purposes, the center sample of a transect would be recorded with a handheld Garmin Rino 530HCx GPS device, with the rest of the transect surveyed with a tape measure and a Brunton compass. Individual samples were collected either within transects or as standalone samples in between transect locations. Samples were transported to UBC and photographed in a controlled environment where UV and paragenetic information was collected. Subsurface Transects The sampling interval distances were calculated from the drill hole orientation and the feature orientation (Figure 3.2). Core boxes for the target transects were laid out and quick logged, marked, and sample locations were photographed and cut for sampling. Cut samples were transported to UBC where the final sample was re-photographed with the collection of UV and paragenetic information.  Figure 3.2: Diagram demonstrating how subsurface transects were sampled. Samples were collected at specific distances from the feature being examined. The distances measured are perpendicular to the orientation of the feature. 31  3.1.2 - Sample Analysis  Samples for isotopic analysis were drilled using a Dremel® rotary drill tool equipped with a tungsten carbide bit, and the resulting powder was collected and stored in ½ dram glass vials. The amount of material collected and stored was dependent on the carbonate content of the sample, with most calcite vein and limestone samples yielding approximately 50-100 mg of material. In between samples, the drill bit was rinsed with 10% dilute HCl until any remnant carbonate was dissolved and then cleaned with an ethanol-soaked Kimwipe®.  Samples of limestone wall rock were drilled at least 1cm from veins, mineralization, or stylolites where possible. The microdrilled powder samples were analyzed for 13C/12C, 17O/16O, and 18O/16O using the Mineral Isotope Analyzer (MIA) in the Earth Science Building at University of British Columbia (UBC). The MIA was developed by Mineral Deposit Research Unit (MDRU) and Los Gatos Research and uses off-axis integrated cavity output spectroscopy (OA-ICOS). The model CCIA-46 was used for the analysis of samples in this thesis. The analytical procedure for sample analysis follows the techniques outlined by Barker et al. (2011), which are outlined below.   Prior to the start of an analytical run, the MIA was turned on for a period of at least 150 minutes to allow the internal components to reach a stable temperature of approximately 46oC. Each carbonate sample was weighed and transferred to clean borosilicate glass vials that were then sealed with butyl rubber septa. The sample material was transferred directly from the ½ dram vials to a fresh weighing paper without the use of a transfer tool, eliminating a potential source of cross-contamination. Depending on carbonate content, between 6 and 30 mg of each carbonate sample was used for each run. The vials were not flushed prior to this transferal due to the relatively small amount of CO2 naturally present within the atmosphere compared to that generated by the sample in question (Barker et al., 2011). Each vial was injected with approximately 0.2 mL of 85 % phosphoric acid (H3PO4) through the rubber septum at an angle to ensure that the acid did not react with any sample material prior to the removal of the syringe needle. The sample vials were then mixed for 2-3 seconds in a mixer and placed in an aluminum heating block at 72°C for at least one hour. Finally, the reacted sample was driven through the MIA through a custom-designed series of Swagelok stainless steel compression fittings, stainless steel tubing, HDPE tubing, and glass cold trap during the automated analysis process.  After the MIA has achieved sufficient temperature, a baseline is run using the internal software included with the analyzer. The operator observes the results of the baseline and determines if the results are within a normal 32  and acceptable range. Prior to carbonate samples, a minimum of two gas samples are run through the system as a preliminary check for analyzer issues and are used in the end of day processing. Two types of standards, synthetic carbonate (Sigma) and carbonate (BN13) are analyzed prior to analyzing unknowns. In order to analyze powdered samples, the analyzer begins a systematic pump down and flushing cycle using laboratory atmosphere passed through a Drierite drying column in order to remove high concentrations of CO2 from the OA-ICOS measurement cell. During this time, the preparatory section of the apparatus is flushed with zero gas (N2) and the operator pierces the septum of the carbonate sample with a syringe attached to a Swagelok fitting until the entire instrument and sampling line were pumped down to a pressure of less than 0.5 kPa (Barker et al., 2011). After this pressure is achieved, the fitting attached to the syringe is opened, which allows the CO2 gas generated by the sample to move through the preparatory section of the line. To avoid ambient moisture in the sample to be processed within the analyzer, the sample gas is run through a glass coil containing a dry ice bath of ethanol at -78°C.  Prior studies observed that water vapor negatively affects the isotopic results, and its removal is critical to accurate analyses (Barker et al., 2011). The sample gas is drawn into a transfer cell inside the analyzer due to the pressure gradient and after a period of 60 seconds, 100 mL of dry laboratory air is mixed with the sample. Following a waiting period of 30 seconds to allow for equilibration, the instrument determines the C and O isotope ratios of the sample CO2 using the isotopologues 12C16O18O, 13C16O16O, and 12C16O18O from measured high-resolution laser absorption spectra (Barker et al., 2011).  3.2 - Establishment of Background Values Thresholds for Carbon and Oxygen Isotopes  3.2.1 - Purpose A necessary precursor to any isotopic study is the collection of background values. Carbonate rocks can vary widely in their original isotopic composition as geologic conditions vary throughout the depositional history of a basin (Ramirez et al. 2015). This variation can have a significant effect on interpretation of results, as an anomalous result in one carbonate unit may qualify as background in another. Regional background samples were collected in order to develop background thresholds for the two main lithological units in this study. Background threshold values were calculated to identify changes in isotopic composition from the established background, aiding the identification of rocks affected by hydrothermal alteration and the presence of potential alteration haloes.   33  3.2.2 - Sample Selection Criteria and Sampling Methodology  Background samples were collected from locations greater than 10km from known intrusions, mineral deposits, or other alteration features. To ensure that the samples represented an unaltered background, regional scale geologic maps were consulted, and three suitable regions were identified. Each of these regions had the same lithologic units as the study area, apparent road access, and no mapped features nearby that would indicate the presence of altered rocks. Prior to the collection of samples, Glencore and Empresa Minera Quenuales S.A. (EMQSA) community relations employees were consulted to ensure that permission and legal access was obtained prior to entry into the communities.  Outcrop was preferred for each sample location; however, permission and access restrictions prevented the collection of outcrop samples at all the target regions. Where direct outcrop samples were unavailable, representative float samples were collected. Representative float samples were only collected in areas where the target location was a ridge of outcrop and the local topography precluded the possibility that the float samples were derived from anything other than the target ridge.  Two main regions were sampled during the 2016 summer campaign, a region approximately 10-15 km north of Iscaycruz near the town of Oyon, and a region 10-17 km south of Iscaycruz along the main highway (Figure 3.3).  34   Figure 3.3: Location of background samples relative to Iscaycruz deposits and nearby mines  Several large intrusive bodies have been mapped to the west of Iscaycruz by the Peruvian government’s geology and mining branch (INGEMMET) and noted in prior studies (Cobbing et. al 1998). The area east of Iscaycruz has suitable geology and would be appropriate for this study, but access and permission restrictions prevented sampling. 35   A total of 17 locations were sampled, and samples were collected from four different lithologic units. The two main units targeted were the Pariahuanca Formation and the Santa Formation. Samples from the Jumasha and the Pariatambo/Chulec formation were also collected; however, the sample results from these units were used solely for reference and not factored into threshold calculations.  3.2.3 - Sample Collection  For outcrop samples, chip samples were collected from a range of 10m to 50m across the outcrop depending on local availability. Chip samples were randomly selected and a total of approximately 2kg of material was collected and put into a sample bag. For float or boulder fields, only samples greater than 5m in diameter were sampled, and only when the float or boulder was representative of cliff faces above. If a single boulder was the only available material, chip samples were randomly collected from across the surface in the same manner as outcrop samples. Upon return to the office at Mina Iscaycruz, the 2kg of material was crushed in a rock crusher. The crusher was washed in between samples. The resultant crushed material yielded pieces that were approximately <6cm in diameter. From this material, half was sent for geochemical analysis at ALS in Lima, and the other half was retained for shipment to UBC for future isotopic analysis. During isotope analysis sample preparation, two limestone pieces with surfaces devoid of apparent weathering were selected and drilled in accordance with the sampling methodology outlined in 3.2.2. Sample locations with calcite veins large enough to be reliably sampled were drilled and analyzed. Calcite samples are denoted with a C at the end of their analysis number and are provided as a reference and not factored into threshold calculations (Appendix A1).  3.2.4 - QA/QC In-house calcite standards BN13 (δ13CVPDB = 1.82‰; δ18OVSMOW = 13.82‰) and Sigma (δ13CVPDB = -14.18‰; δ18OVSMOW = 10.22‰) were inserted into the analytical sequence every seven samples and blanks in the form of pure CO2 gas were inserted every five samples in order to normalize errors and calculate instrument drift respectively. A minimum of six Sigmas and BN13’s were analyzed during each run day, and two gas blanks were inserted at the beginning and end of each run day.  36  To achieve a greater confidence level in results, all regional background samples were analyzed at least twice (denoted with an A or a B at the end of their analysis number), with the two analyses conducted on different run days. Analysis results were averaged and are displayed as single data points throughout Section 3.2.6.   3.2.5 - Comparison to Global and Greater Regional Background  To develop an appropriate background for Early Cretaceous carbonate rocks, a comparison to comparable carbonate rocks was conducted. The two main lithologies that host mineralization at Iscaycruz are the carbonate and siliciclastic rocks of the Pariahuanca Formation (Late Aptian to Early Albian) and the Santa Formation (Valanginian). Carbon and oxygen isotope data from similarly-aged carbonate rocks and global seawater data (Veizer et al., 1999) was compared to the study data. A comparison of lithologies used in the study is summarized in Table 3.1. Table 3.1 - A comparison between Valanginian carbonates and Late Aptian-Early Albian carbonates    37  Lithologic formations included in the comparison were selected due to their similarity to the Iscaycruz carbonate sequences in lithology, age, thickness, and regional setting. The studies used in this comparison also utilize microdrilling to achieve powdered samples for carbon and oxygen isotope analysis.   3.2.6 - Results  Carbon and oxygen stable isotope results from collected samples are displayed alongside data from the reference literature (Figure 3.4). Values for δ13C and δ18O were plotted using the geostatistical program ioGas® and are given in per mil (‰) with δ13C values listed with respect to Vienna Pee Dee Belemnite (V-PDB) and δ18O values with respect to Vienna Standard Mean Ocean Water (V-SMOW). Sample results were reported to 0.01‰ for background calculations but are presented below as rounded values to the nearest 0.1‰. For background calculations, only samples from the carbonate wall rock were used.  Figure 3.4: δ18O vs δ13C scatterplot of all background and reference samples. Colored fields denote observed range of background compositions. Inca reference samples assigned δ18O value of 33‰ for plotting purposes 3.2.6.1 - Santa Formation (Valanginian) Santa Formation background samples have isotopic values ranging from -3.5‰ to 1.7‰ δ13C, and 23.7‰ to 34.4‰ δ18O (Figure 3.4). Sample results for δ13C are evenly spread across the full range of values, whereas 38  δ18O results cluster tightly around 25‰, with one outlier sample at 34.4‰ (SFCRS005). Data from Valanginian reference samples also demonstrate a similarly restricted range of δ18O values (23.7‰ to 25.6‰) and a less varied range of δ13C values (-0.7‰ to 2.0‰) indicating that sample results from the Santa Formation conform well to other unaltered shallow marine carbonates in the greater region (Silva-Tamayo et al., 2016). Data selected from the global seawater curve for the Valanginian display a smaller range for both δ13C and δ18O, with δ13C values ranging from 0.0‰ to 1.5‰ and δ18O values ranging from 29.1‰ to 31.4‰ (Veizer et al., 1999).   3.2.6.2 - Pariahuanca Formation (Late Aptian-Early Albian) Pariahuanca Formation background samples have isotopic values ranging from 2.2‰ to 3.8‰ for δ13C, and 21.9‰ to 30.3‰ for δ18O (Figure 3.4). The Pariahuanca Formation samples have a small range of δ13C values and δ18O values are evenly spread across the observed range. Data from Late Aptian-Early Albian reference samples show a similarly restricted range of δ13C values (2.9‰ to 4.4‰) with a wide range of δ18O values (15.2‰ to 24.4‰). Reference sample δ18O distribution is similar to that of Pariahuanca Formation samples; however, the overall range is approximately 6‰ lower. Finally, three samples from the Inca Formation were analyzed by Ramirez et. al. 2015 for δ13C only. The Inca Formation is the northern Peru equivalent of the Pariahuanca Formation, and the nomenclature difference is solely a result of prior classification (Benavides 1956, Jaillard 1987, Scherrenberg et al. 2012.) The Inca Formation displayed values of 1.5‰, 1.8‰, and 1.8‰ where they fall below the bottom of the range for Pariahuanca samples analyzed in this study. For plotting purposes, these three samples have been arbitrarily assigned a δ18O value of 33.0‰ so that they can be plotted on a δ18O vs δ13C diagram. Data selected from the global seawater curve for the Albian include two samples with δ13C values of 5.5‰ and 4.6‰ and δ18O values of 28.6‰ and 30.6‰ (Veizer et al., 1999).   3.2.6.3 – Removal of Outliers and Threshold Value Determination Of the 17 locations sampled, 16 were suitable for inclusion into the background dataset. A float sample SFCRS013 of the Santa Formation limestone collected approximately 500 meters from a small hot springs resort facility was removed. The float sample was interpreted to have come from the ridges above the collection point, and not from the visibly altered rock near the resort. However, the analytical results from two sets of analyses for SFCRS013 yielded δ13C results of -0.9‰ and -0.9‰, and δ18O results of 16.6‰ and 17.4‰. Given the 39  proximity to active hot springs, it was determined that these samples were not representative of unaltered background and were not included. The second outlier sample, SFCRS005, was analyzed four times to rule out analytical error. As outlined above, two small pieces of limestone were randomly selected from each sample location and analyzed twice, and in the case of SFCRS005, four times. The data show that within this sample location, a large but consistent range in δ13C and δ18O values is present. The two separate pieces are designated A and B, and the results of the analyses are summarized below in Table 3.2.   Table 3.2 - SFCRS005 Analytical Results – Santa Limestone Analysis ID Analysis Date δ13C VPDB s.d. δ13C δ18O VSMOW s.d. δ18O RS005A 1/30/2017 -2.96 0.28 35.43 0.49 RS005A 2/28/2017 -2.78 0.08 30.85 0.31 RS005A 5/5/2017 -2.63 0.16 35.45 0.25 RS005A 5/5/2017 -2.75 0.16 34.63 0.25 RS005B 1/30/2017 0.34 0.28 26.84 0.49 RS005B 2/28/2017 0.12 0.08 24.63 0.31 RS005B 5/5/2017 0.34 0.16 27.72 0.25 RS005B 5/5/2017 0.12 0.16 27.80 0.25  As demonstrated by the data in Table 3.2, the results from sample SFCRS005 are internally consistent and appear to represent a large degree of heterogeneity within one sample location. In contrast to SFCRS013, this location lacks geological justification for removal therefore the results are included in background calculations.   3.2.6.4 - Santa Formation Background Threshold Value - δ18O Seven of the eight Santa Formation limestone samples yielded δ18O values within a range between 23.7‰ and 27.1‰, with RS005A yielding a value of 34.4‰ (Figure 3.4). Data from this background study and all reference samples form tight clusters in δ18O values, with both groups of samples providing a lower end range of 23.7‰ δ18O. A lower end threshold value of 23.7‰ is therefore proposed for δ18O.   40  3.2.6.5  - Santa Formation Background Threshold Value - δ13C Determination of background δ13C values in Santa Formation limestone is complicated by the broad range of values within the dataset (Figure 3.4). A review of sampling locations indicates that sub-regional scale changes in δ13C could potentially explain the broad range of values, as the two highest values (1.6‰, 1.7‰) came from the only Santa Formation sample collected from the southern region. Similarly, SFCRS006 and SFCRS007 are located 300m apart and sample the bottom and top of the unit respectively and display a δ13C range of -3.5‰ to -0.9‰. The broad range of values for Santa Formation limestone provides no indication that any single observation is a statistical outlier, and thus the lowest observed value of -3.5‰ is proposed for the threshold value. 3.2.6.6 - Pariahuanca Formation Background Threshold Value - δ18O Of the ten Pariahuanca limestone samples, eight yielded δ18O values within a range between 26.1‰ and 30.3‰ (Figure 3.5). The remaining two samples came from SFCRS016, a float sample of Pariahuanca from a 5m solitary boulder that displayed significantly lower δ18O values compared to the remaining Pariahuanca samples. RS016A yielded a δ18O value of 21.9‰, while RS016B had a δ18O value of 23.0‰. When compared with the remaining Pariahuanca Formation samples, SFCRS016 has δ18O values approximately 5‰ lower on average and may signify the presence of fluid:rock interaction or a difference in diagenetic environments (Morse et al., 2007). SFCRS016 is a single boulder, and therefore no additional context regarding proximity to a fault, vein, or mineralization is available.  41   Figure 3.5: δ18O Plot for Pariahuanca background samples displaying the lower values for SFCRS016 compared to the other samples analyzed. Average δ18O background for the Pariahuanca Formation is 26.3‰ which accommodates the slightly higher values of sample SFCRS015 and lower values of SFCRS016. Samples SFCRS015 and SFCRS016 yielded values that deviated from their duplicate by >1‰, indicating a consistent degree of variation among certain samples in the Pariahuanca Formation. The lowest observed background value of 21.9‰ is proposed as the threshold for δ18O.  Early Albian rocks from reference samples displayed δ18O results that were significantly lower than results from Pariahuanca Formation limestone (Madhavaraju 2013). These reference samples were derived from two lithologic units, the El Caloso Formation and the Canova Formation (Figure 3.4). The El Caloso Formation limestone had an average δ18O value of 16.7‰, whereas the Canova limestone had an average of 21.6‰ (Madhavaraju 2013). At their minimum values, they reported 15.2‰ and 18.6‰ respectively (Madhavaraju  2013). The reason for the lower δ18O content relative to both Pariahuanca Formation limestone and Albian 42  seawater (Veizer 1999) is not clear. Mural Formation rocks were not factored into threshold value determinations for this study.  Prior studies conducted at Iscaycruz have designated 20‰ as a threshold value for δ18O in Pariahuanca and Santa Formation rocks based on relatively fresh rock samples collected near the community of Rapaz (Escalante and Hart, 2012). The previously used value of 20‰ is similar to the threshold value proposed for this study, and thus interpretations from the prior studies can be accommodated with this new context.  3.2.6.7 - Pariahuanca Formation Background Threshold Value - δ13C Results for δ13C in Pariahuanca Formation rocks from this study were broadly distributed across a range of 2.2‰ to 3.8‰. When Inca Formation (north Peru Pariahuanca equivalent) carbonate data from Ramirez et. al. 2015 is included, the range expands to 1.5‰ to 3.8‰. The broad distribution of δ13C values could be indicative of changing depositional environments throughout the Early Albian or local diagenetic variation (Morse et al., 2007). A drop in both carbonate and organic δ13C values is correlated with the Paquier Level from the European Tethys during the Early Albian (Ramirez et. al. 2015). As identified by Ramirez et al. (2015), this widespread carbon isotope event during the mid-Cretaceous is well documented and the observed drop in Peru is comparable with the mid-Pacific (Jenkyns and Wilson, 1999, Ramirez et. al. 2015). The three values from the Inca Formation occur prior to the Paquier Level drop, and values for δ13C in the Chulec Formation that overlies the Inca (Figure 3.4) return to values of approximately 1.5‰ to 2‰ after this event (Ramirez et al., 2015). Additionally, the three Inca samples were collected approximately 500km northeast of the study area, and as such, their lower δ13C composition relative to the 2016 samples could be indicative of varied conditions in northern Peru during the Early Albian. Given this contrast to Pariahuanca Formation limestone in central Peru, the three Inca samples were not factored into threshold value determinations for this study.  The variability in δ13C background indicates that no single value is an outlier, and thus the lowest observed value of 2.2‰ is proposed as the background threshold.   43  3.2.7 Discussion and Comparison  Primary study samples were also reviewed to determine if the thresholds calculated were similar in composition to visibly unaltered rocks around Iscaycruz. For these comparisons, wall rock refers to limestone, dolostone, calcareous sandstone, and skarn altered rock. Wall rock isotope data from surface and subsurface transects of Santa and Pariahuanca limestone at Iscaycruz was compared to regional background and reference samples (Figure 3.6). Primary study samples are displayed with an X and labeled as “isotope”, with background samples from this chapter labeled as “background isotope.”  Figure 3.6: δ18O vs δ13C scatterplot of background, reference, and all study results. As an additional comparison, data from Figure 3.6 was viewed as a point density map, where clusters of samples of similar composition are represented as hot colors, and compositions with few similar samples were represented as cold colors or not colored (Figure 3.7).  44   Figure 3.7: δ18O vs δ13C scatterplot of background, reference, and all study results. Results demonstrate that the threshold values utilized meaningfully separate background values from depleted values in δ13C and δ18O for both the Santa and the Pariahuanca Formation wall rock samples (Figures 3.6 and 3.7). The data also demonstrates the importance of separating background values by lithologic unit, as the background clusters shown in Figure 3.2.7C show the contrast in δ13C between the two units. A small population (Santa Population 2) of Santa Formation wall rock samples occur in a small compositional range of -2.9‰ to -3.5‰ δ13C and 26‰ to 27‰ δ18O, whereas the larger “Santa Background Cluster” has a range of -0.8‰ to 1.1‰ δ13C and 23.8‰ to 28.2‰ δ18O (Figure 3.7). The variation in δ13C content for Santa background samples resulted in the selection of the lowest observed value as the threshold value; however, it is possible that additional unaccounted factors have resulted in two background populations. The Santa Formation background δ13C mean is -1‰ which may approximate the apparent lower end value for the “Santa Background Cluster”.  3.2.8 Summary  Regional background samples were collected in order to calculate stable isotope δ13C and δ18O background threshold values for the two main lithological units (the Santa Formation and Pariahuanca Formation), to identify rocks affected by hydrothermal alteration in the study area. Samples were analyzed for 13C/12C, 17O/16O, and 18O/16O using the MIA. Calculated background threshold values are summarized in Table 3.3. Results from the background calculations demonstrate the importance of separating the relevant lithologies and generating 45  a background value for each. The results also show that δ13C and δ18O can vary significantly between lithologies that are close spatially or temporally (Ramirez et al. 2015, Silva-Tamayo 2016). Pariahuanca samples from northern Peru yielded significantly different δ13C values from those in central Peru, and globally identified events like the Paquier Level drop highlight the importance of time-correlated isotopic data (Ramirez et al 2015). Table 3.3 - δ13C and δ18O Thresholds for Pariahuanca and Santa Formation Rocks Santa Formation Isotopic Ranges Lower Bound Upper Bound Lower Bound Upper Bound δ13C (‰) δ13C (‰) δ18O (‰) δ18O (‰) Extremely Depleted -8 -5 8 12 Moderately Depleted -5 -4.3 12 17.9 Slightly Depleted -4.3 -3.5 17.9 23.7 Background -3.5 > -3.5 23.7 >23.7      Pariahuanca Formation Isotopic Ranges Lower Bound Upper Bound Lower Bound Upper Bound δ13C (‰) δ13C (‰) δ18O (‰) δ18O (‰) Extremely Depleted -8 -5 8 12 Moderately Depleted -5 -1 12 17 Slightly Depleted -1 2.2 17 21.9 Background 2.2 > 2.2 21.9 > 21.9   3.3 Pb Isotopes  Thirty-one samples were analyzed for Pb isotopic compositions by J.E. Gabites at the Pacific Centre for Isotopic and Geochemical Research at the University of British Columbia. Samples were collected in three phases and targeted the main ore bodies at Iscaycruz (27 samples), the Palpas tonalite (2 samples) and the skarn 46  mineralization spatially associated with the Palpas tonalite (2 samples). Detailed sampling procedure and raw data are provided in Appendix A6.1. The combined isotopic data for all studies were plotted on “uranogenic” (207Pb/204Pb vs. 206Pb/204Pb) and “thorogenic” (208Pb/204Pb vs. 206Pb/204Pb) diagrams, as well as 208Pb/206Pb vs. 207Pb/206Pb diagrams. The combined isotopic data are also displayed on Tukey Box plots (Section 3.7.2) and cumulative probability plots (Section 3.7.3).  3.4 U-Pb Dating    One sample from the Palpas tonalite was dated for the study by R.M. Friedman at the Pacific Centre for Isotopic and Geochemical Research (PCIGR), who carried out the sample preparation, analysis, and interpretation of results. An explanation of the methodology used for sample analysis is found below. The methodology and analysis protocol utilized is outlined in Appendix A6.1.  Zircons were separated from rock samples using standard mineral separation procedures.  Zircons were selected for analysis by handpicking in alcohol; they were subsequently mounted in epoxy, along with reference materials. Grain mounts were then wet ground with carbide abrasive paper and polished with diamond paste. Next, cathodoluminescence (CL) imaging was carried out on a Philips XL-30 scanning electron microscope (SEM) equipped with a Bruker Quanta 200 energy-dispersion X-ray microanalysis system at the Electron Microbeam/X-Ray Diffraction Facility (EMXDF) at the University of British Columbia. An operating voltage of 15 kV was used, with a spot diameter of 6 μm and peak count time of 30 seconds.  After removal of the carbon coat the grain mount surface was washed with mild soap and rinsed with high purity water.  Prior to analysis the grain mount surface was cleaned with 3 N HNO3 acid and again rinsed with high purity water to remove any surficial Pb contamination that could interfere with the early portions of the spot analyses.   Analyses were conducted using a Resonetics RESOlution M-50-LR, which contains a Class I laser device equipped with a UV excimer laser source (Coherent COMPex Pro 110, 193 nm, pulse width of 4 ns) and a two-volume cell designed and developed by Laurin Technic Pty. Ltd. (Australia).  This sample chamber allowed for the investigation of several grain mounts within one analytical session. The laser path was fluxed by N2 to ensure better stability. Ablation was carried out in a cell with a volume of approximately 20 cm3 and a He gas stream that ensured better signal stability and lower U-Pb fractionation (Eggins et al., 1998). The laser cell was 47  connected via a Teflon squid to an Agilent 7700x quadrupole ICP-MS housed at PCIGR. A pre-ablation shot was used to ensure that the spot area on grain suface was contamination-free.  Samples and reference materials were analyzed for 202Hg, Pb (204Pb, 206Pb, 207Pb, 208Pb), 232Th, and U (235U and 238U) with a dwell time of 0.02 seconds for each isotope. The U-Pb ratios were determined on the same spots along with trace element concentration determinations. The settings for the laser were: spot size of 34 μm with a total ablation time of 30 seconds, frequency of 5 Hz, fluence of 5 J/cm2, power of 7.8 mJ after attenuation, pit depths of approximately 15 μm, He flow rate of 800 mL/min, N2 flow rate of 2 mL/min, and a carrier gas (Ar) flow rate of 0.57 L/min.   Reference materials were analyzed throughout the sequence to allow for drift correction and to characterize downhole fractionation for U-Pb isotopic ratios.  For U-Pb analyses, natural zircon reference materials were used, including Plešovice (Sláma et al., 2008) or 91500 (Wiedenbeck et al., 1995, 2004) as the internal reference material and Temora2 and/or 91500 as monitoring reference materials; the zircon reference materials were placed between the unknowns.  KL is an in-house reference zircon from the Kloochlimmis Pluton, with a TIMS age of 196.72±0.23 Ma.  Raw data was reduced using the Iolite 3.4 extension (Paton et al., 2011) for Igor ProTM yielding U/Pb ages, and their respective uncertainties.  Final interpretation and plotting of the analytical results employed the ISOPLOT software of Ludwig (2003).  Mineral separation was conducted by H. Lin, grain selection, mounting and imaging by T. Ockerman, and laser set-up by M. Amini.   3.5 Luminescence  A total of 877 rock samples collected from surface and subsurface transects around the Santa Este deposit and at surface prospects to the north at Santa Este Norte were examined for their UV fluorescent and phosphorescent responses. From those 877 samples, a total of 1,125 analyses were run using the Mineral Isotope Analyzer.   All UV photographs were taken with a Nikon DX - AF-S Nikkor 18-55mm DSLR camera, fitted with a UV filter, with manual settings at: ISO = 200, WB = Auto, AFA = F3.8, and Quality = Fine. The camera was situated 17cm above the black felt stage where the samples were located. UV photographs were taken with 2 second exposures with the Superbright II shortwave UV lamp (254nm) at 17cm above the stage to ensure consistent 48  brightness. All photographs were taken on the cut, washed, and dried surface of the sample. Surface weathering where lichen or other organic molecules are present results in a false positive UV response. Lichen and other bacteria are strongly fluorescent and phosphorescent, emitting a vibrant green color. Care was taken to ensure that no organic material was present on the cut surface during photography, and that any smeared organic material generated during the rock cutting process was polished away. Cut rock surfaces were wiped clean with a damp Kimwipe and then left to air dry. Wet, reflective rock surfaces interfere with consistent UV photography. Surfaces were air dried to prevent dry Kimwipe remnants, which have a strong white fluorescence, from remaining on the sample during the photography step.  Fluorescent intensity in carbonate veins and limestone was tracked using a subjective visual scale from 0-3, with a “0” designating no fluorescent response, and a “3” designating bright fluorescent response (Figure 3.8). To maintain consistency during identification, three samples were selected as definitional samples for the three intensity levels of fluorescent response, with FS2, FS91, and FS102 representing 1, 2, and 3 respectively. Two non-luminescent rocks were used as a null comparison. An intensity of 0.5 was assigned for cases where a sample’s luminescent response appeared to be between the null rocks and FS2’s response. FS2, FS91, and FS102 were situated on top of the photography rig during the sample analysis process for comparison.  Phosphorescent response in carbonate veins and limestone was tracked using a subjective visual scale from 0-4. Phosphorescent response is more difficult to assess than fluorescence as the UV lamp must be turned off to observe it, and weak response is difficult to detect. Generalized definitions of phosphorescent and fluorescent intensities, along with methods for determination are given below. Figure 3.9 shows a sample with 3F white fluorescence and 4P green phosphorescence and Figure 3.10 shows sample FS90, a sample of Santa limestone with varying intensities and colors of both fluorescence and phosphorescence in calcite veins and limestone wall rock. Luminescent Intensities:  For Limestones  Fluorescence • 0F – Nonfluorescent. •         1F - Faint, nonzero color. Usually dull, reddish brown if present 49  • 2F - Clear UVF color • 3F - Intense UVF color, similar to 3F for minerals  Phosphorescence • 1P - Nonzero UVP response after 2+ seconds of UV exposure at <2" from lamp • 2P - Moderate intensity UVP response, decay time of 2-5 seconds after 1 second of UV exposure at <2" from lamp  • 3P - Intense UVP response, decay of 2-5 seconds after <0.5 seconds of UV exposure at <2" from lamp. Recordable via photography. • 4P - Like 3P, but detectable for 10+ seconds. Detectable response in photos after two back to back 2 second exposures.    For Carbonate Minerals  Fluorescence • 0F – Nonfluorescent. • 0.5-1F - Nonzero UVF response, ultrafaint to faint, as compared with reference samples defined as a 0F. Colors are muted, usually prefixed with dull  • 2F - Moderate intensity UVF response, colors are discernible, has a wide range.  • 3F - Highly intense UVF response, colors are clear and bright, mineral is illuminated even from indirect UV exposure  Phosphorescence • 0P – Nonphosphorescent. • 1P - Nonzero UVP response after 2+ seconds of UV exposure at <2" from lamp • 2P - Moderate intensity UVP response, luminescent decay time of 2-5 seconds after 1 second of UV exposure at <2" from lamp • 3P – Intense UVP response, decay of 2-5 seconds after <0.5 seconds of UV exposure at <2” from lamp. Recordable via photography. • 4P - Like 3P, but detectable for 10+ seconds. Detectable response in photos after two back to back 2 second exposures  50  Phosphorescence can only be captured in photography with an intensity of 3P or above, and is used to differentiate between 2P and 3P, as well as 3P and 4P.  Figure 3.8: Diagram illustrating organization and domaining of luminescent samples. Fluorescence intensity ranges from 0.5-3F, and phosphorescent intensity ranges from 0.5-4P. The color fill indicates the four major color categories observed 51   Figure 3.9: Sample FS142, calcite displaying 3F white intensity UV fluorescence (left) and 4P green UV phosphorescence (right).   Figure 3.10: FS90 – Sample with 3F white, blue, and yellow fluorescent calcite, 3P green and 4P blue phosphorescent calcite, and 3P phosphorescent, Santa limestone wall rock. 3.6 Data Display  3.6.1 Isotopic Values: δ18O and δ13C Isotopic values were plotted using the geostatistical program ioGas® and are given in per mil (‰) with δ18O values recorded with respect to Vienna Standard Mean Ocean Water (VSMOW) and δ13C values recorded with respect to Vienna Pee Dee Belemnite (VPDB).  3.6.2 Box Plots  Box Plots divide the ordered values of the data into four equal parts, by finding the median and then the 25th and 75th percentiles. The range between the 25th and 75th percentiles is referred to as the inter quartile range (IQR). The median is represented by a horizontal line within a box that spans the IQR and contains approximately 50% of the data. The mean is represented by a large black circle. The fence/whisker is defined as 52  the IQR extended by 1.5 times the length of the box towards the maximum and the minimum, with the fence as the boundary and the whisker as the range. The upper and lower whiskers are drawn from each end of the box to the fence position.  Samples outside of the fence are classified as outliers and those that are three times the central box length from the upper or lower quartile boundaries are considered far outliers (Tukey 1975). 3.6.3 Probability Plots   A probability plot shows the distribution of a data variable and how it relates to a normal distribution. The values plotted along the X axis are in units of standard deviation. These N-score values represent the z-scores from a standard normal distribution (mean = 0 and a standard deviation = 1).  3.7 Isotopic Results Nomenclature For the results and subsequent discussion sections, the terms depleted and enriched are isotopic value adjectives that are used when comparing two compositions. Sample results are enriched or depleted relative to other rocks, background thresholds, or reference materials. Isotopic results relative to the background thresholds are reported as slightly, moderately, and extremely depleted. Results are classified as extremely depleted if the values are within or below the range of calcite in equilibrium with magmatic water (Bowman 1998); equal to -8‰ through -5‰ δ13C and 8‰ through 12‰ δ18O. Isotopic values above the respective background thresholds are not depleted and considered background values. Values between background and extremely depleted are divided into two parts, slightly and moderately depleted, and these ranges are presented in Chapters 4 and 5.    53  Chapter 4:  Santa Este 4.1 – Introduction  The Santa Este deposit at Iscaycruz (Figure 1.1) is a distal, Zn-rich carbonate replacement deposit hosted in carbonate and siliciclastic rocks of the Santa, Chimu, and Carhuaz formations (Flores 1990, Escalante and Hart 2011, 2012). The deposit is located on the eastern limb of the Iscaycruz anticline and has a similar mineralization style to Limpe Centro; however, it has fluid characteristics indicating slightly lower temperature and salinity (Escalante and Hart 2012). The purpose of the investigation at Santa Este has three primary goals: to identify the isotopic signature of distal mineralizing fluids at Santa Este, to evaluate the 9-km trend to the northwest of Santa Este for evidence of similar signatures, and to evaluate the effectiveness of different sampling and analysis strategies in order to compare results from other isotopic studies and advise industry best practices. The results of various methodologies and techniques can be evaluated for effectiveness across a range of geological environments. Successful application of stable isotopes in the exploration environment can serve as a proof of concept for future explorers.  Santa Este provides an excellent study area due to extensive historical studies, availability of drill hole, surface sample geochemistry, and geochemical and textural markers of buried mineralization (Escalante and Hart 2012) all of which complement the current isotopic study. Study results can aid in reducing exploration targets when conventional or historical indicators of mineralization present too many equally prospective targets for a given area.  4.2 – Santa Este Background and Location The Santa Este deposit is located on the eastern flank of the Iscaycruz anticline, hosted primarily in Santa Formation limestone with portions of the deposit in Carhuaz Formation siliciclastic rocks (Figure 4.1). The Zn-rich deposit was intermittently mined as an open pit from approximately 2013 to 2016 when operations were suspended (Figure 4.2). The deposit was discovered via the presence of goethite-rich gossans on the surface at the contact between the Santa and Chimu Formations (Escalante and Hart 2012). Similar gossans are variably present at or near this contact for 9km to the northwest of the Santa Este deposit. 54   Figure 4.1: Santa Este deposit showing the nearby surface transects and drill holes included in the study. 55   Figure 4.2: Iscaycruz area looking NW. Open pit outlines are yellow dashed lines, boundary of Santa Formation is shown as black dashed lines 4.2.1 – Geology and Structural Elements The local geology at Santa Este consists of calcareous and siliciclastic rocks from the Carhuaz, Santa, and Chimu formations (Figure 4.3). Bedding attitudes at Santa Este are similar to those on the west side of the anticline at Limpe Centro, albeit with the younging direction reversed (Figure 4.4) (Escalante and Hart 2012). The Santa Formation is the main host rock for mineralization and comprises gray to black, micritic to fossiliferous limestones with minor shale laminations. Bands of dolomitization and recrystallized limestone are present in both surface and subsurface rocks throughout the Santa Formation and are typically found proximal to structures. Two generations of stylolites are observed; an earlier, thin stylolite generation, characterized by minor chloritization on the margins, and a later, higher intensity stylolite generation with undulating bands of fine-grained black pyrite to disseminated pyrite and locally recrystallized, fluorescent/phosphorescent calcite nodules (Figure 4.5).  56  Figure 4.3: Iscaycruz geologic model highlighting the location of the Santa Este deposit. 2015 and 2016 surface isotope transects are rendered as small white circles.  57   Figure 4.4: Adapted interpreted cross-section of Iscaycruz deposits from Escalante and Hart 2012. Previous studies suggest that Santa Este mineralization stemmed from the same central mineralizing event centered below Tinyag, however structural relationships do not preclude an eastern source.   58    Figure 4.4: Sample EXGR00008108 from SE-L4-1 displaying a stylolite with localized alteration and recrystallized calcite nodules with intense UV fluorescence and phosphorescence. Prior studies have identified two primary structural systems; a north to north 30° west trending fault/fold system related to the Incaic Phase of Andean tectonic development, and a later phase of north-south extension yielding east-west normal fault and tilting of the sedimentary units (Flores 1990, Escalante and Hart 2010, 2011). Mineralized ore bodies at Santa Este are observed to be cut by staircase-geometry listric faults that dip to the east (Empresa Minera Quenuales S.A. (EMQSA) geologists). Bedding-parallel faulting within the Santa/Chimu contact is observed, as are high-angle west-dipping faults that resulted in a vertical displacement of lithologic contacts.  4.2.2 – Mineralization and Paragenesis Mineralization at Santa Este is stratabound and proximal to the contact between Santa Formation limestones to the east and Chimu Formation sandstones and quartzites to the west (Flores 1990, Escalante and Hart 2012,). Prior studies have indicated up to eight phases of sulfide/oxide mineral deposition at Santa Este:  magnetite → pyrite I ± chalcopyrite I → pyrite II → pyrrhotite → sphalerite I ± chalcopyrite → chalcopyrite II + sphalerite II + pyrite III → bornite → galena →covellite + hematite + digenite + specularite (Escalante and Hart 2012). Decarbonization, dolomitization, and chloritization alteration patterns have been noted in prior studies and observed in both surface and subsurface samples (Escalante and Hart 2012). Patterns at Santa Este are similar to alteration observed at Limpe Centro and other deposits west of the Iscaycruz anticline (Escalante and Hart 59  2012). Oxidized gossans are observed on the surface, and shallow portions of drill holes are characterized by goethite, Al-goethite, chlorite-serpentine, sphalerite, phosgenite, and lepidocrocite (Escalante and Hart 2012). Sphalerite, galena, pyrite, and chalcopyrite were abundant enough to be readily identified in study samples; however, many of the less prevalent ore minerals were not identified in this study. The evidence for the less prevalent ore minerals is derived from the drilling logs and geochemical assays provided by EMQSA or from previous studies (Escalante and Hart 2010, 2011, 2012).  4.3 – Sample Collection 4.3.1 – Surface Transects  Surface transect samples were collected from two areas designated Santa Este (SE) and Santa Este North (SEN), and followed the surface transect sampling methodology outlined in Chapter 3. SE is the area encompassing the Santa Este mine and the surface areas within approximately 1-km to the north and south of the open pit (Figure 4.6). SEN is the area to the north of SE for 9-km along the Santa/Chimu Formation contact (Figure 4.6). Samples collected consisted primarily of unmineralized to weakly mineralized rocks, as previous studies focused on alteration mineralogy, fluid inclusions, and ore paragenesis at Santa Este (Escalante and Hart 2012). 60   Figure 4.5: Santa Este deposit, Santa Este transects, and Santa Este North transects. 4.3.1.1 – Santa Este The transect locations in Santa Este were selected for their proximity to the Santa Este open pit. Two transects were collected from within the pit, one transect was collected to the north of the pit, and two transects were collected to the south. The transects to the north and south were selected as being the nearest outcrop locations of visibly unaltered Santa limestone. Outcrop is scarce in these areas, and as a result, the transects have few samples, and not all of them are centered on a geologic feature of interest. Results from surface sampling are evaluated against other surface transects and nearby drill holes.  The two southern surface transects are SE-L2-1 and SE-L3-1. The two transects are single samples of Santa limestone from outcrops along the ridge. SE-L4-1 is north of Santa Este is and is centered on a small, west-dipping gossan contact (Figure 4.7). The Santa limestone at that location is brown to black with 20-40% stockwork calcite veins up to 8mm, interlayered with discontinuous chert layers. The gossan contact to the east 61  contains abundant hematite and goethite along with traces of Mn oxides. A secondary transect at this location was centered on a subvertical E-W fault cutting through the outcrop (Figure 4.8).  Figure 4.6: Overhead view of Santa Este pit showing locations of SE-L1-2, SE-L4-1, and SE-L5-1 transects. 62   Figure 4.7: SE-L4-1 transect, centered on gossan contact (red dotted line) and a secondary transect centered on fault contact (yellow dotted line) 4.3.1.2 – Santa Este North A total of 54 locations were sampled in Santa Este North (Figure 4.6). A summary of the 54 locations is provided in Table 4.1.  Transects were centered on several geological features: lithological contacts, faults, oxidation/mineralization, calcite veins/breccia and dolomitized contacts. Descriptions of these features in the Santa Este North area are given below. 63  Table 4.1 - Summary of Santa Este North surface transects  Santa Este North Transect Types Lithologic Contact Santa Formation limestone within these transects ranges from gray to black, dull micritic to sugary-recrystallized, with consistent lenticular tectonic calcite veins and variable presence of up to three later calcite vein generations. Two generations of stylolites were observed; an earlier, thin stylolite event with minor chloritization on the margins of the stylolites, and a later, higher intensity stylolite event with undulating bands of fine-grained black pyrite to disseminated pyrite and locally recrystallized, fluorescent/phosphorescent calcite nodules (Figure 4.5). The orientations of the high-angle lithologic contacts were consistently northwest/southeast as the dip direction varied locally from east to west (Figure 4.9). 64    Figure 4.8: Stereonet of the 18 lithologic contacts from Santa Este North   Fault The region to the east of the Iscaycruz anticline contains faults from three main types: bedding parallel, north to north-northwest, and east-west. Bedding-parallel faults within the sedimentary units are common, especially within contacts that have high rheological contrast (Flores 1990, Escalante and Hart 2011). A pre to intra-mineral north to north-northwest structure related to the Incaic phase has been identified as one of the fluid conduits for mineralization at Iscaycruz (Escalante and Hart 2010.) Finally, the intra to post-mineral extensional east-west faults are observed at Santa Este North where they are generally present as east-dipping 65  listric to normal faults (Escalante and Hart 2012, EMQSA pers. Comm.) All three fault types were targeted and represented in the sampling transects and displayed a wide variety of orientations (Figure 4.10).  Figure 4.9: Stereonet of the 17 faults sampled from Santa Este North Gossan/Oxidation/Mineralization Four transects sampled various types of oxidized, gossanous, or sulfide mineralized outcrops. Three of the locations were sampled at or near the contact with Carhuaz Formation rocks and contained either fracture controlled or bedding controlled hematite + goethite + jarosite + minor pyrite mineralization. The fourth location sampled a mineralized structure near the Santa/Chimu contact. Three of the four gossanous transects occur within 200m of a structural intersection of NNW faults with an E-W fault displaying 500m of sinistral displacement named the Mancacuta Fault (Figure 4.11).  66   Figure 4.10: Location of SEN transects near a large structural intersection with 500m sinistral displacement. Gossanous transects were mostly associated with bedding parallel and stockwork structures. Calcite Vein/Breccia Five locations sampled a specific calcite vein or calcite-matrix breccia. SEN-L2-2 is a transect centered on a 2cm-thick calcite vein perpendicular to bedding within an outcrop of recrystallized limestone. Samples in the transect contain calcite-matrix breccias with calc-silicates, marble, and chloritized fragments (Figure 4.12). SEN-L2-5 is a transect centered on an outcrop of Santa limestone with bedding parallel and bedding perpendicular calcite veins with blue phosphorescence. 67   Figure 4.11: Sample photos from the SEN-L2-2 transect centered on a brecciated, skarn altered outcrop with a 2cm calcite vein perpendicular to bedding. Dolomitized Contact Layers of dolomite within less altered limestone rocks were observed throughout the Santa Este Norte region, and previous studies have identified dolomitization as a necessary precursor to sulfide mineralization at Iscaycruz (Escalante and Hart 2012). Six transects in Santa Este North were centered on dolomitized layers or contacts. Dolomitized contacts ranged from abrupt to gradational, 10-30 cm thick, and were accompanied by recrystallized limestone + globular calcite + pyrite + sphalerite.   4.3.2 –Subsurface Transects Subsurface transects were sampled in a manner consistent with the surface transects in Santa Este and sampled lithologic contacts, faults, and mineralization contacts. Within these categories, more specific features were sampled, such as dolomitized faults, bedding parallel faults, pervasive CRD-style mineralization, the Santa-Chimu contact, and intercalated shale transitional zones. Transect targets were selected for fluid pathway potential, accessibility, competence, and availability of remaining core. The sampling interval distances were calculated from the drill hole orientation and the feature orientation (Figure 3.2). Sampling followed methodology procedures outlined in Chapter 3. A total of 11 drill holes were relogged and sampled (Figure 4.13).  68   Figure 4.12: Santa Este drill holes sampled during the 2015 and 2016 field seasons A total of 75 transects were selected and sampled within Santa Este drill holes, and a summary of the 75 transects is presented on Table 4.2. Detailed descriptions of individual subsurface transects are presented in Appendix A3.  Table 4.2 – SE Subsurface Transects Transect Transect Summary Description SE-1-15-01 Transect #1 Transect is centered on 60 deg fault @449.15 that separates the mixed rock types (SS,CSS, Sh, LS) above with the LS below. SE-1-15-01 Transect #2 The second transect is centered on the large fault zone beginning around 507. A smaller precursor fault zone is located at 503.1. The small zone is ~20 deg, the large zone is ~30. SE-13-06 Transect #1 Centered on contact with shale, also small FAULT ZONE from 73-74m SE-13-06 Transect #2 Centered on edges of small FAULT ZONE SE-13-06 Transect #3 Centered on high angle, west dipping (?) fault 69  Transect Transect Summary Description SE-13-06 Transect #4 Centered on mod high angle, west dipping (?) fault after contact with shale, extends intermittently until 86.75 SE-13-06 Transect #5 Centered on Mineralized FAULT ZONE, relatively high angle, west dipping (?), fault is near transition to bxa'd LS fault zone. Transect ends in fault gouge near contact with shale. SE-13-07 Transect #1 Centered on Limestone with pyrite, first logged limestone in hole, separated from next transect by several meters of shale SE-13-07 Transect #2 Centered on fault zone from 49-56m, no true orientations or controlling structures. Shale+LS layer SE-13-07 Transect #3 Centered on 63deg fault associated with slightly mineralized zone w/ pyrite and siderite SE-13-07 Transect #4 Centered on fault zone from 73-74m SE-13-07 Transect #5 Centered on weakly mineralized zone associated with 82.75-86 fault zone SE-13-07 Transect #6 Centered on fault zone from 90-92m, LS with pyrite and calcite veins SE-13-07 Transect #7 Centered on weakly mineralized zone associated with 97.75-99 fault zone SE-13-07 Transect #8 Centered on mineralized fault zone preceding lithologic contact change from LS->qtzt, increasing mineralization as contact approaches SE-14-05 Transect #1 The intermittent nature of the shallow LS's, and the near complete removal of all core after 45m led to this hole being point sampled w/ no true transects. Mineralized zones were reviewed, and samples were collected wherever sufficient material remained SE-14-07 Transect #1 Centered on dolomitized mineralized LS and 70 deg fault zone SE-14-07 Transect #2 Centered on intersecting 70deg and 45 deg fault zone dolomitized mineralized LS SE-14-07 Transect #3 Centered on 45deg fault zone, mineralized LS w/ sph SE-14-07 Transect #4 Centered on 45-50 deg fault zone locally dolomitized mineralized SE-14-07 Transect #5 Centered on dolomitized mineralized fault zone 50deg SE-14-07 Transect #7 Centered on mineralized dolomitized zone. Distances to fault bounds, no measurements SE-14-07 Transect #8 Centered on partially dolomitized mineralized fault zone SE-14-07 Transect #9 Centered on dolomitized mineralized fault zone preceding mineralized, Manto SE-14-07 Transect #11 Centered on mineralized zone around fault @90m SE-14-07 Transect #12 Centered on fault zone contact with quartzite, partially dolomitized and mineralized SE-14-08 Transect #1 Centered on faulted LS in between shale layers SE-14-08 Transect #2 Centered on weakly mineralized LS in between shale layers SE-14-08 Transect #3 Centered faulted CSS in contact with shales, weakly mineralized 70  Transect Transect Summary Description SE-14-08 Transect #4 Centered on weakly mineralized LS with dolomite veins with fault @43.08, contact with mineralized and dolomitized CSS@45.6 SE-14-08 Transect #5 Centered on dolomitized, partially mineralized CSS with 3% Zn contact with LS @45.6, centered on 25 deg FAULT ZONE, shares contact sample with T3 SE-14-08 Transect #7 Centered on fault contact with manto @56, small Zn-rich zone @57.9-59.56m, and fault contact @65.9 SE-14-08 Transect #8 Centered on mineralized fault contact with CSS @65.9, and next LS->CSS contact @68.9 SE-14-08 Transect #9 Centered on LS+CSS->Bit LS->Dolomitized LS, ending in fault contact with shale before strongly mineralized zone SE-14-08 Transect #10 Strongly mineralized zone, select samples where available SE-14-11 Transect #1 Centered on dolomitized, mineralized LS cut by 3-4 fault zone's SE-14-11 Transect #3 Centered on weakly mineralized multiple faulted section of LS SE-14-11 Transect #4 Centered on bounded fault contacts with shales, 2nd shale is Zn mineralization rich SE-14-11 Transect #5 Centered on faulted LS, partially mineralized, bounded by shale and mineralized manto SE-14-11 Transect #6 Mineralized manto, select samples SE-14-11 Transect #7 Centered on brecciated LS, mineralized faulted LS SE-14-14 Transect #1 Centered on LS before contact with bituminous shale SE-14-14 Transect #2 Centered on silic'd ls+sh, fault bounded  bituminous shale and fault@117.98 SE-14-14 Transect #3 Centered on faulted ls, 117.98-121.38 SE-14-14 Transect #4 Centered on mineralization' chl'd ls w/ fault @122.5, associated w/ zn mineralization SE-14-14 Transect #5 Centered on heavily mineralized, dolomitized ls SE-14-16 Transect #1 Centered on faulted zone of LS, w/ trace sphalerite SE-14-16 Transect #2 Centered on fault @121.09 in bituminous LS SE-14-16 Transect #3 Centered on fault @126.6 before contact with silicified LS SE-14-16 Transect #4 Centered on silicified LS contact with fault @137.1 SE-14-16 Transect #5 Centered on last section of LS, in fault contact with shale, in between shale and quartzite SE-7130-05H Transect #2 Centered on multiple fault zones across LS section SE-7130-05H Transect #3 Centered on fault @30.35 SE-7130-04 Transect #1 Centered on mineralized fault zone at 20.9m, with small dolomitized lenses on either side of the fault SE-7130-04 Transect #2 Centered on dolomitized section with a fault at 28.05 SE-7130-04 Transect #3 Centered on small fault zone at 39.60m, with some dolomitized zones and some silicified zones 71  Transect Transect Summary Description SE-7130-04 Transect #4 Centered on heavily faulted zone with silicification and dolomitization    4.4 – Analysis and Results Samples for isotopic analysis were prepared and analyzed following the detailed methodology outlined in Chapter 3. Prepared powder samples were analyzed for 13C/12C, 17O/16O, and 18O/16O using the MDRU’s MIA. Isotopic values are plotted as calcite-equivalent plots in per mil (‰) with δ18O values recorded with respect to Vienna Standard Mean Ocean Water (VSMOW) and δ13C values recorded with respect to Vienna Pee Dee Belemnite (VPDB). The threshold values in Santa rocks for δ13C and δ18O are -3.5‰ and 23.7‰ respectively (refer to Chapter 3).   For the results and subsequent discussion sections, the terms depleted and enriched are isotopic value adjectives that are used when comparing two compositions. Sample results are enriched or depleted relative to other rocks, background thresholds, or reference materials. Isotopic results relative to the calculated background thresholds are reported as slightly, moderately, and extremely depleted. Results are classified as extremely depleted if the values are within or below the range of calcite in equilibrium with magmatic water (Bowman 1998); equal to -8‰ through -5‰ δ13C and 8‰ through 12‰ δ18O (Figure 4.4). Isotopic values above the background thresholds are not labeled as depleted and considered background values. Values between background and extremely depleted are divided into two parts, slightly and moderately depleted. Slightly depleted values are within the range of -4.3‰ through -3.5‰ δ13C and 17.9‰ through 23.7‰ δ18O. Moderately depleted values are within the range of -5‰ through -4.3‰ δ13C and 12‰ through 17.9‰ δ18O (Table 3.2.8, Figure 4.14). 72   Figure 4.13: Comparison of background Santa Formation rock composition and range of calcite in equilibrium with magmatic fluid (Bowman et. al 1998). Adapted from Escalante 2008. The specific type of sample was recorded for each sample location, and calcite vein paragenesis was evaluated where possible. Up to six calcite vein generations were observed based on cross-cutting relationships and textural characteristics. Paragenetic relationships were evaluated using a hand lens, photography, and UV response; however, limited thin section petrography was conducted. As a result, the confidence level for calcite vein generation identification was highly variable. The broader categories of analysis spot types, along with their qualitative definitions are outlined in Table 4.3. Table 4.3 – Spot Type categories and the associated qualitative definition for each group Type Definition Calcareous Sandstone Sandstone with visible HCl response. Calcite White to gray carbonate vein, blotch, or globule. Vigorous HCl reaction. Carbonate Vein Vein of carbonate material of unknown type. Variable HCl reaction Dolomitized Limestone Bands or layers within limestone package that are visibly tan or brown and respond to HCl only after scratching. Dolomite Vein Vein of carbonate material that responds to HCl only after scratching. Limestone Gray to black carbonate rock. Moderate to vigorous HCl reaction. 73  Recrystallized Limestone Gray carbonate rock with coarse-grained recrystallized texture. Moderate to vigorous HCl reaction. Skarn Marble or calc-silicate material with HCl reaction. 4.4.1 – Surface Results Results from surface transects are displayed as point data on geologic maps and summarized on box plots (Figure 4.15). Summaries of transects are presented in this section whereas all individual transect data is displayed in Appendix A3. Parallel XY plots showcase individual transects with sample distance in meters from the center of a transect against either δ13C or δ18O.  The transect direction is indicated by annotation, and for two-sided transects, one direction was arbitrarily assigned a negative value for the distance from the center for plotting purposes.  Figure 4.14: δ13C (left) and δ18O (right) results for all Santa Este surface results 74  Results for δ13C and δ18O from all Santa surface and subsurface transects were plotted on a Tukey plot for comparison (Figure 4.16). A total of 620 surface and 508 subsurface samples were evaluated. Subsurface samples were located proximal to the Santa Este deposit and yielded lower average δ13C and δ18O values than surface samples.   Figure 4.15: δ13C vs δ18O results from Santa samples by surface vs subsurface. Subsurface samples were located proximal to the Santa Este deposit and yielded lower average δ13C and δ18O values than surface samples. 4.4.1.1 - Surface Results by Spot Type Surface results were plotted in Tukey Box plots by the spot type categories of the microdrilled sample, which included limestone and three variants (dolomitized, fossiliferous, and recrystallized), calcite and dolomite, and skarn (Figure 4.17). Results for both δ13C and δ18O show markedly lower values for samples collected from calcite (n = 270) vs. samples collected from limestone wall rock (n = 254). Recrystallized limestone (n = 16) samples yielded results that were slightly lower on average than regular limestone for both δ13C and δ18O. Dolomite veins (n = 5) had the third-lowest δ18O and the third-highest δ13C values. Conversely, dolomitized limestone (n = 49) displayed the highest average results for δ18O and the third-lowest average results for δ13C. Fossiliferous limestone (n = 16) yielded the second-highest average δ18O values and the highest average δ13C 75  values. Finally, skarn samples (n = 4) displayed the lowest average δ13C values and δ18O values that roughly in the middle of the observed groups.  Figure 4.16: Surface sample results by broad spot type. Calcite vein samples yielded the lowest average values for both δ13C and δ18O. Dolomitized limestone displayed the largest contrast between δ18O and δ13C results. Samples were evaluated with an ultraviolet light, and their fluorescent response was recorded. Results for δ13C and δ18O vary significantly depending on the color type observed (Figure 4.18). 76   Figure 4.17: δ13C vs δ18O results by their UV fluorescent color. Non-fluorescent samples are not shown. Generalized compositional fields are drawn around samples of the same color. Lines indicate threshold values, dashed = background -> depleted, dotted = extremely depleted threshold Samples with pink and white fluorescent response displayed the widest variation of results, whereas tan and orange were the least varied. A total of two red and two yellow fluorescent samples were observed in surface rocks. The two red samples were extremely depleted in δ18O and above background in δ13C.  4.4.1.2 - Feature Type Surface results for δ13C and δ18O were plotted in Tukey Box plots by the transect feature they belong to (Figure 4.19). Results had similarities to the spot type evaluation with dolomitized contacts yielding relatively low δ13C values and higher δ18O values. Gossans and mineralized contacts had the lowest average δ18O values; however, faults and lithologic contacts contained samples with lower extreme values. 77   Figure 4.18: δ13C vs δ18O results by transect feature (Surface samples only). Mineralized transects, gossans, lithologic contacts, and faults yielded the lowest results for δ18O. 4.4.1.3 - Distance from Feature Isotopic results from all surface transect types show a correlation between distance from the lithologic or fault contact and isotopic depletions in both δ13C and δ18O (Figure 4.20). The majority of transects show that the samples closest to the contact had lower values for δ13C and δ18O. Beyond the nearest sample, the relationship appears more complicated. For both δ13C and δ18O, average values increase for the first 5m from the contact; however, they decrease between 5-10m. For δ18O, a further decrease is observed between 10-15m, with the 15m samples yielding a lower average than samples at the transect center. The median for this group is 23.3‰, indicating that the average is strongly influenced by several extremely low values. Distances beyond 15m each have fewer than ten samples, and the distance group may be represented entirely by a single transect. As a result, limited interpretation is warranted for these distance groups. Transects centered on gossans were observed to have the opposite correlation, with the least depleted samples at the center of the transect (Figure 4.21). Four transects were centered on gossans, one of which was SE-L4-1, a transect to the east of the Santa-Chimu contact near the Santa Este pit. At SE-L4-1, distances farther from the gossan transect center equate to samples collected closer to the Santa-Chimu contact, which is a buried contact approximately 60m to the west. Removing SE-L4-1 from the analysis eliminates the observed inverse correlation.  78   Figure 4.19: Tukey Plots for isotopic results for all Santa Este surface transects showing δ13C and δ18O (‰) against sample distance from transect center  Figure 4.20: Tukey Plots for isotopic results for Santa Este surface gossan transects showing δ13C and δ18O (‰) against sample distance from transect center. 4.4.1.4 - Sample Location Evaluating all the surface transects by their location indicates that there are four regions in Santa Este North with a depleted isotopic signature that spans multiple transects (Figures 4.22 and 4.23).  Significant depletions in δ13C and δ18O occur near oxidized faults/contacts, E-W trending faults, and late-stage calcite veins 79  associated with either the faults or oxidized zones. The large-scale control on fluid flow appears to be related to the E-W faults, as all four zones can be related to their proximity to large structures.  The first three zones are located to the south of the large ENE fault near Mancacuta (Mancacuta Fault) with a dextral displacement of approximately 500m. The fourth zone is centered on transects on the north and south ends of an E-W fault with sinistral displacement of the same magnitude. Taken together, these represent a large block displaced to the east. This displacement is mirrored on a small scale in Zone 1 (Figure 4.24). In both scales, isotopic depletions in δ13C and δ18O occur in greater degrees to the north and south of the blocks displaced east.   Figure 4.21: δ13C vs δ18O results in the southern portion of Santa Este North. The three highlighted regions contain multiple transects with extremely depleted results. 80   Figure 4.22: δ13C vs δ18O results in the northern portion of Santa Este North. The highlighted regions contain multiple transects with extremely depleted results. Zone 1:  Transects in this area are adjacent to NE/SW trending faults and center on skarn contacts, gossans, faults, and veins and represent an area approximately 650m long with moderately to extremely depleted values for δ13C and δ18O in every transect sampled. The fault transects on the north and south have depletions mostly in limestone samples, whereas the fault and calcite vein transects in the central displaced block yielded depletions primarily in calcite veins. Late-stage veins in this area are nearly all depleted (22 of 25) in δ18O, and all but one of the pink UV fluorescent samples from this subset are depleted.    81   Figure 4.23: Zone 1 highlighting smaller scale displacement in Santa limestone from east-west faults. Transects in dashed boxes had isotopic depletions mostly in limestones, whereas samples in the solid box were depleted mostly in calcite veins.  Zone 2: The two southern transects in this area (SEN-L7-4, and 5) are lithologic contacts with mostly background values, however, moderately to extremely depleted values were present in E-W trending late-stage 82  calcite veins near the center of the Santa formation (Figure 4.25). The northern two transects are located near a mapped E-W fault with an adjacent gossan and contain moderately to extremely depleted values in δ18O in both the fault transect (SEN-L7-6) and the lithologic contact (SEN-L7-7). Depleted δ18O values were observed primarily on the north side of the fault transect despite both sides displaying skarn alteration features. Extremely depleted values in δ13C were present in the lithologic contact, including a late-stage, bright red UV fluorescent, E-W trending calcite vein, observed cross-cutting a late stage vein.   Figure 4.24: Zone 2 highlighting depletions in δ18O adjacent to E-W structures and depletions in δ13C at lithologic contacts. Pink and red UV fluorescent calcite veins were depleted in δ18O in E-W oriented, late-stage calcite veins. Zone 3: The transects in this area range from single point samples, to brecciated zones, lithologic contacts, and faults. Isotopic depletions in δ18O occur adjacent to E-W structures, with increasing quantity and consistency as sample locations approach the Mancacuta fault (Figure 4.26). Isotopic depletions in δ13C appear to be less reliant on E-W structures as most of the depletions are at or near lithologic contacts, in both veins and wall rock. Results for δ13C and δ18O mainly contrast with one another except in SEN-L5-3, 4, and 5 transects near the Chimu-Santa contact. This area is a mix of E-W fault transects and lithologic contact transects.  83  Proximity to gossans had a variable correlation with δ13C and δ18O depletions. SEN-L3-2 and 9, L5-10, and L6-4 and 7 all are gossans or have nearby gossanous samples without corresponding depletions in δ13C or δ18O.   Figure 4.25: Zone 3 displaying δ13C (left) and δ18O (right) results and locations of pink and red UV fluorescent calcite veins. Depletions in δ18O are focused around E-W faults whereas δ13C depletions are localized near lithologic contacts south of the Mancacuta Fault. Between Zone 3 and Zone 4, several isolated lithologic contacts have individual samples with extremely to moderately depleted values; however these values are distributed amongst samples proximal to contacts, distal to contacts, in fluorescent pink calcite veins, in non-fluorescent veins, and do not have any immediately apparent larger control on isotopic composition. Zone 4: Samples from transects in this area yielded extremely depleted values in both δ18O and δ13C in lithologic contact and dolomitized contact transects (Figure 4.27). In contrast to samples in Zone 3, extremely depleted values were observed in transects both north and south of the large E-W structure. Skarn outcrops, gossans, and oxidized vein samples were not observed in this area, and only recrystallized limestone and dolomitized limestone textures were present. 84   Figure 4.26: Zone 4 transects displaying results for δ13C (left) and δ18O (right). Lithologic contact and dolomitized contact samples on the north and south sides of the E-W fault yielded extremely depleted values. 4.4.2 - Subsurface Results 4.4.2.1 – Subsurface Results by Drill Hole Subsurface results are displayed on parallel downhole plots showing δ13C and δ18O (‰) alongside sample spot type, background thresholds, and key features (Figures 4.28-4.33). Drill hole results plotted within geologic cross-sections and against fluorescent intensity and vein density are located in Appendix A3. Subsurface results for δ13C and δ18O in the drill holes evaluated showed similar relationships and trends to surface samples. Proximity to the Chimu-Santa lithologic contact, sulfide mineralization, and critical faults were the most common features related to isotopic depletion. 85  Figure 4.27:  Parallel downhole plots for SE-1-15-01 and SE-14-05 showing δ13C and δ18O (‰), alongside sample spot type, background thresholds, and key features    86  Figure 4.28: Parallel downhole plots for SE-14-07 and SE-14-11 showing δ13C and δ18O (‰), alongside sample spot type, background thresholds, and key features.    87  Figure 4.29: Parallel downhole plots for SE-14-08 and SE-14-1 4 showing δ13C and δ18O (‰), alongside sample spot type, background thresholds, and key features.   88  Figure 4.30: Parallel downhole plots for SE-14-16 and SE-13-06 showing δ13C and δ18O (‰), alongside sample spot type, background thresholds, and key features.   89   Figure 4.31: Parallel downhole plots for SE-13-07 and SE-7130-04 showing δ13C and δ18O (‰), alongside sample spot type, background thresholds, and key features.  90   Figure 4.32: Parallel downhole plots for SE-13-07 and SE-7130-04 showing δ13C and δ18O (‰), alongside sample spot type, background thresholds, and key features.  4.4.2.2 – Subsurface Results by Spot Type Subsurface results are displayed on XY scatter plots and shape/color coded by the spot type categories of the microdrilled sample (Figure 4.34). Results from subsurface samples yielded similar categories to surface samples. Limestone samples displayed the widest variation of isotopic composition and calcite samples yielded lower average δ18O values. The largest difference between the surface and subsurface results is the number of extremely depleted dolomite samples within subsurface ore zones. Ore zones within Santa Este are 91  characteristically dolomitized (Appendix A3) and as a result, limestone samples from the ore zones were less common.  Figure 4.33: Subsurface Santa results by spot type. Lines indicate threshold values, dashed = background -> depleted, dotted = extremely depleted threshold 4.5 –Discussion 4.5.1 – Surface Results Previous studies have identified lithologic contacts and faults as probable fluid conduits (Morris 1986, Escalante 2008, Barker 2013). In the case of Santa Este and similar deposits related to distally derived fluids (Escalante and Hart 2012), the degree of fluid:rock interaction would be expected to be higher proximal to these fluid pathways (Bowman 1998, Escalante 2008, Beinlich et al. 2019). In cases where the fluid is magmatic in origin the result would be depleted host rocks and veins proximal to fluid pathways (Bowman 1998, Barker and Dipple 2019). Studies at similar CRD deposits have indicated that calcite veins related to mineralizing fluids have an isotopic composition that is closer to the composition of the parent fluid due to the lower degree of fluid:rock mixing (Beinlich et al. 2019). Evaluating the results for only calcite and dolomite veins reveals a pattern similar to the combined samples, with the highest concentration of extremely depleted δ18O results in the <0.5m distance group (Figure 4.35). The 15m distance group (n = 13) has lower average δ18O values than the <0.5m 92  group (n = 153); however, they contain significantly different sample counts. Despite the low sample count, the 15m distance group is split across six different surface transects spanning 5-km along strike, and thus cannot be attributed to a single local factor that may be providing a bias to the results. The vein results for δ13C show a broader distribution of values with no statistical outliers in any distance group. This is most likely explained by the broad distribution observed in background and study host rocks, as the range of δ13C values was consistently wide regardless of proximity to Iscaycruz mineralization.  Figure 4.34: Tukey Plots for calcite and dolomite vein isotopic results for all Santa Este surface transects showing δ13C and δ18O (‰) against sample distance from transect center. Distances beyond 15m contained a total of seven samples and are not shown. Spot Type Results showed significant differences among sampled materials across a variety of transect types. Dolomitized limestone consistently yielded high δ18O values, with only two samples below background. The fluid:rock interactions between limestones and dolomites are similar; however the kinetics of oxygen isotope exchange for dolomites result in a reduced degree of dissolution-precipitation reactions; which can result in a minimized depletion signature in systems that have distal fluids like Santa Este (Benezeth 2013, Barker and Dipple 2019).  The fluorescent color of calcite veins provided a range of isotopic compositions, with some colors yielding only depleted results and other colors yielding only background values. The two red calcite veins yielded extremely 93  depleted values for δ18O, which is consistent with previous studies (Escalante 2008) and results from Mina Chupa (Chapter 5, Chapter 8). For the surface samples, tan and orange fluorescence were almost universally correlated with background δ18O values, with 25 of 25 samples and four of five samples above background, respectively. No studies were found discussing tan or orange fluorescence in limestone. Mentions of orange fluorescence in calcite veins from other studies were restricted to combinations with red calcite veins (Escalante 2008). The majority of fluorescent pink calcite vein samples occur near E-W structures; however the distribution of δ18O values for fluorescent pink calcite veins did not show any relation to distance from transect center, transect feature, or geographic location. As shown in Figure 4.18, pink fluorescent calcite veins displayed a wide variety of isotopic compositions. Pink calcite veins appear to be related to a specific vein generation, designated CV4 (Appendix A1), with 16 of 26 pink fluorescent samples belonging to that group. An additional four samples were questionable identifications to other late-stage calcite veins, and therefore could be CV4 as well. The remaining pink samples belonged to individual point samples from the FS series, and therefore, their paragenesis was not evaluated. It is probable, however, that these remaining six samples belong to the CV4 vein generation.  Detailed paragenesis was conducted at each sample location; however, the results are not consistently reliable due to the textural similarity of the vein generations. Cross-cutting relationships with stylolites and early calcite vein generations were the primary method of confidently establishing paragenesis. Grouping the calcite vein generations into middle (CV2 and CV3) and late (CV4, CV5, and CV6) was chosen as a method to simplify this issue and evaluate the vein generations with a higher degree of confidence (Figure 4.36).  94   Figure 4.35: δ13C vs δ18O results by their simplified spot ID. Detailed paragenesis at individual sample locations can vary, but simpler cross-cutting relationships between middle and late calcite veins was more reliable. The FS series of samples consisted of isolated sampling spots and did not provide enough context to make a confident paragenetic identification, as such the samples are designated “FS – Calcite” to indicate this lack of classification. For both δ13C and δ18O, late calcite veins yielded the most depleted results in spot types, followed by FS calcite veins and middle calcite veins (Figure 4.36). Given that the FS series was most likely sampling a mix of middle and late veins, the results from this sample group would be expected to be between the two. The results highlight the value of identifying paragenetic relationships and vein generations for isotopic studies. Effectively identifying later calcite veins, which at Iscaycruz are associated with lower isotopic values, can provide real-time exploration insights during the sample collection period. Sample Location Previous studies have noted that sulfide deposition was controlled by the intersection of ENE and NNW structures, with the ENE structures being reactivated multiple times and displacing mineralization (Escalante and Hart 2011).  These results (Section 4.4.1.4) indicate that the regions within the displaced block received a reduced intensity of fluid flow carried by the faults. The fault contact on the southern end of Zone 1 is marked by skarn 95  mineralization, and the north contact has nearby mineralized gossans, and both ends contain pink UV fluorescent late-stage veins (Figure 4.24). In contrast, the transects in the displaced block only display recrystallization textures and weakly UV fluorescent (white) calcite veins.   Zone 2: These transects (Figure 4.25) provide further indication that a late-stage event utilized E-W structures to channel mineralizing fluids, and the bright red UV fluorescent vein could be indicative of a Mn-rich fluid escape structure common to carbonate-hosted systems (Megaw 2018, Escalante 2008).  Zone 3: Most of these sample locations (Figure 4.26) are adjacent to the Santa-Carhuaz lithologic contact, which could indicate that this contact did not experience the same degree of fluid flow as the Chimu-Santa contact. Additional research by EMQSA geologists has noted that there are late-stage (4.2-5.1 Ma) mineralization events that deposited vein-controlled, north-south oriented mineralization much later than the main Iscaycruz event (EMQSA, Escalante and Hart 2012). The cluster of mineralized veins and oxidation at the Santa-Carhuaz contact near the north is mostly vein-controlled. Field notes for these outcrops note silicification and white clay minerals, which suggest that mineralization could be related to the ~4.2-5.1 Ma high sulfidation event observed in areas around Iscaycruz (Escalante and Hart 2011, 2012).  Overall, the increase in depleted samples approaching the intersection of the Mancacuta fault with the Chimu-Santa contact, combined with the association of depleted values near other E-W faults indicates that this location was a significant fluid pathway for isotopically depleted fluids. Associated mineralization is not consistently present along these structures, which could represent the E-W structures channeling fluid during both mineralizing phases and post-mineral reactivation phases (Escalante and Hart 2011). Zone 4: Samples from transects in this area (Figure 4.27) yielded extremely depleted values in both δ18O and δ13C in lithologic contact and dolomitized contact transects. In contrast to samples in Zone 3, extremely depleted values were observed in transects both north and south of the large E-W structure. Skarn outcrops, gossans, and oxidized vein samples were not observed in this area, and only recrystallized limestone and dolomitized limestone textures were present. Distance from Feature Samples from this study were separated into four groups, samples within the Santa Este pit, and proximal (<2km), medial (2-4km), and distal (>4km), with distances along strike relative to the Santa Este pit (Figure 96  4.37). This grouping broadly highlights the spatial extent of the isotopic alteration around Santa Este, most notably in δ18O.  Figure 4.36: δ13C vs δ18O results by their distance to the Santa Este deposit The stark contrast in average isotopic composition between Santa Este Pit and proximal samples shows that the observed relation between distance from fluid pathway and isotopic completion at the outcrop scale is represented at the deposit scale as well.  4.5.2 – Subsurface Results Chimu-Santa Lithologic Contact One of the principal goals of the investigation was to evaluate whether the Chimu-Santa contact acted as a pathway for mineralizing fluids. Four of the drill holes utilized in the study did not reach the lithologic contact (SE-13-06, SE-7130-04, SE-7130-05H, and SE-14-14), and an additional four drill holes did not yield sample results at the contact due to sulfide mineralization (SE-14-05, SE-14-07, SE-14-11, SE-14-08, and SE-13-07). Samples from the remaining drill holes (SE-1-15-01, SE-14-16) yielded mixed results. SE-1-15-01 displayed the most significant contrast between carbon and oxygen, with no clear relationship between proximity to the lithologic contact and depletions in δ13C. Results for δ18O showed a consistent wall rock/vein combination of background to slightly depleted wall rock and slightly depleted calcite veins for most of the drill hole. A 97  gradational downward trend in isotopic values within 20m of the contact mirrors surface observations in both extent and intensity, as the most significant decreases in isotopic composition, occur within 5m.  Results from SE-14-16 are difficult to disentangle from the influence of nearby sulfide mineralization; however, no wider halo is present approaching the contact. The consistent presence of sulfide mineralization at or near the Chimu-Santa contact serves as direct evidence of mineralizing fluids; therefore, greater emphasis should be provided to surface sample results when evaluating the isotopic alteration of wall rocks and veins near this contact. Proximity to Sulfide Mineralization The isotopic signature around sulfide mineralization was shown to be one of three cases; pervasive and extreme isotopic depletion, gradational depletion, and isolated depletion. Examples for pervasive and extreme isotopic depletion are shown in SE-14-07, SE-14-11, and to a lesser extent, SE-14-08 (Figures 4.29-4.30). This signature is characterized by the majority of all wall rock and vein samples near to or in between zones of sulfide mineralization yielding moderately to extremely depleted results (in either δ13C or δ18O) with minor variation. Gradational depletion signatures are present in SE-14-07, SE-14-08, SE-14-16, SE-13-07, and SE-7130-04. This signature is characterized by a gradational decrease in isotopic values (in either δ13C or δ18O) for 5-20m, with moderate variability. The signature, in this case, is less consistent than the pervasive signature and has a higher amount of variation, especially in vein samples. Isolated depletion signatures are present in SE-14-14 and SE-14-16 where contrasting wall rock vein pairs do not display a consistent relationship to the distance from sulfide mineralization (Figures 4.30-4.31). SE-14-07 has a combination of these signatures where results in δ13C are pervasive, and δ18O is gradational. However, the majority of the drill holes had linked signatures with δ18O displaying a larger magnitude of variation. Proximity to Faults The isotopic signature around faults was shown to be one of three cases; unaffected, footwall depletion, or depleted in footwall and hanging wall rock. The footwall of bedding parallel faults showed depletion in either δ13C, δ18O, or both in all drill hole groups except for SE-1-15-01, which is a much deeper hole than the others studied. The bedding parallel faults have been identified in previous studies as plausible pathways for mineralizing fluid (Escalante and Hart 2011, 2012), and the consistent isotopic depletion in carbonate wall rock and veins observed adjacent to these faults supports that hypothesis. Combined with the observation of direct 98  mineralization or isotopic depletion adjacent to the Chimu-Santa contact, which itself is often marked by bedding parallel faults, provides further support to this hypothesis. The extent of isotopic alteration around bedding parallel faults, combined with the discrepancies between wall rock and vein composition indicate that fluids traveling along these pathways were in the rock-dominated region of isotopic exchange (Barker and Dipple 2019). When wall rock/vein pairs were examined, the isotopic composition of the carbonate veins was significantly more depleted in the majority of cases. This observation conforms with an expectation of isotopic exchange in a system with distal, rock-buffered fluids where the veins represent a composition closer to that of the original fluid (Barker and Dipple 2019).  4.5.3 – Fluid Evolution Results from surface and subsurface samples indicate that rocks at and to the north of Santa Este interacted with two fluids of differing compositions (Figure 4.43). The composition of Fluid 1 is evidenced by calcite veins and carbonate rocks within or adjacent to sulfide mineralization in Santa Este drill holes SE-14-07, SE-14-08, and SE-14-11. The compositional range of Fluid 2 is visible in the point density plot (Figure 4.38) as a region from -6‰ to -3.5‰ δ13C and 16.5‰ to 22‰ δ18O. Surface samples were evaluated by transect to classify their isotopic trend as either Fluid 1 or Fluid 2 (Figure 4.39). A portion of the transects contained too few samples, or too few depleted samples to be classified and were designated “Background/Unknown.” Ten transects displayed isotopic trends that appeared to be a mix between the paths for Fluid 1 and Fluid 2 and may represent a mixture of samples from both fluids. The seven transects classified as Fluid 1 are evenly split between lithologic contacts (Santa/Chimu) and fault transects sampling E-W structures, with five located south of the Mancacuta fault.   99   Figure 4.37: δ13C vs δ18O results for Santa surface and subsurface samples. Colors displayed are a point density cloud with colder colors (blue to green) indicating low to moderate point density, and warmer colors (yellow to red) indicating high point density.  Figure 4.38: δ13C vs δ18O results for Santa surface samples highlighting the two main fluid evolution paths. Transects were attributed with the fluid evolution path that best represented their sample distribution. 100  A consistent correlation between Fluid 1 transects and ultraviolet fluorescence (UVF), ultraviolet phosphorescence (UVP), or vein generation was not observed. No correlation was observed for Fluid 2 for proximity to mineralization, lithologic contacts, E-W structures, UVF, UVP, or proximity to transects with Fluid 1 composition. The evolution path for Fluid 2 approximates what would be expected for normal calc-silicate decarbonation at lower temperatures (300o) from Rayleigh distillation (Bowman 1998). The fluid channeling capacity from E-W structures interacting with bedding-parallel structures would likely remain during subsequent reactivations of the fault system (Escalante and Hart 2011). The plumbing system would allow a low-temperature barren fluid to isotopically alter the carbonate rocks and potentially overprint original isotopic signatures from the mineralizing event. 4.6 – Exploration Implications Carbonate replacement deposits (CRDs) have been noted for the challenges they pose to exploration geologists, and more specifically, their limited visible alteration signatures (Titley 1993, Megaw 1998). Results from surface and subsurface transects demonstrate the potential for stable isotopes as a tool for exploring in carbonate host rock environments. These results indicate that surface sampling of comparable contacts in similar lithologic conditions could provide value for exploration companies looking for buried skarns and other carbonate-related deposits. Surface results from the Santa Este North region highlight the potential value of stable isotopes within a brownfields exploration program, as the region has over a dozen exploration prospects with gossans or minor mineralization at the surface (Figure 4.40). In these types of exploration scenarios, the isotopic signature can be used as a potential prospect filter, given that the degree and extent of isotopic depletions can help signal the size and quantity of fluid flow to the explorer (Barker et al. 2013, Bowman et al., 1994, Dipple 1998). The extent of the signatures at Iscaycruz may be deposit specific; however, the overall results from the study can inform exploration techniques for carbonate-hosted deposits in Cordilleran environments. The surface sampling results indicate that wide, high sample count transects centered on lithologic or fault contacts may not be necessary for practical exploration. The orientation of the primary controlling lithologies results in isotopic signatures that necessarily extend farther along strike. Therefore, a higher number of smaller transects is most likely the optimal sampling strategy in similar environments. The spatial extent combined with the results from Section 4.5.1 – Distance from Feature demonstrate the asymmetric isotopic signature 101  present around Santa Este, and why any exploration program would benefit from an understanding of that asymmetry.  Figure 4.39: Locations of transects with gossans, oxidized samples, or skarn alteration in Santa Este North. Gossans and skarn samples were only observed south of the Mancacuta Fault. The successful application of stable isotopes as an exploration tool requires an evaluation of the exploration program’s geology, structural controls, and lithological characteristics. Surface sampling programs should consider bedding orientation of the host rocks during sample collection. The samples at Santa Este were collected exclusively in a high-angle bedding environment, and thus conclusions are not universally applicable; however, the results demonstrate the significant effect that lithology and bedding may have in shaping isotopic haloes. Isotopic alteration haloes identified at Santa Este indicate that a sampling strategy involving samples collected along short, focused transects across features potentially involved in fluid flow can be applied throughout the central Peruvian polymetallic belt to aid in the discovery of buried mineralization in this region. 102    4.7 - Conclusions Previous comprehensive studies of CRDs highlight their propensity for manto geometry, and results from studies at Iscaycruz highlight the diversity of mineralization characteristics at different distances from the hypothesized magmatic fluid source (Escalante and Hart 2011, 2012). High angle lithologic contacts at Iscaycruz combined with distal fluids have created a very narrow isotopic halo at both the surface and subsurface, which corresponds to tightly bound ore bodies along the Chimu-Santa contact at depth (Escalante and Hart 2011, 2012). Detailed parsing of the potential fluid pathways also demonstrated the importance of identifying separate events within the same system that have contrasting compositions. The two events at Santa Este utilized the same pathways and were difficult to separate, however this would not necessarily be the case at all deposits. Results from this study demonstrate that in Cordilleran environments where carbonate rocks have high-angle contacts with other lithologies, the potential for diagnostic isotopic signatures is possible even when the mineralizing source is distal. The observed asymmetric isotopic signature along strike was both consistently observed and would be expected from the dynamics of isotopic exchange in carbonates (Barker and Dipple 2019).    103  Chapter 5:  Mina Chupa 5.1 – Introduction, Purpose, and Previous Studies The Mina Chupa deposit at Iscaycruz (Figure 5.1) is a Cu-Zn skarn deposit hosted in carbonate and siliciclastic rocks of the Pariahuanca and Farrat formations (Flores 1990, Escalante and Hart 2011, 2012). The deposit is the only zone of mineralization within Iscaycruz that is hosted in Pariahuanca Formation limestone, and the skarn characteristics provide a deposit-type contrast to characteristics of the carbonate replacement style deposits of Santa Este, Limpe Centro, and others at Iscaycruz (Flores 1990, Escalante and Hart 2011, Escalante and Hart 2012; Chapter 4, this study). The purpose of the investigation is to identify how the mineralizing fluids from the larger Iscaycruz system interacted with the different host rock composition, as well as the ore geometry, fluid source proximity, and structural controls at Mina Chupa. Descriptions of Mina Chupa geology and mineralization are given in Chapter 2. Results and insights from Mina Chupa can be combined with results from Santa Este (Chapter 4) to identify key observations that are common to both environments and to offer guidance for exploration geologists working in similar environments elsewhere. The practical application of stable isotopes as an exploration tool requires an investigation into sampling and analysis techniques in different environments. The results of different methodologies and techniques can be evaluated for effectiveness across a range of geological environments. Successful application of stable isotopes in the exploration environment can serve as a proof of concept for future explorers. 104   Figure 5.1: Iscaycruz Mine in Central Peru with locations of Mina Chupa and associated drill holes (black labels) relative to other deposits and prospects in the mine area. 5.2 – Mina Chupa Background and Location The Mina Chupa deposit is located west of the Iscaycruz anticline to the south of a topographic depression (Figure 5.1). The orebody at Mina Chupa has a semi-cylindrical shape with dimensions approximately 600m x 200m (Escalante and Hart 2011). Mina Tinyag is the nearest deposit, located ~400m to the northeast of Mina Chupa. A detailed summary of the Mina Chupa deposit is located in Chapter 2.  5.2.1 – Geology and Structural Elements The local geology at Mina Chupa consists of the transition between the upper calcareous group rocks of the Iscaycruz district and the lower clastic dominated rocks of the Goyllarisquizga Group (Figure 5.2, Flores 1990). The Mina Chupa deposit is located beneath a topographic high, where the Pariahuanca Formation carbonates contact the sandstones and quartzites of the Farrat Formation.  105   Figure 5.2: Leapfrog model of study area. Image is looking to the S, showing location of Iscaycruz deposits (colored circles). Drill hole collars (black circles) and surface transects (blue circles) are also displayed. The transition zone grades from black, micritic Pariahuanca Formation limestone in the west, through a gray-tan calcareous sandstone in the contact, to a tan sandstone and quartzite of the Farrat Formation to the east (Figure 5.3). The main structural trends in the Mina Chupa area consist of the larger, N-N300W trending, bedding-parallel structure which forms the central Iscaycruz anticline, and an E-NE trend of primarily strike-slip structures with some apparent vertical displacement (Escalante and Hart 2011). At Mina Chupa, mineralization is present primarily at the intersection of these two controlling trends, with mineralization bounded by a subvertical fault (Fault 1) in the north and a low angle fault (Fault 2) in the south (Escalante and Hart 2011). 106   Figure 5.3: Overview of Mina Chupa area lithologies, looking  NW atop ridge above mine entrance The NNW structural trend is the result of regional scale compression, followed by a tilting of the fold axis, and later bedding parallel normal faulting; whereas the E-NE trending structures are interpreted to be the result of multiple events related to regional extension (Escalante and Hart 2011). Bedding-parallel faulting is observed at the Pariahuanca/Farrat contacts throughout Mina Chupa.   5.2.2 – Mineralization and Paragenesis Mineralization at Mina Chupa is primarily stratabound within the Pariahuanca Formation along the contact with the Farrat Formation. Ore minerals consist of coarse-grained sphalerite and massive chalcopyrite in centimeter to meter scale layers and veins with minor to trace pyrrhotite and pyrite, and fill the interstices of tremolite, actinolite, and ilvaite rich layers (Escalante and Hart 2011). The surface above the underground mine contains scattered Fe and Cu oxide showings that occur parallel to bedding (Escalante and Hart 2011). Layers of ore mineralization are interbedded with limestone, chloritized calcareous sandstone, dolomitized limestone, and grey, brown, and green marble (Escalante and Hart 2011). Skarn alteration of host rocks is not pervasive within the host units and occurs near ore minerals and mineralizing veins. Textural relationships indicate that 107  an initial phase of pyrite deposition was followed by pyrrhotite + chalcopyrite + sphalerite, followed by the second phase of pyrite (Escalante and Hart 2011). The primary alteration assemblage at Mina Chupa includes andradite, Mg-calcite, Mg-chlorite, clinochlore, epidote, muscovite, phlogopite, rutile, and talc, whereas lower levels of the mine contained andradite + magnetite + titanite + ilvaite (Escalante and Hart 2011). Retrograde alteration at Mina Chupa partially replaced garnet minerals in an initial phase of epidote + clinochlore + phlogopite + johannsonite, followed by a second phase of actinolite + stilpnomelane + calcite (Escalante and Hart 2011). Dickite, illite, followed by kaolinite, occur as millimeter scale haloes around ore minerals (Escalante and Hart 2011).  5.3 – Sample Collection Samples collected consisted primarily of unmineralized to weakly mineralized rocks as previous studies focused on alteration mineralogy, fluid inclusions, and ore paragenesis at Mina Chupa (Escalante and Hart 2011). Surface transect samples were collected from two areas designated Mina Chupa South (MCS) and Mina Chupa North (MCN) (see Chapter 3 for detailed transect sample collection). MCS is the region encompassing the Mina Chupa mine and the surface areas to the south (Figure 5.4). MCN is the region north-northwest of the Mina Chupa mine, located along the contact between the Pariahuanca Formation and the Farrat Formation (Figure 5.4). A summary of the surface transects is presented in Table 5.3. Full transect descriptions are presented in Appendix A4. Table 5.1 – Mina Chupa Transects Transect ID Type Transect Feature Unit MCS-L1-1 Multiple Sample Gossan/Oxidation Pariahuanca MCS-L1-2 Multiple Sample Lithologic Contact Pariahuanca MCS-L1-3 Multiple Sample Fault Pariahuanca MCS-L2-1 Multiple Sample Lithologic Contact Pariahuanca MCS-L2-2 Multiple Sample Lithologic Contact Pariahuanca MCS-L2-3 Multiple Sample Lithologic Contact Pariahuanca MCS-L3-1 Multiple Sample Lithologic Contact Pariahuanca MCS-L3-2 Multiple Sample Lithologic Contact Pariahuanca MCS-L3-2 Multiple Sample Point Sample Pariahuanca MCS-L4-1 Multiple Sample Lithologic Contact Pariahuanca MCS-L4-2 Multiple Sample Lithologic Contact Pariahuanca MCN-L1-1 Multiple Sample Lithologic Contact Pariahuanca MCN-L1-2 Multiple Sample Lithologic Contact Pariahuanca  108   Figure 5.4: Mina Chupa South and North Regions with locations of surface transects (white labels) and drill holes from study (black labels).  5.3.1.1 - Mina Chupa South The transect locations in Mina Chupa South were selected for their proximity to the Mina Chupa deposit, beginning with transects that targeted surface mineralization and gossans and moving south towards less altered outcrops. The location of the surface transects is also related to drill hole data collected. Results from 109  surface sampling are evaluated against other surface transects and nearby drill holes. Detailed descriptions of Mina Chupa South transects are presented in Appendix A4. 5.3.1.2 - Mina Chupa North The two transect locations in Mina Chupa North were selected for their distal location to the Mina Chupa deposit (Figure 5.4). The lithologic contacts sampled by these transects mirrored those of the Mina Chupa South transects for comparison purposes. The location of the surface transects is also related to collected drill hole data collected. The MCN-L1-1 transect (Figure 5.4) is centered on the transitional contact between the dull black, micritic limestone of the Pariahuanca Formation and the grey to tan, well sorted, fine-grained calcareous sandstone of the Farrat Formation. The MCN-L1-2 transect (Figure 5.4) is centered on the sharp contact between the dull black, micritic limestone of the Pariahuanca Formation and the quartzite and sandstone of the Farrat Formation. Detailed descriptions of Mina Chupa North transects are presented in Appendix A4. 5.3.2 – Subsurface Transects Subsurface transects were sampled in a manner consistent with the surface transects in Mina Chupa and sampled lithologic contacts, faults, and mineralization contacts. Transect targets were selected for fluid pathway potential, accessibility, competency and availability of remaining core. The sampling interval distances were calculated from the drill hole orientation and the feature orientation (Figure 3.2). Sampling followed methodology procedures outlined in Chapter 3. A total of five drill holes were relogged and sampled (Figure 5.4). A summary of the five drill holes is presented in Table 5.1. Full descriptions of the drill holes and individual transects is presented in Appendix A4. Table 5.2 – Summary of Mina Chupa drillholes  Small LargeLow-AngleMineralized FaultFarrat-> PariahuancaPariahuanca -> Chulec SkarnPervasive DolomitizationCHS-7-14-01Pyrite + Sphalerite + PyrrhotiteX X X XCHS-7-14-02Pyrite + Sphalerite + ChalcopyriteX X X X XPA-12-02 None X X X XCH-8-12-05Pyrite + Sphalerite + ChalcopyriteX X X XCHN-13-02 Pyrite only X X X XDrill Hole MineralizationFaults Lithologic Contacts Other110  5.4 – Analysis and Results Samples (n = 668) were first micro-drilled and the resulting powder was analyzed for 13C/12C, 17O/16O, and 18O/16O using the MIA. Isotopic values are are given in per mil (‰) with δ18O values recorded with respect to Vienna Standard Mean Ocean Water (VSMOW) and δ13C values recorded with respect to Vienna Pee Dee Belemnite (VPDB). Threshold values for δ13C and δ18O were derived from an evaluation of background samples outlined in Chapter 3. The threshold values in Pariahuanca rocks for δ13C and δ18O are 2.2‰ and 21.9‰ respectively. For the results and subsequent discussion sections, the terms depleted and enriched are isotopic value adjectives that are used when comparing two compositions. Sample results are enriched or depleted relative to other rocks, background thresholds, or reference materials. Isotopic results relative to the background thresholds are reported as slightly, moderately, and extremely depleted. Results are classified as extremely depleted if the values are within or below the range of calcite in equilibrium with magmatic water (Bowman 1998); equal to -8‰ through -5‰ δ13C and 8‰ through 12‰ δ18O (Figure 5.5). Isotopic values above the background thresholds are not depleted and considered background values. Values between background and extremely depleted are divided into two parts, slightly and moderately depleted. Slightly depleted values are within the range of -1‰ through 2.2‰ δ13C and 17‰ through 21.9‰ δ18O. Moderately depleted values are within the range of -5‰ through -1‰ δ13C and 12‰ through 17‰ δ18O (Figure 5.5). These divisions are partially arbitrary and were selected under the assumption that the skarn at Mina Chupa was the result of magmatic fluids. 111   Figure 5.5: Illustration of background and depleted values for Pariahuanca rocks. Modified from Escalante 2008.  5.4.1 – Surface Results Results from surface transects are shown on an XY scatterplot (Figure 5.6) and summarized on box plots and parallel XY plots showcasing the sample distance from the center of a transect against either δ13C or δ18O (Figures 5.7 and 5.8).  Transect direction is indicated by annotation, and for two-sided transects, one direction was arbitrarily assigned a negative value for the distance from the center for plotting purposes. Detailed results for individual transects are presented in Appendix A4. Isotopic results from all surface transect types show a correlation between distance from the lithologic or fault contact and isotopic depletions in both δ13C and δ18O (Figure 5.6, Figure 5.8). The majority of transects show that samples closer to the feature contact had lower values for δ13C and δ18O (Appendix A4). The spot type of a sample showed a correlation to isotopic results, with calcareous sandstone samples yielding the lowest δ13C and δ18O values and calcite veins yielding lower average values than their host rock (Figure 5.7) 112   Figure 5.6: Isotopic results for all Mina Chupa surface transects showing δ13C and δ18O (‰) against sample distance from transect center.  Figure 5.7: δ13C vs δ18O results for Mina Chupa surface samples colored and shape coded by spot type. 113   Figure 5.8: Tukey Plots for isotopic results for all Mina Chupa South surface transects showing δ13C and δ18O (‰) against sample distance from transect center 5.4.2 – Subsurface Results Subsurface results for δ13C and δ18O are displayed within parallel downhole plots for each drill hole (Figures 5.9, 5.10, and 5.11). Cross-sections showcasing geology and transect features alongside results are presented in Appendix A4. Wall rock and carbonate vein samples are designated with an A and B at the end of an analysis number (Appendix A2). Select sample locations with two or more carbonate veins of interest have additional results designated with a C or D.  Subsurface results displayed similar correlations to transect features to those of surface samples and Santa Este drill holes. Proximity to sulfide mineralization, bedding parallel faults at lithologic contacts, and pervasively dolomitized intervals were the features most consistently depleted in δ13C and δ18O (Figures 5.9, 5.10, and 5.11). Isotopic depletions were observed to be localized the mineralized faults, dolomitized zones, and lithologic contacts, with values shifting from extremely depleted to background within several meters of the feature tested (Figures 5.9, 5.10, and 5.11, Appendix A4). Wall rock and vein pairs examined were mostly correlated in isotopic composition, however, they showed disparities between wall rock and vein composition within all feature types other than pervasively dolomitized and mineralized zones.   114  Figure 5.9: Parallel downhole plots for CHS-7-14-01 and CHS-7-14-02 showing δ13C and δ18O (‰), alongside sample spot type, background thresholds, and key features.   115   Figure 5.10: Parallel downhole plots for PA-12-02 and CHN-13-02 showing δ13C and δ18O (‰), alongside sample spot type, background thresholds, and key features.    116    Figure 5.11: Parallel downhole plot for CH-8-12-05  showing δ13C and δ18O (‰), alongside sample spot type, background thresholds, and key features. 5.5 –Discussion 5.5.1 – Isotopic Haloes and Distance to Feature Isotopic results from surface and subsurface samples show a correlation between distance from the lithologic, mineralization, or fault contact and isotopic depletions in both δ13C and δ18O (Figure 5.6, Figures 5.8-5.11). Host lithology also has a significant effect on isotopic composition with calcareous sandstone samples yielding 117  average δ13C values approximately 2‰ lower than limestone and recrystallized limestone. Calcareous sandstone samples are closer to the transect center on average, which could explain the observed difference in δ13C composition. Sample results were compared by lithology for only samples within five meters or less from the transect center (Figure 5.12). A significant difference in δ13C values between calcareous sandstone and other lithologies was again observed (Figure 5.12). Values for δ18O in this subset of the data demonstrate that calcareous sandstone has a broader range of values, but the mean value for the three rock types (limestone, calcareous sandstone, and recrystallized limestone) ranged from approximately 15.5‰ to 16.5‰. Similar subsetting of the data at two, one, and <1 meter showed similar patterns in both δ13C and δ18O.  Figure 5.12: Tukey Plots for isotopic results for all Mina Chupa surface transects showing δ13C and δ18O (‰) for samples within 5 meters from transect center. Calcareous sandstone samples show significantly lower δ13C values than other lithologies. Samples collected at distances <1 meter from the transect center have average δ13C and δ18O values of 1.8‰ and 14.4‰ respectively. Samples from two to ten meters away from the center displayed variable results, including a contrast between δ13C and δ18O behavior. The average value for δ13C at two meters is above background at 2.6‰, whereas the average value for δ18O is slightly depleted at 19.6‰. Samples at five meters display values of δ13C with a lower average than <1m; however, this is due to a single outlier (EXGR00008129) with -4.8‰. Removing this sample changes the δ13C average for samples at five meters from 1.6‰ to 2.1‰. 118  Removing this sample makes a small change to the δ18O average as the value for EXGR00008129 δ18O was 14.5‰. PA-12-02 Results from PA-12-02 show similarities to surface and other subsurface transects with depletions in δ18O occurring at and around the lithologic contact between the Farrat and Pariahuanca formations. The most substantial depletion, however, is surrounding the fault zone around the Chulec/Pariahuanca contact, with δ18O results in wall rock within 50m on either side of the fault zone grading from background to extremely depleted at the center of the fault zone (Figure 5.13). The results from this drill hole indicate that both faults and lithologic contacts can yield potentially valuable information about fluid flow.    Figure 5.13: δ18O results (‰) and threshold values within PA-12-02, highlighting the wall rock results surrounding  the fault zone in Transect 2 CHN-13-02 The results from CHN-13-02 do not demonstrate significant patterns in δ13C as the Pariahuanca carbonate samples yielded results above background. The results for δ18O yielded mildly to moderately depleted values in wall rock and veins at the fault-bounded transition from shale to limestone, grading discontinuously towards background values for 40 meters after the contact. The presence of 40 meters of shale in between the Farrat calcareous sandstone and the Pariahuanca limestone may have affected the fluid flow along this contact. The 119  non-calcareous shale could not be sampled due to insufficient carbonate content, and thus any gradational changes in isotopic composition between the calcareous sandstone and the limestone could not be determined.  5.5.2 – Isotopic Haloes Perpendicular to Bedding Overall, the results suggest that the contact between the two lithologies is a reliable focal point for the concentration of fluid flow, likely due to the contrasting rock properties of the calcareous sandstone of the Farrat Formation and the primarily micritic limestone of the Pariahuanca Formation (Escalante and Hart 2012). This is demonstrated by the extreme sensitivity to sample distance exhibited in δ18O results from lithologic contacts (Figure 5.6, Figures 5.8-5.11). Lithologic contacts show a clear contrast between the <1m and 1m distance groups, with the <1m group yielding a mean value of 13.1‰ δ18O. The difference between <1m and 1m distances indicates that isotopically depleted fluids moving along this contact are rock buffered and rapidly reach background levels with larger distances from the contact (Barker and Dipple 2019).  Samples collected at distances greater than one meter have variable responses that broadly correlate with distance but allow for local variation. Beyond ten meters from the contact, the average sample is above background for both δ13C and δ18O. A potential explanation for this relatively small range of apparent influence relates to the hypothesized role of mineralizing fluids at Iscaycruz. Previous studies have been unable to locate a causative intrusion for Iscaycruz mineralization, and a deep intrusion centered below the Tinyag area has been proposed as the most likely location (Escalante and Hart 2012). Fluid inclusion studies at Iscaycruz indicate that mineralizing fluids cooled down at greater distances from the system center (Escalante and Hart 2011, 2012). This zonation and temperature gradient is also potentially related with the metal distribution at Iscaycruz, with Cu-Zn skarn bodies at Mina Chupa and Zn-rich carbonate replacement bodies and mantos at Santa Este, Limpe Centro, and others (Escalante and Hart 2012, Meinert and Chang 2017). Ore bodies at Mina Chupa are structurally controlled and bounded by two faults; however, the mineralizing fluids related to the ore body formation had the potential to continue to surface along fluid pathways (Escalante and Hart 2012). The rheological contrast between more competent calcareous sandstone, sandstone, and quartzite lithologies against limestones has been proposed as an explanation for bedding parallel faults observed at Iscaycruz, and these faults could have also served as crucial fluid pathways for spent or rock-buffered mineralizing fluids (Figure 4.4; Escalante and Hart 2010, 2011.) The consistency of isotopically depleted sample 120  results at the lithological contacts may reflect this transport of rock buffered mineralizing fluids to the surface. The large distance from the mineralizing source could explain the diminished permeation of these fluids into the rocks around these contacts. The buffering effect of the calcareous sandstone and limestone rocks at the lithologic contacts near the surface, combined with the minimal remaining heat and contrast in isotopic composition of the fluids could result in a limited isotopic halo perpendicular to the orientation of the contact (Barker and Dipple 2019). Reactive transport models for stable isotopes as a result of contact metamorphism with igneous rocks shows a similar asymmetric gradient of isotopic values grading away from the contact (Bowman et al. 1994, Dipple 1998). The change in isotopic values observed on surface transects on the 0-10m scale range mimics the observed changes in isotopic values from the 0-1,000-meter range for the Alta isotopic aureole (Bowman et al., 1994, Dipple 1998).   5.5.3 – Isotopic Haloes Parallel to Bedding The fluid channeling capacity of the lithologic contact may be related to the contrasting rock properties, and as a result, the capacity would not be expected to vary significantly to the north and the south of Mina Chupa. Therefore, the isotopic halo surrounding the Mina Chupa deposit could potentially be present at a greater scale parallel to bedding compared to the limited scale observed perpendicular to bedding. δ18O results for surface transects were compared against their distance from the Mina Chupa deposit along the contact between the Farrat and Pariahuanca Formations. For this comparison, three subsets of the data were used and plotted on a fence section (Figure 5.14); the first sample from each transect (Figure 5.15), the average sample results from 0-10 meters from the transect center (Figure 5.16) and the average sample results from >10 meters (Figures 5.17). δ13C results were also compared for these three subsets; however, the data showed mixed results due to the different δ13C composition of the calcareous sandstones and limestones around Mina Chupa. δ13C results for the first sample from each transect are displayed in Appendix A4. 121   Figure 5.14: Fence section across Mina Chupa area with MCS and MCN transect locations. 122   Figure 5.15: Fence section of the Mina Chupa area with all surface transects displaying the δ18O value for the first sample in each transect.  Figure 5.16: Fence section of the Mina Chupa area with all surface transects displaying the average δ18O value for sample results from 0-10 meters from the transect center. 123   Figure 5.17: Fence section of the Mina Chupa area with all surface transects displaying the average δ18O value for sample results from >10 meters from the transect center. The results show the most depleted values in surface transects above and to the south of the deposit, with increasing values to the north and south. The two comparisons also show that the largest depletions in δ18O occur over the approximate location of the deposit (Figures 5.15 and 5.16). Sample results within one meter of the contact yield a broader range of values relative to results from 0-10 meters, however, both subsets show a δ18O anomaly in the rocks above the deposit. Local variation in host rock composition and fluid flow can yield extremely depleted values up to 1,000 meters from the deposit as evidenced by MCS-L4-2 (Appendix A4: Figure 5.4.1.2H). Considering the results of both comparisons, the averaging of samples from 0-10 meters may provide a more robust isotopic halo by diminishing the effect of far outliers in a transect. Comparison between the average sample results for δ18O from >10 meters from the transect center show that the isotopic halo above the approximate deposit location is no longer clearly distinguishable (Figure 5.17). Transect MCS-L1-2 shows the most significant change throughout the three compared intervals ranging from 7.8‰ to 13.6‰, and 26.7‰. Results for δ13C from the first sample in each transect show a similar pattern to δ18O results albeit with a smaller total footprint (Appendix A4: Figure 5.5.3E). Only five of the surface transects yielded δ13C values below background for the first sample in their transect, compared to 11 of 12 transects for δ18O results.  124  These comparisons demonstrate that isotopic haloes in δ18O at Mina Chupa can be considered a reliable indicator for buried mineralization. The results suggest that characteristics of the Mina Chupa deposit and the related host rocks have apparently channeled isotopically depleted fluids along the Pariahuanca/Farrat Formation contact up the to present surface. The zonation of isotopically depleted samples to the north and south of Mina Chupa suggest that the fluids were able to significantly influence host rocks at the contact up to 500m from the deposit. The farthest transect from the deposit, MCN-L1-2, consistently yielded δ18O values above background (Figures 5.15-5.17). The orientation and nature of the lithologic contact combined with the apparent fluid volume and composition of mineralizing fluids has resulted in an asymmetric isotopic halo approximately 10m (east/west) x 500m (north/south) along the transition from Pariahuanca limestone to Farrat calcareous sandstone. The applicability of δ13C as an isotopic vectoring tool appears to be more limited than δ18O. As demonstrated by the background threshold calculations in Chapter 3, the values for δ18O do not appear to vary significantly across carbonate lithologies in similar depositional periods (Gonzalez-Leon et al. 2008, Madhavaraju et al. 2013, Ramirez et al. 2015, Silva-Tamayo 2016). δ13C varies significantly from Pariahuanca limestone to Santa limestone, as well as from Farrat calcareous sandstone to Pariahuanca limestone. The local variations and transitional nature of the lithologic contacts at Iscaycruz could make them especially sensitive to influence from host rock differences in δ13C composition. Given that the most responsive sampling interval for δ18O was the first sample at the contact, this lithologic bias is problematic for implementation of δ13C as a reliable vector for fluid flow.  5.5.4 – Isotopic Haloes adjacent to Mineralization CHS-7-14-01 and 02  Results from CHS-7-14-01 and 02 also show depletions in δ13C and δ18O at lithologic and fault contacts. However, the presence of mineralized and dolomitized zones provide additional information about fluid flow and isotopic alteration. Results show significant meter-scale changes in isotopic composition at the boundaries of mineralized zones. The greatest depletions in δ13C and δ18O for both drill holes are in mineralized zones and the contact between calcareous sandstone and limestone. This association coincides with results from the surface transects, indicating that lithologic contacts are fluid pathway targets on both the surface and in the subsurface. The limited extent of isotopic alteration in wall rock was also observed, with results above background in δ13C and δ18O less than 10m from mineralized contacts. In CHS-7-14-01 however, there is a 125  gradual decrease in δ18O from a high of 25‰ at 70m from the contact, to slightly depleted values <5 meters from the contact (Figure 5.10). CH-8-12-05 Results from CH-8-12-05 yielded similar patterns in extremely depleted δ13C and δ18O with samples within or adjacent to mineralized zones. Results for δ13C had a limited alteration halo with samples returning to background values in less than five meters from mineralization. Patterns in δ18O showed depletions on both sides of the fault-controlled zones of mineralization, with values grading towards background over ~30 meters towards the end of the hole (Figure 5.18).   Figure 5.18: δ18O results (‰) and threshold values within CH-8-12-05. Alteration haloes surround both the mineralized skarn and the mineralized fault 5.5.5 – Relationship to Luminescence Wall rock and calcite vein sample results for δ13C and δ18O are mostly correlated; however, there are several exceptions. Calcite samples adjacent to faults from CHS-7-14-02 show a disparity between wall rock and calcite vein composition (Figure 5.9). Wall rock samples in this drill hole return to background values after 5 meters below the first mineralized fault zone; however, all of the calcite vein samples from these rocks were extremely 126  depleted in δ18O and displayed bright red UV fluorescence. Previous studies have identified bright red UV fluorescing calcite veins as fluid escape structures in CRD systems, which indicates that δ18O depleted red UV calcite veins at Iscaycruz may represent the same phenomenon (Meinert and Chang 2017, Megaw 2018).  PA-12-02 is the farthest sampled drill hole from Mina Chupa, does not contain any mineralized sections and only has two locations with bright red UVF calcite veins (Figure 5.10). The presence of red UV calcite alongside extremely depleted values suggests that the fault channeled fluids of the same composition as the mineralizing fluids from CHS-7-14-01 and 02. 5.5.6 – Fluid Evolution and Composition A characteristic of skarn deposit development is the depletion of δ18O values from the background carbonate composition to a value that approaches the composition of the causative magmatic intrusion (Nabelek 1991, Bowman 1998). Results from surface and subsurface samples (Figure 5.19) indicate an isotopic trend from the calculated background region (>2.2‰ δ13C and >21.8 δ18O) towards the composition of samples collected from Mina Chupa ore zones (-9.5‰ to -6‰ δ13C, 0‰ to 5.25‰ δ18O, CHS-7-14-01 and CHS-7-14-02). The fluid evolution of Pariahuanca rocks indicates that δ13C equilibrated separately from δ18O (Barker and Dipple 2019). The composition of carbonates from Mina Chupa ore zones (Figure 5.20) plot in a range that is more depleted than what would be expected for calcite in equilibrium with fluids derived from a nearby igneous intrusion (Bowman 1998, Barker and Dipple 2019). Mina Iscaycruz does not have a causative intrusion to sample; therefore, results from Mina Chupa can only be compared to similar deposits from other studies. Isotope systematics within multi-stage skarn systems have demonstrated different isotopic compositions for host rocks affected by the anhydrous prograde skarn stage and the hydrous retrograde stage (Bowman 1998). The prograde stage is dominated by magmatic fluids, resulting in altered host rocks that are fluid buffered and contain an isotopic composition similar to that of the nearby intrusion (Bowman 1998).  127   Figure 5.19: δ13C vs δ18O results for all Mina Chupa samples. Fluid evolution paths for Pariahuanca rocks in and around Mina Chupa are shown. Surface samples are closer to background values than subsurface samples, and calcareous sandstone samples are more depleted on average.  Both Mina Chupa and Santa Este ore zones yielded a similar field of isotopic composition. The retrograde stage can involve a significant component of meteoric water, which has a much wider range of isotopic composition and tends to lead to an overall decrease in δ18O composition (Bowman 1998, Hoefs 2009). For systems that are dominated by meteoric fluids, the δ18O composition of the hydrous stage can be significantly lower than the composition of the causative intrusion (Bowman 1998). Mina Chupa and Tinyag are most likely closer to the hypothesized center of the system than other Mina Iscaycruz deposits (Escalante and Hart 2012); however, the δ18O composition of the ore zones at Mina Chupa suggest that it represents a distal, meteoric water dominated hydrothermal system since its true distance from the source of the mineralizing fluid is unknown. Other studies have observed that postmineral exchange with δ18O-depleted meteoric water can decrease skarn isotopic composition below the range for the causative intrusion (Brown et al. 1985, Hall et al. 1988). Meteoric fluids depleted in δ18O could have interacted with Mina Chupa carbonates either during mineralization or after mineralization to produce the isotopic composition observed. 128   Figure 5.20: δ13C vs δ18O results for all Mina Chupa samples. Composition of Mina Chupa ore zone, ore zone calcite, and igneous calcite in equilibrium with magmatic intrusions from skarn deposits (Bowman 1998). Meteoric fluids interacting with ore zone rocks either during or after mineralization can significantly the reduce δ18O composition of host carbonates. 5.5.7 – Vein and Wall Rock Pairs The paired δ18O compositions of carbonate wall rock and calcite veins (Table 5.3) were evaluated to determine if significant correlations to fluid pathways or hydrothermal centers were present. Results for δ18O indicate that pervasive isotopic alteration only occurred in the areas proximal to the hypothesized hydrothermal center (Escalante and Hart 2012). Of the 30 samples that yielded a pair of extremely depleted δ18O values, 28 were subsurface samples, all of which were from proximal drill holes. Carbonate sample pairs where the wall rock was extremely depleted, and the calcite vein was not, were extremely rare (2 of 32). Similarly, surface samples with an extremely depleted calcite vein were also extremely rare (3 of 50). Given that late, meteoric dominated fluids can alter host rock composition in skarn systems, the background wall rock/extremely depleted calcite vein pairs do not appear to be reliable vectors for mineralization (Bowman 1998, Hall et al. 1988). The δ18O depletion in calcite veins by itself does not appear to correlate with proximity to mineralization. In contrast, pervasive and intense depletion of the carbonate wall rock, especially when paired with calcite vein depletion, is a reliable vector for proximity. For carbonate wall rock samples without a corresponding calcite vein, the results were similar. The vast majority of extremely depleted samples were proximal to the deposit and primarily in the subsurface, and the surface samples of this group were mostly at the start of contacts. 129  Table 5.3 - Summary of Carbonate Wall Rock and Vein Pair δ18O composition from Mina Chupa   5.6 – Exploration Implications and Conclusions Results from surface and subsurface transects demonstrate the potential for stable isotopes as a tool for exploring in carbonate host rock environments. Surface sampling for stable isotopes can identify large scale haloes and help distinguish otherwise equally prospective surface targets. The degree and extent of isotopic depletions can help signal the size and quantity of fluid flow to the explorer (Barker et al. 2013, Bowman et al. 1994, Dipple 1998) and environments with an asymmetric alteration halo similar to Mina Chupa can be used to inform drill programs. High angle lithologic contacts at Mina Chupa, combined with distal fluids have created a very narrow isotopic halo, both on the surface and in the subsurface, which corresponds to tightly bound ore bodies along the lithological contact at depth (Escalante and Hart 2011, 2012). These results indicate that surface sampling of comparable contacts in similar lithologic conditions could provide value for exploration companies looking for buried skarns and other carbonate-related deposits.  The results from PA-12-02 are of particular interest to explorers as it represents the distal signature of the isotopic alteration pattern. 130  In an active exploration project, PA-12-02 would most likely be disregarded as an exploration “miss” given the absence of mineralization or any visible alteration. However, isotopic depletions observed at the fault contact and UVF results of calcite veins at PA-12-02 are two types of cryptic alteration that are present. Results have shown that isotopic and UVF analysis can be used as valuable vectors towards mineralization in otherwise ‘barren’ drill holes (Escalante 2008). Results indicate that sampling can be limited to only lithological contacts and faults in order to detect isotopic haloes, providing a fast and low-cost method using MIA to detect cryptic alteration patterns. Isotopic alteration haloes indicative of magmatic fluid flow identified at an unmineralized fault or lithological contact can provide valuable information regarding the origin of the fluid and the orientation of fluid flow, which may be used for vectoring in subsequent drilling attempts.  Surface sampling programs should consider bedding orientation of the host rocks during sample collection. The samples at Mina Chupa were collected exclusively in a high-angle bedding environment, and thus conclusions are not universally applicable; however, the results demonstrate the significant effect that lithology and bedding may have in shaping isotopic haloes. The central Peruvian polymetallic belt has areas with similar terrain, host rocks and mineralization styles to Mina Chupa. Isotopic alteration haloes identified at Mina Chupa indicate that a sampling strategy involving samples collected along short, focused transects across features potentially involved in fluid flow can be applied throughout the central Peruvian polymetallic belt to aid in the discovery of buried mineralization in this region.    131  Chapter 6:  Pb Isotope Classification of Mineralizing Events 6.1 – Introduction and Purpose Understanding the metallogenic potential of individual periods of magmatism within an orogenic belt is critical to strategic exploration for new mineral deposits. Previous studies in the Peruvian Cordillera have identified several discrete periods of Cenozoic magmatism that correspond to known mineral deposits (Bissig et al. 2008, Bissig and Tosdal 2009). The Middle Miocene was considered to be particularly favorable to ore deposit generation, as evidenced by the Colquijirca, Chungar, Vitis, Cerro Tunshu, Cerro de Pasco, Huaron, and other deposits (Bissig and Tosdal 2009). The majority of Paleogene-Neogene mineral deposits in the Peruvian Cordillera have intrusive rocks that are genetically, spatially, and/or temporally linked to mineralization (Bissig and Tosdal 2009). Several prominent deposits in the northern region of central Peru, however, including Iscaycruz and Cercapuquio, are an exception to this association (Bissig and Tosdal 2009). The intrusive source rocks for the mineralization at Iscaycruz have not been previously identified and their composition and location are unknown. Understanding the exact timing of mineralization at Iscaycruz is crucial for the development of proximal exploration strategies, in order that nearby intrusions of similar age can be prioritized for subsequent exploration.  Despite the inability to sample the causative intrusions directly, stable Pb isotopes can be used to identify the main source reservoirs for mineralizing fluids and contained metals. Lead isotopes can also help evaluate the relative proportions of Pb in a deposit that was derived directly from a crystallizing magma versus from crustal assimilation and relate specific ore bodies to particular magmatic and mineralizing events (Bissig and Tosdal 2009). Previously published studies compared the Pb isotopic composition of central Peruvian igneous rocks, Mesozoic sediments, and Precambrian basement rocks to those of sulfide minerals from Cenozoic ore deposits (Gunnesch 1990, Macfarlane et al. 1990, Bissig and Tosdal 2009.) The challenge of identifying the obscured source intrusion for mineralization is not unique to Iscaycruz, and thus successful efforts can serve as further proof of concept and a model for other systems.   Fluid inclusion, alteration mineralogy, and stable isotopic evidence suggests that Iscaycruz represents a single mineralizing system centered above Tinyag (Escalante and Hart 2011, 2012, Chapters 4 and 5 of this study).  Mineralization at Santa Este, however, is located on the eastern side of the Iscaycruz anticline and is cut by east-dipping faults (Chapter 1, Figure 1.1). Appropriate identification of fluid sources using Pb isotopes could 132  indicate whether or not Santa Este represents a second mineralizing source located to the east of Iscaycruz,