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Age, paleotectonic setting, and common Pb isotope signature of the San Nicolás volcanogenic massive… Danielson, Thomas J. 2001

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AGE, PALEOTECTONIC SETTING, AND COMMON PB ISOTOPE SIGNATURE OF THE SAN NICOLAS VOLCANOGENIC MASSIVE SULFIDE DEPOSIT, SOUTHEASTERN ZACATECAS STATE, CENTRAL MEXICO THOMAS J. DANIELSON B.S., University of Wisconsin - Eau Claire, 1998 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF M A S T E R OF SCIENCE in THE F A C U L T Y OF G R A D U A T E STUDIES (Department of Earth and Ocean Sciences) by THE UNIVERSITY OF BRITISH C O L U M B I A December 2000 © Thomas J. Danielson, 2000 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department of £tL(sfk asirf fleets-^ Se /c^c^^ The University of British Columbia Vancouver, Canada Date DE-6 (2/88) Abstract The San Nicolas deposit is a world class polymetallic volcanogenic massive sulfide (VMS) deposit located in southeastern Zacatecas State, central Mexico. The location of the deposit in a metallogenic province previously thought to be barren with respect to V M S mineralization, coupled with the large size of the deposit, make understanding the framework within which the deposit formed important for further mineral exploration efforts in the region, as well as for providing a more complete understanding of the geology of the region. A geological, geochronological, lithogeochemical, and isotopic study was undertaken to address the age of the volcano-sedimentary succession which hosts the San Nicolas deposit and the paleotectonic setting in which it formed. U-Pb zircon dating of felsic volcanic units within the volcano-sedimentary succession indicates that the volcanic rocks, and by extension, the associated sulfide mineralization, formed in Late Jurassic time, at -148 Ma. The succession comprises Upper Jurassic submarine, mafic and felsic flows, volcaniclastic rocks, and hypabyssal intrusions. A limited number of thinly bedded, fine-grained clastic sedimentary rocks are locally intercalated with the volcanic units. Volcanic rocks within the lower part of the succession (stratigraphically below and interfingering with sulfide mineralization) display geochemical and isotopic characteristics which are consistent with formation within a back-arc basin. Geochemical and isotopic characteristics of volcanic rocks within the upper part of the succession (stratigraphically above V M S mineralization) reflect a change in paleotectonic setting to a relatively juvenile volcanic arc environment. Isotopic and geochronologic constraints suggest that the entire package was formed above attenuated continental crust, perhaps as old as Precambrian in age. In a separate, but related study, sulfide Pb isotope ratios were examined as a means of distinguishing between styles of mineralization within central and southwestern Mexico. Pb isotopic compositions from syngenetic and epigenetic mineralization in the region form two distinct fields, suggesting that Pb isotopic compositions can be used as an inexpensive and effective tool for discriminating between syngenetic and epigenetic mineralization. The consistency of Pb isotopic compositions of sulfide minerals from syngenetic and epigenetic deposits may also indicate a similar basement for all of central and southwestern Mexico. iii Table of Contents Abstract ii Table of Contents iv List of Tables vii List of Figures viii Acknowledgements x Chapter 1: Introduction 1 Introduction 1 Methodology 3 Presentation 4 Chapter 2: Age and Paleotectonic Setting of the Polymetallic San Nicolas 6 Volcanogenic Massive Sulfide Deposit, SE Zacatecas State, Mexico: Constraints from Isotopic and Geochemical Studies Introduction 6 Central Mexico: Terranes and Regional Geology 8 Geology of the Main Study Area 11 Age Constraints on Volcanic Stratigraphy and Sulfide Mineralization 16 Samples and Methods 16 Age Constraints from the San Nicolas Deposit 20 Age Constraints from the El Salvador Occurrence 27 Age Constraints from Surface Exposures of the Main Study Area 30 Age Constraints from Regional Samples 33 Discussion - Age Constraints 36 iv Paleotectonic Setting 37 Geochemistry 37 Samples and Methods 37 Immobile Elements 41 Lithogeochemical Stratigraphy 48 Tectonic Discrimination 51 Radiogenic Isotopes 57 Samples and Methods 57 Isotopic Stratigraphy 59 Tectonic Discrimination 64 Discussion - Paleotectonic Setting 67 Conclusions 70 References 73 Chapter 3: Common Lead Isotopes as an Exploration Tool: A Case Study from 77 Zacatecas State, Central Mexico Introduction 77 Geologic Background 79 Samples and Analytical Methods 81 Results 85 Viability as an Exploration Tool in Central Mexico 88 Extrapolation throughout the Guerrero Terrane 92 Conclusions 92 V References 97 Chapter 4: Conclusion 98 Conclusion 98 References 102 Appendix A 103 vi List of Tables Table 2.1 U-Pb sample information 19 Table 2.2 U-Pb analytical data 21 Table 2.3 Geochemical sample information 43 Table 2.4 Geochemical analytical data 44 Table 2.5 Isotope sample information 58 Table 2.6 Sm-Nd analytical data 60 Table 2.7 Whole rock Pb-Pb analytical data 61 Table 3.1 Sulfide Pb-Pb sample information 84 Table 3.2 Sulfide Pb-Pb analytical data 86 Table 3.3 Range of previously published Pb isotope ratios 93 Table 5.1 Lithogeochemical Standard Analyses 104 vii List of Figures Figure 1.1 Geologic map of southeastern Zacatecas State 2 Figure 2.1 Geologic map of southeastern Zacatecas State 7 Figure 2.2 Tectonostratigraphic terranes of Mexico 9 Figure 2.3 Geologic map of the main study area 12 Figure 2.4 Schematic stratigraphic column of the San Nicolas deposit area 14 Figure 2.5 Surface U-Pb sample locations 17 Figure 2.6 Schematic U-Pb sample locations: San Nicolas deposit area 24 Figure 2.7 U-Pb concordia plots: San Nicolas deposit area 25 Figure 2.8 Schematic U-Pb sample locations: E l Salvador occurrence area 28 Figure 2.9 U-Pb concordia plots: E l Salvador occurrence area 29 Figure 2.10 U-Pb concordia plots: surface exposures - main study area 31 Figure 2.11 Regional U-Pb sample location 34 Figure 2.12 U-Pb concordia plot for regional sample 35 Figure 2.13 San Nicolas area schematic column with sample locations 38 Figure 2.14 El Salvador area schematic column with sample locations 39 Figure 2.15 Surface geochemical sample locations: main study area 40 Figure 2.16 Surface geochemical sample locations: Zacatecas City area 42 Figure 2.17 Plots of Nb/Y vs. Zr /Ti0 2 and S i 0 2 distribution 49 Figure 2.18 Plot of Zr vs. Y 50 Figure 2.19 Plot of chondrite normalized REE patterns 52 viii Figure 2.20 Plots of Zr-Ti-Y, Zr-Nb-Y, and Ti-V: main study area 54 Figure 2.21 Plots of Zr-Ti-Y, Zr-Nb-Y, and Ti-V: Zacatecas City vicinity 55 Figure 2.22 Plots of SNd for the main study area 62 Figure 2.23 Plot of 2 0 6 Pb/ 2 0 4 Pb ratios: San Nicolas deposit area 65 Figure 2.24 Plot of Pb isotope growth curves 68 Figure 2.25 Schematic model for the evolution of the study area 71 Figure 3.1 Zacatecas State 78 Figure 3.2 Physiographic provinces of Zacatecas State 80 Figure 3.3 Generalized map of sulfide sample locations 83 Figure 3.4 2 0 6 Pb/ 2 0 4 Pb vs. 2 0 7 Pb/ 2 0 4 Pb plot - central Mexico 89 Figure 3.5 2 0 6 Pb/ 2 0 4 Pb vs. 2 0 8 Pb/ 2 0 4 Pb plot - central Mexico 90 Figure 3.6 2 0 7 Pb/ 2 0 6 Pb vs. 2 0 8 Pb/ 2 0 6 Pb plot - central Mexico 91 Figure 3.7 Guerrero Terrane comparison: 2 0 6 Pb/ 2 0 4 Pb vs. 2 0 8 Pb/ 2 0 4 Pb 94 Figure 3.8 Guerrero Terrane comparison: 2 0 6 Pb/ 2 0 4 Pb vs. 2 0 8 Pb/ 2 0 4 Pb 95 IX Acknowledgements This project has benefited greatly from the guidance and support of a number of people. First and foremost, I would like to thank my supervisor, Jim Mortensen, for bringing this project to my attention and for providing a guiding hand when necessary. Richard Friedman and Janet Gabites are thanked for their endless patience and assistance in the laboratory, both in the analysis of samples and in the interpretation of those results. I am convinced that without their guidance I would have long since developed a number of nervous system disorders. My thanks are also extended to Steve Piercey, with whom I shared an office. He proved a great fountain of knowledge with regard to geochemistry and he never allowed me to take myself to seriously. I would like to thank Teck Corporation for allowing me the opportunity to work on the San Nicolas deposit, a world class V M S deposit. Minera Teck, S.A. de C.V. provided generous financial and logistical support throughout the course of the project and I would particularly like to extend my gratitude to Brad Johnson, Mario Canela-Barboza, and Antonio Montante-Martinez for sharing their knowledge of the geology of the deposit and of the region. Those who are the most responsible for this work reaching fruition, and to whom I owe the deepest gratitude, are my wife, Antje, who hasn't had the opportunity to spend a summer with me in three years, and my son Alex, for his endless patience (at least from his perspective as an eleven year old). C H A P T E R 1 Introduction The San Nicolas deposit is a world class (-100 Mt), polymetallic volcanogenic massive sulfide (VMS) deposit located in southeastern Zacatecas State, central Mexico (Figure 1.1). The deposit is located in a metallogenic province that was previously thought to be barren with respect to V M S mineralization. This fact, coupled with the large size of the deposit, make understanding the stratigraphic and paleotectonic framework within which the deposit formed critical for guiding future mineral exploration efforts in the region. In addition, the pre-Tertiary geology of this part of central Mexico is currently not well understood, and any additional information concerning the nature, age, and origin of pre-Tertiary rock units will help constrain the Mesozoic tectonic evolution of central Mexico. A field based geochronologic, lithogeochemical, and isotopic study was undertaken in an effort to constrain the age of the host stratigraphy and sulfide mineralization in the San Nicolas deposit area and identify the paleotectonic setting in which the deposit and its host rocks formed. In conjunction with efforts to identify the paleotectonic setting of the San Nicolas deposit, it was hoped that a chemostratigraphic framework for the deposit could also be established. As an outgrowth of this study, the viability of using Pb isotope compositions of sulfide minerals as isotopic "fingerprints" for different styles of mineralization in central Mexico was also investigated. This research was funded solely by Teck Corporation Ltd. through their Mexican subsidiary, Minera Teck, S.A. de C.V.. A total of eight U-Pb age determinations, 31 major, trace, and rare earth element analyses, 21 Sm-Nd analyses, and 55 Pb isotopic measurements were completed during the course of this study. 1 2 Methodology Field Studies Field studies were carried out by the author within the main study area, incorporating the work of Minera Teck, S.A. de C.V. geologists, to provide a geologic framework for detailed geochronologic, lithogeochemical, and isotopic studies. Detailed logging of core from a limited number of drill holes associated with the San Nicolas deposit, and a cursory reexamination of core from many of the other drill holes, provided a background for the systematic geochronologic, lithogeochemical, and isotopic sampling from the deposit stratigraphy. In addition, geologic traverses across portions of the main study area provided additional information on which to base the interpretation of analytical results from surface samples. U-Pb Geochronology U-Pb dating methods were selected for this study due to the robust nature of the U-Pb isotopic system and the presence of zircon-bearing felsic volcanic rocks within the study area. U-Pb zircon ages obtained by the author during the study constrain the age of the volcano-sedimentary succession in the study area, and sulfide mineralization contained within that succession. A l l U-Pb analyses were carried out in the Geochronology Laboratory at the University of British Columbia. 3 Lithogeochemistry Lithogeochemical studies using relatively immobile high field strength and rare earth elements were employed to interpret the paleotectonic setting of volcanic sequences in the study area. Samples were prepared by the author and geochemical analyses, using a combination of X-ray fluorescence and inductively coupled plasma mass spectrometry, were done by Chemex Labs in North Vancouver, British Columbia. Isotopic Studies Isotopic studies are commonly used to provide insights into the paleotectonic settings of ancient volcanic sequences. Both the Sm-Nd and Pb-Pb isotopic systems were used in this study. As with the lithogeochemistry samples, all sample preparation was done by the author. Sm-Nd analyses were done at the University of Alberta, and common Pb analyses of whole rock and sulfide samples was carried out by the author and J. Gabites in the Geochronology Laboratory at the University of British Columbia. In both laboratories, thermal ionization mass spectrometry was used to determine isotopic ratios. Presentation Results of this study are presented here as two research papers that will be submitted to referred journals for publication (Chapters 2 and 3). Chapter 2 includes the major body of work carried out during this study. It is focused on the age and paleotectonic setting of V M S mineralization in the study area, particularly of the San Nicolas deposit and the El Salvador occurrence. Age constraints presented here for the volcano-sedimentary succession within 4 the study area were determined on geologically well constrained felsic volcanic and intrusive rocks. Geological control for lithogeochemical and isotopic sampling was provided by detailed maps and drill logs prepared by Minera Teck, S.A. de C.V. geologists, supplemented by observations by the author. Chapter 3 examines the viability of using Pb isotopes as an exploration tool within central and southwestern Mexico; specifically the potential for providing Pb isotopic "fingerprints" for determining styles and relative ages of mineralization in the region. Analytical data was obtained from sulfide minerals for known styles of mineralization from deposits visited throughout central Mexico. These data are interpreted together with published and unpublished Pb isotopic data from mineralization in central and southwestern Mexico. Chapter 4 presents a brief summary of the conclusions presented in Chapters 2 and 3. Additional questions raised during the course of this study and potential avenues of investigation to address those questions are also discussed. 5 C H A P T E R 2 Age and Paleotectonic Setting of the Polymetallic San Nicolas Volcanogenic Massive Sulfide Deposit, SE Zacatecas State, Mexico: Constraints from Isotopic and Geochemical Studies Introduction The San Nicolas deposit is a stratiform, polymetallic (Cu-Zn-Ag-Au) volcanogenic massive sulfide (VMS) deposit hosted within Upper Jurassic to Lower Cretaceous volcanic and volcaniclastic rocks in southeastern Zacatecas State, central Mexico (Figure 2.1). Both the San Nicolas deposit, which was discovered in late 1997, and the previously discovered (1996) but much smaller E l Salvador V M S occurrence, are contained within the E l Salvador property, a joint venture property held by Teck Corporation and Western Copper Holdings Ltd. The E l Salvador property (hereafter referred to as the main study area) is located -60 kilometers east-southeast of the city of Zacatecas and lies along the southern margin of the Mesa Central or Altiplano physiographic province. Historically the region of the Mesa Central province which encompasses the main study area has been referred to as the Faja de Plata or Mexican Silver Belt due to the epigenetic Ag-Au-Pb mineralization which has been mined in the area since colonial times (Consejo de Recursos Minerales, 1992). Recognition in 1996 that the E l Salvador occurrence (<1 Mt) comprises syngenetic rather than epigenetic mineralization, however, highlighted the VMS potential of the region. This potential was confirmed in 1997 with the discovery of the San Nicolas deposit, which is now recognized as the largest V M S deposit in Mexico, with resources currently estimated at approximately 100 Mt at 1.36% Cu, 1.64% Zn, 0.15% Pb, 0.41 g/t Au, and 24.0 g/t Ag (Johnson et al., 2000). 6 7 V M S exploration within much of the southeastern Mesa Central province is hampered by extensive Tertiary and Quaternary cover, poor exposure, and a relatively poor geologic understanding of the region. In view of the tendency of V M S deposits to occur as clusters along favorable stratigraphic horizons, establishing the age of formation of the San Nicolas deposit and its host rocks and achieving a better understanding of the tectonic setting in which the deposit formed was desirable in order to improve V M S exploration models for the region. The results of U-Pb dating, lithogeochemical, and radiogenic isotope (Sm-Nd and Pb-Pb) studies are presented here and are used to constrain the age and paleotectonic setting of the San Nicolas deposit. Central Mexico: Terranes and Regional Geology In 1983, Campa and Coney published the first attempt at defining the terranes of Mexico. They identified fourteen discrete tectonostratigraphic terranes and two major overlap assemblages (Figure 2.2). Of the terranes defined by Campa and Coney (1983), the Guerrero and Sierra Madre terranes are the only terranes present within the study area. The Guerrero Terrane is a composite terrane which is interpreted to be allochthonous with respect to nuclear Mexico. It has been subdivided into three major subterranes and a number of lesser subterranes. Major subterranes include the Zihuatanejo subterrane, exposed in the western and southern parts of the terrane along the west coast of Mexico, the Huetamo subterrane, which composes the south-central inland portion of the terrane, and the Teloloapan-Ixtapan subterrane, which makes up the southeastern portion of the terrane. The main study area lies within the relatively poorly studied northeastern portion of the Guerrero Terrane, near the margin of the adjacent Sierra Madre Terrane (Figure 2.2). 8 Figure 2.2 Tectonostratigraphic terranes of Mexico modified from A) Campa and Coney (1983) and B) Sedlock et al. (1993). 9 Sedlock et al. (1993) also attempted to define the terranes of Mexico. Although most of the terrane nomenclatures changed, many of the terrane boundaries are consistent between the two studies, particularly in northern and southern Mexico. The area of greatest discrepancy is in central and southwestern Mexico, in the area identified as the Guerrero Terrane by Campa and Coney (1983). According to Sedlock et al. (1993) this region of Mexico is composed of three separate terranes; the Tepehuano Terrane, encompassing the western Sierra Madre and eastern Guerrero Terranes of Campa and Coney, the Tahue Terrane which corresponds to the northern part of the Zihuatanejo subterrane of Campa and Coney, and the Nahuatl Terrane which corresponds to the Huetamo, Teloloapan-Ixtapan, and southern Zihuatanejo subterranes of Campa and Coney. According to the Sedlock et al. (1993) terrane distribution model the main study area lies within the Tepehuano Terrane (Figure 2.2). Although the work of Sedlock et al. postdates that of Campa and Coney by ten years the Campa and Coney terminology is still used by persons working in the area. We therefore follow convention and use the terrane terminology of Campa and Coney throughout the remainder of this work. Regardless of terrane nomenclature, Campa and Coney (1983) and Sedlock et al. (1993) agree on the geology of the Mesa Central physiographic province, the region which encompasses the study area. The oldest rocks exposed within the region are Upper Triassic phyllite, slate, quartzite, marble, and meta-conglomerate of undetermined thickness which are assigned to the Zacatecas Formation. Jurassic marine flysch of the Noria de Angeles Group and Upper Jurassic to Lower Cretaceous volcano-sedimentary rocks of the Chilitos Formation unconformably overlie the Zacatecas Formation. The Chilitos Formation, the 1 0 formation of primary interest in this study, consists of a succession of pillowed basalt to basaltic-andesite flows and intercalated marine, non-fossiliferous siliciclastic, epiclastic, and calcareous sedimentary rocks (de Cserna, 1976; Consejo de Recursos Minerales, 1992). Overlying the Chilitos Formation is a thick succession of limestone, shaly limestone, and shale of the Cuesta del Cura, Indidura, and Caracol formations. Tertiary subaerial volcanic rocks and Quaternary alluvium, colluvium, and caliche cover much of the province leaving only isolated exposures of pre-Cenozoic units. Geology of the Main Study Area The majority of the main 30 km 2 study area is covered by Quaternary alluvium and caliche (Figure 2.3). Scarce exposures of pre-Quaternary rocks are typically low and restricted areally with the exception of exposures associated with human activities (quarries, exploration trenches, and mine workings). Pre-Quaternary units exposed on the main study area are dominated by sub-aqueous, Upper Jurassic - Lower Cretaceous volcanic and volcaniclastic rocks with subordinate sedimentary rocks, all of which are overlain by sub-aerial, Tertiary volcanic and volcaniclastic rocks. The Mesozoic rocks mainly comprise mafic flows, felsic flows and tuffs, mafic to felsic dikes, and epiclastic sediments (volcanic lithic wackes). Less abundant mudstone, chert, and limestone are locally intercalated with the volcanic units. The Mesozoic volcano-sedimentary sequence dips gently to the southwest over much of the main study area. It has been tentatively correlated with the volcano-sedimentary Chilitos Formation, which is interpreted to be Tithonian to Hauterivian in this area (Johnson et al., 2000). Surface exposures of Tertiary units comprise felsic flows, tuffs, and volcanogenic breccias (predominately aphanitic volcanic fragments with less abundant 11 Figure 2.3 Geologic map of the main study area. Modified from Johnson et al., 2000. L E G E N D QUATERNARY I I Undivided TERTIARY j — | Rhyolitic Ash Flow Tuff, Volcaniclastic Breccia U. JURASSIC - L. CRETACEOUS 0 Lithic Wacke, Lithic Tuff, Chert ~ \ Chert 1 I Rhyolite Flows and Felsic Intrusion _ Mafic to Intermediate Flows and Related Intrusions Mudstone. Chert, Limestone A VMS Occurence ^ Strike and Dip of Strata 4. 43 5 5 ^ tonalite, homblende-diorite, crystal tuff, and chert fragments). The Tertiary magmatism may be correlative with volcanism associated with either the mid-Tertiary Sierra Madre Occidental or the late Tertiary Trans-Mexican Volcanic Belt. Due to the extensive Quaternary alluvium and caliche cover in the immediate area, stratigraphic and structural interpretations of the host rocks for the San Nicolas deposit and El Salvador occurrence are derived almost entirely from drill intersections. The stratigraphy of the larger and more completely drilled off San Nicolas deposit is described in the following section. Eight distinct units have been identified in the immediate vicinity of the San Nicolas deposit, based on lithologic variations and observed contact relationships. Four of these are in the footwall relative to sulfide mineralization and four are in the hangingwall of the deposit. Each of the units associated with the San Nicolas deposit are briefly described here and shown schematically in Figure 2.4. A more detailed description of the host stratigraphy can be found in Johnson et al. (2000). The structurally lowest and presumably oldest unit in the host sequence consists of black to medium gray, thinly bedded graphitic mudstone ("SED" unit). This unit has been moderately to strongly deformed relative to the remainder of the host succession suggesting either that deformation preceded deposition of the younger units or, more probably, that strain was preferentially partitioned into the less competent sedimentary unit. Overlying the graphitic sediments is a succession of sub-aqueous mafic flows, lithic-crystal- tuffs, and intercalated sedimentary rocks ("MVT" unit) with a unit thickness of up to 120 meters. Flows are massive, aphanitic to feldspar phyric and locally contain relict pyroxene and amygdules. Tuffs contain 10-20% feldspar fragments and lesser quartz 13 Figure 2.4 Schematic stratigraphic column for the San Nicolas deposit area. 0 so 100 150 200 L E G E N D • TC TUF MFB 1 M V S MS F V M V T 1 QRY SED TC Unit - Polymict volcanogenic breccia with angular to sub-rounded , mafic and felsic volcanic and less abundant chert clasts. TUF Unit - Plagioclase-phyric and amygdaloidal mafic flows and volcaniclastic rocks. MFB Unit - Pyroxene- and plagioclase-phyric and amygdaloidal mafic flows and volcanic breccias. MVS Unit - Aphanitic and locally amygdaloidal mafic flows and intercalated mudstones. MS - Massive to semi-massive sulfides. FV Unit - Flow dome complex composed of massive, aphanitic to locally feldspar-phyric felsic flows and hyaloclastite. QRY Unit - Quartz- and feldspar-phyric felsic dikes tentatively correlative with the FV Unit. MVT Unit - Aphanitic to feldspar-phyric mafic flows, lithic- crystal-tuffs, and intercalated siliceous sediments. SED Unit - Moderately to strongly deformed, thinly bedded graphitic mudstone. 14 fragments (1-10%) as well as 10-15% volcanic lithic fragments. Intercalated sedimentary rocks are massive and siliceous in nature and include less abundant medium to fine grained wackes. Stratigraphically overlying and locally interfingering with the M V T unit is a thick succession (up to 300m) of felsic (rhyolite to rhyodacite) flows and hyaloclastites ("FV" unit) that together form a flow-dome complex. Flows are massive and aphanitic to locally feldspar phyric and are more abundant in the lower portion of the complex. Hyaloclastites are autoclastic and consist of sub-angular to sub-rounded fragments of massive to flow banded rhyolite. Fragment size generally decreases up section within the flow-dome complex. The complex forms the southwest footwall of sulfide mineralization and is interpreted to be at least in part contemporaneous with sulfide mineralization. The fourth distinct unit identified in the footwall of the San Nicolas deposit is a quartz-(5-10%) and feldspar- (1-10%) phyric rhyolite ("QRY" unit) which locally cross-cuts the M V T unit. Based on the absence of this unit in the hanging wall, the presence of alteration typical of other footwall units, and lithologic and geochemical similarities to the F V unit, this unit is thought to predate sulfide mineralization and is tentatively interpreted as a sub-volcanic equivalent of the F V unit. The sulfide mineralization at San Nicolas was not examined in this study; for a detailed discussion of the San Nicolas deposit sulfides the reader is referred to Johnson et al. (2000). Mafic flows and intercalated sedimentary rocks ("MVS" unit), with a variable unit thickness of up to 80 meters, immediately overlie the sulfide deposit. Flows are massive, aphanitic, and locally amygdaloidal. Intercalated sedimentary rocks are massive to laminated, siliceous to slightly carbonaceous mudstones. This unit interfingers with the 15 upper portion of the felsic dome complex to the southwest, suggesting the dome complex was active during the entire span of mineralization. A thick (up to 100 meters) succession of mafic flows and volcanic breccias ("MFB" unit) overlies the M V S unit. Flows and breccia fragments are pyroxene (1-10%) and plagioclase (10-15%) phyric and contain abundant amygdules. Pyroxenes are in part replaced by calcite, epidote, and / or amphibole and the amygdules are commonly filled with chlorite, calcite, or quartz. The uppermost unit within the Mesozoic sequence in the San Nicolas area comprises an unknown thickness of mafic flows and volcaniclastic rocks ("TUF" unit). Mafic flows are characterized by the presence of fine plagioclase phenocrysts and amygdules. Volcaniclastic rocks include tuffs of a similar composition, as well as tuffaceous sandstone and wacke. The volcaniclastic component distinguishes this unit from the M F B unit. Massive Tertiary volcanogenic breccias overlie the TUF unit above an angular unconformity. The breccia is polymict with angular to sub-rounded, mafic and felsic volcanic and less abundant chert clasts, and appears to be a mass flow deposit. Age Constraints on Volcanic Stratigraphy and Sulfide Mineralization Samples and Methods A total of nine samples were collected for U-Pb dating from either drill core or surface exposures of the main study area. Four samples were collected from San Nicolas deposit drill core and two from El Salvador occurrence drill core. Three additional samples were collected from surface exposures in the La Virgen, El Tepozan, and San Nicolas cemetery areas (Figure 2.5). Although not immediately associated with the main study area, a sample 16 Figure 2.5 U-Pb sample locations for samples collected from surface exposures in the main study area. Map modified from Johnson et al., 2000. L E G E N D QUATERNARY I 1 Undivided TERTIARY I—| Rhyolitic Ash Flow Tuff, Volcaniclastic Breccia U . JURASSIC - L. CRETACEOUS C D Lithic Wacke, Lithic Tuff, Chert C Chert I I Rhyolite Flows and Felsic Intrusion _ Mafic to Intermediate Flows and Related Intrusions E55B8 Mudstone, Chert, Limestone A V M S Occurence Strike and Dip of Strata S A N N I C O L A S C E M E T A R Y A R E A ( 9 8 T D 1 4 7 ) 17 from the La Blanca pluton was also collected for U-Pb dating. This body was sampled to examine the possibility of a comagmatic relationship between the La Blanca pluton and volcanic rocks on the main study area. If a comagmatic relationship is temporally permissible, then the La Blanca pluton may have provided the heat source necessary to drive sulfide mineralization throughout the property. Sample numbers, sample locations, and brief lithologic descriptions are shown in Table 2.1. Zircon were separated from 10 to 15 kilogram samples using conventional crushing, grinding, and Wilfley table techniques, followed by final concentration using heavy liquids and magnetic methods. Mineral fractions for analysis were selected based on grain morphology, quality, size, and magnetic susceptibility. Unless otherwise noted all zircon fractions were abraded prior to dissolution to minimize the effects of post-crystallization lead loss, using the technique of Krogh (1982). A l l geochemical separations and mass spectrometry were conducted in the Geochronology Laboratory at the University of British Columbia. Samples were dissolved in concentrated HF and H N O 3 in the presence of a mixed 233 235 205 U - Pb tracer. Separation and purification of Pb and U employed ion exchange column techniques modified slightly from those of Parrish et al. (1987). Pb and U were eluted separately and loaded together on a single Re filament using a phosphoric acid-silica gel emitter. Isotopic ratios were measured using a modified single collector VG-54R thermal ionization mass spectrometer equipped with a Daly photomultiplier. Most measurements were done in peak-switching mode on the Daly detector. U and Pb analytical blanks were in the range of 1-3 pg and 7-15 pg, respectively, during the course of this study. U fractionation was determined directly on individual runs using a mixed " U tracer, and Pb isotopic ratios were corrected for a fractionation of 0.12%/amu and 0.43%/amu for Faraday and Daly 18 Table 2.1 U-Pb Geochronology Samples, Locations, and Rock Types Sample Location Rock Type Main Study Area San Nicolas Deposit 98TD153 SAL47 324.4-337.3m (FV Formation) 98TD11 SAL35 538.6-544.0m (QRY Formation) 98TD10 SAL24 230.9-236.5m (FV Formation) 98TD148 SAL34 313.5-324.Om (FV Formation) El Salvador Occurrence 98TD12 SAL 12 199.8-204.3m 98TD150 SAL2 232.8-241.lm Surface Exposures 98TD01 La Virgen Area (UTM 2503950N 196100E) 98TD02 El Tepozan Area (UTM 2503030N 194750E) 98TD147 San Nicolas Cemetery Area (UTM 2499600N 191425E) Aphanitic Felsic Volcanic Quartz Phyric Volcanic or Hypabyssal Aphanitic Felsic Volcanic Felsic Hyaloclastite Quartz and Feldspar Phyric Volcanic Quartz Phyric Volcanic Aphanitic Felsic Volcanic Quartz Phyric Volcanic Polymict Volcanic Breccia Regional Sample 98TD13 La Blanca Pluton Hornblende- Biotite- Granitoid runs, respectively, based on replicate analyses of the NBS-981 Pb standard and the values recommended by Todt et al. (1996). A l l analytical errors were propagated through the entire age calculation using the numerical technique of Roddick (1987). A l l errors are reported at the 2a level. Analytical results are reported in Table 2.2. Age Constraints from the San Nicolas Deposit FV Unit - San Nicolas Deposit (98TD153): This sample is a massive, aphanitic felsic volcanic rock representative of the F V unit and was collected from drill hole SAL47 between 324.4 and 337.3 meters (Figure 2.6). Abundant fine (<74 um), clear, colorless, stubby, and prismatic zircon grains of poor to moderate quality were recovered. Nearly all grains contained colorless to brown, rod-, tube-, and bubble- shaped inclusions. Five zircon fractions were analyzed. Fractions A , D, and F returned concordant results and fractions C and E returned discordant results (Figure 2.7). Based on the Pb/ U ages of the three concordant fractions a conservative igneous crystallization age of 146.5 +/- 2.2 Ma can be assigned. However, assuming fractions A and F have experienced simple Pb-loss following crystallization, we assign a crystallization age of 148.3 +/- 0.4 Ma based on the Pb/ U age of fraction D. Fraction E is discordant and gives a slightly older Pb/ Pb age (179.0 +/- 6.4 Ma), indicating the presence of a minor inherited zircon component. Based on volcanic textures (flow foliation, hyaloclastite, peperite along basal contacts) observed in this and other drill intersections, the sampled unit is interpreted to be an extrusive, submarine flow that is intercalated with, and locally replaced by, massive sulfides. 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II -I o o o ~ B O ^ H W V E CN O N i n 00 i n in cn CN cn i n in i n NO i n 0 0 0 0 O 0 CN 0 O CN 0 O 0 O O d d d d d d 0 d d 0 d d d d d 0 d d C N B O N % a E " E E E I g 1 4 ia 4 5 <5- s 4 H a" n," o K M- H,- to < n" Q w* u; o K ; o. q, D . 1 CN CN CN^  in in E E 2 03 CN O O N — —.' O — •3 O N & O . O . O D 0) C t ) C H Q , CJ H) —" —" C C C C C C c c o, o" o" o C J 2 ^  c 0 o 3 o C CN c H 2 2 o o d T 3 a o o d c o T 3 O B * f § a s =3 c b O § _o g « 8 3 ?. 03 so CO O • C CN C + 2 S o c o u cj -a e .p (2 S CM C P H 'a. H O O H O O H ° c o o 'B. E 3 E o o .2 U M G 4 2 O H H O G O T 3 •a P * + 00 O H # ° O H U J<! op c ca .2 HO J O O H <2 ^ O H t o O 23 Figure 2.6 Schematic stratigraphic column for the San Nicolas deposit area with U-Pb sample locations. L E G E N D TC TUF MFB MVS MS FV MVT QRY SED S A M P L E D L O C A T I O N S A M P L E L O C A T I O N S (schematic) I 98TD10-FVUnit 98TD148 - FV Unit 98TD153 -FVUnit I 98TD11 - QRY Unit 24 0.0239 0.0227 P CO oo < V XI NO o c\] 0.0215 0.0203 0.140 150 y S -A 140 yS Sample 98TD153 (FV Unit -148.3 +/-i i i Aphanitic Felsic Volcanic) 0.4 Ma 0 . 1 4 8 2 0 7 0.156 0.164 2 3 5 P b / U Figure 2.7 U-Pb concordia plots for rocks of the San Nicolas deposit area: A ) sample 98TD153, B) sample 98TD11 mineralization. These results therefore suggest that sulfide mineralization formed, at least in part, at 148.3 +/- 0.4 Ma. QRY Unit - San Nicolas Deposit (98TD11): This sample is a quartz phyric, non-foliated dike representative of the QRY unit and was collected from drill hole SAL35 between 538.6 and 544.0 meters (Figure 2.6). Abundant clear, colorless, stubby, and prismatic zircon grains of moderate to good quality were recovered. Three fractions were analyzed and each returned concordant results (Figure 2.7). The spread of 2 0 6 P b / 2 3 8 U ages is interpreted to reflect post-crystallization lead loss. An age of 148.9 +/- 1.4 Ma is assigned based on fractions A and C. There is no indication of an inherited component in any of the zircon fractions. Although this unit is hypabyssal in nature and discordant to the rest of the San Nicolas stratigraphy there are several reasons to believe the intrusion of this unit predates sulfide mineralization. This unit has been subjected to the same style of alteration as the rest of the footwall stratigraphy at San Nicolas. Additional indications of intrusion timing relative to sulfide mineralization include; presence of the unit only within the footwall stratigraphy, voluminous disseminated pyrite throughout the unit, and a lithogeochemical signature similar to that of the F V unit. Based on these arguments the sulfide mineralization associated with the San Nicolas deposit can be no older than 148.9 +/- 1.4 Ma. FV Unit - San Nicolas Deposit (98TD10 & 98TD148): Two additional aphanitic, felsic volcanic samples were collected from the FV unit (Figure 2.6) in drill holes SAL24 (230.9 -236.5 meters) and SAL34 (313.5 - 324.0 meters) but did not yield dateable minerals. 26 Age Constraints from the El Salvador Occurrence Footwall - El Salvador Occurrence (98TD12): This is a quartz- and feldspar- phyric intermediate to felsic volcanic rock collected from drill hole SAL12 between 199.8 and 204.3 meters (Figure 2.8). Abundant clear, colorless to pale pink to pale yellow, stubby, prismatic zircon with simple terminations was recovered. Four fractions were analyzed with two fractions returning concordant results (Figure 2.9). Based on the 2 0 6 P b / 2 3 8 U ages of concordant fractions B and D, a crystallization age of 150.6 +/- 0.7 Ma is assigned. Fraction E is interpreted as having lost lead and fraction C, with an older 2 0 7 Pb/ 2 0 6 Pb age (253.7 +/-97.6 Ma), is interpreted to have contained a minor inherited zircon component, either as xenocrystic grains or as cryptic cores that were not detected visually. The sampled unit is interpreted to be extrusive in nature and occurs in the footwall of sulfide mineralization. As the stratigraphy at E l Salvador appears to be structurally intact, the crystallization age of the sampled unit indicates that sulfide mineralization must be younger than 150.6 +/- 0.7 Ma. Footwall - El Salvador Occurrence (98TD150): This sample is a quartz-phyric felsic volcanic rock which was collected from drill hole SAL2 between 232.8 and 241.1 meters (Figure 2.8). Abundant clear, colorless to pale yellow, stubby, and prismatic zircon was recovered. Fractions A through C represent the slightly coarser grains and fractions D through H represent the finer grains. Of the eight fractions, four yielded concordant results (Figure 2.9). The spread of Pb/ U ages for fractions D through H is interpreted to reflect post-crystallization lead loss. A crystallization age is assigned based on the oldest concordant fraction (E), with a 2 0 6 P b / 2 3 8 U age of 147.9 +/- 0.4 Ma. Fractions A, B, and C are 27 Figure 2.8 Schematic stratigraphic column for the El Salvador occurrence area with U-Pb sample locations. L E G E N D S I L I C E O U S S E D I M E N T S I N T E R M E D I A T E T O M A F I C F L O W S , A S S O C I A T E D D I K E S , A N D I N T E R C A L A T E D M U D S T O N E S M A S S I V E A N D S E M I - M A S S I V E S U L F I D E S F E L S I C F L O W S A N D T U F F S I N T E R M E D I A T E T O M A F I C T U F F S S A M P L E D L O C A T I O N S A M P L E L O C A T I O N S (schematic) I 98TD12 98TD150 Figure 2.9 U-Pb concordia plots for rocks of the El Salvador occurrence area: A) sample 98TD12, B) sample 98TD150 all discordant and are interpreted to contain an inherited zircon component. Fraction C is a single grain fraction which yielded a Early to Middle Proterozoic 2 0 7 Pb/ 2 0 6 Pb age (1598.5 +/-3.2 Ma). This unit is also interpreted as extrusive in nature and occurs in the footwall of sulfide mineralization. Results from this sample require that sulfide mineralization in the El Salvador occurrence must be younger than 147.9 +/- 0.4 Ma. Although these results are from unabraded zircon they are consistent with results from other samples in this study. Age Constraints from Surface Exposures of the Main Study Area La Virgen Area (98TD01): This samples is a non-foliated, aphanitic rhyolite flow collected from a low, discontinuous exposure west of the La Virgen workings (Figure 2.5). Abundant clear, pale yellow, stubby to elongate prismatic zircon grains of moderate to good quality were recovered. Four zircon fractions all yielded concordant results (Figure 2.10). A conservative crystallization age of 148.0 +/- 1.3 Ma can be assigned based on the entire range 206 238 of Pb/ U ages. Assuming that only post-crystallization lead loss has affected the sample, the preferred age of 148.7 +/- 0.6 Ma is assigned based on fractions A and D. No evidence of an inherited component is present in any fraction. Without stratigraphic controls on emplacement with respect to sulfide mineralization, this sample provides no direct constraints on the age of sulfide mineralization at either the San Nicolas deposit or the E l Salvador occurrence. Based on petrologic, petrographic, and lithogeochemical similarities, however, the sampled unit is tentatively correlated with the FV Unit within the San Nicolas deposit. The interpreted age is consistent with this correlation. Figure 2.10 U-Pb concordia plots for rocks from surface exposures of the main study area: A) sample 98TD01, B) sample 98TD02, C) sample 98TD147 31 El Tepozan Area (98TD02): This sample is a quartz-phyric rhyolite and was collected from a low, discontinuous subcrop in the El Tepozan region of the main study area (Figure 2.5). Zircon grains recovered from this sample form clear, colorless to pale yellow to pale pink, stubby prisms of moderate to good quality (fractions A, B, E - J). A limited population of zircon grains were similar in clarity and color but were multifaceted to rounded (fractions C and D). Unfortunately all of the analyses are discordant (Figure 2.10) and therefore a crystallization age cannot be assigned. A regression of all fractions gives an imprecise lower intercept age of-158 +/- 20 Ma which overlaps the crystallization ages of rocks from both the San Nicolas deposit and El Salvador occurrence. San Nicolas Cemetery Area (98TD147): This sample was interpreted in the field to be a massive, aphanitic felsic flow with a locally brecciated flow margins. It was collected from an extensive area of subcrop north-northwest of the San Nicolas cemetery on the main study area (Figure 2.5) and was taken from the massive, aphanitic part of the exposure. Abundant clear to cloudy, colorless to pale brown, stubby, and prismatic to anhedral zircon grains of moderate quality were recovered. The cloudy appearance of grains is attributed to an abundance of bubble- and rod-shaped inclusions. Most of the anhedral grains were spherical with some showing signs of resorption. Eight zircon fractions were analyzed and all were highly discordant with Proterozoic to Late Archean 2 0 6 Pb/ 2 0 7 Pb ages (Figure 2.10). A small quantity of dark red to brown rutile was also recovered from this sample and two unabraded fractions were analyzed. Fraction RI is composed of relatively coarse anhedral grains and fraction R2 is composed of finer, sub- to euhedral acicular grains. Both rutile fractions are nearly concordant with a range of 2 0 6 P b / 2 3 8 U ages of -1455 - 1466 Ma (Figure 2.10). 32 The lack of any concordant fractions and the presence of rounded grains suggests the unit may be sedimentary rather than volcanic in nature. The relationship of the sampled unit to the stratigraphy in the San Nicolas and E l Salvador areas is unclear, however the 2 0 7 Pb/ 2 0 6 Pb ages for fractions F and G are none the less interesting. Fraction F returns a Paleoproterozoic age (-2375 Ma) and fraction G returns a Late Archean age (-2650 Ma). This is the first indication of an Archean detrital or igneous zircon component in central Mexico. Age Constraints from Regional Samples La Blanca Pluton (98TD13): This sample is a medium to coarse grained, hornblende-biotite- granitoid and was collected from a low exposure 2.7 km east of the town of La Blanca (Figure 2.11). A large quantity of poor to moderate clarity, colorless to light yellow, acicular, and prismatic zircon was recovered. Most grains contained abundant clear, bubble-and rod-shaped inclusions. Eight fractions were analyzed with only one returning a concordant result (Figure 2.12). Although all fractions were strongly abraded, the data suggests that all post-crystallization Pb-loss effects were not eliminated. Based on the single concordant analysis a crystallization age of 75.1 +/- 0.2 Ma is assigned. Fractions A , E, F, and G are interpreted as being affected by post-crystallization lead loss and fractions B, C, and D are interpreted to have contained an inherited zircon component. The Late Cretaceous crystallization age is consistent with a previously reported K - A r for this body, and eliminates the La Blanca pluton as a potential heat source for sulfide mineralization within the main study area. 33 34 Figure 2.12 U-Pb concordia plot for the La Blanca pluton (98TD13) Discussion - Age Constraints U-Pb results reported here provide constraints on the age of sulfide mineralization in both the San Nicolas deposit and the E l Salvador occurrence. Based on the interfingering of the FV unit with sulfide mineralization and both footwall (MVT) and hangingwall (MVS) units the age of 148.3 +/- 0.4 Ma for the FV unit is interpreted as both a crystallization age and the age of sulfide mineralization associated with the San Nicolas deposit. This is consistent with the maximum age of sulfide mineralization provided by the 148.9 +/- 1.4 Ma crystallization age for the pre-mineralization intrusive (QRY) unit. Results of this study do not provide an absolute minimum age for sulfide mineralization at San Nicolas. Similarly, only a maximum age constraint has been determined for sulfide mineralization in the E l Salvador occurrence. U-Pb age constraints indicate sulfide mineralization at E l Salvador occurrence must be younger than 150.6 +/- 0.7 Ma and may be younger than 147.9 +/- 0.4 Ma. Ages reported in this study are broadly consistent with the Upper Jurassic to Lower Cretaceous ages reported for V M S deposits of the Cuale District (Miranda-Gasca, 1995), the Guanajuato District (Ortiz-Hernandez, 1991), the Ray de Plata District (Burckhardt, 1930, Campa-Uranga et al., 1974), and the Campo Morado District (Oliver, 1997). Ages for deposits in the Cuale, Guanajuato, and Ray de Plata districts are based on fossil data with the exception of K - A r age data for the Guanajuato District (114-157 Ma). Volcanic rocks that host V M S deposits in the Campo Morado district have been assigned U-Pb zircon ages ranging from 143 Ma. to 150 Ma. (Mortensen, personal communication). The only V M S deposit within the Guerrero Terrane with a reported age of other than Upper Jurassic to Lower Cretaceous is the Tizapa deposit with a Triassic to possibly Upper Jurassic age (Elias-Herrera and Sanchez-Zavala, 1992) based on loosely constrained stratigraphic correlations 36 and varying degrees of deformation. U-Pb dating of a post-mineralization dike provides a minimum age of sulfide mineralization of 138.7 +/- 1 Ma (Rhys et al., 2000). Paleotectonic Setting Geochemical studies have been used to classify igneous rocks by composition and magmatic series (e.g., Irvine and Baragar, 1971, Winchester and Floyd, 1977, MacLean and Barrett, 1993) and to characterize the paleotectonic setting in which magmatism occurred (e.g., Pearce and Cann, 1973, Shervais, 1982, Meschede, 1986, Jenner et al., 1991, Santos Zalduegui et a l , 1996, Thompson et a l , 1997, Ustaomer, 1999). Major, trace, and REE geochemical and radiogenic isotope (Sm-Nd and Pb-Pb) data for igneous and sedimentary rocks from the study area are used to provide insights into the paleotectonic setting in which the San Nicolas deposit formed. GEOCHEMISTRY Samples and Methods Thirty-one samples were collected for lithogeochemical analysis; twenty-six of these were from the main study area and five were from regional exposures. Samples collected from the main study area include twelve from San Nicolas deposit drill core, two from El Salvador occurrence drill core, and twelve from surface exposures. Representative samples were collected from all of the volcanic units on the main study area, both from drill core and surface samples. Drill core sample locations are shown schematically in Figures 2.13 and 2.14 and surface sample locations are shown in Figure 2.15. Five regional samples were 37 SAMPLE LOCATIONS (schematic) < 98TD106 - TUF Unit (sediments) < 98TD40-MFB Unit • • 99TD29-MFB Unit < 98TD41 -MFB Unit * < 99TD28 - MVS Unit (sediments) < 98TD42 - MVS Unit (volcanics) • 99TD27 - MVS Unit (volcanics) 98TD2l3-FVUnit < 98TD45 - FV Unit 98TD152 - FV Unit 98TD148 - FV Unit 98TD153 - FV Unit 98TD136-Mafic Dike 4 98TD56-Mafic Dike 98TD58 - FV Unit 98TD154-FVUnit 98TD138 - MVT Unit + < 98TD60 - QRY Unit 98TD145 - QRY Unit 98TDl39-M\TUnit 98TD146-QRY Unit • < 99TD25 - SED Unit Figure 2.13 Schematic stratigraphic column for the San Nicolas area with lithogeochemical and isotopic sample locations. L E G E N D TC TUF MFB M V S MS FV MVT QRY SED • G E O C H E M I S T R Y • S M - N D < P B - P B 38 S A M P L E L O C A T I O N S (schematic) • 98TD184-Mudstone • 98TD187- Mafic Dike • 98TD189- Intermediate Flow + 98TD196 - Intermediate Tuff • 98 TD208 - Felsic Flow • 98TD149 - Felsic Flow • 98TD150- Felsic Flow L E G E N D S I L I C E O U S S E D I M E N T S I N T E R M E D I A T E T O M A F I C F L O W S , A S S O C I A T E D D I K E S , A N D I N T E R C A L A T E D M U D S T O N E S M A S S I V E A N D S E M I - M A S S I V E S U L F I D E S F E L S I C F L O W S A N D T U F F S I N T E R M E D I A T E T O M A F I C T U F F S • G E O C H E M I S T R Y • S M - N D Figure 2.14 Schematic stratigraphic column for the E l Salvador area with lithogeochemical and isotopic sample locations. Figure 2.15 Surface lithogeochemical and isotopic sample locations from the main study area. Geologic map modified from Johnson et al., 2000. L E G E N D QUATERNARY I I Undivided TERTIARY I—| Rhyolitic Ash Flow Tuff, Volcaniclastic Breccia U. JURASSIC - L. CRETACEOUS CD Lithic Wacke, Lithic Tuff, Chert ~\ c h e r t I Rhyolite Flows and Felsic Intrusions, Mafic to Intermediate Flows and ~l Related Intrusions E l Mudstone, Chert, Limestone E S A G R A D O C O R A Z O N A R E A ( 9 8 T D 1 6 2 ) E S A G R A D O C O R A Z O N A R E A ( 9 8 T D 1 6 1 ) E L SOCORRO A R E A ( 9 8 T D 1 4 0 ) A VMS Occurence Strike and Dip of Strata S A N N I C O L A S C E M E T A R Y A R E A ( 9 8 T D 1 4 7 ) 40 collected from north of the city of Zacatecas (Figure 2.16). Table 2.3 provides sample numbers, sample locations, and brief rock descriptions for all samples. Weathered surfaces were removed from surface samples prior to crushing. Samples were crushed and powdered using a standard jaw crusher and a tungsten carbide ring mill. A l l major, trace, and rare earth element (REE) analyses were done at Chemex Labs Ltd. in North Vancouver, British Columbia. X-ray fluorescence (XRF) was used in the determination of major element concentrations and inductively coupled plasma emission mass spectrometry (ICP-MS) methods were used to determine trace and rare earth element concentrations. In-house standards provided quality control and documented excellent reproducibility (Appendix A). Major, trace, and REE element concentrations are reported in Table 2.4. Immobile Elements One of the major considerations when using lithogeochemical data is the degree of mobility that elements within the system may have experienced, particularly in altered and metamorphosed rocks. The Upper Jurassic to Lower Cretaceous marine volcanic and sedimentary rocks of the Mesa Central province generally display lower greenschist facies mineral assemblages and have been subjected to either low grade regional metamorphism or alteration consistent with that of modern sea floor settings. Samples collected from surface exposures of the main study area contain mineral assemblages (sericite, chlorite, amphibole, epidote, and carbonate) which record similar metamorphic and / or alteration conditions. Samples collected from the San Nicolas deposit and El Salvador occurrence drill core are mineralogically similar to the regional sample suite but also display mineral assemblages and textures consistent with moderate to intense hydrothermal alteration related to V M S 41 I I 2540000N Q U A T E R N A R Y MESOZOIC Alluvium B Gabbro TERTIARY • Rhyolite Tuff Rhyolite Andesite to basalt flows, locally pillowed Andesite Tuff Andesite to Basalt Flows and Intercalated Sediments H i Andesite Tuffs and Local Andesite Flows |H Slate, Phyllite, Quartzite, Chert • J Slate, Sericite Phyllite, Fine-grained Schist Figure 2.16 Surface lithogeochemical and isotopic sample locations from the vicinity of Zacatecas City. Table 2.3 Geochemistry Sample Numbers, Sample Locations, and Brief Descriptions Sample Location Description Main Study Area San Nicolas Deposit 99TD27 Sal 47 233.0m Aphyric mafic flow (MVS) 99TD29 Sal 47 210.3m Mafic flow / breccia (MFB) 98TD136 SAL57 354.7-357.0m Mafic dike (MVS) 98TD138 SAL35 487.0-489.0m Mafic lithic tuff (MVT) 98TD139 SAL35 515.6-516.9m Mafic lithic tuff (MVT) 98TD145 SAL59 298.5-308.8m Quartz-phyric volcanic (QRY) 98TD146 SAL34 527.4-539.0m Quartz-phyric volcanic (QRY) 98TD148 • SAL34 313.5-324.0m Felsic hyaloclastite (FV) 98TD152 SAL47 312.7-322.2m Chloritized felsic volcanic (FV) 98TD153 SAL47 324.4-337.3m Chloritized felsic volcanic (FV) 98TD154 SAL49 407.1-416.1m Lt gry aphanitic volcanic (FV) 98TD213 SAL40 268.8-281.5m Lt gry hyaloclastite (FV) EI Salvador Occurrence 98TD149 SAL2 204.8-217.0m Quartz- feldspar-phyric volcanic 98TD150 SAL2 232.8-241.lm Quartz-phyric volcanic Surface Expos ures 98TD01 La Virgen Area (UTM 2503950N 196100E) Felsic aphanitic volcanic 98TD02 El Tepozan Area (UTM 2503030N 194750E) Quartz-phyric volcanic 98TD102 N . of El Tepozan Area (UTM 2505510N 195045E) Hblde(?) phyric flow 98TD103 N . of El Tepozan Area (UTM 2505850N 195400E) Amygdaloidal flow 98TD140 El Socorro Area (UTM 2507410N 195055E) Plagioclase-phyric volcanic 98TD141 El Suavecito Area (UTM 2501075N 193275E) Hblde plag phyric vole 98TD142 El Suavecito Area (UTM 2501075N 193275E) Hblde phyric volcanic 98TD147 San Nicolas Cemetery Area (UTM 2499600N 191425E) Lt grn aphanitic volcanic 98TD151 SW. of El Socorro (UTM 2507120N 194635E) Pnk/wht silicic vole 98TD160 E. of Sagrado Corazon Area (UTM 2507110N 194420E) Qtz + feld phyric vole 98TD161 E. of Sagrado Corazon Area (UTM 2507135N 193295E) Lt grn aphanitic volcanic 98TD162 E. of Sagrado Corazon Area (UTM 2507185N 193070E) Dk gry qtz? phyric vole Regional Samples Zacatecas Area 99TD18 NW. of Panuco (UTM 2533625N 751750E) Intermediate Tuff 99TD22 Loma Chaparruda Area (UTM 2521800N 743495E) Mafic Sub-volcanic 99TD30 W. of Cerro Alto (UTM 2526250N 751250E) Pillowed Mafic Flow 99TD31 W. of Veta Grande (UTM 2526800N 750000E) Pillowed(?) - Mafic Flow 99TD32 N . of Veta Grande (UTM 2528800N 750900E) Ugly Green Rock Table 2.4 Lithogeochemical Data for Samples Analyzed in Support of this Study1 Sample Majors A1203 CaO Cr203 Fe203 K 2 0 MgO MnO Na20 P205 Si02 Ti02 LOI 98TD01 12.32 0.16 0.01 1.02 3.6 0.02 0.01 4.62 0.03 76.34 0.18 0.58 98TD02 12.05 0.11 0.01 0.61 5.1 0.34 0.01 1.89 0.03 76.48 0.21 1.57 98TD102 14.66 1.55 0.01 6.49 3.8 5 0.08 3.41 0.27 58.35 0.91 4.49 98TD103 12.9 5.64 0.01 7.61 0.22 5.89 0.07 2.53 0.09 59.08 0.82 4.1 98TD136 18.39 8.15 0.01 8.11 1.46 8.83 0.12 2 0.11. 45.59 1.03 4.79 98TD138 14.74 3.54 0.01 11.98 1.62 7.53 0.17 2.36 0.21 47.4 2.26 7.25 98TD139 15.09 4.09 0.01 8.92 3.17 8.4 0.25 1.24 0.16 46.37 1.56 8.99 98TD140 16.32 3.7 0.01 4.93 4.15 2.91 0.06 2.5 0.12 58.52 0.66 3.68 98TD141 14.92 11.51 0.01 8.59 1.56 6.55 0.13 2.29 0.36 47.19 0.91 4.8 98TD142 15.93, 9.7 0.01 9.03 0.44 5.81 0.16 3.72 0.54. 48.86 0.84 3.48 98TD145 10.95 0.31 0.01 1.4 7.88 0.78 0.02 0.08 0.01 75.2 0.13 1.44 98TD146 11.33 0.73 0.01 2.32 2.78 0.99 0.05 0.17 0.01 76.48 0.17 3.97 98TD147 9.25 1.79 0.01 1.85 0.14 0.8 0.04 4.71 0.05 78.03 0.28 2.17 98TD148 10.18 0.1 0.01 3.31 2.05 4.1 0.05 0.05 0.03 75.55 0.19 3.48 98TD149 12.25 0.24 0.01 1.22 1.92 0.06 0.01 5.5 0.04- 76.71 0.2 1.05 98TD150 14.61 0.89 0.01 2.94 3.83 1.91 0.03 1.35 0.04 68.83 0.25 4.04 98TD151 2.19 0.07 0.01 0.53 0.19 0.19 0.01 0.41 0.01 94.19 0.07 0.79 98TD152 12.22 0.08 0.01 2.07 9.16 0.46 0.03 0.66 0.04 73.1 0.21 0.74 98TD153 10.3 0.38 0.01 1.91 6.32 0.67 0.03 1.15 0.03 77.27 0.12 1.17 98TD154 •' 9.99 1.34 0.01 1.6 6.98 1.01 0.03 0.01 0.03 75.03 0.11 2.95 98TD160 11.99 0.08 0.01 0.79 5.79 0.32 0.01 2.23 0.04 76.43 0.16 0.86 98TD161 10.78 0.14 0.01 2.6 8.21 1.15 0.04 0.01 0.08 74.46 0.35 1.4 98TD162 9.09 0.23 0.01 1.29 0.64 0.15 0.01 4.69 0.04 77.52 0.17 1 98TD213 8.4 0.07 0.01 7.15 0.37 7.32 0.08 0.01 0.01 71.15 0.15 4.31 99TD18 16.5 2.3 0.01 4.99 0.8 3.88 0.03 4.59 0.12 62.51 0.63 2.96 99TD22 14.79 5.68 0.01 9.29 2.2 2.78 0.22 3.71 0.35 53.13 1.35 5.8 99TD27 16.11 7.19 0.01 7.67 1.29 6.93 0.16 2.92 0.44 44.89 1.72 9.91 99TD29 11.03 15.14 0.05 7.4 1.28 10.2 0.12 1.69 0.23 39.92 0.55 11.49 99TD30 14.43 8.34 0.03 7.68 0.26 7.7 0.13 3.36 0.06 54.09 0.54 2.44 99TD31 14.06 9.3 0.01 7.11 0.14 6.55 0.11 3.45 0.08 55.9 0.52 2.01 99TD32 15.99 9.96 0.01 8.33 1.26 6.44 0.12 3.65 0.22 46.75 1.48 5.2 'Analyses on all samples were conducted by Chemex Labs Ltd. in Vancouver, British Columbia ''Major element abundances are reported as oxide weight %. ""Trace and REE abundances are reported as parts per million (ppm). 44 Table 2.4 (corit.) Sample Total Trace / REE'' Ba Ce Cs Co Cu Dy Er Eu Gd Ga Hf Ho 98TD01 98.89 567 53.0 3.6 33 10 10.1 6 1.5 9.3 19 9 2.4 98TD02 98.41 1730 103.0 4.5 20 5 6.5 4 1 8.5 18 6 1.4 98TD102 99.02 663 54.0 4.1 29 25 3.4 1.8 1.3 4.5 16 5 0.6 98TD103 98.96 98 11.0 4.9 28 30 4.6 3.2 0.9 3.5 12 1 1 98TD136 98.59 75 7.0 10.7 29 65 3.3 2.2 1 2.8 15 1 0.7 98TD138 99.07 729 17.0 28.4 31 45 6.1 4.3 1.5 5.9 20 3 1.4 98TD139 98.25 981 13.5 29.5 33 40 5 3 1.4 4.7 15 3 1 98TD140 97.56 908 27.0 2.7 20 15 2.6 1.4 1 3 18 3 0.5 98TD141 98.82 569 38.0 2.1 34 110 3.8 2.1 1.6 5.2 17 3 0.8 98TD142 98.52 263 40.0 0.8 31 55 4.2 2.4 1.7 5.2 16 2 0.8 98TD145 98.21 641 48.5 6.6 12 5 9.4 7.1 1.4 9.5 17 6 2.3 98TD146 99.01 946 49.0 23.1 13 35 12.9 8.5 1.6 11 17 8 3 98TD147 99.12 479 14.5 1.0 14 30 4.4 3.3 0.5 3.6 9 4 1 98TD148- 99.1 786 61.0 8.4 9 170 7.8 5.4 0.6 8.5 14 6 1.8 98TD149 99.21 470 54.5 3.9 18 10 7.1 3.9 1.2 7.7 14 6 1.4 98TD150 98.73 784 54.5 12.6 7 15 7.3 5.1 1.2 8.6 20 8 1.8 98TD151 98.66 82 7.5 1.2 37 30 0.8 0.6 0.1 1 2 1 0.1 98TD152 98.78 609 ' 53.5 2.4 25 10 11.7 8.3 1 9.2 16 8 2.6 98TD153 99.36 371 63.0 3.6 27 5 12.2 8.4 1.9 11.7 18 10 2.6 98TD154 99.09 338 49.5 9.9 18 15 10.4 7.1 1.9 10.7 16 9 2.5 98TD160 98.71 1920 74.0 10.6 15 10 6.9 4 0.6 6.2 14 5 1.5 98TD161 99.23 2340 29.0 3.0 23 30 7.6 5.2 1.2 6.1 11 5 1.7 98TD162 94.84 457 17.5 1.2 27 15 5.1 3.7 0.9 4.5 10 3 1.1 98TD213 99.03 343 36.5 3.0 17 5 7.8 5 0.3 6.4 13 6 1.7 99TD18 99.31 317 29.0 3.8 13 5 3.6 2.2 1.6 3.8 21 3 0.7 99TD22 99.3 387 24.5 19.0 27 30 5 3.6 1.6 5.3 19 3 1.1 99TD27 99.24 1010 20.0 14.4 34 50 7.8 5.2 1.9 7 20 4 1.6 99TD29 99.1 362 21.5 6.2 43 75 2.2 1.5 1.1 3.1 12 1 0.5 99TD30 99.06 62 5.5 0.6 33 65 2 1.5 0.5 1.9 14 1 0.4 99TD31 99.24 33 7.0 0.2 37 25 2 1.7 0.6 2.2 14 1 0.5 99TD32 99.4 276 19.5 7.7 40 55 3.8 2.4 1.3 4.1 17 3 0.8 Table 2.4 (cont.) Sample La Pb Lu Nd Ni Nb Pr Rb Sm Ag Sr Ta Tb Tl 98TD01 23.0 5 0.9 30 10 9 7.6 44.4 8.3 1 48 3 1.6 0.5 98TD02 53.0 15 0.6 44 25 10 12.3 140.0 8.9 1 62 2 1.3 0.5 98TD102 26.0 5 0.3 25 15 12 6.5 102.0 5.4 0.1 153 0.05 0.6 0.05 98TD103 5.5 5 0.5 7 50 2 1.6 7.6 2.6 1 327 0.5 0.7 0.5 98TD136 2.5 5 0.3 7 85 0 1.2 23.4 2.2 0.1 227 0.05 0.6 0.05 98TD138 6.5 1 0.6 15 15 3 3.0 36.0 4.5 0.1 123 0.05 0.9 0.05 98TD139 5.5 10 0.4 12 90 2 2.2 60.0 3.4 1 96 0.5 0.7 0.5 98TD140 13.0 1 0.2 14 5 5 3.5 87.0 3.3 0.1 317 0.5 0.5 1.5 98TD141 16.5 10 0.3 23 30 1 5.1 28.8 5.4 1 814 0.5 0.7 0.5 98TD142 17.5 15 0.4 26 40 2 5.8 6.6 5.7 0.1 580 0.05 0.8 0.05 98TD145 19.5 15 1.2 30 10 7 6.9 57.2 8.3 1 29 1 1.7 0.5 98TD146 19.5 35 1.4 34 1 8 7.4 69.4 9.5 0.1 69 1.5 2.1 0.5 98TD147 9.0 20 . 0.6 11 5 2 2.3 2.0 2.9 1 47 0.5 0.6 0.5 98TD148 28.0 35 0.9 34 10 6 8.1 47.4 7 1 21 0.5 1.4 0.5 98TD149 ' 29.0 15 0.5 31 5 7 7.6 26.8 7.1 1 60 1.5 1.3 0.5 98TD150 26.0 10 0.9 29 10 9 6.4 99.8 6.3 1 65 0.5 1.2 0.5 98TD151 4.0 30 0.01 4 1 1 1.1 7.2 0.8 0.1 20 . 2.5 0.1 0.05 98TD152 23.0 25 1,3 29 5 8 7.4 71.4 7.9 1 33 2 1.8 0.5' 98TD153 25.0 5 1.4 40 5 10 9.1 46.6 10.4 1 53 2.5 1.9 0.5 98TD154 20.0 10 1.1 31 10 10 7.2 46.6 8.8 1 60 1.5 1.7 0.5 98TD160 36.5 15 0.6 32 5 8 9.0 118.5 7 1 73 1.5 1.1 0.5 98TD161 15.0 10 0.7 20 25 5 4.5 44.2 5.8 0.1 23 1.5 1.2 0.05 98TD162 8.0 . 25 0.6 13 5 2 2.9 4.0 3.6 0.1 59 2 0.7 0.05 98TD213 15.0 10 0.8 21 5 6 5.4 8.0 5.6 1 11 0.5 1.2 0.5 99TD18 15.0 5 0.3 16 5 4 3.8 16.6 3.4 1 387 0.5 0.7 0.05 99TD22 10.0 5 0.5 16 10 6 3.5 53.0 4.8 1 389 0.5 1 0.05 99TD27 7.5 5 0.8 16 105 3 3.3 52.0 5.5 1 312 0.5 1.3 5 99TD29 10.0 5 0.1 13 345 1 3.1 35.6 3.1 1 524 0.5 0.5 0.05 99TD30 2.0 5 0.2 4 125 1 0.8 5.4 1.4 1 194 0.5 0.3 0.05 99TD31 3.5 5 0.3 6 160 1 1.2 3.4 1.7 1 190 0.5 0.4 0.05 99TD32 8.5 5 0.3 13 95 7 3.0 24.6 3.6 1 749 0.5 0.7 0.05 Table 2.4 (cont.) Sample Th Tm Sn W U V Yb Y Zn Zr 98TD01 5 0.8 3 360 2 30 5.5 51.5 45 230.0 98TD02 12 0.5 7 227 4.5 30 3.7 35.0 25 151.0 98TD102 10 0.2 2 34 3 135 1.7 16.5 70 163.5 98TD103 1 0.5 1 74 0.5 160 3.5 31.0 75 48.5 98TD136 0 0.3 2 15 0.05 155 1.8 18.0 150 53.5 . 98TD138 0 0.5 1 22 0.05 390 4.1 35.5 115 117.5 98TD139 1 0.4 1 17 0.5 210 2.6 25.5 65 70.5 98TD140 5 0.2 1 63 1.5 95 1.4 13.5 105 113.5 98TD141 3 0.3 1 32 1.5 270 2.1 19.5 65 73.0 98TD142 5 0.4 1 54 1.5 215 2.3 22.0 70 79.0 98TD145 3 1 3 128 1 20 8 62.0 85 173.0 98TD146 5 1.4 4 144 1 1 9.3 78.0 395 261.0 98TD147 2 0.4 1 107 0.5 50 2.9 28.5 30 96.0 98TD148 5 0.8 3 77 1.5 20 5.7 49.5 65 158.0 98TD149 6 0.5 1 . 183 3.5 20 3.5 28.5 15 160.0 98TD150 8 0.8 3 67 2.5 10 5 43.0 85 216.0 98TD151 1 0.01 0.1 453 0.05 1 0.5 4.5 25 58.0 98TD152 6 1.2 2 279 2 20 8.3 65.5 55 238.0 98TD153 4 • 1.2 4 274 1 25 8.9 62.5 75 234.0 98TD154 4 1.2 4 182 1 25 7.8 57.0 50 211.0 98TD160 12 0.6 3 152 3.5 30 4.2 36.0 5 108.5 98TD161 5 0.7 0.1 187 1.5 20 4.7 44.0 25 152.5 98TD162 2 0.6 0.1 298 1.5 1 3.9 29.0 40 90.5 98TD213 5 0.8 4 75 1.5 5 5.9 47.5 55 148.5 99TD18 3 0.3 1 49 1.5 40 2.2 20.0 105 94.0 99TD22 1 0.5 2 47 0.5 210 3.2 29.0 100 132.5 99TD27 1 0.8 1 22 3.5 275 5.1 48.5 150 140.5 99TD29 1 0.2 1 26 0.5 185 1.2 12.5 40 50.5 99TD30 1 0.2 1 54 0.5 270 1.4 11.0 40 36.0 99TD31 1 0.2 1 86 0.5 255 1.4 12.5 30 32.5 99TD32 1 0.3 1 37 0.5 220 2.4 20.5 50 117.0 mineralization (extensive development of sericite, chlorite, carbonate, and barite with only rare epidote and amphibole). The abundance of hydrous alteration minerals in all samples suggests at least some degree of post-crystallization elemental mobility and requires caution when making any interpretations based solely on lithogeochemistry. Elements which have been typically found to remain immobile during hydrothermal alteration associated with V M S mineralization are the high field strength elements (HFSE) and the REE (Barrett and MacLean, 1994, Jenner, 1996) and most of the following interpretations are based on those elements. Lithogeochemical Stratigraphy Volcanic host rocks from the both the San Nicolas and E l Salvador areas display a strongly bimodal distribution of geochemical composition, both in terms of trace element ratios and absolute silica abundance (Figure 2.17). A l l units sampled fall within the range of subalkaline basalt to basaltic andesite and rhyodacite to rhyolite (Figure 2.17). Samples from the San Nicolas section yield Zr/Y ratios consistent with the tholeiitic magmatic series, using the discriminate of Barrett and MacLean (1994), whereas the two samples from the El Salvador section yield compositions transitional between the tholeiitic and calc-alkaline series (Figure 2.18). Although no obvious major and trace element trends were observed throughout the stratigraphic succession, a significant change was observed in the normalized REE patterns of samples collected from the mafic units. Samples collected from mafic units immediately adjacent to sulfide mineralization (MVS and M V T units) displayed relatively flat REE patterns whereas the sample from the M F B unit displayed a depletion in the heavy 48 ~\ I I I I I ! o t — s-N # San Nicolas deposit samples • El Salvador occurrence samples A Main study area surface samples Rhyolite Rhyodacite/Dacite .01 .001 Andesite/ . ^ + Basalt SubAlkaline Basalt Alk-Bas Bsn/Nph J i i i i i i 1 1 i i J i i .01 .1 N b / Y 10 B 15 San Nicolas Deposit / El Salvador Occurrence N = 25 Surface Samples 10 a. r/2 O 35 45 55 65 75 85 SiO, Figure 2.17 Plots of A) Nb/Y vs. Zr/Ti02 after Winchester and Floyd (1977) and B) Si02 distribution. For samples collected from both drill core and surface exposures. Figure 2.18 Plot of Zr vs. Y defining magmatic affinity for samples from the main study area (after Barrett and MacLean, 1994). Zr/Y ratios of 2-4.5, 4.5-7, and 7-20 correspond to the tholeiitic, transitional, and calc-alkaline affinities, respectively. rare earth elements (HREE) and an enrichment in the light rare earth elements (LREE) (Figure 2.19). Samples collected from surface exposures also indicate a bimodal geochemical composition distribution in terms of trace element ratios and absolute silica abundance (Figure 2.17), with two exceptions. The two exceptions are samples 98TD102 and 98TD140 which fall within the trachyandesite and andesite ranges, respectively. No lithologic or petrologic difference which would account for the intermediate composition of these samples was discernable. Similarly, samples collected from throughout the property also have Zr/Y ratios consistent with the tholeiitic magmatic series with the exception of samples from the El Socorro and El Tepozan areas which yield Zr/Y ratios indicative of the calc-alkaline magmatic series (Figure 2.18). Normalized REE patterns of samples collected from mafic units at the surface generally display depletion in the HREE and enrichment in the LREE with the exception of a single sample from north of the E l Tepozan area which displays a relatively flat REE pattern (Figure 2.19). Tectonic Discrimination The most obvious indication of the magmatic setting for rocks of the main study area is the broadly bimodal geochemical character of the volcanism (Figure 2.17). Volcanic sequences where sampling has been sufficiently thorough as to demonstrate a truly bimodal geochemical distribution are almost universally accepted to have formed in extensional environments (Shinjo and Kato, 2000). As the volcanic succession at San Nicolas appears to be intact, this succession is believed to represent a bimodal geochemical distribution, and by inference, to have formed in an extensional setting. 51 A 100 Figure 2.19 Chondrite normalized rare earth element (REE) patterns for mafic volcanic rocks from A) the San Nicolas deposit area, B) surface exposures of the main study area, and C) the Zacatecas City vicinity. B i - and multi-element discrimination diagrams have also proven to be useful in identifying the paleotectonic setting of magmatism. Most tectonomagmatic discrimination diagrams have been constructed based on mafic rocks; therefore the following analysis is based mainly on mafic units of the main study area. The tectonomagmatic discrimination diagram of Pearce and Cann (1973) is used to distinguish between various within-plate and plate margin settings. Analyses from the main study area indicate that all of the mafic units, with the exception of the slightly alkalic samples, were generated at a divergent plate margin setting, either at a mid-ocean ridge or back-arc basin spreading center (Figure 2.20). Samples from the M F B unit and from surface exposures are moderately depleted in Ti relative to the M V S and M V T units, perhaps reflecting an arc component. The Zr-Nb-Y diagram of Meschede (1986) is also commonly used in the discrimination between various within-plate and plate margin settings. This diagram again indicates that, with the exception of the slightly alkalic samples, all of the mafic units in the main study area were formed at either a mid-ocean ridge or back-arc basin spreading center (Figure 2.20). A plot of Ti vs. V supports a mid-ocean ridge or back-arc basin spreading center setting for formation of the M V S and M V T units and suggests a shift in magmatic setting to more of an arc related environment for the overlying M F B unit and surface samples (Figure 2.20). The geochemical composition of a limited number of mafic volcanic rock samples collected from exposures north of the city of Zacatecas are equivocal with regards to environment of formation. Based on the Zr-Ti-Y, Zr-Nb-Y, and Ti-V discrimination diagrams (Figure 2.21) samples plot in both the mid-ocean ridge / back-arc basin and arc tholeiite fields (Figure 2.21), similar to the distribution seen in the samples from the main study area. The juvenile volcanic arc interpreted for two of the samples from this study is 53 54 consistent with the signature of mafic volcanic rock samples reported by Centeno-Garcia et al. (1993) and Centeno-Garcia (1994) for much of Guerrero Terrane. Rare earth elements are generally considered to be amongst the most immobile of the elements under most alteration conditions and therefore normalized REE abundance diagrams (spider diagrams) are commonly used to identify the paleotectonic setting in which volcanic units formed. Relatively flat chondrite normalized REE patterns for samples of both the M V T and M V S units are consistent with formation within a mid-ocean ridge or back-arc basin setting (Figure 2.19). The normalized REE pattern for a sample from the M F B unit shows a marked enrichment of light REEs and a depletion of heavy REEs (Figure 2.19). This pattern is more typical of a volcanic arc basalt and supports a change in paleotectonic setting from an ocean floor or arc marginal basin spreading center setting to a volcanic arc setting prior to eruption of the M F B unit. Samples collected from surface exposures on the main study area display volcanic arc basalt normalized REE patterns similar to that of the M F B unit whereas samples from the Zacatecas city vicinity display normalized REE patterns typical of both ocean floor and volcanic arc settings (Figure 2.19). The use of simple immobile element ratios is a useful way to classify altered volcanic rocks because it largely eliminates the effects of mass loss or gain during alteration. Most of the elemental ratios examined, particularly the heavy elements, are consistent with an ocean floor paleotectonic setting for the M V T and M V S units and a volcanic arc setting for the M F B unit. Some elemental ratios, however, suggest both volcanic arc (e.g. (Th/Nb)n > 1, (Nb/La)n < 1) and non-arc (e.g. (Th/Nb)n < 1, (Nb/La)n > 1) paleotectonic settings for different portions of the M V T and M V S units. Although this mixed geochemical character is not typical for rocks which formed at mid-ocean ridge environments, it is not an uncommon 56 geochemical feature of rocks which formed within an arc margin basin spreading center setting (Pearce, 1996). RADIOGENIC ISOTOPES Samples and Methods Sm-Nd and Pb-Pb isotopic analyses were conducted on a suite of volcanic and sedimentary rock samples from both the study area and regional exposures. Sm-Nd samples collected from the study area include nine from the San Nicolas strata, five from the El Salvador strata, and three from surface samples. Four additional Sm-Nd samples were collected from regional exposures. Nine samples were collected from the San Nicolas strata for Pb-Pb analysis. Locations for Sm-Nd and Pb-Pb samples taken from drill core are shown on schematic stratigraphic columns in Figures 2.13 (San Nicolas deposit area) and 2.14 (El Salvador occurrence area). Locations of regional Sm-Nd samples are shown in Figure 2.15 (main study area) and Figure 2.16 (Zacatecas City area). Sample numbers, sample locations, and brief rock descriptions for all isotopic samples are given in Table 2.5. Sample preparation for Sm-Nd samples was identical to that for the lithogeochemical samples. Isotopic analyses were carried out in the Geochronology Laboratory at the University of Alberta using thermal ionization mass spectrometry and methods as described by Creaser et al. (1997). A l l sample preparation, geochemical separations, and isotopic measurements for Pb-Pb analyses were done in the Geochronology Laboratory at the University of British Columbia. A l l weathered surfaces were removed from samples and the unweathered fragments were subsequently crushed by hand. Approximately 50 milligrams of finely crushed rock from 57 Table 2.5 Sm-Nd and Pb-Pb Isotope Sample Numbers, Sample Locations, Brief Lithologic Description, and Type of Isotopic Analysis Sample Location Rock Type Analysis 98TD01 La Virgen (UTM 2503950N 196100E) felsic-aphanitic Sm-Nd 98TD40 SAL47 (194.3m) Dk grn autobreccia Pb-Pb 98TD41 SAL47 (217.0m) Lt gry autobreccia Pb-Pb 98TD42 SAL47 (231.2m) Grn aphanitic volcanic Pb-Pb 98TD45 SAL47 (276.5m) Lt grn aphanitic volcanic Pb-Pb 98TD56 SAL47 (400.2m) mafic-intrusive (shallow) Sm-Nd Pb-Pb 98TD58 SAL47 (418.8m) hyaloclastite Sm-Nd 98TD60 SAL47 (439.8m) qtz-phyric intrusive Sm-Nd Pb-Pb 98TD103 N . El Tepozan (UTM 2505850N 195400E) mafic flow Sm-Nd 98TD106 SAL25 (119.5m) argillite Sm-Nd Pb-Pb 98TD152 SAL47 (312.7m) felsic - aphanitic flow Sm-Nd 98TD161 El Soccorro (UTM 2507135N 193295E) mafic - hornblende phyric flow Sm-Nd 98TD184 SAL5 (20.8m) cherty seds Sm-Nd 98TD187 SAL5 (37.8m) mafic dike Sm-Nd 98TD189 SAL5 (59.3m) intermediate volcanic Sm-Nd 98TD196 SAL5 (110.5m) intermediate crystal tuff Sm-Nd 98TD208 SAL5 (180.4m) intermediate volcanic Sm-Nd 99TD16 Taures (UTM 2522075N 752590E) siliceous seds Sm-Nd 99TD21 L. Chapparuda (UTM 2521850N 743500E) seds (looked to be felsic) Sm-Nd 99TD22 L. Chapparuda (UTM 2521800N 743495E) mafic intrusive Sm-Nd 99TD25 SAL47 (454.3m) lower graphitic sediments Sm-Nd Pb-Pb 99TD27 . SAL47 (233.0m) mafic-aphanitic (mvs) Sm-Nd 99TD28 SAL47 (227.0m) blk seds in mvs Sm-Nd Pb-Pb 99TD29 SAL47 (210.3m) mafic-px phyric (mfb) Sm-Nd 99TD31 W. Veta Grande (UTM 2526800N 750000E) mafic volcanic Sm-Nd 58 each sample was leached in dilute HF and HB acid and dissolved in concentrated HF. Following ion exchange chemistry, approximately 10-25 nanograms of lead in chloride form was loaded on rhenium filaments using a phosphoric acid-silica gel emitter. Isotopic ratios were determined with a modified VG54R thermal ionization mass spectrometer in peak-switching mode on a Faraday detector. Measured ratios were corrected for instrumental mass fractionation of 0.12%/amu based on repeated measurements of NBS 981, and the values recommended by Todt et al. (1996). Errors were numerically propagated throughout all calculations and are given at the 2a level. Sm-Nd and Pb-Pb analytical results are given in Tables 2.6 and 2.7, respectively. Isotopic Stratigraphy Measured Nd ratios for all volcanic rock samples from the San Nicolas strata yielded 1 4 3 N d / 1 4 4 N d ratios from 0.512846 to 0.513068 and s N d values of 4.9 to 8.5 (calculated for a model age of 155 Ma). Mafic units yield relatively primitive signatures with 1 4 3 N d / 1 4 4 N d ratios between 0.512949 and 0.513073 and sNd values between 7.2 to 8.5, with the stratigraphically higher M F B unit being slightly more radiogenic than that of the M V S unit and coeval dike (Figure 2.22). Felsic volcanic units from San Nicolas yield slightly more evolved signatures than the mafic units with 1 4 3 N d / 1 4 4 N d ratios between 0.512846 and 0.512990 with SNd values between 4.9 and 6.8 (Figure 2.22). The more radiogenic signature of the felsic units may reflect a longer crustal residence time and / or the assimilation of older crust. Analyses from sedimentary rock units of the San Nicolas sequence yield 1 4 3 Nd/ 1 4 4 Nd ratios from 0.512546 to 0.512373 and end values of -0.8 to -3.9, becoming increasingly radiogenic up-section (Figure 2.22). 59 Table 2.6 Sm-Nd Data for Samples Analyzed in Support of this Study1 Sample Sm ppm Ndppm 1 4 7 Sm/ ' 4 4 Nd l 4 3 Nd/ ' 4 4 Nd uncert. T D M eNdT2 98TD01 8.62 33.07 0.1575 0.512881 0.000007 0.77 5.51 98TD56 2.89 8.49 0.2057 0.513073 0.000008 1.71 8.31 98TD58 4.42 12.85 0.2032 0.51299 0.000008 2.49 6.75 98TD60 9.11 33.6 0.164 0.512934 0.000007 0.7 6.43 98TD103 ' 2.68 8.59 0.1889 0.512877 0.000011 1.76 4.81 98TD106 4.1 19.05 0.1302 0.512373 0.000008 1.44 -3.86 98TD152 8.34 32.51 0.1551 0.512846 0.000009 0.82 4.89 98TD161 4.67 18.2 0.1553 0.512749 0.000007 1.16 2.39 98TD184 ..3.78 16.99 0.1346 0.512556 0.000008 1.23 -0.95 98TD187 2.5 5.91 0.2554 0.513144 0.000011 -0.07 8.71 98TD189 4.33 18.88 0.1387 0.512614 0.000009 1.11 0.69 98TD196 7.17 28.01 0.1493 0.512739 0.000021 1 2.9 98TD208 7.07 27.71 0.1542 0.512772 0.000008 1 3.47 99TD16 3.21 16.26 0.1194 0.512637 0.000007 0.85 1.51 99TD21 5.58 24.79 0.1297 0.512613 0.000007 0.84 0.51 99TD22 4.99 15.65 0.193 0.512996 0.000022 1.23 7.06 99TD25 4.46 18.38 0.1467 0.512546 0.000022 1.4 -0.81 99TD27 4.97 15.85 0.1897 0.513068 0.000008 0.61 8.52 99TD28 0.91 3.93 0.1407 0.512497 0.000013 1.39 -1.65 99TD29 2.92 12.91 . 0.137 0.512949 0.000009 . 0.43 7.25 99TD31 2.09 7.06 0.1795 0.512992 0.000008 0.76 7.25 ' A l l analyses conducted at the Geochronology Laboratory, University of Alberta ^Calculated at T M a of 155 Ma > N -o 3 00 V, o 00 T 3 N > N C E 00 a. S OH C-o N O O N CN O N N O O N co » n O N N O CO ro « n o i n ro ro o 00 O N o CO O <N o o o O O o O N O d d d d d d d d d O N oo CN O N ro o 00 N O CN CO o CN CN i n o CN r - O N CO f~ CN 00 i n i n r - N O N O i n N O i n o o o O N O O o o p O N CN CN CN CN CN CN CN C C 00 O N •<t CO • f 00 o i n 00 N O CO ro O N i n CO O N CN N O CN r-~ N O i n co CN o i n CN O N N O O o O o o O o d d d d d d d d d N O O N O N i n N O o o O N i n 00 00 CO i n O N CN 00 N O N O CO CO o ro M - ro CO CO 00 oo oo oo oo oo oo 00 00 d d d d d d d d d O N N O N O i n CO o CO O N i n N O o o N O CN oo O N ro 00 N O ON i n ro CN O N N O ro o 00 CN — o o O '—1 o ro oo d d d d d d d d d o o o o O N CO i n CN i n N O N O N O o CO i n 00 O ro O N o o ro i n N O ro OO OO O N i n 00 O N O N CN O OO N O O N i n O N N O i n '—i i n 00 r-^ 00 t ~ 00 00 00 oo C O ro ro ro ro ro ro ro ro CN r- N O CN 00 CN CN N O r-~ O N o o o O N CN r - o - 3 - N O C O CN OO o ro CN OO CN •—i o O o <—i O '—1 d d d d d d d d d i n o o i n N O o o r- N O O N r~- O N ro ro CN o o i n ro o OO CN ro o 00 O N N O ro O N ro ro O N N O N O i n N O N * N O N O 00 i n i n i n i n i n i n i n i n i n N O O N n r o r -i n oo itf- — O N C N r ~ o r o r o C N C O C N O N —• i n r o C N — O O " O O O O o — d © O N O •sf CN — — , — n h O CN N O o d d CN CO N O CO. t • 00 OO CO CN N O r - C N C N i n C N O N — oo O N r o r -— c N O N O o o o c N r ~ - r - -T T M - H V O N t M h ^ - ' o d o o O N O O o o o o o o o d J 3 o J 3 CQ Q OH o o o at o •A H to "5 E E E E E E E E E CO o CN i n CN 00 i n CO o r-^ —H" N O d O N O N r ~ O N CO r~- o r o i n CN CN CN CN • f CN r - r - t— i n r -••sl- " 3 - CN J •4 < < < < < < < < < 00 00 oo 00 oo 00 00 o o oo o —< CN i n N O O o i n 00 •sf m N O CN CN Q Q Q Q Q a a a Q H H H E-i H (- H H H 00 00 00 00 00 00 00 O N O N O N O N O N O N O N O N O N O N O N a, E O "3 •s * > N II XI v. -a 5 a, E _>N 13 61 s a g . .a .a o? £ " c CS o <3 3 V. cu OO >* < e MODE! ZACAT THIS S • • n M fi I ii i i i; i; i h i i i i i i i i i i i i i i i M i i n i i h i i n i i ents) ilcanic ients) g > *S « CO M E. 'S Unit (s Uni Unit (s fe OO P m 75 > a @ 00 CO 3 Q •o fa ca O &. > s w CO - o - j •4-> M CO O D H EX'S e 0 0 O N oo o OO o * ,53 -_ U »lr5 -5 co £ c oo c 3 <D c3 > T3 cZ o 0 0 u o c t o o o t-o T3 CO > OO —2 £ V CO <s W <s •£ .2 T3 "a, 3 OO CO X) o CO T3 62 Neodymium isotope ratios for felsic units from the E l Salvador sequence range from 0.512772 in the lower part of the sequence to 0.512614 in the upper part of the sequence, with corresponding SNd values ranging from 3.5 to 0.7 (Figure 2.22). These results are more radiogenic than the felsic units in the San Nicolas strata and are consistent with modern bulk silicate earth values suggesting a possible mixing of depleted mantle and continental crust isotopic signatures. One sample from a mafic dike from El Salvador yields a 1 4 3 Nd/ 1 4 4 Nd ratio of 0.513144 and an SNd value of 8.7 (Figure 2.22), a value consistent with the mafic dike from the San Nicolas strata and the M V S unit. Siliceous sediments in the uppermost part of the sequence at E l Salvador yield a 1 4 3 N d / 1 4 4 N d ratio of 0.512556 and an £ N d value of-1.0 (Figure 2.22). An aphanitic felsic volcanic rock from a surface exposure in the La Virgen region of the study area (Figure 2.15) yields a 1 4 3 N d / 1 4 4 N d ratio of 0.512881 and an s N d value of 5.5, similar to the F V unit in the San Nicolas sequence. Mafic samples from the El Soccorro and El Tepozan areas (Figure 2.15) yield 1 4 3 N d / 1 4 4 N d ratios of 0.512749 and 0.512877 respectively with corresponding SNd values of 2.4 and 4.8. These results are more radiogenic than those from mafic units in either the San Nicolas and El Salvador strata. Samples of mafic volcanic and intrusive rocks from the vicinity of the city of Zacatecas (Figure 2.16) yield 1 4 3 N d / 1 4 4 N d ratios and SNd values consistent with those of the M F B unit in the San Nicolas strata (Figure 2.22). Siliceous and tuffaceous sediments from the same area yield 1 4 3 N d / 1 4 4 N d ratios of 0.512637 and 0.512613 respectively with e N d values of 1.5 and 0.5, both less radiogenic than sedimentary units in the main study area. Siliceous to tuffaceous sediments from the vicinity of the village of San Rafael yield a 1 4 3 N d / 1 4 4 N d ratio 63 of 0.512551 and an 8Nd value of -0.6, values consistent with the sedimentary rocks in the uppermost sequence at El Salvador. In summary, volcanic rocks from the main study area and from the Zacatecas City vicinity yield 1 4 3 N d / 1 4 4 N d ratios and 8Nd values which reflect a relatively primitive source. Felsic volcanic rocks throughout the study area yielded slightly more evolved than the mafic volcanic rocks, perhaps reflecting greater assimilation of basement rocks. Sedimentary rocks are definitely more evolved than the volcanic units and require a component derived from an older source. Lead isotope compositions from the San Nicolas sequence whole rock samples yielded a range of values. For all samples, measured 2 0 6 Pb/ 2 0 4 Pb ratios range from 18.401 to 20.177 (Figure 2.23), the 2 0 7 Pb/ 2 0 4 Pb ratios range from 15.471 to 15.869, and the 2 0 8 Pb/ 2 0 4 Pb ratios range from 37.890 to 38.692. Although the Nd signatures of sedimentary rocks in the San Nicolas succession become progressively more radiogenic up-section, lead isotope ratios do not show a similar trend. The moderate scatter in Pb isotope ratios may reflect the greater mobility of lead in hydrothermal systems related to V M S mineralization. The majority of samples yield lead isotope signatures similar to that of sulfide samples from the San Nicolas deposit (see Chapter 3, this study). This similarity in signatures suggests either magmatic fluids contributed lead to the sulfide deposit or lead associated with sulfide mineralization has subsequently been mobilized throughout the surrounding host rocks. Tectonic Discrimination Samples of mafic flows and dikes of the M V S unit in the San Nicolas deposit area and a mafic dike in the E l Salvador occurrence area yield relatively juvenile £Nd values similar to 64 D Sampl Sampl JO "B. Volcanic Volcanic entary Sa Mafic Felsic Sedim • • o O C N od X 5 o .PH o > o E c p ffl XI c O "3 > o CO 00 > Q u ca C ca o > a > UH > - 5 Q W a co rt T3 to O C J M H OH C O bo co O co a, o T3 W co ^ cO cn O 45 o o C O 00 fa 3 <o « ^ I i co O O "+J CO CO O ^ '5 jD g ^ PH HQ " PH • r*> 1 ^ CO S i "0 a S .5? & to 2 65 basalts from modern mid-ocean ridge and / or island arc settings (Figure 2.22). These compositions may reflect derivation by partial melting of a depleted mantle source and possible incorporation of a small crustal component. A sample from a mafic flow of the M F B unit in the San Nicolas deposit area is slightly more radiogenic and resembles compositions of igneous rocks in modern island arc settings (Figure 2.22). These observations are consistent with the older (pre-MFB) units having formed in an arc marginal basin setting such as a back-arc basin, followed by emplacement of the slightly more radiogenic M F B unit, perhaps due to migration of the locus of arc magmatism. The more radiogenic signature of intermediate and felsic volcanic units in both the San Nicolas deposit and E l Salvador occurrence areas is also consistent with formation in a back-arc basin environment where depleted mantle magmas assimilated crustal rocks, with more evolved Nd signatures, during crustal residence times more protracted than for the mafic units. Mafic volcanic rock samples from the El Socorro and El Tepozan regions of the main study area, which are tentatively correlated with the M F B unit from the San Nicolas deposit area, yield £N q values which are consistent with values recorded from modern island arc settings. As with the M F B unit, this increase in the radiogenic Nd component may reflect a shift from back-arc basin magmatism to arc magmatism. Clastic sedimentary rock units in both the San Nicolas deposit and El Salvador occurrence areas yielded considerably more radiogenic SNd values than volcanic rock units from the same areas (Figure 2.22). This indicates that the sediments were not entirely derived from the coeval volcanic succession, but contained a component of detritus derived from a more evolved source, perhaps a distal cratonal source. Mafic units from the vicinity of Zacatecas City yield ENd values similar to surface samples from the main study area and the M F B unit of the San Nicolas deposit area. These values are similar to 8Nd values for basalts formed in modern island arc settings; this interpretation supports results from previous isotopic studies of volcanic rocks in the region by Centeno-Garcia (1994). Siliceous to tuffaceous sedimentary rocks from the same region are less radiogenic than sedimentary units from the main study area which suggesting a less radiogenic source area. Lead isotope compositions of volcanic and sedimentary rock samples from the San Nicolas deposit area are shown in Figure 2.24. The majority of samples analyzed in support of this study plot within the field Zartman and Doe (1981) defined as the 'orogene' reservoir, a reservoir interpreted as a mixture of isotopic signatures from three other reservoirs (upper crust, lower crust, and mantle). Unfortunately Pb isotopic compositions do not discriminate between volcanic rocks formed in primitive arc, mature arc, and some mid-ocean ridge environments, therefore the paleotectonic setting of formation for the San Nicolas deposit cannot be determined based on Pb isotope data alone. Analyses of volcanic rocks exhibit a dichotomy in isotopic ratios with samples roughly evenly distributed between the mantle and upper crust growth curves (Figure 2.24). This distribution of lead isotope signatures is not inconsistent with formation in a back-arc basin setting. Discussion - Paleotectonic Setting Geochemical and isotopic data presented here constrain the tectonic setting of formation for the volcano-sedimentary succession that hosts the San Nicolas deposit. The bimodal geochemical distribution of volcanic units strongly suggests formation within an extensional 67 Cu - O 16.00 15.80 15.60 15.40 15.20 15.00 13 15 17 19 21 ^ P b / ^ P b W H O L E R O C K S A M P L E S • Mafic Samples • Felsic Samples A Sedimentary Samples RESERVOIRS Mantle [7T| Orogene Upper Crust Lower Crust B Cu C u 2 0 6 P b / 2 0 4 P b Figure 2.24 Plots of Pb isotope ratios for whole rock samples taken from units within the San Nicolas deposit stratigraphy: A) 2 0 6Pb/ 2 0 4Pb vs. 2 0 7Pb/ 2 0 4Pb and B) 2 0 6Pb/ 2 0 4Pb vs. 2 0 8Pb/ 2 0 4Pb. Mantle growth curves of and Doe (1981) and the fields from which they were derived are shown. environment. The distribution of elements on bi- and multi-element discrimination diagrams, immobile element ratios, and normalized REE patterns are all consistent with pre-MFB unit volcanism having occurred within an intra-arc or back-arc basin setting. Sm-Nd data support the geochemical observations and further indicate a progressively more evolved source for sedimentary units within the main study area. Pb isotopic compositions are more radiogenic than would be expected from a juvenile arc related environment or from an ocean floor environment and suggest some influence from continental crust. This is a reasonable premise based on the abundance of Precambrian crust interpreted to underlie much of central, eastern, and southern Mexico (Schaaf et al., 1994, Ortega-Gutierrez et al., 1995, Lawlor et a l , 1999, Weber and Kohler, 1999), and is further supported by a Precambrian inherited zircon component present in several of the dating samples. The aforementioned geochemical criteria suggest an arc setting for emplacement of the M F B unit and possibly correlative mafic units exposed elsewhere in the main study area. Neodymium isotopic data for the M F B and correlative units supports the geochemical interpretation with SNd values consistent with formation within an arc environment. Samples of volcanic rocks from the vicinity of the city of Zacatecas yield equivocal geochemical signatures with regards to paleotectonic setting. Some samples display geochemical characteristics consistent with formation in a back-arc basin setting whereas other samples display geochemical characteristics consistent with formation in an arc environment. Isotopic data is similarly ambiguous with samples yielding SNd values which are equally consistent with volcanism in both an intra-arc or back-arc basin setting and an arc environment. Stratigraphic relationships between the various rock units in this region are uncertain therefore it is unclear whether a progression from a mid-ocean ridge / back-arc 69 basin setting to an arc setting, as documented above for the San Nicolas area, is also present in the Zacatecas City succession. Conclusions Based on the results of U-Pb dating, lithogeochemical, and radiogenic isotope (Sm-Nd and Pb-Pb) studies presented here, together with regional constraints from Sedlock et al. (1993), we propose the following sequence for the formation of rocks within the San Nicolas deposit area (schematically represented in Figure 2.25): 1) Eastward subduction of oceanic crust below the edge of attenuated cratonal Mexico during Middle Jurassic time, leading to the development of a relatively juvenile, west-facing, continental (or continent-fringing) magmatic arc. 2) Further attenuation of the cratonal crust through the onset of slab rollback in Late Jurassic time leading to the initial development of a back-arc basin. Deposition of the SED Unit within the developing back-arc basin. 3) Continuing back-arc basin development and eruption of the M V T through M V S Units within the San Nicolas deposit area. Sulfide mineralization forms coincident with emplacement of the felsic dome complex. 4) Continued sporadic back-arc basin volcanism with deposition of fine-grained clastic sedimentary rocks during periods of volcanic quiescence, perhaps reflecting the cessation of back-arc basin spreading. 5) Eastward migration of the axis of arc magmatism, perhaps related to shallowing of the subducting slab, and emplacement of the arc-related M F B and TUF units. 70 Figure 2.25 Schematic model for the formation of rocks in the San Nicolas deposit area: A) Back-arc basin magmatism represented by units M V T through M V S in the San Nicolas stratigraphy and coeval sulfide mineralization, and B) Migration of the locus of arc magmatism represented by the M F B unit. Data presented here also provide potential keys to further V M S exploration efforts within the Mesa Central province of central Mexico. The Late Jurassic age constraints on mineralization at San Nicolas and E l Salvador, consistent with the age of V M S deposits throughout Mexico, allow exploration geologists to focus on a temporally narrow range of rock units. Furthermore, the continental back-arc basin paleotectonic setting for V M S mineralization at the San Nicolas deposit and El Salvador occurrence is an environment which tends to produce geochemical and isotopic signatures which are distinctive. These signatures, along with the apparent geochemical and isotopic marker horizon defining the progression from back-arc basin setting to arc setting, may allow exploration geologists to vector into portions of central and southwestern Mexico prospective for additional V M S mineralization. Throughout the modern record, back-arc basins tend to develop as elongate, linear features. In light of the data presented here, and documented Late Jurassic back-arc basin rocks in the Guanajuato area (Sedlock et al., 1993), exploration efforts should be concentrated along a NNW-SSE axis within central Mexico. 72 References Barrett, T.J. and MacLean, W.H., 1994, Chemostratigraphy and hydrothermal alteration in exploration for V H M S deposits in greenstones and younger volcanic rocks, in Lentz, D.R., ed., Alteration and Alteration Processes associated with Ore-forming Systems: Geological Association of Canada, Short Course Notes, v. 11, pp. 433-467 Burckhardt, C , 1930, Etude synthetique sur le Mesozoique mexicain, Soc. Paleontologique Suisse Mem., 50, pp 157-158 Campa-Uranga, M.F., Campos-Flores, R., and Oviedo, R., 1974, La secuencia mesozoica volcano sedimentaria metamorfizada de Ixtapan de la Sal, Mexico-Teloloapan, Guerrero, Bol. Soc. Geol. Mex., 35, pp 7-28 Campa, M.F., and Coney, P.J., 1983, Tectono-stratigraphic terranes and mineral resource distributions in Mexico, Canadian Journal of Earth Sciences, 20, pp. 1040-1051 Centeno-Garcia, E., 1994, Tectonic evolution of the Guerrero terrane, western Mexico, University of Arizona, Ph.D. thesis Centeno-Garcia, E., Ruiz, J., Coney, P.J., Patchett, P.J., and Ortega-Gutierrez, F., 1993, Guerrero terrane of Mexico: Its role in the Southern Cordillera from new geochemical data, Geology, 21, pp. 419-422 Consejo de Recursos Minerales, 1992, Geological-Mining Monograph of the State of Zacatecas, Secretaria de Energia, Minas e Industria Paraestatal, Subsecretaria de Minas e Industria Basica, Publication No. M-2e Creaser, R.A., Erdmer, P., Stevens, R.A., and Grant, S.L., 1997, Tectonic affinity of Nisutlin and Anvil assemblage strata from the Teslin tectonic zone, northern Canadian Cordillera: Constraints from neodymium isotope and geochemical evidence, Tectonics, 16, pp. 107-121 de Cserna, Z., 1976, Geology of the Fresnillo area, Zacatecas, Mexico, Geological Society of America Bulletin, 87, pp. 1191-1199 de Cserna, Z., 1989, An outline of the geology of Mexico, in Bally, A.W., and Palmer, A.R., eds., The Geology of North America - An overview: Boulder, Colorado, Geological Society of America, The Geology of North America, A Elias-Herrera, M . , and Sanchez-Zavala, J.L., 1992, Tectonic implications of a mylonitic granite in the lower structural levels of the Tierra Caliente Complex (Guerrero Terrane), Southern Mexico, Revista, 9, pp. 113-125 Irvine, T.N., and Baragar, W.R.A., 1971, A Guide to the Chemical Classification of the Common Volcanic Rocks, Canadian Journal of Earth Sciences, 8, pp. 523-548 73 Jenner, G.A., 1996, Trace element geochemistry of igneous rocks: geochemical nomenclature and analytical geochemistry, in Whman, D.A., ed., Trace Element Geochemistry of Volcanic Rocks: Applications For Massive Sulphide Exploration: Geological Association of Canada, Short Course Notes, v. 12, pp. 51-77 Jenner, G.A., Dunning, G.R., Malpas, J., Brown, M . , and Brace, T., 1991, Bay of Islands and Little Port complexes, revisited: age, geochemical and isotopic evidence confirm suprasubduction-zone origin, Canadian Journal of Earth Sciences, 28, pp. 1635-1652 Johnson, B.J., Montante-Martinez, J.A., Canela-Barboza, M . , and Danielson, T.J., 2000, Geology of the San Nicolas Deposit, Zacatecas, Mexico, in EDITORS, V M S Deposits of Latin America, Geological Association of Canada Krogh, T.E., 1982, Improved accuracy of U-Pb zircon ages by the creation of more concordant systems using an air abrasion technique, Geochimica et Cosmochimica Acta, 46, pp 637-649 Lawlor, P.J., Ortega-Gutierrez, F., Cameron, K . L . , Ochoa-Camarillo, H. , Lopez, R., and Sampson, D.E., 1999, U-Pb geochronology, geochemistry, and provenance of the Grenvillian Huiznopala Gneiss of Eastern Mexico, Precambrian Research, 94, pp. 73-99 MacLean, W.H., and Barrett, T.J., 1993, Lithogeochemical techniques using immobile elements, Journal of Geochemical Exploration, 48, pp. 109-133 Meschede, M . , 1986, A Method of Discriminating Between different Types of Mid-Ocean Ridge Basalts and Continental Tholeiites with the Nb-Zr-Y Diagram, Chemical Geology, 56, pp. 207-218 Miranda-Gasca, M.A. , 1995, The Volcanogenic Massive Sulfide and Sedimentary Exhalative Deposits of the Guerrero Terrane, Mexico, Ph.D. thesis, University of Arizona, 294 p. Monod, O., Faure, M . , and Thieblemont, D., 1994, Guerrero terrane of Mexico: Its role in the Southern Cordillera from new geochemical data, Comment, Geology, x, pp. 477 Oliver, J., Payne, J., and Rebagliati, M . , 1997, Precious-metal-bearing Volcanogenic Massive Sulfide Deposits, Campo Morado, Guerrero, Mexico, Exploration and Mining Geology, 6, pp. 119-128 Ortega-Gutierrez, F., Ruiz, J., and Centeno-Garcia, E., 1995, Oaxaquia, a Proterozoic microcontinent accreted to North America during the late Paleozoic, 1995, Geology, 23, pp. 1127-1130 Ortiz-Hernandez, L.E. , and Lapierre, H., 1991, Field, petrological, and geochmical evidences for the intraoceanic environment of the upper Jurassic- early Cretaceous Palmar Chico-Arcelia arc sequence, southern Mexico, Convencion sobre la evolucion geologica de 74 Mexico, Primer congreso mexicano de mineralogia, Memoria, Pachuca, Hgo., pp 147-148 Parrish, R., Roddick, J.C., Loveridge, W.D., and Sullivan, R.W., 1987, Uranium-lead analytical techniques at the geochronology laboratory, Geological Survey of Canada. In Radiogenic Age and Isotopic Studies, Report 1, Geological Survey of Canada, Paper 87-2, pp 3-7 Pearce, J.A., 1996, A User's Guide to Basalt Discrimination Diagrams, in Wyman, D.A., ed., Trace Element Geochemistry of Volcanic Rocks: Applications for Massive Sulphide Exploration, Geological Association of Canada, Short Course Notes, 12, pp. 79-113 Pearce, J.A., and Cann, J.R., 1973, Tectonic Setting of Basic Volcanic Rocks Determined Using Trace Element Analysis, Earth and Planetary Science Letters, 19, pp. 290-300 Rhys, D.A., Enns, S.G., and Ross, K . V . , 2000, Geological Setting of Deformed VMS-Type Mineralisation in the Azulaquez-Tlanilpa Area, Northern Guerrero State, Mexico, in EDITORS, V M S Deposits of Latin America, Geological Association of Canada Roddick, J.C., 1987, Generalized numerical error analysis with application to geochronology and thermodynamics, Geochimica et Cosmochimica Acta, 51, pp 2129-2135 Santos Zalduegui, J.F., Scharer, U . , Gi l Ibarguchi, J.I., and Girardeau, J., 1996, Origin and evolution of the Paleozoic Cabo Ortegal ultramafic-mafic complex (NW Spain): U-Pb, Rb-Sr and Pb-Pb isotope data, Chemical Geology, 129, pp. 281-304 Schaaf, P., Heinrich, W., and Besch, T., 1994, Composition and Sm-Nd isotopic data of the lower crust beneath San Luis Potosi, central Mexico: Evidence from a granulite-facies xenolith suite, Chemical Geology, 118, pp. 63-84 Sedlock, R.L., Ortega-Gutierrez, F., and Speed, R.C., 1993, Tectonostratigraphic Terranes and Tectonic Evolution of Mexico: Boulder, Colorado, Geological Society of America Special Paper 278 Shervais, J.W., 1982, Ti-V plots and the petrogenesis of modern ophiolitic lavas, Earth and Planetary Science Letters, 59, pp. 101-118 Shinjo, R., and Kato, Y. , 2000, Geochemical constraints on the origin of bimodal magmatism at the Okinawa Trough, an incipient back-arc basin, Lithos, 54, pp. 117-137 Thompson, G.M. , Malpas, J., and Smith, I.E.M., 1997, The geochemistry of tholeiitic and alkalic plutonic suites within the Northland ophiolite, northern New Zealand; magmatism in a back arc basin, Chemical Geology, 142, pp. 213-223 Todt, W., Cliff, R.A., Hanser, A. , and Hofman, A.W., 1996, Evaluation of a 2 0 2 Pb- 2 0 5 Pb 75 double spike for high-precision lead isotope analysis, Geophysical Monograph, 95, pp 429-437 Ustaomer, P.A., 1999, Pre-Early Ordovician Cadomian arc-type granitoids, the Bolu Massif, West Pontides, northern Turkey: geochemical evidence, International Journal of Earth Sciences, 88, pp. 2-12 Weber, B., and Kohler, H. , 1999, Sm-Nd, Rb-Sr and U-Pb geochronology of a Grenville Terrane in Southern Mexico: origin and geologic history of the Guichicovi Complex, Precambrian Research, 96, pp. 245-262 Winchester, J.A., and Floyd, P. A. , 1977, Geochemical Discrimination of Different Magma Series and Their Differentiation Products Using Immobile Elements, Chemical Geology, 20, pp. 325-343 Wood, D.A., 1980, The Application of a Th-Hf-Ta Diagram to Problems of Tectonomagmatic Classification and to Establishing the Nature of Crustal Contamination of Basaltic Lavaz of the British Tertiary Volcanic Province, Earth and Planetary Science Letters, 50, pp. 11-30 Zartman, R.E., and Doe, B.R., 1981, Plumbotectonics - The Model, Tectonophysics, 75, pp. 135-162 76 C H A P T E R 3 Common Lead Isotopes as an Exploration Tool: A Case Study from Zacatecas State, Central Mexico Introduction The state of Zacatecas (Figure 3.1) has enjoyed a rich mining and mineral exploration history dating to pre-Columbian times when copper carbonates and silicates were mined for their mystical properties (Consejo de Recursos Minerales, 1992). The advent of the colonial period established the state of Zacatecas as a major producer of silver with mining and mineral exploration responsible for the foundation of many of the modern cities of Zacatecas including the state capital - the city of Zacatecas. A variety of epigenetic carbonate replacement and low sulfidation epithermal systems have been the primary sources of Ag, Pb, and Zn within the state, from colonial times through to the present. Discoveries of the El Salvador occurrence (1996) and San Nicolas deposit (1997), however, established central Mexico as a prospective region for syngenetic mineralization as well. With the emergence of a new style of mineralization target, the mineral exploration strategies used within the region must be adapted to quickly and accurately distinguish between the different styles of mineralization. Both epigenetic and syngenetic mineralization are hosted predominately by Mesozoic volcano-sedimentary sequences; thus the difficulties inherent in exploring in a region of poor exposure and extensive Tertiary and Quaternary cover requires an effective means of distinguishing between epigenetic and syngenetic mineralization based on limited surface exposures. Although hand sample examination of the nature of sulfide mineralization (grain size, texture, etc.) in many cases permits geologists to determine the Figure 3.1 The central Mexican state of Zacatecas with the location of Zacatecas city, the state capital. Inset shows the location of Zacatecas (colored) within Mexico. Modified from Sedlock et. al., 1993. 78 style of mineralization with some confidence, in areas of extensive weathering or metamorphism this may not be possible. In this study we examine the possibility of using Pb isotope ratios from sulfide minerals as a rapid and inexpensive means of distinguishing between predominately Tertiary epigenetic mineralization and Upper Jurassic to Lower Cretaceous syngenetic mineralization within central Mexico. We further compare the results from this study with previous Pb isotope work within central Mexico and then examine the viability of extrapolating the results from the study in central Mexico to the remainder of the Guerrero terrane. Geologic Background The state of Zacatecas has been divided into three physiographic provinces by the Consejo de Recursos Minerales; the Sierra Madre Oriental, the Mesa Central, and the Sierra Madre Occidental provinces (Figure 3.2). This study focuses on mineral deposits primarily within the southernmost Mesa Central and northernmost Sierra Madre Occidental provinces (Figure 3.2). Although much of this portion of central Mexico is covered by Tertiary terrigenous volcanic rocks and Quaternary alluvium, colluvium, and caliche, there are local exposures of pre-Tertiary rocks and it is within these pre-Tertiary rocks that most of the regions mineral deposits can be found. The oldest rocks within the study area are submarine Upper Triassic phyllite, slate, quartzite, marble, and meta-conglomerate of undetermined thickness which are exposed in the central and southeastern portions of the study area. Marine flysch of the Noria de Angeles Group and overlying volcano-sedimentary rocks of the Chilitos Formation are also locally exposed in the central and southeastern portions of the study area. The 79 Figure 3.2 Physiographic provinces of Zacatecas State and mining districts discussed in this paper. Modified from the Consejo de Recursos Minerales, 1992. Chilitos Formation is composed of a succession of pillowed basalt to basaltic-andesite flows and intercalated, marine, non-fossiliferous siliciclastic, epiclastic, and calcareous sedimentary rocks (de Cserna, 1976, Consejo de Recursos Minerales, 1992). U-Pb zircon dating of felsic volcanic rocks which host the San Nicolas deposit and El Salvador occurrence have yielded Late Jurassic crystallization ages which are consistent with limited faunal data from the Chilitos Formation (Chapter 2, this study). Cretaceous strata are more widespread throughout the study area but exposures are still limited. The Cretaceous strata, which host most of the epigenetic mineral deposits within this region, are almost exclusively composed of limestone, shaly limestone, and shale. Pre-Mesozoic rocks are not exposed within the study area but can be inferred to be present based on isotope model ages of granulite xenoliths from Cenozoic volcanic rocks (Sedlock et a l , 1993) and the presence of inherited zircon in mid-Mesozoic volcanic rocks (Chapter 2, this study). The study area falls along the northeastern margin of the Guerrero terrane, a terrane defined by Campa and Coney (1983) as a composite terrane that is allochthonous with respect to nuclear (cratonal) Mexico. Samples and Analytical Methods Lead isotope analyses were conducted on a suite of sulfide minerals (galena, pyrite, sphalerite, and mixed sulfides) collected from both epigenetic and syngenetic mineral deposits of west-central, central, and southeastern Zacatecas state and northern Aguascalientes state. The majority of samples considered to represent epigenetic mineralization are from the area immediately north of the city of Zacatecas where numerous historically mined Ag rich veins are present. Other samples of epigenetic mineralization 81 from Zacatecas State include one from south of the city of Zacatecas and two each from the Francisco I. Madero and Sabinas deposits in central and west-central Zacatecas, respectively. Four epigenetic samples from southeastern Zacatecas state and northern Aguascalientes state were also analyzed. Samples of definite syngenetic mineralization were from the San Nicolas deposit (nine samples) and the E l Salvador occurrence (four samples). Four additional samples of mineralization an unknown style were collected from the vicinity of the San Nicolas deposit and E l Salvador occurrence. General sample locations for all samples are shown in Figure 3.3 and sample numbers, sample locations, and minerals analyzed are given in Table 3.1. A l l sample preparation, geochemical separations, and isotopic measurements were conducted at the Geochronology Laboratory at the University of British Columbia. For trace lead sulfide samples (pyrite, sphalerite, and mixed sulfides), approximately 10-50 milligrams of hand picked minerals were first leached in dilute hydrochloric acid to remove surface contamination and then dissolved in dilute nitric acid. Samples of galena required no leaching and were directly dissolved in dilute hydrochloric acid. Following ion exchange chemistry, approximately 10-25 nanograms of lead in chloride form was loaded on rhenium filaments using a phosphoric acid-silica gel emitter. Isotopic ratios were determined with a modified VG54R thermal ionization mass spectrometer in peak-switching mode on a Faraday detector. Measured ratios were corrected for instrumental mass fractionation of 0.12%/amu based on repeated measurements of NBS 981 and the values recommended by Todt et al. (1996). Errors were numerically propagated throughout all calculations and are given at the 2a level. 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The four analytical results for minerals collected from mineralization of an unknown style are also presented here and in Table 3.2. Mineral analyses for samples collected from known or probable epigenetic mineralization generally yield consistent 2 0 6 Pb/ 2 0 4 Pb, 2 0 7 Pb/ 2 0 4 Pb, 2 0 8 Pb/ 2 0 4 Pb, 2 0 7 Pb/ 2 0 6 Pb, and 2 0 8 Pb/ 2 0 6 Pb ratios (Table 3.2). Exceptions include analyses on pyrite from samples collected in the Taures and La Plomosa areas and analyses on sphalerite from a sample collected in the Francisco I. Madero deposit, all of which are less radiogenic then analyses from the remainder of the analyses of epigenetic mineralization. Excluding the above mentioned exceptions, the 2 0 6 Pb/ 2 0 4 Pb ratios exhibited by mineral analyses of epigenetic mineralization spans the range from 18.742 to 18.926. The narrow range in 2 0 6 Pb/ 2 0 4 Pb ratios is mirrored in the other Pb ratios with 2 0 7 Pb/ 2 0 4 Pb, 2 0 8 Pb/ 2 0 4 Pb, 2 0 7 Pb/ 2 0 6 Pb, and 2 0 8 Pb/ 2 0 6 Pb ratios yielding ranges from 15.585 to 15.724, 38.537 to 39.047, 0.830 to 0.835, and 2.055 to 2.068, respectively. Mineral analyses for samples collected from known syngenetic mineralization (San Nicolas deposit and E l Salvador occurrence) also yield consistent 2 0 6 Pb/ 2 0 4 Pb, 2 0 7 Pb/ 2 0 4 Pb, 2 0 8 Pb/ 2 0 4 Pb, 2 0 7 Pb/ 2 0 6 Pb, and 2 0 8 Pb/ 2 0 6 Pb ratios with values of 18.557 to 18.686, 15.600 to 15.646, 38.393 to 38.487, 0.836 to 0.841, and 2.056 to 2.070, respectively. Of the four samples collected from mineralization of an unknown mineralization style, three mineral analyses yield a narrow range of 2 0 6 Pb/ 2 0 4 Pb, 2 0 7 Pb/ 2 0 4 Pb, 2 0 8 Pb/ 2 0 4 Pb, 2 0 7 Pb/ 2 0 6 Pb, and 2 0 8 Pb/ 2 0 6 Pb ratios with values of 18.627 to 18.638, 15.639 to 15.649, 38.513 85 PH 18 OH PH 3 cn H H •3- CN r - o o ^1- 00 i n r - N O i n i n N O r - i n CN CN CN cn o O N i n o o o o o CN o o o o o o o o O CN O O o O o o o o o o o o o o o o o o o o o o o o o o O d O d d d d d d d d d d d d d d d d d d d d d d d cn N O C-- t-- 00 N O o CN i n NO cn o oo 00 CN CN CN O N oo 00 00 o r- O N N O i n N O i n i n i n N O N O v-» i n N O r - N O r - i n N O N O N O i n i n i n N O i n i n o O O o o o o o o o o o o o o o o o o o o o o o o CN r-i CN CN CN CN CN CN CN CN CN CN CN CN CN CN r-i CN CN CN CN CN CN CN CN cn i n m CN O cn O N N O N O ^ - cn O N i n i n i n r- i n CN 00 CN o r- oo O o O o o O CN •3- o O o o o O o O cn O o o . o o o o O o o o o o o O o o o o o o o O o o o o o d d d d d d d d d d d d d d d d d d d d d d d cn CN CN o CN CN CN o CN CN CN CN o cn cn cn cn cn cn cn cn cn cn cn cn i n cn m cn m cn cn cn m cn cn cn 00 oo oo 00 00 00 oo 00 oo oo 00 oo oo 00 00 00 00 oo 00 00 00 oo oo 00 oo d d d d d d d d d d d d d d d d d d d d d d d d d i n o o 00 N O cn N O CN CN ON o O N CN o r - o i n o CN N O i n O o cn cn o o CN o o o O o o O O CN o o O o o O o o o o O o o d d d d d d d d d d d d d d d d d d d d d d d d d CN e'- cn N O N O 00 o o r - r - ON o oo r - r - m o i n O N ON o en r - O N o O N ON m O N e'- NO O N i n N O N O o r - m r - 00 i n CN oo i n o r~; t-; r~; o 00 r - N O o en r - oo oo O N O N r - r-; r - CN oo OO 00 00 O N oo oo oo 00 OO 00 00 O N od 00 r~ od od od 00 00 od od 00 cd od 00 cn cn cn cn cn cn cn cn cn cn cn cn cn cn cn cn cn cn cn cn cn cn cn cn cn O N i n O N N O N O oo N O CN cn o oo i n N O 00 00 O N O N oo N O r— cn cn oo 00 O o o o o CN i n i n o o o o o o O o o cn o o O —< O o o o o o O o CN o o o o o o O O o o o o o o d d d d d d d d d d d d d d d d d d d d d d d d cs i n CN cn ON i n O N cn en c^ - i n N O NO o cn N O oo N O O N 00 i n N O cn N O O N 00 CN CN r- N O cn oo o o i n i n i n m oo i n N O N O i n t-~ N O N O N O N O N O N O N O r - i n N O i n N O c~- r - N O N O N O N O i n N O N O N O un un i n i n i n i n i n i n i n i n i n i n i n i n i n i n i n i n i n i n i n i n i n i n i n N O cn o CN r - CN O N i n CN 00 i n ON CN r - O N O N o CN O N N O O N CN o o O CN i n i n O o O O o o cn CN cn o o CN o o o o O o o o CN o o o O O o o o o o O o o o d d d d d d d d d d d d d d d d d d d d d d d d CN 00 o O N N O CN O N i n O N N O O N cn i n i n cn cn O N CN o CN O N CN N O ON 00 i n CN cn i n M - CN CN CN cn 00 O N O N N O cn CN CN 00 r -00 r - oo 00 oo 00 oo 00 oq 00 O N i n oo CN 00 00 00 f- oo 00 OO N O 00 00 00 00 00 00 00 00 00 00 00 00 00 od od od 00 00 00 00 00 od od od od od 00 00 —H —< >—1 ^ H H H — ' '—1 • — i —-< '—1 '—1 '—1 <—i •—1 —< ^ H ^ H •—i •-H — " OH £2 X C H W) M M O . 3 ST1 3 O J 3 o o ca oo 111 I CJ u ^ ^ c cj CS oo a a/ .Sf 'S, CJ cfl 2 „ , * X « CJ OJ o I-H - g J 3 JU O O £ 8 co C3 CL> oo H cd cd cd fH cj cj <u - G J 3 o o o a C o o o > > > td N) o ed ^> < 00 >-C N cn r ~ 00 O N O N O C N cn i n r - o cn cn cn o O o o O O O C N C N C N C N Q Q Q Q Q Q Q Q P Q Q Q Q Q Q Q D Q H H H H H H H H H H H H H H t-H H O N O N O N O N O N O N O N O N O N O N O N O N O N ON ON O N O N O N O N O N O N O N O N O N O N O N ON O N O N O N O N ON ON O N O N O N 3 § S. oo OO1 oo <-> H N O N CJ CJ CJ •o -o -o I I I t-0 o <-0 •X3 ~< CN CN 8 § 2 § s s -3 'HS 0^ o D H _ O N O N OO O N O N a •S -3 NO o D H IN <3 ^ ON 86 i f 1 PH PH IS? 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O E _3 O O . c cn •c CQ •c X ! a o ° a >N J5 E E g > . ^ N >> g O H O H O . C H C H O H >^ O - O - C O H C "Sb "oh " M . 3 o o P - PH Co C tu a ' — ^ E cd CO a -5. co g " a •- § S a, 5o co H ^ O N C N cn C N C N C N C N Q Q H H 00 00 O N O N Z Z Z Z Z Z Z Z Z Z c c c c P3 Cd Cd co co co co c c c c c cd cd cd cd cd co co co co co ox < Q e K H > N CJ oo CO Cn O N N O r~ O N O N O O O r -Q Q Q Q H H H H OO 00 oo oo O N O N O N O N Q Q ca CO O u O lH o •o T3 -a cd cd cd > _> _> *cd "cd "ra CO co co s 3 W H ^ ^ ^ H - - S 5j ^ H CN cn H J w w W m Q Q Q Q H H H O N O N O N O N O N O N O N O N CO c « .a 2 5^ I I & &H H - V ° -S E <8 P 71 ^ Ch E f-S cd cd H r -o o O S 5 ^j- H O o U W P Q Q H H H oo O N O N O N O N O N 0 0 _o o c o E N =3 c < < to 38.567, 0.8396 to 0.8397, and 2.068 to 2.069, respectively. Mixed pyrite and chalcopyrite minerals collected from the fourth sample (99TDCT) of indeterminate origin yield 2 0 6 p b / 2 0 4 p b ; 2 0 7 p b / 2 0 4 p b ) 2 0 8 p b / 2 0 4 p b ; 2 0 7 p b / 2 0 6 p b j ^ 2 0 8 p b / 2 0 6 p b ^ Q f ^ ^ ^ ^ 38.007, 0.832, and 2.042, respectively. Based on these results, two distinct ranges of Pb isotope ratios are observed. One range of results corresponds to samples collected from epigenetic mineralization and the other corresponds to samples collected from syngenetic mineralization. Of the results from the samples of unknown mineralization style, three yielded Pb isotopic compositions consistent with results from known syngenetic mineralization and one sample yielded Pb isotopic compositions inconsistent with results from either epigenetic or syngenetic mineralization. Viability as an Exploration Tool in Central Mexico Plots of Pb isotopic compositions for minerals from all samples collected for this study, with the exception of the outliers identified above, clearly establish distinct fields for both epigenetic and syngenetic mineralization (Figures 3.4, 3.5, and 3.6). In both 2 0 6 Pb/ 2 0 4 Pb vs. 2 0 7 p b / 2 0 4 p b a n d 2 0 6 p b / 2 0 4 p b y g 2 o 8 p b / 2 0 4 p b g p a c e i s o t o p i c compositions of epigenetic mineralization are definitely more radiogenic than those of syngenetic mineralization, a distinction that clearly establishes Pb isotope ratios as an effective exploration tool in central Mexico. Whether this distinction in isotopic ratios is a function of drawing lead from distinctive reservoirs, a temporal evolution of the same reservoir, or a combination of the two is not clear. For the purposes of this study however this is not a critical distinction. 88 Figure 3.4 2 0 6Pb/ 2 0 4Pb vs. 2 0 7Pb/ 2 0 4Pb plot for sulfide samples collected during the course of this study. 15.9 15.7 Mass Fractionation Trend S 2WPb Error Trend Taures (99TD15) OH 3 . 0 -15.5 La Plomosa (99TD20) Cerro Tamara (99TDCT) 15.3 15.1 18 18.2 18.4 18.6 18.8 2c*Pb/ 204Pb Legend % San Nicolas Deposit Sulfides # El Salvador Occurrence Sulfides % El Salvador Property Sulfides % Zacatecas Area (South) Sulfides # Zacatecas Area (North) Sulfides _ Aguascalientes Sulfides _ Real de Angeles Sulfides A Sabinas Sulfides # Francisco I. Madero Sulfides Figure 3.5 2 0 6Pb/ 2 0 4Pb vs. 2 0 8Pb/ 2 0 4Pb plot for sulfide samples collected during the course of this study. 38.9 38.4 PH # 37.9 37.4 36.9 - i r— Mass Fractionation Trend w "Pb Error Trend Taures (99TD15) • La Plomosa (99TD20) • —1 r-0 Cerro Tamara (99TDCT) _ l I ' 1 ' 1— 18 18.2 18.4 18.6 18.8 19 2 0 6 Pb/ 2 0 4Pb A A A Legend San Nicolas Deposit Sulfides El Salvador Occurrence Sulfides El Salvador Property Sulfides Zacatecas Area (South) Sulfides Zacatecas Area (North) Sulfides Aguascalientes Sulfides Real de Angeles Sulfides Sabinas Sulfides Francisco I. Madero Sulfides Figure 3.6 2 0 7Pb/ 2 0 6Pb vs. 2 0 8Pb/ 2 0 6Pb plot for sulfide samples collected during the course of this study. 2.09 2.07 OH -a 2.05 2.03 * Mass Fractionation Trend Taures (99TD15) _ A La Plomosa (99TD20) Cerro Tamara (99TDCT) .82 .83 .84 7Pb/ 2 f*Pb .85 .86 A A A Legend San Nicolas Deposit Sulfides El Salvador Occurrence Sulfides El Salvador Property Sulfides Zacatecas Area (South) Sulfides Zacatecas Area (North) Sulfides Aguascalientes Sulfides Real de Angeles Sulfides Sabinas Sulfides Francisco I. Madero Sulfides 91 Extrapolation throughout the Guerrero Terrane Given that a clear distinction between Pb isotopic compositions of epigenetic and syngenetic mineralization within the main study area exists, we then compared our data with Pb isotope data from mineral deposits elsewhere in the Guerrero Terrane to determine if our results could be extrapolated throughout not only Zacatecas State but the entire terrane. We used published data on epigenetic mineralization from the Fresnillo and Guanajuato districts (Lucio, 1990 and Mango et al., 1991) and on syngenetic (VMS) mineralization from the Campo Morado and Cuale districts (Cumming et al., 1979 and Miranda-Gasca, 1995). Ranges in isotopic ratios from each of the districts are provided in Table 3.3 and are shown in Figures 3.7 and 3.8. As with results from the study in central Mexico, epigenetic mineralization from throughout the Guerrero Terrane is consistently more radiogenic than syngenetic mineralization. This consistent distinction suggests that Pb isotope ratios of sulfide minerals can be effectively used to distinguish between epigenetic mineralization and syngenetic mineralization throughout the Guerrero Terrane and therefore represents an effective exploration tool for additional V M S mineralization in central and southwestern Mexico. Conclusions Sulfide minerals from Tertiary epigenetic mineral deposits in central Mexico yield Pb isotope ratios which are consistently more radiogenic than sulfide minerals from mid-Mesozoic syngenetic mineral deposits within the same region. This consistent discrepancy suggests Pb isotope ratios can be an effective means of distinguishing between the two styles of mineralization within central Mexico. 92 Table 3.3 Representative Lead Isotope Data for Guerrero Terrane Districts / Deposits '. • 206.,, / 204 n u 207,,, /204,,, 208,,. /204.,, " District / Deposit Pb/ Pb Pb/ Pb Pb/ Pb Reference Fresnillo District 18 819 15 628 38 706 Lucio, 1990 Fresnillo District 18 861 15 669 38 838 Lucio, 1990 Guanajuato District 18 782 15 632 38 666 Mango, 1991 Guanajuato District 18 813 15 666 38 763 Mango, 1991 Cuale District 18 648 15 631 38 594 Cummings, 1979 Cuale District 18 653 15 665 38 621 Cummings, 1979 Campo Morado District 18 594 15 639 38 514 Cummings, 1979, Miranda-Gasca, 1995 Campo Morado District 18 668 15 677 38 698 Cummings, 1979, Miranda-Gasca, 1995 Epigenetic Min. - Central Mexico 18 742 15 585 38 537 This Study Epigenetic Min. - Central Mexico 18 926 15 724 39 047 This Study Syngenetic Min. - Central Mexico 18 557 15 603 38 384 This Study Syngenetic Min. - Central Mexico 18 686 15 646 38 486 This Study Figure 3.7 Ranges of 2 0 6Pb/ 2 0 4Pb vs. 2 0 7Pb/ 2 0 4Pb ratios for mineralization in epigenetic and syngenetic districts within the Guerrero Terrane. 15.78 J 2 OH OH 15.68 2 (*Pb/ 2 MPb I—| Syngenetic Mineralization - This Study I—I Cuale District Campo Morado District Epigenetic Mineralization - This Study Guanajuato District Fresnillo District Figure 3.8 Ranges of 2 0 6Pb/ 2 0 4Pb vs. 2 0 8Pb/ 2 0 4Pb ratios for mineralization in epigenetic and syngenetic districts within the Guerrero Terrane. PH O .Cu 2m?b 1 204Pb I—I Syngenetic Mineralization - This Study I—I Cuale District Campo Morado District Epigenetic Mineralization - This Study Guanajuato District Fresnillo District The differences in Pb isotopic signatures observed in epigenetic and syngenetic mineral deposits of central Mexican is similarly reflected in epigenetic and syngenetic mineral deposits from throughout the Guerrero Terrane, providing a rapid and inexpensive means of distinguishing between the two styles of mineralization which could prove to be particularly useful in regions of extensive metamorphism or deep weathering. 96 References Campa, M.F., and Coney, P.J., 1983, Tectono-stratigraphic terranes and mineral resource distributions in Mexico, Canadian Journal of Earth Sciences, 20, pp. 1040-1051 Consejo de Recursos Minerales, 1992, Geological-Mining Monograph of the State of Zacatecas, Secretaria de Energia, Minas e Industria Paraestatal, Subsecretaria de Minas e Industria Basica, Publication No. M-2e de Cserna, Z., 1976, Geology of the Fresnillo area, Zacatecas, Mexico, Geological Society of America Bulletin, 87, pp. 1191-1199 Cumming, G.L., Kesler, S.E., and Krstic, D., 1979, Isotopic composition of lead in Mexican mineral deposits, Economic Geology, 74, pp. 1395-1407 Lucio, J.A., 1990, A Pb and Sr isotope study of the Fresnillo mining district, Zacatecas, Mexico, MSc. thesis, Dartmouth College, 136 p. Mango, H . , Zantop, H . , and Oreskes, N . , 1991, A Fluid Inclusion and Isotope Study of the Rayas Ag-Au-Cu-Pb-Zn Mine, Guanajuato, Mexico, Economic Geology, 86, pp. 1554-1561 Miranda-Gasca, M.A. , 1995, The Volcanogenic Massive Sulfide and Sedimentary Exhalative Deposits of the Guerrero Terrane, Mexico, Ph.D. thesis, University of Arizona, 294 p. Sedlock, R.L., Ortega-Gutierrez, F., and Speed, R.C., 1993, Tectonostratigraphic Terranes and Tectonic Evolution of Mexico: Boulder, Colorado, Geological Society of America Special Paper 278 Todt, W., Cliff, R.A., Hanser, A. , and Hofman, A.W., 1996, Evaluation of a 2 0 2 Pb- 2 0 5 Pb double spike for high-precision lead isotope analysis, Geophysical Monograph, 95, pp 429-437 97 C H A P T E R 4 Conclusion The San Nicolas V M S deposit and E l Salvador V M S occurrence are hosted in volcano-sedimentary successions that comprise mafic flows, tuffs, and dikes, felsic flows and dikes, and intercalated fine-grained clastic sedimentary rocks. Geochronologic, lithogeochemical, and isotopic data produced during the course of this study place constraints on both the age and paleotectonic setting of the San Nicolas deposit and E l Salvador occurrence. Results of the study also provide new insights into the geology and tectonic evolution of central Mexico. U-Pb zircon ages for volcanic rocks that host the San Nicolas deposit together with observed contact relationships, indicate that sulfide mineralization comprising the San Nicolas deposit formed, at least in part, at 148.3 +/- 0.4 Ma. Although this study was unable to determine the exact age of sulfide mineralization in the E l Salvador occurrence, dating of volcanic rocks in the footwall of the sulfide body provides a maximum age of 150.6 +/- 0.7 Ma. for the mineralization. Based on immobile element ratios, bi- and multi-element discrimination diagrams, and chondrite normalized REE patterns, the pre-MFB units within the San Nicolas deposit area are interpreted as having formed in a back-arc basin setting. The geochemical criteria also document the transition from a back-arc basin setting to an arc setting prior to formation of the M F B unit. An initial back-arc basin setting is supported by Sm-Nd isotopic data from volcanic rock samples, as is the subsequent transition to an arc setting. The moderately to highly radiogenic nature of Sm-Nd ratios from sedimentary units within the volcano-98 sedimentary succession suggests that the sediments were derived from an evolved source, probably exposed continental crust of nuclear Mexico. The highly radiogenic nature of Pb isotope ratios and the presence of Precambrian inherited zircon components in several U-Pb dating samples support the presence of an evolved basement. Together, these data suggest the Upper Jurassic volcano-sedimentary succession formed initially in a back-arc basin setting adjacent to a continental (or continent-fringing) magmatic arc, and the subsequent (post sulfide mineralization) transition to a volcanic arc setting. Lead isotopic compositions were determined for a large suite of sulfide minerals from both mid-Mesozoic syngenetic deposits and Tertiary epigenetic deposits of central Mexico. A distinct clustering is observed in the data with a clear distinction between isotopic compositions from syngenetic sulfide deposits and those from epigenetic sulfide deposits. A comparison of data generated in this study to published and unpublished Pb isotopic data from syngenetic and epigenetic deposits throughout southwestern Mexico indicates that the distinction between syngenetic and epigenetic mineralization in central Mexico applies throughout southwestern Mexico as well. Lead isotopes can therefore provide isotopic "fingerprints" for syngenetic and epigenetic mineralization and provide an effective exploration tool for this region. As with most scientific endeavors, this study has answered a number of questions but has raised many more. Perhaps the most obvious observation drawn from this study is that the geology of central Mexico is still poorly understood and represents a gap in the current overall understanding of North American geology. Whereas no single study will be able to address this deficiency immediately, a number of well constrained stratigraphic, geochronologic, and chemostratigraphic studies on a local basis will go a long way to 99 resolving some of the geologic uncertainty associated with the region. Although, due to the extensive Cenozoic cover, much of the region is not amenable to such work there are a number of areas which would be suitable for studies of this type. In particular, some of the established mining districts have extensive mine workings (shafts, adits, trenches, and drill core) which allow access to three dimensional exposures not available from surface exposures. The volcano-sedimentary succession which hosts the San Nicolas deposit has been tentatively correlated with the Chilitos Formation as defined by de Cserna (1976) in the Fresnillo area. This correlation is only tentative however, and requires further investigation. A detailed stratigraphic and geochronological study, perhaps coupled with a biostratigraphic study, as well as a thorough geochemical and isotopic characterization of volcanic rocks exposed in the type area of the Chilitos Formation would provide a much more detailed basis for the correlation between the Chilitos Formation and the San Nicolas deposit host stratigraphy. As with the relationship between the Chilitos Formation and the volcano-sedimentary succession which hosts the San Nicolas deposit, the relationship between the volcano-sedimentary succession in the Zacatecas area and that in the San Nicolas deposit area is also unconstrained. Geochronologic, geochemical, and stratigraphic studies similar to those outlined above may establish the temporal and tectonic relationship between the two areas. The consistency of Pb isotope ratios from sulfide minerals of a given mineralization style throughout the Guerrero Terrane suggests a Pb isotope reservoir common to the entire terrane. Does this suggest that the basement is the same throughout the entire terrane? Based on lithogeochemical and isotopic observations discussed in the preceding chapters, as 100 well as the presence of Precambrian zircon within geochronology samples, this would require that continental crust underlie the entire terrane. If the Guerrero Terrane is a coherent body underlain by continental crust how can a back-arc basin develop that far (-320 km) from the interpreted subduction zone? Does this suggest that the basin and range style extension of northern Mexico and the west-central United States extends into central and southwestern Mexico? Isotopic and geochronologic studies of crustal xenoliths in Tertiary volcanic rocks of the Trans Mexico Volcanic Belt and the Sierra Madre Occidental provinces may help outline the nature of the basement. A recommended future study of a more specific nature involves further lithogeochemical and isotopic work on the volcano-sedimentary succession in the San Nicolas deposit area. The documentation of a transition from back-arc basin setting to arc setting represented in this study is soundly based; however it is based on a limited number of samples. A more extensive lithogeochemical, particularly employing high field strength and rare earth elements, and isotopic study may provide further insights into the nature of the volcano-sedimentary succession and the exact paleotectonic setting in which the San Nicolas deposit formed. 101 References de Cserna, Z., 1976, Geology of the Fresnillo area, Zacatecas, Mexico, Geological Society of America Bulletin, 87, pp. 1191 -1199 102 Appendix A To examine the precision of obtained analytical data, a Mineral Deposit Research Unit (MDRU) in-house standard was submitted with lithogeochemical samples analyzed during the course of this study. Analytical data obtained from this sample was compared to a mean of all available previous analyses done by the same methods. For the vast majority of the elements measured, the difference in concentrations between the mean values and those measured during this study fall within the 2 sigma range (Figure 5.1). Based on this observation we feel that the analytical, lithogeochemical data obtained during the course of this study reflects a high degree of precision. 103 o o o o o o o o o O o o o o o ro o o o r- ro r- r-<—1 o o o ro o o p o ro NO NO o o d r ~ d d d d i n ro d d NO ro ,4> I i ca C < ca -a c ca ca o "5. 3 Q if) — 3 H O O O O co r~- — in ro o i n i n d —' d oo o o o oo ro o o NO o o o o o o o O O CN O l CN CN o o C N oo r- oo ci ci ci ci ci O O O O O O O O NO NO CN O co ro ro ro O O O o o d CN o o o o o o o r ~ ro o o o r - r--f-^ i n ro p i n Net- N O . ro ro ro O O d —• O O O O O O O O O O O O O O O O CN p i n p i n p i n 0 - - 0 0 — • co in NO NO — NO — o o o o o o o o ro ro in O ro — r o —; d i n —' ro oo d O O O O O O O O O O O O O O O O O r o ^ - i n r ^ r o — t^- — O N C N O N r - o o o o - H ^ p p p p p p c N p i n o p i n p p ' s j ; d d d d d d d d d c i c i c i c i — * c i c i 0 0 a o o o o o o o o o i n t N O ^ O O O c O f N O — » <N <N O — ( N v o c o o r -C N O ' O O O — O O O O O O O O O 0 0 —< CN O p r-- — cx) o o o oo o o o o o o o o o o o o o o o o o o o o c~--^f ^ 0 > ^ O N ^ ^ r ^ o o o r ^ r ^ r ^ r n o r n o r - -• - ; O O p ^ p O O O O O m o o c N c o O O C N O O O O O O O O O O O O C N O O ^ O O O O 0 0 T 3 0 0 0 0 0 0 0 0 O 0 O ' X ) 0 \ o o o a \ — ^ o o o C N 0\ • ^ • ' O i n o c N — ' O L O - ^ O L T I ^ d ^ f - r f — ^ ( N o ^ f o ^ o o v d r ^ o o o o o o o o o o O m r * - i o o o m o r - m i n m c o o o o o i n ^ O r o d o o o — ^ o o o — ' o O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O r ^ i o o i n c ^ c \ r n o o i n o o r o i n o o 0 ^ 0 ( N ^ O r o c ^ r o o " c o i n O < / " i © c o - c o — —< m — rt O O O O O O O O 0 \ C\ CN in vo O —• —' —' O O O O O O O O O O C O C N ^ o O r n r n r n r n O ' m ro < * vo ^ o -O O O O O O O O O O O O _ . o r - o r - r o o o o r - ^ o r o c o o ^ ^ O N X N o n . i n q q c N O n m <N Q fNj CN CQ U o •< U U co co O O o O O oo 2 2 "iJ CN CN a. m h J cd cu co O 3 CQ CJ O O O u. 3 TJ B ^ O Q w w o a a: K T 3 J D z z z 104 

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