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Low-temperature thermochronology of the northern Carlin Trend : evidence for Late Cretaceous exhumation.. 2012

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 LOW-TEMPERATURE THERMOCHRONOLOGY OF THE NORTHERN CARLIN TREND: EVIDENCE FOR LATE CRETACEOUS EXHUMATION OF NORTHERN CENTRAL NEVADA AND SHORT DURATION OF CARLIN ORE-FORMING HYDROTHERMAL SYSTEMS by Moira Cruickshanks  MSci, Imperial College London, 2008  A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF  MASTER OF SCIENCE in THE FACULTY OF GRADUATE STUDIES (Geological Sciences)  THE UNIVERSITY OF BRITISH COLUMBIA (Vancouver) September 2012  © Moira Cruickshanks, 2012  ii Abstract Samples were collected from Jurassic intrusions in the northern Carlin Trend to determine the thermal history of the region that was strongly affected by Late Cretaceous exhumation and Eocene hydrothermal fluid flow. Low-temperature apatite fission track and apatite and zircon (U-Th)/He thermochronology of the northern Carlin Trend records 3-4 km exhumation since the Late Jurassic. At least 1-2 km exhumation occurred during a phase of rapid exhumation between 85-60 Ma that is coincident with extensional collapse of the overthickened hinterland of the Sevier orogeny. Late Cretaceous exhumation ages provide a background against which ~40 Ma thermal anomalies associated with the formation of the Carlin-type Au deposits can be observed. In the northern Carlin Trend, a thermal halo defined by apatite fission track resetting was observed up to 400 m out from major fluid flow conduits around the Goldstrike Stock that is immediately south of the multi-million ounce Betze-Post deposit. A lack of hydrothermal alteration within the stock shows that Eocene hydrothermal fluid flow was concentrated around its edge, and that heating of the stock was predominantly conductive. Finite element modelling and forward modelling of time-temperature paths constrain the duration of a hydrothermal fluid event to on order of 10 kyr. Multiple phases of this duration of fluid flow are possible, but were either thermally isolated or were shorter- lived. The high rates of fluid flux associated with short fluid flow durations are similar to those observed in hydrothermal systems driven by forced convection or pressure-driven flow, suggesting fluid flow responsible for the formation of Carlin-type Au deposits was driven by a magmatic source or seismicity rather than free convection.   iii Table of Contents Abstract .......................................................................................................................... ii Table of Contents ......................................................................................................... iii List of Tables................................................................................................................. ix List of Figures ................................................................................................................. x Acknowledgements ..................................................................................................... xiv Chapter  1: Introduction ............................................................................................... 1 1.1 Thesis goals and fundamental concepts .............................................................. 1 1.1.1 Background exhumation study ................................................................... 4 1.1.2 Thermal mapping ........................................................................................ 5 1.1.3 Apatite as a chemical indicator of hydrothermal fluid flow ....................... 6 1.2 Thesis structure ................................................................................................... 6 Chapter  2: Geological background to Carlin-type Au deposits .............................. 10 2.1 Introduction....................................................................................................... 10 2.2 Geological setting of the northern Carlin Trend ............................................... 10 2.2.1 Stratigraphy of the northern Carlin Trend ................................................ 10 2.2.2 Igneous geology ........................................................................................ 11 2.2.3 Structure .................................................................................................... 12 2.2.4 Geological history ..................................................................................... 13 2.3 Carlin-type mineralization in the northern Carlin Trend .................................. 16 2.4 Alteration in the northern Carlin Trend ............................................................ 17 2.5 Characteristics and source of mineralizing fluids ............................................. 18 2.6 Ore deposit formation models .......................................................................... 19   iv Chapter  3: Thermochronology overview .................................................................. 24 3.1 Principles of low-temperature thermochronology ............................................ 24 3.2 Apatite fission track dating ............................................................................... 26 3.2.1 Annealing .................................................................................................. 27 3.2.2 Fission track length distribution ............................................................... 28 3.2.3 AFT age calculation .................................................................................. 29 3.2.3.1 EDM and single grain age calculation .................................................. 29 3.2.3.2 LA-ICP-MS and single grain age calculation....................................... 30 3.2.3.3 Sample AFT ages ................................................................................. 31 3.3 (U-Th)/He dating .............................................................................................. 32 3.3.1 Diffusion behaviour of helium ................................................................. 33 3.3.2 Crystal size and shape ............................................................................... 33 3.3.3 Alpha-ejection corrections ........................................................................ 34 3.3.4 Zoning ....................................................................................................... 36 3.3.5 Radiation damage ..................................................................................... 37 Chapter  4: Late Cretaceous exhumation of northern central Nevada: evidence from low-temperature apatite and zircon (U-Th)/He and apatite fission track thermochronology ........................................................................................................ 50 4.1 Introduction....................................................................................................... 50 4.1.1 Tectonic setting of northern central Nevada ............................................. 52 4.1.1.1 Cretaceous contraction and synconvergent extension .......................... 52 4.1.1.2 Cenozoic extension ............................................................................... 53 4.1.2 Metallogeny of northern central Nevada .................................................. 55 4.1.3 Local geology of the northern Carlin Trend ............................................. 56  v 4.2 Previous thermochronology of the northern Carlin Trend ............................... 58 4.3 Methods ............................................................................................................ 59 4.3.1 Sampling ................................................................................................... 59 4.3.2 (U-Th)/He dating ...................................................................................... 61 4.3.3 Apatite Fission Track (AFT) dating ......................................................... 62 4.3.4 HeFTy modelling ...................................................................................... 64 4.4 Results .............................................................................................................. 67 4.4.1 Apatite and zircon yields .......................................................................... 67 4.4.2 Apatite fission track results ...................................................................... 67 4.4.2.1 Apatite fission track data analysis ........................................................ 67 4.4.2.2 Apatite fission track ages ...................................................................... 68 4.4.2.3 Apatite fission track lengths and length distribution ............................ 69 4.4.3 (U-Th)/He results ...................................................................................... 71 4.4.3.1 Intrasample variation ............................................................................ 71 4.4.3.2 Zircon (U-Th)/He ages ......................................................................... 72 4.4.3.3 Apatite (U-Th)/He ages ........................................................................ 73 4.4.4 Zircon fission track data ........................................................................... 74 4.4.5 HeFTy inverse modelling ......................................................................... 74 4.5 Interpretation..................................................................................................... 76 4.5.1 Assumptions ............................................................................................. 76 4.5.2 Time-temperature-depth histories ............................................................. 78 4.5.2.1 Little Boulder Basin Stock ................................................................... 78 4.5.2.2 Vivian Sill ............................................................................................. 82 4.5.2.3 Goldstrike Stock ................................................................................... 82  vi 4.5.2.4 Central Tuscarora regional samples ..................................................... 86 4.5.3 Interpreted timing and motion on the Post Fault ...................................... 87 4.5.4 Interpreted exhumation of the northern Carlin Trend............................... 90 4.6 Discussion .......................