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Petrophysical and physicochemical controlling parameters on stable isotope depletion patterns in carbonate rocks from auriferous hydrothermal fluid infiltration at the Long Canyon sediment-hosted gold deposit : NE Nevada Lepore, William Adamas


The Long Canyon deposit is a sediment-hosted gold deposit located in northeastern Nevada, over 150 km east of the Carlin Trend. The deposit geology, geochemistry and mineralogy suggest that it is a Carlin-type deposit. This project tests the extents of auriferous hydrothermal fluid infiltration through carbonate rocks at Long Canyon using patterns of ¹⁸O/¹⁶O depletion to define the limits of fluid-rock interaction. Patterns of isotope depletion are used to assess the structural and lithologic controls on fluid flow, and the lateral extent of fluid rock interaction beyond the limits of trace element halos genetically associated with gold mineralization. Carbon and oxygen isotope ratios were measured using the Mineral Deposit Research Unit’s Mineral Isotope Analyzer (MIA) at the University of British Columbia. Gold mineralization at Long Canyon occurs predominantly within deeply oxidized, limestone-hosted solution breccias, located within necklines of an 80 metre thick boudinaged dolomite unit. Alteration associated with gold mineralization comprises decalcification of limestones>>dolomite, fine-grained pyrite growth within Fe-rich host rocks, argillization and silicification. Clay minerals include dickite within the core of the system, flanked by illite and kaolinite. Assuming typical Carlin-type fluid temperatures of 180° to 240°C, δ¹⁸O values of ore fluids are calculated at ≤1.6 to 6.3‰. Calculated δ¹⁸OH₂₀ that deposited dickite within the core of the mineralized system was calculated at 2.3‰ for a 200°C fluid. In this study several scales of sampling were tested including: contiguous drill assay pulps over cross sections; surface sampling along traverses; closely spaced hand sample coverage down drillholes; and micro-drilled hand samples. Presented here are the results of over 2,800 unique ¹⁸O/¹⁶O carbonate sample compositions from across the deposit. Results indicate stable isotope depletion mapping around carbonate-hosted ore bodies with sufficient sample density can provide far field vectors to fluid flow paths beyond what traditional lithogeochemistry alone may show. Additionally, it is evident from the pattern of oxygen isotope depletion that hydrothermal fluid flow responsible for mineralization was largely constrained to brittle damage zones within boudin necks, or incipient boudin necks, within massive dolomite units. There was minimal lateral flow within or vertically across stratigraphic units without pre-existing structural damage.

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