2009 Portland GSA Annual Meeting (18-21 October 2009)

Paper No. 6
Presentation Time: 2:30 PM


MUNTEAN, John L., Nevada Bureau of Mines and Geology, University of Nevada Reno, Reno, NV 89557-0178, CASSINERIO, Michael D., Department of Geological Sciences and Engineering, University of Nevada Reno, Reno, NV 89557, AREHART, Greg B., Department of Geological Sciences and Engineering, Univ of Nevada Reno, Reno, NV 89557, CLINE, Jean S., Department of Geoscience, University of Nevada, Las Vegas, Las Vegas, NV 89154-4010, LONGO, Anthony A., Department of Geoscience, University of Nevada Las Vegas, 4505 Maryland Pkway, Las Vegas, NV 89154-4010 and HICKEY, Kenneth A., Mineral Deposit Research Unit, University of British Columbia, Vancouver, BC V6T 1Z4, munteanj@unr.edu

Fluid pathways at the Turquoise Ridge Carlin-type gold deposit are deduced from patterns of lithology, structure, hydrothermal alteration, Au grade, carbonate δ18O values, and trace elements. Auriferous fluids migrated up the NNW-striking Getchell fault and entered antithetic and NE-striking small-displacement faults in the hanging wall, which, along with a pre-ore dike, served as conduits to high-grade ore bodies located up-dip. Flow was mainly fracture-controlled; evidence for extensive pervasive fluid flow is lacking. Zones of decalcification/argillization/silicification typically contain grades of >0.34 g/t Au. The HGB zone, which constitutes the bulk of the ore, occurs above the antithetic fracture zones and straddles the dike in a reactive sedimentary breccia unit, which occurs between carbonaceous rocks (>0.5% org. C) and overlying, less carbonaceous rocks. The HGB also occurs just above a transition from underlying calcite to ferroan calcite (>0.1 to 0.5 wt% Fe) that is interpreted to be a pre-ore feature. Visually unaltered carbonates, with undepleted δ18O values (>17.5‰), are commonly within meters of ore zones, making exploration difficult. Fluid flow in the visually unaltered rocks surrounding ore was limited to narrow bleached fractures. Fracture coatings commonly have As concentrations that are an order of magnitude higher than the visually unaltered adjacent rock, based on analyses using a hand-held portable XRF. Compared to background ages of >64 Ma, visually altered, Au-bearing dikes have apatite fission track dates between 33-44.5 Ma, which is consistent with the published 39 Ma age for mineralization. Visually unaltered dikes <75 m from ore are only partially reset to 55-56 Ma, supporting short-lived, fracture-controlled fluid flow. Abundant late-stage realgar is more extensive on the west side of Turquoise Ridge, along a zone sub-parallel to the Getchell fault, and likely represents incursion of meteoric water during collapse of the hydrothermal system. The patterns of fluid flow agree with percolation theory, whereby at low strain, flow along the Getchell fault linked deeply sourced fluids and metals with sites of discharge in reactive host rocks. Small strain changes likely occurred during the change from shortening to extensional tectonics in the Eocene in northern Nevada.