Abstract

Many Carlin-type gold deposits in northern Nevada are found adjacent to steeply dipping faults that channeled hydrothermal fluids upward into mineralized regions. However, some Carlin deposits comprise gently dipping tabular ore zones lacking any obvious large, steeply dipping, feeder structure beneath them. The goal of this paper is to test whether fluid flow into such ore zones also involved largely vertical upwelling, or whether lateral fluid flow, particularly along low-angle fault structures, may have played a significant role in their formation. In this study we use carbon and oxygen isotopes in conjunction with trace element geochemistry to investigate pathways taken by hydrothermal ore-forming fluids into the tabular subhorizontal ore zones of the Pipeline deposit, a giant carbonate rock-hosted Carlin-type deposit in Nevada. We sampled deep drill holes within, below, and to the side of the main ore zone to assess the extent of lateral versus vertical fluid flow up to 600 m below mineralization.

The main ore zone at the Pipeline deposit is dominated by carbonate host rocks having δ18O and δ13C values that are depleted in the heavier 18O and 13C isotopes compared to global background values for rocks of the same age. Depletion results from the progressive buffering of the rock by an infiltrating auriferous hydrothermal fluid. There is significant heterogeneity in the spatial pattern and relative degree of 18O and 13C depletion through the ore zone, most likely reflecting multiple flow paths with different total integrated fluid fluxes, path lengths, rates of fluid cooling, and possibly fluid mixing.

A lack of a pervasive reduction in δ18O and δ13C values, and low concentrations of trace elements in the rocks immediately beneath the main ore zone indicate that the deposit was not a product of large-scale vertical upwelling of auriferous hydrothermal fluid directly into the mineralized region. Rather, flow was focused along preexisting low-angle thrusts, particularly the Abyss fault, with fluids flowing laterally underneath and then up into the area of the main ore zone. Enhanced permeability along these low-angle structures most likely derived from fault reactivation and the generation of fracture networks in damage zones peripheral to a relatively impermeable fault core. Such reactivation would have required suprahydrostatic fluid pressures, relatively shallow crustal depths, and a lack of more optimally oriented, high angle faults (>50°) that would otherwise have preferentially failed and focused fluid flow steeply upward.

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