At Buckskin Mountain (elev 2,650 m, 8,743 ft), Humboldt County, Nevada, a hydrothermal system, imposed on a middle Miocene volcanic sequence with contrasting permeabilities and tensile strengths, produced alteration assemblages controlled by elevation, from Hg-mineralized sinter to subjacent precious metal veins over a vertical distance exceeding 790 m. Sinter and epiclastic deposits, interpreted to be remnant paleosurface basinal strata enclosed by 16.6 to 16.1 Ma rhyolites, overlie older volcaniclastic basinal deposits and were part of a regional fluvial-lacustrine system developed among ca. 16 to 12 Ma basalt-rhyolite eruptive centers throughout the northern Great Basin. Because of contrasting erosional resistance among altered and unaltered rocks, Buckskin Mountain represents inverse topography with sinter and silicified epiclastic deposits at the summit.
Sinter and veins, correlated by common elements, similar mineralogy, age constraints, textures, S isotope compositions, and fluid inclusion microthermometry, were deposited by sinter-vein fluid, the first of two sequential hydrothermal fluid regimes that evolved in response to magmatism, tectonism, hydrology, and topography. Thermal quenching of distally derived sinter-vein fluid in planar conduits caused deposition of banded quartz-silicate-selenide-sulfide veins ~270 to > 440 m below sinter at 16.1 Ma; veins were initially enveloped by zoned selvages of proximal K-feldspar + K-mica + quartz + pyrite and distal illite + chlorite + calcite + pyrite. Mixing of sinter-vein fluid with local meteoric water in saturated basinal deposits caused deposition of silica, Hg-Se-S-Cl minerals, and precious metals in sinter and epiclastic deposits. Elevated ∑Se/∑S in sinter-vein fluid, and the relatively large stability fields of reduced aqueous selenide species in the temperature range of 250° to <100°C, enabled (but was not the cause of) codeposition of selenide-sulfide minerals and common element associations in veins and sinter.
Acid-sulfate fluid of the second fluid regime was derived from oxidation of H2S and other volatiles exsolved from sinter-vein fluid. Acid-sulfate fluid produced (1) a subhorizontal zone of partially leached basinal deposits and rhyolite from the paleosurface to a depth of ~60 m, and (2) laterally pervasive zones, ~100 to 200 m thick, of quartz + alunite ± hematite and quartz + kaolinite + pyrite in volcaniclastic deposits immediately beneath partially leached rocks, but this fluid did not decompose selenide-sulfide-precious metal phases in sinter. Paragenetically late vein and wall-rock assemblages, including marcasite + pyrite, calcite, and kaolinite-replaced K minerals, record deeper transition of sinter-vein fluid into acid-sulfate fluid in vein conduits. This transition occurred as regional subsidence, manifested by the Goosey Lake depression immediately east of Buckskin Mountain, lowered the pieziometric surface at Buckskin Mountain, terminated sinter deposition, and caused boiling and/or degassing of sinter-vein fluid. The timing of subsidence is recorded by a decrease in alunite ages, from ca. 15.8 to 15.6 Ma, with depth below sinter. Lateral replacement of sinter and partially leached epiclastic deposits and rhyolite by opal-A marks the termination of the two hydrothermal regimes that lasted ~0.5 m.y. and followed rhyolitic volcanism of similar duration.
Veins and sinter display textures that attest to plastic deformation, spalling, and gravitational settling, and indicate fluid-flow direction, velocity, and density stratification which, with conduit topology, may have influenced precious metal tenor in the veins. Components of sinter and veins were transported as colloids, formed in supersaturated sinter-vein fluid, that aggregated or coagulated as incompetent gelatinous layers in shallow pools and in underlying, near-vertical conduits in rhyolite and initially crystallized as opal and chalcedony. The low thermal conductivity of homogeneous, structurally isotropic, and relatively impermeable rhyolite, and limited hydraulic permeability of altered overlying volcaniclastic deposits, insulated conduits and maintained increments of sinter-vein fluid in a nearly isothermal environment (250° ± 20°C) over vertical distances of hundreds of meters. The rhythmic layering of sinter beds and vein bands reflects (1) flow impedance by silicification of volcaniclastic deposits at the tops of conduits and by coagulation of semisolid silica gel in vents and subsinter channels, and (2) flow restoration by over-pressured fluid that ruptured silica gel, clearing conduit blockages and creating new fluid pathways. These pressure cycles enabled sinter-vein fluid to rapidly fill conduits, supersaturate, nucleate colloids, and be flushed upward from conduits into shallow paleosurface depressions, leading to simultaneous deposition of paired vein bands and sinter beds in overlying pools. These cycles also minimized fluid boiling and degassing during sinter-vein deposition.
Miocene hydrodynamics at Buckskin Mountain imply stable topographic recharge systems of several hundred thousand years duration throughout the northern Great Basin from ca.16 to 12 Ma when basalt-rhyolite volcanism and numerous Se-rich, precious metal hydrothermal systems were active. Hydrothermal fluids thatcontain elevated ∑Se/∑S may deposit Hg selenide-sulfide minerals and precious metals that can be detected by close examination of near-surface alteration assemblages and sinter, providing guides to subjacent precious metal veins. Topographic effects on hydrologic discharge and recharge, hydraulic conductivity and structural contrasts within the stratigraphic section, and temperature gradient and amplitude imposed by magma, were the main controls of the morphology of the Buckskin Mountain system and ultimately determined the existence of sinter and veins and degree of metal concentration in other middle Miocene hydrothermal systems.