Ledge formation at the Sandstorm and Kendall gold mines, Goldfield, Nevada

Abstract

Gold ore at the adjacent Sandstorm and Kendall mines occurs in silicified fissures or ledges that formed by both open-space deposition and replacement of Miocene rhyolites 2 mi (3.2 km) north of Goldfield, Esmeralda County, Nevada. The ledges, which are approximately 21 Ma, alteration selvages, and volcanic stratigraphy at Sandstorm-Kendall are similar to those of the main district, immediately northeast of the town of Goldfield, where more than 95 percent of past gold production occurred. From 1903 to 1947 4.2 M oz (136,360 kg) of gold was produced from the Goldfield district.En echelon Sandstorm-Kendall ledge segments consist of six assemblages which from oldest to youngest are: (1) ledge replacement quartz, (2) barite + sulfides, (3) quartz + pyrite + barite, (4) quartz + barite + kaolinite breccia, (5) vuggy quartz, and (6) lateral replacement quartz. A large range in homogenization temperatures (292 degrees -100 degrees C), salinities (0.2-7.9 wt % NaCl), isotopic compositions (delta D, +3 to -137ppm; delta 18 O, +4.2 to -17.6ppm) and gas concentrations (3.8-27.5 wt % CO 2 + H 2 S + SO 2 + N 2 ) is observed in quartz and barite of the six assemblages. Isotopic temperatures calculated for delta 34 S (sub pyrite-alunite) and delta 34 S (sub pyrite-gypsum) in wall-rock mineral assemblages range from 200 degrees to 356 degrees C.Gold occurs as inclusions in copper sulfosalts (mainly luzonite and famatinite) and barite of assemblage (2), and in quartz and barite of assemblage (4). It is analytically detectable in assemblages (1) and (3). Rhyolites enclosing the six assemblages are altered to quartz + kaolinite or alunite adjacent to ledge segments, and to illite and montmorillonite-dominated zones with distance from the ledge. Quartz and kaolinite are the stable silicates in the ledge.According to fluid isotope compositions and salinities, assemblages (1), (2), and (3) precipitated from heavy, pre-Miocene formation water, probably modified seawater, mixed with Miocene meteoric and magmatic water. Assemblage (4) was deposited by pre-Miocene formation water that either mixed with magmatic water or was enriched in delta 18 O by exchange with Paleozoic metasedimentary rocks that underlie Goldfield. Assemblage (5) largely precipitated from Miocene meteoric water. Assemblage (6) fluid, which laterally replaced extensive volumes of andesite with quartz, is also Miocene meteoric water, but silicified andesitc beneath laterally replaced andesite formed from delta 18 O-enriched pre-Miocene formation water or increments of magmatic water. Sulfur isotope compositions permit derivation of barite from underlying Paleozoic metasedimentary rocks, which contain bedded barite deposits elsewhere in southwestern Nevada, and sulfide sulfur from Tertiary intrusions. Small amounts of organic matter in barite may also have been derived from Paleozoic rocks.Spheroidal weathering of andesite preserved by assemblage (6) quartz and limited erosional exposure of andesite prior to mineralization indicate that ledge segments in the Sandstorm-Kendall area may have formed within hundreds of feet below the paleosurface. However, ledge fluid inclusion temperatures exceeding several hundred degrees centigrade at such shallow depths require a combination of thicker premineralization cover, fault displacements, lithostatic pressure, and more thorough evaluation of fluid inclusion phase ratios and gas compositions.Distinguishing features of Sandstorm-Kendall ledge formation--near-surface fluid inclusion temperatures exceeding 250 degrees C, high-temperature gradients, disparate and distal water sources, internal ledge brecciation and sedimentation, and gas-rich fluids containing H 2 S + SO 2 --are consistent with rapid, very shallow emplacement of magma beneath Goldfield. Since ledges are essentially coeval with rhyodacite, latite, and andesite in the main district, subvolcanic intrusions or diapirs of these rocks probably supplied thermal energy for mineralization both there and at Sandstorm-Kendall.