STEAM-HEATED ALTERATION AT THE FLORIDA CANYON EPITHERMAL GOLD DEPOSIT, NEVADA: ASSEMBLAGES, TIMING AND POSSIBLE FLUID SOURCES

The Florida Canyon deposit is a large, disseminated, low sulfidation epithermal gold deposit located along the western range-bounding fault of the Humboldt Range in north-central Nevada. Mineralization and alteration is hosted by Triassic siliciclastic metasedimentary rocks and is most intensely developed at the intersection of the N-S range-bounding fault and a NE-trending shear zone. Remote sensing, XRD, and petrographic data document the alteration assemblages and zonation in the oxidized upper part of the deposit. Alunite>kaolinite grades successively outward to kaolinite>alunite, kaolinite+illite, illite+chlorite+kaolinite and, at the periphery of the deposit, smectite+chlorite+kaolinite. Hematite±goethite is widespread except locally in fault-controlled zones of friable, sandy quartz with minor fine-grained alunite, kaolinite, and sporadic traces of native sulfur. These features characterize intense steam-heated alteration where downward percolating, acid-sulfate fluids leach the rocks of most components and precipitate alunite at and below the paleowater table. Alunite from one occurrence yielded a ⁴⁰Ar/³⁹Ar plateau date of 2.12 ± 0.06 Ma.  

Oxygen isotope fractionations between SO₄ and OH in fracture-filling alunite (n = 8) yield plausible to somewhat elevated temperatures (120 – 190 °C) for the steam-heated environment. Alunite-forming fluids exhibit a pronounced shift in d¹⁸O and dD values from -21.2 and -144 ‰, respectively, similar to that of present day meteoric water, to -2.3 and -97 ‰, respectively. Fluids related to alteration clays (kaolinite, dickite, illite; n = 4) display a subparallel trend of d¹⁸O and dD values from -10.9 and -173 ‰, respectively, to -1.5 and -137 ‰, respectively. These data and trends when combined with regional geologic constraints suggest possible fluid sources and evolutionary paths: 1) clay fluids with dD values < -140 ‰ imply the involvement of Plio-Pleistocene meteoric groundwater that precipitated during a cooler (glacial?) paleoclimate; 2) the alunite fluid trend implies the mixing of meteoric water and isotopically heavier, moderately saline lake water (such lakes were present in restricted basins along the Humboldt River corridor 2 - 4.5 Ma ago); or 3) the input of magmatic vapor and sulfur directly into the steam-heated environment.