2005 Salt Lake City Annual Meeting (October 16–19, 2005)

Paper No. 13
Presentation Time: 5:00 PM


SAMAL, Abani R., ER&P, Dept. of Geology, Southern Illinois University, Carbondale, 1259 Lincoln Drive, Mailcode 4324, Carbondale, IL 62901, FIFAREK, Richard H., Dept. of Geology, Southern Illinois University at Carbondale, 1259 Lincoln Drive, Mailcode 4324, Carbondale, IL 62901 and THOMASON, Robert E., RTGEO LLC, 5015 Snowy Mountain Drive, Winnemucca, NV 89449, arsamal@siu.edu

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.