Doctoral Dissertations

Changsheng Lu

Date of Award

8-1994

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Geology

Major Professor

Kula C. Misra

Committee Members

Larry Taylor, Ted Labotka, Craig Barnes

Abstract

Gold mineralization in the Brewer mine in the Carolina slate belt, South Carolina, occurs in an intensely silicified zone, which constitutes the central part of hydrothermal alteration system more than 2 kilometers across. Mineralogic and petrographic studies indicate the following concentric alteration pattern: a silicic breccia center (quartz-pyrite-gold- enargite); an advanced argillic inner zone (quartz-alunite-andalusite); a sericitic outer zone (quartz-sericite); and a patchy propylitic outermost zone

(chlorite-epidote-quartz) that grades into unaltered metavolcanic rocks. Geochemical analysis of the host volcanic rocks, including volcanic tuffs, flows, and breccias, indicates a rhyolitic composition within a tholeiitic suite. Compared to the unaltered volcanic rocks, the hydrothermally altered volcanic rocks are relatively enriched in SiO2 and depleted in K2O and Na20 . The altered volcanic rocks also have relatively higher abundances of Sr, Cu, Mo, Pb, and Zr. SiO2, Cu, and Mo are enriched toward the center of the system, whereas K2O, Na2O, Y, and V are depleted in the same direction.

Alunite in the Brewer mine is quite heterogeneous in composition and has extensive solid solution between alunite and natroalunite with compositional zonation of Na increasing from center to rim, as revealed by XRD and electron microprobe analyses. Sulfur isotope analyses of alunite and pyrite indicate a hypogene origin for the a limits and a magmatic hydrothermal environment for the acid sulfate alteration, which was broadly content contemporaneous with the gold mineralization.The δ34S values of the alunites, occurring as veinlets, replacements and disseminations, range from 17.7 to 26.8 per mil. The δ34S values of the pyrites of various modes of occurrence-euhedral crystals and finely disseminated in the groundmass of the pyrite-quartz rocks and breccias, and quartz-pyrite veins-- range from 0.5 to 5.7 per mil. The narrow range of values in pyrite suggests that the sulfur was derived from the same source. That δ34S values of the alunites are generally 15 to 22 0∞ larger than those of the pyrites suggests that alunite was a direct product of magmatic hydrothermal, acid sulfate alteration in an epithermal environment. Sulfuric acid was probably produced by the disproportionation of SO2 with decreasing temperature, and equilibrium between aqueous H2S and SO4 formed by this disproportionation resulted in the alunite being enriched in 34S relative to pyrite. Pyrite-alunite sulfur isotopic geothermometer gives temperatures of about 300-500 °C.

Microthermometric and microchemical analyses of fluid inclusions in the quartz reveal that the inclusion fluids are dominantly CO2-bearing aqueous solutions. The majority of the primary and pseudosecondary inclusions are COj2bearing two-phase (L-1-LCO2) or three-phase (L+LCO2+VCO2) inclusions at room temperature. Some CO2- rich and aqueous primary and pseudosecondary fluid inclusions are also present, but the aqueous inclusions are mostly secondary in origin. Homogenization temperatures of primary and pseudosecondary fluid inclusions indicate that the hydrothermal fluids had temperatures between 160 and 330 °C. The widespread range probably reflects the effect of multiple- brecciation and hydrofracturing events during mineralization. The fluids have low to moderate salinity (<15 wt% NaCl. equiv.) and contain many trace components, including K, Ca, Ti, Cr, Mn, Fe, Ni, Cu, Zn, As, and Ba, as indicated by synchrotron X-ray fluorescence microprobe analysis. Laser Raman microprobe analysis on the fluid inclusions indicates that the fluids contain SO4-2in addition to CO2.

The fluids for the Brewer gold mineralization differ compositionally from those of typical epithermal gold deposits and porphyry Au-Cu-Mo deposits, and a magmatic source appears to be the most likely source for the Brewer mineralizing fluids. Hydrofracturing and brecciation in the Brewer mine suggest that these processes most likely accompanied the release of aqueous fluids from a deep-seated felsic magma. The magma provided both the heat and fluid source for the mineralization. The Brewer hydrothermal system may represent a slightly deeper environment than the typical epithermal systems, with magmatic fluid playing a more important role than in the case of those in shallow environments. It is likely that meteoric water was involved in the process by mixing with the magmatic fluid, especially during the later stage of mineralization. Considering the widespread aluminosilcate alteration and other features in the Brewer mine, it is proposed that the Brewer hydrothermal system is a magmatic hydrothermal acid sulfate system which overlies or is related to a deep porphyry system.

Reaction-path calculations of the ore-forming processes indicate that fluid-rock reactions and mixing of magmatic fluids with cooler and diluted fluids were the dominant processes that contributed to the mineralization. These calculations, in response to the increase in pH and the decrease in temperature, produced sulfide mineral assemblages— pyrite as the most abundant mineral and various amounts of bornite, chalcopyrite, enargite, covellite, tetrahedrite, and tennantite— that are similar to those observed in the Brewer mine area. Speciation calculations show that chloride complexes are the dominant species for Fe, Cu, Zn, Pb, and Ag. Au(HS)2- is the more abundant species for Au and AUCI2- becomes more abundant only during the later stage of fluid-rock reaction. The bisulfide complex of Au appears to be the dominant complex of gold under reduced conditions and chloride complex appears to be the more stable species under oxidized conditions. The primary mechanisms of gold precipitation appear to have been desulfidation resulting from precipitation of sulfide minerals and increase in pH due to either fluid-rock reaction or mixing with less acidic meteoric water.

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