Robert K. Amissah, MSc
R. K. Amissah1, S. M. Brueckner1
1Harquail School of Earth Sciences, Laurentian University, Greater Sudbury, Ontario, Canada
The Neoproterozoic, low-sulfidation Haile epithermal deposit in South Carolina has been metamorphosed to greenschist facies conditions and deformed during the Paleozoic Taconian and Alleghanian orogenies related to the formation of the Appalachians. The primary hosts for mineralization at Haile are late Proterozoic meta-sedimentary and meta-volcanic rocks of the Persimmon Fork Formation. The deposit has total estimated reserves of 45.4 Mt with 2.6 Moz Au @ 1.8g/t and 3.2 Moz Ag @ 2.2g/t. The complex deformational history and metamorphic conditions of the deposit yields questions about the degree and extent of epigenetic processes that led to the distribution and remobilization of the mineralization dominated by pyrite, minor sphalerite, arsenopyrite, galena, chalcopyrite, pyrrhotite, and traces of native gold ± silver ± lead tellurides.
In order to constrain the impact of metamorphism and deformation on the ore phases, detailed mineralogical, textural and compositional investigations were conducted on pyrite due to its known close association with precious metals. Petrographic and micro-structural results show three growth generations of pyrite that are from oldest to youngest: (1) syngenetic very fine to medium grained, idiomorphic to subhedral pyrite, denoted Py1a, that is concordant with foliation S2; (2) syngenetic fine-grained to medium-grained, anhedral pyrite (Py1b) with a spongy, open vugs texture that commonly has irregular grain boundaries; (3) syntectonic, coarse, euhedral, and inclusion-free pyrite (Py2) that forms either around Py1b or forms annealed aggregates with weakly convex-concave grain boundaries.
While gold generally occurs as “invisible gold” in pyrite, high-grade gold occurs either as native gold along cataclastic cracks in Py1b and Py2 or as inclusion in Py1b. Less abundant, gold also occurs with Ag ± Pb tellurides interstitially between Py2. Also, arsenic concentrations is, on average, highest in Py1a (0.30 wt.%, n=56) and Py1b (0.30 wt.%, n=59); however, As-rich zoning developed in Py2 can be enriched in As of up to 2.94 wt.%. Further, the trace elements Ag, Au, Bi, Sb, and Te are more enriched in Py1a and Py1b.
These textural and geochemical observations indicate a complex epigenetic history where precious and semi-metals were deposited syngenetically with pyrite. However, metamorphism and deformation, likely during the Taconic orogeny, resulted in the partial destruction of coarser Py1b by relatively reduced, acidic fluids, and the formation of Py2 by dissolution-reprecipitation around Py1b. Further, Ag, Au, Sb and Te were liberated from Py1b, remobilized and deposited along cataclastic cracks (native gold) or interstitially between Py2 by the metamorphic fluid and sulfide melts (tellurides).