A comparison of alteration assemblages associated with Archean gold deposits in Western Australia and Paleozoic gold deposits in the southeast United States: Discussion

Abstract

Ririe states that the purpose of his comparison of the mineralogy and chemistry of gold deposits from Western Australia (Sons of Gwalia and Mount Celia) and the Carolina slate belt (Haile and Brewer) is illustrate that alteration in these two terranes is the result of similar processes that were also responsible for the introduction of gold into the altered (p. 1561). The alteration assemblages contain highalumina minerals such as andalusite, kyanite, pyrophyllite, kaolinite, and sericite, plus local alunite. Ririe interprets these rocks as having ... been altered by acid hydrothermal solutions prior to regional (p. 1561) based upon relations [that] indicate that the rocks had their alteration composition by the time the suite of aluminosilicate minerals was forming (p. 1574) and the absence of documented low-pH regional metamorphic fluids capable of producing alunite plus a suite of high-alumina minerals such as andalusite-pyrophyllite (p. 1574). He concludes that the best analogs for these gold deposits are acid-sulphate epigenetic gold deposits such as Goldfield, Nevada, and modern geothermal systems. The important question to be asked is, Does the presented evidence convincingly support the interpretation? In this writer's opinion, the presented evidence does not positively answer this question. ~easons why the two principal arguments proposed above by Ririe inadequately support his interpretation follow below. The textural evidence consists of photomicrographs of randomly oriented andalusite porphyroblasts, fractured and rotated andalusite crystals, and folded quartz-pyrophyllite + pyrite banding in the Western Australia deposits (Fig. 3, p. 1564), randomly oriented andalusite porphyroblasts at Brewer, and deformed and altered argillite at Haile (Fig. 7, p. 1568). It is reasonable to suggest that the high-alumina assemblages reflect metamorphism of a previously altered and 1 aluminous rock. But that is all that it proves; the products of hydrothermal alteration have been metamorphosed! However, I the critical questions is the timing of hydrothermal alteration relative to metamorphism and regional deformation? and What is the metamorphic and deformation history of the surrounding country rocks? are not addressed. ree ens chi st facies regional metamorphism is implied for Western Australia or stated (Carolina slate belt, p. 1563) with only one period of deformation and metamomhism imolied. ow ever. more than one period of deformationkd metahorphism occurred in both the Norsemen-Wiluna greenstone belt (Archibald et al. 1978; Bickle and Archibald 1984) and the Carolina slate belt (Williams and Hatcher 1983).