Abstract

Integrated petrological and geochemical examination of metasedimentary assemblages associated with mesothermal lode gold mineralization in five major centers of past and present production (Ballarat, Percydale, Tarnagulla, Maldon, Fosterville) in central Victoria, Australia, reveal that hydrothermal alteration in the hosting Cambro-Ordovician turbidites is much more pronounced and extensive than has previously been recognized. Characteristically, the auriferous reef structures are surrounded by bleached zones up to several tens of meters wide. Sericitization, carbonatization and carbonate spotting, and pyrite and arsenopyrite porphyroblasts—within and beyond zones of visible bleaching—are the most obvious features of alteration. Quantitative XRD analysis illustrates that these marked bleached halos are due to the replacement of albitized plagioclase clasts by white mica and the breakdown of metamorphic chlorite to form secondary carbonates, and to a lesser extent, kaolin. The negative correlation between metamorphic chlorite and hydrothermal carbonate phases provides a reliable mineralogical alteration index useful to exploration. Geochemical reconnaissance profiles also indicate the development of systematic trends in response to hydrothermal alteration involving mainly SiO2, K2O, Na2O, CO2, As, Au, and S at constant bulk rock volume. Calculation of a geochemical alteration index given by the ratio K2O + CO2 / K2O + CO2 + Na2O + Al2O3 suggests that this can be a reliable indicator of alteration, in particular where concentrations of individual invariable oxides such as Al2O3 vary too much in response to lithological variations. Saturation indices that are based on ratios of 3K/Al and CO2/(Mg + Fe + Ca) are less reliable, because muscovite and carbonate are not always the dominant alteration mineral species of relevance.

The alteration assemblages, as well as mineralogical and geochemical trends at the five ore systems investigated herein, are common to many slate belt-hosted gold deposits. The results of this investigation show that, although the detection of such systematic spatial variations relative to mineralization can be complicated, for example by postore modification of the wall rocks due to contact metamorphism (e.g., at Maldon) or where mineralization is not associated with massive to laminated quartz veins (e.g., at Fosterville), recognition of extensive, though not always conspicuous, alteration halos around mesothermal lode gold mineralization represents a potentially powerful tool for exploration in slate belts.

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