Carbonate-sulfide equilibria and “stratabound” disseminated epigenetic gold mineralization: a proposal based on examples from Alleghany, California, U.S.A.

Mineralogy, geochemistry, and structural controls of a disseminated gold-bearing alteration halo around the schist-hosted Bullendale orogenic gold deposit, New Zealand

“…Barium has apparently been depleted. The overall style of alteration, dominated by carbonation and sulphidation, resembles some of those described from other goldfields by Kishida and Kerrich (1987), Böhlke (1988), Gao and Kwak (1997), Eilu and Mikucki (1998) and McCuaig and Kerrich (1998). However, unlike most of these examples, the Bullendale alteration has no evidence for addition or depletion of alkalis (Fig.…”

“…Pyrite is essentially a replacement mineral arising from addition of sulphur to iron-bearing rock (cf. Böhlke, 1988). This added sulphur contributes to the elevated LOI of altered rocks (Figs.…”

“…Epidote decomposition may also have contributed some iron for pyrite formation. Pyrite formation from iron released during carbonate formation in high Fe/(Fe + Mg) rocks is reputedly a key feature facilitating gold deposition during alteration (Böhlke, 1988).…”

“…Recognition of the existence of alteration zones is important, as this increases the size of the target mineralised zone during mineral exploration (Bierlein and Crowe, 2000). Also, some of the altered host rocks contain anomalous (ppm scale) concentrations of gold, far-removed (metres to tens of metres) from the main vein zones (Böhlke, 1988;Eilu and Mikucki, 1998;Craw et al, 1999). Thus, the volume of potential ore may be greater when alteration zones can be identified (Craw et al, 1999).…”

“…Turbidite-hosted mesothermal (orogenic) veinhosted gold deposits have traditionally been assumed to have negligible wall rock alteration zones, but research in the past 20 years has shown that these deposits typically have hydrothermal alteration haloes (e.g., Böhlke, 1988;Eilu and Mikucki, 1998;Bierlein and Crowe, 2000;. Recognition of the existence of alteration zones is important, as this increases the size of the target mineralised zone during mineral exploration (Bierlein and Crowe, 2000).…”