Gold distribution in supracrustal rocks from Archean greenstone belts of southern Africa and from Paleozoic ultramafic complexes of the European Alps; metallogenic and geochemical implications

Source of lead in the gold-bearing quartz-fuchsite vein at the Dome mine, Timmins area, Ontario, Canada

“…If gold and lead came from a common source, then, according to the lead isotope signature of the galenas, either the bulk of the gold was the product of the degassing of a typical greenstone belt sequence, or gold was derived from local quartzfeldspar porphyries with a possible small input from local chemical sediments and volcanic rocks. However, unaltered igneous rocks ranging in composition from ultramafic to felsic commonly have gold contents between 2 and 5 ppb (Tilling et al 1973;Kwong and Crocker 1978;Saager et al 1982;Crocker and MacRae 1986), whereas in the Dome mine, the altered quartz-feldsplar porphyries and the mafic to ultramafic volcanic rocks have gold abundances which are 102 to 104 times higher Moritz 1988). Consequently gold must have been introduced into these rocks rather than leached from them during mineralization and alteration.…”

The scale of ore-depositional systems: an important restraint on epigenetic vs remobilized syngenetic origins for Archaean mesothermal gold deposits

“…MACDONALD, 1983;PHILLIPS et al, 1984;FUCHTER & HODGSON, 1986;WYMAN et al, 1986;GROVES et al, 1988c) and those models that implicate remobilization of gold from locally enriched source rocks (e.g. FRIPP, 1976a, b;ANHAEUSSER, 1976;HUTCHINSON, 1976;BARNETT et al, 1982;SAAGER, et al, 1987;STONE et al, 1988) for specific BIF-hosted deposits.…”

“…SAAGER et al (1982) and MEYER & SAAGER (1985) provide high-quality data to suggest that such rocks in the greenstone belts of southern Africa are enriched in Au (>10 ppb), whereas GROVES et al (1988c) present data that indicate normal background Au values (1-2 ppb) in most BIF units in the Yilgarn Block of Western Australia. Even if mineralized BIFs are enriched in Au, it is important to establish whether they are primary high values related to volcanogenic (exhalative) processes or are secondary gold haloes surrounding mineralization as in other gold deposits.…”

“…Conceptual (lateral)-secretion models, involving local derivation of Au and other metals, are highly unlikely given the very low Au contents of most of the host rocks, normally below 1 ppb and seldom above 2 ppb Au (e.g. SAAGER et al, 1982;MEYER & SAAGER, 1985), and the very sharp cut-offs between ore grades and background Au values in enclosing rocks.…”

“…The major object of this paper, however, is to demonstrate that the deposits should be viewed not only at this scale, where interpretations are often equivocal (see, for example the gold deposits of the Gwanda belt, Zimbabwe; FUCHTER & HODGSON, 1986;SAAGER et al, 1987), but also at the regional (province or even craton) scale in the context of other gold deposits and the regional geology. In Western Australia, at least, these regional observations provide definitive evidence for an epigenetic origin, and demonstrate that local secretion models are inappropriate at the scale of the mineralizing system.…”