Metallogeny of Precambrian gold deposits; space and time relationships

Abstract

A study of metallogenic relationships among Precambrian gold deposits provides some broad new insights regarding genetic problems. Three types of deposits are important: greenstone-hosted lodes, paleoplacers, and graywacke- or turbidite-hosted ores. Epithermal deposits are present but are rare and of minor importance.Important gold lodes occur in greenstone belts of all Archean cratons and are a worldwide element ofArchean metallogeny. Their temporal distribution, however, exhibits some enigmatic irregularities. They are apparently more important in late Archean than in early Archean belts, and much less so in otherwise similar early Proterozoic greenstone belts. Early Proterozoic oxygenation of the atmosphere-hydrosphere may explain the latter situation. The deposits are lacking or are very rare in middle-Proterozoic rocks, but they reappear occasionally in late Proterozoic and Phanerozoic orogenic belts. Their origin remains controversial, but broad metallogenic considerations favor original sea-floor hydrothermal exhalative deposition followed by extensive metamorphogenic redistribution rather than a single-stage epigenetic process.Paleoplacer deposits also occur on most Precambrian cratons at the base of Proterozoic successions which rest unconformably on older, deformed, and partially denuded Archean rocks containing the greenstone-hosted lodes. Unlike the latter, however, these deposits are important only in the great Witwatersrand district of South Africa. If weathering of Archcan lodes in a hinterland terrane has provided the gold in the Witwatersrand deposits, then the spatial restriction of this paleoplacer type is unique. Moreover, only in the Witwatersrand district are the paleoplacer ores of an absolute age (2.8-2.6 b.y.) and associated with thick tholeiitic basalts similar to Archean lodes in other cratons. These metallogenic relationships suggest a temporal and perhaps also a genetic link between the otherwise apparently very different paleoplacer and greenstone-hosted ores.The graywacke- or turbidite-hosted lodes occur in late Proterozoic and Paleozoic sequences that are sediment dominated and lack the thick volcanic component of the greenstone-hosted type. Otherwise, they resemble the latter in their geologic, mineralogical, and geochemical characteristics and may represent a later evolutionary variant of it. However, they also exhibit many geologic similarities to deposits of the sediment-hosted or Carlin type, suggesting that they may belong to this last group, rather than to the epithermal deposits with which they are coregional and therefore commonly classified.Epithermal deposits, although rare in Precambrian terranes, are represented by the high-grade, silver-rich veins of the Cobalt and other similar Canadian districts. Their rarity in Precambrian rocks is due partly to erosional removal of these shallow, subaerially formed deposits but partly also to their later, evolutionary development within stabilized continental cratons. Gold-rich strata of the older types within these cratons may have been essential metal sources for the epithermal deposits which formed from hydrothermal systems related to widespread Tertiary subaerial continental igneous activity, either subvolcanic or extrusive.