Gold, Lamprophyres, and Porphyries: What Does Their Association Mean?
Nicholas M. S. Rock, David I. Groves, Caroline S. Perring, Suzanne D. Golding, 1989. "Gold, Lamprophyres, and Porphyries: What Does Their Association Mean?", The Geology of Gold Deposits: The Perspective in 1988, Reid R. Keays, W. R. H. Ramsay, David I. Groves
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Extremely intimate space-time associations between caic-alkaline (shoshonitic) lamprophyres and mesothermal gold deposits are now confirmed worldwide and from Archean to Tertiary times. They include the late Archean gold deposits of the Superior province (Canada) and the Norseman-Wiluna belt (Western Australia), which hosts the world’s most golden square mile (Kalgoorlie) and almost certainly one of the richest gold deposits outside South Africa, (Porgera, Papua New Guinea). In an increasing number of areas, lamprophyres are found to be the only igneous rocks emplaced at the same time as the gold, and economic status has been found to correlate quantitatively with the presence of lamprophyres. Many lamprophyric rocks appear to be enriched in Au relative to other igneous rocks, with contents of tens of ppb Au being common. Evidence that such enrichments could be primary includes the following, although this will remain impossible to prove for lamprophyres in major gold fields: (1) the persistence of high Au contents in calc-alkaline lamprophyres which lie outside the alteration halos of large-scale gold systems (e.g., British Caledonides) and more especially in lamprophyric rocks which are not associated with gold deposits at all (e.g., lamproites); (2) the plausible explanation for Au enrichment that exists in the lamprophyres’ exceptionally deep origins in presumed Au-rich regions of the earth (>150 km), high F, K, Ba, and Rb, moderate S contents, and H2O/(H2O + CO2 ratios, and fluidized condition, which make them uniquely similar to auriferous ore fluids in their element abundances and possibly in their physical state, and thus, well suited to transporting gold into the crust; and (3) detailed statistical analysis of Au data, notably for the Superior province, comparing fresh versus altered and proximal versus remote samples from gold mineralization to rule out any overall addition of Au to these lamprophyres from external sources. The widespread presence of lamprophyres suggests a much more significant role in gold deposition for large-scale crust-mantle events and for certain (e.g., oblique subduction) tectonic regimes than has hitherto been generally recognized. A part-genetic, part-structural interpretation is preferred, in which lamprophyres may contribute at least some Au or fluids from deep sources into mesothermal systems, which then redeposit the Au according to the metamorphic model in its broadest sense. This is in accord with stable isotope evidence, which argues against a direct relationship between lamprophyric and gold-depositing fluids but by no means precludes an indirect one.