Preferential magma extraction from K- and metal-enriched source regions in the crust |
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| Authors: | Andrew G Tomkins Roberto F Weinberg Chris R M McFarlane |
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| Institution: | (1) School of Geosciences, Monash University, PO Box 28E, Victoria, Australia, 3800;(2) Research School of Earth Sciences, Australian National University, A.C.T., Australia, 0200 |
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| Abstract: | We compare melting of potassic alteration zones in metamorphosed gold deposits with that of unaltered rocks of the same protolith
to examine their relative contributions to crust-derived magmas and to investigate the implications for ore genesis. Potassic
hydrothermal alteration, at the crustal levels where orogenic gold deposits form, stabilizes a higher proportion of muscovite
and biotite than is possible in unaltered rocks at high metamorphic grades. Because these micas contain water, they control
the melt fraction generated through dehydration melting in that a greater proportion of micas permits more extensive melting.
Orogenic gold deposits, in which mineralization is typically encapsulated by potassic alteration, form at deep-enough crustal
levels to survive repeated tectonic activity, which can lead to their being metamorphosed. In the vicinity of this metamorphosed
gold mineralization, the greatest proportion of felsic melt is generated in the more metal- and sulfur-rich rocks because
of the associated potassic alteration. Ore minerals dissolve and are physically incorporated into the resulting felsic melt,
which thereby becomes metal- and sulfur-enriched. Since melt fraction is the dominant control on strain partitioning and melt
mobilization, increased melting in K-altered mineralized rocks implies that these sites will be the first to experience melt
escape and will continue to be the focus of melt escape during ongoing metamorphism. This strain partitioning promotes shear
zone development, and once shearing is localized to K-altered mineralized domains, it may attract external magma, allowing
extension and linking with nearby active shear zones. In this way, mineralized zones may connect to a regional network of
magma transfer, allowing metal enrichment of migrating magmas. Terrains that underwent widespread K alteration associated
with mid-crustal gold enrichment are likely, when metamorphosed, to produce significant volumes of reduced, relatively metal-
and sulfur-enriched felsic magma. The ages and relative tectonic preservation potential of different K alteration-associated
ore types implies that Au, Ag, As, Sb, Bi, Te, and W may be recycled within the crust through this mechanism, whereas Cu and
Mo are unlikely to be recycled and require mantle sourcing to form new intrusion-related ores. Silicate melt derived from
preexisting zones of gold enrichment in the lower crust may contribute significantly to the metal budget of intrusion-related
gold systems, and possibly some gold-rich porphyry deposits. |
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| Keywords: | Ore genesis Source Anatexis Granite metallogeny Intrusion-related gold |
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