Lithophile Element Systematics of Gold Vein Deposits in Archean Greenstone Belts: Implications for Source Processes
R. Kerrich, 1989. "Lithophile Element Systematics of Gold Vein Deposits in Archean Greenstone Belts: Implications for Source Processes", The Geology of Gold Deposits: The Perspective in 1988, Reid R. Keays, W. R. H. Ramsay, David I. Groves
Download citation file:
Potassic alteration domains of greenstone belt lode gold deposits, typified by the Kerr Addison mine, Ontario, are characterized by systematic partitioning between different groups of incompatible elements such that Al, Ga, Th, U, Ti, and V are decoupled from K, Rb, Ba, Cs, Li, and Tl. The former group of elements have concentrations and interelement ratios in alteration domains which reflect host-rock control, implying isochemical behavior. In contrast, the lithophile elements K, Rb, and Ba are generally coenriched and linearly correlated over three orders of magnitude in abundance, where K/Rb = 230 to 380, and K/Ba = 35 to 100. K/Rb and K/Ba ratios trend toward higher values with respect to increasing concentrations of Rb and Ba, possibly as a result of mixing between host rock and hydrothermal reservoirs of the lithophile elements. K/Cs and K/Tl are weakly correlated, and lithium abundances and Rb/Sr ratios are erratic in altered rocks. These interelement trends, collectively, are present in deposits variously hosted by ultramaflc, mafic, or felsic volcanic rocks, and sediments or granitoids.
Magmatic processes involving crystal fractionation of biotite, K feldspar, and plagioclase generate trends to systematically diminished K/Rb (≤50), K/Li, K/Cs, K/Tl, and Al/Ga but enhanced K/Ba (≥8 x 103) and Rb/Sr in most late-stage differentiates. Such late-stage trends are the rule in magmatophile deposits including the Archean Cadillac molybdenite deposit, Phanerozoic Cu, and Mo porphyry deposits, Sn-W greisens, and most pegmatites. Accordingly, magmatic processes of this type can be ruled out as the dominant source of volatiles for gold-forming systems. In most granulites, especially Archean examples, lithophile element depletion is a primary feature rather than being acquired during carbonic metamorphism. Consequently there is no complementarity between large ion lithophile element-depleted granulites and the K, Rb enrichment characteristic of gold deposits. Moreover, the high K/Rb, K/Cs, and Rb/Cs ratios but low K/Ba ratios of most depleted granulites are not reflected in the gold deposits, where K/Rb and K/Ba ratios approximate average crustal values. The compliance of K/Rb and K/Ba ratios in potassic alteration domains of Au deposits with values characteristic of main trend igneous rocks, or average crust, implies that K, Rb, and Ba were partitioned into the hydrothermal ore-forming fluids in approximately the same ratios as in the source rocks. Dehydration reactions in the source, or equilibration of fluids with source rocks under conditions of low water/rock ratio, rather than purely magmatic or granulitization processes, may satisfy the requirement for proportional K, Rb, and Ba coenrichment.