Lithophile-element systematics of Archean greenstone belt Au–Ag vein deposits: implications for source processes

Abstract

Potassic alteration domains of greenstone belt lode gold deposits are characterized by systematic partitioning between lithophile elements. K, Rb, and Ba are generally co-enriched and linearly correlated over almost three orders of magnitude in abundance, where K/Rb = 220–400 and K/Ba = 30–85; K/Cs (~104) and K/Tl (~2 × 104) are also correlated, though more weakly. Lithium abundances and Rb/Sr ratios are erratic in altered rocks. These interelement trends, collectively, are present in deposits variously hosted by ultramafic, 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, and K/Tl but enhanced K/Ba (≤8 × 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. 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 rocks or equilibration of fluids with source rocks under conditions of low water/rock ratio, rather than magmatic processes, may satisfy the requirement for proportional K, Rb, and Ba co-enrichment.