Archean gold: Relation to granulite formation or felsic intrusions?

Abstract

Two current models for Archean Au deposits appeal to granulitization or magmatic processes, but new data reveal that neither of these schemes is viable. Most tonalitic Archean granulites are characterized by primary large ion lithophile element (LILE) depletion, and so there is no complementary relation to LILE enrichment in Au deposits; nor are K/Rb, K/Ba, K/Li, K/Cs, and Th/U systematics of granulites and gold deposits complementary. Published C-isotope systematics and C/He flux calculations do not support the model of granulitization by influx of mantle C0 2 , nor is the total population of δ 13 C values of Au deposits comparable to mantle CO 2 or the granulite C reservoir. Archean tonalite, trondhjemite, granodiorite (TTG) magmas are characterized by uniform 87 Sr/ 86 Sr, whereas the deposits range between 0.701 and 0.720, and average δ 13 C ranges between -9‰ and -0.4‰ with provinciality of Sr, O, and C isotopic signatures. Moreover, fractionated granites trend to normal K/Rb, but higher Rb/Sr and K/Ba ratios, whereas the gold deposits are characterized by main trend or higher K/Rb and K/Ba ratios, coupled with little Cs, Li, or Tl enrichment and scattered Rb/Sr ratios. Many of the deposits postdate the stocks with which they are associated by 20 to 30 m.y. The variable Sr, Pb, C, and O isotope systematics, but uniform LILE trends, reflect equilibration of internally or externally derived fluids, with compositionally variable crust, at low water:rock ratios, in transpressive accretionary regimes.