Magmatic sulphide/volatile interaction as a mechanism for producing chalcophile element enriched, Archean Au-quartz, epithermal AuAg and Au skarn hydrothermal ore fluids

Abstract

A combination of available geological, geophysical and geochemical evidence indicates that significant parts, or components, of Archean Au-quartz vein, epithermal AuAg and Au skarn ore fluids were magmatically derived, and that Au, Ag and associated, enriched elements were also magmatically derived (e.g., Giggenbach, 1986; Henley and Hoffmann, 1987; Sillitoe, 1989; Sillitoe and Bonham, 1990; Spooner, 1991 a,b). There is a significant core of commonality in element occurrence in Archean Au-quartz vein, Archean Hemlo-type, epithermal AuAg and Au skarn systems which suggests a shared genetic process. For instance, 44% ( n=12) of 27 elements occur in more than seven of ten ore deposit subdivisions: Au, Ag, Fe, Cu, Pb, Zn, Hg, Mo, Sb, As, Te and S (nine metals; two metalloids: one non-metal). Addition of B and W gives 52% ( n=14) in more than five subdivisions. The frequency of occurrence of enriched chalcophile/siderophile elements is high at 85%; 23 out of 27. The converse is also high at 74% (23 of 31): all significant chalcophile/siderophile elements occur enriched in Archean Au-quartz vein, Archean Hemlo-type, epithermal AuAg and Au skarn deposits. However, if magma volatile saturation occurred after gravitational loss of magmatic sulphide melt/suspended solids, then Au, Ag and associated chalcophile elements would have been effectively removed from the magma because of known high sulphide-melt/silicate-magma partition coefficients of ∼ 10 2–10 4 (e.g., Au at ∼ 3×10 4; Se at ∼ 10 3). Hence, it is proposed that Archean Au-quartz vein, Archean Hemlo-type, epithermal AuAg and Au skarn ore fluids derive their characteristic element enrichments by high-temperature fluid/vapour dissolution of magmatic sulphide-liquid droplets/solids ( T<∼ 1010–1050°C) enriched in Au and associated elements (e.g., 1–13 ppm Au in MORB sulphide globules; Mathez and Peach, 1989) in mafic/intermediate igneous compositions before sulphide loss. The key components of the mineralization enrichment process are sulphide melt/silicate magma partition, during which ore-grade Au concentrations can be produced from non-anomalous background Au abundances of ∼ 1 ppb (enrichment factor=∼ 10 4), high aqueous-phase sulphide solubilities at high temperatures (e.g., Hemley et al., 1986; to 600°C), and selective Au precipitation. Support for this hypothesis is provided by: (a) the element association in Au skarns which shows 81% and 75% overlaps with associations in epithermal AuAg and Archean Au-quartz vein systems, respectively; (b) being able to explain enrichments of a variety of low-abundance elements simply because they are chalcophile (e.g., Se, Tl; Sb, Hg, Bi, As, Te; Ni, Co, Cd, Pd, Pt; Mo, W as scheelite); and (c) being able to explain a tendency for certain large AuAg systems to occur associated with volatile-rich (e.g., subduction zone magmas, 3–7 wt% H 2O; sodic alkaline magmas, ∼ 0.5–1 wt% H 2O; c.f., ∼ 0.1 wt% H 2O in MORB), more mafic/intermediate magma compositions (e.g., Au skarns, Meinert, 1989; Hollinger-McIntyre, ∼ 995 tonnes Au; Ashanti, ∼ 710 tonnes Au; Cripple Creek, ∼ 590 tonnes Au; Ladolam, ∼ 570 tonnes Au; Porgera, ∼ 410 tonnes Au; Kerr Addison-Chesterville, ∼ 335 tonnes Au; Emperor, Fiji, ∼ 120 tonnes Au). On the basis of well-defined igneous intrusive associations, the typical presence of H 2OCO 2 low-salinity ore fluids, and particularly the presence of carbonate ( δ 13C=−4.5±0.6‰) and a chalcophile element association (AuAgSFeCuPbZnMoWAsSbBiTe) which is very similar to those of Archean Au-quartz vein, epithermal AuAg and Au skarn ore systems, a magmatic origin for Cordilleran Au-quartz vein ore systems may be assessed (e.g., Sierra Nevada Au province, California; ∼ 3,150 tonnes Au; Clarke, 1970). This proposal can explain why Archean Au-quartz and, particularly, epithermal AuAg systems occur associated with several different magma series such as calc-alkaline, shoshonitic, sodic alkaline with and without TiNbTa subduction depletions, and components of the Archean tonalite-trondhjemite-granodiorite (TTG) association. These series share a common property of being relatively volatile-rich, thus causing magmatic volatile saturation in more mafic compositions before loss of magmatic sulphide. This process is essentially unavoidable, and should have repeated in space and time. Implications for mineral exploration include a justification for large-tonnage, low-grade Au targets in, or related to, mafic/intermediate intrusions in Archean terrains (e.g., Moss Lake, N.W. Ontario; diorite hosted, >65 mtonnes at ∼ 1 g/tonne Au; >65 tonnes Au) and a justification for evaluating mafic, sodic and potassic alkaline complexes.