Gold and metal enrichment in natural granitic melts during fractional crystallization

Abstract

Metal evolution in a composite granitic pluton was tracked by analyzing melt inclusions in 11 quartz samples from 7 zones at the Timbarra gold deposit, Australia. We present the first quantitative microanalyses of gold (Au) in granitic silicate melt inclusions using laser ablation inductively coupled plasma mass-spectrometry and show how Au and other metals become enriched during fractional crystallization in a granite intrusion. Au was enriched during fractionation from a monzogranite to a highly fractionated alkali-feldspar granite. Similar enrichment behavior for other metals implies that no gold-enriched precursor melt is required and fractional crystallization can enrich the Au concentration to economic levels. The low content of accessory oxides and sulfides, the absence of early Clbearing fluids, the volatile content in the melt, and a prolonged crystallization constitute important factors for extensive metal enrichment during crystal fractionation. These characteristics play a crucial role in felsic, highly fractionated plutons and their associated deposits such as intrusion-related Au deposits. The gold enrichment during fractionation also implies that Au is directly sourced from the granites.

The partitioning behavior of As and Au in S-free and S-bearing magmatic assemblages

“…Arsenic was not quantified in the vapor inclusions. Assuming a Au concentration on the order of 0.1 lg/g in the parental melt at Bajo de la Alumbrera (Mustard et al, 2006) suggests a value for the vapor/melt partition coefficient for Au on the order of 5. These fluid inclusion data from porphyry-type ore deposits, coupled with chemical data from fumarole emissions, demonstrate that significant quantities of As and Au may be scavenged by magmatic volatile phases and transported into the overlying superjacent environment.…”

Gold solubility in oxidized and reduced, water-saturated mafic melt

“…(2) the non-mutually exclusive process of siderophile element enrichment in residual liquids, driven by the crystallization and differentiation of sulfide-free basaltic magmas (Mustard et al, 2006); and (3) the large degree of partial melting of asthenospheric mantle that fully incorporates the stable mantle sulfide phase(s) into a sulfide undersaturated silicate liquid. Mantle derived silicate liquids are widely accepted as the primary agent of chemical mass transfer for Au and other siderophile elements from mantle to crustal reservoirs.…”

Direct Analysis of Ore-Precipitating Fluids: Combined IR Microscopy and LA-ICP-MS Study of Fluid Inclusions in Opaque Ore Minerals

“…12b-f) can be interpreted as due to heterogeneous trapping of solid inclusions in which gold is present. A similar "nugget effect" for high Au values was recently reported for Au-bearing magmatic quartz-hosted melt inclusions from the Timbarra gold deposit, Australia, also studied by LA-ICP-MS (Mustard et al, 2006) and in hydrothermal fluid inclusions from the Famatina Cu-Mo-Au system, Argentina (Pudack et al, 2006). Considering the suite of elements that correlate positively with Au, we conclude that the trapped microparticles are AgAu tellurides or selenides.…”