Ore metal redistribution by hydrocarbon–brine and hydrocarbon–halide melt phases, North Range footwall of the Sudbury Igneous Complex, Ontario, Canada

• Published in: Mineralium deposita 2005-11, Vol.40 (3), p.237-256
• Main Authors: Hanley, J. J., Mungall, J. E., Pettke, T., Spooner, E. T. C., Bray, C. J.
• Format: Article
• Language: English
• Subjects: Brines; Crystallization; Hydrocarbons; Metal concentrations; Metals; Quartz; Sodium chloride; Sulfides; Trapping; Veins (geology)
• ISSN: 0026-4598; 1432-1866
• DOI: 10.1007/s00126-005-0004-z

We report methane-dominant hydrocarbon (fluid) inclusions (CH^sub 4^±C^sub 2^H^sub 6^-C^sub 2^H^sub 2^, C^sub 3^H^sub 8^) coexisting with primary brine inclusions and secondary halide melt (solid NaCl) inclusions in Au-Pt-rich quartz-sulfide-epidote alteration veins associated with the footwall-style Cu-PGE (platinum-group element)-Au deposits at the Fraser Mine (North Range of the Sudbury Igneous Complex). Evidence for coentrapment of immiscible hydrocarbon-brine, and hydrocarbon-halide melt mixtures is demonstrated. A primary CH^sub 4^-brine assemblage was trapped during quartz growth at relatively low T (min. T ^sub trapping^145-315°C) and P (max. P ^sub trapping^500 bar), prior to the crystallization of sulfide minerals in the veins. Secondary inclusions contain solid halite and a mixture of CH^sub 4^, C^sub 2^H^sub 6^-C^sub 2^H^sub 2^ and C^sub 3^H^sub 8^ and were trapped at a minimum T of 710°C. The halite inclusions may represent halide melt that exsolved from crystallizing sulfide ores that texturally postdate (by replacement) early alteration quartz hosting the primary, lower T brine-CH^sub 4^ assemblage. Laser ablation ICP-MS analyses show that the brine, hydrocarbon and halide melt inclusions contain significant concentrations of Cu (0.1-1 wt% range), Au, Bi, Ag and Pt (all 0.1-10 ppm range). Cu:Pt and Cu:Au ratios in the inclusions are significantly (up to 4 log units) lower than in the host alteration veins and adjacent massive sulfide ore veins, suggesting either (1) early Cu loss from the volatiles by chalcopyrite precipitation or (2) enhanced Au and Pt solubilities relative to Cu at the temperatures of entrapment. Concentration ratios between coexisting brine and CH^sub 4^ inclusions(ProQuest: Formulae and/or non-USASCII text omitted; see image) are lower for Cu, Au, Bi and Ag than for other elements (Na, Ca, Fe, Mn, Zn, Pb) indicating that during interaction with the brine, the hydrocarbon phase was enriched in ore metals. The high concentrations of ore metals in hydrocarbon, brine and halide melt phases confirm that both aqueous and non-aqueous volatiles were carriers of precious metals in the Sudbury environment over a wide range of temperatures. Volatile evolution and magmatic sulfide differentiation were clearly part of a single, continuous process in the Sudbury footwall. The exsolution of H^sub 2^O-poor volatiles from fractionated sulfide liquid may have been a principal mechanism controlling the final distribution of PGE and Au in t