Links between abiogenic-methane and gold, indicated by Ar, Ne and Cl in fluid inclusions
M.A. Kendrick1, M Honda2, J.L. Walshe3 and K.J. Petersen4
1 The School of Earth Sciences, The University of
Melbourne, VIC 3010, Australia
The processes that mobilise gold through the Earth's crust and concentrate it in ore deposits have been widely debated. Most workers emphasise the importance of CO2-H2O fluids and phase separation or wall-rock reaction as depositional mechanisms. However, the possible importance of mantle components and the significance of CH4-dominated fluid inclusions have been subject to speculation. The Yilgarn Terrane of Western Australia is richly endowed with gold-only ore deposits that have a low concentration of base metals, and are associated with quartz ± carbonate veins in regionally-significant, mid-crustal (3-15 km), shear zones that formed in an arc or back arc setting. Recent mapping of mineral alteration assemblages in the world class St Ives Gold Camp, in Western Australia, has shown high grade gold occurs preferentially at the intersection of 'oxidised' pyrite-magnetite-hematite-anhydrite bearing veins that are dominated by H2O-CO2 fluid inclusions; and 'reduced' pyrrhotite-pyrite bearing quartz veins that are dominated by CH4 fluid inclusions
H2O- and CO2-dominated fluid inclusion assemblages have maximum 40Ar/36Ar values of ~21,000 (Fig. x-a) and ppb 36Ar concentrations, consistent with the involvement of magmatic fluids. Based on the fluid inclusion abundances, this suggests Cl must be present as HCl in CH4 as well as NaCl in rare H2O fluid inclusions. Provided Cl has a lower abundance in CH4 than in the H2O-CO2 fluid inclusions, this measurement also suggests CH4 has the lowest 36Ar concentration. As wall-rock reaction increases the 36Ar concentration of the volatile phase, this inference precludes a CH4 source by localised reduction of CO2. Instead, the high 40Ar/36Ar value favours an abiogenic CH4 origin in the deep-crust or mantle. The Ne isotope data reveal a mantle component in pre- to early-gold pyrites, but the quartz-hosted H2O and CO2 fluid inclusions are dominated by atmospheric and crustal Ne (Fig. x-b). If all these fluids had a magmatic origin, this pattern is consistent with the changing style of regional magmatism from bimodal (mafic-flesic) prior to mineralisation to dominantly Ca-poor granite during the main-stage of gold deposition. The maximum 21Ne/22Ne value of 0.55 determined for CH4-dominated fluid inclusions corresponds to the maximum 40Ar/36Ar value of ~50,000. The highest 21Ne/22Ne values require a source in which U is hosted by a mineral with an O/F value of greater than the upper-crustal average for U minerals. If CH4 was generated by serpentinisation of deep-crustal mafic intrusions, the Ne data would be consistent with precursor H2O-CO2 fluids derived from lower-crustal rocks in which zircon or pitchblende were important U hosts.
These Ar and Ne data suggest CO2-H2O was derived from a lower crustal magmatic source and conclusively demonstrate that CH4 had an independent 'abiogenic' origin. As CH4 fluid inclusions or graphite are found in many gold-only ore deposits, we suggest that oxidation of abiogenic CH4, possibly sourced from as deep as the Earth's mantle, has been a critical and overlooked control on the formation of many of the planets largest gold deposits.