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
2 Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia
3 CSIRO, ARRC, 35 Stirling Highway, Kensington, WA 6151, Australia
4 School of Earth and Geographical Sciences, The University of Western Australia, WA 6009, Australia

Figure 1. Fluid inclusion noble gas data for pre- to syn-gold minerals of the St Ives Gold Camp, Western Australia: a) Cl/³⁶Ar versus ⁴⁰Ar/³⁶Ar; b) ²¹Ne/²²Ne versus ²⁰Ne/²²Ne. The high Cl/³⁶Ar values determined for CH₄ fluid inclusions suggest Cl is present as HCl and imply a parts per trillion 36 Ar concentration. CO₂-H₂O fluid inclusions have higher ³⁶Ar concentrations. The convergence of mixing trends and increase in ³⁶Ar concentration is interpreted to result from fluid interaction with mafic host-rocks rich in seawater-derived noble gases. Note that few of the deeply-derived fluids have Ne isotope compositions within the light grey envelope that could be explained by mixing atmospheric or mantle Ne with 'average-crustal' Ne.  This suggests Ne-isotope heterogeneity in the lower crust; assuming an atmospheric intercept, the best fit slope for CH₄ indicates a source region in which U-minerals have an O/F value of close to the average crustal O/F value of 752.<

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.