Untitled Document
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.