Thermodynamic modelling of lode-gold deposits in Archaean granitoids: Woodcutters and Lady Bountiful mines, Kalgoorlie region, Western Australia
Title:
Thermodynamic modelling of lode-gold deposits in Archaean granitoids: Woodcutters and Lady Bountiful mines, Kalgoorlie region, Western Australia
Author:
Mason, D. R.
Appeared in:
Australian journal of earth sciences
Paging:
Volume 51 (2004) nr. 3 pages 431-438
Year:
2004-06
Contents:
Granitoid rocks host relatively few Archaean lode-gold deposits, leading to the suggestion in the literature that granitoids may be geochemically less suitable host rocks because of lower Fe content compared with greenstones. Thermodynamic modelling is used to clarify alteration assemblages including gold deposition in two Archaean lode-gold deposits in the Kalgoorlie district, Western Australia: Woodcutters and Lady Bountiful. Modelling of the Lady Bountiful deposit reproduces the proximal alteration assemblage characterised by sericite-chlorite-carbonate-pyrite at the published fluid composition and P-T conditions (P ∼200 MPa, T ∼300°C). In contrast, modelling of the Woodcutters deposit fails to produce the proximal muscovite-biotite-pyrite alteration assemblage at the published temperature of 270°C, but successfully generates that assemblage at T = 440°C, which is consistent with a reinterpretation of the published fluid-inclusion data and with known stability of biotite at and beyond middle greenschist conditions. Epidote-bearing alteration 'halos' enclosing the Woodcutters structures are shown to be incompatible with the CO2-bearing mineralising fluid, and are reinterpreted as having formed at an earlier time from CO2-poor fluid. Evaluation of the effect of variable abundance of FeOtotal on deposition of native gold and pyrite in Woodcutters granodiorite and monzodiorite indicates that virtually identical abundances of native gold at significant grade are formed in both granitoids, and that maximum deposition of native gold and associated pyrite is achieved in both granitoids at relatively low Fe content (FeOtotal ∼1.0 wt%). These results confirm that most granitoids represent suitable geochemical traps for deposition of lode-gold mineralisation and therefore represent exploration targets, given the presence of appropriate structures for ingress of mineralising fluid.
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