Authors / CoAuthors
Mernagh, T.P. | Bastrakov, E.N.
Abstract
Until recently, the development of better constrained models for gold mineralisation in the western Lachlan Fold Belt has been hindered by the paucity of data on the chemistry of the ore-bearing fluids, the temperatures and pressures at the time of mineralisation, and the extent and nature of wallrock alteration. Newly available data, however, has allowed us to use thermodynamic mass transfer modelling to test some of the models proposed for gold mineralisation in the western Lachlan Fold Belt and to investigate whether the results of the modelling correlate with the styles of mineralisation and alteration assemblages observed in these turbidite-hosted gold deposits. The first model (cf. Keays, 1987) invokes initial seawater alteration of tholeiitic rocks, followed by leaching of gold from these rocks by a metamorphic fluid. This fluid then ascends into the overlying turbidite succession where fault-valve behaviour leads to phase separation and progressive water-rock interaction. Phase separation results in gold precipitation but the associated mineral assemblage contains significant amounts of feldspar, epidote and prehnite, which is not in accord with observed vein and alteration mineralogies. The third model (cf. Gray et al., 1991; and Cox et al.,1995) proposes that the ore-bearing fluids originated from metamorphic devolatilisation process occurring in deep level crustal rocks. This fluid then ascends to upper crustal levels as before. Phase separation leads to a relatively large decrease in the activity of sulfur in the fluid, resulting in gold precipitation and the precipitation of quartz, muscovite, arsenopyrite with chlorite, feldspar and pyrite forming at lower water-rock ratios. Therefore, this model is the one that best reproduces the vein and alteration assemblages commonly observed in the western Lachlan Fold Belt.
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33408
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- External PublicationAbstract
- Australian and New Zealand Standard Research Classification (ANZSRC)
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- Earth Sciences
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2000-01-01T00:00:00
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