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Open File Report 30

The mineralogical and geochemical effects of weathering in mafic and ultramafic profiles, Mt Magnet, WA

Scott, K.M. and Martinez, A.

Material from five drill holes located up to 300 m west of the Parkinson Pit have provided a suite of mafic volcanic rocks with which the effects of proximity to mineralization and alteration in the weathered zone can be determined. Fresh barren rocks consist of assemblages of calcite + dolomite + albite + chlorite ± minor mica and more proximal rocks, dolomite + chlorite + paragonite + muscovite whereas altered and mineralized assemblages consist of siderite + dolomite + chlorite + muscovite. With weathering, the carbonates, chlorite and albite break down to Fe oxides, kaolinite + smectites but micas remain essentially unaffected. Thus, within weathered profiles, increasing proximity to mineralization is reflected by the progression minor mica -> abundant paragonite + muscovite -> abundant muscovite i.e. increasing K contents. The relatively low abundances of other Au pathfinders (As, B, Mo, Sb and W) at Mt Magnet suggests that high muscovite (or K) contents are probably the best guide to mineralization in this area.

The absence of talc high in a weathered ultramafic profile suggests, by comparison with ultramafic profiles from the Eastern Goldfields, the presence of alteration. This is confirmed by the presence of some K-rich zones within the profile and elevated abundances of the Au pathfinders, As, Mo, Sb and Sn, in the surficial calcrete zone of the profile.

Last updated: Tuesday, January 04, 2000 03:54 PM


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