Origin of breccias in the Mount Painter area, South Australia

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Abstract
Field and petrographic studies on granitic, hematitic and chloritic breccias in the central portion of the Mount Painter Inlier, South Australia, indicate that: (i) breccias and brecciated basement extend to depths exceeding 400 m and have gradational contacts; (ii) clasts are mainly autochthonous and contain fine‐scale hematite, chlorite or quartz veinlets and fractures; (iii) K‐metasomatism preceded hematitisation and chloritisation; (iv) hematitic breccia intrudes a pegmatite dyke correlated with the Ordovician Arkaroola Pegmatite; and (v) U, F and REE‐containing minerals are present in the Proterozoic basement rocks, and concentrated in the breccias. With a single exception, δ34S values for pyrite from the breccias and brecciated granites fall in the narrow range —2.9% to +3.5%, implying formation from magmatic emanations or reducing fluids that leached sulphide minerals of magmatic derivation. δ34S values for three barite samples are all close to +16%o, and firm conclusions cannot be drawn from these data. Calcites from the same rock‐types as the pyrite have δ13C values of — 22.3%o to —4.2%o and δ18O values of —4.0%o to +23.1%., with an inverse δ13C/δ18O relationship. The more 13C‐depleted calcites probably incorporated CO2 from organic C, and their δ18O values are compatible with precipitation from magmatic or metamorphic fluids; mixing of such fluids with meteoric waters is implied by the calcites with variably lower δ18O values. The above features indicate that the major processes leading to brecciation and associated metasomatism were hydraulic fracturing and hydrothermal activity resulting from ascent of granitic magmas to shallow crustal levels during late stages (late Ordovician‐?Silurian) of the Delamerian Orogeny. Tectonic and sedimentary processes appear to have played relatively minor roles in breccia formation.