Fluid evolution and origins of iron oxide Cu-Au prospects in the Olympic Dam district, Gawler Craton, South Australia.

Geological observations indicate two stages of hydrothermal activity at a number of currently sub-economic iron oxide-Cu-Au systems in the Olympic Dam district, which contain high- and moderate- to low-temperature Fe oxide-rich hydrothermal alteration. The mineral assemblages include magnetite-calc-silicate-alkali-feldspar +/-Fe-Cu sulphides and haematite-sericite-chlorite-carbonate +/-Fe-Cu sulphides +/- U,REE minerals. In all prospects the minerals of the haematitic assemblages replace the minerals of the magnetite-rich assemblages. Most of the sub-economic Cu-Au mineralisation is associated with the haematitic alteration assemblages. Significant amounts of Cu were transported by the early-stage high-temperature fluids responsible for the magnetite-rich alteration. Sulphur in both alteration assemblages was derived either from cooling magmas and/or crystalline igneous rocks carried by relatively oxidised fluids or from crustal sedimentary rocks. The lack of significant Cu mineralisation associated with magnetite- forming fluids that carried Cu suggests that either there was no effective mechanism for saturation of Cu minerals or the quantity of fluids was not sufficient to produce appreciable mineralisation. Association of the Cu-Au mineralisation with the haematitic alteration can be explained by a two-stage model in which preexisting hydrothermal magnetite with minor associated Cu-Au mineralisation was flushed by late- stage oxidised brines that had reacted with sedimentary or metamorphic rocks.
Geological observations indicate two stages of hydrothermal activity at a number of currently sub-economic iron oxide-Cu-Au systems in the Olympic Dam district, which contain high- and moderate- to low-temperature Fe oxide-rich hydrothermal alteration. The mineral assemblages include magnetite-calc-silicate-alkali-feldspar +/-Fe-Cu sulphides and haematite-sericite-chlorite-carbonate +/-Fe-Cu sulphides +/- U,REE minerals. In all prospects the minerals of the haematitic assemblages replace the minerals of the magnetite-rich assemblages. Most of the sub-economic Cu-Au mineralisation is associated with the haematitic alteration assemblages. Significant amounts of Cu were transported by the early-stage high-temperature fluids responsible for the magnetite-rich alteration. Sulphur in both alteration assemblages was derived either from cooling magmas and/or crystalline igneous rocks carried by relatively oxidised fluids or from crustal sedimentary rocks. The lack of significant Cu mineralisation associated with magnetite- forming fluids that carried Cu suggests that either there was no effective mechanism for saturation of Cu minerals or the quantity of fluids was not sufficient to produce appreciable mineralisation. Association of the Cu-Au mineralisation with the haematitic alteration can be explained by a two-stage model in which preexisting hydrothermal magnetite with minor associated Cu-Au mineralisation was flushed by late- stage oxidised brines that had reacted with sedimentary or metamorphic rocks.