An investigation into chalcopyrite and dolerite dykes at the George Fisher Pb-Zn-(Ag) Mine, NW Queensland, Australia

Abstract

The George Fisher Mine is situated in the Western Fold Belt of the Mount Isa Inlier, NW Queensland, and comprises two separate ore deposits of stratiform and breccia-hosted Pb-Zn-(Ag) mineralisation; Hilton and George Fisher. Both deposits are hosted within the Proterozoic Urquhart Shale and contain anomalous, and potentially economic Cu mineralisation. Chalcopyrite (CuFeS2) is spatially associated with galena (PbS) and sphalerite (ZnS), and gangue Fe-sulphides pyrite and pyrrhotite. The genesis of these deposits has previously been compared to that of the giant Mount Isa Cu-Pb-Zn-(Ag) deposit located 22km to the south, along strike. Two metallogenic models are proposed for ore genesis at George Fisher: syngenetic Pb-Zn (‘SEDEX’) overprinted by epigenetic Cu; or epigenetic Cu-Pb-Zn mineralisation.

This study investigates the mineralogy, textures and association of Cu with the Pb-Zn-(Ag) system and concludes that chalcopyrite is texturally coeval with the main galena and sphalerite mineralisation stage, favouring a co-genetic Cu-Pb-Zn model. Chalcopyrite (Cd:Zn) geothermometry estimates an emplacement temperature of 280—350°C, with a temperature gradient decreasing to the north. Additional Cu is unlikely to be present north of George Fisher however, potential remains for additional (deep) structurally controlled Cu mineralisation at Hilton and south along strike towards the Mount Isa deposit.

This study also investigates the presence and geochemistry of the dolerite dykes within the George Fisher and Hilton deposits. The dolerites are sheared and strongly altered by pre-mineralisation hydrothermal activity and host minor Pb-Zn and Cu mineralisation providing evidence for their emplacement prior to ore mineralisation. Low Ti and overlapping Y/Nb-Zr/Nb and REE fields infer the dolerites share the same mantle source as the Eastern Creek Volcanics (ca. 1779 Ma) but have been produced by a later stage of tectonically-induced melting, most likely associated with deformation during the D2 stage of the Isan Orogeny (~1575 Ma). The presence of pre-ore hydrothermally altered and sheared dykes provides considerable field evidence for epigenetic Cu-Pb-Zn ore genesis, concurring with recent studies of the Mount Isa deposit.