Proterozoic geology and mineralisation of the Greenmount Cu-Au-Co deposit, Cloncurry District

The Greenmount deposit is hosted by mid-Proterozoic graphitic and carbonaceous slates of the Marimo Slate near the contact with the calcareous and evaporitic metasediments of the Staveley Formation in the southern Marimo Basin, some 40 km south of Cloncurry. A diorite intrudes the sequence and is altered and veined but not mineralised. Late brittle faults fragmented the geology and earlier tight D2 folds. The area around Greenmount is particularly disjointed and structurally complex. Alteration and mineralisation was localised in a \flat\" ramp within a reverse fault/shear regime and veining and mineralisation was in a dominantly brittle to brittle-ductile regime. The Marimo Slate the Staveley Formation and the diorite underwent alkali-rich metasomatism dominated by microcline with subordinate albite sericite (retrogressed microcline) and lesser hematite rutile tourmaline quartz dolomite ¬¨¬± sulphides ¬¨¬± magnetite. Episodic veining comprises a stockwork of millimetre to metre wide veins which in the Marimo Slate is dominated by microcline with lesser quartz albite phlogopite apatite ferroan dolomite and sulphides and in the Staveley Formation is dominated by microcline dolomite calcite with lesser albite muscovite pyrite biotite magnetite and chalcopyrite. Vein density generally decreases in the Staveley Formation away from the contact with the Marimo Slate. Mineralisation occurred syn-to post-veining and is characterised by elevated Au Cu Co As Mo W ¬¨¬± Pb ¬¨¬± Zn ¬¨¬± Bi ¬¨¬± Cr ¬¨¬± U ¬¨¬± Rb ¬¨¬± Ba. The most important economic metals are Au Co and Cu. Sulphide mineralogies are dominated by pyrite and chalcopyrite with lesser cobaltite and minor marcasite. Almost all of the mineralisation is hosted by the Marimo Slate. Mineralisation and vein related sulphur isotope values generally cluster between 0 to +16%0 (mean 8.4%0) while diorite hosted sulphur is relatively lighter (mean -3.8%0) and diagenetic black slate hosted sulphur has a Proterozoic sea water signature (mean 19.1%0). The hydrothermal system was sulphur deficient. Fluid inclusions contain H20 ¬¨¬± CO2 (dominant) ¬¨¬± CH4 (minor)-¬¨¬± complex daughter salts. Combined with the wide range of minimum homogenisation temperatures lack of phase separation evidence and two salinity populations it would appear that fluid mixing of two different fluids occurred. One would have been a hotter (‚ÄövÑvÆ 320 - 500¬¨‚àûC) hypersaline fluid and the other was a cooler (220 - 360¬¨‚àûC) lower salinity fluid. The overall trend appears to be one of decreasing temperature and salinity with increasing carbonate deposition later in the paragenesis. Although the Cloncurry district black shale hosted deposits share some characteristics with the other slate belt and black shale hosted deposits they do generally differ as a group. The reason for the differences are that the ore fluids in the district are oxidised and highly saline as a group which supplies alkalis for the extensive alkali alteration and veining. Another fundamental difference is that gold transport is via a chloride rather than bisulphide complex and the high chloride content also permits Cu¬¨¬±Co transport. Pb - Zn - Ag deposition is restricted by the amount of sulphur available in wallrocks. Veining and mineralisation is generally post-folding in a dominantly brittle regime. These deposits thus form a distinct subclass which may be described as oxidised alkali-rich slate belt Au¬¨¬±Cu-Co deposits. Other deposits such as Muruntau Bidjovagge and Blackbird may be included in this subclass."