GEOLOGY AND MINERALIZATION OF THE GREENMOUNT CU-AU-CO DEPOSIT, SOUTHEASTERN MARIMO BASIN, QUEENSLAND

The Greenmount deposit is hosted by mid-Proterozoic graphitic and carb onaceous slates of the Marimo Slate within tens of metres of the conta ct with the calcareous and evaporitic metasediments of the Staveley Fo rmation in the southern Marimo Basin, some 40 km south of Cloncurry. A diorite intrudes the sequence and is altered and veined but not miner alised. The area around Greenmount is particularly disjointed and stru cturally complex, Late brittle faults fragmented the rocks and earlier tight D-2 folds. Alteration and mineralisation was localised in or ne ar to a `flat' ramp (subparallel to the formational boundary) within a reverse fault/shear regime, and veining and mineralisation was in a d ominantly brittle to brittle-ductile regime. The Marimo Slate, the Sta veley Formation and the diorite underwent regional- and deposit-scale alkali-rich metasomatism dominated by microcline with subordinate albi te, sericite (retrogressed microcline) and lesser hematite, rutile, to urmaline, quartz, dolomite +/- sulfides +/- magnetite. The underlying Staveley sequence contains 'red-rock' (hematite-microcline) alteration and intraformational breccias, The Marimo Slate often hosts `whiteroc k' (microcline-quartz-pyrite) alteration, These extensive alteration z ones are overprinted by episodic vein assemblages at Greenmount. Episo dic veining comprises a stockwork of millimetre-to metre-wide veins, w hich in the Marimo Slate is dominated by microcline with lesser quartz , albite, phlogopite, apatite, ferroan dolomite and sulfides, and in t he Staveley Formation is dominated by microcline, dolomite, calcite wi th lesser albite, muscovite, pyrite, biotite, magnetite and chalcopyri te. Vein density generally decreases in the Staveley Formation away fr om the contact with the Marimo Slate. Mineralisation occurred syn- to post-veining. The most important economic metals are Au, Cu and Co. Su lfide mineralogies are dominated by pyrite and chalcopyrite with lesse r cobaltite and minor marcasite.