Petrogenesis of the Squirrel Hills granite and associated magnetite-rich sill and vein complex: lightning creek prospect, Cloncurry district, Northwest Queensland

The Lightning creek Fe-oxide Cu–Au prospect is hosted within a Mesoproterozoic granitoid batholith, composed dominantly of high-K, calc-alkaline, porphyritic quartz monzodiorite. The quartz monzodiorite contains enclaves of quartz diorite and has been intruded by more felsic granitoids (monzogranite and alkali-feldspar granite) and a series of subhorizontal sills. The quartz monzodiorite crystallised at crustal depths in excess of 10 km (Pnot, vert, similar4 kb, T=800–850°C, melt XH2O=>4 wt.%, fO2not, vert, similarNNO buffer), and was probably fluid-saturated (XH2O:XCO2 of >0.5). As might be expected, the more fractionated granitoids crystallised at progressively lower temperatures (quartz monzonite not, vert, similar800°C, alkali-feldspar granite 2.5 kb) and temperatures greater than 500°C. Phase separation occurred in the fluid phase, producing in a CO2-rich vapour and a metal-rich (Fe, Cu) brine