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

Abstract

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 ( P∼4 kb, T=800–850°C, melt X H 2O =>4 wt.%, f O 2 ∼NNO buffer), and was probably fluid-saturated ( X H 2O : X CO 2 of >0.5). As might be expected, the more fractionated granitoids crystallised at progressively lower temperatures (quartz monzonite ∼800°C, alkali-feldspar granite <800°C). Subsolidus re-equilibration took place at temperatures between 700 and 600°C under increasingly oxidising conditions (amphiboles become increasingly TiO 2-poor and MgO-rich). Although there is abundant petrographic evidence for magma mingling in the petrogenesis of this granitoid suite, the curvilinear nature of many of the chemical trends indicates an additional contribution from crystal fractionation. Sm–Nd isotopic characteristics ( εNd=−2.7 to −3.3) indicate an older crustal component in the source region. Sill emplacement was contemporaneous with the development of a large magnetite-rich vein system. The sills are remarkable for their mineralogical and textural complexity. In addition to aplitic rocks, there are zones of albite–magnetite–quartz rock, which display a variety of unusual spherulitic textures. These Fe-rich portions of the sills are marginally younger than the aplites and are mineralogically similar to the magnetite-rich veins. The aplites are interpreted as late-stage differentiates of the plutonic suite, with which they share many mineralogical and compositional characteristics. It is argued that the textural and compositional variation within the sills reflect crystallisation under different degrees of melt saturation, and that the Fe-rich spherulitic rocks reflect the transition from magmatic to hydrothermal conditions within the crystallising pluton. Fluid inclusion studies indicate that this transition occurred at (hydrostatic?) pressures in excess of 1.5 kb (and possibly >2.5 kb) and temperatures greater than 500°C. Phase separation occurred in the fluid phase, producing in a CO 2-rich vapour and a metal-rich (Fe, Cu) brine.