The Lightning Creek sill complex, Cloncurry district, northwest Queensland: A source of fluids for Fe oxide Cu-Au mineralization and sodic-calcic alteration

The Lightning Creek Cu-Au prospect is hosted by a cogenetic suite of pluton ic, I-type granitoids. The dominant rock type is a porphyritic quartz monzo diorite that is intruded by more fractionated rocks, including monzogranite and alkali feldspar granite. A series of flat-lying sills are interpreted to be late-stage differentiates, based on their timing, mineralogy, and che mistry In parts of the prospect there is pervasive sodic-calcic alteration (pyroxe ne after amphibole, albite after K feldspar and oligoclase) of the plutonic rocks. This alteration predates sill emplacement and is unrelated to veini ng or fracturing of any kind. The presence of small amounts of carbonate in the altered rocks suggests that the fluids were CO2, bearing. Quartz and f eldspar separates from these altered rocks have oxygen isotope compositions similar to those from fresh quartz monzodiorite, suggesting that the fluid s were hot and of magmatic composition. Sodium and Ca were added, and K, Fe , Cl, and Cu were stripped during what is interpreted as an autometasomatic event. The sills display considerable textural and mineralogical complexity and ev olved from equigranular, quart-zofeldspathic rocks (aplites), with magmatic chemistry, to unusual Fe-rich rocks (albite-magnetite-quartz) that exhibit a range of bizarre spherulitic textures. Some of the albite and magnetite in the sills is secondary. Albite forms pseudomorphs after K feldspar (Na-F e +/- Ca alteration) along sill margins and within sills, at the contacts b etween different textural zones. Halos of disseminated magnetite + clinopyr oxene (Fe-Ca +/- Na alteration) are developed adjacent to early magnetite v eins. Fluid inclusion studies indicate that these rocks crystallized at temperatu res in excess of 500 degrees C and at pressures in excess of 1.5 kbar. The range of spherulitic textures is taken to indicate crystallization under hy drous conditions with the episodic release of a fluid phase. This magmatic fluid phase was dominated by H2O, CO2, and chlorine and underwent phase sep aration into a CO2-rich vapor and a hypersaline brine (33-55 wt % NaCl equi v). The hypersaline fluid was enriched in Fe (similar to 10 wt %) and Cu (s imilar to 1 wt %, PIXE analysis), in addition to Na, K, and Ca. Where this fluid was retained within Fe-rich portions of the sills, it caused Ca-Fe +/ - Na alteration (pyroxene-albite +/- magnetite growth at the expense of qua rtz). Where the fluid was expelled from the sills, it produced quartz-magne tite +/- clinopyroxene +/- albite veins (broadly coeval with the early magn etite veins). Although rich in Cu, these granitoid-derived magmatic fluids did not generate significant Cu(-Au) mineralization, perhaps because of the high temperatures involved and/or a lack of reduced sulfur in the fluids o r host rock. However, the amount of iron present is estimated (from the aer omagnetic anomaly) to be in excess of 2,000 million tonnes (Mt). A later generation of calcite +/- chlorite +/- pyrite +/- chalcopyrite vein s contain traces of Cu-Au mineralization. Fluid inclusion and stable isotop e work indicate that these veins probably crystallized from cooler (<200 de grees C), more dilute (15-28 wt % NaCl equiv) fluids, perhaps generated by the admixture of a meteoric component. The conclusions reached in this study have implications for understanding t he genesis of Fe oxide Cu Au deposits and related sodic-calcic alteration. The study indicates the potential for CO2-rich granitoid magmas to evolve h ypersaline, Fe- and Cu-rich fluids capable of causing intense magnetite vei ning and Cu(-Au?) mineralization. Autometasomatic sodic-calcic alteration o f the granitoids may be an important precursor to mineralization, contribut ing Fe, K, Cu, and Cl to the magmatic fluids.