The evolution of apatite in iron-oxide-copper-gold mineralization of the Olympic Cu-Au Province: unraveling magmatic and hydrothermal histories through changes in morphology and trace element chemistry

Abstract

Iron-oxide-copper-gold (IOCG) mineralization is expressed in various forms across some 700 km of the eastern Gawler Craton throughout the N-S striking Olympic Cu-Au Province. In all instances, IOCG mineralisation and the rocks that host it contain variable concentrations of apatite with varying morphological and chemical characteristics. A large body of work has demonstrated apatite's ability to chemically reflect the physiochemical conditions under which it formed and act as tracers of magmatic and hydrothermal processes. This is confirmed throughout the IOCG deposits and prospects studies as part of this work. Magmatic apatite hosted within the Roxby Downs Granite, the dominant host to the Olympic Dam deposit displays characteristics indicative of a complex magmatic history. Namely, nano-scale, oriented inclusions of pyrrhotite and fluorite within the cores of apatite closely associated with mafic enclaves are indicative of the granites protracted interaction with mafic melts. Their chondrite-normalized rare earth element (REE) fractionation trends are light REE (LREE) enriched and vary when altered by hydrothermal fluids along with the concentrations of several other elements. Magmatic apatite hosted in other intrusives displays similar behaviour when altered, but contains higher concentrations of Cl, Sr, and lower Mn which vary systematically with regards to bulk rock basicity. Many of the deposits and prospects within the Olympic Cu-Au Province exhibit a chemical and mineralogical zoning grading from early, reduced and later, oxidized hydrothermal assemblages as evidenced by changes in the dominant Fe-oxide, Cu-Fe-sulphide species and as we report herein, changes in apatite. Within the early, reduced, high-temperature expressions of IOCG mineralisation throughout the Province, apatite is abundant, making up, alongside magnetite, the bulk of the mineralisation. Such apatite is dominantly near end-member fluorapatite characterized by LREE-enriched chondrite-normalized signatures and variable but measurable concentrations of S and Cl. Overprinting of the magnetite-dominant reduced assemblages by later oxidised hematite-sericite altering fluids results in LREE-loss within the early, hydrothermal and magmatic apatite. Such hematite-sericite altered zones along with newly formed apatite display middle REE (MREE) enriched signatures and are devoid of many of the other trace elements present in magmatic and early hydrothermal apatite, such as S and Cl. This behaviour is observed within the Olympic Dam deposit, as well as the Wirrda Well and Acropolis prospects. Late apatite hosted within the high-grade massive bornite mineralisation of Olympic Dam and within chalcopyrite-barite-rich zones of the Acropolis prospects displays extreme MREE-enriched REE-signatures with positive Eu-anomalies. The latter characteristic is unique amongst all other apatite examined as part of this study and highly anomalous globally. Numerical modeling shows that the evolution in apatite trace elements, and in particular REE signatures is the direct result of fluid evolution within IOCG systems. Given this association, the various assemblages within IOCG systems are classified according to REE-signature and the use of apatite in mineral exploration and as a petrogenetic tool is discussed in detail. The modeling of REE behaviour in hydrothermal fluids typical of IOCG mineralised systems has offered important insights into the transport and deposition of REE within Olympic Dam and possibly other IOCG systems. Specifically, REE are transported primarily as REE-Cl species and deposited under conditions of suppressed REE-Cl activity. The propensity of the LREE to occur as Cl-complexes explains both their significantly greater enrichment in IOCG systems when compared to the HREE, as well as their preferential depletion in apatite during hematite-sericite alteration.