Feldspar Petrography and Geochemistry: Insights into the Magmatic-Hydrothermal Evolution of IOCG Systems from South Australia

Abstract

The iron-oxide copper-gold (IOCG) class of mineral deposits is characterized by, among other properties, widespread alkali alteration and an enrichment of multiple trace elements, including REE and U. The ability of feldspar replacement reactions to alter physical and chemical properties of host rocks invokes the idea that the early widespread alkali alteration stage within IOCG genesis may prove critical in the subsequent enrichment of trace elements within such magmatic-hydrothermal systems. Based on detailed petrographic investigations and geochemistry, the link between these two fundamental IOCG properties is tested in one of the largest IOCG metallogenic belts in the world, the Olympic Cu-Au Province. Firstly, the temporal evolution of a deeper-style IOCG system (Moonta-Wallaroo) is tracked from protolith through to early and late mineralization stages using mineral reactions associated with regional alkali alteration (namely, albitization), their relative textures and chondrite-normalised REE+Y (hereafter REY) fractionation patterns. Different stages can be distinguished based on variation in REY-anomalies (e.g., Eu, Y), slope (LREE/HREE) and ΣREY concentration. The results also highlight how feldspar alteration is expressed across various lithologies within a single IOCG terrane, which is largely dependent on the style of fluid/rock interaction. In contrast to Moonta-Wallaroo, mineralization at Olympic Dam is hosted largely within a single lithology; the Roxby Downs Granite. Thus, changes in feldspar textures and geochemistry from magmatic to hydrothermal stages within a single lithology can be tracked based on proximity to mineralization within the Olympic Dam Breccia Complex. From observations and analyses of cryptoperthitic alkali feldspars, patch perthite, plagioclase and albite, it is evident that feldspars preserve the major element exchange (loss of Na -> gain of K and Fe -> Cu-U-Au-Ag mineralization) occurring within the Olympic Dam deposit. Moreover, results also highlight the ability of widespread feldspar replacement reactions (with a coupled dissolution-reprecipitation mechanism) to produce significant microporosity within the host rocks that facilitate subsequent fluid/rock interaction and mineralization. The contribution and distribution of REE elements within the Roxby Downs Granite is also investigated and compared to other IOCG terranes within the Olympic Cu-Au province, as well as pre-Hiltaba Suite lithologies, such as the Donington Suite granite. Chondrite-normalized REY fractionation patterns obtained from magmatic and hydrothermal feldspars, show that feldspars in the Roxby Downs Granite are significantly enriched in lattice-bound REY relative to Donington Suite granite. Moreover, although it appears that hydrothermal albite and K-feldspar inherit REY concentrations from the respective parent phases, variation in ΣREE (~1-100 ppm) at Olympic Dam indicate an increase in mobility of REE from feldspars with proximity to the Olympic Dam Breccia Complex. Nanoscale characterization is also undertaken on feldspars undergoing transformation from early post-magmatic to hydrothermal stages in the Roxby Downs Granite and the Hillside Granite. Numerous textures and phases are identified, including complex perthitic textures, anomalously Ba-, Fe-, or REE-rich compositions, and REE-fluorocarbonate + molybdenite assemblages which are observed to replace feldspars. Epitaxial orientations between cryptoperthite (magmatic), patch perthite (deuteric) and replacive albite (hydrothermal) support interface-mediated reactions between pre-existing alkali-feldspars and pervading fluid(s). In addition, nanoscale inclusions are observed within feldspars, such as epidote (Olympic Dam) and titanite + andradite (Hillside), which are strong hosts of REE and highlight the grain-scale remobilization of such elements during alkali alteration. Consequently, it appears that changes in REE fractionation trends can also be attributed to, and thus controlled by, sub-μm-scale inclusions within feldspars.