Albitisation - from mineral grains to giant ore deposits

The Mesoproterozoic Iron-Oxide-Copper-Gold (IOCG) -Uranium-REE super-giant Olympic Dam has an estimated resource of all ores of 7.74 billion tonnes and is still open to depth. Large mineral accumulations like this require large scale, fluid dominated leaching — transport — and depositional processes which inevitably leave a chemical footprint in the source region. Identifying and interpreting this footprint is one of the keys conceptual exploration models for hidden ore deposits. The most frequently observed footprint associated with the REE anomalous IOCG deposits is a regional sodic alteration (albitisation) with accompanying formation of calc-silicate breccias and metasomatism. While readily identifiable as a regional geochemical anomaly, the challenge for mineral exploration is to quantify the spatial variability of this anomaly, to determine the sequential leaching of trace elements and to infer the direction of fluid flow to deduce vectors to mineralisation. The answer to this lies in the micro- and sub-micro scale analysis of the involved mineral replacement reactions. The Palaeo- and Mesoproterozoic metasedimentary/metavolcanic units of the Willyama Supergroup of the South Australian Curnamona Province are a prime example of regional scale albitisation. Petrographic, geochemical and SEM-BSE/edx studies show that mineral replacement in the albitised units initially affects plagioclase, followed by K-feldspars, micas and finally Fe-oxides. It thereby progressively reduces K, Ca, Mg, Fe and Ti, the traces of lithophiles (Ba, Sr, Rb, Th, Nb), siderophiles (Co, Ni) and chalcophiles (Cu, Zn, Pb) by orders of magnitude. With progressing intensity of albitisation the light REE are depleted preferentially over the heavy REE but in intense albitisation all REE are depleted. On the other side, Na (and Al) are enriched while the mobility of Si is conservative and dependent on the affected lithologies, Zr appears the least mobile element. The end products are a trace element poor, Na-rich end-member albite and a highly charged, metal-rich fluid. The course these fluids take can be traced in the form of breccia pipes and dikes with a distinct FeOx-rich, calc-silicate (actinolite, clinopyroxene) bearing matrix. These breccias, with analogue occurrences in the Cloncurry (Qld, Australia) and Wernecke Mts (Canada), are distinctly enriched in Fe, Mg, Ca and K and are strongly positively anomalous in Ba, Sr and Rb, Cu, Pb and Zn as well as the REE elements, emphasizing the mineralising potential of the involved fluids. Electron microprobe and Laser Ablation ICP-MS analysis show that these metasomatic reactions gradually affect individual grains while the influx of Na-rich fluids occurs along microfractures and crystal cleavage plains. Our study so far shows that the geochemical characteristics of the footprint left by albitisation in the source rock are an intriguing "inverted image" of IOCG-U-REE deposits .