THE COMPOSITION OF HYPERSALINE, IRON-RICH GRANITIC FLUIDS BASED ON LASER-ICP AND SYNCHROTRON-XRF MICROPROBE ANALYSIS OF INDIVIDUAL FLUID INCLUSIONS IN TOPAZ, MOLE GRANITE, EASTERN AUSTRALIA

High-temperature (> 550-degrees-C) hypersaline (> 50 wt% salts) fluid inclusions, representative of the earliest hydrothermal fluids associated with the Sn-W-Cu-Ph-Zn-mineralised Mole granite of eastern Australia, are well developed in topaz from the Fielders Hill locality. Methods based on Inductively Coupled Plasma Emission Spectroscopy following laser ablation and on Synchrotron X-Ray fluorescence microanalysis are described and applied to the semiquantitative point analysis of these inclusions. Crush-leach analysis provides further information as well as highlighting the importance of point methods when several generations of inclusions are present. The laser-ICP results confirm the dominance of Fe, K, and Na in these early high-temperature fluids. The mean Fe:K:Na atomic ratios (0.95:0.79:1.00) are entirely in agreement with published experimental data on the composition of chloride brines in equilibrium with synthetic granite at magmatic temperatures and support the view that these fluids are direct products from a cooling granite magma. A number of trace and minor elements have also been detected in the inclusions. These include Ca, Mg, Li, B, Bc, Ba, Sr, and several of the orc metals. Order of magnitude estimates of the ore metal contents of these fluids, based on combined XRF-microprobe and laser-ICP analysis, are in the percent range for Fe, Mn, and Zn, in the range from several hundred to several thousand ppm in the case of Sn, Cu, and Pb, and less than 600 ppm for Mo and W. These results have important implications for ore genesis in granite environments and point to the very high ore-carrying potential of high-temperature, hypersaline, chloride-rich brines exsolved from cooling granite magmas.