Abstract:

A cost-effective, simple, and time-efficient method to determine the bulk composition of fluid inclusions is evaporate mound analysis. This method is semi-quantitative and determines inclusion composition by integrating SEM imaging with energy-dispersive analysis of precipitates, or mounds, produced by thermal decrepitation of fluid-inclusions. The method is applicable to magmatic-hydrothermal systems where fluid inclusions contain solute ions (e.g., Na, K, Ca, Cl, F). In order to assess the application of this method for evaluating hydrothermal evolution and metal fertility with regards to intrusion-related mineralisation, a test study is being conducted on the large (7800 km2) and variably mineralized (e.g., Sn, W, Cu, U, Mo, Ta) South Mountain Batholith (SMB) of Nova Scotia.<br/>Decrepitate mounds were analyzed using a LEO 1450VP (SEM) imaging system linked to an Oxford X-Max 80 mm2 SDD detector energy-dispersive detector. Based on decrepitating over a range of temperatures, from 325°C to 500°C, it appears that T = 500°C is optimal to produce large, well-shaped, and readily identifiable mounds. To optimize analysis time and, hence, increase research efficiency while maintaining result accuracy, data were collected with 5, 10, and 30 second acquisition times. The number of analyses required to produce representative results was also tested by comparing the results for 4, 8, 16, 32, and 64 mound analyses for individual samples. Results indicate that optimal procedures require multiple (N = 12) point-mode analyses on individual decrepitate mounds to substantiate mound heterogeneity, and that in order to accurately reflect in-situ fractionation a single, 10 second raster-mode analysis is the best approach.<br/>These optimal analytical protocols are being applied to a regional study of the SMB to determine their suitability as mineral fertility indicator and/or vector to ore mineralisation. This test case is the first of its kind conducted on a batholithic scale, with the resulting methodological protocols being readily exportable for the mineral fertility assessment of other regions. Data interpretation protocols integrate a granitic petrographic alteration index, fluid inclusion types, density and evaporate mound chemistry. Samples are chosen such that the entire batholith may be assessed, with all mapped lithologies represented. Fluid compositions determined thus far include brines with 5-20 % fluorine, which is quantitatively indeterminable using other methods, and has been linked as primary control on the transport and deposition of ore in porphyry- and greisen-style mineralisation.