|Résumé||(disponible en anglais seulement)|
An innovative approach for measuring the volatile trace-element suite (As, Cd, Hg, In, Sb, Tl, etc.) of hydrothermally altered rocks from the footwall and
hanging wall of the Bathurst Mining Camp massive sulphide deposits is established in this study. The developed methodology for micro-analyses of major phyllosilicate minerals specifically white mica and chlorite, by in situ laser ablation ICP-MS
includes an evaluation of optimized ablation conditions, choice of external standards, internal standardization and data reduction strategies (Iolite™ 2.5 Trace Element Data Reduction Scheme). Mineral assemblages of the BMC host rocks are
predominantly composed of chlorite, white mica, and subordinate quartz, barium feldspar, plagioclase, septechlorite (greenalite), stilpnomelane, etc. Chlorite is a major silicate constituent, associating with the massive sulphides (disseminated to
submassive sulphides) and exhalative sedimentary rocks along mineralized horizons. Optically, chlorite is typically pale-green or grass-green to yellowgreen in colour with typical Berlin-blue interference colour for Fe-rich varieties. Results of
LA-ICP-MS of chlorite indicate average values of As=131 and Sb=589 ppm. Average contents of Tl (11.9 ppm), In (5.35 ppm), Hg (1.31 ppm), and Cd (0.43) were also established. White mica is ubiquitous in potassic (sericitic) alteration zones in
quartzofeldspathic volcaniclastic, clastic and exhalative sedimentary rocks. In host rocks and semi-massive sulphide lenses, muscovite is typically associated with chlorite, forming fine-grained fibrous intergrowths in the groundmass. Based on
previous studies, the composition of white mica is defined as solid solution between muscovite and celadonite (phengitic). White mica shows extensive systematic substitutions (e.g., Tschermaks substitution) allowing it to host a wide range of trace
elements. Results from this study yielded an average As content of 90 ppm (ranging from 1.62 to 700 ppm) with the highest value recorded in the Halfmile lake deposit. Similarly, thallium is more enriched in this deposit ranging from 0.02-43.5 ppm.
Other volatile trace elements occur in lower concentration in muscovite, including In (6 ppm), Sb (3.2 ppm), Hg (1.9 ppm), and Cd (0.17 ppm). Phyllosilicate chemistry can be used to distinguish the hydrothermal alteration zoning associated with
massive sulphide deposition (e.g., proximal and distal halos). The knowledge obtained from the trace-element content of phyllosilicate minerals can, therefore, help to elucidate the processes controlling mineralization and the effects that
metamorphism and deformation have on the dispersion of the volatile trace elements. Furthermore, the enrichment of volatile trace elements toward the upper stratigraphic sections of the drill cores examined to date can be used to map hydrothermal
systems and to aide in VMS exploration.