Ben Gaudet, BSc
B. Gaudet1, D. Leung1, S. Perrouty1, P. Lypaczewski2
1Harquail School of Earth Sciences, Laurentian University, Sudbury, Ontario, Canada
2LithologIQ, Montreal, Quebec, Canada
The Alleghany district, California, is located within the Sierra Nevada foothills and has historically produced 89 tons of gold. It is located adjacent to the Melones fault zone, which acts as the main pathway for mineralizing fluids in the region. When gold-bearing H2O-CO2-rich fluids interact with ultramafic country rocks, a distinct distal-to-proximal alteration profile is produced: talc-carbonate, talc-carbonate-quartz, and carbonate-quartz-muscovite mineral assemblages. Chlorite, which is present across the alteration profile, has been analysed in Neoarchean ultramafic hosted-gold settings (e.g., Kerr-Addison) and shows changes in chemistry attributable to distance from fluid pathways. Specifically, chlorite shows a proximal increase in Al3+, suggesting that Tschermak substitution ([6]Mg + [4]Si ↔ [6]Al + [4]Al) is occurring.
This study aims at testing if similar trends can be documented in the Phanerozoic ultramafic hosted-gold Alleghany district. Half-core and grab samples of altered ultramafic rocks from the Alleghany district (n = 20) will be analyzed using:
(1) hyperspectral analysis (mineralogy and trends in mineral chemistry),
(2) whole-rock geochemistry (protolith information and elemental gains and losses),
(3) optical microscopy (modal mineralogy and textures), and
(4) SEM-EDS (mineral chemistry and textures).
Petrographic thin-section observations suggest that chlorite is present across the alteration profile. Early SEM results have shown that gold grains are only present in the most altered mineral assemblage (carbonate-quartz-muscovite), showing a direct link between fluid interaction and gold mineralization. Mineral chemistry variations will be documented by semi-quantitative SEM analyses and hyperspectral imaging of least altered and mineralized samples. Ultimately, this study will demonstrate if chlorite chemistry can vector towards gold mineralisation in ultramafic-hosted gold systems, using time and cost-effective hyperspectral analyses that can easily be implemented in exploration programs.