Rapid Oxidation and Secondary Mineral Growth on Drill Core, Goldrush Complex Carlin-type Gold Deposit, Eureka County, Nevada

摘要

Rapid oxidation and growth of secondary minerals are observed on drill core from the Goldrush Complex Carlin-type gold deposit in Eureka County, Nevada. Samples were obtained from within the mineralized envelope of the Goldrush deposit, from unit 5 of the Devonian Wenban Formation, an important host of mineralization containing abundant fine fossiliferous material. The secondary minerals exhibit several growth habits including bladed, disc, film-like, and radial fibrous forms. New mineral growths form within hours of exposing fresh sample surfaces to the atmosphere or introducing moisture to dry samples. The study included analyses of (1) secondary mineral growths on rough samples, (2) secondary minerals removed from drill core samples, (3) secondary minerals grown on polished sections in the laboratory under humid conditions, (4) mineral substrates on which secondary minerals formed, and (5) mineral surfaces that did not support secondary mineral growth. Samples were analyzed by semi-quantitative methods using a scanning electron microscope, quantitative analyses using electron probe microanalysis, and X-ray diffraction analyses. Results show that secondary minerals with disc, film, and radial fibrous growth habits are comprised of Fe, S and O in atomic ratios consistent with the hydrated Fe sulfate minerals szomolnokite (FeSO_4 ? H_2O) and rozenite (FeSO_4 ? 4H_2O). Rozenite was only identified in secondary minerals grown on drill core in the lab. Secondary minerals exhibiting single and radiating clusters of blades contain Ca, S, and O in ratios consistent with gypsum. Secondary mineral growth appears unrelated to variations in trace element chemistry of arsenian pyrite; instead, the presence of secondary minerals appears to be related to the size of substrate pyrite crystals. Arsenian and arsenic-free pyrite crystals>~40 μm did not support secondary mineral growth, whereas secondary minerals did form on finely crystalline, < 2-40 μm arsenian to As-free pyrite inter-grown with quartz. Secondary mineral growth was initiated by exposure of finely crystalline pyrite to humid, oxidizing conditions.