Mineralogy, Fluid Inclusion, and Stable Isotope Constraints on the Genesis of the Roudny Au-Ag Deposit, Bohemian Massif

Abstract

Orogenic gold deposits with gold of low-fineness are relatively rare in the Bohemian Massif but occur locally within the late Variscan Blanice graben structure in the Moldanubian zone. Among them, the Roudný deposit has been the most important, with total production of about 6 t of gold between 1895 and 1930. The ore is related to an irregular quartz stockwork and is represented by sulfide and electrum impregnations within the quartz gangue and the altered host rocks. Ore grade varies from 4 to 25 g/t Au. The gold is present as macro-and microscopic electrum (fineness 658–665) and also as “invisible gold” in the crystal structure of early sulfides (As-rich pyrite and Sb-rich arsenopyrite). Three mineralization stages have been distinguished: (1) the largely barren pre-ore stage, (2) the main-ore stage with economic Au-Ag mineralization, (3) the late stage with subeconomic Ag-Pb-Zn mineralization. Fluid inclusion and mineral thermometry indicate that the three stages were formed at about 400° to 330°, 350° to 250°, and 250° to 70°C, respectively. Early mineralization stages are related to complex H2O-CO2 ± CH4-N2 low salinity (<3.9 wt % NaCl equiv) fluids with isotopic signatures consistent with a metamorphic origin (δ18Ofluid = 4.7–2.9‰ SMOW). Most of the main ore-stage minerals, including the electrum, precipitated from aqueous fluids (2–4 wt % NaCl equiv). During the late mineralization stage, the salinity of aqueous fluid was higher (7–9 wt % NaCl equiv). The isotopic composition of the late fluid, however, also records gradually increasing proportions of meteoric water (δ18Ofluid = +2 to −1‰ SMOW). Elevated arsenic contents (120–210 ppm As) were found in aqueous-carbonic fluids by the LA-ICP-MS technique. Models of fluid mixing at the transition of the main to late mineralization stages are used to estimate the temperature and paleogeothermal gradient. Possible ages of electrum mineralization (~300 to ~ 275 Ma) and Ag-Pb-Zn vein-type mineralization (~275 to ~265 Ma) are discussed. Overall, the Roudný deposit shows striking similarities with ~300 Ma gold deposits of the French Massif Central.