Mineralized structures from the Alhué mining district, Chile: Using fluid inclusions and textures

Abstract

The Alhué mining district (AMD) of the Coastal Range in Central Chile is situated approximately 70 km southwest of the city of Santiago and hosted in stratified Jurassic and Early Cretaceous sequences which are common in central Chile. More than 79 faults and veins are recognized in the AMD hosted in andesites and tuffs with clear structural control showing two distinct trends. Precious‐metal mineralization locally extends over strike lengths of up to 1000 m along these structures with grades averaging at 5 ppm gold and 23 ppm of silver. In this study, a University of Chile – YAMANA GOLD joint research project was created to better understand this particular deposit and explore the origin of gold anomalies found in the district using drill core, mine, and surface samples. Wide internal variations in homogenization temperature and electrolytes were observed at all scales: on the district scale, in mineralized structures intersected in drill core, and in hand specimens. This variability may reflect the diversity of ore-forming conditions in the mining district in addition to the different processes (including fault motion over time) and geochemical components within the district. Textural analyses and fluid inclusion assemblages (FIA) provide evidence for boiling and flashing mechanisms at several localities. Raman spectroscopy and LA-ICP-MS techniques allowed a better characterization of the fluid composition and mineral paragenesis. The presence of gold (∼3.8 ppm) and silver (∼1424 ppm) in both vapor-rich only FIAs (i.e. that contain 99 % vapor) hosted in quartz and liquid-rich FIA hosted in sphalerite implies that gold was transported as sulfide and chloride complexes. The AMD shows a transition from a high temperature environment (sometimes containing calc-silicate minerals) to a low temperature (epithermal) system. This mineralization depended on the type of vein system which tend to be mineralized in strike slip faults e.g. Maqui and Lorena structures. The consequence of flashing documented at dilational jogs, structural intersections and fault bends was likely an important outcome leading to the precipitation of metals at the AMD.Coexisting liquid-rich and vapor-rich fluid inclusions contained in a FIA were observed at the deepest levels in the Chilco vein and the proportion of samples showing boiling increases with depth, indicating that additional precious metal resources may be present below current exploration depths. In summary, our new results reveal that gold, silver and base metal veins found at the AMD in Chile are unique hydrothermal polymetallic ore deposits that are structurally controlled and related to an orogenic environment. A general model for the AMD is one where high temperature fluids from deep crustal mafic magmatic systems, probably as products of a devolatilization and far-from-equilibrium conditions, reach subsurface conditions along faults and are mineralized along these faults. Because having a truly unique ore deposit in the geologic record is very unlikely, the AMD forms an important case study that helps understand structurally controlled ore deposits. Thus our work can form a model for future exploration in the AMD and provide insight into exploration in other sites in Chile and along continental margin settings worldwide, in both poorly characterized known deposits and at greenfield sites.