Discovery of a suspected intrusion-related polymetallic Mo-Bi-Te-Au vein-stockwork system in the southern Chilean Andes: The Trapananda prospect, Patagonia, Chile

• Gold-bearing, Mo-Bi-Te vein-stockwork prospect associated with a granite-metasedimentary contact in Patagonia, Chile. • Early bismuth mineralization is overprinted by late molybdenite with abundant native bismuth and Bi-Te-S sulfosalts; Mo and Au correlate. • An intrusion-related polymetallic style, similar to a reduced intrusion-related Au system, previously unreported in the Chilean Andes metallogenic province. • Gold concentrated by Bi-Te-S sulfosalt (semi-metal) melts coeval (immiscible) with aqueous fluids. The Trapananda prospect in Rio Soler, Aysén, XI th Region, Patagonia, Chile, contains quartz-molybdenum (Mo)-bismuth (Bi)-tellurium (Te)-gold (Au) vein arrays within, and near, a granite apophysis of the North Patagonian Batholith that intrudes carbonaceous metasedimentary rocks (phyllites and schists) of Late Paleozoic to Triassic age. It was first interpreted as a porphyry-type Mo deposit, and some of the quartz vein arrays have some similarities with those in metamorphogenic turbidite-hosted orogenic Au deposits. However, the Trapananda mineralization is comprised of molybdenite, native bismuth, and Bi-Te-S sulfosalts suggesting a different genetic model. The dating of molybdenite by the Re-Os isotopic method yielded a model age of 143.5 (±0.6) Ma (latest Jurassic), confirming a genetic association with the adjoining batholith. Two-phase fluid inclusions in quartz show no evidence of boiling (phase separation) but are coeval with molybdenite, Bi-Te-S sulfosalt minerals, and trails of Bi-Te-S semi-metal melt inclusions. The fluid inclusions have an average of salinity of 13.4 wt% eq. NaCl and T h of 210°C (n = 39 inclusions in 8 fluid inclusion assemblages). Fluid inclusions were trapped between ∼270 and ∼400°C at a minimum P of 0.7–2 kbar kbar (min ∼2.5–7 km depth), constrained by integrating isochore data with Bi-Te-S phase relation compositional constraints for fluid-coeval semi-metal melt inclusions. Together with the fluid inclusion systematics, the geochemistry of the igneous rocks, the vein textures, mineralogy and metal association (Mo-Bi-Te-Au) including semi-metal melts (which may have played a role in concentrating Au), and the stable isotope systematics (quartz δ 18 O + 10 ‰; sulfide-sulfosalt δ 34 S −0.1 to −1.0 ‰; host schist δ 13 C = –22 ‰) at the Trapananda occurrence are most compatible with metasediment-hosted, reduced intrusion-related Au system (RIRGS). However, some features are not consistent with this classification and thus, an intrusion-related polymetallic type of mineralization is favoured as a more general classification. Importantly, this type of Mo-Bi-Te-Au mineralized system has not previously documented in Chile. The lack of native Au and abundance of Bi-Te-S sulfosalt minerals that may contain dissolved “invisible Au”, concentrated during the suspected migration of semi-metal melts, may explain why the deposit eluded discovery by traditional prospecting (panning) methods in this remote, glaciated region.