The granite-hosted Variscan gold deposit from Santo António mine in the Iberian Massif (Penedono, NW Portugal): constraints from mineral chemistry, fluid inclusions, sulfur and noble gases isotopes

Abstract

The study area is located in the Central Iberian Zone, a major tectonic unit of the Iberian Massif (Variscan belt). In this region the basement is composed of Cambrian-Ordovician sedimentary and minor volcanic rocks that underwent deformation and metamorphism during the Carboniferous. These metamorphic rocks host ca. 331-308 Ma granitic plutons emplaced during the D 2 extensional and D 3-D 4 contractional deformation phases. The gold-bearing quartz veins from the Santo António mine (Penedono region) occur in granite formed at 310.1 ± 1.1 Ma and postdated the peak of metamorphism. Gold-silver alloy is included in quartz, but mainly occurs in spaces between grains or micro-fractures within arsenopyrite of all three generations and less in pyrite. Late sulphides and sulphosalts were deposited along fractures mainly in arsenopyrite, and locally surrounding the gold-silver alloy grains. Ferberite, scheelite and stolzite replace arsenopyrite. The abundant aqueous carbonic fluids and the occurrence of a low-salinity fluid and their minimum possible entrapment temperature of 360-380 °C suggest that this gold-forming event began during the waning stages of the Variscan orogeny. The mean δ 34 S values of arsenopyrite and pyrite are − 4.7‰ and − 3.8‰, respectively. HeAr -Ne isotopic data suggest a crustal origin. The ascent of the granite magma has provided the heat for remobilization of gold, other metals and metalloids from the metamorphic rocks. This gold-arsenopyrite deposit has thus similar characteristics as other selected gold-arsenopyrite deposits from the Iberian Massif, but it contains tungstates.

Genesis of the telescoped Eocene silver and Oligocene gold San Dimas deposits, Sierra Madre Occidental, Mexico: Constraints from fluid inclusions, oxygen - deuterium and noble gases isotopes

“…Taking into account these observations the difference in the maximum depth formation needed to overlap the Eocene and the Oligocene "bonanza" levels could be estimated at 2-3 km. Such conditions are in agreement to those reported in literature for comparable plutonic related vein deposits (Prokofiev and Pek, 2015;Nieva et al, 2019). At this depth, the silver mineralization trapping temperature, corrected for the hydrostatic pressure, could be estimated at ~350 °C (Steele-MacInnis et al, 2012).…”

“…It is reasonable to suppose that the batholith building process not only provided the heat, part of the fluids and the metals, but also generated shallow extension and the formation of mineralized tension gashes during its ascent. The Piaxla batholith shallow position in the stratigraphic column suggest the enrichment of the magma in precious metals and metalloids by assimilation of the Cretaceous porphyry bodies (Montoya et al, 2019a), as recognized in other granite-related deposits in Northwest Europe (Nieva et al, 2019;Vallance et al, 2003). Furthermore, during the batholith emplacement the thermal anomaly remained elevated for a long period of time.…”