MINERALOGY, FLUID INCLUSIONS AND ISOTOPIC DATA FROM THE SALAMÓN SEDIMENT-HOSTED GOLD DEPOSIT (LEÓN, N OF SPAIN)

In the Salamón prospect there are several occurrences of As, Sb, Cu, Ni, Co, Hg and U ores, with the most promising as Au deposits found in the As-rich zones. The mineralization is hosted mainly by limestone interbedded with bituminous shales, Upper Carboniferous age, close to a small dyke of igneous rocks of intermediate to basic composition which is spatially and genetically related. The hydrothermal alterations of the host rocks related to the orebodies are mainly decarbonization, dolomitization, silicification and minor, but locally noticeable, argillation or carbonatisation.

The ore deposition took place in two phases: an early, dominant and extensive stage, with very fine grained gold-bearing sulphides, mainly As-bearing pyrite and arsenopyrite, in a matrix of quartz-chalcedony (jasperoid) and dolomite, and a latter stage, of a larger crystal size, which replaced the early stage or in pockets and veins, with greater mineralogical variety. Gold is essentially present in refractory ores, without mineralogical expression, but as a minor element in arsenopyrite and As-pyrite of the early, main ore stage. Chemical U-Pb dating on hydrothermal, Th-depleted uraninite gives 269±10 Ma and which is consistent with the isotopic ages for other gold deposits of the Cantabrian zone and with Rb-Sr dating of intrusive rocks associated with several ore deposits in the area

The deposit has been formed as a consequence of a strong fluid-rock interaction under nearly hydrostatic conditions. Boiling is locally inferred from high-temperature behaviour of primary fluid inclusions in quartz from the silicified igneous dykes. For early phase, fluid inclusion studies show low to moderate salinity, temperatures in the range 140-280ºC and pressure of <600bars. The latter phase is formed in shallower levels, with moderate salinity and temperatures from 100ºC and pressure of <100bars. Stable isotope data suggests reduction of sulphur from marine sulphates and buffer of carbon in the fluid by the host carbonates.