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