Lithological control on the distribution of ores in W-Mo porphyry systems: An example from the Ochtiná-Rochovce deposit (Western Carpathians, Slovakia)

In W-Mo porphyry deposits, tungsten and molybdenum ore bodies are often spatially separated due to the decoupling of W and Mo during hydrothermal processes and fluid-rock reactions. The Ochtiná-Rochovce W-Mo deposit has variable host lithologies affected by hydrothermal alteration mostly in well-defined vein selvedges. The Ochtiná-Rochovce W-Mo deposit was studied in detail by geochemical and mineralogical analytical methods on samples derived from a recent exploration campaign. Biotitic phyllite and mafic metavolcanites in upper parts of the deposit are the main W host, with abundant scheelite, accompanied by minor wolframite. Deeper metasandstone is enriched in molybdenite. Mineralisation is hosted by quartz stockworks and veins, and developed during biotite-wolframite, scheelite-molybdenite-pyrite and K-feldspar-calcite stages. Tungsten minerals precipitated by an increase in pH aided by wall-rock reactions and loss of CO₂ from early fluids on decompression. Amphibole and plagioclase replacement by biotite in vein selvedges in metavolcanites provided Fe, Ca, and Mg for precipitation of W minerals. The trend of increasing Mg/(Mg + Fe), F and decreasing Ti in biotite in all rocks suggest its hydrothermal origin. The Mg-rich wolframite preferentially occurring in metavolcanites documents a high aMg in the early fluids related to biotite alteration. Molybdenite deposition was promoted by cooling of later fluids, related to decompression, combined with decreasing fO₂ and increase in pH, resulting in quartz-sericite ± pyrite selvedges. Scheelite precipitation and replacement of wolframite are related to a progressive decrease in aFe/aCa following the biotite crystallisation, accompanied by increase in fS₂ associated with the saturation with respect to molybdenite. Progress of fluid-rock reactions and cooling invoked a further drop of acidity and fO₂, enrichment in S(−II), and liberation of Fe(II) into the fluid, thereby generating abundant pyrite. The presented model, that links the nature of lithologies to ore precipitation processes, can aid in a more efficient exploration for this type of mineralisation worldwide.