Mineralogy and petrogenesis of scheelite mineralization from tungsten skarn deposit Saő Pedro das Aguías, NW Portugal

Abstract

Scheelite, the most important tungsten ore mineral, is widely abundant in the calc-silicate rocks in the area of Saõ Pedro das Aguías, NW Portugal. This master thesis is a summary of the study on the mineralogy, petrography, texture and compositional characteristics of scheelite ore and its associated minerals from the Tabuaço Tungsten Project operated by Colt Resources Inc. The deposit is located between two Variscan age units: Beiras granite batholiths and the Douro Valley Schist-Greywacke Complex in Central Iberian Zone. Metasomatism of the carbonates has produced the two main skarn units comprising scheelite: Main Skarn and Lower Skarn, cut by multiple aplitic intrusions. The skarn mineral assemblage comprises scheelite with gangue minerals: vesuvianite, grossular, fluorite, fluoroapatite, clinopyrocene, titanite, albite plagioclase K-feldspar, calcite and quartz with sulphides, namely pyrite, chalcopyrite and arsenopyrite not observed in economic grades. Scheelite occurs in various forms: thick veins, remobilized along fluid paths, very fine, dispersed mineralization; almost all crystals are broken, with undulatory extinction indicative of deformation processes and stress leading to subgranulation on the grains contact. There are at least two to three generations of scheelite, connected with influxes of different chemistry during the skarnification event. Detailed petrographic observations show that the occurrence of scheelite in calc-silicate rocks is always associated with the growth of F-rich minerals formed by externally derived Si-Al-Fe-W-F-bearing fluid. Higher fluorine content in the metasomatic fluid led to the enrichment in tungsten, because fluorine increases the solubility of tungsten, allowing the tungsten transport in the skarn system.