W-mineralization from greisen at Cetoraz near Pacov, Czech Republic

Studied locality is situated about 750 m NE from the Cetoraz village. The small tungsten deposit is surrounded by mostly moldanubian rocks, especially Pacov orthogneiss and sillimanite-biotite paragneiss. W-minerals occur in quartz veins and lenses with accompanying greisen. Greisenization is developed in endocontact of the orthogneiss, it has form of silicification and muscovitization. Ore mineralization of this greisen is represented by wolframite, scheelite, gahnite, pyrite, molybdenite, chalcopyrite and native bismuth. Cassiterite, galena and gold are only accessoric. Wolframite in quartz gangue is associated with scheelite and albite. Wolframite forms black tabular aggregates with size up to 10 cm. It is usually accompanied by scheelite crystals and aggregates of yellow-brown color and size from few mm to 1 cm. Microscopic study revealed wolframite, scheelite and uncertain secondary phase of Fe and W, which is by chemistry closely related to some pyrochlore group minerals. This phase was temporarily labeled as ferritungstite. Wolframite in BSE-image is chemically homogenous and forms subhedral grains from 0.08 to 0.5 mm in size. Wolframite is composed of dominating ferberite (72 – 75 %), lower hubnerite (21 – 24 %) and small but unusual amount of rare huanzalaite (4 %). Average chemical composition is (Fe0,752Mn0,227Mg0,040Ca0,001K0,001)E1,021 (W0,987Nb0,004V0,004)E0,995O4. Wolframite grains are often partially or fully replaced by ferritungstite. Unaltered wolframite is rare and sometimes contains inclusions of bismuth. Anhedral grains of scheelite are from 0.04 to 0.45 mm in size. Scheelite is chemically homogenous and close to its ideal formula. Scheelite is replaced by ferritungstite too.