The scheelite skarn deposit of Salau (Ariege, French Pyrenees)

Between 1971 and 1986 the scheelite deposit of Salau, located in the French Hercynian Pyrenees, produced 930,000 metric tons of ore with 1.50 percent WO 3 (13,950 metric tons of WO 3 ).The skarn deposit is hosted in a Devonian carbonate and partly detrital series intruded by a late Carboniferous stock. In the Devonian series (the lithostratigraphy and geochemistry of which is described in some detail) a unit of so-called Barregiennes (i.e., rythmic alternations of limestones and silts) appears to be very favorable for extensive development of skarns and overprinted ores.The Hercynian orogeny involved polyphase deformation, regional metamorphism of greenschist facies, and late intrusions of granite-granodiorite. Deformation included two coaxial stages of tight isoclinal folding and one important later stage which partly controlled the intrusion of the granodiorite stock, followed by three stages of brittle deformation. Ore deposition is related to faulting stages 0 and 1 and orebodies are cut into slices by stage 2 reverse faults.The Salau intrusion includes various rock types belonging to two slightly different calcalkaline trends. One of these trends, characterized by its low K contents, is unusual in the Pyrenees. The local shapes of the intrusion, apophyses and embayments, are important controls of the orebodies.After isochemical contact metamorphism, the skarns and ores developed in two main stages related to the infiltration of two quite different types of fluids. During the first stage (540 degrees -450 degrees C), anhydrous skarn formation was followed by an intergranular development of pyrrhotite-scheelite-quartz-calcite in equilibrium with the earlier hedenbergite. The resulting tungsten grade is uneconomic. The associated fluids, probably of magmatic origin, had a high salinity. In the second stage (450 degrees -350 degrees C), no new skarn formed; transformations of the silicate rocks are observed. A first substage of epidotization was followed by the development of intermediate grossularite-almandine-spessartite garnet and finally by the main ore stage with scheelite, pyrrhotite, arsenopyrite, and other sulfides with some gold. Two types of fluid inclusions are found in this stage; one is similar in composition to the first stage, the other is characterized by low salinity and high CO 2 and CH 4 contents.