The rutile composition as indicator of greisen-forming conditions in the Sn-W Mazan district, La Rioja province

Abstract

The mineral deposits of the old mining district of Mazan consist of small quartz veins that contain cassiterite, wolframite, and minor scheelite, hosted by a peraluminous Ordovician-Silurian? granite. This granite was intruded by a Carboniferous? granite, which is probably related to greisen and quartz-cassiterite-wolframite veins. On the basis of field characters two classes of quartz-muscovite greisen were distinguished: «greisenized granite» unrelated to mineralization, and «vein-related greisen». These greisens have accessory rutile with different shapes and optical characteristics. Microprobe studies carried out on the rutile crystals present in both greisen types allow the recognition of distinctive contents of Fe, Sn, Nb, Ta and W and of the different evolution of the traceelement substitution in the rutile of each greisen type. Based on these differences it is proposed that rutile from the vein-related greisen was mainly formed from hydrothermal Ti-bearing solutions. These differences can be explained under rock buffering and fluid buffering conditions. Cassiterite, wolframite and rutile were deposited under a fluid-dominated system.