Textural and chemical features of sphalerite from the Palai-Islica deposit (SE Spain): implications for ore genesis and color

Abstract

Sphalerite in the Au-Cu volcanic-hosted, Palai-Islica deposit appears in three locations with differences in chemistry, mainly in the Fe content: a) included in pyrite (Fe: 0.49 – 5.47 at. %) within the quartz veins; b) disseminated or in crustiform bands, also within the quartz veins; and c) disseminated in hydrothermally altered volcanic rocks from the deepest part of the deposit (Fe: 0.28 – 1.12 at. % - the only one which is cathodoluminescent). Disseminated- or crustifom band-sphalerite is the most abundant type, with two varieties: dark (Fe: 3.16 – 8.66 at. %) and light (Fe: 0.08 – 2.52 at. %). The former is associated with zones rich in gold and other metals. The Fe content of sphalerite reflects an evolution in fs2 of the hydrothermal fluids. Fe-rich, “dark” sphalerite could be related to a mixing process triggering noble- and base metal sulphide precipitation. Different types of sphalerite have significant amounts of minor elements, such as Cu (up to 1.34 at. %), Sb (up to 0.67 at. %), Sn (up to 0.31 at. %), Ge (up to 0.29 at. %), Cd (up to 0.24 at. %), In (up to 0.18 at. %), Mn (up to 0.15 at. %) and Ga (up to 0.12 at. %), some of which are elements not traditionally recognized in sphalerite. Among them, Sb, Sn, Ga-Ge and In are proportional to Cu content, and the following charge balanced coupling substitutions have been demonstrated: Sb3+ + Cu+ + Cu 2+ 3Zn2+; Sn4+ + 2Cu+  3Zn2+; 2Ge2+ + Ga3+ + 2Cu2+ + Cu+  6Zn2+. The first two substitutions have been shown to correlate with red coloration in Fe-poor sphalerite. The latter substitution, could be related to incorporation into the hydrothermal system of Ga-Ge bearing fluids from the basin. The presence of cathodoluminescent sphalerite seems interesting since it could reflect distinctive trace element content, and could help to distinguish a different type of mineralization and fluid/metal source manifested in an unexplored part of the deposit.