Mineralization and REE geochemistry of hydrothermal quartz and calcite of the Helmesi vein-type copper deposit, NW Iran

The Helmesi copper deposit is a typical carbonate-silicic vein-type epithermal mineraliza- tion that developed within Eocene trachy-andesitic host rocks in northwestern Iran. The mineralized veins/veinlets and patches of breccia zones in the Helmesi area occur within a fault zone and are spatially associated with propylitic, phyllic, argillic, and silicic alteration zones. The mineralization in veins and accompanying breccia zones consists of both hypogene (chalcopyrite, bornite, chalcocite, enargite, quartz, and calcite) and supergene (covellite, malachite, azurite, goethite, limonite, hematite, Mn-oxides, and native copper) minerals. Rare earth element (REE) abundances in hydrothermal quartz and calcite intimately affiliated with copper mineralization have ranges of 13.43-68.18 ppm and 4.72-28.65 ppm, respectively. The distribution patterns of (REE+Y) in vein quartz indicate LREE enrichment over HREE (LREE/HREE = 5.59-13.98) and show negative Eu (Eu/Eu* = 0.11-0.35) and weak positive Ce (Ce/Ce* = 0.77-0.94) anomalies. Similarly, vein calcite displays weak LREE enrichment (LREE/HREE = 2.03-4.44) and weak to strongly negative Eu (Eu/Eu* = 0.20-0.74) and robust negative Ce (Ce/Ce* = 0.17-0.32) anomalies. The narrow range of variation in Y/Ho of quartz (43.71-49.25) and calcite (44.88-49.33) with corresponding values in aplitic dikes (45.23-47.06) provide convincing evidence for a genetic relationship between copper mineralization and Oligocene aplites in the study area. The negative Eu and Ce anomalies in both hydrothermal quartz and calcite suggest that the copper-bearing veins at Helmesi developed from relatively low-temperature (< 250 °C) fluids of an oxidized and strongly acidic nature.