بررسي رفتار ژئوشيميايي عناصر نادر خاكي ( REE) در زون هاي دگرساني گرمابي منطقه اوچ پلنگ (شمال شرق كاشمر، ايران مركزي)

Geochemical behavior of rare earth elements (REEs) during hydrothermal alteration in the Euch- Palang area (NE Kashmar, East of Central Iran)

The Euch-Palang area located in the NE of the Kashmar city, is part of Central Iran structural Zone. Eocen volcanic and pyroclastics rocks with intermediate composition (andesite to latite) associated with are exposed in the area. Based on field, petrography and geochemistry, propylitic, quartz- phyllic (sericitic) and argillic- silicic alteration zones could be identified in this area. These zones show notable differences in REE behavior. The propylitic alteration zone contains mineral assemblage of chlorite, epidote, carbonate, sericite, apatite, iron oxides, titanite, zeolite, clay minerals and pyrite. It can be divided into two sub-zones of carbonate-chlorite and carbonate-epidote. In the carbonate-chlorite sub-zone, REE are commonly depleted, with the LREE relatively more depleted than the HREE. Also compared to their fresh rock equivalents, all the REE fractionation ratios (La/Yb)n, (La/Sm)n and (Gd/Yb)n (3.76, 1.7 and 1.6, respectively) are low in this sub-zone, which indicate low REE fractionation during hydrothermal alteration. There are no notable changes of the REE behavior in the carbonate-epidote sub zone. The phyllic zone is identified by the presence of quartz, sericite and pyrite which may be accompanied by some chlorite, illite and rotile. The overall obtained results show that REEs (except for Eu) was added to the phyllic alteration zone. The Eu/Eu* is depleted compared to the fresh rock equivalents. The silicic-argillic zone is characterized by the presence of quartz and clay minerals. Behavior of REEs in this alteration zone is controlled by pH, frequency of complexes such as S04-2 and presenting of clay minerals which can act as REE-host. The LREE are enriched in comparison to HRRE elements. In additional, the REE fractionation ratios of (La/Yb)n, (La/Sm)n and (Gd/Yb)n are the same as their fresh rock equivalents, demonstrating that no REE fractionation occurs during hydrothermal alteration in this zone.