The mineralogy of copper-bearing skarn to the east of the Sungun-Chay river, East-Azarbaidjan, Iran

摘要

A calcic copper-bearing skarn zone in East-Azarbaidjan, NW of Iran is located to the east of the Sungun-Chay river. Skarn-type metasomatic alteration and mineralization occurs along the contact between Upper Cretaceous impure carbonates and an Oligo-Miocene Cu-bearing granitoid stock. Both endoskarn and exoskarn are developed along the contact. Exoskam is the principal skarn zone enclosed by a marmorized and skarnoid-hornfelsic zone. The skarnification process occurred two stages: (1) prograde and (2) retrograde. The prograde stage is temporally and spatially divided into two sub-stages: (a) metamorphic-bimetasomatic (sub-stage Ⅰ) and (b) prograde metasomatic (sub-stage Ⅱ). Sub-stage Ⅰ began immediately after the intrusion of the pluton into the enclosing impure carbonates. Sub-stage Ⅱ commenced with segregation and evolution of a fluid phase in the pluton and its invasion into fractures and micro-fractures of the marmorized and skarnoid-hornfelsic rocks developed during sub-stage Ⅰ. The introduction of considerable amounts of Fe, Si and Mg led to the development of substantial amounts of medium- to coarse-grained anhydrous calc-silicates. From texture and mineralogy the retrograde metasomatic stage can be divided into two discrete sub-stages: (a) early (sub-stage Ⅲ) and (b) late (sub-stage Ⅳ). During sub-stage Ⅲ, the previously formed skarn zones were affected by intense multiple hydro-fracturing phases in the Cu-bearing stock. In addition to Fe, Si and Mg, substantial amounts of Cu, Pb, Zn, along with volatile components such as H_2S and CO_2 were added to the skarn system. Consequently considerable amounts of hydrous calc-silicates (epidote, tremolite-actinolite), sulfides (pyrite, chalcopyrite, galena, sphalerite, bornite), oxides (magnetite, hematite) and carbonates (calcite, ankerite) replaced the anhydrous calc-silicates. Sub-stage Ⅳ was concurrent with the incursion of relatively low temperature, more highly oxidizing fluids into skarn system, bringing about partial alteration of the early-formed calc-silicates and developing a series of very fine-grained aggregates of chlorite, clay, hematite and calcite.