Geochemistry of metamorphic rocks and mineralization in the Golgohar iron ore deposit (No. 1), Sirjan, SE Iran: Implications for paleotectonic setting and ore genesis

摘要

The Golgohar iron mine is located southwest of Sirjan in the Sanandaj-Sirjan structural zone. The Golgohar ore deposit occurs mainly in the forms of massive, dispersed in altered rocks and rarely stratabound mineralizations, which is related to a fault zone. The mineralization is hosted by metapelites and metabasites. To clarify the paleotectonic, P-T condition of metamorphism and the genesis of the Golgohar ore deposit we report ICP-MS data of metamorphic rocks and EPMA data of feldspar, biotite, amphibole, and magnetite of host rocks and the mineralization. The protolith of metapelites are classified as shale and Fe-bearing shale. The chondrite-normalized REE patterns show enrichment in LREE, fairly flat HREE and negative Eu anomaly. The REE and other geochemical features of the shales support an undifferentiated arc precipitation environment. Based on the relative age of the protoliths, the shales formed during the Late Precambrian-Early Cambrian Pan-African orogeny. The protoliths of amphibolites were calc-alkaline basalt to andesite, which formed in a continental island arc setting. Geochemical data of the amphibolites suggest that the magma was derived from a depleted mantle that was contaminated by melts of subducted sediments. The igneous rocks were also formed during the Pan African orogeny. At the orogenic episode, the deep faulting generated a hydrothermal conduit to form the mineralization in this area. The compositional variations of the magnetite in the mineralization zones and the altered rocks were controlled by the compositions of fluid and the host rocks, the fugacity of oxygen and sulfur, the temperature, and the solubility of elements in the fluid. Large volume of the mineralization crosscutting the host rock is clearly controlled by structure. The presence of dispersed alteration zones, geochemical features such as high values of transitional elements (Co, V) in the magnetite mineral, the presence of xenoliths with high V and Ti contents in the mineralization zones and the stable isotope data indicate that the Golgohar ore deposit formed from high-density magmatic fluids in a subduction zone. The amphibolite host rock contains repidolite, magnesio-hornblende, pargasite, tschermakite, actinolite, phlogopite, andesine-oligoclase-albite, K-feldspar, and quartz. The sedimentary, igneous, and mineralized rocks had undergone metamorphism in the upper greenschist and amphibolite facies (450-700 degrees C and 6-10 kbar) at the Middle Jurassic, during the Middle Cimmerian tectonic phase. After these events, the deposit has been exposed, oxidized, weathered, and then buried by the Quaternary sediments. Thus, sedimentation and magmatism events occurred during Proto-Tethys subduction in the Cadomian arc, which is related to the Pan-African orogeny. Subsequently, in the late stage of subduction, magmatic fluids were responsible for the multiphase mineralization in the Golgohar area, while at the lateral stage of mineralization, they were accompanied by the mixing of sulfur-rich fluids (evaporative sulfate).