Mineral chemistry, petrogenesis and evolution of the Ghorveh-Seranjic skarn, Northern Sanandaj Sirjan Zone, Iran

Abstract

The Ghorveh-Seranjic (GS) skarn is located in the northern part of the Sanandaj-Sirjan zone, NW Iran, which is part of Alpine-Himalaya orogenic belt. The GS metamorphic complex is the oldest unit in the GS area composed of marble, dolomitic marble, greenschist, and amphibolite of Early Jurassic age. The complex is intruded by NW-SE trending Late Jurassic peraluminous granitoids, which caused contact metamorphism and resulted in the development of skarn, hornfels and crystallization of marble. The skarn is showing distinct textural, mineralogical and geochemical zonation. At least four stages of skarn development have been recognized; stage I, clinopyroxene+garnet±vesuvianite±quartz±calcite±scheelite± pyrrhotite; stage II, garnet+clinopyroxene +vesuvianite+scheelite±apatite+calcite±pyrrhotite; stage III, amphibole+vesuvianite+epidote+chlorite±quartz±calcite±pyrrhotite±pyrite±chalcopyrite and stage IV, quartz+calcite±amphibole±epidote±chlorite±pyrite±chalcopyrite. Scheelite occurs in stages 1 and 2 together with garnet and clinopyroxene, and its abundance slightly increases with vesuvianite growth. In general, mineral chemistry of the GS skarn shows enrichment in Ca, Al and Mg. Two types of garnet, clinopyroxene and vesuvianite are identified in the prograde stage within the GS skarn. Variable Mg:Mn:Fe proportions in clinopyroxene of the early prograde stage suggest formation from a relatively homogeneous F-rich volatile phase. Mineralogical documentation of the GS skarn indicates that presence of the F-rich volatile phase affected zoning patterns and mineral abundances. Addition of fluorine increases the solubility of Al in the hydrothermal fluid by forming strong Al-F complexes, causing an increase in vesuvianite instead of clinopyroxene during the late prograde substage, resulting in high garnet/clinopyroxene ratios. The presence of granditic (grossular-andradite) and subcalcic (grossular-almandine-spessartine) garnet during the skarn evolution suggests variable Fe/Mn and Fe2+/Fe3+ ratios during the prograde stage of the skarn formation. Subcalcic garnet formed in a relatively reduced environment compared to the granditic garnet. Paragenetic reconstructions indicate that clinopyroxene, garnet and scheelite grew together during the prograde stage. These minerals were stable and coexisted at temperatures between 580 °C and 400 °C and at a logfO2 = −18 to −28.