Analysis of the infiltrative metasomatic relationships controlling skarn mineralization at the Abbas-Abad Fe-Cu Deposit, Isfahan, north Zefreh Fault, Central Iran

Abstract

The Abbas-Abad volcano-sedimentary-hosted Fe-Cu skarn deposit, NE Isfahan, is one of the most important skarns in the central Urumieh-Dokhtar magmatic arc. It was formed along the contact with the Dorojin granitoid massif next to the Zefreh Fault. The plutonic rocks are of I-type, volcanic arc affinity with normal-K, metaluminous, calc-alkaline to calcic quartz diorite, tonalite, and granodiorite, similar to many other Fe-type skarn-related granitoids worldwide. The parent magma involved in this skarn system is high temperature and relatively oxidized. Amphibole geobarometery in quartz diorite yielded a crystallization pressure lower than 200 MPa (2 kb, ~7 km) at 724° to 785 °C. Granitoid compositions around the ore deposit are mainly granodiorite. U-Pb zircon dating yields Early Miocene ages of 23.0 ± 1.6 Ma for a quartz dioritic rock and 21.3 ± 1.5 Ma for a tonalitic sample. The injection and cooling of the granodiorite produced a hornblende hornfels aureole with endoskarn.Paragenetic relationships and microprobe data indicate that Abbas-Abad calcic skarn evolution can be subdivided into three stages as follow: (I) Prograde skarn associated andradite-rich garnet (Adr93-98Grs0-4Spe1-2) and pyroxene, (II) Retrograde skarn starting with garnet (Adr53-69Grs28-43Spe2-4), magnetite, and sulfide minerals associated with calcic-alteration, and (III) Post-ore with pyrite, chalcedony, epidote, quartz, calcite, and zeolite veinlets. Textural and compositional studies of garnet and magnetite from the garnet-bearing exoskarn zone reveal the multiple events associated with skarn formation. Garnets are characterized by low TiO2 and relatively high CaO that are indicative of a calcareous wall-rock among Eocene volcaniclastic rocks. They are grouped into garnet-1 (low w/r) and garnet-2 (inverse zoning at high w/r) with notable Cu-contents (up to 743 ppm). Petrographically, magnetite morphology is divided into fine-grained granular, needle-like, and polycrystalline aggregates. Mineral chemistry of needle-like type reveals impure components (Al2O3, CaO, and SiO2). This type formed from dissolution-reprecipitation processes during a stage of reequilibration in the skarn system. Mixing with cooler external fluid (rich in oxygen and poor in Fe2+) is reflected in individual features during infiltration metasomatism during garnet and magnetite growth, such as oscillatory zoning and needle-like textures. Thus, we infer increasing pH (decreasing acidity) and decreasing T related to carbonate neutralization reactions affecting Fe- and Cu-chloride complexing as the main controls on mineralization.The structural studies of the area show that movement of the dextral transtensional Zefreh Fault provide local zones for emplacement of Dorojin granitoid during the Early Miocene. Consequently, the dextral transtensional Zefreh Fault and dextral transpression associated with the Marbin-Rangan Fault uplifted the skarn and host units and Dorojin body under the roughly N-S directed maximum compression direction. Furthermore, the interplay of Zefreh and Marbin-Rangan faults within the N-S regional compressional regime formed an anticlinal structure that exposed the Dorojin body within the core.