Alteration-mineralization, and radiometric ages of the source pluton at the Sangan iron skarn deposit, northeastern Iran

Abstract

The world-class Sangan iron skarn deposit with a proven reserve of>1000Mt iron ore @ 53% Fe, is located in the Khaf-Kashmar-Bardaskan Volcano-Plutonic Metallogenic Belt (KKB-VPMB) of northeastern Iran along the E–W trending regional Doruneh Fault, north of the Lut Block. Skarn mineralization occurs at the contact of the 39.1±0.6Ma to 38.3±0.5Ma Middle Eocene syenite to syenogranite porphyry source pluton with Cretaceous carbonate rocks. The source pluton is part of an I-type, calcalkaline granitoid series with potassium to shoshonitic affinity, metaluminous to slightly peraluminous character. The granitoids are enriched in LILE (Cs, Rb, Ba, K, Th, and U) and LREE (La and Ce), are depleted in HFSE (Nb, Y, Ta, and Ti), HREE (Yb and Lu) and Eu, which together with the low Sr (96 to 362ppm) and (La/Yb)N contents suggest a metasomatized, slab-derived mantel source within the Cenozoic volcanoplutonic continental arc of northeaster Iran. The location of the skarn orebodies is controlled by the regional E–W Doruneh Fault structure, and the spatial distribution and the two, calcic and magnesian, skarns is controlled by the composition of the carbonate protolith that consists of limestone in the west and dolomite in the east. The chemistry of the skarn minerals reflects the composition of the protoliths. The western skarn is dominated by ca-rich calcsilicates that consist of prograde Ca-rich garnet (Adr50–97Grs0–45Sps-Alm2-7), Ca–Fe-rich pyroxene (Hd75.6Di20 Jo4.5), K–Cl–F-bearing Fe–Ca-rich amphibole ferrohastingsite (Fe4.4Ca2Mg 0.5K0.4Na0.3)(Si6.5Al1.8 O22)(Cl, F, OH)2 in endoskarns, and retrograde ferroactinolite (Fe4.2Ca2MgMn0.04Na0.1K0.06)(Si7.8Al0.2O22)(F, Cl, OH)2 and Fe-rich chlorite ripidolite (Fe4.3Mg1.1 Ca0.06Mn0.01)(Al2.8Si3O10)(OH)8 in the exoskarns. Wollasonite, plagioclase, and K-feldspar have been additionally formed together with ferrohastingsite in the endoskarns. The eastern magnesian skarn is typically phlogopite-rich (K0.9Mg2.7Fe0.2 Na0.02)(Si2.9Al1.2O10) (OH, F)2, and consists of prograde forsterite (Fo97.0Fa2.7), diopsidic pyroxene (Hd0.2Di0.88), and retrograde Mg-rich actinolite, Mg-rich chlorite clinochlore (Mg4.5Fe0.1K0.05)(Al1.8Si2.7O10)(OH)8, serpentine and talc. Iron mineralization overprints the calcsilicate skarns in both the western and the eastern skarns and is magnesian in the eastern orebodies (3.65wt.% MgO). The iron ore consists of a number of high grade, replacement, and magnetite orebodies with minor amounts of pyrrhotite, chalcopyrite, and pyrite indicating a large scale metasomatic transfer of iron bearing fluids during Middle Eocene magmatic activity at the KKB-VPMB.