Geochemistry of intrusive rocks, petrology of skarn, and mineralogy and chemistry of ore bodies in the Senjedak-I area, east of Sangan mine, Khaf, NE Iran

The Senjedak-I prospect area is one of the six eastern anomalies of Sangan iron mine. Geologic units in the area consist of Jurassic shales and sandstones, skarn rocks and Tertiary biotitemonzonite and biotitesyenogranite intrusive rocks. Due to severe alteration of biotitemonzonite porphyry intrusive, geochemical studies have focused on the biotitesyenogranite. This granular intrusive consists of K-feldspar, plagioclase, quartz, biotite and accessory minerals such as zircon, titanite, apatite and magnetite, with weak sericitic, argillic and silicic alterations. Biotitesyenogranite is rich in silica (68.7 to 77.2 wt.%) and is chemically peraluminous which is formed by fractional crystallization an I-type granitic magma poor in P (average 0.1% P2O5). This intrusive rock falls in the range of I-type granites and is oxidized. REE values indicate negative Eu anomaly, mild enrichment of LREE, a positive pattern close to flat HREE and negative anomalies of Ba, Sr, La, Ce, Ti, and Eu. On the basis of Rb, Nb, Yb, Hf, and Ta contents, tectonic setting of samples fall in Volcanic Arc Granite (VAG) and Post Collision Granite (post-COLG) divisions. The most important event in the Senjedak-I area was infiltration of Fe-bearing fluids through the carbonate rocks, their recrystallization, skarnification (prograde and retrograde), and iron ore deposition that could be explained by occurrence of calc-silicate minerals. The skarn has been separated into 4 zones on the basis of abundance of the calc-silicate minerals: garnet skarn, phlogopiteskarn, epidoteskarn, and amphibole skarn. According to Electron Probe Micro Analysis (EPMA), the composition of garnets is andradite-grossular (An 100-42.6 Gr 0-55.32 Sps 0-1.39) and pyroxenes are diopside-hedenbergite (Di 63-92 Hd 4-35 Jo 0.5-3.9). Mineralization in this prospect area formed as stratabound and massive bodies in the carbonate rocks. The main ore mineral is magnetite (40%) with minor amounts of pyrite. Secondary minerals are hematite and malachite. The FeO content in the magnetite is 91.7% and the S content is 0.03%. The Senjedak-I anomaly is located along the eastern part of the Dardvay deposit (in central division of the Sangan mine). The geochemical similarities of intrusive rocks, chemistry of skarn minerals, and pyroxene and garnet composition confirm that the Senjedak-I is a part of Dardvay, which is separated by a main fault with southeast-northwest trend.