Geochemistry and petrogenesis of the Alamkandi granitoid body and Fe skarn (west of Mahneshan, the Zanjan province)

Fe skarn deposits are the largest skarn deposits which are exploited for Fe as well as by-products of Cu, Co, Ni and Au (Meinert et al., 2005). They are one of the most important Fe deposits in the Zanjan province which have been exploited in recent years. The Alamkandi Fe deposit is one of these Fe skarn deposits which is located at 35 km west of the Mahneshan within the Takab-Takht-e-Soleyman subzone, northern Sanandaj- Sirjan zone. In this area, alternation of amphibolite, amphibole schist and biotite schist with intercalations of marble belonging to Paleozoic and intruded by late Oligocene alamkandi granitoid exist. This intrusion has caused contact metamorphism and formation of Fe mineralization. Some of the Fe skarn deposits in the Zanjan province were studied during the past years (i.e., Nabatian et al., 2017; Mokhtari et al., 2019) and valuable information is present about their geological and mineralization characteristics. However, the Alamkandi granitoid and Fe deposit have not been studied until the present. In this research study, geochemistry and petrogenesis of the Alamhandi granitoid along with mineralogy, textures and geochemistry of Fe deposit and thermodynamic conditions for formation of contact metamorphic rocks have been studied.

This research can be divided into two parts including field and laboratory studies. Field studies include recognition of different parts of granitoid intrusion and skarn aureole along with sampling for laboratory studies. During field work, 65 samples were selected for petrographic and analytical studies. 19 thin sections and 13 polished thin sections were used for petrographical and mineralogical studies. For geochemical studies, 15 samples from granitoid and ore skarn sub-zone were analyzed by XRF and ICP-MS methods at the Zarazma laboratory, Tehran, Iran. 
  

Based on petrographic studies, the Alamkandi granitoid is composed of granodiorite, quartz diorite and porphyritic diorite. Granodiorites with hetrogranular texture are composed of plagioclase, quartz, K-feldspar, hornblende and biotite. Quartz diorites indicate porphyroid to seriate and hetrogranular textures and are composed of plagioclase, clinopyroxene, hornblende and quartz. Porphyritic diorites have porphyritic texture with plagioclase and amphiboles phenocrysts. The Alamkandi granitoids demonstrate calc-alkaline to high-K calc-alkaline affinity and can be classified as metaluminous I-type granitoids. Primitive mantle-normalized (McDonough and Sun, 1995) trace elements patterns for the Alamkandi granitoids indicate LILE and LREE enrichment along with negative HFSE anomalies and positive Pb anomaly. Chondrite-normalized (McDonough and Sun, 1995) REE patterns for these rocks demonstrate LREE enrichment (high LREE/HREE ratio). Based on tectonic setting discrimination diagrams, the Alamkandi granitoids were formed in the active continental margin.Fe mineralization in the Alamkandi area crops out in discrete places as massive and lens-shaped bodies. The Northern outcrop body has 150m length and up to 50m width, while the southern outcrop body has 100m length and up to 20m width.  Microscopic studies reveal that the skarn zone at the Alamkandi granitoid is composed of garnet skarn, pyroxene skarn, epidote pyroxene skarn, serpentine skarn, and ore skarn sub-zones. Magnetite is the main ore mineral along with some pyrite and chalcopyrite. Garnet, clinopyroxene, olivine, serpentine, epidote, actinolite, calcite and quartz are present as gangue minerals. Based on the field and microscopic studies, the Alamkandi Fe deposit has massive, banded, disseminated, brecciated, vein-veinlets, replacement and relict textures. Based on mineralogical and textural studies, the skarnization processes in the Alamkandi deposit can be divided into 3 stages including: (1) isochemical metamorphic stage, (2) prograde metasomatic stage and (3) retrograde metasomatic stage.

Based on skarn mineralogy, the XCO2 vs. T and T vs. logƒO2 diagrams were used to determine the possible physio-chemical conditions. According to these diagrams and considering mineralogical and textural evidence, maximum temperature for formation of olivine in XCO2≈0.1 and P=1kb was about 450-600°C. Furthermore, garnet and clinopyroxene were formed simultaneously at 430-550°C and ƒO2 equal 10-18 to 10-22. In temperatures less than 450°C, olivine was replaced by serpentine while in temperatures less than 430°C and increasing ƒO2, garnet and clinopyroxene were replaced by epidote + quartz + calcite and actinolite + quartz + calcite, respectively. In temperatures less than 430°C, fluids in equilibrium with granitic intrusion and with relatively high sulfidation (ƒS2>10-6), were not in equilibrium with andradite. Therefore, andradite was replaced with quartz + calcite + pyrite. With reducing ƒS2 (<10-6), andradite was replaced by quartz + calcite + magnetite. During the early retrograde stage, magnetite and pyrite were formed along with quartz and calcite. Mineralogical studies indicate that pyrite was formed after magnetite. In this regard, it seems that metasomatic fluids probably had ƒS2≈10-6.5 and less than 430°C temperature in the beginning of the retrograde stage. Presence of hematite lamella within the magnetite demonstrates that ƒO2 was probably about 10-22 in the beginning of retrograde stage.