Petrology and Geochemistry of the Qozlou Granitoid and Related Fe skarn (west Zanjan)

Fe skarn deposits are one of the important Fe deposits in the Zanjan province which have been exploited in recent years. The Qozlou Fe deposit is one of these Fe skarn deposits which is located at 65 km west of Zanjan. In this area, alternation of micro-sparitic limestone, marly limestone, shale and sandstones of Upper Cretaceous were intruded by Late Eocene granitoids. This event caused to metamorphism contact and it caused the formation of Fe mineralization. Some of the Fe skarn deposits in the Zanjan province were studied during the past few years (e.g. Nabatian et al., 2017) and valuable information is present about their geological and mineralization characteristics. However, Qozlou granitoid and Fe deposit have not been studied yet. In this research, petrology and geochemistry of the Qozlou granitoid along with petrographic characteristics, mineralogy, structure and texture of Fe deposit and thermodynamic conditions for formation of contact metamorphic rocks have been studied.

This research study can be divided into two parts including field and laboratory studies. Field studies include The recognition of different parts of granitoid intrusion and skarn aureole along with sampling for laboratory studies. Thus, 50 samples were selected for petrographic and analytical studies. 16 thin sections and 16 thin-polish sections were used for petrographical and mineralogical studies. 13 samples from granitoid and ore skarn sub-zone were analyzed by XRF and ICP-MS methods at the Zarazma laboratory, Tehran for geochemical studies.

Based on petrographic studies, the Qozlou granitoid is composed of porphyritic granite-granodiorite and quartz monzodiorite. Porphyritic granite-granodiorite have porphyritic to porphyroidic, micro-graphic and felsophyric textures and are composed of plagioclase, quartz, K-feldspar, hornblende and biotite phenocrysts within quartz-feldspatic groundmass. Quartz monzodiorites indicate porphyroidic texture and they are composed of plagioclase, hornblende, quartz and K-feldspar. The Qozlou granitoid demonstartes high-K calc-alkaline affinity and it is classified as metaluminous I-type granitoids. Trace elements normalized by primitive mantle (Sun and McDonough, 1989) for Qozlou granitoid indicate LILE and LREE enrichment along with negative HFSE anomalies and distinctive positive Pb anomaly. Chondrite-normalized (Nakamura, 1974) REE patterns for the Qozlou granitoid demonstrate LREE enrichment (high LREE/HREE ratio). Based on tectonic setting discrimination diagrams, the Qozlou granitoid were formed in active continental margin. Microscopic studies reveal that the skarn zone in Qozlou is composed of garnet skarn, garnet-pyroxene skarn, pyroxene skarn, epidote skarn, and pyroxene-bearing marble sub-zones. The Ore zone is present as massive and lens-shaped with 300m length and up to 30m width. Magnetite is the main ore mineral along with some pyrite, chalcopyrite and pyrrhotite. Garnet, clinopyroxene, epidote, actinolite, calcite and quartz present in skarn zone. Based on field and microscopic studies, the Qozlou Fe deposit indicates massive, banded, disseminated, brecciated, vein-veinlets, replacement and relict textures. Based on mineralogical and textural studies, skarnization processes in the Qozlou deposit can be divided into 3 stages including: (1) isocheimal metamorphic stage, (2) prograde metasomatic stage and (3) retrograde metasomatic stage. Chondrite-normalized (Sun and McDonough, 1989) REE and trace element patterns for different skarn samples and porphyritic granite demonstrate similar patterns.

Since all of minerals present in the Qozlou skarn aureole are located in Ca-Fe-Si-C-O-H system, we used the temperature vs. logƒO2 diagram (Einaudi, 1982) to determine possible physico-chemical conditions for skarn formation in the Qozlou. Based on this diagram and considering mineralogical and textural evidence, garnet and clinopyroxene were formed simultaneously in 430-550°C and ƒO2 equal 10-23 to 10-26. In the temperature less than 430°C and increasing ƒO2, garnet and clinopyroxene replaced by epidote, actinolite, quartz and calcite, respectively. Furthermore, in temperature of 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 by quartz, calcite and pyrite. With reducing ƒS2 (<10-6), andradite was replaced by quartz, calcite and 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. Based on this, it seems that metasomatic fluids probably had ƒS2≈10-6.5 and had less than 430°C temperature in the beginning of the retrograde stage. Presence of hematite lamellae within the magnetite demonstrates that ƒO2 probably was 10-22 in the beginning of retrograde stage.