Carboniferous Volcanism And Fe Mineralization At The Zhibo Iron Deposit In The Western Tianshan

Voleanogenic iron deposit is an important type of iron deposit, and has recently been the subject of research. The association between volcanisim and Fe mineralization is the key to undertand this ore type. In this contribution, we choose Zhibo iron deposit, a recently discovered voleanogenic iron deposit in the Western Tianshan, as the typical deposit to study. An integrated study of field investigation, mineralogy, mineral chemisty, LA-ICP-MS in situ trace element analysis of magnetite, whole-rock geochemistry, zircon U-Pb dating, titanite U-Pb dating, microthermometric studies of fluid inclusions and stable isotope was carried out to investigate the metallogenic tectonic setting, geological characteristics, the age of mineralization, source of metal, ore genesis and metallogenic model. Combined with previous data, we propose a preliminary metallogenic model of metallogenic belt.The Zhibo iron deposit is hosted by Carboniferous volcanic and volcaniclastic rocks. Most magnetite orebodies are mainly tabular, stratoid and lenticular in shape. Ore types include massive, disseminated, banded, and brecciated ores. Ore minerals are predominantly magnetite, with traces of hematite. Pyrite is the most common sulfides. Magnetite usually occurs as euhedral-subhedral crystals or as dendritic and platy forms. Associated alteration assemblage is mainly characterized by Na-Ca alteration and K-Ca alteration, which produce various gangue minerals including diopside, albite, actinolite, K-feldspar, and epidote. Four paragenetic stages are recognized:stage I, characterized by albite-diopside-magnetitie assemblages; stage II, represented by widespread actinolite-K-feldspar-magnetitie assemblages; stage III, dominated by epidote-pyrite±magnetite veins; and stage IV, occurring chiefly as hematite-calcite-quartz veins.Magnetite from Zhibo have low Ti content. Magnetite is depleted in elements that are relatively immobile in hydrothermal fluids (e.g., Ti, Al, Cr, Zr, Hf, Zn, Co, Sc), but is enriched in elements that are highly incompatible into magmatic magnetite (e.g., Si, Ca, Y). In addition, Ni and Cr of magnetite are decoupled and Ni/Cr ratio is high (>1), with an average value of8.6. The trace element characteristics indicated that the ore magnetite was formed by Fe-rich magmatic-hydrothemal fluids.The volcanic host rocks of Zhibo display a compositional continuum from basalt to rhyolite, dominated by basalt, basaltic andesite, andesite, dacite, and tuffaceous rocks. Geochemical analyses indicate that the volcanic host rocks are mainly of calc-alkaline to high-K calc-alkaline affinity, enriched in LILEs (e.g., Rb, K) and LREEs, and depleted in HFSEs (e.g., Nb, Ta, Ti) and HREEs, supporting a subduction origin for the volcanic rocks. The most likely origin of the parent magma is partial melting of a mantle wedge that had been fertilized by fluids released from subducted slab. The parental magma was slightly contaminated by crust during ascent. The iron mineralization were suggested to have formed in extensional zones of continental arc-setting.LA-ICP-MS U-Pb dating of igneous zircon from an andesite (12ZB56) and a mineralized lava (12ZB06) yielded crystallization ages of328.7±2.1Ma and329.9±1.5Ma, respectively. A dacite (ZB382) and a diorite (ZB360) have been dated at300.3±1.1Ma and305±1.1Ma, respectively. In situ LA-ICP-MS U-Pb dating of hydrothermal titanite in thin sections of three ore sapmle yielded ages of310.0±2.1Ma,310.6±2.6Ma,315.2±2.8Ma, considered to represent the age of mineralization (310--315Ma). Iron mineralization at Zhibo and Chagangnuoer have formed more-or-less coevally in a single metallogenic episode generated by magmatic-hydrothermal activity within the same volcano. Geochronological data indicated that there were at least two episodes of volcanism at Zhibo, i.e. pre-mineralization volcanism (ca.330-316Ma) and post-mineralization magmatic intrusions (ca.307-295Ma). The timing of mineralization at Zhibo was bracketed between310Ma and315Ma.Stable isotope compositions of pyroxene indicate a magmatic source for early mineralization stages:calculated δ18O composition of fluids in equilibrium with pyroxene ranging between4.0and10.0%o. Calculated δ18O composition of fluids in equilibrium with actinolite (9.9-12.6%o) and K-feldspar (7.9-18.9%o), probably resulted from the isotope reequilibration between Fe-rich fluids with host rocks. Calculated δ18O composition of fluids in equilibrium with epidote (-1.7-4.2%o) and quartz (-3.7-1.4%o) may indicate ingress of meteoric fluids in the late stage alteration. We show that magnetites from Zhibo have δ18O values between0.5and8.8%o, with an average value of3.3%o, indicative of magmatic origin. The δ18O values for pyrite from Zhibo vary from-2.4to0.3%o, with an average value of-0.8%o, consistent with a deep-seated sulfur source. Homogenization temperatures of epidote, quartz and calcite are between131℃and391℃with salinities of3.23-22.44wt.%NaCleq.and31.85-37.40wt.%NaCleq., indicate an involvement of low-T and medium to high-salinity chloride fluids in the late stage alteration. Taken together, fluid inclusion and stable isotope data suggest that iron is predominantly magmatic in origin, sharing common parental magma with volcanic host rocks.Fe mineralzation at Zhibo was attributed to the regional magmatic-hydrothemal during Carboniferous. The most likely origin of the ore-related magma is partial melting of a mantle wedge that had been fertilized by fluids released from subducted slab. Fe-rich melts may be separated from the parent magma by liquid immiscibility or differentiation in low oxygen fugacity. Fe-rich melts were then channeled along major faults and fractures within a volcano into the volcanic hostr rocks, forming iron ores. The intrusive of Fe-rich melts is the dominant mineralization style, whieh formed massive ores, with little alteration, having sharp contacts with host rock, and dendritic or platy forms of magnetite. The banded and disseminated ores resulted from the metasomatic reaction between Fe-rich fluids with host rocks. The brecciated ore can plausibly be interpreted as resulting from an explosion of Fe-rich melt, caused by a sudden release of volatiles in response to decreasing pressure during ascent to near-surface levels. The Zhibo ore is typical of volcanogenic iron deposit formed in a volcanic magmatic-hydrothermal system.Combined with previous data, we propose a preliminary metallogenic model of Awulale iron metallogenetic belt (ATMB) and suggest that iron deposit within AIMB are relevant ores that formed in a single metallogenic episode. Fe mineralizaion was controlled by volcanic-hydrothermal and volcanism-sedimentation. The metal source was magmatic in origin.