Metallogenesis Of Iron Polymetallic Deposits In Altay, Xinjiang

In recent years, many iron polymetallic deposits are discovered in Altay, Xinjiang. These deposits are lack of systematic study on metallogenesis, leading to the limitation of further prospecting work. We choose Qiaoxiahala, Laoshankou, Akexike, and Qiaxia as typical deposits to study. Detailed geological characteristics investigation, geochemistry of ore deposit, geochronology are carried out to discuss the metallogenic tectonic setting, evolution and source of ore fluids, source of ore-forming material, ages of intrusion and mineralization, metallogenic process. Based on previous researches, the metallogenic regularity of iron polymetallic deposits in Altay was summarized to propose a regional metallogenic geodynamical model in this paper.Orebodies in Qiaoxiahala and Laoshankou Fe-Cu-Au deposits have stratiform, podiform, vein, and lenticular shapes, and are hosted in the skarn zone located at the contact between the Beitashan Formation marbles, intermediate to mafic volcanics, pyrocalstic rocks and diorite porphyry. Magnetite formed in retrograde stage, whereas copper and gold formed in quartz-sulfide-carbonate stage. Orebodies in Akexike Fe-Au deposit have stratiform, vein, and lenticular shapes, and are hosted in volcanics and tuffs of the Nanmingshui Formation. Fe and Cu mineralization formed in volcanic-sedimentary period and regional metamorphic period, respectively. Orebodies in Qiaxia Fe-Cu deposit have layered, stratiform, and lenticular shapes, and are hosted in volcanic-sedimentary rocks of the Kangbutiebao Formation, with weak wall-rock alteration. Fe and Cu mineralization formed in volcanic-sedimentary period and magmatic-hydrothermal period, respectively."Dissolution-reprecipitation" was found in Qiaoxiahala and Laoshankou deposits, e.g. Mag2contain much lower SiO2、K2O、Al2O3、MgO、Na2O and higher FeOT compared to Mag1, indicative of multi periods of magmatic-hydrothermal activities. This structure is an important mechanism of metallogenesis, and provides us direct evidence on hydrothermal genesis. Chromites appeared in both deposits, and were probably drived from the lower part of the Beitashan Formation. Native gold, silver, electrum, hessite appeared as fine veined and allotriomorphic granular structure, and were hosted in pyrite, chalcopyrite, and cobaltine.In the Qiaoxiahala deposit, H and O isotope data imply that the ore fluids of retrograde stage are mainly magmatic water. Ore fluids of quartz-sulfide-carbonate stage are characterized by increasing meteoric water and moderately low temperature, moderately low salinity, and low density. S and Re data indicate that the source of ore-forming material is derived from mantle. Ore fluids of volcanic-sedimentary period in Akexike deposit are mixture of sea water and magmatic water, and are characterized by moderate temperature, low salinity, and moderately low density, with minor CO2, CH4and SO2, whereas ore fluids of hydrothermal period are mixture of metamorphic water and deeply circulated meteoric water, containg more carbon component and lower salinity than volcanic-sedimentary period. S of volcanic-sedimentary period has a sedimentary origin, whereas S of hydrothermal period has a metamorphic origin. Temperature, salinity, daughter-bearing inclusions, concentration of Na+and Cl-increased, with declining of density, in regional metamorphic period compared to volcanic-sedimentary period under an oxidation environment in the Qiaxia Fe-Cu deposit. S of volcanic-sedimentary period has a sedimentary origin, whereas S of magma-hydrothermal period has a deep origin.Three periods of magmatic activities are identified in Qiaoxiahala district, based on zircon LA-ICP-MS U-Pb dating:the first period formed in the Middle Devonian (377.6±1.4Ma) in a subduction-related setting, the second period formed in the Late Devonian (355.8±3.4Ma) in a subduction to post-collision transitional setting, the third period formed in the Early Permian (289.5±1.1Ma) in a post-collision setting, implying the magmatic activities lasted for88Ma. The Middle Devonian diorite porphyry is closely related to mineralization. Molybdenite Re-Os ages show that Laoshankou and Qiaoxiahala formed in383.2±4.5Ma and375.2±2.6Ma, repectively. Qiaoxiahala and laoshankou have similar spatial and temporal distribution and tectonic setting with porphyry copper deposits in Kalaxiange’er, indicating that they are probably a part of regional porphyry metallogenic system.The geological and geochemical evidence presented here suggests that Qiaoxiahala and Laoshankou Fe-Cu-Au deposits are skarn type, and are related to late-stage magmatic-hydrothermal activity of the diorite porphyries. Fe mineralizations in Akexike and Qiaxia deposits belong to submarine volcanic type, whereas Cu and Au are late overlied mineralizations.The iron polymetallic deposits in Altay can be devided into5genetic types:volcanic type, skarn type, magmatic-hydrothermal type related to granite, magmatic type, and sedimentary metamorphic type. Mineralization ages can be devided into5periods:Middle-Late Ordovician, Early Devonian (407～385Ma), Middle Devonian (383～375Ma), Early-Middle Permian (289～261Ma), and Triassic (235～234Ma). Various types of iron polymetallic deposits formed in the subduction stage, with a peak mineralization age at the Early-Middle Devonian. Skarn type, magmatic-hydrothermal type related to granite, and magmatic type deposits formed in the post-collision stage in the Early-Middle Permian. Skarn Fe-Mo mineralization occoured in the intracontinetal evolution stage in the Triassic, and overlied on the early orebodies in the Maizi Basin.