Geological Characteristics And Ore Genesis Of The Chagangnuoer Iron Deposit In The Western Tianshan Mountains, Xinjiang, NW China

As one of the important components of the Central Asian Orogenic Belt, Western Tianshan Mountains experienced complex accretionary orogenic process, and Western Tianshan metallogenic belt is also one of the famous Fe-Cu-Au poly-metallic regions. From2004on, great progress has been achieved in iron exploration in the Awulale metallogenic belt of Western Tianshan Mountains due to the discovery of several medium to large-size Fe deposits. Located in the eastern Awulale metallogenic belt, north side of Western Tianshan Mountain, the large-scale Chagangnuoer iron de-posit is hosted in the volcanic rock termed by Carboniferous Dahalajunshan Formation, which may be the product on the continental arc created by the subduction of the Junggar Ocean southward beneath the Yili plate in the late Early Carboniferous. This iron ore consists of two predominate ore bodies named as Fe I and Fe II, with one lentoid marble as footwall rock beneath the main ore bodies Fe I which exhibit as lamellar, stratoid and lenticular, controlled by NW, NWW, NEE strike faults and circular faults. Mineralizations take place along fissures and fractures in the wall rocks. Wall rock alterations chiefly exhibit garnetization, actinolitization, chloritization, epidotization and so on. On the basis of field geological survey and indoor ore microscopy, using by these tec-hnical means of electron microprobe, inductively coupled plasma mass spectrometry, isotope mass spectrometry and mircothermometry, doing some studies included mineralogy, trace element geoch-emistry, stable isotope, fluid inclusion and Sm-Nd geochronology etc., and compared geological ch-aracteristics of the Chagangnuoer iron deposit with those of other typical iron deposits, to search ore-forming source, to explore ore genesis, and to build metallogenic model, in order to further summarize mineralization mechanism and metallogenic regularity of iron ores in the Awulale Belt, this thesis has proposed several points as follows.(1) This iron deposit is hosted in the upper-middle sections of andesite and andesitic volcanic-lastics of Carboniferous Dahalajunshan Formation. Ore minerals are mainly consisted of magnetite, and subordinately pyrite and chalcopyrite while the gangue minerals are composed of garnet, actin-olite, chlorite, epidote, tremolite, and calcite et al. Ore structures mainly occur as brecciated,spotted and mottled,"leopard pattern", massive, disseminated and banded, whereas ore textures dominantly display as anhedral, allotrio-hypidiomorphic, metasomatic, filling and intersertal.(2) The alteration zonation is similar with typical hydrothermal deposits. According to ore fab-ric and mineral paragenesis, this deposit can be divided into two ore-forming stages, which are magmatic stage and hydrothermal stage (included prograde sub-stage and quarts-sulfide sub-stage), and could be further subdivided into magnetite-diopside phase, chlorite-pyrite phase and magnetite-garnet-actinolite phase, propylitic phrase, sulfide phase and quartz-carbonate phase.(3)Electron microprobe analyses show that componential characteristics of garnet and pyroxene are quite similar with those in calcic skarn from the major large iron deposits, and probably are resulted from skarnization. In the Ca+Al+Mn vs Ti+V discriminant diagram showing spot analyses of magnetite and hematite, almost of data from the Chagangnuoer ore are fall into the district of skarn type of iron deposit. In addition, in the ternary plots of TiO2-Al2O3-MgO of magnetite, many data from the Chagangnuoer ore are seated in the sedimentary metamorphogenic and contact meatasomatic trending region while less amount of those dropping into magmatic mafic-ultramafic trending region. (4) In the magmatic stage, REE in magnetite is very low, rich in LREE and HREE but depl-eted in MREE with a U type pattern. This kind of magnetites has a higher Ti, V, Cr, indicating that Fe might come from the crystallization differentiation of andesitic magma. On the other hand, in the prograde sub-stage, magnetites have a lower REE content, a bit rich in LREE but other REE strongly depleted. Compared with the magnetites in magmatic stage, these magnetites are po-or in Ti, V but a bit abundant in Ni, Co and Cu content. Garnets in barren and ore-bearing skarn distribute the same REE patterns, having a relatively high REE content, enriched in HRRE but depleted in LREE, and with a not pronounced positive Eu anomaly, which displays the feature of garnet with metasomatic origin in the calcic skarn. And this hints that the magnetites, which have a paragenesis relationship with ore-bearing garnets, should be also a product of hydrothermal fluid replacement with wall rocks, and most of the mineralizing materials (Fe) probably are derivate fr-om andesitic strata.(5) Oxygen isotopes of magnetite show that the818O values display a decreasing trend, which reflects that wall alteration may change the ore-forming fluid. Both the sulfur isotope components between magmatic stage and prograde stage mainly distribute the magmatic feature, yet certain mi-nor strata sulfur or oceanic sulfur might be mixed in the former. In the late ore-forming stage, the δ13CPDB-δ18Osmow ratios of calcite show a positive linear correlation, probably attributing to the mixture of different concentrations of NaCl fluid or the water-rock reaction between ore-forming fluid and wall rocks, and marble may contribute to partial mineralization materials.(6)In the late ore-forming stage, fluid inclusions in the calcite are chiefly gas-liquid two phase type, indicating that ore-forming fluids may belong to NaCl-H2O system. Plot showing homogeni-zation temperature versus salinity of fluid inclusions in calcite provides the information that the sa-linity of late ore-forming fluids decreased with the reaction of mineralization, and that this process might be related with the isothermal mixing and mixture of various fluids with different temperat-ures and salinities.(7) Garnet, which has a closely paragenesis relationship with magnetite, yields the Sm-Nd iso-chron age with316.8±6.7Ma, which represents the formation epoch of high hydrothermal alteration. This result indicates that magnetite intergrown with garnet formed at the late stage of Early Carb-oniferous. Therefore, the iron metallogeny and high hydrothermal alteration should result from the metasomatism between the post-magmatic hydrothermal derivated from eruption of Dahalajunshan Formation volcanics and the underlying marble, rather than the skarnization caused by the magmat-ic hydrothermal from Permian intrusion with marble in ore district.(8) In combination the ore deposit geology, ore fabric features with mineralogy of typical mi-nerals, stable isotopes, and rare element chemistry, compared with typical skarn iron deposits and volcanic iron deposits (included magmatic type), conclusions have been drawn that the Chagangnu-oer iron ore is one polygenetic deposit with the skarn type (predominated) superposition upon the magmatic type, and that skarnizations would play an quite important role on the deposition of this iron deposit..