Alteration mineralogy,mineral chemistry and genesis of Zhibo iron deposit in western Tianshan Mountains,Xinjiang
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The world-class Sarbai, Kachar and Sokolovsk iron ore deposits of the Turgai belt, in the Carboniferous Valerianovskoe arc of northwest Kazakhstan, contain an aggregate of more than 3 billion tonnes of mineable massive magnetite. The Valerianovskoe arc is the possible westward extension to the South Tien Shan arc that is host to the giant Almalyk Cu-Au porphyry system in Uzbekistan. The magnetite bodies of the Turgai belt replace limestone and tuffs, and are distal to locally proximal to the contacts of gabbro-diorite-granodiorite intrusive complexes. Three main stages of alteration and mineralisation can be recognised at these deposits, namely: (1) pre-ore; (2) the main magnetite forming; and (3) post ore phases. The pre-ore stage is characterised by high temperature, metamorphic/metasomatic calc- and alumino-silicates. The main magnetite ore phase formed when hot, sulphur poor, acidic, iron-, silica- and aluminium rich fluids were structurally focused to dissolve and replace the dominantly limestone hosts. This was accompanied by a skarn assemblage gangue of epidote, calcic-pyroxenes, calcic-garnet and calcic-amphiboles, minor sulphide minerals and high field strength element (HFSE)-bearing accessory minerals such as titanite and apatite. This magnetite-skarn
mineralisation was followed by a late sulphide phase, when comparatively cooler fluids, which produced distinctive and extensive alteration assemblages of sodium-rich scapolite, albite, chlorite and K feldspar, accompanied by chalcopyrite, pyrite and minor sphelarite and galena. The post-ore phase, is characterised by cross cutting barren veins composed of calcite, lesser albite and K feldspar, and minor quartz, and by widespread alteration comprising scapolite, albite and silica, which surrounds the deposit, and extends for several kilometers into the host rock. Many of the geological and mineralogical features of these deposits closely resemble those of IOCG deposits and provinces around the world.
However, as the copper sulphide mineralisation is sub-economic, they may only be classified as either IOCG-style or IOCG-related deposits. Stable isotope (C, O, S) studies have been carried out on a range of sulphides, carbonates and silicates related to the mineralisation. Preliminary results from sulphides intergrown with magnetite support a magmatic source for the sulphur. Oxygen isotope data from associated silicates and iron oxides also support an igneous, or igneous rock equilibrated source for the mineralising fl uids. Carbon and oxygen isotope data from gangue carbonates suggest
carbonate is derived from the interaction of igneous-derived or igneous-equilibrated fl uids with host limestones.
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Located in the eastern Awulale metallogenic belt of western Tianshan Mountains,the large-scale Chagangnuoer iron deposit is hosted in the andesite and andesitic volcaniclastics of the Lower Carboniferous Dahalajunshan Formation,with one lentoid marble as footwall rock beneath the main ore bodies which exhibit as lamellar,stratoid and lenticular.The alteration zonation is similar with typical hydrothermal deposits.According to ore fabric 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).In the magmatic stage,REE in magnetite is very low,rich in LREE and HREE but depleted in MREE with a U type pattern.In addition,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 poor 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 from andesitic strata.In combination geological characteristics with trace element geochemistry,we hold that the Chagangnuoer iron ore is probably one polygenetic deposit with the skarn type(predominated) superposition upon the magmatic type.
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The large Dahongshan Fe-Cu-(Au-Ag) deposit in the Kangdian iron oxide copper-gold (IOCG) metallogenic province, southwest China, contains approximately 458.3 Mt of ore at 41.0% Fe, 1.35 Mt Cu (metal) at 0.78% Cu, and significant amounts of Au (16 t), Ag (141 t), Co (18,156 t), and Pd + Pt (2.1 t). The deposit consists mainly of two types of ores: (1) lenses of massive or banded magnetite-(hematite) hosted in extensively Na metasomatized metavolcanic rocks, metaarenite, and brecciated rocks, and (2) strata-bound disseminated, stockwork, and banded magnetite-chalcopyrite-(bornite) in mica schist and marble. Both types of orebodies and country rocks underwent extensive hydrothermal alteration, resulting in a similar paragenesis. Pervasive stage I sodic alteration formed widespread albite and local scapolite. It was subsequently replaced by Ca- or K-rich minerals represented by actinolite, K-feldspar, biotite, sericite, and chlorite of stages II and III. Magnetite is slightly younger than and partly overlaps the sodic alteration assemblages. Hematite is texturally later than magnetite, is locally abundant within the massive Fe oxide orebody, and is closely associated with sericite. Copper sulfides are coeval with quartz, biotite, sericite, and chlorite in stage III assemblages. Widespread siderite and ankerite predominate in stages II and III, respectively. Quartz-calcite veins mark the result of waning stage IV hydrothermal alteration. In addition to widespread alteration during the major ore-forming event, the deposit has also undergone extensive overprinting and remobilization during post-ore magmatic and metamorphic events. The Dahongshan orebodies are intimately associated with abundant doleritic dikes and sills that have hydrothermal mineral assemblages similar to those in the ore-hosting rocks. One dolerite sill that cuts a massive Fe orebody has a laser ablation-inductively coupled plasma-mass spectrometry zircon U-Pb age of 1661 ± 7 Ma, which is, within uncertainty, consistent with the age of 1653 ± 18 Ma determined for hydrothermal zircons from stockwork chalcopyrite-magnetite ore. The zircon U-Pb ages are thus considered to mark the timing of major mineralization that formed the Dahongshan deposit. Post-ore modification is recorded by an Re-Os isochron age of 1026 ± 22 Ma for pyrite in discordant quartz-carbonate-sulfide veins, and by younger Neoproterozoic mineralization dated at ca. 830 Ma using Re-Os isotopes on molybdenite. The former age is contemporaneous with late Mesoproterozoic magmatism in the region, whereas the latter is coeval with regional Neoproterozoic metamorphic events in southwest China. Carbon and oxygen isotope values of albitized marble are between those of mantle-derived magmatic carbon and dolostone end members. The ore-forming fluids that equilibrated with stage II magnetite have δ 18 O values of 9.1 to 9.5‰, whereas fluids linked to the deposition of quartz and ankerite during stages III and IV have lower δ 18 O values of 2.9 to 7.3‰. The oxygen isotope data indicate that the ore-forming fluids related to stage II are chiefly magmatically derived and mixed with abundant basinal brine during stages III and IV; this interpretation is consistent with sulfur isotope values of sulfides in the deposits. Pyrite and chalcopyrite from the Dahongshan deposit have a large range of δ 34 S values from −3.4 to +12.4‰, implying mixing of magmatic and external sulfur (likely from basinal brines) in sedimentary rocks. The Dahongshan deposit formed in an intracratonic rift setting due to underplating by mafic magmas that induced large-scale fluid circulation and pervasive sodic-calcic metasomatism in country rocks. Ore metals were derived mainly from a deep-seated magma chamber and partly from country rocks. Hydrothermal brecciation of the country rocks formed at the top of the dolerite intrusions and along zones of weakness within the country rocks owing to overpressure imposed by the ore fluids. Magnetite and hematite precipitated early near the dolerite intrusions, whereas Cu sulfides formed later in country rocks where sulfide saturation was favored. We propose that this genetic model may be widely applicable to Precambrian IOCG deposits elsewhere that formed in intracratonic rift settings.
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