Ore-forming Age, Metallogenic Geodynamic Setting And Genesis Of The Dahongliutan Iron Ore Deposit, West Kunlun, Xinjiang

The Western Kunlun orogenic belt, located along the northwest periphery of the Tibetan plateau and southwest part of Tarim Basin, represents a key tectonic junction between the Pan?Asia and the Tethyan tectonic domains. The belt is subdivided into the North Kunlun, South Kunlun, Tianshuihai and Karakorum terranes. This orogenic belt contains Precambrian to Cenozoic sedimentary rocks, as well as numerous igneous and metamorphic rock units, which reflect the immense complexity of its tectonic assemblages. The long?term complex geological evolution had facilitated the formation of a wide variety of ore systems and shows excellent exploration potential. The Dahongliutan Fe?ore deposit is a newly discovered large?scale hematite?rich Fe deposit in eastern segment of the Tianshuihai terrane. On the basis of detailed description on the geological characteristics of the Dahongliutan deposit, the author done some research included mineralogy, major?trace element geochemistry, geochronology, stable isotope, and then carried out the study for ore?forming age, metallogenic geodynamic setting, geochemistry. After this, the author compares the Dahongliutan Fe deposit with typical BIF deposits in the world, finally synthesizes genesis of the iron deposit and establish the metallogenic model. The main advance achievements from this study are as followings.1. The deposit is hosted in the Tianshuihai Group, a suite of neritic, siliciclastic and sedimentary carbonate rocks metamorphosed to greenschist facies. The protoliths of the Tianshuihai Group contain a suite of muddy clastic?carbonate sequence. The host rocks mainly consist of chloritoid?muscovite schist, muscovite?quartz schist, silicified calcitic marble and ferroan dolomitic marble. The most important host rocks are ferroan dolomitic marble and chloritoid?muscovite schist. Most of the Fe orebodies are laminate, quasi laminate and lenticular in form, mainly interlayered with ferroan dolomitic marbles on the field outcrops. The deposit contains TFe grade ranging from 18 ? 53%(average 35%). Ore minerals are mainly hematite with minor siderite, limonite, chalcopyrite, chalcocite. Gangue minerals include quartz, dolomite, ferroan dolomite, ankerite, calcite, muscovite and chloritoid. The ores display granoblastic, lepidoblastic, blastopsammitic, metasomatic textures, and banded, massive, corrugation and impregnation structures.Based on the different combination of the ore and gangue minerals, the Fe ores are sub?divided into four types: Type 1(Quartz?hematite) ore, Type 2(Quartz?dolomite?calcite?muscovite?hematite) ore, Type3(Quartz?ankerite?hema? tite) ore, Type 4(Quartz?siderite?hematite) ore, Type 5(siderite?hematite? sulfide) ore. Type 1 ore is the principal ore type, accounting for 90% of the total ore reserves. According to field geological characteristics, mineral combination, mineral chemical composition and contact relationship, the author divided the mineralization into four stages: primary depositional?diagenetic, hydrothermal, metamorphic and supergene oxidation stages.2. U?Pb dating results indicate that the medium?grained porphyritic biotite?quartz monzonite, medium?fine grained biotite?quartz monzonite and metagabbro intrusion occurred during the Cambrian?Early Ordovician(i.e., 484.5 ± 2.7 Ma, 527.9 ± 2.8 Ma and 532.3 ± 3.1 Ma), which indicates the latest ore?forming age is 532 Ma. The youngest zircon grain yielded ages of 593 ± 7 Ma from chloritoid?muscovite schist and silicified calcitic marble, which represents the oldest ore?forming age. Thus, the author constrains the Dahongliutan Fe deposit formation age to be ca. 532?593 Ma. According to metamorphic ages(485 ± 8Ma and 489 ± 6Ma) from metagrabbro, the emplacement age of medium?grained porphyritic biotite?quartz monzonite and Caledonian intense metamorphism in the Tianshuihai terranes, the author infer that a later regional magmatic or metamorphic event may have occurred at ca. 480 Ma, and last to 440 Ma.3. 163 detrital zircon analyses from the Dahongliutan marbles and schists define five major age populations, namely: 2561?2329 Ma(a peak of 2442 Ma), 2076? 1644 Ma(two peaks at 2051 Ma and 1835 Ma), 1164?899 Ma(a major peak at 943 Ma and a subordinate peak at 1142 Ma), 869?722 Ma(a major peak of 830 Ma and a subordinate peak of 754 Ma) and 696?593 Ma(a prominent peak of 637 Ma), indicating that the Tianshuihai terrane experienced several major tectono?magmatic events(corresponding to the global growth of continental nuclei, the assembly of the Columbia supercontinents, the Rodinia assembly and breakup, and the Gondwana assembly). Based on the old age of 2.7Ga~3.2Ga, crustal model age(TDM2) of 1.6Ga~3.6Ga and Archean?Palaeoproterozoic ages reported from the high?grade metamorphic? and volcanic rocks, the author suggest that there is Precambrian basement in Western Kunlun with unique geological structural evolutional history. The age peaks of 1142 Ma, 943 Ma, 830 Ma, 754 Ma and 637 Ma and positive εHf(t)values are comparable to other areas in the world, indicating that Western Kunlun is a part of the Rodinia and the Gondwana supercontinent.4. The metallogenic geodynamic setting of the Dahongliutan Fe deposit is related to the Gondwana assembly. Based on the tectonic of the Western Kunlun orogenic belt, the author proposed that the evolution is as followings. During the Late Neoproterozoic to Early Cambrian(532?593 Ma), the Dynamic model of Western Kunlun the back?arc extension following the the Gondwana assembly, which is supported by continental arc?related settings or a back?arc setting for the schists. Following constant tension, South Western Kunlun Ocean, a part of the Proto?Tethys formed between the South Kunlun and Tianshuihai terranes. During the early stage, the mature oceanic crust do not form, which is supported by neritic–littoral marbles. Until the later stage, the mature oceanic crust has developed, which is supported by metagrabbro formed in Mid?Ocean Ridge, with the emplacement age of 532 Ma. During Early Cambrian to Early Ordovician(532?485Ma), South Western Kunlun Ocean subducted toward north and south continents. The Tianshuihai and Karakorum terrane belongs to post?collision tectonic environment at ca. 480 Ma, which is supported by the medium?fine grained biotite?quartz monzonite in a subduction setting with the emplacement age of 528 Ma, and the medium?grained porphyritic biotite?quartz monzonite in post?collision tectonic setting with the emplacement age of 485 Ma.5. Type 1 ore, Type 2 ore, Type 3 ore and Type 4 ore cantain low Ni/Zn ratio(0.03~0.85) and Co/Zn ratio(0.01~0.35), which are similar to Ni/Zn ratio(0.08~0.78) and Co/Zn ratio(average 0.15) of hydrothermal deposit. The concentration of REE in iron ores is lower than PAAS. Post Archean Australian Shale(PAAS) normalized REE patterns for iron ores displayed depletion of Light Rare Earth Elements(LREEs) relative to Heavy Rare Earth Elements(HREEs), no obvious anomalies of La and Ce, and slightly positive anomalies of Y. Type 1 ore and Type 4 ore show intensive positive anomalies of Eu(average 2.07 and 4.28, respectively, >1.8) and relatively low concentration of REE. Type 2 and Type 3 ores show positive anomalies of Eu(average 1.42 and 1.46, respectively, <1.8) and relatively high concentration of REE. However, Type 1 ore and Type 4 ore contain lower Y/Ho ratio(average 28.48 and 27.88, respectively) than Y/Ho ratio(average 29.18 and 29.43, respectively). The characteristics of trace element and REEY for Type 1 ore and Type 4 ore profiles resemble that of themixture of high?temperature hydrothermal fluid and seawater. However, Type 2 ore and Type 3 may be attributed to contribution of Fe partly through low?temperature hydrothermal solutions.6. Type 1, Type 2,Type 3 and Type 4 ores cantain uneven Si O2/Al2O3 ratio(1.53~1603.33), Th/U ratio(0.19~5.34), Ni/Co ratio(1.44~6.33), and low Sr/Ba ratio(0.05~1.56). Al2O3 displays good positive correlation against Ti O2, Na2 O, K2 O, P2O5, As, Sc, V, Cr, Co, Cu, Rb, Zr, Nb, Cs, REE and Th. Al2O3 and Ti O2 display negative correlation against TFe2O3, Fe O and Mn O. TFe2O3 and Fe O show negative correlation against Ca O, Na2 O, K2 O, As, Zr, Cs and Nb. The characteristics of iron ores are interpreted as the result of the incorporation of a detrital component. However, Type 2 ore show slightly lower Si O2/Al2O3 ratio than Type 3 ore, but obviously lower than Type 1 ore and Type 4 ore. Type 2 ore contain higher Zr, Cu, Cs, Th and Cr than Type 1 ore, Type 3 ore and Type 4 ore. The characteristics of iron ores are consistent with incorporation of higher amounts of clastic components in Type 2 ore. Type 3 ore contain slightly lower detrital input. Type 1 ore and Type 4 ore have lowest detrital component admixtured with their original chemical precipitates.7. With the exception of negative anomalies of Ce from few Type 2 ores, the other iron ores display no obvious anomalies of Ce. The δ13C values of silicified calcitic marble, ferroan dolomitic marble and Type 2 ore consistent with those of normal marine carbonates, and show similar to those of carbonates from Wittenoom Formations, Hamersley Basin, Western Australia and Archean?Palaeoproterozoic laminated dolomite. Type 3 ore and Type 4 ore, with negative δ13C values, is comparable to ankerite and siderite from BIFs, Hamersley Basin, Western Australia. Type 3 ore has slightly higher δ13C values than Type 4 ore. The characteristics of host rocks and iron ores suggest that silicified calcitic marble, ferroan dolomitic marble and Type 2 precipitated in shallow oxidized waters, while Type 1 ore and Type 4 ore precipitated in deeper anoxic waters, however, Type 3 ore precipitated in transition zone.8. Type 1 ore shows negative δ18OV-SMOW varying from ?3.9‰ to?0.9‰, average being ?2.1‰, which is very similar to those of high?grade BIF?hosted iron ore deposits. The oxygen isotope geochemistry of hematite indicated that metamorphic and meteoric fluids were responsible for the hydrothermal enrichment of banded iron formations to highgrade iron ores. δ34S values from barite,chalcopyrite and chalcocite vary from +4.7‰ to +10.1‰, +8.0‰ to +12.7‰ and +9.5‰ to +11.4‰, respectively, average being +7.25‰, +10.28‰ and +10.58‰, respectively. The characteristics of sulfate and sulfide show that the source of S in the deposit is from sulfate of igneous rocks or the admixture of marine sulphate sulfur and magma sulfur.9. Based on geological and geochemical characteristics of The Dahongliutan Fe?ore deposit, comparing with typical BIF deposits in the world, the author infers that genesis type of this deposit is special. The ore-forming age is Late Neoproterozoic to Early Cambrian, while the geological and geochemical characteristics resemble to those of Superior BIFs.10. On the basis of geological characteristics, metallogenic geodynamic setting, and ore deposit geochemistry, the author established metallogenic model. An four?stage metallogenic model is proposed here:(1) depositional ? diagenetic stage(593?532Ma)(2) hydrothermal stage(532?485Ma)(3) metamorphic and deformation stage(485?440Ma)(4) supergene oxidation stage.