The Late Paleozoic Magmatism And Corresponding Iron And Copper Polymetallic Metallogenic In Kuokuqueke-Halegati District,Chinese Northwestern Tianshan

Halegati-kuokuqueke area is located in the west of Neopaleozoic Boluokenu metallogenic belt, northwest Tianshan Mountain. It is the significant area of metal mineral resources with the geotectonic location lying in the Neopaleozoic Bolukenu island arc where is between the Juggar block and Central Yili block. With the subduction and closes of the north Tianshan ocean, plenty of Neopaleozoic magmatic rocks and iron, copper, molybdenum and gold polymetallic deposits (spots) appear in this region accompanied with multiple magma activities from the middle Ordovician to early Permian. There are some arguments about the petrogenesis of the abundant granite and the late Carboniferous tectonic background of this region. This research chose the iron copper polymetallic deposits and the related igneous rocks as objects. Based on the field investigation, the study about the late Devonian Aenaxia rocks, Halegati rocks and early Carboniferous Shatedaban rocks and Aimujin rocks like petrography, zircon U-Pb dating, lithogeochemical analysises and Sr-Nd-Hf isotope analysises will be carried out. The origin and the diagenesis progress of the two stages magma will be discussed. Combined with the previous research results of regional geochronology, petrology and economic geology, the geodynamic settings and ore control factors will be discussed and the prospecting direction will be proposed. The main results and conclusions are as follows:1. Kuokuqueke and Halegati iron copper polymetallic deposits are formed under the similar metallogenic backgrounds which are the late Devonian granites intruded into the Neopaleozoic marble and formed the skarn type deposits by contact metasomatism. The main ore minerals of Kuokukueke deposits are magnetite, hematite, chalcopyrite, malachite, galena, sphalerite and so on, and the main gangue minerals are garnet, quartz and carbonate. The main ore textures are massive and brecciated structures. The main alterations are skarnization, chloritization, silicification, potassic and hornfelsic alteration. The ore minerals are formed mainly in the late skarnization stage and sulfide stage. The main ore minerals of Halegati deposits are magnetite, hematite, chalcopyrite, malachite, bornite, chalcocite, galena, sphalerite and molybdenite, and the main gangue minerals are garnet, pyroxene and epidote. The main alterations are skarnization and carbonatation. The disseminated and massive structures are the main ore textures, other ore structures like inclusion structures, stockwork structures also can be found in this region.2. The field work and petrography study showed that the late Devonian granite mass are mainly formed by moyite, monzonitic granite and granodiorite. The zircon U-Pb dating results showed that the emplacement age of Akenaxia moyite, Kuokuqueke granodiorite and Halegati granodiorite are 361±3 Ma、367±3 Ma and 368±2 Ma respectively, which are the same with granite in the Boluokenu island arc. The Akenaxia moyite has the SiO2 contents ranging from 64.05～75.10 wt.% with high K2O+ Na2O contents (7.37～8.78 wt.%), which indicate that the rocks belong to high potassium calc-alkaline series with much higher A/CNK values of 0.92～1.11. It is the peraluminous serious with the characters like medium fractionation of LREE and HREE, strongly negative Eu anomaly, rich in LILE (Rb、Th、K etc.), depleted in HFSE (Nb、Ta、 Zr、Ti etc.), much lower content of compatible elements (Cr、Ni etc.), average contents of whole rock (87Sr/86Sr)i (0.7032～0.7055), εNd (t) ranging from 0.1 to 1.6 and much higher εHf (t) (2.1～4.5). Elements contents of huokuqueke and Halegati granodiorite both range in a slight range, the SiO2 contents ranging from 61.62-66.95 wt.%, the K2O+Na2O contents ranging from 6.78～8.28 wt.%, the K2O contents ranging from 3.31～4.52 wt.%, which indicate that the rocks belong to middle-high potassium calc-alkaline series with middle A/CNK values of 0.81～1.01. It is the metaluminous-weak peraluminous serious, with characters like much higher Mg# of 39～60, medium fractionation of LREE and HREE, weakly negative Eu anomaly, rich in LILE(Rb、 Th、K etc.), depleted in HFSE (Nb、Ta、Zr、Ti etc.), much lower content of compatible elements (Cr、Ni etc.), lower values of (87Sr/86Sr); (0.7058～0.7079), and higher εNd(t) (-1.5～-0.5) and εHf (t) ranging from 1.5 to 4.3, TDM2ranging from999 Ma to 1159 Ma. The results of the whole rock major and trace elements and Sr-Nd-Hf isotope indicate that both the moyite and granodiorite are I type granite. There are residual garnet exist in the magma source of Kuokuqueke and Halegati granodiorite. Besides the residual hornblende and little plagioclase exist in the magma source of Akenaxia moyite. Hornblende, pyroxene and apatite fractional crystallized during the magma rising progress. The magma source of granodiorite has the high partial melting degree and the main magma source is the Neoproterozoic mafic lower crust with a little mantle-derived material.3. The Late Carboniferous granites from the study area are mainly distributed in the Sailimu plate located in the north of the fault zone from northern slope of Mount Boluokenu. Zircon LA-ICP-MS U-Pb dating yield weighted mean 206Pb/238U ages of 302±2 Ma and 315±2 Ma for Shatedaban biotite monzonite granite and Aimujin biotite monzonite granite, respectively. The inherited zircons are present in Aimujin granite, and give ages of 363 Ma and 380 Ma. The whole-rock geochemistry results show that these two granites have high SiO2, Al2O3, Na2O, K2O contents, but low MgO content and Mg# values, indicative of high-K calc-alkaline, weak peraluminous compositions. The granites are enriched in large ion lithophile elements and depleted in high field strength elements, and show distinct negative Eu anomalies (0.22～0.36 and 0.11～0.62, respectively) with low (La/Yb)N ratios (2.16～6.58 and 0.96～6.23, respectively). These granites have consistent εNd(t) values (0.1～3.8 and 0.1～3.5, respectively), and Aimujin granite has (87Sr/86Sr)i ratios from 0.7009 to 0.7057. The zircon εHf(t) values of these granites are relatively high, ranging from 6.9 to 9.land from 2.6 to 7.4, respectively. Their corresponding two-stage Hf model ages are 683～804 Ma and 788～1051 Ma. The evidences derived from major and trace element, and Sr-Nd-Hf isotope compositions argue that Shatedaban and Aimujin granites are weak peraluminous I-type granites, and they were derived from the partial melting of juvenile crust that formed during the underplating of early subduction-related arc magmas. The plagioclase and amphibole were present in the residue. During the upwelling, the resultant melts experienced different degree of fractional crystallization and crustal contamination.4. Combined with previous studies, this study suggests there were three episodes of Paleozoic magmatism in the northwest of Tianshan, namely:Early Paleozoic (466-447 Ma), represented by diorite and gabbro intrusions in Sailimu plate; Late Devonian-Early Carboniferous (395～346 Ma), characterized by volumetrically dominant I-type granites in Boluokenu island arc and intermediate-felsic volcanic rocks from Dahalajunshan Group; Late Carboniferous-Early Permian (320～278 Ma), during this period the magmatism rolled back to the Sailimu plate, and formed the I-type granitic batholith in east part and stocks in west part, associated with contemporaneous Nb-rich basalt, high-Mg andesite, adakite in Alatau region. The diagenesis and mineralization in this region were controlled by North Tianshan oceanic subduction and collision processes. During the Late Devonian to Early Carboniferous, the North Tianshan oceanic crust subducted beneath the Yili plate, and resulted in the first magmatism and related mineral deposits. To the Early Carboniferous (346-320 Ma), the tectonic setting changed to flat-slab subduction stage and induced the quiescent period of magmatism, with no ore deposits formed in this stage. Until the Late Carboniferous, the subduction angle of north Tianshan oceanic plate changed from flat subduction to oblique subduction, rather than the previous proposals that North Tianshan oceanic subduction was considered to be finished and evolved into intracontinental orogenic processes.5. Depending on various parameters, such as the emplacement depths of the Late Paleozoic intrusions, physical and chemical conditions of surrounding rock, and styles of ore-controlling structure, different types of ore deposits formed and constituted a unified metallogenic system. The iron-copper skarn deposits formed through contact metasomatism, when magmas intruded the Paleozoic carbonate sedimentary rocks; the porphyry copper-molybdenum deposits formed at the top (edge) of intrusions, or contact zone, or adjacent breccias, when magmas intruded Paleozoic clastic rock; the related hydrothermal vein deposits might form by filling and replacement in clastic rocks when hydrothermal fluids migrated along structural fracture zone to the distal part. In the Halegati ore concentration area, the metallogenic system shows iron (copper)-lead and zinc-molybdenum (copper)-copper (molybdenum) horizontal mineralization zoning from Halegati to the periphery. The spatial distribution of ore-related intrusions was controlled by main regional fault and its secondary structure. The contact zones controlled the positioning of ore bodies; the magmas were the key internal factors controlling ore formation; and the strata showed a metallogenic specialization. This paper proposed that the mineral prospecting work in this region should focus on the Late Devonian-Early Carboniferous intrusions in Boluokenu island arc, analyze the characteristics of metallogenic system, and operate the absence prospecting.