Geochemical Characteristics and Metallogenesis of the Qingkuangshan Ni-Cu-PGE Mineralized Mafic-Ultramafic Intrusion in Huili County, Sichuan Province, SW China

The Qingkuangshan Ni-Cu-PGE deposit, located in the Xiaoguanhe region of Huili County, Sichuan Province, is one of several Ni-Cu-PGE deposits in the Emeishan Large Igneous Province （ELIP）. The ore-bearing intrusion is a mafic-ultramafic body. This paper reports major elements, trace elements and platinum-group elements in different types of rocks and sulfide-mineralized samples in the intrusion. These data are used to evaluate the source mantle characteristics, the degree of mantle partial melting, the composition of parental magma and the ore-forming processes. The results show that Qingkuangshan intrusion is part of the ELIP. The rocks have trace element ratios similar to the coeval Emeishan basalts. The primitive mantle-normalized patterns of Ni-Cu-PGE have positive slopes, and the ratios of Pd/Ir are lower than 22. The PGE compositions of sulfide ores and associated rocks are characterized by Ru depletion. The PGE contents in bulk sulfides are slightly depleted relative to Ni and Cu, which is similar to the Yangliuping Ni-Cu-PGE deposit. The composition of the parental magma for the intrusion is estimated to contain about 14.65 wt% MgO, 48.66 wt% SiO2 and 15.48 wt% FeOt, and the degree of mantle partial melting is estimated to be about 20%. In comparison with other typical Ni-Cu-PGE deposits in the ELIP, the Qingkuangshan Ni-Cu-PGE deposit has lower PGE contents than the Jinbaoshan PGE deposit, but has higher PGE contents than the Limahe and Baimazhai Ni-Cu deposit, and has similar PGE contents to the Yangliuping Ni-Cu-PGE deposit. The moderate PGE depletions in the bulk sulfide of the Qingkuanghan deposit suggest that the parental magma of the host intrusion may have undergone minor sulfide segregation at depth. The mixing calculations suggests that an average of 10% crustal contamination in the magma, which may have been the main cause of sulfide saturation in the magma. We propose that sulfide segregation from a moderately PGE depleted magma took place prior to magma emplacement at Qingkuangshan, that small amounts of immiscible sulfide droplets and olivine and chromite crystals were suspended in the ascending magma, and that the suspended materials settled down when the magma passed trough the Qingkuangshan conduit. The Qingkuangshan sulfide-bearing intrusion is interpreted to a feeder of Emeishan flood basalts in the region.

Geochemical and Petrological Studies on the Early Carboniferous Sidingheishan Mafic-Ultramafic Iintrusion in the Southern Margin of the Central Asian Orogenic Belt,NW China

The Sidingheishan mafic-ultramafic intrusion is located in the eastern part of the Northern Tianshan Mountain, along the southern margin of the Central Asian Orogenic Belt in northern Xinjiang autonomous region of China. The Sidingheishan intrusion is mainly composed of wehrlite, olivine websterite, olivine gabbro, gabbro and hornblende gabbro. At least two pulses of magma were involved in the formation of the intrusion. The first pulse of magma produced an olivine-free unit and the second pulse produced an olivine-bearing unit. The magmas intruded the Devonian granites and granodiorites.An age of 351.4±5.8 Ma(Early Carboniferous) for the Sidingheishan intrusion has been determined by U-Pb SHRIMP analysis of zircon grains separated from the olivine gabbro unit. A U-Pb age of 359.2±6.4 Ma from the gabbro unit has been obtained by LA-ICP-MS. Olivine of the Sidingheishan intrusion reaches 82.52 mole% Fo and 1414 ppm Ni. On the basis of olivine-liquid equilibria, it has been calculated that the MgO and FeO included in the parental magma of a wehrlite sample were approximately10.43 wt% and 13.14 wt%, respectively. The Sidingheishan intrusive rocks are characterized by moderate enrichments in Th and Sm, slight enrichments in light REE, and depletions in Nb, Ta, Zr and Hf. The εNd(t) values in the rock units vary from +6.70 to +9.64, and initial87Sr/86Sr ratios range between 0.7035 and0.7042. Initial206Pb/204Pb,207Pb/204Pb and208Pb/204Pb values fall in the ranges of 17.23-17.91,15.45-15.54 and 37.54-38.09 respectively. These characteristics are collectively similar to the Heishan intrusion and the Early Carboniferous subduction related volcanic rocks in the Santanghu Basin, North Tianshan and Beishan area. The low(La/Gd)PMvalues between 0.26 and 1.77 indicate that the magma of the Sidingheishan intrusion was most likely derived from a depleted spinel-peridotite mantle.(Th/Nb)PMratios from 0.59 to 20.25 indicate contamination of the parental magma in the upper crust.Crystallization modeling methods suggest that the parental magma of the Sidingheishan intrusion was generated by flush melting of the asthenosphere and subsequently there was about 10 vol%contamination from a granitic melt. This was followed by about 5 vol% assimilation of upper crustal rocks. Thus, the high-Mg basaltic parental magma of Sidingheishan intrusion is interpreted to have formed from partial melting of the asthenosphere during the break-off of a subducted slab.

Geochronology,Petrology and Geochemistry of Xingdi No.3 Mafic-Ultramafic Intrusions in the Northeastern Tarim Craton,NW China

The Xingdi mafic-ultramafic intrusions occur in the northeastern margin of the Tarim craton. The Xingdi No. 3 intrusion is the smallest of four intrusions, with an exposed area of 1.7 km2, and the zircon U-Pb age of the intrusion is 752±4 Ma. The intrusion consists of gabbros, pyroxenites and peridotites, and exhibits a crystallization sequence of the main rock-forming minerals as olivine, orthopyroxene, clinopyroxene and plagioclase. Mineralization occurred at or near the boundaries of the intrusion between pyroxenites and peridotites, and appears as a layered or lenticular shape about 500 m long and 4–15 m wide. The primary sulfides have a relatively simple mineralogy dominated by pyrrhotite-pentlandite-chalcopyrite assemblages, which occur as droplet, star-like and graphic texture and locally sideronitic structures. Geochronological and geochemistry investigations suggest that the Xingdi mafic-ultramafic intrusions and coeval volcanic rock in the Kuluktag area of the Tarim craton formed in an intracontinental breakup environment. Based on the composition of the dominant rockforming minerals and covariant relationships of other oxides versus Mg O, the parental magma of the Xingdi No.3 intrusion belongs to high-Mg tholeiitic basaltic magmas with Mg O of 10.78 wt%. The Xingdi No.3 intrusive rocks are characterized by light REE enrichment relative to heavy REE, negative Nb-Ta anomalies, low 143Nd/144Nd ratios（from 0.511183 to 0.511793） and high initial 87Sr/86Sr ratios（from 0.7051 to 0.7113）. The magma was derived from the enriched-lithospheric mantle and was contaminated during emplacement. According to rock assemblages, mineralization, olivine characteristics, geochemical characteristics and mass balance, there are better copper-nickel ore prospects in the Xingdi No.3 intrusion than in the other three intrusions in the area.

Geochemical characteristics and tectonic setting of the Tuerkubantao mafic-ultramafic intrusion in West Junggar,Xinjiang,China

Mineral chemistry, whole-rock major oxide, and trace element compositions have been determined for the Tuerkubantao mafic-ultramafic intrusion, in order to understand the early Paleozoic tectonic evolution of the West Junggar orogenic belt at the southern margin of the Central Asian orogenic belt. The Tuerkubantao mafic-ultramafic intrusion is a well-differentiated complex comprising peridotite, olivine pyroxenite, gabbro, and diorite. The ultramafic rocks are mostly seen in the central part of the intrusion and surrounded by mafic rocks. The Tuerkubantao intrusive rocks are characterized by enrichment of large ion lithophile elements and depleted high field strength elements relative to N-MORB. In addition, the Tuerkubantao intrusion displays relatively low Th/U and Nb/U （1.13-2.98 and 2.53-7.02, respectively） and high La/Nb and Ba/Nb （1.15 4.19 and 37.7-79.82, respectively）. These features indicate that the primary magma of the intrusion was derived from partial melting of a previously metasomatized mantle source in a subduction setting. The trace element patterns of peridotites, gabbros, and diorite in the Tuerkubantao intrusion have sub-parallel trends, suggesting that the different rock types are related to each other by differentiation of the same primary magma. The intrusive contact between peridotite and gabbro clearly suggest that the Tuerkubantao is not a fragment of an ophiolite. However, the Tuerkubantao intrusion displays many similarities with Alaskan-type mafic-ultramafic intrusions along major sutures of Phanerozoic orogenic belts. Common features include their geodynamic setting, internal lithological zoning, and geochemistry. The striking similarities indicate that the middle Devonian Tuerkubantao intrusion likely formed in a subduction-related setting similar to that of the Alaskan-type intrusions. In combination with the Devonian magmatism and porphyry mineralization, we propose that subduction of the oceanic slab has widely existed in the expansive oceans during the Devonian around the Junggar block.

SHRIMP Zircon U-Pb Age of the Sidingheishan Mafic-Ultramafic Intrusion in the Southern Margin of the Central Asian Orogenic Belt,NW China and its Petrogenesis implication

Objective The Sidingheishan mafic-ultramafic intrusion is located in the eastern part of the North Tianshan Mountains. This work used zircon U-Pb age data, bulk rock major and trace elements, Sr-Nd-Pb isotope data to assess mantle source characteristics and crustal assimilation of the parental magma of the Sidingheishan intrusion. We have also discussed the tectonic evolution of the southern margin of the Central Asian Orogenic belt in the Late Paleozoic.

Trace Element Geochemistry of the Baishiquan and Pobei Intrusions,Xinjiang,NW China:Petrogenisis Implications

The Baishiquan and Pobei Early Permian mafic-ultramafic intrusions were emplaced into Proterozoic metamorphic rocks in the Central Tianshan and the Beishan Fold Belt,northern Xinjiang,NW China.The Baishiquan intrusion comprises mainly gabbro,and mela-gabbro sills occur within and along the margins of the gabbro body.In the Pobei intrusion,two distinct gabbroic packages,a lower gabbro and the main gabbro,are intruded and overlain by small cumulate wehrlite bodies.

Textures and mineral compositions of the Xinjie layered intrusion,SW China:Implications for the origin of magnetite and fractionation process of Fe-Ti-rich basaltic magmas

The Xinjie layered intrusion in the Panxi region,SW China,hosts both Fe-Ti oxide and platinum-group element（PGE） sulfide mineralization.The intrusion can be divided,from the base upward,into UnitsⅠ,ⅡandⅢ,in terms of mineral assemblages.UnitsⅠandⅡare mainly composed of wehrlite and clino-pyroxenite, whereas UnitⅢis mainly composed of gabbro.PGE sulfide-rich layers mainly occur in Unit I, whereas thick Fe-Ti oxide-rich layers mainly occur in UnitⅢ.An ilmenite-rich layer occurs at the top of UnitⅠ.Fe-Ti oxides include magnetite and ilmenite.Small amounts of cumulus and intercumulus magnetite occur in UnitsⅠandⅡ.Cumulus magnetite grains are commonly euhedral and enclosed within olivine and clinopyroxene.They have high Cr2O3 contents ranging from 6.02 to 22.5 wt.%,indicating that they are likely an early crystallized phase from magmas.Intercumulus magnetite that usually displays ilmenite exsolution occupies the interstices between cumulus olivine crystals and coexists with interstitial clinopyroxene and plagioclase.Intercumulus magnetite has Cr2O3 ranging from 1.65 to 6.18 wt.%, lower than cumulus magnetite.The intercumulus magnetite may have crystallized from the trapped liquid.Large amounts of magnetite in UnitⅢcontains Cr2O3（＆lt;0.28 wt.%） much lower than magnetite in UnitsⅠandⅡ.The magnetite in UnitⅢis proposed to be accumulated from a Fe-Ti-rich melt.The Fe-Ti-rich melt is estimated to contain 35.9 wt.%of SiO2,26.9 wt.%of FeOt,8.2 wt.%of TiO2,13.2 wt.%of CaO, 8.3 wt.%of MgO,5.5 wt.%of Al2O3 and 1.0 wt.%of P2O5.The composition is comparable with the Fe-rich melts in the Skaergaard and Sept Iles intrusions.Paired non-reactive microstructures,granophyre pockets and ilmenite-rich intergrowths,are representative of Si-rich melt and Fe-Ti-rich melt,and are the direct evidence for the existence of an immiscible Fe-Ti-rich melt that formed from an evolved ferro-basaltic magma.

Paleoproterozoic emplacement and Cambrian ultrahigh-temperature metamorphism of a layered magmatic intrusion from the Central Madurai Block,southern India:From Columbia to Gondwana

The Madurai Block in the Southern Granulite Terrane(SGT)of Peninsular India is one of the largest crustal blocks within the Neoproterozoic Gondwana assembly.This block is composed of three sub-blocks:the Neoarchean Northern Madurai block,Paleoproterozoic Central Madurai block and the dominantly Neoproterozoic Southern Madurai Block.The margins of these blocks are well-known for the occurrence of ultrahigh-temperature(UHT)granulite facies rocks mostly represented by Mg-Al metasediments.Here we report a dismembered layered mafic–ultramafic intrusion occurring in association with Mg-Al granulites from the classic locality of Ganguvarpatti in the Central Madurai Block.The major rock types of the layered intrusion include spinel orthopyroxenite,garnet-bearing gabbro,gabbro and gabbroic anorthosite showing rhythmic stratification and cumulate texture.The orthopyroxene-cordierite granulite from the associated Mg-Al layer is composed of spinel,cordierite and orthopyroxene.The pyroxene in both rock units is high-Al orthopyroxene formed under UHT metamorphic conditions.Conventional thermobarometry yields near-peak metamorphic conditions of 9.5–10 kbar pressure and a minimum temperature of 980℃.We computed P–T pseudosections and contoured for the compositional as well as modal isopleths of the major mineral phases,which yield temperature above 1000℃.FMAS petrogenetic grid,Al-in-orthopyroxene isopleth,conventional thermobarometry and calculated pseudosection reveal a clockwise pressure–temperature(P–T)path and near isothermal decompression.The U–Pb data on zircon grains from the layered magmatic suite indicate emplacement of the protolith at ca.2.0 Ga and the metamorphic overgrowths yield weighted ^(206)Pb/^(238)U mean ages ca.520 Ma.Monazite from the garnet-bearing gabbro and Opx-Crd granulite yielded ^(206)Pb/^(238)U weighted mean ages of ca.532 Ma and 523 Ma marking the timing of metamorphism.We correlate the layered intrusion to a Paleoproterozoic suprasubduction zone setting,defining the Ganguvarpatti area as part of a collisional suture assembling the Northern and Central Madurai Blocks.The Paleoproterozoic magmatism and late Neoproterozoic-Cambrian UHT metamorphism can be linked to the tectonics of the Columbia and Gondwana supercontinents.

Tectonic Setting of the Cu-Ni Sulfide-Bearing Mafic-Ultramafic Complexes in Northern Jilin Province, NE China

There are series of Cu-Ni sulfide-bearing mafic-ultramafic intrusions widespread in north JilinProvince,Northeastern China. The intrusions formed in Xing’an-Mongolian Orogenic Belt near to the northeastern margin of North China Craton. The complexes were formed in almost same period according to the zircon U-Pb dating reported recently, which means that the complexes were formed in same tectonic period and belong to one tectonic magmatic event. The rock assemblages are different from the ophiolite type and Yidun type in orogenic belt. The mafic-ultramafic complexes formed in the range from 217 Ma to 232 Ma coeval with A-type granites in the area, which formed bimodal igneous rock assemblage. According to the regional angular unconformities, there were existed the orogenies of Caledonian, Hercynian, Early Indosinian, Late Indosinian and Yanshanian. The Early Indosinian coeval with orogenic I-type granites and sanukitie that suggesting the lithosphere thickening in the extrusion tectonic setting of orogenic processes, however the Late Indosinian coeval with bimodal igneous rock assemblage that suggesting the lithosphere thinning in the extension tectonic setting of post-orogenic processes in the Xing’an-Mongolian Orogenic Belt. Chemical composition of the mafic-ultramafic rocks has the characteristics of high-Mg and low-K tholeiites related with inter-continental post-orogenic tectonic setting. The trace elements indicate their formed in conditions of continental extension belt or initial rift and has the characteristics of revolution from oceanic island arc, volcanic arc of continental margin to continental extended belt. The low initial Sr isotopic ratios and positive εNd(t) values suggest that the initial magma of the complexes come from the parting melting of depleted lithospheric mantle. The depleted ithospheric mantle was new formed supported by zircon Hf isotope in Hongqiling complex. The depleted lithospheric mantle may be caused by the asthenosheric mantle upwelling and underplating in the tectonic setting of extension during the Late Indosinian post-orogenic processes.

Petrogenesis and Economic Potential of the Sangong MaficUltramafic Intrusion in the Eastern Tianshan, Central Asian Orogenic Belt: Constraints from Mineral, Whole-Rock, and PGE Geochemistry

The Sangong Cu-Ni sulfide mineralized mafic-ultramafic intrusion is located on the southern margin of the Bogeda-Harlik belt,eastern Tianshan,China.The intrusion is a well-differentiated complex and is comprised of leucogabbro,gabbro,olivine gabbro,Pl-bearing peridotite,and Pl-bearing pyroxenite.The Pl-bearing pyroxenite hosts both irregularly disseminated sulfide and round droplet sulfide.The intrusive rocks have a wide range of SiO_(2)(42.1 wt.%-50.48 wt.%)and MgO(6.21 wt.%-22.11 wt.%),and are enriched in light rare earth elements(LREE),large-ion lithophile elements(LILE;e.g.,Rb,Ba,Sr,and Pb),and palladium platinum group elements(PPGE)but depleted in high-field-strength elements(HFSE;e.g.,Nb,Ta,and Ti)and iridium PGEs(IPGE).These geochemical characteristics indicate that the Sangong mafic-ultramafic intrusion was derived from high degree of partial melting of depleted mantle and interacted with subduction-related material.The low Pd/Ir(3.21-27.44)but high Ni/Cu(1.64-24.16)ratios,combined with the olivine crystals with low Fo(60.88-78.65)and Ni(54.99 ppm-1688.87 ppm)concentrations suggest that the parental magma of the Sangong intrusion were likely high MgO basaltic in composition that experienced extensive evolution prior emplacement.The Ce/Pb ratios(5.8-13.6)and Nb/U ratios(11.6-30.3)of the intrusive rocks all range between MORB and crustal values,the Nb/Yb and Th/Yb values are close to the lower crust values,together with the low Se/S ratios[(17-100)×10^(-6))]suggest that the magma experienced assimilation not only in mantle source but also in conduit,but the degree of crustal contamination is limited.The Cu/Pd ratios of the rocks range from 3.9×10^(4) to 10.8×10^(4),and the Cu/Zr ratios of Pl-bearing pyroxenite in the Sangong intrusion are>1,combined with the presence of sulfide droplets in the Pl-bearing pyroxenite,indicating the parental magma experienced sulfide saturation and the economical ore bodies may present in the depth of the intrusion.Furthermore,given the discovery of the Baixintan and Yueyawan deposits,we propose the DananhuHarlik belt as an essential prospecting target for Cu-Ni mineralization in North Xinjiang.

新疆坡北基性-超基性岩带Ⅰ号岩体Sm-Nd和SHRI MP U-Pb同位素年龄及其地质意义

坡北基性_超基性岩带Ⅰ号岩体位于塔里木板块东北部北山裂谷带内,侵位于下石炭统红柳园组。该岩体规模较大、分异良好,是坡北岩带中寻找铜镍硫化物矿床最具代表性岩体之一。前人根据侵位地层的时代,间接推断该岩体形成于中、晚石炭世。笔者首次对该岩体不同岩相进行了同位素年代学研究,获得岩体Sm_Nd等时线年龄为(307±32)Ma(95%可信度,MSWD=0.12),辉长岩锆石SHRIMPU_Pb年龄为(278±2)Ma(95%可信度,MSWD=1.17)。鉴于铜镍矿化无论从时间上和空间上都与岩体的侵位密切相关,矿床成因类型为岩浆熔离型,说明坡北Ⅰ号铜镍矿区的成岩成矿作用发生于晚石炭世—早二叠世,是后碰撞构造背景下幔源岩浆上侵的产物。

新疆坡北镁铁-超镁铁质岩体地球动力学背景探讨

坡北镁铁-超镁铁质岩体位于塔里木板块东北缘的北山地区，主要由坡一、坡三、坡七和坡十等十几个侵入体组成，已发现坡一和坡十岩体赋存大型Cu—Ni硫化物矿床，目前对坡北含矿岩体成岩成矿构造背景存在不同认识。本文对坡一岩体各类岩石的矿物、主量和微量元素组成进行了系统分析，坡北岩体橄榄石FO值范围为83．17～90．2，斜方辉石主要为古铜辉石，单斜辉石主要为普通辉石和透辉石；铬尖晶石D#和Mg#值变化范围分别为0．45--0．69和0．11--0．41，具有由富铝、富镁向富铬、富铁方向演化的趋势。微量元素具有明显的Nb、Ta和Ti负异常，Pb正异常。岩体主量元素变化特征主要受橄榄石、辉石和斜长石分离结晶／堆晶作用控制。模拟计算表明，岩体母岩浆MgO含量为16．09％，FeO含量为10．38％，岩浆结晶温度为1331～1411℃，为高温苦橄质岩浆。坡北岩体是早期受俯冲流体交代的岩石圈地幔在塔里木地幔柱作用下发生较高程度部分熔融的产物。

塔里木东北缘坡北基性-超基性岩体形成时代及岩浆演化

坡北基性-超基性岩体位于塔里木板块东北缘,出露面积约180 km^2,岩浆分异充分,主要岩性包括橄榄岩、橄榄辉长岩、辉长苏长岩、辉长岩等。锆石U-Pb测年结果显示,辉长岩结晶年龄为277.1±2.2 Ma,辉长苏长岩结晶年龄为273.5±2.9 Ma,橄榄辉长岩结晶年龄为275.5±1.5 Ma,含斜长石橄榄岩结晶年龄为280±2 Ma,不同类型岩石形成年龄集中在270~280 Ma,与塔里木地幔柱在时间和空间上具有一定耦合性。矿物地球化学数据显示,纯橄榄岩中橄榄石F o(镁铁比值)值最高,达到90.9%;单斜辉石成分在不同类型的岩石中相似,MgO和CaO含量连续变化,且呈负相关关系,指示其具有相似的原始岩浆成分;不同类型岩石的微量元素蛛网图与稀土元素配分图变化一致,表明其为同源岩浆演化;Sr-Nd同位素地球化学图解显示,坡北岩体源自亏损地幔,经历了4%~15%下地壳物质的混染。综合研究认为,坡北基性-超基性杂岩体不同岩相具有过渡或侵入接触关系,形成时代相近,具有同源岩浆演化特征,但是岩浆就位顺序不同,地壳混染程度存在差异。

新疆坡北杂岩体西端镁铁--超镁铁质岩体成因的稀有气体同位素制约

新疆坡北镁铁-超镁铁质杂岩体由一个辉长岩体以及二十多个超镁铁质侵入体组成,其中坡一超镁铁质岩体稀有气体同位素组成揭示存在地幔柱的贡献。坡北杂岩体西端的坡一、坡四、坡十和坡十四等几个超镁铁质岩体的稀有气体同位素对比分析结果表明,岩浆矿物的~3He/~4He值(0.26~2.79Ra)分布于地壳与地幔值之间,较高的^(20)Ne/^(22)Ne和较低的^(21)Ne/^(22)Ne值分布于Ne质量分馏线(MFL)和L-K线之间,^(40)Ar/^(36)Ar=295~598。~3He/~4He与^(40)Ar/^(36)Ar比值揭示坡北杂岩体西端不同超镁铁质岩体形成过程中地幔(柱)、地壳和大气组分的贡献不同,岩体成因也可能不同。其中,坡一岩体具有地幔柱作用的贡献,其他三个岩体的岩石圈地幔及地壳流体组分的贡献较大。岩浆地幔源区由深部地幔柱物质叠加俯冲流体交代的岩石圈地幔物质所组成,大气与地壳物质组分可能由俯冲再循环洋壳带入到岩浆地幔源区以及围岩物质的混入。