Diagenesis And Mineralization Of The Hongshishan Mafic-ultramafic Rock Belt,Western Beishan,Xinjiang

A series of Permian mafic-ultramafic rocks and related Ni-Cu mineralization occur in the Hongshishan area,western Beishan Terrane,NW China.The ore is mainly sparsely disseminated with low tenor.There is no breakthrough in recent exploration work.The tectonic setting is on debate.The origin and evolution of parental magma is still unknown.In terms of mineralization,the sulfide segregation,metallogenic condition and regularity are lack of systematical study.In this thesis,detailed geological survey is first presented.The systematical petrological,mineralogical,geochronological and geochemical studies are then conducted.The thesis aims to discuss the tectonic setting,diagenesis and mineralization,and summary metallogenic regularity.Finally,a metallogenic model is established and proposed for following exploration work.The mafic-ultramafic intrusions occur along south of the Hongliuhe-Yigezitage deep fault and as cluster.The wall rocks are mainly lower Carboniferous Hongliuyuan formation and lower Cambrian Shuangyingshan formation.The lithofacies consist of norite,gabbro,olivine gabbro,troctolite,clinopyroxene peridotite,wehrlite and dunite.The sulfide mainly occurs in the late wehrlite.The rocks are characterized by low Si,Ca,alkaline and high Mg content and are classified as sub-alkaline and tholeiitic series.Generally,they have low REE content with no obvious differentiation between LREE and HREE.The rocks are enriched in HFSE(Zr,Hf,Th,U)and Pb,Sr and depleted in Rb,Nb and Ta.The PGE are mainly controlled by magmatism rather than later hydrothermal events.The fractional crystallization is the main magmatism,together with significant crustal assimilation and strong alteration and weathering.The olivine is mainly classified as forsterite with highest Fo=90.3.The clinopyroxene mainly consists of augite and diopside.The orthopyroxene is mainly clinoenstatite.The chromite that crystallizes earlier than silicate minerals and occurs in olivine gabbro and troctolite is more likely primary and is therefore appropriate to evaluate nature of parental magma.Two episodes of mafic magmatism in the Beishan area have been recognised.Most intrusions are early Permian(271-284Ma).The Cantoushan,Xuanwoling,and mafic dikes in Luodong belong to late Permian(261-266Ma).Coexisted middle-acid rocks have ages from 271-280 Ma.The early Permian mafic rocks have relatively wider Rb-Sr isotopic composition.The initial(87Sr/86Sr)i is 0.70295-0.7127 and(143Nd/144Nd)i=0.5122-0.5128 with ?Nd(t)=-1.29-11.2.The zircon 176Hf/177Hf=0.28274-0.28282,?Hf(t)=4.9-7.8,TDM1=618-742 Ma.The parental magma originated from depleted mantle which suffered from metasomatism and modification by subductionrelated fluid.The mantle under Beishan area undergone more modification than that under the East Tianshan and Middle Tianshan.The parental melt of the early Permian mafic rocks originated from 16.5-17.0% partial melting of depleted mantle in relatively shallow depth.The parental magma has an estimated composition of 14.5 wt% MgO,12.3 wt% Al2O3 and 1.9 wt% TiO2 and is characterized by high temperature,picritic affinity,hydrous nature and high Mg contents.The late Permian rocks has similar nature but is of lower degree of partial melting and more modification.In consideration of petrology,mineralogy,geochronology and geochemistry of mafic rocks and comparison with related rocks in the East Tianshan and Tarim area.The large scale early Permian mafic-ultramafic magmatism in the Beishan area are neither subduction-related arc setting,nor mantle plume in origin.The tectonic background seems more likely slab break-off and consequent upwelling of asthenosphere mantle in post-orogenic setting.The upwelling magma near surface results in calc-alkaline volcanic assemblage such as basalt and dacite.Meanwhile,A-,I-,S-types granite also emplaced.The mafic-ultramafic magmatism continues to late Permian.The orebodies mainly occur in the bottom of late wehrlite.The 3D magnetic inversion indicates that magma conduit is from SW to NE.Medium-low temperature elements(Se,Te)anomalies are consistent with orebody,maybe imply that low temperature hydrothermal event has some extent contribution to enrichment of oreforming elements.Sulfide droplets imply sulfide segregates during magma evolution.The Cu/Zr ratio,PGE geochemistry and simulation of olivine composition indicate that sulfur had undergo saturation and 0.015-0.020% segregation after 4% olivine crystallization,resulting in sparsely disseminated ore with low tenor.It is different from the Pobei mafic-ultramafic belt.Sulfide segregation in Pobei occurred prior to olivine crystallization and continued to olivine crystallization,leading to vein type mineralization and sparsely disseminated ores,respectively.Assimilation of calcareous strata makes for segregation,whereas the contribution is limited.The crystallization of chromite,olivine reduces the FeO content and sulfur solubility and finally results in sulfur saturation and sulfide segregation.The model “deep segregation+ in site segregation+ multi periods pulsating” is proposed to explain mineralization in the Beishan area based on comprehensive evidences from tectonic setting,nature of parental magma,sulfide segregation simulation.The intersection site of deep fault and secondary fault is appropriate to seek mafic-ultramafic intrusions.The following geological information should be paid more attention,i.e.differentiated lithofacies,ultramafic rocks such as wehrlite,relative low Ni content in olivine.Some geochemical indicators are also available to evaluate ore-bearing property of mafic intrusions,such as tholeiitic parental magma(>11.5% MgO),high m/s,m/f and low TiO2 content(<1%),the relatively high degree of partial melting(>12-15%),high solidation index(>40).