| In 2015,Huangyangshan graphite deposit hosted by granite pluton in the Kalamaili district 160km away on the north of Qitai County,Changji State,Xinjiang,was discovered by Xinjiang Exploration Squad of Building Metarials.Total 8 ore bodies were recognized.The reserve of crystalline graphite is estimated to be 72.64Mt,which is a super-large scale.Besides,90%of the graphite in this deposit occurs in orbicular structure with diameter up to 20cm,which is rare on earth.Unlike the common graphite deposits resided in metamorphic rocks,the Huangyangshan deposit is a rare type of graphite deposit with super-large scale in orbicular structure hosted by granite.The research state of this deposit is relatively low,this contribution unravels on the forming mechanism of graphite orbicules through petrography,geochemical composition,diagenetic geochronology,Sr-Nd-Hf isotope,ore-forming fluid,provenance,and mineral compositions of this deposit.The ore bodies of Huangyangshan graphite deposit are hosted by Huangyangshan granite pluton,distributing along the boundaries of comagmatic sequences.The metallogeny could be divided into one ore-forming period and two stages,namely the fluid period including the magmatic fluid replacement stage and the hydrothermal superposition stage.The magmatic fluid replacement stage is the predominant metallogenic stage,when graphite orbicules composed of core and shell were formed.The diagenetic minerals of orbicule cores comprise alkali-feldspar,plagioclase,quartz,hornblende and minor boitite,which are in fine-grained granitic texture and are severely altered.The diagenetic minerals of orbicule shells and matrix consist of alkali-feldspar,plagioclase,quartz,clinopyroxene and hornblende,which are in medium-grained granitic texture and apparently underwent alteration.In orbicule shells clinopyroxene and hornblende are highly concentrated,while these ferromagnesian minerals are relatively scarce in matrix.Graphite and sulfides mainly develop in the orbicules,replacing the silicates.Most ferromagnesian minerals are altered into chlorite,and feldspars sericite.Graphite formed in the hydrothermal superposition stage is in vein structure.Graphite crystals of both metallogenic stages are in colloform and flake texture.Results of laser Raman spectroscopy and X ray diffraction connote graphite of both ore-forming stages have higher degree of order and crystallization contrasting to graphite in Yundukala Formation in the region.Through a series of analyses,acquiring understandings as below:1.Through the two aspects including petrography and electronic micro-probe analysis demonstrates the core of the graphite orbicules in this deposit is the fragment of the granitic host rocks,and the shell and the matrix of the graphite orbicules are the products of the ore-forming magma by liquid immiscibility.2.Through the three aspects including geochemistry,fluid inclusion and H-O isotope demonstrates graphite and sulfides in the graphite orbicules in this deposit are formed through the replacement?or alteration?by the magmatic hydrothermal fluid.3.Through the aspects including microthermometry on fluid inclusions,laser Raman analysis,XRD analysis,C isotope,S isotope and Sr-Nd-Hf isotope demonstrates graphite and sulfides in the graphite orbicules are formed through alteration reaction as the following at about 630?,2000bar,and-18log10f O2:3CH4+2SO2+2(Fe2+)7[Si4O11]2?OH?2?hornblende?=3C?graphite?+2Fe S?pyrrhotite?+8Si O2?quartz?+2?Fe3Si4O10?OH?2+Fe3?OH?6??chlorite?.CH4 in the above reaction is decomposed from the organic matters in the stratum assimilated by the ore-forming magma,while SO2 is derived from the deep mantle.4.Based on the results of all analyses,envisage the graphite orbicules in this deposit are formed through the three steps,such as the incorporation of granitic host rock fragments and H2O,the liquid immiscibility and the replacement by the late magmatic fluid.The forming mechanism of the graphite orbicules are postulated as follows:?1?Ore-forming magma originated from partial melting of depleted mantle contaminated by minor crustal substances and migrated upwards entraining deep mantle SO2.Organic matters were captured by ore-forming magma and decomposed into CH4 dissolving in the magma.At Late Carboniferous,the ore-forming magma intruded the partially crystallized Huangyangshan pluton through the fractures at the boundaries between comagmatic sequences,and myriad granitic host rock fragments containing H2O fluxed into the ore-forming magma.?2?The ore-forming magma was H2O saturated,resulting in liquid immiscibility with segregation of orbicular mafic melt with felsic residual melt.Most mafic melts segregated with felsic residual melt,enclosing granitic host rock fragments.So,granitic host rock fragments graded into orbicule cores,and mafic melt evolved into orbicule shells,and felsic residual melt developed into matrix.?3?At about 2000bar,630?,and-18log10f O2,CH4-and SO2-bearing late magmatic fluid exsolved from mafic melt replaced silicate minerals through the following deuteric alteration:3CH4+2SO2+2(Fe2+)7[Si4O11]2?OH?2?hornblende?=3C?graphite?+2Fe S?pyrrhotite?+8Si O2?quartz?+2?Fe3Si4O10?OH?2+Fe3?OH?6??chlorite?.As this reaction proceeded,colossal graphite and sulfides propagated,and zonation transpired in orbicules. |
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