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Geology And Geochemistry Of Huanggang Sn-Fe Deposit, Inner Mongolia

Posted on:2012-11-10Degree:DoctorType:Dissertation
Country:ChinaCandidate:Z H ZhouFull Text:PDF
GTID:1100330335476368Subject:Mineralogy, petrology, ore deposits
Abstract/Summary:PDF Full Text Request
The Huanggang Sn-Fe deposit in Inner Mongolia is one of the important large polymetallic deposits in the polymetallic belt of south Daxinganling, the largest tin-based multi-metal deposit in the north of the Yangtze River, and also it is the second largest iron deposit in Inner Mongolia Autonomous Region. In this paper, we choose Huanggang Sn-Fe deposit as a typical deposit to study, which were based on the full collection and summary of the previous studies. Through the methods and means as field investigation, electron microprobe analysis, major and trace elements component analysis, Sr-Nd-Pb-Hf isotopic tracer, fluid inclusion, stable isotope, etc., we mainly studied the aspects of the host rocks characterics, skarn bodies characterics, source of ore-forming material, source of ore-forming fluid, genesis of the deposit, mineralization process and metallogenic geodynamics background, etc, established the mineralization model of Huanggang deposit, preliminary discussed the regional metallogenic rule in the study area, the main achievements obtained were as follows:(1) Detailed field geological survey indicates that lower Permian Dashizhai Formation and Huanggangliang Formation are the main ore-hosting layers. Ore bodies are largely concordant with the bedding of the strata, and the distribution of the ore bodies is spatially related to skarn. Main types of ore minerals are major of laminated, massive skarn ores, followed by the banded, agglomerate ore produced in the marble, with clear contact lines between wall rocks and ore bodies. There are various types of ore minerals, commonly developed rock alteration, and skarns with obvious zoning. The formation of Huanggang deposit experienced a period composed of skarn phase, retrograde eroding phase, quartz sulfide phase and carbonate phase, among which the second and third phase are the main mineralization stages.(2) The deposit was temporally and specially related to the late Yanshanian granites, the Huanggang granites are characterized by SiO2 content (66.81~77.93%), Al2O3 content (11.07~14.54%), and significant depletion of magnesium, high ALK (5.65~10.67%), the K2O/Na2O values format a range of 0.32 to 10.53, averaging 2.26. The chondrite-nomalised REE pattern shows LREE enrichment, strong negative Eu anomalies, and 8Eu at 0.03 to 0.24. The high field strength elements such as Zr, Hf and lithophile elements such as Rb, U and Th are enriched, whereas the elements P, Ti, Ba and Sr are significantly depleted and their have similar Y/Nb values(>1.2) to those of oceanic island basalts. These features are coincident with the typical A1 within-plate anorogenic granite. (3) The isotopic composition characteristics show that (87Sr/86Sr)i values of the Huanggang granites range from 0.70211 to 0.70729, theεNd(t) values and Nd model ages(TDM) vary from-0.8 to 0.9 and 855 to 993 Ma respectively; The whole rock Pb isotope values are relatively high, the 206Pb/204Pb values range from18.974 to 26.107, with 207Pb/204Pb values between 15.554 and 15.914, and 208Pb/204Pb values between 38.894 and 39.890, which suggested the lead with a mixed source; The 176Hf/177Hf values range from 0.282744 to 0.282922, with the correspondingεHf(t) values ranging from 1.9 to 18.3, and Hf model ages (TDM2) from two-stage between 561 and 888Ma, young Nd, Hf isotope model ages imply that this area took place an important crustal growth event during the late Proterozoic. The Huanggang granites derived from the partial melting of newborn lower crust originated from depleted mantle, and maybe with the contamination of a small amount ancient continental crust. We conclude that the formation mechanism of Huanggang granites as following:the separation of mafic magma, produced from the partial melting of mantle wedge metasomatized by the precipitated fuilds from subducted oceanic crust or the decompressed depleted mantle, could induce lithospheric delamination, mantle upwelling and magma underplating, which promoted the remelting, differentiation and continued evolution of mafic primary crust, resulting in a large number of granitic magma, and its geodynamic setting was consistent with the regional Basin and Range tectonic setting.(4) Electron microprobe analyses showing that the components of garnet and pyroxene are Adr28.69~96.44Grs2.00~67.38(Prp+Sps)0.67~5.69 and Di11.8~94.12Hd4.08~81.28JO1.79~20.02, respectively, their large range of composition character reflects that skarns were not formed under the totally enclosed equilibrium condition. The amphibole mostly belong to magnesium-ferric-calcium amphibole, only some individuals are ferric-hornblende, larger change of composition is probably due to the change of redox conditions resulting in different degrees of AlⅥSi←→(Na, K) displacement, which belongs to the transforming tendency under solidus. Tetrahedral Si, Al, octahedral Al, Ti and cations in A site of amphibole change greatly, which may be caused by the composition difference of the magma in contact metasomatic process or the change of physicochemical condition during crystallizing. Mn-enriched pyroxene skarns were formed by infiltration metasomatism of magmatic fluid along fracture zones between layers, Mn-rich pyroxene could be the indications for looking for Sn, Cu, Zn and many other metal ores in this area, and the outer contact zone of skarn and its vicinal marble is the favorable position for polymetallic mineralization.(5) Fluid inclusion studies show that the inclusion types of Huanggang deposit were complex, mainly of silicate melt inclusions, vapor-rich liquid inclusions, liquid-rich liquid inclusions, daughter mineral-bearing inclusions, CO2-rich inclusions and CO2-bearing mutiphase inclusions, in which the liquid-rich inclusions as the main. Inclusions in early mineralization phase mostly contain silicate melt inclusions and H2O-NaCl type inclusions, a small amount of CO2-H2O±H4 type inclusions and CO2-H2O-NaCl type inclusions occurs in late mineralization phase. Homogenization temperatures of four phases from early till late are (257~432℃,>550℃),322~403℃,202~304℃,153~221℃respectively; salinities are (12.13~19.88%,>66.8%),16.43~22.34%,1.74~14.77%,1.74~11.9% respectively. Homogeneous temperature and salinity of ore-forming fluid concentrated in 220~432℃and 1.74~22.34%, which belong to the deposit type of medium-high temperature and medium-low salinity. Gas composition of fluid inclusions mostly contain CO2 and H2O, followed by the N2, O2, and CH4, a small amount of C2H2, C2H4 and C2H6; cation of liquid composition mostly contain Na+, K+, followed by Ca2+, Mg2+, while anions with Cl-, SO42- as major, followed by the F-, and also contain a small amount of Br-, NO3-. Buck boiling and phase separation of fluid as well as fluid mixing may be the main reasons of mineralization.(6) H-O isotopic tracing show that 8Dv-smow values of gangue minerals in different mineralization stages vary greatly, mainly of -116~-73‰, averaging -98‰, the individual samples range from-182‰to -187‰.δ18OH2O values ofⅠtoⅣstage were 7.4~9.8‰,-3.3~8.6%‰,-6.0~4.9‰and -10.9~-1.6‰, respectively, which indicate that the ore-forming fluids were mainly of magmatic water, there also being meteoric water mixing and post-magmatic hydrothermal superimposed mineralization.δ34SV-CDT values of ore minerals range from -9.0~4.5‰, averaging -1.87‰, which have wider range relative to the averageδ34S values of mantle, showing a mixture character of sulfur through transformation effect; Re isotopic content of molybdenites were high, change range of 209.7~300.6×10-6, an average of 260.5×10-6, which were far higher than the Re contents of most molybdenites, indicating that the ore-forming minerals of Huanggang deposit were mainly came from the mantle and having a part of the crustal material added.(7) LA-ICP-MS zircon U-Pb dating results show that the K-feldspar granite and granite-porphyry in the Huanggang rocks were formed at 136.7±1.1Ma and 136.8±0.57Ma, respectively. Re-Os isotopic dating of the molybdenites symbiosis with magnetites obtained isochron age of 135.3±0.70Ma. The host rock and mineralization ages of Huanggang deposit were consistently, both occurred in early Cretaceous, suggesting that their have a close genetic relationship. Ore-forming ages data statistical analysis show that the mineralization of the Da Hinggan Mts. area were mainly in Mesozoic Yanshanian, there are two ore-forming eruptive periods, i.e., c.140-130 Ma and 180~160 Ma, while the tin-lead-zinc-copper-silver polymetallic deposits related with the small intrusion in Yanshanian under lithospheric extensional and thinning environment mainly occurred in c.140~130 Ma; the molybdenum-lead-zinc-copper-aurum polymetallic deposits related with the post-collision orogeny of Siberia plate and North China plate in early Yanshanian mainly occurred in c.180~160Ma. Huanggang tin-iron deposit is the product of large-scale mineralization under the paleo-Pacific plate subduction environment.(8) Based on the detailed study described above, we compared the typical deposits coexisting with tin and iron at home and abroad, established the metallogenic mode of the Huanggang tin-iron deposit, and preliminary discussed the regional metallogenic rule in the study area.
Keywords/Search Tags:Skarn, A-type granite, Geochemical characteristics, Fluid inclusions, Metallogenic mechanism, Host rock and mineralization ages, Huanggang Sn-Fe deposit
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