Study On Mineralogy And Ore-forming Fluids Of The Hengdong Cobalt Deposit In Northeastern Hunan Province Of South China

The Hengdong cobalt ore deposit located in Northeastern Hunan province, South China, a segment of the Jiangnan Orogen, has been controlled by the Changsha-Pingjiang ductile shear fault. It’s the first case that the protogenetic cobalt ore deposit has been discovered in Hunan province. Based on field geological survey, the study of petrography, mineralogy, mineral chemistry, isotopic geochemistry and geochronology, and fluid inclusion has been carried out. Then the geological characteristics have been summed up, the genesis has been discussed, a metallogenetic mode has been established and a prospecting direction has been put forward.1) By the detailed geological survey, combined with regional mineralization, we have studied on geological characteristics, metallogenic conditions and ore-controlling factors.2) By the detailed observation on field and analysis indoors, there are two types of ores found in Hengdong deposit, including altered tectonic breccias type and silicalite type. The ore minerals mainly are pyrite, minor pyrrhotite, chalcopyrite, sphalerite, galena, etc... The gangue minerals include quartz, chlorite, and minor sericite, calcite, kaolinite and rutile. The wall rock alteration is well developed, including the tectonic hydrothermal alteration zone, compressed fracture zone and migmatization zone in sequence from the ore body to wall rocks. The ore formation of the Hengdong Co deposit has been divided into a pre-ore tectonic stage and three mineralization stages.3) The in situ EPMA and LA-ICPMS analysis has been carried out on sulfides. The result shows that Co elements present in sulfide lattices with the form of isomorphism; S/Se ratios imply that the deposit genesis is constrained by the magmatic endogenic process; Co/Ni ratios imply that the mineralization is related to moderate-high temperature environment.4) The study of genetic mineralogy from chlorite shows that the chlorite in second stage mainly comprises ripidolite but brusvigite in third stage. The calculated temperatures from chlorite chemical composition are from 280℃~347℃in second stage to 280~114℃in third stage. The study of rutile implies that Ti4+, Si4+, Al3+, Mg2+, Fe2+, etc. have exchanged during the mineralization because of the varying physicochemical condition. The Cr/Nb ratios and Nb contents of rutile shows that the rutile originates from metamafic rocks, which closely relative to the PaleoproterozoicArchean strata.5) Through systematic petrographic observation, the ore-bearing quartz vein and the quartz hosted in mylonite have been studied in details to investigate the fluid evolution of the Hengdong cobalt deposit. Four types of fluid inclusions have been recognized in ore-bearing quartz, which include vapor-rich or pure vapor(V-type) inclusions, liquid-rich(L-type) inclusions, daughter mineral-bearing(S-type) inclusions, and pure liquid(PL-type) inclusions. Microthermometric analysis shows that the fluid of pre-ore tectonic stage is characterized by the occurrence of L-type inclusions associated with homogenetic temperature of 169 to 223℃. The ore-forming fluid of the first mineralization stage is characterized by the occurrence of L-, and V-type inclusions associated with homogenetic temperature(250 to 329 ℃). The ore-forming fluid of the second mineralization stage is characterized by S-, L-, and V-type inclusions. S-type inclusions in the second stage contain mainly halite, sylvite, sulfide and other unidentified daughter minerals with estimated homogenization temperatures of 275 to 340℃. V-type inclusions(coexisting with the S-type inclusions) homogenize to vapor phase at 257-366℃, suggesting that the ore-forming fluid may have experienced boiling in the second stage. The third stage is characterized by the occurrence of L-type inclusions and low homogenetic temperature(150 to 234℃). Representative inclusions were systematically measured by the Laser Raman Spectroscopy and the corresponding results shows that vapor phase composition of the second stage V-type inclusions was primarily H2 O. PL-type inclusions also showed distinct characteristics peak for H2 O, and H2 O was the major composition in the vapor phase of the abundant different stages L-type inclusions. The vapor phase composition of the different stages S-type inclusions was also primarily H2 O. Besides, the fluid inclusion microthermometer is generally in accordance with the estimated chlorite temperatures.6) Sulfur isotope ratios of sulfide minerals are measured in this thesis, yielding δ34SCDT values relatively large ranging from-1.5 to-15.9 per mil, with a mean of-10.8 per mil. Because sulfides from the Lengjiaxi Group host rocks have a main range of δ34SCDT values from-13.1 to-6.3 ‰, we believe that this stratum most likely provided sulfur for the Hengdong Co mineralization. Nevertheless, the broad range of the isotopic data also indicates multiple sources of sulfur. In present study, there are two δ34SCDT values(-1.5 ‰ and-1.7‰) which are very close to the typical ―magmatic‖ values of sulfides. This probably suggests a contribution of magmatic rocks to the Hengdong deposit. The lead isotopic composition shows that 208Pb/204 Pb is between 38.47~39.17, 207Pb/204 Pb is between 15.64~15.65 and 206Pb/204 Pb is between 18.16~18.76, which suggests that the source of Pb isotopic is stable. In the diagram of "lead structural model" proposed by Zartman et al.(1981), the metallogenetic materials of Hengdong deposit have characteristics of mixed sources from both deep mantle and crust.7) Both the Lianyunshan pluton and Changping fault constrains on the Hengdong deposit. The LA-ICPMS zircon age of Lianyunshan pluton is 145.17±0.85 Ma(2σ, MSWD = 1.9) and the muscovite Ar-Ar plateau age of silicified rocks hosted in Changping fault is 130.3±1.4 Ma(2σ, MSWD = 1.06). So we hypothesized that the formation age of Hengdong deposit may be 145 ~ 130 Ma, which belongs to the product of the large-scale tectonic/magmatic/hydrothermal activities in Mesozoic era, South China.8) Compared to the typical Co deposit at home and abroad, we consider that there are obvious similarities and differences for Hengdong deposit, such as tectonic environment, the relationship with magmatic rocks, geological characteristic, elemental and mineral composition, wall rock alteration and mineralization age, etc... Then the Hengdong is a typical moderate-high temperature hydrothermal reformation type ore deposit.9) The genesis of the Hengdong deposit has been discussed, a metallogenic model has been established and a prospecting direction has been put forward, then we deem that a potential of large-size Co ore may be found in the northeastern Hunan province, South China.