The Mineralization Of The Jinshandian Skarn Fe Deposit,hubei Province

Jinshandian skarn Fe deposit located in the west part of the Edong district,which was within the westernmost part of the Middle-Lower Yangtze River metallogenic belt(MLYRMB),Two major mineralized localities named Zhangfushan and Yuhuashi have been discovered,which are located in the southern and northwestern contact zone of the Jinshandian intrusion with Triassic carbonate and clastic formations,respectively.Host rocks of the deposit are Lower Triassic Daye Formation,Lower to Middle Triassic Jialingjiang Formation,and Middle Triassic Puqi Formation.Ore bodies occur as lenticular and vein shape in profile along NW to NWW-striking fault at the contact zone between the intrusion and country rocks.In this study,we present research on deposit geology,compositions of skam mineral,and origin and evalution of the ore-forming fluid,in order to constain the relationship of the evaporites with ore forming process,timing of the intrusion and ore forming process and dynamics of mineralization.Specific improvement has been made as follows:(1)Mineralization and alteration process of Jinshandian skarn Fe deposit can be divided into four stages,which are prograde skam stage,magnetite stage,vein ship skam stage,anhydrite-pyrite stage and gypsum-calcite stage.Skarn minerals of Jinshandian deposit include pyroxene,scapolite,phlogopite,amphibolite,garnet,titanite and epidote etc.Pyroxene mainly belongs to diopside(Di68.15-99.53Hd0.4-31.21Jo0.06-0.88),scapolite is chlorine-rich mizzonite,and amphibole belongs to calcium amphibole series,and mainly hastingsite and magnesiohastingsite.Two generations of garnet have been recognized.Early garnet is mainly grossular and grossular-andradite series(And25.94-87.96Gro10.90?43.62 Spe0.94?3.28?And33.10?37.62Gro59.40?63.22Spe2.29?3.74),while late garnet belongs to andradite-grossular series(And62.71?97.69Gro1.96?34.00Spe0.35?4.22).Iron content of garnet increase from early to late generation,suggesting that iron content and oxygen fugacity of the fluid was increased gradually as time went on.(2)Oxygen isotope values of minerals form prograde skarn stage to anhydrite-pyrite stage of the Jinshandian deposit fluctuate in a narrow range,and with a trend of anhydrite(+11.6‰?+15.4‰)>scapolite(+8.6‰?+11.2‰)>phlogopite(+8.9‰?+10.4‰)>magnetite(+5.1‰?+7.6‰).Hydrothermal calcite shows a large range of ?18O(+10.4%o?+22.8‰),but homogeneous carbon isotope values(-0.7‰?+0.9‰).Oxygen and carbon isotope values of the marbles both fluctuate in a narrow range,with?18O values ranged from+25.0‰ to+27.8‰,and ?13C values ranged from+2.8‰ to+3.7‰.Oxygen isotope values of fluid which equilibriumed with minerals from different stages indicated a mixing of multiple fluid sources in Jinshandian hydrothermal system,with originally dominated by magmatic water during the pre-ore stage,followed by significant signals of evaporite materials during the ore forming stage and notable influx of ?18O-depleted meteoric water after ore formed.Sulfur bearing minerals of different stages were both enriched in heavy ?34S(anhydrite,gypsum,?28‰;pyrite,?19‰),also suggesting the involvement of evaporite materials into the skarn system of the Jinshandian deposit.(3)Intensive reaction of the evaporites with the magmatic-hydrothermal had given rise to a series related geological and geochemical phenomena(evidence),including mineralogy,stable isotope geochemistry and structure etc.The hydrothermal system of the Jinshandian deposit probably experienced significant incursion of evaporites before or during the early magmatic-hydrothermal stage,and with features of heterogeneity and periodicity.The involvement of evaporites into the skarn system had significant impact on the mineralization process,mainly in:? by incrusion of large amounts of strong oxidizing mineral anhydrite,the oxygen fugacity of the mineralization system would be improved obviously,resulting in the formation of plenty Fe3+,in order to form large amounts of magnetite;?output of large amounts of Cl-rich mineral indicated that the ore forming fluids had high concentration of chlorine,which would hace played important roles for migration and enrichment of Fe;?production large amounts of gypsum bearing breccia would not only conducive to water-rock interaction between the ore-forming fluid and host rocks,but also would provide the ore-host space.(4)Zircon U-Pb LA-MC-ICPMS dating research shows that formation age of the Jinshandian pluton range from 131 to 127 Ma,within the error of hydrothermal titanite U-Pb LA-ICP-MS age(130.4 ±1.1?127 ± 12 Ma)and phlogopite 40Ar-39Ar age(128.6 ± 1.5 Ma)form the skarn Fe deposits of Jinshandian orefield.Therefore,the magmatic-hydrothermal event in Jinshandian ore filed belongs to the second stage of mineralization events of the MLYRMB,suggesting that Jinshandian skarn Fe deposit formed during the large-scale lithospheric extension and thinning background of the eastern part of China at early Cretaceous.40Ar-39Ar age of phlogopite which cut cross the massive magnetite ore in this study also indicated that the Edong district could have experienced another new magmatic-hydrothermal event(?120Ma)after the two stages of main mineralization.However,the extent and metallogenic significance of this newly recognized magmatic-hydrothermal event is temporarily unclear.(5)Jinshandian deposit has favorable intrusion,wall-rocks and structure condition,which resulted in extension water-rock interaction between the Jinshandian pluton and wall-rocks,as well as formation of the Jinshandian deposit.Iron of the deposit was mainly derived from the magmatic-hydrothermal.High-grade massive iron ore may be related to a long-term stable fluid convective circulation system under favorable geological conditions,whereas high-grade powdery iron ore probably related to the reform of massive or disseminated ores by post-ore structure and groundwater.(6)Compared with other Daye type skarn Fe deposit in Edong district,Jinshandian deposit has a distinct specificity,mainly in:the evaporites strongly involved in mineralization process;ore bodies controlled by the combination of the fault and contact zone.