Ore-forming Processes And Genesis Of The Gold Deposits In Pangjiahe Region,Fengxian,Shanxi Province

Pangjiahe region located in northwestern cornor of the famous Feng-tai Pb-Zn-Au mineral cluster zone in Qinling orogen,Fengxian,Shanxi province.Former researchers did great job on ore genesis and metallogeny in Feng-tai mineral cluster zone.Here,three gold deposits with different mineralization style in the Pangjiahe region,including the disseminated Pangjiahe gold deposit,the quartz-vein type Zuojiazhuang gold deposit and the Matigou deposit with both styles,have been chosed to study.They have been discovered for almost 20 years,except the newly found Matigou deposit.The research work of them are far away behind other deposits in Feng-tai mineral cluster zone.In this paper,the geological features,mineral assembleges,whole rock geochemistry,property of ore fluid and its evolution,H-O-S-Pb isotope,in-situ trace element and isotope analysis,and geochronology of the three deposits have been addressed to discuss their inner connection.Moreover,the Pb-Zn-Au mineralization in the Feng-tai mineral cluster zone will be compared with the Pangjiahe region to further summarize the metallogeny and offer theory-based exploration stratage to this area.During the processes,several results have been accomplished below.Pangjiahe gold deposit hosts in the Devonian phyllite,and is strictly controlled by the EW stricking foliation belt.Zuojiazhuang gold deposit hosts in the auriferous quartz-sulfide veins,and is strictly controlled by the EW stricking shear-fracture zone in the Hejiazhuang pluton.Matigou gold deposit hosts in both Devonian phyllite and auriferous quartz-sulfide veins,which depends on the ore fluid interacting with the phyllite rocks,or deposited in second-level en echelon crack of EW shear.Generally,gold deposits in Pangjiahe region were affected by the EW shear happened in early Indosinian.The alteration mineral assemblages are similar in the three deposits,including pyrite,arsenopyrite,quartz,sericite,and minor carbonate,galena,sphalerite and sulfosalt minerals.By comparison the major and trace elements in the fresh and mineralized phyllite and granite porphyry rocks in Pangjaihe and Matigou deposits,we find that Si?K?Al and S are highly enriched in mineralized rocks which coupled with the strong pyritization,silicification and sericitation during mineralization.Na?Ca?Mg and Fe are depleted in mineralized phyllite which coupled with the fading alteration in phyllite.However,Fe is enriched in mineralized porphyry rocks,indicating that the ore fluid transport Fe from phyllite rocks to granite porphyry.In terms of trace elements,Au?As?Ag?Sb?Cu and Pb are highly enriched during mineralization.And the ore fluid causes stong Sr negative anomaly in mineralized granite porphyry.Elements like Lu?Tm?Ho?Tb?Yb?Ta?Er?Eu?Dy?Gd?Sm?Hf?Y?Nb?Pr?U?Th?Nd?Ga?V?La?Cr?Ce and Zr keep constant during alteration.Fluid inclusion research indicate the three gold deposits have similar low tempreture?120-300??,low salinlity?1.2-12.9%NaCleq?,low density?0.75-1 g/cm³?,and CO₂enriched epizonal?0.8-3.2km?ore fluid.But,the Baguamiao gold deposit sitted in the deep level in Feng-tai mineral cluster display medium-high tempreture?177-580??,medium-low salinlity?2.8-21.4%NaCleq?,low density?0.6-1.1 g/cm³?,and CO₂-CH₄-N₂ enriched mesozonal?0.8-3.2km?ore fluid.H-O isotope study of gold deposit in both regions display similar composition,indicating a primary metamorphic water mixed with minor magmatic water.Which means the ore fluid in both regions may have same and homogeneous fluid source,they firstly trapped in the deep,when the geological background changed to relatively extension,the ore fluid would periodically pump into the favorable structures.If water-rock interaction plays an important role,Au will precipitate in arsenian pyrite in phyllite like Pangjiahe and Matigou deposit.But the sharp decrease of fluid pressure will cause pase sepretation,and Au will precipitate as free gold inclusions in pyrite and their cracks in quartz veins like Zuojiazhuang deposit.Pyrite in Pangjiahe region record a long history from Devonian sedimentation Py0to diagenetic Py1,to early Indosinian metamorphic hydrothermal Py2,to ca.240Ma magmatic hydrothermal Py3,to ca.230Ma ore stage hydrothermal Py4.Pyrite in each stage have their distinct petrological features.In-situ sulfur isotope and trace element analysis show that the ore fluid is normally enriched in ³⁴S(?³⁴S focused between+8‰and+12‰),Au,As,Cu,Sb,Ag.That is totally different with Py0 to Py3,which further exclude the?carbonaceous?phyllite?hosting Py0-Py2?and granite porphyry?hosting Py3?on site as the source of sulfur and other trace elements,including Au.According to the former researchers,the SEDEX mineralization,hosted in the boundary of middle to upper Devonian in the deep Feng-tai garben basin,is enriched the same elements as the ore fluid in the upper gold deposit.Therefore,we propose that the strong deformation and metamorphism during orogenic process in Indosinian may extract sulfur,gold and other trace elements from the SEDEX sequence,and transport through the thrust faults,and finally precipitated in the favorable structures.The granite porphyry formed before gold mineralization display U-Pb age at ca.240Ma in Matigou and Pangjiahe deposits.And the mafic dykes formed after the gold mineralization dated at ca.220Ma.Moreover,some of the zircons in mineralized granite porphyry display hydrothermal zircon features in CL image and trace elements,and they formed another group of age at ca.230Ma.The high U-Th-Pb-Zr-Hf?10²-10⁴ppm?rim around late ore stage Py4c further prove the possibility of hydrothermal zircons formed during strong alteration,and further restrict the lower limit of mineralization age at ca.230.The hydrothermal sercite in Pangjieh deposit display plateau age at 231.7±0.2 Ma,and in Matigou deposit display a similar plateau age at 234±0.2 Ma.The age in different methods prove each other and provide accurate age of gold mineralization in Pangjiahe region.To comparison the gold mineralization age around the Qinling orogen,we propose that the gold deposits in Pangjiahe region record the earliest gold mineralization events in Qinling orogen,and they are the first gold mineralization response to the orogen from a relatively compression to extension background.As with the large-scale delamination in deep Qinling,the ore fluid has broadly released and continuely formed hundreds of gold deposits around the Qinling oregen.Through the comparison with worldwide orogenic gold deposit and carlin gold deposit,we favored that gold deposits in Pangjiahe region could be classified as epizonal orogenic gold deposit.