Research On Magmatic Fluids And Gold Mineralization Of The Late Mesozoic Epithermal Gold System In The Northern Lesser Xing’an Range

The Northern Lesser Xing’an Range is located in the middle part of Heilongjiang Province,estern Xing’an Mongolian orogenic Belt, northern rim of Songliao Basin or Songnen Massif, andbetween Xing’an Massif and Jiamusi Massif, which is a complex tectonic region that experiencedevolution of the Paleo-Asian Ocean, orogenesis of the Xing’an Mongolian and close of theMongoian-Okhotsk, as well as subduction of the Passific Plate. The multi-period andcomplicated geological process formed enrichment of supernormal gold and multi-metalmetallogenic system, especially epithermal gold deposit. Since the Tuanjiegou larger epithermalgold deposit was discovered in the late1950s, a number of deposits have been discovered in theregion, including the Dong’an and Sandaowanzi medium-and large-scale epithermal golddeposits, Gaosongshan, Fuqiang, Pingdingshan, Malian and Dujiahe medium-and small-sizedepithermal gold deposits. Epithermal gold deposits have been the focus of hot research topic inthe study area by many economic geologist from China and globally., In order to deeply revealedregularity regularity of epithermal deposits in the study area, combining previous research results,we chose important and representative deposits, such as Sandaowanzi, Dong’an and Tuanjiegouepithermal gold deposits to undertake system study from ore deposit geological characteristics,ore-froming fluids, geochemistry of mineral deposits and metallogenic chronology. The mainadvance achievements from this study area are as followings.1. According to research on geology of ore deposit and mineralography, we further indicatethat the Sandaowanzi Te-Au deposit is a low-sulphidation epithermal Te-Au deposit. Theorebodies for Sandaowanzi Te-Au deposit occurs as veined and lenticular bodies, and are hostedby the Tamulangou group volcanics. Four stages of mineralization have been identified, rangingfrom white quartz-pyrite, through gray quartz-polymetallic-sulfide and dark gray quartz-gold-telluride, to a final carbonate stage. The Dong’an Ag-Au deposit is alow-sulphidation Ag-Au deposit. Orebodies mainly occurs as veined bodies, and hosted byrhyolite porphyry, fine-grained alkali granite and middle-to coarse-grained alkali granite.Ore-forming processes within the deposit can be divided into four mineralization stages:quartz-pyrite, white silicification-adularia, offwhite silicification-adularia and chalcedonicquartz-fluorite. The Tuanjiegou Sb-Ag-Au deposit is a low-sulphidation epithermal Sb-Ag-Audeposit. Orebodies occurs as veined and lenticular bodies, and are hosted within an apophysis ofthe granite complex, with some mineralization within a contact zone between the graniteporphyry apophysis and rocks of the Heilongjiang Group. The main orebearing wall-rocks arerocks of plagioclase granite porphyry, granodiorite and granite porphyry. Four stages ofmineralization have been identified, ranging from an early veiny quartz–euhedral pyrite stage,through pyrite–marcasite–gray–black chalcedonic quartz and fine granular pyrite stages, to afinal carbonate stage.2. The mineral fluid inclusions reveal that fluid inclusions from the Sandaowanzi, Dong’anand Tuanjiegou gold deposits mainly are aqueous two-phase inclusions, with minor pure liquidinclusions and pure volatile inclusions. Fluid inclusions form the Sandaowanzi Te-Au depositmainly are aqueous two-phase inclusions, with minor pure liquid inclusions and pure volatileinclusions. The homogenization temperature for the Sandaowanzi Te-Au deposit is range from350³20℃, through310²90℃and280²50℃, to240²00℃. Salinities is in order of1.39⁵.09%NaCleq,1.39⁴.32%NaCleq,1.56⁵.09%NaCleq and1.22⁴.63%NaCleq.The homogenization temperature for the Dong’an Ag-Au deposit are range from360³10℃,through300²70℃and250²10℃, to190¹70℃, and salinities is in order of4.01⁶.14%NaCleq,2.89⁶.14%NaCleq,0.70⁴.32%NaCleq and2.06³.21%NaCleq. Thehomogenization temperature for the Tuanjiegou Sb-Ag-Au deposit are range from360²80℃,270²30℃,220¹90℃and180¹50℃, and salinities is in order of0.70⁹.99%NaCleq,1.22⁶.58%NaCleq,1.22⁴.79%NaCleq and0.35⁴.32%NaCleq.3. Gas phase composition for fluid inclusions, and Hydrogen and oxygen isotopecharacteristics for group inclusions indicate that the initial ore-forming fluid for the SandaowanziTe-Au deposit is H₂O-NaCl magmatic fluid of CO₂-bearing and minor CH4. The fluid translateinto reduced magmatic fluid of CO₂-poor after CO₂escaping in the ore-forming stage. Theore-forming fluid approximates recycled water with continous influxing of meteoric waters inthe late ore-forming stage. The initial ore-forming fluid for the Dong’an Ag-Au deposit ischaracterized by medium to high temperature, low salinity H₂O-NaCl magmatic fluid, and minorCO₂and H₂-bearing H₂O-NaCl magmatic fluid in the metallogenic stage, however, ore-forming fluid approximates recycled water with continous influxing of meteoric waters. The initialore-forming fluid for the Tuanjiegou Sb-Ag-Au deposit is characterized by CO₂-bearingH₂O-NaCl magmatic fluid, and minor hydrocarbon-bering （such as C2H₂, C2H4and C2H6）H₂O-NaCl magmatic fluid in the metallogenic stage, however, influxing of meteoric waters inthe late ore-forming stage.4. Study on diagenetic and metallogenetic epoch, and geological setting show thatmetallogenesis of the Sandaowanzi Te-Au deposit formed at118.9¹25.3Ma, Dong’an Ag-Audeposit fromed at109.7¹05.14Ma, Tuanjiegou Sb-Ag-Au deposit formed at102¹00Ma orafter102.07±0.84Ma. Related magmato-thermal event for the Sandaowanzi Te-Au deposit isclosely related to quasi-aluminous, high-K calc-alkaline and potassium volcanics, as well asquasi-aluminous, calc-alkaline and sodium diabase porphyrite. Related magmato-thermal eventfor the Dong’an Ag-Au deposit is closely related to quasi-aluminous to peraluminous, high-Kcalc-alkaline and potassium to high potassium Ningyuancun group volcanics. Relatedmagmato-thermal event for the Tuanjiegou Sb-Ag-Au deposit is closely related toquasi-aluminous to peraluminous, high-K calc-alkaline and potassium to high potassiumNingyuancun gourp volcanics, as well as peraluminous, calc-to high-K calc-alikaline andpotassium granite complex.5. According to study on geochemistry for volcanics and hypabyssal rocks that areassociated with the formation of the deposit, showing that rare earth and rare elements forigneous rock that is closely related to ore-forming is characterized by enrichment in light rareearth elements （LREE）, large ion lithophile elements （LREE）, depletion in high field-strengthelement （HFSE） and Nb depletion is not obvious. Researh on Sr-Nd and Lu-Hf isotope show thatmelt of the Tamulangou group volcanics are associated with the formation of SandaowanziTe-Au deposit was possibly derived from decompression melting of an enriched continentallithospheric mantle, which had been previously metasomatized by fluids derived from subductedslabs, during the closure of the paleo-Asian and/or Mongolia-Okhotsk Oceans. Moreover,However, low (⁸⁷Sr/⁸⁶Sr)iand positive εNd（t） values suggest that depleted asthenospheric mantlehad been involved in the sources of the Tamulangou volcanics. The Ningyuancun groupvolcanics are associated with the formationg of Dong’an Ag-Au deposit were derived from thedifferentiation end-product of andesitic magmas. The granite complex are associated with theformationg of Tuanjiegou Sb-Ag-Au deposit were derived from end-product of interactionbetween magma related subduction and lower mafic crust.6. Combing with magmatism and fluid evolution, we further confirm that ore-forming fluidis derived from calc-alkaline basaltic magma formed by partial melting of an enriched continental lithospheric mantle, which had been previously metasomatized by fluids derivedfrom subducted slabs. In the late evolution stage under stretching environment, mgma descentwith reducing of temperature and pressure and unload ore-forming elements, and then formedveined and lenticular orebodies. The ore-forming fluid for Dong’an Ag-Au deposit is derivedfrom arc calc-alkaline magma formed by partial melt of lower mafic crust, and incentive islithosphere delamination caused by subduction of the Pacific Palte, which triggered upwelling ofasthenosphere and occurred partial melting of lower mafic crust. In the late evolution stage understretching environment, mgma descent to crust at shallow levels and mix with meteoritic water.After that, solubility of Au and Ag in the ore-forming fluid is gradually dropped and precipitatedwith variation of physical and chemical conditions. The ore-forming fluid for the TuanjiegouSb-Ag-Au deposit is derived from subduction-derived magmatic fluid, which is closely related tocrust-matle interaction in the Mesozoic. The earlier stage magma is characterized by arccalc-alkaline volcanics, which is end-product of interaction of underplated asthenosphere andlow crust, while the late stage magma is characterized by arc calc-alkaline and adakite-likehypabyssal intrusive rock, which is end-product of interaction of adakite-like magma and lowcrust. The mineralizing fluids that formed the Tuanjiegou gold deposit were exsolved frommagmas that formed during both of these stages, with the deposit forming after fluids ascendedthrough the crust in the Late Cretaceous.7. Combing with Mesozoic regional dynamics background, considering that the formationof Sandaowanzi Te-Au deposit is related to subduction of the Izanagi plate towards NNWtrending, and its metallogenic thermal source is basaltic magma derived from partial melting ofan enriched continental lithospheric mantle, which had been previously metasomatized by fluidsderived from subducted slabs. The formation of Dong’an Ag-Au deposit is closely related tosubduction of the Izanagi plate from NNW trending to NW trending, and its metallogenicthermal source is andesitic magma derived from partial melting of low crust. The formation ofthe Tuanjiegou Sb-Ag-Au deposit is associated with subduction of the Izanagi plate towards NWtrending, and its metallogenic thermal source is magma derived from interaction of adakite-likemagma and low crust.