Mineralization Of Hadamengou Gold Deposit,Inner Mongolia,China

The Hadamengou gold deposit is a super-large gold deposit in the Wulashan-Daqingshan gold mineralization cluster on the northern margin of the North China Craton,with proven gold reserves of more than 100 t（average grade of 4.22 g/t）.This deposit is hosted by Archean gneiss,and the mafic-felsic dykes are widely distributed in mining area.Although researchers have done a lot of research work and achieved more scientific research results for Hadamengou deposit,some key scientific issues such as the source of ore-forming materials and the genesis of ore deposits are still controversial.Thus,we divided the metallogenic stages and established the mineral generation sequence of different stages by detailed geological survey,core logging,petrography and mineralogy observation in Hadamengou gold deposit.On this basis,we carried out in-situ trace element and sulfur isotope analysis of gold-bearing pyrite from different generations,in-situ analysis of trace elements in fluid inclusions of quartz from different ore-forming stages,and Re-Os isotopic dating of molybdenite,aiming to study the source of ore-forming materials,fluid evolution process,and genesis of the Hadamengou gold deposit.（1）Based on the deformation characteristics,intersection relationships of orebodies,and wrapping relationship of minerals,the Hadamengou gold deposit was divided into five mineralization stages（S1-S5）,and five generations of pyrite（Py1-Py5）were identified.The earliest stage of pyrite（Py1）occurs as random disseminations within the limited volume of modified quartz-pyrite veins（S1）and was formed during or prior to the last episode of metamorphism.Py2,from the potassic and chlorite-sericite alteration zones（S2）,is related to the activity of potassic fluids,and contains inclusions of native gold,galena,melonite,electrum,and petzite.Py3,without inclusions,occurs as randomly-disseminated grains within the limited volume of grey quartz-pyrite veins（S3）.A few native gold grains are observed along fractures of quartz associated with Py3.Py4 occurs as veinlets or aggregates in the milky quartz-pyrite veins（S4）and contains the highest Au concentrations.Abundant native gold,calaverite,and electrum occur along the grain boundaries and fractures,or as inclusions within Py4 and quartz.Py5 occurs as rounded and cataclastic shapes within late quartz-carbonate veins（S5）and is depleted in Au.（2）The LA-ICP-MS trace element analysis of the pyrite shows that Py1 and Py5 has a low content of Au（0.02-0.56 ppm,0.04-0.24 ppm）and other trace elements（Ag,Te,Pb,Cu,Bi,and As）.Potassic alteration,grey quartz-pyrite veins,and milky quartz-pyrite veins make a very large contribution to the gold endowment,with a relatively high gold content in pyrite,including Py2（0.14-4307 ppm）,Py3（0.41-31.1 ppm）,and Py4（0.04-39.9 ppm）.Other trace elements（e.g.,Ag,Te,Pb,Mo,and Bi）are also significantly enriched within Py2,Py3 and Py4.The invisible gold is mainly in the form of Au-Ag-Te-Pb-Bi complex of submicron or nano inclusion in pyrite.Based on the volume of orebodies and Au content of pyrite from different stages within the Hadamengou deposit,the potassic alteration and milky quartz-pyrite veins were major contributors to gold mineralization.（3）The LA-MC-ICP-MS in-situ sulfur isotope analysis of different generations of pyrite（Py2-Py5）provides important information on ore-forming material source.These pyrites have negative values and a narrow range of variation ofδ³⁴S（-13.0～-5.7‰）.A small amount of magnetite and barite in the ore veins indicates that this deposit was formed in a relatively oxidized environment.Combined with the previous barite S isotope data,it is estimated that the total sulfur value is-4.8～1.4‰,which is basically consistent with the magmatic sulfur,indicating that the ore-forming material originated from the relatively oxidized magmatic hydrothermal system.In addition,in situ trace elements of pyrite（Py2-Py5）showed high Ag/Au ratio（0.14-4.36）and high Te concentrations（0.17-282 ppm）,suggesting an alkaline magma source.（4）The fluid inclusion petrography show that S2 stage are mainly liquid-rich inclusions（VL-type）and pure-liquid inclusions（L-type）.In S3 and S4 stage,there are liquid-rich inclusions（VL-type）,pure-liquid inclusions（L-type）,and a small amount of liquid CO₂inclusions（C-type）and pure-vapor inclusions（V-type）.The fluid inclusion microthermometry and in-situ LA-ICP-MS analysis results show that the fluids of S2 stage are characterized by high homogenization temperatures（324.5-428.7°C）,moderate salinities（12.85-18.3 wt%Na Cl eqv.）,and high Fe,Pb,Cu,Zn,Ba contents.With the hydrothermal evolution,fluid temperature of S3 stage decreases（263-338.8°C）,but have large salinity changes and are characterized by fluid boiling.The fluid was significantly enriched in Cu,Pb,Mo,Ag content.The fluid temperature of S4 stage is basically consistent with the S3 stage.The Pb,Cu,Ba contents are significantly decreased in the fluid,but has higher Mo and Ag contents.The ore-forming fluids in stage S2-S4 are characterized by high alkali metals and base metal contents,low Ca contents,high Zn/Na and K/Na ratios,and small variation range of Cs/（Na+K）and Rb/（Na+K）ratios,indicating that the ore-forming fluids originated from homogeneous magmatic hydrothermal system.During the evolution of magmatic hydrothermal fluid,the fluid cooling,water-rock reaction and fluid boiling triggered precipitation of gold and other sulfides.（5）Re-Os dating of molybdenite from Au-bearing veins shows that the Hadamengou gold mineralization was formed in the middle-Late Devonian.The ages of mafic-felsic dykes are mainly concentrated in 2.52 Ga,2.45-2.44 Ga and 1.99 Ga.There is no direct genetic relationship between gold mineralization and these dykes,but the structure controlling the dykes provides a good channel for the migration of ore-forming fluid.（6）Combining fluid evolution features,magmatic activity and regional geodynamic settings,the metallogenic model of Hadamengou gold deposit is as follows:during or prior to the last episode of metamorphism,Au-bearing quartz-pyrite veins were filled along cracks in pegmatite dykes,and precipitated Py1 and little gold.In the middle-late Devonian,Au-rich potassic fluids derived from the deep-lying alkaline magma ascended along the EW-trending faults,and together with the wall-rocks reaction and fluid cooling formed potassic alteration and chlorite-sericite alteration zones,resulting in the precipitation of Py2 and abundant gold（tellurium）minerals.Subsequently,Au-bearing Si O₂-CO₂-rich fluids separated from the deep-lying alkaline magma/melts continues to rise along the secondary fracture,continued increases caused fluid pressure to exceed the capacity of the potassic and chlorite-sericite alteration zones,resulting in rupture of previous alteration zone and escape of the volatiles（e.g.CO₂）.The subsequent sudden pressure drop initiated fluid boiling and deposition of large amounts of Py3,Py4 and gold（tellurium）minerals to form Au-bearing quartz-pyrite veins.Finally,limited Si O₂-Ca CO₃-rich fluids penetrated fractures of the previous alteration zones and quartz-pyrite veins,resulting in the precipitation of Py5.Therefore,the Hadamengou gold deposit belongs to the magmatic hydrothermal deposit related to alkaline rock.