Magmatism And Ore-Forming Process Of Chaihulanzi Gold Deposit In Inner Mongolia

The northern margin of the North China Craton(NCC)experienced complex tectonic evolution from Archean to Cenozoic.Multi-stage magmatic-hydrothermal events led to formation of an important Au,Mo,Cu,Pb,and Zn polymetallic metallogenic belt in this area.The Chaihulanzi gold mine,situated 60 km west of Chifeng City,is an important part of this metallogenic belt in the northern margin of NCC,with gold resources of over 20 tons.The studies on ore-forming process of this deposit is limited,and the genesis of the deposit is still controversial.Based on detailed field geological,mineralogical petrological,geochemical and geochronological studies,we discuss the relationship between magmatism and gold mineralization at Chaihulanzi gold deposit,and the metal source of this ore-forming system.This study shows that the so-called Jianping Group in Chaihulanzi area is composed of intrusive rocks with different ages.The granodioritic and dioritic gneisses formed at ca.2.57-2.59 Ga are TTG-like,representing a continental growth event of the NCC.The granitic gneisses emplaced at ca.2.50 Ga are potassic granite and represents a crustal reworking event of the NCC.The carbonatite and olivine hornblendite were formed at ca.1.85 Ga during breakup of Columbia supercontinent.The olivine hornblendite is Au,Co,Ni enriched and has higher gold content(up to 0.19 ppm)than the average crust and Archean gneisses in study area.LA-ICP-MS trace element analysis shows that gold mainly distributes in pentlandite,chalcopyrite,and pyrrhotite in this rock.The sulfur isotopic ratios of the sulfides in the gold ores are similar with those of sulfides in Paleoproterozoic ultramafic rocks worldwide.In addition to Au,the ores are also enriched in Co and Ni,implying that gold may be sourced from the Paleoproterozoic mantle-derived magmatic rocks.This study may provide a new insight for understanding the metal source of gold deposits in the northern margin of the NCC.Phanerozoic magmatic rocks in Chaihulanzi area are mainly Paleozoic intrusions and Mesozoic and Cenozoic volcanic rocks.According to the alteration and mineralization style,and the crosscut relationship among hydrothermal veins and these magmatic rocks,two stages of gold mineralization are confirmed in Chaihulanzi area.The early stage mineralization occurs as quartz-veins and altered wall rocks,and is the main gold mineralization stage.Silicification,actinolitization and sericitization are closely related to early stage mineralization,with native gold as the ore mineral.The Ar-Ar chronology results of hydrothermal muscovite constrains the gold mineralization at 263.67±0.99 Ma.This metallogenic age is consistent with the age of aplite emplacement(264.5±1.3 Ma),and genetic link between them is manifested by their close spatial relationship.The ore fluid related to early gold mineralization is medium-to high-temperature and medium-salinity magmatic water,with CO2 and CH4 as the gaseous species.The precipitation of gold may be caused by interaction between the ore-fluid and wall rocks.The geochemical characteristics of the ore-related aplite indicate that the magma was formed under a collision-extension transition setting during the closure of the Paleo Asian Ocean.Zircon Hf isotopic data of aplite imply that the magmatism may be sourced from partial melting of the ancient craton basement.The late stage of gold mineralization is characterized by gold-bearing quartz-carbonate-clay veins.The late stage and related hydrothermal veins crosscut the early stage mineralization and Mesozoic volcanic rocks,indicating a Mesozoic(or later)metallogenic event.The mineral assemblage and ore fabrics show the characteristics of low-sulfidation epithermal mineralization.The main ore mineral of late mineralization stage is gold-silver tellurides,which are different from those of the early stage.The ore fluid related to this stage is low-temperature and low-salinity,and the volatiles in the fluid are mainly H2O with minor CO2.Blade calcite and boiling fluid inclusion assemblages indicate that fluid boiling occurred during ore formation and may cause the gold precipitation.Calcite C isotopic results show signatures of mantle-derived C,implying that the late gold mineralization stage may be related to Mesozoic mantle-derived magmatic rocks,which formed during destruction of NCC.