The Hadamengou Mine: A Typical Gold Deposit in the Archean Granulite Facies Terrane of the North China Craton

Abstract

Gold deposits in Archean high-grade terranes are either syngenetic or epigenetic. In the North China craton, the high-grade metamorphic rocks are considered to be the main source of gold for the formation of gold deposits formed during the Proterozoic or Mesozoic eras. The Hadamengou mine is located in the E-W-trending Archean Daqingshan granulite belt at the northern margin of the North China craton. In the mine area, the Archean Wulashan Group rocks were metamorphosed to granulite facies under temperatures of 750-850°C and pressures of 7-9 kbar during the late Archean and subsequently were affected by greenschist to epidote-hornblende facies retrograde metamorphism during the late Proterozoic. Metamorphosed banded iron formation (BIF), i.e., magnetite-quartz granulite and magnetite-garnet-plagioclase- quartz gneiss, is spatially closely associated with gold mineralization. Gold-bearing quartz-K- feldspar veins are the main type of mineralization. A fluid inclusion study demonstrated that K+ and Na+ (with some Mg2+ and Ca2+) were the main cations and Cl−, F− and SiO4 2− the main anions of the mineral fluid which, in most cases, is a single H2O-rich + CO2 vapor phase. Isotopic geochemistry suggests that the mineralizing fluid is weakly alkaline and oxidizing (pH = 8.3−8.52 and Eh = 231.6–299.2 millivolts) and suitable for gold precipitation. The high-grade meta-morphic rocks in the mine area, which contain high background values of gold, are the most probable source of gold (73%) for the formation of the gold deposit. Granitic magmatism during the late Proterozoic provided additional heat energy and also may have contributed to the mineralizing fluid that resulted in the liberation and eventual precipitation of gold in shear zones. The temperature of mineralization is higher than 200-450°C, as is indicated by a fluid inclusion study. Gold mineralization in the mine area is structurally controlled and all gold lodes are restricted to the hanging wall or north side of an augen-gneissic mylonite zone. This ductile shear zone developed under high-temperature conditions and acted as a conduit for later fluid flow. Late brittle-ductile shear zones, accompanied by retrograde metamorphism of greenschist to epidote-hornblende facies during late Proterozoic, acted as the host structures of gold mineralization. The payshoots are distributed in the foliated zones or fill the fractures in the deformational zones. The Hadamengou deposit is clearly epigenetic and the source of the gold is probably the high- grade metamorphic rocks. In the North China craton, more than 30 gold deposits are located in the high-grade rocks. These deposits formed during late Proterozoic or Mesozoic. It is suggested that Archean high-grade terranes are of similiar importance for gold mineralization and exploration as greenstone belts.