The Bilihei gold deposit in Inner Mongolia： the first large-sized high-grade concealed porphyry Au deposit discovered on the northern margin of North China plate

Abstracts：The Bilihei gold deposit（No. II ore zone） in Inner Mongolia, found by Gold Geological Institute of CAPF in 2006-2008, is one of the fatal breakthrough deposits supported by the Project of Supersedable Resource Prospecting of Critical Mines in China. It is located in the Mesozoic (Jurassic) continental facies volcanic basin on the northern margin of North China plate. The main characteristics of the deposit are as follows：(1) the concealed ore body occurs in the contact zone (mainly in the inner-contact zone) between granodiorite porphyry and volcanic rocks; (2) a large ore body 500m long, 300m wide and averagely 52.85m thick was identified, which contains 21916 kg Au at the average grade of 4.5×10-6 (the highest grade being 54.76×10-6)；(3) ores can be classified into altered granodiorite porphyry type and andesite porphyrite type, with the former type having typical unidirectional solidification texture (UST). There are only small amounts (<1%) of metallic minerals, such as pyrite, chalcopyrite and molybdenite, in the ore； gold mainly occurs in lumpy or veinlet quartz formed during alteration; (4) just like gold-rich porphyry copper deposits, the alteration shows obvious ring-shaped zoning; the concentric alteration zones from the inner part outwards and from early to late stages are successively the K-silicate zone, the quartz-sericite zone, and the propylitic zone； the structure-controlled argillic alteration locally occurs as patches, superimposed by other alteration zones； (5) the early ore-forming stage has very high temperature (>550℃), and the temperature of the middle-late stage is 108～375℃, 194℃ on average. It is concluded that this deposit is a large-sized and high-grade independent and concealed porphyry Au deposit and is the first deposit of this kind discovered on the northern margin of North China plate.