Genesis Of The Wulitu Pb-Zn Deposit At Kulun County, Inner Mongolia:Its Significance To Ore Prospecting

The Wulitu lead-zinc deposit at Kulun County, Inner Mongolia, is situated on the southern margin of the Central Asian Orogenic Belt, which is adjacent to the North China Continental Block to the south. It is a large skarn-type deposit recently found and prospected.Prospecting between drill lines No.30 and No.34 obtained (121b+122b+333) metal resources/reserves of 0.19 Mt Pb at an average grade of 1.27%, and 0.29 Mt Zn averaging 1.90%. These ores also contain recoverable Ag of 278 t at 18.18g/t, and Cu of 12235 tat 0.08%.Main sedimentary rocks affiliated to mineralization include limestones, siltstones and mudstones of the Upper Carboniferous Shizui Formation. These strata general dip towards NW at 55°～65° ormation. Tadamellite is the main intrusive rock type in the deposit area.Ores in this deposit can be divided into the Pb-Zn-bearing marble, Pb-Zn-bearing granite and Pb-Zn-bearing skarn types. Ore minerals are dominated by sphalerite, galena and pyrite, with less pyrrhotite, arsenopyrite, magnetite and chalcopyrite. There are also marcasite, bornite and argentite. Gangue minerals are mainly diopside, epidote, chlorite, quartz, calcite, dolomite and ankerite, with lesser tremolite, actinolite and garnet. Alterations are represented by diopsidization, epidotization, chloritization, silisification, carbonatization and pyritization, with local garnetization, tremolitization and actinolitization. The ores are characterized by granular textures with varying idiomorphism. Miscellaneous metasomatic textures are also present. Mineralization styles are typified by veins, networks, patches, breccia fillings and disseminations. Most mineralized veins range from millimeters to 3 cm in width. Massive ores can only be seen locally.The mineralization-affiliated biotite adamellite intrusion can be divided into two mutually connected parts:1) the vertically emplaced main stock in the east; 2) the tongue-shaped portion protruding out and upwards on the western side of the stock. Being inclined towards SE, the general conformation of its occurrence with the Shizui Formation suggests that magma ascent and emplacement of this granite tongue were constrained by interlayer fractures and brecciated zones of the strata. This granite tongue measures about 0.05 km3 in volume.Based upon drill core analyses, more than 40 lead-zinc orebodies of different size have been delineated. Three orebodies largest in ore tonnage and highest in grade (No.1, No.2 and No.3) occur at the lower contact zone of the tongue-shaped granite. Mineralization of varying extent also occurs between these main orebodies. By contrast, only small orebodies and sub-economic mineralizations can be found at or above the upper contact of the granite tongueIn spite the fact that the main lead-zinc orebodies occur very close to the tongue-shaped granite, it is unlikely that ore-forming fluids and metals came directly from the granite tongue. There is, so far, no credible mechanism to support that hydrothermal fluids at higher temperatures were capable to overcome buoyancy and flowed downwards, and finally unloaded metals beneath the tongue-shaped granite. Meanwhile, this granite tongue is too small in volume to have provided sufficient lead and zinc for so large orebodies. It seems also difficult that the quickly cooling magma for such a small intrusion had, prior to its consolidation, accepted large volumes of ascending fluids from the magma body of the stock.The author would like to suggest that the ore-forming fluids and metals for the deposit came from the granite stock flanking the tongue-shaped body or from an even greater depth. Essential mechanism for ore-formation underneath this granite tongue could be the blocking of the earlier cooled and consolidated granite tongue on the up-rising fluids from the crystallizing magma in the stock. The stock magma at that moment still held the capacity to release fluids. The fluids rising from deeper parts of the stock along fractures could have been blocked by the granite tongue and have gathered below it. Subsequent unloading of metals resulted in final formation of economic orebodies.Bed-like and tongue-shaped granite bodies could occur commonly at higher crustal levels corresponding to the formation depth of skarn-type deposits. In case such intrusions coexisted with up-rising fluids from depth, much attention should be paid to promising skarn-type ores beneath these intrusions. The author believes that the fluid-blocking mechanism is also workalbe on mesothermal, epithermal and other hydrothermal deposit types, and hence should take a wider application to mineral prospecting.This paper also put forward some promising areas for further prospecting inside the Wulitu area.