Abstract:The Xifieshan Pb-Zn deposit is one of the large-sized massive sulfide deposits prospected successfully in the Northwest China in the late 1950's. This paper describes the geological background and characters of the deposit, and also makes an approach to its genetic classification on the basis of the geological reconnaissance. Geotectonically the Xitieshan Pb-Zn deposit is situated at the northern border of Qaidam Basin, belonging to the Saishitenshan-Luliangshan-Xitieshan Caledonian eugeosynclinal foldbelt, and also on the southern side of Qilian geosyncline system. The ore-bearing greenstone series, which consists mainly of sodium-rich intermediate-basic volcanic rocks intercalated with intermediate-acid rocks and some sedimentary rocks, extends as a belt in NW-SE direction. In accordance with the data of conodonts and Rb-Sr isotope age (464.6 Ma) in Xitieshan area, the greenstone series should be of Upper Ordovician and can be correlated with the Tanjianshan Group of Saishitengshan region, with its overlying terrain being sandy gravel sediments of upper Devonian and Lower Carboniferous system. The greenstone series can be divided into three lithologic members: the lower member of intermediate-acid-intermediate-basic volcanic rocks, the middle member of sedimentary rocks (composed of volcaniclastic rock. carbonate rock, silicolites and ferruginous sillcolites, phyllite, slate, etc., and the Upper member of intermediate-basic volcanic rocks. In this area, from north to south, the rock sequence occurs as a monocline without any significant magmatic activity except for frequent volcanism-intrusions. The basic characters of the deposit are as follows: (1) Ore belt occurs in the lower part of the middle lithologic member along a definite horizon which consists of carbonate rocks and tuffaceous phyllite slate and carbonaceous schist. However, the ore-bearing rocks are of great diversity and tend to occur in a transitional part of a layer or a lithologic unit. The main ore bodies take stratiform, stratoid or lenticular forms and are conformable with the overlying and underlying strata with some of them locally intersecting the stl atification or schistosity. The boundary of ore bodies is rather irregular. The pocket-like or vein-like ore bodies, though small, can also be observed. Ores are in great varieties. The lower ore bodies composed mainly of massive ore are rich in Pb-Zn, while the upper ore bodies composed mainly of the striped and veinlet-disseminated ore are usually deficient in Pb-Zn. The metallic mineral assemblage in the ore is quite simple, consisting mainly of pyrite(pyritogelite), sphalerite and galena, locally associated with chalcopyrite. The structure and texture of the ore are rather unusual. The stripes of the ore trend essentially in accordance with the stratification and schistosity of wall rocks. There are remainders of pyritogelite which have been notably replaced by other metallic minerals. In addition, there are macrocrystalline and pegmatophyrm textures appearing against the background of the fine-grained structure, and the veinlet-disseminated texture occurring widely in the tuffaceous phyllite slate. In short, the ores are characterized by both contemporaneous deposition and post-magmatic hydrothermal reformation, i. e., the ores have experienced autochthonous activation and transference for a short distance. (3) The mean ratio of lead to zinc (Pb/Zn=0.82) tends to decrease along the NW-SE direction. One of the important characters of the ore deposit is its relatively high contents of gold and silver. The mean grade of gold and silver in primary ore is Au 0.5 g/t, Ag 25-27 g/t, with uneven distribution(and local concentration). In addition, the ores are associated with relatively high Sn, Cd, Ga, In, As, Cr and less Ni, Co, Se. (4) The homogenization temperature (220℃±) of gas-liquid inclusions in sphalerite, which is essentially concordant with the data of the thermo-acoustic analysis of sphaierite, should belong to a middle-low temperature mineralization. The temperature, however, only reflects a possible state of mcro-crystallization during the mobilization stage of the ore deposit. (5) Some related stable-isotope data indicate: 1)the S-isotopic composition in ore minerals(δS34 generally being+1一+5‰, deviation extent<6‰) is close to the sulfur in meteorite in content. 2) According to the Pb-isotopic composition in ore minerals(Pb206/Pb204 being 17.65—18.467, Pb207/Pb204 15.106一16.813, Pb208/Pb204 36.927—39.30), the lead in the ore must be a polystage evolutionary product instead of a normal lead. In conclusion, the Xitieshan Pb-Zn deposit is characterized by stratabound volcanogenesis and multiple-reformation. The source of metallogenic materials and the processes of mineralization may also be associated with volcanism. The ore deposit, therefore, should be called volcanic exhalation-sedimentary and hydrothermal superposition-reformed massive sulfide ore deposit.
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邬介人.1985.青海锡铁山块状硫化物矿床地质特征[J].矿床地质,4(2):1~12.1985.Geological characteristics of the Xltieshan massive sulfide deposit, Qinghai province[J].Mineral Deposits4(2):1~12
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