桂北地区金属矿床成矿历史演化程式

THE EVOLUTIONARY HISTORY OF METALLIC DEPO- SITS IN NORTHERN GUANGXI, CHINA

桂北地区是中国重要的稀有、稀土及多金属成矿区。区内存在着五个矿床成矿系列和两个矿床成矿亚系列,其成矿作用在地质历史中具有多旋回螺旋式上升的演化规律。对于各旋回而言,从早到晚成矿物质从以幔源为主到壳源。从整个地质历史演化来看,也有幔源组成趋向减少,壳源组成逐渐增高;成矿元素组合由少到多,成矿组分趋于复杂。成矿作用在空间上显示出从古老基底向外迁移,即前寒武纪从西向东递变,显生宙以来从北向南过渡,燕山期内又自东向西演化的规律。

Northern Guangxi is located in the transitional belt between the Yangtze platform and the South China fold system. The ore-forming process in the region developed throughout the geological history from the Precambrian to Yanshanian in different degrees. During the Early Proterozoic, a group of Cu-Ni (Co) suifide deposits and some Pt-Pd mineralization occurrences associated with mafic-ultramafic magmatism as well as tin mineralization related to submarine exhalation (about 2219 Ma B. P.) occurred in/near the intrusive-volcanic complex in the Jiuwandashan-Yuanbaoshan area of the north part of the studied region. Then the Xuefengian (Late Proterozo-ic) tin polymetallic mineralization (900 to 700 Ma B. P.) associated with biotite granite of crustal anatexis-type extensively developed in the same area as above. The mineralization exhibits a zoning of metallic elements, i, e. the Sn-W zone, Su-Cu zone, Cu-Pb-Zn zone and Sb zone from the granite intrusion outwards. In the same period, Cu-Ni suifide mineralization related to mafic-ultramafic magmatism occurred in the Longsheng-Mahai area of the northeast part of the studied region, which strikes in a NNE direction. But deposits of economic value have, up to today, not been found there. After then, a series of W, Sn and Cu deposits associated with Caedonian granitic magmatism originated by remelting of the crust around the Yuechengling and Miao'ershan intrusions east of the Longsheng-Mahai area. Pre-Calidonian mineralizations occurred in the Yangtze platform. Henceforth, all the metallic mineralization developed in the South China fold system. First of all, a lot of Pb, Zn, Ag, Sb, (Hg), and pyrite deposits represented by the Laochang, Siding and Beishan deposits are distributed in a depression zone on the south margin of the Yangtze platform. Geophysical studies indicate that several blind granite intrusions exist beneath the major base-metal ore deposits. The orebodies therein occur in the stratified form or as veins. After then, the strongest mineralization occurred in the area outside the depression zone mentioned above. The mineralization of this period ore characterized by Sn, W polymetallic deposits and are mainly distributed in the Fuhezhong area and Danchi area. Several crust-derived granite intrusions range in age from 240 to 88 Ma, which are considered to be emplaced in the Guidong depression area in the southeast of the studied region. W, Sn, Nb, Ta, REE, Zn and Sb deposits of porphyry type, skarn type, and hydrothermal vein type are distributed in endo-or exocontact zones of the granite intrusions. These groups of deposit also show mineralization zoning, i. e. Nb-Ta-REE, W-Sn and Zn-Sb zones centered around the granite intrusions outward. Late Yanshanian tin polymetallic deposits are distributed in the Danchi depression zone striking NNW in the southwest of the studied region. The orebodies are either stratoid or vein-like. In terms of time, the mineralizations can be divided into those of skarn-greisen stage, cassieritet-sulfide stage and tungsten-antimony stage. Mineralization zoning around the late Yanshanian crustal granite intrusions (ranging in age from 149 to 80 Ma) shows consequent zoning, i. e. the Cu-Zn-Mo-W, Sn-Pb-Zn-Ag-Bi, Zn-Pb-Sb-Ag and Hg zones and reversed zoning of W and Sb, As there occurred multistage structural activities, carbonate rocks acted as host rocks and the granite intrusions experienced strong differentiation and less disintegration. Several giant deposits, such as the Dachang and Mangchang deposits, were formed and are preserved there. On the whole, the metallogenesis in the northern Guangxi region shows a poly-circlic spirial development regularity throughout the geological history. As far as each circle is concerned, mantle-derived substances were reduced while crust-derived substances increased and ore elements became more and more varied from early to late. As for the whole geological evolutionary history, the mantle-derived substances gradually got less, while crust-derived ones became more plentiful. In terms of space, the metallo