Concentration Mechanism of Ore-Forming Fluid in Huize Lead-Zinc Deposits, Yunnan Province
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摘要: 云南会泽铅锌矿床位于扬子板块西缘川-黔-滇铅锌银多金属成矿域的中南部, 严格受断裂带的控制.长期以来, 对于该矿的成矿流体来源存在着较大的争论.研究表明, 矿石中脉石矿物方解石的C、O同位素组成相对均一, 其δ13C (PDB) 为-2.1×10-3~-3.5×10-3、极差-1.4×10-3、均值-2.8×10-3, δ18O (SMOW) 为16.7×10-3~18.6×10-3、极差1.9×10-3、均值17.7×10-3, 不同矿体(不同标高)、不同产状以及相同矿体不同产状方解石的C、O同位素组成不具明显差别; 除了纯液相包裹体(L) 和富液相的气液两相包裹体(L+V) 外, 还存在含子晶的三相包裹体(S+L+V) 和不混溶的CO2三相包裹体(VCO2+LCO2+LH2O), 流体包裹体均一温度介于110~400℃之间, 具有双峰现象; 矿床的(87Sr/86Sr) 0 (0.713676~0.717012) 不仅明显高于地幔(0.704±0.002) 和峨嵋山玄武岩(0.703932~0.707818;85件样品) 的(87Sr/86Sr) 0, 也相对高于矿区赋矿地层(C1b) 的(87Sr/86Sr) 0 (0.70868~0.70931;3件样品), 但明显低于基底岩石的(87Sr/86Sr) 0 (0.7243~0.7288;5件样品), 且成矿过程中流体基本没有发生Sr同位素分馏现象.因此, 成矿流体为均一流体, 是不同性质流体的混合产物, 具有多源性.而从气液两相包裹体盐度-均一温度图解可以看出, 在300~400℃区间, 包裹体盐度基本被孤立为两群: 一群为5%~6% (w (NaCl)), 另一群为12%~16% (w (NaCl)).而在100~300℃特别是150~250℃区间, 包裹体盐度则基本均匀分布在7%~23% (w (NaCl)) 之间.断裂带形成压力为(50~320) ×105Pa, 矿体上覆岩石压力为(574~640) ×105Pa, 矿床成矿压力为(145~754) ×105Pa.流体在上升到断裂带后压力的剧降, 导致了沸腾作用的发生.在混合作用和沸腾作用的双重影响下, 受狭窄断裂带控制的成矿流体高度浓缩, 金属矿物得以大规模地从流体中沉淀出来, 形成品位极高的铅锌矿石.Abstract: The Huize Pb-Zn ore deposits, Yunnan Province, located in the southern-center of the Sichuan-Yunnan-Guizhou Pb-Zn-Ag multimetal mineralization district, are strictly controlled by faulted zones. There has long been controversy about the sources of ore-forming fluid in these ore deposits. Calcite is one of the gangue minerals in the ores of these ore deposits. Their δ13C (PDB) values vary from -2.1×10-3 to -3.5×10-3, and their δ18O (SMOW) values are 16.7×10-3 to 18.6×10-3. There are no obvious difference in the δ13C (PDB) values and the δ18O (SMOW) values of calcites from different orebodies, different occurrences and different elevations. Besides pure liquid inclusions (L) and gas-liquid inclusions with rich liquid (L+V), three-phase inclusions containing a daughter (S+L+V) and immiscible three-phase CO2 inclusions (VCO2+LCO2+LH2O) occur in the minerals. The homogenization temperatures of all these inclusions vary from 110 ℃ to 400 ℃, and have two peaks. (87Sr/ 86Sr) 0 ratios of these ore deposits (0.713 676-0.717 012) are obviously higher than those of the mantle (0.704±0.002) and Emeishan basalt (0.703 932-0.707 818; 85 samples), and slightly higher than those of the Baizuo Formation of Huize lead-zinc ore deposits (0.708 68-0.709 31; 3 samples). But all these ratios are lower than the ones of the basement rocks (0.724 3-0.728 8; 5 samples). The (87Sr/ 86Sr) 0 ratios of these ore deposits also show that the isotopic fractionation of Sr does not occur in the ore-forming fluid during the precipitation of minerals. So, the ore-forming fluid is homogeneous and a product of different fluid mixing.Gas-liquid inclusions can be obviously separated into two groups in the range of 300-400 ℃ according to their salinities: a group with the salinity of 5%-6% (w (NaCl)) and the other group with the salinity of 12%-16% (w (NaCl)). However, the salinities of inclusions vary from 7% to 23% (w (NaCl)) in the range of 100-300 ℃, especially the range of 150-250 ℃. The pressures of faulted zones are 50×105-320×105 Pa and the pressures of overlying rocks on the ore bodies are 574×105-640×105 Pa, and those of the ore-forming processes vary from 145×105 to 754×105 Pa. When the ore-forming fluid flows into the faulted zones, its pressure sharply reduces. Then boiling occurs in the ore-forming processes. As a result of fluids mixing and boiling, the ore-forming fluid is highly concentrated. When the fluid is saturated or oversaturated, metallic minerals begin to precipitate on a great scale. So, the grade of lead-zinc ores is very high.
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Key words:
- fluid mixing /
- boiling /
- relief of pressure /
- high grade
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图 4 会泽铅锌矿床成矿深度(据Hass, 1971, 1976)
Fig. 4. Plot of mineralization depth in Huize lead-zinc ore deposits
表 1 会泽超大型矿床Sr同位素组成
Table 1. Sr isotopic compositions of minerals in Huize lead-zinc ore deposits
表 2 会泽铅锌矿床各成矿阶段流体性质
Table 2. Characteristics of fluid in Huize lead-zinc ore deposits
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