| 其他摘要 | Shangmanggang gold deposits are located southwestern corner of Yunnan, China. The deposit occurs along the southwest-trending Shangmanggang Fault. Near tile surface, The hanging wall of the fault consists of sandstone and mudstone of the Middle Jurassic Mongga Formation (J_(2m)), and also severs as the hanging wall of the gold orebodies. The footwal1 of the fault is composed by dolomitite locally intercalated with medium-bedded limestone of Lower Permian Sazipo Formation (P_(1s)). In the deep, the fault cut into strata of Sazipo Formation (P_(1s)). The deposit is characterized by intense clay alteration, which is similar to the Mercur Gold Deposit, Utah of U.S.A., and by unique hydrothermal pinnacle karst, which serves as the footwall of the gold orebodies. The muinable ores are mainly earth-red clay ore in which the amotmt of illite reaches to ～70%. The main gold mineralization is associated with kaolinite-pyrite-alteration, illite-kaolinite-pyrite-alteration. Gold is disseminated in the ore with the element association of Au, Ag, Sb, As, Hg, and Cu. Five stages of alteration and gold mineralization are identified in Shangmanggang gold deposits. Stage VI is the most important to gold mineralization. Silicification, kaolinite-pyrite and kaolinite-illite-pyritealteration superimposed are favorable for ore. The program CHILLER was used both to calculate the speciation and concentration of metals in solution and to model ore-depositional processes. Thermodynamic data used are contained within the SOLTHERM database (Spycher and Reed, 1998, pers. com.). The deposition mechanisms considered were (1) isobaric cooling, (2) isothermal diiustion, (3) fluid mixing, and (4) isothermal reaction with host rocks and (5) combinations of wall-rock reaction and fluid mixing. Of these, only reaction path 1, 3, 4 accurately reproduce both the paragenetie sequence of synore alteration assemblages and the proportions of mineral assemblages. ore-deposifional processes in Shangmanggang gold deposits can be described as following: ① the starting ore-fluid (I) (200 ℃) ascended along Shangmanggang Fault to the shallower place (900 ～ 500m deep), and mixed with the cold water (25 ℃) in the fault braccia mainly in Sazipo Formation (P_(1s)), with quartz (>97%) + carbon (graphite) + pyrite assemblages precipitating, which occurred as jasperoid because of quenching. This is Stage I, silicification-jasperoid(I). Jasperoid(I) is characterized by containing carbon up to 2.75% so that the rock is black, and lower Au, As, Sb, Cu, Ph. ② The derived ore-fluid(II)(190 ℃) continued to interact with dolomitite of Sazipo Formation (P_(1s)). With calcite dissolving, dolomite + Mg-bearing clay (Beidellite) + pyrite + quartz + carbon (～0.24%) (+ stibnite) deposited. Comparing to the protolith, grey-dark dolomite, the altered rocks are gray ～ graylish as the carbon leached, and the rocks is sandy and loose as dolomite crystallized and calcite dissolved. This is stage II, dolomitization (decaleitization). ③ The altered ore-fluid(III) got cooling from 190 to 165 ℃ as it made its way to contact with rocks of Mongga Formation (J_(2m)) or in any favorable fissures and cavities. The products occurs as quartz stockwork or jasperoid(II) with the assemblage of quartz + carbon + pyrite + stibnite and trance of As-Sb-Cu-bearing sulfosalt This is stage III, quartz vein (II). ④ Now come to the main ore-forming stage, Stage IV, the ore-fluid(III) interacted isothermally with the sandy mudstone of Mongga Formation (J_(2m)). When the ratio of water/rock was less than 250, the mineral assemblage consists of quartz + kaolinite + pyrite + gold (± carbon ± stibnite ±As-Sb-Cu-bearing sulfosalt), which represents the early Stage IV, and as the ratio of water/rock reached to 250 ～ 25, the assemblage is quartz + kaolinite + illite + pyrite + siderite + dolomite + gold + barite, which is the later Stage 1V. ⑤ with the stockwork of crystalline quartz depositing mineralization is waning. This is stage V. The chemical-reaction-path of Shangmanggang gold deposit is obviously different from that of the Jerdtt Canyon gold deposits. It is inferred that there might be no identical mechanisms and chemical-reaction-path models of ore-depositional processes in the Carlin-type gold deposits. Also the Fig. 2 shows that gold (silver) always deposited later than Sb-As-bearing sulfosalt and sulfide in the Shangmanggang gold deposit. The relationship of Au (Ag) with Sb and As in the Carlin-type gold deposits is superimposition other than paragenesis. So Al usually did not get into these minerals, e.g. stibnite, but occurred in their fissures and their surface by adsorption. |
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