| 其他摘要 | Based on previous fruitful results of Gejiu Sn-Pb polymetallic placer deposit in southwest China’s Yunnan Province, Ban Nameung Cu-Pb-Zn-Ag polymetalic deposit in central Lao PDR’s Xaysomboun Special Zone and basalt weathering crust on southern Lao PDR’s Boloven Plateau all under tropical-subtropical monsoon climate, the material sources of supergene mineralization, its mechanism, supergene geochemical characteristics and industrial potentiality of ore recovery were discussed, aiming to study the following three scientific questions: (1) supergene enrichment or mineralization rules of Sn, Pb, Mn, Al, Cu, Zn, Ag, REE, Nb, Ta and Ga under tropical-subtropical climate, (2) supergene metallogenetic mechanism and industrial potentiality of ore recovery and (3) supergene depletion and enrichment of some elements, through empirical examples that were dolomite weathering crust, Sn-Pb polymetallic placer deposit, manganese nodule, gravity separating dressing in Gejiu, supergene initial oxidizing of sulphide ore in Ban Nameung, basalt weathering crust in Boloven etc. Necessary analyses were utilized including thin section observation, major element geochemistry, trace element geochemistry, REE geochemistry, heavy mineral, chemical speciation and microarea probing and so on.
The principle conclusions are as follows.
1. Chiselled karst feature largely distributed in Gejiu has been formed because of ceaseless uplift of Gejiu since Yanshanian movement and typical subtropical monsoon climate. Concomitantly, much leaching of CaO and MgO in carbonate rocks has given Sn-Pb placer deposit tremendous mineralization hosting space. Meanwhile, primary sulphide ores has been easily decomposed in alkaline environment. Clay minerals, iron-manganese oxides and heavy minerals resistant to weathering like cassiterite have been formed and then enriched with other mineralized elements during weathering of dolomite, granite, basalt, skarn and primary sulphides some of which might not be mineralized. The adsorption of elements and traping of fine minerals by clay minerals and iron-manganese oxides are supergene mineralizing mechanism of placer deposit. Sn, Pb, Mn, Ga, Cd, Ag and In were derived from above mentioned rocks, sulphide ores and oxides.
2. The supergene mineralized mechanism of Gejiu Sn-Pb placer deposit includes: (1) the enrichment of primary heavy minerals like cassiterite and magnetite. (2) the enrichment of relic sulphide like galena, chalcopyrite and pyrite etc. (3) the enrichment of secondary minerals such as cerusite, malachite, native lead and native copper. (4) the adsorption and trap of Pb, Ga and other elements or fine minerals by iron-manganese oxides. (5) the adsorption and trap of elements or fine minerals by manganese nodule. (6) the isomorphism between Ga and Al, Cd and Zn. (7) the enrichment of heavy minerals by flowing groundwater in karst holes or caves. Supergene mineralization is not attributed to only one factors of the above mentioned mechanism, but to comprehensive factors.
3. The so called manganese nodules in Gejiu placer deposit are actually manganese-iron concretions with main component of Fe2O3, Al2O3, SiO2 and MnO in which Mn, Sn, Pb, Ag, Ga, Cd and In are enriched to mineralization while Cu and Zn are enriched. Many minerals such as hematite, calcite, mica, quartz, montmorillonite, kaolinite, dolomite, potash feldspar are trapped (included) and Mn, Sn, Pb, Ag, Ga, Cd, In, Cu and Zn are adsorbed in manganese nodules. Mn, Pb and REE are all more enriched in manganese nodule than in placer ore.
4. There are many complicated components composed of cassiterite, galena, cerusite, chalcopyrite, nativel lead, native copper, malachite, pyrolusite, scheelite, magnetite, hematite and so on in Gejiui Sn-Pb placer ore. Natural process of gravity enrichment of minerals in placer formation was simulated by mean of actual gravity separating dressing process in Yuanxing Dressing Plant. Results show that general recovery rate of Pb, Ga, Mo, Cd, In, Cu and Zn is only from 3.037% to 6.439% in concentrate, while most of these elements left in tailings. Meanwhile, Ag and Mn were also analyzed in only two shaking tables, and the results show both have also lower recovery from 0.66% to 2.9%. In the two shaking table process, Ag is more enriched in sulphide phase than in iron-manganese oxide phase, while Mn is more enriched in carbonate phase than in iron-manganese oxide phase. In whole dressing process, Pb, Mn, Cu and Zn are more enriched in carbonate phase than in iron-manganese oxide phase. What is more, magnetite with potential value in concentrate is not evaluated. The results of process of gravity enrichment show that carbonate minerals of Pb, Mn, Cu and Zn would be more enriched than their sulphides in course of placer ore formation in karstification.
5. In initial stage of oxidization of sulphide ores in Ban Nameung Cu-Pb-Zn-Ag polymetallic deposit, Cu, Ag, Pb and Zn are leached while SiO2, K2O and CaO are enriched in weathered material. Au in sulphide phase which is main phase in sulphide ore is partially transformed into organic and carbonate phases. Ag in sulphide phase which is main phase in sulphide ore is partially transformed into carbonate and organic phases. Cu in sulphide phase which is main phase in sulphide ore is partially transformed into iron-manganese and carbonate phases. Pb in sulphide phase, adsorption phase, carbonate phase and iron-manganese phase which are main phases are partially transformed into irom-manganese phase, carbonate phase and organic phase. Zn in sulphide phase which is main phase is partially transformed into its irom-manganese phase, organic phase, carbonate phase and adsorption phase. All the ratios of phases would be changed while weathering strenghthened.
6. The Boloven basalt in south Lao PDR’s Champasak Province is characterized by subalkaline and Nb,Ta and Ga rich features. In special parts of the basalt weathering crust, REE, Nb, Ta and Ga are enriched to supergene mineralization where ∑REE, (Nb2O5+Ta2O5) and Ga reach their maximums at 775×10-6~1003×10-6, 641.9×10-6~655.8×10-6 and 81.6×10-6 respectively. Furthermore and significantly, there is no less than 2 meters in the basalt weathering profile in which (Nb2O5+Ta2O5) is over cut off grade. REE might mostly occur in P and Ti hosting minerals, and independent xenotime might be formed in the profile. Nb-Ta might occur in Ti, U, Zr and Th hosting minrals. Ga might occur in Ti hosting minerals by way of isomorphism of Ti. Nb, Ta and Ga might is enriched in only several Ti, REE and Zr hosting minerals. So the supergene enrichment of these elements in parts of the weathering crust may have great deal to do with the result of residual enrichment of heavy minerals, and not much to do with adsorption of clay minerals and isomorphism in gibbsite. |
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