| 其他摘要 | The Bainiuchang deposit is a super-large Ag-polymetallic deposit, which is characterized by large-scale Pb, Zn and Sn reserves with a lot of dispersed elements (In, Cd, Ge, Ga, etc.) in addition to the Ag reserves of >7000t. The Bainiuchang Ag-polymetallic deposit lies geographically between the Gejiu ore field and the Dulong ore field in southeastern Yunnan Province and occurred geologically in the Middle Cambrian fine clastic-rock formations at the northwestern terminal of the Caledonian fold system in South China. The mining district, which is separated from the Yangtze Platform by the Mile fault in the northwest, abuts on the Ailaoshan block at the Honghe fault in the southwest, links with the North Vietnam ancient land in the south and is in line with the Nanling fold system at the Wenma fault in the east, is a Caledonian fault depression basin. The mining district is relatively complicated in structure, magmatic activity and mineralization. And the evolution of every structural unit has made a certain contribution to the formation of the Bainiuchang deposit.
Since the Bainiuchang Ag-polymetallic deposit was found in late 1980's, many scientific researches have been performed by several research institutions or many scholars. But it is debatable in terms of source of the ore-forming material and fluid, genesis of the deposit and the relations between the deposit and magmatic batholith and so on. Detailed studies on the regional geology and the local geology of the Bainiuchang deposit have been carried out in this study. And systematic microscopic observation of the ores and the quantitative electron-probe analysis of the ores and minerals have been done. Meanwhile, the contents of major elements, trace elements, rare earth elements and the Pb, S, C, O isotopic compositions of the ores and rocks have been analyzed. Based on these, several conclusions drawn from this study are as follows:
1. The existing form and forming stage of silver mineral. Ag mainly appears in the form of solfosalt minerals or complex solfosalt minerals and galena is the main carrier mineral of Ag. Protosomatic silver mineral is obviously divided into two groups: one group includes the minerals in the family of tetrahedrite and the antimony solfosalt minerals of Ag, which occur in galena's interior or combines tightly with galena like a whole. This group bears similar source of ore-forming materials and it belongs to exhalative sedimentary origin. Another group is tin complex solfosalt minerals. Ag generates from the edge of galena, and always generates with cassiterite, stannite and franckeite. It is clear that stannite replaces cassiterite on the edge of cassiterite to form purfle texture, and it implies magmatic hydrothermal origin.
2. The characteristics of granite batholith and the relations between the granite batholith and the deposit. Granite in the Bainiuchang mining district possess the characteristics of silicon and alkaline enrichment, high-K and low-Na and belong to peraluminous and calc-alkaline granite similar with transformation type granite in south China, and lie in syn-collison S-type granite range. Granite mainly come from the remelting of basement metamorphic rocks and enriches Metallogenetic elements Ag, Pb, Zn, Sn etc. By comparing the Chondrite-normalized REE distribution pattern of granite with that of ores, we find that the REE distribution pattern of granites resemble that of ores in Chuanxindong, Duimenshan, Awei ore blocks in southeastern，but show obvious difference with that of ores in Baiyang ore block in northwestern, which implies that the magmatic hydrothermal superimposition mainly occurs in the southeastern of the Bainiuchang mining district.
3. Geochemical characteristics of ore-bearing formation in the deposit. We have analyzed systematically major elements, trace elements and rare earth elements of the ore-bearing formation. By comparing preceding three sets values of the Bainiuchang Ag-polymetallic deposit with corresponding values of hot-water sedimentary siliceous rocks in the Dachang and the Gejiu deposits, rich-tourmaline banded hydrothermal sediments in the Canada Sullivan deposit and banded chert in iron-bearing formations in the Canada Agnico-Eugle deposit in addition to modern sea-floor hydrothermal sediments. We offer exhalative sedimentary evidences for the ore-bearing formation and prove that extensive hot-water exhalative sedimentary activities took place in the Bainiuchang mining district in mid-Cambrian.
4. The source ore-forming materials and fluids. Pb and S isotope compositions of the Bainiuchang Ag-polymetallic deposit show that the metallic ore-forming material and sulfur mainly derived from basement metamorphic rocks in lower curst by leaching in mid-Cambrian, and magmatic hydrothermal fluid also provided partly ore-forming material such as Sn for the deposit in southeastern in Yanshanian. In addition, the characteristics of C, O isotopic compositions and rare earth elements of ores indicate that the ore-forming fluid mainly came from the deep magma reservoir in mid-Cambrian and was composed of magmatic hydrothermal fluid in remelting process of basement metamorphic rocks in Yanshanian.
5. The characteristics of dispersed elements. The average contents of dispersed element In, Cd and Ga are 33.21×10-6, 389.8×10-6 and 10.18×10-6 in the deposit, respectively. Based on this, we estimate that In, Cd and Ga reserves could exceed 180t, 20000t and 600t, respectively. And sphalerite is main carrier mineral of In, Cd and Ga.
6. A possible genetic model for mineralization of the deposit. The deposit shows the characteristics of multi-sources of ore-forming material and goes through a relatively long time mineralization process, and possesses the extensive and important mid-Cambrian exhalative sedimentary mineralization, which formed the stratiform, stratoid orebodies, and Yanshanian magmatic hydrothermal superimposition, which mainly occurs in the southeastern of the deposit. Based on above of all, the Bainiuchang Ag-polymetallic deposit belongs to “hot-water exhalative sedimentary + magmatic hydrothermal superimposition” origin. |
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