Igneous Rock Series And Metallogenic Series Of Tin Polymetallic Deposits In Jiuwandashan-Yuanbaoshan Region, North Guangxi

The metallogenic region of Jiuwandashan-Yuanbaoshan tin polymetallic deposits in North Guangxi is located in the marginal active belt of Yangtze oldland where there is a great deal of various igneous rocks and a very special rock-forming and metallogenic environment. Some gelolgists have pointed out that the main characteris-tics of gelolgical evolution of South China was that the crust grew from the margin of Yangtze oldland outward. So if the oldland margin is well studied, it is possible to get a key for knowing the geological evolutional history and rock-forming and metallogenic processes in South China. Many geologists have come to the region and obtained some precious and interesting data on the petrology and ore deposits through their research works.Based on the previous studies, this work is mainly aimed at the geneses and evo-lutions of the igneous rock series and the metaliogenic series by the study on regional geology, strata geochemistry, petrology, model mineralogy, petrochemistry, trace ele-ments, isotopes and fluid inclusion, and some new scientific results are obtained as fol-low:1. The Jiuwandashan-Yuanbaoshan region is a geochemical anomalous field en-riched in metallogenic elemcnts Sn, W, Cu, Pb, Zn, Cr, Co, Ni and Sb, alkali cornpenents K and Na, volatilcs B, F and Cl, as well as incompatible clcmcnts Rb, Sr, Li, Ba, U and Th. An apparent feature of the anomalous field is that elements metioned above are enrichcd eighcr in the crust or in the mantle, which is thought to be the basis for the metallogenesis or tin poly-mctallic deposits.2. A new rock-forming age 2412 Ma. for the Sibao group is got hcrc by Sm-Nd isochron method dating the mafic-ultramafic whole rocks. In this way, the oldcst stra-ta in the Yangtze Massif is the early Protcrozoic in stead of middle-late Protcrozoic in-ferred before.3. Tha layered basic-ultrabasic intrusions called before are confirmed as a suite of mafic-ultamafic volcanic rocks, of which its part is identified to be komatiite by studieson the rcglonal rock sequence, petrology, rock structures, geochemistry and trace ele-ments.4. The magmatic liquation type Cu-Ni sulfide deposits called before have been proved to be volcanic Cu-Ni deposits associated with pyroxenic komatiite, which is a new type Cu-Ni deposit in China.5. A series or criteria of recognizing tin-bearing graniteis set up by collecting the data on related petrology, mineralogy, geochemistry, trace elements, transitional ele-ments, incompatible elements and rare earth elements. Furthermore, with the criteria the two series of granitoids (granodiorite series and biotite granite series) in the region are well recognized.6. The biotite granite intrusions, emplacemented in two stages during the Xuefeng movement, is a dominant factor for the metallogenesis of tin polymetallic deposits. The biotite granite intrusions were formed through remelting of the crust and show two im-portant contributions to the minerogeneses. Firstly, during formation and evolution, the granite intrusions experienced a strong fractionation and concentrated the scattered tin polymetals into their cupolas, which lay a good fundament for the metallogenesis. Secondly, when the biotite granite intrusions emplacemented, their heating energy made the ground water to form a series of circular systems of hydrothermal brines and then many base metal sulfide deposits were formed.7. The Beidong and other similar granodiorite intrusions have been firmly believed as a classical differential granitoids derived from the mantle for long time. Recently some gelolgists suggest that the granodiorites belong to I type granite, and constitute a pair granitoids with Sanfang, Yuanbaoshan, Pingying S type granite at the margin of Yangtze oldoand. In this paper the suit of granodiorite is prelimnarily proved to be remelt type granite in which the major rock-forming substances are derived from the crust and a few from the mantle by study on petrology, genetic mineralogy, geochemistry, trace elements, rare earth elments and isotopes combining with the re-gional geological evolution and geological chronology. The Sm-Nd isotopic model age of the granodiorite shows a consistency with the Sm-Nd isotopic isochronous age of the Sibao group, which displays the former being born from the latter. The key reason for no mineralizations aroud the granodiorite intrusions is that they experienced very poor differentiation, and that they were too small in volume to form the hydrothermal brine deposits.8. The laminated tourmalite tin mineralization in strataform and lentiform bodies are discoverd in the Sibao group. This new discovery not only moves up tin metallogenic age in China from 800 Ma. to 2412 Ma. but also certificates the multicycle and inheritance of tin mineralization in the region; moreover, it provides an instance for marine exhalative tin mineralization associated with mafic magmatic activity.9. The tin polymetallic deposits related to the Xuefeng biotite granites are studied in this thesis according to the academic idea of metallogenic series. The deposits in the metallgenic series show two kinds of zoning from the intrusive body outward.1) the normal zonning in order of greisen type tin deposit" tourmalite type tin deposit→cassiterite quartz type tin deposit→cassiterite sulfide tin deposit→copper-lead-zinc sulfide deposit→lead-zinc sulfide deposit→antimony deposit, which may be related to the high-level emplacement of the intrusions; 2) the reversed zoning in order of lead-zinc deposit→copper-lead-zinc deposit→tin polymctallic deposit, which may be result from the descending of heat centre of the intrusions with their cold and solidification. The metallogenic series in time can be divided into three metallogenic pe-riods (cassiterite silicate period, cassiterite sulfide period and base metal sulfidr period) and six metallogenic stages.10. As for the metallogenesis in inland, some geologists with the viewpoint of magmatic hydroghermal metallogenesis look upon granite intrusion as an only source of metallogenic substance, the others with the viewpoint of stratabound metallogenesis only look upon granite intrusion as a heat source. In the region it is preliminarily certi-fied that some of metallogenic substances (Sn, W, Cu and ect.) are related to the biotite granite, another (Cu, Pb, Zn, Sb and ect. ) are derived from the host strata (es-pecially the mafic-ultramafic rocks). The hydrothermal solution to form the base met-al sulfide deposits is from meteoric water. Thus it can be seen that there exists the dialectical unity between endogenetic and epigenenic mineralizations in the metallogenic series.11. The conception of metasomatic system is put forward here. The metasomatism and metasomatic phenomena in the region are studied with the academic idea of metasomatic series and formations. The chemical compositional changes, conditions, evolutional directions and final results of various metasomatic daughter-series in the metasomatic network are studied from the point of view of mineralized-altered period taken as meridian and metasomatic series as latitude.12. The minerogenetic environment and compositional evolution of the metallogenic series of tin polymetallic deposits related to biotite granite are studied by the methods of genetic mineralogy, geochemistry of ore, altered rocks, isotopes and in-clusions.13. A new idea for the polymetallic metallogenesis, including the geochemical anomalous field of tin polymetallic elements as a prerequisite, the tetonic movement as a leading role and the strong differential evolution and heating effect of the biotite granite intrusions as a dominant factor, is put forward in this thesis. Furthermore, the metallogenic model of the metallogenic series in the region is set up here.14. A new mineral with Cu, 28.52%, Ni, 64, 81% and Fe, 6.58% (preliminarily named Baotanite) is discovered together with Mr. Zhoukezi.