| 粤北地区一个再沉积同生矿床的探讨 |
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| 引用本文：.1983.An Investigation Of A Redeposition Type Syngenetic Deposit In Northern Guangdong[J].Mineral Deposits,2(4):69~77 |
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| An Investigation Of A Redeposition Type Syngenetic Deposit In Northern Guangdong |
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| Abstract:Located at the forefringe of the foreare vertex of the Northern Guangdong "(-type" tectonic system, the mining district is also situated on the east flank of a secondary syncline belonging to the Beijiang syncline and positioned on the southern part of its western flank close to its core. The ore-bearing strata are within the middle and upper parts of the middle Devonian Qiziqiao zu, composed of a set of fossiliferous algal reef biolithite, dolomitic limestone, marble and ore-bearing breccia. The last one, formed as a result of the accumulation of the high density detrital flow, makes up the basic unit of the ore-bearing sedimentary sequence Some redepositional characteristics have been noticed for this deposit: 1) Certain stratigraphically horizontal and spatial relationships seem in existence between the deposit and the middle Devonian Qiziqiao zu; ore in all the four orebodies is characterized by the brecciated structure and the flexible variation of the breccia; the strata between the orebodies are continuous considerably in strike so that they can be well compared at different positions. 2) The ore-bearing breccia shows redepositional characteristics in composition, structure and texture; the breccia cemented by the same substance as itself has such features as wearing, flexible variation, mobility, sorting and depositional cycle, which are products of redepositional origin, undoubtedly typical of intraclast breccia. 3) Graded bed and diagonal bedding composed of worn crystals of pyrite are characteristic of redeposited sediments and also provide the direct evidence for mechanical sedimentation. 4) Minute quartz in brecciated ore, winnowed out from the foreshore sediments, supplies another proof for redeposition. 5) No alterations have been observed except decolorization of the wall rocks. 6) Minor element analyses of individual pyrite show Cu/Ni≈0, Se≈0.8 ppm and V=2~ 44 ppm. 7) Sulfur isotope determinations indicate δS34= +13.3~ 18.8‰ and S32/S34=21.810. In the forefringe of the algal reef carbonate rocks, the fossil assemblage consists predominalatly of stromatolite, brachiopoda, gastropoda, stromatopora and zoarium(?), while intraclast biolithite, dolomitic limestone and marble make up the major sedimentary rocks The grains of sediments are mainly redeposited skeleton or ore-bearing intraclast breccia with gigantic slumped blocks in between. Their lowest boundary is separated by the submarine erosional surface. In addition, there also exist graded bed and diagonal bedding made up of pyrite crystals. All this shows convincingly that: they are special sediments formed in the environments of fore-reef slope and fringe of shelf and constituting forefringe of fore-reef subfacies. Such a stratabound accumulation of sulfides must have developed on the flank or the top of an acting dome, but where the accumulation exceeded the critical stable level, the gradient of the sediment pile was likely to cause slumping. Especially when the growth rate of the reef highland and the calcareous rock highland was greater than the deposition rate of the basin sediments, slumping might have occurred almost exclusively by the influence of the gradient thus formed. It is conceivable, therefore, that where the deposition of ore-bearing bed reached a certain depth and the growth rate of the algal reef exceeded its deposition rate, periodic slumping was sure to take place. This accounts for the fact that brecciated ore is characteristic of every orebody in the mining district. It should be pointed out that the reduction environment of the fore-reef slope is by no means the optimum condition for ore deposition, only through slumping, short-distance transportation, sorting and concentration can small but economically profitable redeposited deposits be formed. This type of deposits are surely not present alone. The fore fringe of the fore-reef subfacies of surrounding-sea algal reef distributed in extensive shelf areas is the place where redeposited deposits form. The lagoon facies on the side of back-reef subfacies is especially promising in search for the syngenetic sedimentary deposit. In northern Guangdong, sufficient attention should also be focused on this sort of areas in mineral exploration. |
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