Study On The Large Ductile Shear Zone In Southern Anhui And Its Relationship With Gold Mineralization

In this paper, with the large ductile shear zone in Southern Anhui as an object and the study on the relationships between the large ductile shear zone and gold mineralization as main point, the tectonic deformation characteristics of the Huangmao-Wucheng-Tunxi ductile shear zone were accurately determined in terms of tectonic deformation theory, tectonic petrology, and tectonic geochemistry, through profile survey of six important tectonic lines including Baishikeng, Tianzikeng, and Hanjia, large-scale geological logging and geological mapping of 17 galleries in the Tianjinshan mining area, and large quantities of laboratory monographic research efforts on microstructures. The sketch map of distribution (1:250000) of the large ductile shear zone in Southern Anhui was drawn by mapping the "Wangcun-Liukou-Hecheng-Yaoli-Ehu-Dabeiwu" regional tectonic profiles. Systematic investigation and comparative study of the Tianjingshan, Xiaohe, Baishikeng, Tianzikeng, Hanjia, Jiumuqiu, Huangjian and Xiaoliankou gold deposits in the study region and the Jinshan and Dabeiwu gold deposits in the adjacent regions, mainly including detailed analysis of the relationships between the large ductile shear zone and gold mineralization, and evaluation of gold-bearing property of the large ductile shear zone, reveal the temporal, spatial and genetic relationships between the large ductile shear zone and gold mineralization, and a geological ore-prospecting model suitable for gold deposits in Southern Anhui was established.In this paper, the following main research achievements are obtained:(1) The existence of a large ductile shear zone, i.e., the Huangmao-Wucheng-Tunxi shear zone in Southern Anhui was accurately determined. Detailed study on the geological characteristics of the large ductile shear zone was conducted in three aspects, i.e., tectonic deformation theory, tectonic petrology, and tectonic geochemistry:1) the Huangmao-Wucheng-Tunxi shear zone is a large ductile shear zone, generally exhibiting bell-shaped distribution in the study region,～2-5 km wide, divergent towards NE, and convergent towards SW. Strong-weak zoning is macroscopically significant, the contact zone between the Proterozoic stratum and Jinningian rock mass is the strongest strain zone, with ultramylonite developed, and the strain strength weakened gradually towards both sides, with mylonitized phyllite and protomylonite developed in the weak strain zones. In the Proterozoic mylonite stratum, Type-A folds, S-C fabric, and developed lenticular quartz could be seen. In the felsic mylonites, rotational porphyroclast formed out of feldspar quartz phenocryst could be seen, both foliation and lineation were relatively developed, and their attitudes show that the ductile shear zone exhibits NE-NNE strike as a whole and is dominated by horizontal displacement; the deformation strength of secondary shear systems in the same period is significantly lower than the primary shear zone, with ore controlled by brittle ductile or brittle shear characteristics; 2) the Huangmao-Wucheng-Tunxi ductile shear zone has microscopic characteristics in agreement with macroscopic characteristics, with protomylonite-ultramylonite distributed, and the macroscopic features (S-C fabric and lenticular quartz) and the microscopic features (rotational porphyroclast and bookshelf structure) show that the Huangmao-Wucheng-Tunxi ductile shear zone have both sinistral and dextral shear characteristics, and the EBSD quartz fabric characteristics reveal that the quartz is superposition of medium- and low-temperature deformation and low-temperature deformation, indicating likewise that the ductile shear zone is characterized by multiple episodes of activity; it was judged from the macroscopic deformation of veins that the ductile shear zone had dextral shear first and then sinistral shear. Based on the geotectonic evolution in this region, it was preliminarily inferred that the dextral ductile shear zone was formed by the intracontinental orogenesis in the Indosinian-early Yanshanian period, whereas the sinistral ductile shear zone was formed in the late Yanshanian period. Microscopically, the quartz in the ductile shear zone has universally undulatory extinction, wiredrawing-like elongation, and prominences including subgranulation, as well as rotational dynamic recrystallization of subgrains, some quartz grains formed pudding and bookshelf structures under micro-rupture, and there are locally pressure shadows, and quartz veins with asymmetrically growing opposite sides, etc., formed by pressure solution, and rotational porphyroclasts, etc., formed by crystal plastic deformation; the dynamic recrystallization is mainly prominence recrystallization, and minor rotational dynamic recrystallization of subgrains; the feldspar has no significant deformation, with minor twin crystal bending, indicating that the ductile deformation temperature of the primary shear zone is not more than 400 ℃. From the metamorphism and deformation characteristics of feldspar and quartz, it can be seen that the metamorphism and deformation mechanisms of the ductile shear zone are mainly dislocation slip, micro-ultramicro-rupture, and pressure solution at medium and low temperatures. The microscopic measurement results of grain size of the new crystal grains of recrystallized quartz show that the grain size of quartz grains recrystallized from mylonite with some alteration did not differ significantly from that from mylonite without alteration, ranging from 0.015 mm to 0.025 mm; the differential stress in the Huangmao-Wucheng-Tunxi ductile shear zone in Southern Anhui was calculated using the method for grain size of new crystal grains from dynamic recrystallization of quartz, ranging from 74.97 MPa to 106.08 MPa; 3) the results from the analysis of quartz fabric in the ductile shear zone using EBSD reveal that the quartz in the ductile shear zone is dominated by medium-and low-temperature rhombohedral slip and low-temperature basal slip, indicating that the ductile shear zone is mainly medium- and low-temperature and low-temperature metamorphic environments; the optical axis of the quartz was manifested as single point maximum parallel to axes b and c, superposed by ac zonal maximum, indicating that the quartz is dominated by prismatic{10-10}<a> slip and rhombohedral{10-11}<a> superposed by basal{0001}<a> slip, with deformation temperature being medium temperature, medium-and low-temperature (400-550 ℃) and low temperature (<400 ℃). Both the plastic deformation mechanism and the EBSD-derived quartz fabric characteristics show that the ductile shear zone had metamorphism and deformation at medium- and low-temperature; 4) the macroscopic and microscopic kinematic characteristics and the EBSD-derived quartz fabric characteristics reveal that at least two ductile shear events occurred in the Huangmao-Wucheng-Tunxi ductile shear zone, and specifically, a dextral ductile shear event occurred first, followed by a sinistral ductile shear event; 5) the metamorphism temperature environment indicated by the deformation mechanisms of the quartz and feldspar and the EBSD-derived quartz fabric characteristics show that the metamorphic facies in the Huangmao-Wucheng-Tunxi ductile shear zone in Southern Anhui are mainly low greenschist facies-high greenschist facies, locally reaching low-medium amphibolite facies.(2) The relationships between the Huangmao-Wucheng-Tunxi ductile shear zone in Southern Anhui and gold mineralization were studied systematically, and ample evidence was obtained for existence of large ductile shear zone and ductile shear zone-type gold deposits in Southern Anhui.1) The Huangmao-Wucheng-Tunxi large ductile shear zone in Southern Anhui and its geometric, kinematic and dynamic characteristics were determined primarily, and the gold-bearing property of the ductile shear zone was evaluated; 2) The comprehensive study results show that the Huangmao-Wucheng-Tunxi shear zone is the overriding ore-controlling structure in the region. The gold mineralization in the region includes mylonite-type mineralization, altered rock-type mineralization, and quartz vein-type mineralization, and all of these three mineralization types occurred spatially in the ductile shear zone and have close genetic relationships with the ductile shear zone; the mylonite-type mineralization is syn-ductile shear zone mineralization, with the occurrence of ore bodies directly controlled by the ductile shear zone; whereas the altered rock-type mineralization and the quartz vein-type mineralization are controlled by the brittle rupture superposed on the ductile shear zone, and they temporally belong to post-ductile shear mineralization but their spatial occurrence is also controlled by the ductile shear zone, especially for quartz vein-type ore bodies with overall attitude consistent with that of the ductile shear zone; 3) The contents of associated elements, including gold, were compared between the Lingshan granite in the Huangmao area not affected by the ductile shear zone and the felsic mylonite in the ductile shear zone, and the results show that the ductile shear zone has significant effect of converging associated elements, including gold; the contents of associated elements, including gold, were compared between the Jingtan Formation stratum in the study region and the Jinningian rock mass, and the results show that the contents of most elements associated with gold in the Jingtan Formation stratum are higher than those of associated elements in the Lingshan rock mass; compared with the Jinningian rock mass, the Jingtan Formation stratum might have provided more ample metallogenic elements for gold mineralization; 4) The analysis results of rare earth elements show that the Jingtan Formation stratum and the Jinningian Lingshan rock mass have consistent distribution patterns, indicating that both of them are possibly cogenetic, but compared with the Jingtan Formation stratum and the Jinningian granite, the Yanshanian granite has significantly different distribution pattern of rare earth elements, showing a pronounced characteristic of enrichment of light rare earth elements and insignificant negative europium (Eu) anomaly; the analysis results of rare earth elements in the gold ore bodies reveal that the distribution pattern of rare earth elements in the gold ore bodies is similar to that of rare earth elements in the Jingtan Formation stratum and the Jinningian granite but differ greatly from that of rare earth elements in the Yanshanian rock mass, indicating that the metallogenic elements are more likely from the Jingtan Formation stratum and the Lingshan rock mass, and have little relationship with the Yanshanian granite; 5) The sulfur isotope σ34S values in the pyrite and arsenopyrite of the Tianjingshan gold deposit are concentrated in two intervals:8%o-10%o, and 1%o-3%o, indicating that the sulfur isotope composition has a characteristic of mixed deep and shallow sources, but the shallow source is dominant; the sulfur isotope δ34S values in pyrite and arsenopyrite are close to each other among three mineralization types, which also indicates that the mineralization process has some continuity or inheritance; 6) The fluid inclusions formed in the hydrothermal mineralization period have three types:gas-liquid two-phase inclusion, pure-gas-phase inclusion, and CO2-bearing three-phase inclusion, out of which the gas-liquid two-phase inclusion is predominant. The temperature measurement results of the inclusions show that the homogenized temperature exhibits obviously two optimum homogenized temperature intervals in the stages Ⅰ and Ⅱ of mineralization, which are concentrated within 125-150 ℃, and 225-275 ℃, respectively, indicating that the ore-forming fluid might have been a mixture of two kinds of fluids; with the proceeding of mineralization, all of the temperature, salinity, and pressure of the fluid exhibited descending trends as a whole from the early stage to the late stage of mineralization, in particular, the temperature and pressure changed significantly; the density of the fluid fluctuated within a small range, slightly exhibiting an ascending trend. The test results of hydrogen and oxygen isotopes in the fluid inclusions show that the δ18OH2O values are -8.93‰-1.5‰ and the δD values are-76.4‰ 62.8‰, and the hydrogen and oxygen isotope values in all of three main stages of mineralization fall into the positions between magmatic water, metamorphic water, and meteoric water lines, indicating that the ore-forming fluid has a characteristic of mixed magmatic water or metamorphic water and meteoric water, and the ratio of meteoric water increased constantly with the mineralization process, and the meteoric water finally became predominant; 7) The spatial relationships between the ductile shear zone and the gold deposits reveal that the ductile shear zone governs the distribution of the gold deposits in the region, and the spatial distribution of different mineralization types, too; and the Xiaohe gold deposit is a strong strain zone developing area, dominated by mylonite-type and altered rock-type mineralization. The gold ore bodies occurring in the primary shear zone mainly exhibit lenticular and vein-like, and occurred in shear alteration zone with intense ductile deformation, with the attitudes consistent with mylonitic foliation, moreover, the plunge direction and plunge angle of the veins are consistent with the lineation on the mylonitic foliation, demonstrating the 3D spatial control of the gold ore bodies by the primary shear zone; the Tianjingshan, Tianzikeng, Baishikeng, Jinbeiwu, and Zhuoma ore blocks occurring in the secondary shear zone in the hanging wall of the primary shear zone are dominated by brittle rupture in weak strain domain, forming gold-bearing quartz vein-type mineralization; 7) The corresponding match relationships between the structure of the ductile shear zone and the gold mineralization enrichment and gold ore body locating were explored in depth, the spatiotemporal relationship and genetic correlation between the ductile shear zone in Southern Anhui and gold mineralization were ascertained primarily, and the ore-controlling mode of the ductile shear zone in Southern Anhui was established. It was believed that the gold mineralization process in this region was consistent with the superposition mode of multiple gold mineralization types in long-term active ductile shear zone proposed by Chen, et al. (1999), so the gold deposits in Southern Anhui should belong to ductile shear zone-type gold deposits.(3) The Tianjingshan gold deposit is a composite gold mineralization system developed with multiple mineralization types, including quartz vein-type mineralization, altered rock-type mineralization, and mylonite-type mineralization, and the spatiotemporal configurations between various mineralization types have close inherent genetic relationships, with significant characteristic of combined occurrence. The macroscopic structure of mineralization evolution is divided into initial deposition stage, tectonic-magmatism pre-enrichment stage, and endogenic hydrothermal enrichment stage, and the microscopic structure of mineralization evolution in the main stage of mineralization is divided into the following stages:Ⅰ. Trace gold-quartz stage; II. Gold-single sulfide-quartz stage; Ⅲ. Gold-polymetallic sulfide-quartz stage; IV. Trace gold-fluorite-calcite-quartz stage. The Tianjingshan gold deposit is a typical ductile shear zone-type gold deposit, but its main stage of mineralization is manifested as ore control by late brittle rupture; the systematic analysis of the ore controlling structure, mineral typomorphism, and deposit geochemical characteristics reveals that the scale of the Tianjingshan gold deposit should be equivalent to the Dabeiwu gold deposit; the reason why the latter becomes a large gold deposit is that the mylonite-type and altered rock-type low-grade ores have been fully exploited and utilized, whereas the Tianjingshan gold deposit is currently only limited to exploitation of shallow quartz vein-type ores.(4) The limitation that the previous explorers prospected gold-bearing quartz vein-type ores only from epimetamorphic clastic rocks in outer contact zones was broken through, and it was clearly proposed that ore prospecting from granite-type mylonite belts in inner contact zones should be focused on. It was found through field work that, in one ductile shear zone, the tectonic deformation of the outer contact zone was mainly embodied as cleavage belt, schistose belt, and phyllite belt, whereas the granitic rocks in the inner contact zone were characterized by mylonite belt and granulate rock belt, so a new understanding that granitic mylonite-type and altered rock-type gold deposits with a certain scale are likely to be found in the gold-bearing ductile shear zone (within 300 m from inner contact zone of rock mass) developed along the inner contact zone was put forward. A 0.6-1.46-m-thick granitic mylonite-type ore body has been seen at a place more than 80 m within the rock mass penetrating from 106 m middle section of the XJ9 of Tianjingshan, which is dominated by arsenopyrite and pyrite, has gold grade of 0.n-n g/t, and the existing engineering controlled extension is> 80 m. It possibly provides a new object for prospecting gold deposits in Southern Anhui, and brews new breakthroughs; two new gold mineralization types were found and evaluated:granitic mylonite type (middle section of 200, and XJ6, etc.), and gold-bearing altered rock type. The latter has become the main exploitation object in mines and the main prospecting object used by geological exploration entities, but due importance has not yet been attached to the former; great efforts have been made to highlight and strengthen exploitation and utilization of low-grade ores, achieving extremely significant social and economic benefits. This is specifically manifested by the fact that the Tianjingshan gold deposit has become the first established gold deposit with reserve over one ton in Southern Anhui and the Tianjingshan gold mine was awarded the title of "Demonstration Enterprise for Conservation and Overall Utilization of Mineral Resources" by corresponding authority of China.