According to the study of Tao Weiping and Zhang Shaohua, silty quartz deposits often form metallogenic series with limestone, dolomite and sepiolite clay, and the combination of silty quartz, limestone and dolomite is the most common, belonging to marine siliceous shale-carbonate formation sedimentary weathering metallogenic series. The Xuanfeng limestone+siltstone deposit assemblage in Pingxiang, Jiangxi Province is a typical example. Ore-bearing formation consists of gray-black medium-thick carbonaceous limestone of lower Permian Bingxia Formation with a small amount of thin limestone, argillaceous limestone and chert strips, dark shale, mudstone with lenticular or thin-bedded siliceous rocks in lower part of Maokou Formation, and dark gray medium-thick bedded limestone, siliceous rock and thin-bedded siliceous rock in upper part. Composition of breccia siliceous rocks. The floor of ore-bearing formation is carbonate rock and sandy shale, and the roof is coal measures of Longtan Formation of Upper Permian. The thickness and quality M of limestone deposits in Qixia Formation are not as good as Maokou Formation. The size of limestone deposits is small and the horizon is unstable. CaO is 50%-52%, MgO is 0.5%-2.09%, and the content of flint is less than 5%, more than 30%, generally between 15% and 20%. The limestone deposit in Maokou Formation has a large thickness, stable horizon and quality, with CaO 53%-S5%. MgOO 7%-1.7%. Powdered quartz orebodies are generally located in the upper part of the ore-bearing formation, especially near the coal-bearing strata. The direct floor of the siltstone orebody and its parent rock (siliceous rock) is dark red mudstone or limestone. The outcrop of the siltstone orebody in the coal measures strata is distributed in two zones, one is from Hengcun to Buwan, with a total length of about 800 m and an average outcrop width of about 40 m; the other is from Tongcun to the foot of Chashan, with an average outcrop length of about lm, with an average outcrop width of 70 m and a floating overburden of the ore bed. Thickness is 0~3m. The ore types of siltstone can be divided into two types according to their parent rock characteristics: weathered siliceous rock type and weathered breccia siliceous rock type. The siliceous rocks of the parent rocks are of lime, grey yellow, grey white and other colors, with fine particle structure, thin to medium-thick layered structures, some with horizontal bedding. The main mineral composition is quartz (97%-99%), others are clay minerals, carbonate minerals, limonite and trace ilmenite. Among them, quartz particles are fine, generally 4 - 30 | xm, equiaxed granular, uneven in size, mosaic between particles, wavy extinction, no biological. Silica is white-gray-white and loose soil-like. With the increase of depth, the non-weathered mass increases, and there is a gradual transition relationship between siltstone and siliceous rocks at the bottom. The content of siltstone-02 is generally 97%-99%, the content of Fe203 is less than 0.4%, and the depth is less than 0.2%.
The Nankeng limestone + siltstone deposit assemblage in Pingxiang, Jiangxi Province is also occurring in the Qixia and Maokou Formations of the Lower Permian. The main characteristics of ore-bearing formation and the scale and quality of limestone deposits are similar to those of Xuanfeng in Pingxiang, Jiangxi Province. The outcrop of siltstone ore body is distributed in Niu-shaped bay, Xiaobutang, Lion-shaped Li and Wutian Bay. The total length of outcrop is about 1000m, the average width is about 80. At present, the mining depth is up to 5m, and the original rock has been seen. powder
The parent rock characteristics and ore types of quartz are similar to those of XuanW. The Liling silty quartz deposit in Hunan Province is an example of the Carboniferous system. Especially in the rock assemblage, the dolomite content increases, but dolomite can not form industrial deposits. Other characteristics of the deposit assemblage are similar to those in Pingxiang area. The silty quartz orebody is tens of meters wide to more than 2_meters wide, extends hundreds of meters and is more than 10 meters thick. Chemical composition of raw ore: SiO: 90.98%, A1203 3.78%, Fe203 2.38% 0
The study on weathering and mineralization mechanism of siltstone deposit is still insufficient. In the sedimentary weathering metallogenic series of marine siliceous shale-carbonate formation, the main metallogenic mechanism is chemical and physical weathering of siliceous rock or siliceous limestone and siliceous dolomite. The siliceous rock is the parent rock of silty quartz, which is composed of fine quartz with particle size of ~30 pm. The siliceous rocks contain more or less carbonate minerals such as calcite and dolomite. It is noteworthy that the fine quartz ore bodies are often located near the coal measures. It is well known that coal measures are rich in sulphur. Under supergene conditions, sulphur is easily oxidized to sulphuric acid. These sulphuric acid can easily dissolve the carbonate components in siliceous rocks and siliceous limestone, making the rock structure loose and further disintegrating into siltstone under physical weathering. Of course, not all siliceous rocks, siliceous limestone and siliceous dolomite can become siltstone deposits. In some areas, even if there are siliceous rocks and siliceous dolomites, the climate, topography and geomorphology conditions are favorable, but there is no weathering into silty quartz. After careful study, it is found that the disintegration of siliceous rocks requires certain structural conditions. Fine quartz crystals without recrystallization and secondary enlargement of mineral particles, crack development, siliceous rocks with linear mosaic contact between particles are conducive to the infiltration of acid solution and groundwater and the leaching of carbonate materials. Then the physical effects of water, such as thermal expansion and cold contraction, lead to the disintegration of siliceous rock particles. Because the supergene conditions are basically acidic environment, siliceous rocks will not dissolve. Silica gel is formed, so the deposit is finally formed by residual in situ.
V. Distribution of ore deposits
Geographically, siltstone deposits are mainly distributed in the south. Up to now, siltstone deposits have been found in Jiangxi, Zhejiang, Fujian, Hunan, Yunnan and other places. According to the analysis of metallogenic conditions, silty quartz deposits can also occur in other provinces in the south, such as Guangdong, Hubei, Guizhou, Sichuan and Chongqing.
From the metallogenic epoch, the siltstone deposits mainly occur in Devonian, Carboniferous and Permian horizons, which are marine sediments in the South and have abundant terrigenous sources.