| Abstract:The formation process of sedimentary deposits might be divided into three minerogenic stages, i.e., syngenetic sedimentation, diagenesis and anadiagenesis (katagenesis). The features of sedimentary ore deposits so far preserved must have been formed chiefly at the last minerogenic stage. Based on formation process and intensity of various minerogenic stages, the author has divided the sedimentary deposits into four genetic subtypes: 1. Sedimentary syngenetic deposit. 2. Sedimentary diagenetic deposit. 3. Sedimentary diagenetic-anadiagenetic deposit; 4. Sedimentary anadiagenetic deposit The siderites present in normal sedimentary deposits and sedimentary rocks were formed either at diagenetic stage or at anadiagenetic stage, with the former called diagenetic siderite, and the latter anadiagenetic siderite. Their chief chalacteristics are given in Table l. The Slay, even impalpable and fine-grained siderites seem likely to be products of diagenesis, as evidenced by following facts: (1) They are closely associated with such organic substances as coal seam, carbonaceous shale, black shale and asphaltite (Fig.6, 7,Table 5) , when the organic carbon content is higher than 0.3%, siderite + chamosite +pyrite facies will be formed (Figs. 6, 7); if it is less than 0.3%, hematite facies occurs (Figs. 8, 9, Table 5), containing a small amount of siderite. (2) Hematite has been commonly replaced by siderite to form lots of replacement fabrics, especially pseudomoiphic fabrics and replacement remnants (Frg.10) . (3) Siderites are in close association with such diagenetic minerals as chamosite, thurinrgite, framboidal and pyrite. The rice-yellow, coarse and large grained siderites are products of anadragenesis, as can be substantiated by following evidence: (i) They have coarse or large grains, abundant fluid inclusions, relatively pure composition and very weak alteration. (2) Veined drusy, comby structures and precipitation textures are frequently seen. (3) The veined orebodies composed of siderites are seen to cut across strata or occur along the exfoliations of strata. (4) Siderites are intimately related to anadiagenetic magnetite, specularite, maghemite, quartz and rock crystal, calcite, pyrite, galena and sphalerite. The replacements, whether they occurred at diagenetic stage or at anadia-genetic stage, are very distinct and of great significance. |
