The Black Shale Series and Ore-bearing Sequences of Upper Sinian-Lower Cambrian, Southwest of China

Key words: 
• black /
• shale /
• ore-bearing

Abstract: The Yangtze area during the Late-Sinian and Early Cambrian is the typical passiv e c ontinental margin basin, which is the ideal basin type for us to study the sedim entary crust and recovery the eustatic curve. Black shales are widely distribute d in the Upper Sinian and Lower Cambrian of the western Hunan, which belongs to the southeastern margin of the Yangtze Platform. The strata of the Upper Sinian - Lower Cambrian can be divided as two super-sequences and three sequences. The sequence of Upper-Sinian and Lower Cambrian is as follows (in ascending order): Upper Sinian-dolostone, black slate and siliceous slate, lenticular barite, blac k slate, phosphorite, black siliceous slate; Lower Cambrian-black slate, concret ionary phosphorites, barite, stone coal, V-Ni-Mo deposits, black slate, black sa ndstone, K-shale, limestone. The series of ore bearing are the black shale-phosp hate-silicalite of Upper Sinian and phosphate-barite-stone coal-metals rich of L ower Cambrian. Advances presented in this thesis can be summarized as follows: 1. There are six types of black shales: black shale, chert, coal stone, phosphor ite, barite and carbonate rich in organic matter. They exhibit different regiona l characteristics in time and space; 2. The geological evidence suggest that the black shales were formed in a platfo rm marginal trough on southeastern margins of the upper Yangtze-Exi Platform, wh ich belongs to a shallow water shelf reductive environment with water depth less than 250 meters. The environment evolved gradually from shallow water during la te Sinian to deeper, and more open during early Cambrian. 3. Many organisms have been discovered in the black shales, including radiolaria s, spicules, and bacteria-algae, which indicate that they have played an importa nt role in the formation of the black shale series. 4. Bio-sedimentation and bio-geochemical processes play a certain leading role. Resulted from the reworked benthos, the organic carbon in the sediments (especia lly in Xinhuang region) is rather abundant. As the upper layer (surface) seawate r is abundant nutrient and rather high biotic productivity, the biomass influx is relatively large. The biologic processes cause the enrichment of metal elemen ts, minor elements and REE, and locally change the pH and Eh values as to form a weak reduction condition resulting in the leaching of the Fe and Mn and remigra tion and re-distribution of metal elements. 5. The rate of sedimentation is very slow and discontinuous. The bottom current eroded and reworked the sediments and hence restrained the accumulation. Therefo re，the sedimentation rate in the area is very low and even some segments witho ut sediments. On the above, the black shale of the Upper Sinian-Lower Cambrian generated from the anoxic event, and the organic matter source is algae and fungal (Thallophy ta). After the organisms degrade slowly, the organic matter absorbed and complex ed many elements. At the early diagenesis stage, the organic matter decomposed, and some element relaxed, i.e.，the element accumulated by many stages, so the e l ements exist in many styles.The generation of Ni-Mo, coal stone, barite and pho sphorite deposits are controlled by the sea-level change, and the ore-bearing se ts developed in the transgressive systems tract and condensed section. Finally, the author concluded that the ore-bearing characteristics controlled by the two aspects of the abundant source of organic matter from algae and fungi and the pr eserved and transverse conditions of the organic matter in the anoxic environmen t.