Composition and origin of Early Cambrian Tiantaishan phosphorite-Mn carbonate ores, Shaanxi Province, China

The Tiantaishan phosphorite-Mn carbonate ores occur in the Early Cambrian T ananpo Formation in complexly folded and faulted rocks located in southern Shaanxi Province. About 65 x 10(6) tonnes of 17% P2O5 ore reserves exist an d Mn-ore reserves are about 8.3 x 10(6) tonnes of +18% Mn. The stratigraphi c sequence in ascending order consists of black phyllite, black to gray pho sphorite ore, black phyllite, rhodochrostone ore, Mn mixed-carbonates, and dolostone. Data are presented from microprobe mineral chemistry, whole-rock chemistry, stable isotopes of carbonates, X-ray mineralogy, petrographic a nd SEM observations, and statistical analysis of chemical data. The dominan t ore-forming minerals are hydroxy- and carbonate fluorapatite and Ca rhodo chrosite, with Mg kutnahorite and dolomite comprising the Mn mixed-carbonat e section. Pyrite occurs in all rock types and alabandite (MnS) occurs thro ughout the rhodochrostone section. The mean P2O5 content of phosphorite is 31% and argillaceous phosphorite is 16%, while the mean MnO content of rhod ochrostone ore is 37%. Phosphorite ores are massive, spheroidal, laminated, and banded, while rhodochrostone ores have oolitic, spheroidal, and granul ar fabrics. The most distinguishing characteristics of the ores are high to tal organic carbon (TOC) contents (mean 8.4%) in the phosphorite and high P 2O5 contents (mean 2.7%) in the rhodochrostone ore. The atypically high TOC contents in the Tiantaishan phosphorite probably result from very strong p roductivity leading to high sedimentation rates accompanied by weak reworki ng of sediments; poor utilization of the organic matter by bacteria; and/or partial replacement of bacterial or algal mats by the apatite. The deposit ional setting of the ores was the margin of an epicontinental seaway create d as a direct consequence of global processes that included break-up of a s upercontinent, formation of narrow seaways, creation of extensive continent al shelves, overturn of stagnant, metal-rich deep-ocean waters, and marine transgression. Water depth increased from deposition of the black phyllite sequence through deposition of the Mn mixed-carbonate sequence, then shallo wed again during deposition of the overlying dolostone sequence. Bottom wat ers were mostly dysoxic to suboxic, but fluctuated from oxic to anoxic. Pro ductivity was high during deposition of the black phyllite sequence, increa sed during precipitation of phosphorite, and then decreased to moderate lev els during precipitation of rhodochrostone ores. Biosilica contributions oc cur in each lithology, but are greatest in rhodochrostone. Changes in sedim entation were determined by changes in water depth, productivity, upwelling , sea-level change, and ventilation of the depositional basin. The source o f the phosphorus was organic matter produced in great quantities during dep osition of the black phyllite and phosphorite sequences in a zone of coasta l upwelling. Organic matter accumulation was rapid. Globally, Mn was suppli ed by overturn of stagnant, metal-rich deep-ocean waters, which were redist ributed to areas of coastal upwelling and seaways; that process may have be en initiated by latest Proterozoic glaciations which would have promoted de nsity stratification and accumulation and storage of metals. Regionally, Mn was supplied by terrigenous input into the shallow seaway and hydrothermal input into the deeper water parts of that seaway. Locally, Mn sources incl uded leaching and transport of metals from the sediment column. Manganese w as stored locally in low-oxygen (not anoxic) seawater prior to Mn-ore forma tion. The source of the carbon in the Mn carbonates and dolostones was predominan tly seawater bicarbonate and secondarily CO2 derived from the oxidation of organic matter in the bacterially mediated diagenetic zone of sulfate reduc tion. (C) 1999 Elsevier Science B.V. All rights reserved.