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The Jixian-type boron–manganese (B–Mn) deposits include both chambersite and rhodochrosite deposits, from which Dongshuichang is economically the best chambersite deposit. Although the ore bodies of the region are relatively small, they contain a rare B–Mn ore that is scientifically significant. Chambersite (Mn3B7O13Cl) is a rare mineral, discovered as a mineral in 1957 and as an ore in 1971 at the Dongshuichang deposit near Jixian, Tianjin City. The deposit forms a monocline with a southwest dip and covers an area of 2 km2. In the mine area, the Gaoyuzhuang Formation, the upper member of the Middle Proterozoic Changcheng System, consists mainly of stromatolitic and muddy dolostone with B–Mn-bearing siliceous shale and dark-gray dolostone. Individual ore lenses range in length from 1 to 4 m with a mean thickness of 0.25 m. The ores can be divided into high-grade and low-grade, the former is composed of spindle and granular chambersite aggregates and the latter of spheroidal chambersite in dark-gray micritic dolostone. The center of spheroids is commonly composed of micritic and columnar chambersite, dolomite, clay minerals, and organic matter, around which columnar chambersite grew, forming a sunflower shape. The organic-carbon content in high-grade ores is 0.63%; in low-grade ores, 0.83%; and in overlying black siliceous shale, 3.75%. Total rare earth element (REE) contents of two chambersite ore samples are 29 and 35 ppm, whereas that of rhodochrosite ore is 22 ppm. The chondrite-normalized REE patterns of chambersite and rhodochrosite ores are similar, with small negative Ce and Eu anomalies and a small positive Gd anomaly. The δ 13 C PDB values of two rhodochrosite ore samples are −4.3‰ and −7.3‰, indicating that the carbon source of Mn-carbonate minerals was mainly seawater bicarbonate with a small contribution from the degradation of organic carbon. The source of the Dongshuichang ore components was mainly submarine volcanic eruptions and a small amount from the weathering of pre-Proterozoic boron deposits. The chambersite precipitated under conditions of high salinity, high B, Mn, and Mg concentrations, and weak alkalinity. Low-oxygen conditions might be favorable for precipitation of chambersite. |
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