Characteristics and origin of the Middle Proterozoic Dongshuichang chambersite deposit, Jixian, Tianjin, China

The Jixian-type boron-manganese (B-Mn) deposits include both chambersite an d 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. Cha mbersite (Mn3B7O13Cl) is a rare mineral, discovered as a mineral in 1957 an d 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 km(2). In the mine area, the Gaoyuzhuang Formation, the upper member of th e Middle Proterozoic Changcheng System, consists mainly of stromatolitic an d 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 form er is composed of spindle and granular chambersite aggregates and the latte r of spheroidal chambersite in dark-gray micritic dolostone. The center of spheroids is commonly composed of micritic and columnar chambersite, dolomi te, clay minerals, and organic matter, around which columnar chambersite gr ew, forming a sunflower shape. The organic-carbon content in high-grade ore s is 0.63%; in low-grade ores, 0.83%; and in overlying black siliceous shal e, 3.75%. Total rare earth element (REE) contents of two chambersite ore sa mples are 29 and 35 ppm, whereas that of rhodochrosite ore is 22 ppm. The c hondrite-normalized REE patterns of chambersite and rhodochrosite ores are similar, with small negative Ce and Eu anomalies and a small positive Gd an omaly. The delta(13)C(PDB) values of two rhodochrosite ore samples are -4.3 parts per thousand and -7.3 parts per thousand, indicating that the carbon source of Mn-carbonate minerals was mainly seawater bicarbonate with a sma ll contribution from the degradation of organic carbon. The source of the D ongshuichang ore components was mainly submarine volcanic eruptions and a s mall amount from the weathering of pre-Proterozoic boron deposits. The cham bersite precipitated under conditions of high salinity, high B, Mn, and Mg concentrations, and weak alkalinity. Low-oxygen conditions might be favorab le for precipitation of chambersite. (C) 1999 Elsevier Science B.V. All rig hts reserved.