Stable isotope, chemical, and mineral compositions of the Middle Proterozoic Lijiaying Mn deposit, Shaanxi Province, China

The Lijiaying Mn deposit, located about 250 km southwest of Xian, is a high -quality ore characterized by low P and Fe contents and a mean Mn content o f about 23%. The ore deposit occurs in shallow-water marine sedimentary roc ks of probable Middle Proterozoic age. Carbonate minerals in the ore deposi t include kutnahorite, calcite, Mn calcite, and Mg calcite. Carbon (-0.4 to -4.0 parts per thousand) and oxygen (-3.7 to -12.9 parts per thousand) iso topes show that, with a few exceptions, those carbonate minerals are not pr istine low-temperature marine precipitates. All samples are depleted in rar e earth elements (REEs) relative to shale and have negative Eu and positive Ce anomalies on chondrite-normalized plots. The Fe/Mn ratios of representa tive ore samples range from about 0.034 to <0.008 and P/Mn from 0.0023 to < 0.001. Based on mineralogical data, the low ends of those ranges of ratios are probably close to ratios for the pure Mn minerals, Manganese contents h ave a strong positive correlation with Ce anomaly values and a moderate cor relation with total REE contents. Compositional data indicate that kutnahor ite is a metamorphic mineral and that most calcites formed as low-temperatu re marine carbonates that were subsequently metamorphosed. The braunite ore precursor mineral was probably a Mn oxyhydroxide, similar to those that fo rmed on the deep ocean-floor during the Cenozoic. Because the Lijiaying pre cursor mineral formed in a shallow-water marine environment, the atmospheri c oxygen content during the Middle Proterozoic may have been lower than it has been during the Cenozoic. (C) 1999 Elsevier Science B.V. All rights res erved.