Geology, mineralogy, and geochemistry of the Middle Proterozoic Wafangzi ferromanganese deposit, Liaoning Province, China

Wafangzi is the largest ferromanganese-ore deposit in north China, consisti ng of southeastern and northwestern regions containing 11 ore blocks (distr icts), Middle Proterozoic and Cambrian sedimentary rocks occur in the mine area, and Jurassic-Cretaceous intrusive igneous rocks subjected the ores to local metamorphism. The ore-bearing sequence is located in the upper part of the Middle Proterozoic Tieling Formation, which is composed of three ore beds and associated rocks. The ore-bearing sequence and the ores display l ithological and mineralogical changes from manganite ore and red and dark-b rown mudstone in the southeast to rhodochrosite ore and black shale and sil ty dolostone in the northwest. The industrial type of Wafangzi ores is ferr omanganese containing 16 to 33% Mn, 15 to 20% Fe, and Mn/Fe ratios of 1 to 2. Genetic types include primary sedimentary and diagenetic type which incl udes manganite and rhodochrosite ores; contact metamorphic type, which incl udes bixbyite-braunite ore, Mn silicate-Mn oxide ore, and pyrosmalite-rhodo chrosite ore; and secondary oxidized type, which includes pyrolusite ore an d psilomelane-vernadite-goethite ore. Mean contents of Co, Ni, and Zn in rh odochrosite ore are two to four times higher than those of the other ores. Strontium and Ba contents in manganite ores are 16 to 27 times higher than in the rhodochrosite ores, indicating that Sr and Ba probably occur in bari te in manganite ores. The mean total REE content of the various ore types i s 129 ppm, LREE/HREE is 6.5, La/Yb is 12.3, Eu/Sm is 0.18, and Sm/Nd is 0.2 2, comparable with those of continental and subcontinental crust. Chondrite -normalized REE patterns of the various ore types have small negative Eu an omalies, whereas the shale-normalized REE curves have weak negative Eu anom alies. Organic carbon contents in rhodochrosite ores range from 1.18 to 1.7 7%, whereas contents in the host rocks range from 0.58 to 0.88%. The H/C vs . O/C diagram shows that organic matter in ores experienced catagenesis and metagenesis. The mean delta(13)C(PDB) Of the rhodochrosite ore is -6.5 par ts per thousand (range -3.9 to -10.5 parts per thousand), and delta(18)O(PD B) is -7.24 parts per thousand (range -4.7 to -9.4 parts per thousand). The following conclusions are made: (1) The source of the ore components is ma inly terrigenous, derived from the weathering of continental crust; (2) bot h manganite and rhodochrosite ores are of primary sedimentary origin, and p recipitated in a shallow-marine basin; (3) the presence of bacterial textur es in the black matrix of spheroidal rhodochrosite ore and C isotope data i ndicate that microbial activity played a role in formation of the ores; (4) some evidence indicates that hydrothermal solutions contributed to the ore s, such as the presence of adularia in manganite ore and pyrosmalite in rho dochrosite ore. Hydrothermal fluids were generated by igneous intrusions. ( C) 1999 Elsevier Science B.V. All rights reserved.