Abstract
Wafangzi is the largest ferromanganese-ore deposit in north China, consisting of southeastern and northwestern regions containing 11 ore blocks (districts). 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 lithological and mineralogical changes from manganite ore and red and dark-brown mudstone in the southeast to rhodochrosite ore and black shale and silty dolostone in the northwest. The industrial type of Wafangzi ores is ferromanganese 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 includes manganite and rhodochrosite ores; contact metamorphic type, which includes bixbyite–braunite ore, Mn silicate–Mn oxide ore, and pyrosmalite–rhodochrosite ore; and secondary oxidized type, which includes pyrolusite ore and psilomelane–vernadite–goethite ore. Mean contents of Co, Ni, and Zn in rhodochrosite 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 barite in manganite ores. The mean total REE content of the various ore types is 129 ppm, LREE/HREE is 6.5, La/Yb is 12.3, Eu/Sm is 0.18, and Sm/Nd is 0.22, comparable with those of continental and subcontinental crust. Chondrite-normalized REE patterns of the various ore types have small negative Eu anomalies, whereas the shale-normalized REE curves have weak negative Eu anomalies. Organic carbon contents in rhodochrosite ores range from 1.18 to 1.77%, 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 δ 13 C PDB of the rhodochrosite ore is −6.5‰ (range −3.9 to −10.5‰), and δ 18 O PDB is −7.2‰ (range −4.7 to −9.4‰). The following conclusions are made: (1) The source of the ore components is mainly terrigenous, derived from the weathering of continental crust; (2) both manganite and rhodochrosite ores are of primary sedimentary origin, and precipitated in a shallow-marine basin; (3) the presence of bacterial textures in the black matrix of spheroidal rhodochrosite ore and C isotope data indicate that microbial activity played a role in formation of the ores; (4) some evidence indicates that hydrothermal solutions contributed to the ores, such as the presence of adularia in manganite ore and pyrosmalite in rhodochrosite ore. Hydrothermal fluids were generated by igneous intrusions.
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