Abstract

Six major and numerous smaller Precambrian iron deposits of magmatic origin occur in the volcanic rocks of the Southeast Missouri Iron Metallogenic Province in an area approximately 70 miles long and 40 miles wide.The Precambrian igneous complex is composed of mainly high silica, with some intermediate, volcanic rocks intruded by granitic rocks and some diorites and gabbros. Biotite schists occur locally. Some of the volcanic and intrusive rocks are characterized by an unusual abundance of alkali metals. Local enrichments in fluorite, calcite, and iron oxides are present.The shapes of the iron deposits range from massive tabular to irregular. Emplacement characteristics vary from cross-cutting masses and breccia cementations to intensive replacements of moderately to steeply dipping volcanic host rocks.The major iron minerals are magnetite and hematite; smaller amounts of martitic hematite, chalcopyrite, and bornite occur. Even less abundant are spinels of the magnetite series: Magnesio-ferrite, Mg-jacobsite, Zn-Fe spinel. Apatite, garnet, pyroxenes, actinolite, tremolite, crocidolite, epidote, biotite, quartz, pyrite, sericite, chlorite, fluorite, calcite, dolomite and barite are more or less common gangue minerals. Relatively rare geikielite, Ca and Mg hydrosilicates and Mg-chamosite are probably present. Alteration minerals of the iron oxides and silicates also occur.Eighty-two purified magnetite, hematite and bulk ore samples were analyzed for Ti, V, Cr, Mn, Co, Ni, Cu, Zn, Mo, Mg and Al by emission spectrography in the ranges of 1-10,000 ppm. Other details were obtained from polished sections, thin sections and X-ray analyses of the minerals.Standard deviation and variance tests showed several statistically significant differences in the trace element population between individual iron deposits. These may partly be explained by mineralogical, geological and genetic differences. A distinct pattern of higher trace element content near the center of the iron deposits decreasing toward the margins occurs in two well sampled bodies. High and low electronegativity pairs such as V/Zn, V/Cu, and Mg/Co show the same pattern and strongly suggest a temperature control of trace element concentration. The range of trace element content, the Ti/V ratio and the V content of purified magnetites resemble those of magmatic injection iron deposits of the world. The geochemical data and the geologic character of the ore bodies suggest that possible ore magma injection, contact metasomatism and hydrothermal replacement were the most important ore forming processes in this Metallogenic Province.

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