Metasomatic-emplacement hypothesis

The author has reached the conclusion that the ore bodies were formed by metasomatic emplacement in the limestone in pre-Cambrian time, before its regional metamorphism. He believes also that they were deposited near the surface under oxidizing conditions, and that they probably consisted largely of the hydrous zinc silicate, calamine, together with hydrous oxides of iron and manganese and perhaps carbonates of zinc and manganese. The depositing solution is believed to have derived its metallic contents from the products of oxidation of the previously existent mass of mixed sulphides.

These oxidized minerals were probably deposited in more or less well layered masses whose form may have been determined by a previous folding of the limestone, which directed the flow of the depositing solution. The minerals at present constituting the deposits were, on this hypothesis, formed by dehydration and recrystallization during later profound and long-continued regional metamorphism of all the pre-Cambrian rocks of the region. Thus the minerals of the ore deposits acquired their characteristic texture and structure, so very like those of the enclosing limestone. The established sequence in the ages of the minerals, the oxides being developed after the silicate, seems entirely constant with the suggested mode of origin.

That the ore bodies originated through metasomatic emplacement was first suggested to the writer by the characters of two other deposits of zinc ores having a related origin. The first is a small-scale illustration and has the handy advantage of being situated in the Franklin area—the calamine deposit at Sterling Hill, described on page 23. There had been accumulated a rich ore deposit made up of parallel layers of calamine, smithsonite, zinciferous clay, and hydrous oxides of iron and manganese, and the layering followed the walls of the cavity in the limestone by whose solution a place for the new ore was formed. The source of this ore was the nearby franklinite-willemite mass, and the new ore was constituted in such a fashion that, had it and the surrounding limestone been subjected to a vigorous deep-seated, high-temperature metamorphism, there might well have resulted from its dehydration and recrystallization a new deposit closely analogous to the parent mass.

The second illustration is drawn from the published descriptions of the well-known deposits of zinc ore at Moresnet, Belgium, where there are vast sphalerite replacement deposits in Paleozoic dolomite. The sphalerite has undergone profound oxidation, and soluble products have been transferred to the nearby dolomite and have there been accumulated in synclinal folds that halted the circulation. Extensive deposits consisting principally of calamine, with some smithsonite and limonite, have thus been formed in bodies whose shape, although in general determined by the structure of the dolomite, is independent in detail of anything except the solvent action of the depositing solution. It seems not unlikely that the Belgian deposits of desulphurized zinc and iron ores are in all respects except in the absence of manganese a good presentment of the New Jersey zinc ores before the great metamorphism.

This hypothesis of metasomatic emplacement of the ore seems to account for the observed facts without essential conflict. The localization in the limestone is necessary, and the duplication is not remarkable. The form of the ore bodies may be quite independent of the sedimentary structure. The pitching synclines conform in attitude to the general structure of the gneiss, but how much of the folding may have been effected during metamorphism cannot be determined. However, that does not matter, as the original form of the deposits may have been anything from a simple layer to a mass whose form was not unlike that now seen. The identical texture of ore and country rock, the layering of the ore, the abrupt passage from ore to barren limestone, the dying out of the ore northward along the strike Mine Hill, the sporadic occurrence of disseminated ore within the fold in the limestone at Sterling Hill—are all satisfactorily accounted for by the suggested process.

The chemical composition of the ore is unusual, chiefly in the association of zinc with both iron and manganese, but metamorphosed metasomatic deposits of iron and manganese ore are not unknown, and the addition of the third metal merely indicates deposition under unusual original conditions. The mineral composition of the ore is conformable to the hypothesis—there is no comparable example for willemite, but franklinite is a spinel and is therefore to be expected as a product of the postulated mode of formation. Light is thrown on the presence of zincite in the recent study by Magnusson (232) of the Langban deposits, where manganosite (MnO) and periclase (MgO with a trace of ZnO) are shown to have been formed from the carbonates of manganese and magnesium, respectively, by high-temperature metamorphism. In such fashion zincite may have been formed by the breaking down of smithsonite in these deposits.

The modifications that have affected the ore body since their primary crystallization in their present form are relatively insignificant and have been sufficiently illustrated in the foregoing paragenetic study. The only clear evidence of the direct introduction of magmatic materials is found in the contact zones about the pegmatites, which were certainly far later than the ore deposits themselves.

This theory, originally outlined about 1915, was first published by the author in 1929. It was followed a month later by the paper of Tarr (258), in which the same mode of origin is postulated, but the effort is made to determine quite exactly the mineralogic nature of the metasomatic deposit before metamorphism. In Tarr’s paper attention is drawn to Rastall’s brief formulation of a similar theory (228), which had escaped the notice of the author.


	Website © by Herb Yeates 1997-2001.
	This page created: January 12, 2001 6:44 PM