The Franklin mining district

General Features

Geology

The geologic relations of the ore deposits were described by A. C. Spencer in the Franklin Furnace folio (no. 161) of the Geologic Atlas of the United States, issued in 1908. The rocks of the district comprise limestone, sedimentary gneiss, igneous gneiss, and intrusive lenses of pegmatite, all of pre-Cambrian age, overlain on the west by the Hardyston quartzite (Cambrian) and Kittatinny limestone (Cambrian and Lower Ordovician). The pre-Cambrian rocks are cut by a few camptonite dikes of post-Ordovician age, and on the east the Kittatinny limestone is faulted down against the Franklin limestone. Glacial deposits of several sorts mask the bedrock of considerable areas.

The structure and composition of the ore bodies and the enclosing rocks are fully described in the folio, with maps and sections showing the extent and form of the ore bodies as known in 1908. The geologic map of the mining district published in the folio is reproduced as plate 1 in this paper, with the addition of the mineral localities most frequently referred to in the description of the minerals.

Little has been published since 1908 regarding the geology, and there has been no detailed description of the extensive mining developments of recent years, but the papers by Ries and Bowen (223) and by Spurr and Lewis (234) contain some valuable new information.

The two main ore bodies are essentially alike in form and composition. Both are tabular masses, folded in warped synclines with hook-shaped outcrops (see plate 1), and both are wholly enclosed in Franklin limestone, their folds pitching northeastward in accord with the general structure of the pre-Cambrian rocks. Both consist chiefly of franklinite, willemite, zincite, and calcite, the first three commercially valuable ores of zinc, manganese, and iron, and the fourth the only important gangue mineral. Normally these minerals are intimately intermixed in granular form, the grains of the ore minerals being in general noticeably rounded and the interlocking calcite grains forming the matrix of the mass. Generally also one or more of the ore minerals is somewhat concentrated in layers roughly parallel to the walls of the deposit, giving a markedly banded appearance to the mass of the ore.


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