Ore deposits

Overview

The two Zn-Mn-Fe orebodies, at Franklin and Sterling Hill, are wholly enclosed within the Franklin Marble. They are home to nearly all of the area’s most interesting minerals. Both were large deposits; the Franklin orebody is estimated to have produced some 22 million tons of zinc ore, and that at Sterling Hill 11 million tons.

	Geologic map of the Franklin-Sterling Hill mining district, showing the outcrop and vertical sections of the orebodies. From Palache (1935). [click for larger image]

It is important to be aware that there are not only two Zn-Mn-Fe deposits at Franklin and Sterling Hill, but several smaller iron-oxide deposits as well. These iron deposits are typical of the rocks of the region, and only the two unusual Zn-Mn-Fe deposits are referred to here.

The Franklin orebody was almost one mile long, and the Sterling Hill orebody roughly half that. Both were plainly exposed at the surface, giving rise to their early exploration.

The unique nature of the ores gave early developers great difficulty in extracting metal, however. An extensive mining history is provided in Dunn (1995).

The length of both deposits follows the northeast-southwest trend of the rocks of the region. Each appears hook-shaped in plan section, and hook-shaped in vertical section.

Though each deposit is actually a single set of contiguous beds, now heavily folded, the east and west portions have long been referred to as east and west ‘veins’ or ‘limbs’. Both deposits plunge to the northeast. See the diagrams of the Franklin orebody and the Sterling Hill orebody for clarification.

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