The Sterling Hill ore body is mineralogically similar to its neighbor to the north in Franklin, N. J. It is an intricately folded, steeply plunging body in Precambrian graphitic, sparsely silicated, coarsely crystalline white marble. The marble is graphite-free within 5 feet of ore. The ore is wrapped around a central core of graphitic marble and an annular gneissic cylinder of mica, feldspar, hornblende, pyroxene, and garnet composition.
Ore minerals are willemite, franklinite, and zincite. Franklinite, tephroite, pyroxene, and biotite compose the leanly mineralized areas. The mineral distribution is zoned. Black willemite with magnetic franklinite forms a band of varying thickness that follows closely the convolutions of the upper surface of the brown willemite-franklinite body. Ore textures are identical with those of the areas of lean mineralization, and the transition is not sharply defined. A thin band of rhodonite is found at this boundary between pyroxenes and willemite.
Paragenetically, it appears that of the ore minerals, willemite formed first in the sequence, with tephroite, zincite, and franklinite following in undetermined order; tephroite and zincite may have been emplaced at the same time. There is abundant evidence of later generations of willemite, franklinite and zincite. Primary zincite has been observed only in the presence of tephroite. There is evidence that tephroite has replaced willemite. Manganese solutions possibly replaced the zinc in willemite to form tephroite and may have resulted in the formation of zincite.
The rock core of the ore body is altered to zinciferous clayey mud, locally containing hemimorphite. This alteration extends from the surface to a depth of 680 feet.
The variation in willemite color is due to color differences in <10 micron franklinite inclusions. Red franklinite is nonmagnetic with a unit cell dimension of nearly 8.51 Å. Black franklinite is magnetic with a unit cell dimension of about 8.42 Å. Macroscopic franklinite cell dimensions are intermediate. Origin of franklinite inclusions is attributed to willemite serpentinization, similar to that commonly observed in magnetite formation by serpentinization of olivine.