GEOCHEMISTRY OF ORE MINERALS AND CALCITE FROM THE FRANKLIN AND STERLING HILL ZINC MINES

The Franklin and Sterling Hill zinc mines, northern New Jersey, were active for more than 100 years. Three hundred and fifty mineral species were found in these mines and 28 are unique to the area. Ninety of these mineral species are fluorescent, usually due to the presence of Mn in the crystal structure. The mineral deposits occur in the Mesoproterozoic Franklin marble and have been interpreted to have formed under oxidizing conditions on the seafloor. The metal-rich carbonate strata were subsequently metamorphosed to granulite facies during the 1.0 Ga Grenville orogeny. The ore-bearing horizons largely consist of the ore minerals franklinite (ZnFe2O4), willemite (Zn2SiO4), zincite (ZnO) and the gangue mineral calcite.

Franklinite, willemite, zincite, and calcite were separated from various ore samples using standard density and electromagnetic separation techniques. The mineral separates were analyzed for more than 30 elements using Instrumental Neutron Activation Analysis (INAA). The ore minerals contain significant amounts of Mn, ranging from several thousand ppm to over a hundred thousand ppm. Scandium, Cr, and Co mainly occur in franklinite and can be used to fingerprint the occurrence of franklinite. Arsenic occurs in all three ore minerals, but Sb only occurs in trace amounts in willemite and zincite. The As/Sb ratio can be used to differential between these minerals, as it is less than 0.5 for franklinite and ranges from 10 to 100 for willemite and zincite. Thus these elements can be used to fingerprint these ore minerals in the environment.

The calcite (gangue) co-existing with the ore minerals contains several thousand to several 10s of thousands ppm Zn. Arsenic, Ag, and Cd also occur at the 10s of ppm level. Rare earth elements (REE) are enriched 10 to 100 times relative to chondrites and show a gently sloping downward pattern toward the heavy REE. There are no Ce anomalies (as might be expected for oxidizing conditions) and slight positive Eu anomalies. Since the calcites are presumed to represent the host rock for the ore deposit, the calcite chemistry may help elucidate the ore-forming environment.