GEOCHEMICAL VARIABILITY OF MARBLES UNDERLYING THE STERLING HILL MINE ORE: CONSTRAINTS ON THEIR ORIGINS

The Sterling Hill mine is a world-class non-sulfide zinc deposit situated in the New Jersey Highlands, within a narrow band of Proterozoic carbonates referred to as the Franklin Marble. It is generally accepted that the Fe-Zn-Mn ore is a sedimentary-exhalative (SEDEX) deposit that formed synchronously with the Franklin marble. Below the ore horizon, there are four mineralogicaly and texturally distinct carbonate layers, interbedded with calc-silicate units, of uncertain origin.

The rock units at the Sterling Hill Site have underwent extensive boudinage and pinch and swell of layers, with inter-boudin gaps reaching several meters. Although highly deformed, the sequence of marble units appear to have originally been stratiform. The ore has been folded into an overturned synform containing the carbonate and calc-silicate rock. The strata on site follow a sequence of: 1)Franklin Marble- white, crystalline, calcic marble, ~600 ppm Fe, ~900 ppm Mn, and negligible Zn; 2) Ore Marble-red-brown, up to 18% Mn, 0.2% Fe, and 330 ppm Zn. This layer also contains the highest levels of S and Pb, ~1% and ~600ppm respectively; 3)Orange Calcite- up to 1% Fe and Mn, 100 ppm Zn, and also abundant in REEs; 4)Crazy Calcite- 1-2% Fe, ~1% Mn, 0.1-0.2% Zn, abundant magnesian calcite (10-20% Mg), some of which is weathering out in relief; 5) Equigranular (Popcorn) Marble- coarsely crystalline, lower levels of ore metals, Si-bearing calcite (3-4% Si); 6) Black marble- ~0.2-0.4% Fe and Mn, ~100ppm Zn, elevated levels of S consistent with observed pyrite weathering.

While there is no direct correlation between distance from the ore and concentration of ore metals, it is likely these units deposited synchronously with a temporally variable exhalative source. The conformability of layers and lack of clear gradational trends away from the ore are more consistent with sedimentation, rather than metasomatism or magmatic processes. A better understanding of the history of the Sterling Hill deposit could be useful in defining exploration criteria for non-sulfide zinc deposits in the future.