INTERDIFFUSION OF ZN-MN-FE IN PYROXENE, SPINEL, AND GARNET DURING GRENVILLE GRANULITE FACIES METAMORPHISM AT STERLING HILL, NJ

The Zn-Mn-Fe orebody of the Passaic Pit at Sterling Hill, Ogdensburg, NJ, is a sedimentary exhalative type deposit that formed at or beneath the surface of the Mid-Proterozoic seafloor. The orebody and overlying sedimentary units were folded into an overturned isoclinal syncline reaching granulite facies peak metamorphic conditions during the Ottowan orogeny. Detailed 1:250 scale mapping revealed dm- to m-scale interbedded pyroxene-spinel gneisses and calcsilicates that are stratigraphically above the orebody. Twenty-four samples were selected to identify mineral phases in the overlying metasedimentary units and to investigate exchange of Zn, Mn, and Fe during peak metamorphic conditions. Mineral phases and microstructures were characterized through a combination of optical petrography, X-ray diffraction, Raman spectroscopy, energy-dispersive spectroscopy, and in-situ electron probe microanalysis (EPMA). Microstructures suggest high-temperature replacement of spessartine along oblate grain boundaries by an assemblage consisting of Zn-rich augite (diopside-hedenbergite-petedunnite solid solution), gahnite, and orthoclase. Augite grains from calcsilicate units show a low plateau of Zn-Mn-Fe contents in the grain core that gradually increase (with a parallel decrease in Mg) towards the boundary with calcite matrix over a distance of approximately 300 µm. Augite grains within gneissic units show similar increasing concentration gradients of Zn in augite approaching gahnite. Zinc contents of augite are highest in units directly adjacent to the ore body (up to 12 wt% ZnO) and decrease to ~1.5 wt% ZnO in calcsilicates that are 10 m from the orebody. These observations provide constraints on the conditions and relative timing of orebody emplacement and the mobilization of Zn-Mn-Fe ions during high temperature metamorphism.