REGIONAL TRENDS IN THE U.S. MID-ATLANTIC PROTEROZOIC IRON OXIDE BELT

Proterozoic rocks of the Reading Prong (PA), New Jersey Highlands (NJ), and Hudson Highlands (NY) host Fe-oxide deposits of two ages: a) Mesoproterozoic magnetite deposits that experienced high grade metamorphism, and b) weakly-metamorphosed Neoproterozoic hematite and magnetite deposits.

Mesoproterozoic deposits include the Dover and Edison (NJ) and Rittenhouse Gap districts (PA), characterized by mt-qtz-feldsp-apatite associated with Na- and/or K-alteration. REE ± U-Th mineralization (Scrub Oaks, Cranberry Lake, NJ) occurs as coarse-grained magnetite-qtz-feldsp ± synchysite ± bastnaesite ± uranothorite "pegmatites." Na-altered rocks contain albite-oligoclase > CPX ± riebeckite, whereas K-altered rocks contain qtz-K-feldsp ± bio. Pyrite ± pyrrhotite ± chalcopyrite occurrences are restricted to the Mesoproterozoic deposits.

Neoproterozoic deposits include those at Fulmer, Titman, Andover, Cedar Hill, and Simpson (NJ) and Kaufmann&Spang and Mt. Pleasure (PA), characterized by hematite > magnetite, gangue qtz-K-feldsp ± sericite ± calcite ± tourmaline, concordance of ore and rift sequence host rocks, and K-alteration.

Regionally, Proterozoic rocks and ores occur in five NE-striking, fault-bounded belts from NW to SE: 1. supracrustal-dominant belt (Bemco, Franklin-Sterling Hill deposits, and K-altered Neoproterozoic deposits); 2. hedenbergite A-type granite belt (hosts K-altered Edison deposit); 3. poorly-mineralized belt with leucocratic albite and qtz-olig rich rocks (Na-alteration); 4. hastingsite A-type granite belt (hosts Dover deposits); and 5. poorly-mineralized belt similar to (3). Belt 1 extends continuously from the Hudson Highlands into Pennsylvania, whereas the other belts are largely restricted to the New Jersey Highlands. Dominance of supracrustal and Neoproterozoic rocks suggests belt 1 constitutes the shallowest structural level exposed. Belts dominated by Na-alteration styles may represent deeper structural levels.