A PROPOSED BANDED IRON FORMATION PROTOLITH FOR QUARTZ-PYROXENE GNEISS AT THE ROOMY MAGNETITE MINE, WANAQUE, NEW JERSEY

The Roomy magnetite mine is hosted by Proterozoic rocks of back-arc basin affinity in the New Jersey Highlands, a portion of the Reading Prong. It is one of over 400 iron mines that were worked in the region from the late 17th through the early 20th century. Detailed mapping around the Roomy mine has delineated a sequence of sedimentary and volcanic strata that have been metamorphosed to granulite facies and folded into a shallowly north-plunging, overturned synform. Facing direction in the sequence is uncertain; from the structural lowest to highest units, the sequence of metasedimentary rocks is a garnet-biotite-quartz-plagioclase gneiss, a phlogopite-dolomite marble, and a pyroxene-scapolite gneiss. The sequence is overlain by a distinctive feldspar-free gneiss that is composed of strongly segregated layers of massive clinopyroxene, massive quartz, and to a lesser extent, pyroxene-scapolite. Layers in this gneiss are generally 3 to 10 cm. A plagioclase-hornblende-clinopyroxene gneiss of basaltic protolith tops the sequence, and the magnetite-bearing ore horizon occurs at the contact between the metabasite and the quartz-pyroxene gneiss. Remarkably similar quartz-pyroxene gneiss occurs in close proximity to the orebody at the Miles Standish magnetite mine near Warwick, New York, some 20 miles north of the study area. The stratiform nature of the quartz-pyroxene gneiss suggests a supracrustal origin, and the plagioclase-hornblende-clinopyroxene gneiss is evidence for a spatial and temporal association with volcanic activity. We propose an Algoma-type banded iron formation protolith for this compositionally unusual unit. The composition of the quartz-pyroxene gneiss can be modeled by the SiO2-CaO-FeO system. The mineral assemblage of the protolith is uncertain but based on mineral assemblages that commonly comprise Algoma-type iron formations of low metamorphic grade, the protolith likely contained chert-(greenalite or minnesotaite)-(calcite or ankerite). The presence of a carbonate mineral in the protolith assemblage is consistent with the presence of a dolomitic marble below the quartz-pyroxene gneiss. The pyroxene-scapolite interlayers require an aluminous component which could have been supplied by episodic input of terriginous sediment or volcanic ash.