GEOLOGICAL SETTING OF THE ROOMY MAGNETITE MINE, NEW JERSEY HIGHLANDS

The Roomy magnetite mine near Wanaque, NJ, is hosted by Mesoproterozoic rocks of 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 late 20th century. Geologists continue to debate when and how the iron ores were formed, with opinions separating into two basic groups. One holds that ores were introduced by hypogene fluids originating from deep-seated granitic magmas emplaced during the Ottawan orogeny and are <1100 million years old. The other group holds that ores were emplaced by exhalative hydrothermal fluids in a marine basin during extensional tectonics presumed to have operated somewhere between 1300 Ma and 1100 Ma. Our field work shows that the Roomy magnetite ore body and a series of small magnetite occurrences are hosted within a sequence of supracrustal rocks that have been metamorphosed to the granulite-facies. The sequence is dominated by amphibolites (metavolcanics) with minor garnet gneiss (metapelites). The iron ore is associated directly with distinctive monomineralic rocks such as pyroxenite and hornblendite. We find that these rocks are interlayered with massive quartz on both sides of the magnetite body; that these same rocks were deformed together with the banded quartz, as exhibited by boudinage, mineral lineations, and a weak foliation; and that granitic dikes cut these rocks. The dikes exhibit the same alaskitic composition of massive outcrops farther away from the mineralized zone, and are presumed apophyses of regional plutons. The mineralized zone occurs near the nose of a closed, overturned syncline and is conformable to the structure of the survey area. Furthermore, the ore is stratiform, and not spatially associated with granite plutons. These field relationships suggest that the iron ore predates regional deformation and the emplacement of Ottawan granite, and is most consistent with the seafloor exhalative model and formation of silica-rich iron formations that were subsequently metamorphosed and remobilized during the Ottawan orogeny.