Origin of Mesoproterozoic Carbonate-Hosted Ore Deposits, New Jersey Highlands

Adam Mansur ('05)

    Metavolcanic and metasedimentary rocks presently exposed in the New Jersey Highlands were subjected to granulite-facies metamorphism during the ca. 1350 to 1000 Ma Grenville orogeny. Primary textures and mineralogy of a pair of unusual marble-hosted Zn-Fe-Mn oxide-silicate deposits in this region (the Franklin and Sterling Hill mines) were obscured during deformation, and the formation of these deposits has been a matter of considerable debate. Recently, a genetic link between the zinc deposits and carbonate-hosted iron deposits in the Highlands has been suggested (Johnson and Skinner, 2003). This study reports the results of a stable-isotope investigation of carbonate samples from eighteen iron deposits, the Franklin zinc deposits, and the barren Limecrest Quarry in the New Jersey Highlands.

The Franklin Marble, host of the zinc and iron deposits, was deposited in a shallow to deep marine, back arc basin setting. Carbon and oxygen isotope ratios from host carbonates at Eastern and Western Highlands iron deposits can be described by fluid-rock interaction between igneous fluids (d¹³C = -8 to -5 ‰, d¹⁸O = 7 to 12 ‰) and the Franklin Marble (d¹³C = -1 to 1 ‰, d¹⁸O = 20 to 25 ‰). This suggests an igneous source for the ore-depositing fluids. Limited sulfur isotope data is ambiguous, but exhibits a range similar to that of modern hydrothermal environments. Ore deposits probably formed initially in seafloor or shallowly buried carbonate sediments. Hydrothermal activity may have been centered int he northwestern Highlands, where the zinc deposits, the majority of the iron deposites, and a large amount of supracrustal amphibolites are present, and stable-isotope ratios are most mantle-like. Distal deposits in the southwest and Eastern Highlands tend to have isotope signatures close to that of the barren Franklin Marble. Zinc deposits may reflect deposition by a slightly cooler fluid derived from the same hydrothermal system. Zinc ore may have been deposited initially as willemite plus zinc oxides and hydroxides.