GENETICALLY-RELATED OXIDE+CARBONATE-FACIES ZN-FE-MN DEPOSITS AND OXIDE-FACIES FE DEPOSITS IN A PROTEROZOIC BASIN SEQUENCE EXPOSED IN THE READING PRONG, NEW JERSEY AND HUDSON HIGHLANDS, NEW YORK

The Reading Prong (RP)-Hudson Highlands (HH) terrane in northern NJ and southern NY contains a metamorphosed sedimentary sequence that accumulated in a late Proterozoic epicratonic sea. Two stratiform ore types have been discovered in this sequence, the Zn-Fe-Mn ores of the Sterling Hill and Franklin deposits, and the Fe ores of the Dover, Sterling Lake-Ringwood, and Franklin-Hamburg districts. Johnson et al. (Econ Geol 85, 1133, 1990) have argued that the Zn-Fe-Mn deposits are oxide- and carbonate-facies metalliferous sediments that formed at or near the sediment-seawater interface, and that the mineral assemblages and structural complexities observed today reflect subsequent metamorphism and deformation during the Grenville orogeny. This hypothesis implies that the RP-HH basin contains an unusual sedimentary-exhalative hydrothermal product in which Zn, Fe, and Mn reside in carbonate, oxide, and silicate minerals, and which has only trace S and Pb. Some of the Fe deposits are associated with metapelites which suggests that these are metamorphosed sedimentary/volcanic Fe formations. A genetic link to the Zn-Fe-Mn deposits is suggested by the occurrence of a thin Zn-free Fe formation immediately beneath the Franklin Zn deposit. The Fe formations may reflect accumulation of dissolved Fe in anoxic bottom waters and subsequent precipitation during a basin ventilation event. Massive metapelite-hosted graphite deposits that occur in the RP are consistent with this model because they provide evidence for the enhanced preservation of organic matter that anoxia would afford. The Zn deposits in the RP-HH are unusual in that (1) metal deposition took place in an H2S-poor environment, and (2) Zn was strongly enriched over Pb and Cu. The first aspect may reflect a depositional environment where redox was controlled by oxygenated basin waters. The second may reflect source rock compositions or chemical fractionation during metal mobilization or deposition. There are hints in the literature that this same mineralization type may exist in other basin sequences where it could present an environmentally-friendly exploration target due to the low acid-generating capacity and low Pb pollution potential of mine wastes.