ZINC ISOTOPE CONSTRAINTS ON THE FORMATION OF SEDIMENTARY EXHALATIVE (SEDEX) ORE DEPOSITS: NEW EVIDENCE FROM THE FRANKLIN, NJ MINING DISTRICT

We report δ⁶⁶Zn for the high temperature metamorphic zinc oxide and silicate minerals franklinite (Fr) (Zn²⁺Fe³⁺₂O₄), zincite (Zc) (ZnO) and willemite (Wlm) (Zn₂SiO₄) from the Franklin, NJ, historic mining district. With reference to the JMC-Lyon standard, δ⁶⁶Zn franklinite ranges from -0.10 to 0.48‰ with an average of 0.20 ±0.17‰ (n=22). δ⁶⁶Zn willemite ranges from 0.23 to 0.48‰ with an average of 0.37 ±0.09‰ (n=7). δ⁶⁶Zn zincite ranges from 0.29 to .060‰ with an average of 0.47±0.12‰ (n=9). These data suggest that the analyzed phases fractionate heavy zinc in the order Fr<Wlm<Zc. Taken as a group, these minerals have an average δ⁶⁶Zn of 0.30 ±0.19‰. This is 0.16‰ heavier than an estimated global mean δ⁶⁶Zn for sphalerite (ZnS) of 0.14 ±0.16‰ for seafloor zinc deposits. Our results are consistent with fractionation factors that predict that Zn oxides and silicates (protoliths of these ores) should be isotopically heavier than sphalerite when precipitated from fluids of the same temperature with similar zinc isotope compositions. Our samples from Sterling Hill are taken from two short transects across the orebody. Calcite (Cal) (CaCO₃) from the same samples has δ¹³C from -0.54 to 1.46 with an average of 0.79±0.51‰ (VPDB), while δ¹⁸O ranges from 9.72 to 15.12 with an average of 12.42±1.35‰ (VSMOW; n=30). Results for these two isotope systems are consistent with earlier studies. δ¹³C decreases smoothly with distance towards the west along both transects; δ¹⁸O in contrast stays very close to its mean on the longer, southern transect but increases with distance to the west along the shorter, northern transect. There is no apparent covariation between δ⁶⁶Zn and either of the other isotopic ratios measured.