ZN-RICH MINERALS OF THE STERLING HILL AND FRANKLIN ORE DEPOSITS, NEW JERSEY

The Zn-Mn-Fe ore deposits of Sterling Hill and Franklin, New Jersey, are mineralogically and geochemically unique localities. These contain more than 360 mineral species, and many are unique to the area or have uncommon properties. For example, micas in this area are known to have abnormally high zinc contents with some containing upwards of 20 wt.% ZnO. We collected gneiss, high-grade ore, and marble samples from the deposits and analyzed many Zn-bearing minerals in the samples including mica, gahnite, amphibole, pyroxene, hardystonite, franklinite, willemite, garnet, and titanite in order to determine if links in Zn contents between these coexisting mineral phases could provide insight into processes forming such Zn-rich micas. The largely phlogopitic micas are trioctahedral with 1-87 % phlogopite, 6-49 % annite, and 0-59 % end-member siderophyllite with an overall trend towards phlogopite. Analyses show mineral grains are generally unzoned, and that different grains of the same phase are similar and homogeneous in composition within the same rock sample. BaO contents in the micas range from 0.4 to 8.8%. The wt. % F in micas ranges from 0 to 5.1, and Cl ranges from 0.01 to 0.8%. When comparing the micas with various combinations of the other Zn-bearing phases, some positive correlations in ZnO content between micas and these minerals are observed, but most other Zn-bearing phases in this study showed no significant variability in ZnO concentration while the micas range from 0.2 to 20.7% ZnO. Atomic proportions of cations filling the octahedral site of these micas show a strong positive correlation between Zn and Mn, a negative correlation between Zn and Mg, and an inverted parabolic trend between Zn and Fe²⁺. According to Goldschmidt’s rules, Zn should readily exchange with Mg in the octahedral site, but instead there is strong replacement trend with Mn. These trends are due to the complex chemistry of the micas, resulting in complex Tschermak’s-style substitutions. The Sterling Hill and Franklin micas fit a Tschermak substitution mechanism involving Zn: ^VI(Mg+Fe²⁺+Mn+Zn)²⁺+2^IVSi⁴⁺← →^VITi⁴⁺+2^IV(Al³⁺,Fe³⁺) with an R² of 0.866.This suggests complex substitution mechanism is a partial explanation for the significant variability of wt. % ZnO in the micas, as Zn exchange is a function of Mg, Mn, Fe, and other cations in these micas.