GSA Annual Meeting in Denver, Colorado, USA - 2016

Paper No. 334-8
Presentation Time: 9:00 AM-6:30 PM


BAHRUTH, M.B., American Museum of Natural History, Department of Earth and Planetary Sciences, Central Park West & 79th St, New York, NY 10024; Wichita State University, Department of Geology, 1845 Fairmount St., Wichita, KS 67260, WEBSTER, James D., American Museum of Natural History, Department of Earth and Planetary Sciences, Central Park West at 79th St., New York, NY 10024 and VERBEEK, Earl R., Franklin Mineral Museum, 32 Evans St., Franklin, NJ 07416,

The Sterling Hill and Franklin Zn-Fe-Mn deposits near Ogdensburg, New Jersey, are home to more than 360 known mineral species, many of them rare or unknown elsewhere. The deposits are hosted by the Franklin Marble and have been subjected to high-grade (granulite facies) regional metamorphism. Limited, prior research shows that many of the rock-forming minerals in and near the orebodies, particularly species of the pyroxene, amphibole, and mica groups, contain uncommonly high amounts of zinc. The Sterling Hill and Franklin micas, for example, have been shown to contain above-average Zn contents (Frondel and Einaudi, 1968), and Franklin is the type locality of hendricksite, a rare trioctahedral mica containing more than 20 wt.% ZnO. Even so, although this is one of the most mineralogically diverse areas in the world, many of the minerals have yet to be studied using modern analytical techniques.

We examined a variety of micas from the Sterling Hill and Franklin deposits with electron microprobe to determine their bulk compositions and, in particular, their zinc contents. Our preliminary results show that biotite and other micas contain 3-14 wt.% ZnO, and that the ZnO concentrations correlate positively with MnO (ranging from 0.01-9 wt.%). Zinc in mica also correlates negatively with MgO, SiO2, and F (ranging from 0.03 to more than 2.5 wt%). BaO does not correlate with ZnO in mica, but the BaO contents of many micas are elevated to concentrations of 1 to nearly 9 wt%. Barium-rich micas of Franklin have been analyzed previously by Tracy (1991). The TiO2 concentrations of mica show a distinct negative correlation with the (Mg/Mg+Fe) ratios.

We will interpret the relative roles of crystal-chemical constraints and that of metamorphic fluids on the incorporation of the elevated abundances of Zn, Ba, and Mn in these micas.

Frondel, C., Einaudi, M. (1968) Zinc-rich micas from Sterling Hill, NJ. Amer. Mineral. 53, p. 1752.

Tracy, R.J. (1991) Ba-rich micas from the Franklin Marble, Lime Crest and Sterling Hill, New Jersey. Amer. Mineral. 76, 1683-1693.

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