LOW OXYGEN ISOTOPE RATIOS IN GARNET FROM THE LEWIS DEPOSIT, ADIRONDACK HIGHLANDS

In the Mesoproterozoic Northeastern Adirondack Highlands, a belt of skarns was formed along the contact between the Marcy Anorthosite Massif of the High Peaks region and the existing carbonate country rock. These skarns have a variety of mineral assemblages with wollastonite-rich lithologies as well as lenses comprised of garnetite. Studies of low oxygen isotope ratios in the skarn belt show that meteoric water had an important role in the formation of these rocks, which is relatively unusual for anhydrous skarns. This study focuses on the Lewis deposit, with the goal of comparing its hydrothermal history to the Willsboro and Oak Hill deposits (Valley and O’Neil, Nature 1981; Clechenko and Valley, JMG 2003).

Samples were collected from the Lewis Mine, located on the Northeastern border of the Adirondack Park. One research approach is to look for zoning of oxygen isotope ratios in cation-zoned garnet crystals in the garnetite rock. Initial results show andradite-rich garnetite compositions And[70-80]Gr[20-30] with δ¹⁸O = -2.0 to -1.8‰ SMOW. Individual garnet crystals from wollastonite-rich units in ~100’ of core from the Lewis deposit have a much wider range of compositions from And[83]Gr[17] to And[20]Gr[80]. Oxygen isotope ratios from these samples range from δ¹⁸O = 0.4‰ to -2.1‰ (n=20), which correlates with garnet chemistry (andradite-rich garnets being the most negative). These data are consistent with meteoric water infiltration during skarn formation and shallow anorthosite emplacement before granulite facies metamorphism in the deep crust. Future work will focus on comparing oxygen isotope ratios and cation zoning in garnet between deposits in the skarn belt, and focused sampling to see if an early magmatic fluid history has been preserved at the Lewis deposit.