MINERALOGY AND GEOCHEMISTRY OF THE ROSE ROAD WOLLASTONITE DEPOSIT, PITCAIRN, NY

Wollastonite-bearing skarns, such as those in the Adirondack lowlands, are formed when calcic rocks undergo contact metamorphism in the presence of Si-rich fluids. Museum quality specimens of diopside and titanite, as well as fluorescent specimens of scapolite, fluorapatite, and wollastonite, are well known from the Rose Road locality in St. Lawrence County, NY. Although Rose Road has been known to geoscientists since the late 19th century, the site has seen rejuvenated interest due to the discovery of a new occurrence of exceptionally well-mineralized rock, dubbed the purple diopside mound (PDM), which opened to collectors in 2013. This study compares mineralogical and geochemical features of the PDM with assemblages more typical of the overall site (wollastonite-bearing exposure, WBE). Samples were taken approximately 90 m apart at this c. 0.16 km² locality. The goal is to attempt to understand the relative influences of protolith variability as compared to fluid composition and evolution.

The WBE is principally composed of white perthitic feldspars, green diopside, and yellow calcite with less abundant titanite and fluorapatite. A pegmatitic zone is present at the WBE, with > 6 cm crystals of feldspar, wollastonite, diopside, and titanite. A calcite vein hosting partial pseudomorphs of diopside after wollastonite, among other minerals, is present in this part of the site. Although more uniform in grain size, the PDM is more heterogeneous in bulk mineralogy than the WBE, ranging from > 90% diopside to > 90% feldspar or calcite within cm. Titanite, fluorapatite, scapolite, and phlogopite occur in variable abundance.

Thus far apatite, a mineral common to both of the examined sites at Rose Road, has been analyzed by solution ICP-MS. Samples of fluorapatite from WBE show higher REE concentrations than in PDM, and slight but measurable distinctions in REE distribution exist. Total lanthanide contents in WBE samples (minus Tb) average c. 2500 ppm, whereas PDM apatite samples average c. 300. Ratios of the REE, which quantify differences in pattern shape, are similarly discernable (La/Yb and La/Sm in WBE average 35 and 6.3 versus 23 and 3.9, respectively, at PDM). Other geochemical distinctions between apatite in the two sites include U/Th (0.7 v. 1.3, WBE v. PDM) and As (c. 70 v. c. 9 ppm, WBE v. PDM).