MINERALOGICAL, GEOCHEMICAL, AND ISOTOPIC CHARACTERIZATION OF THE POWERS FARM TOURMALINE OCCURRENCE, ST. LAWRENCE CO., NEW YORK

The Powers Farm tourmaline occurrence, located approximately two km NW of Pierrepont, St. Lawrence Co., NY, has long been recognized as a source of large (several cm-wide), black, euhedral tourmaline crystals, although a clear picture does not exist regarding the timing and conditions under which this tourmaline occurrence formed.

The tourmaline (tour) occurs with grey quartz (qtz), white calcite (cal), pyrite (py), and mica; textural relationships suggest the cal + py is younger than the tour + qtz. Feldspar is not present. The entire assemblage appears to occur as discrete, localized bodies (veins? dikes?) within the surrounding host rocks; on-site excavations of this mineral assemblage include sub-parallel trenches (≤1m wide, ≤10 m long) and pits aligned with one another, possibly indicating structural control on mineral deposition/emplacement.

Transmitted and reflected light microscopy and electron microprobe analysis reveal an older generation of anhedral, optically unzoned tour with qtz, monazite, and biotite inclusions overgrown by optically zoned, euhedral tour + qtz. The optically unzoned tour contains Ca and Fe; optically zoned tour contains internal, alternating bands that are either relatively enriched or depleted in Ti. The qtz exhibits undulose extinction.

Fractures within and spaces between the euhedral tour + qtz crystals are filled by cal (± Mg, Fe) + py (± inclusions of chalcopyrite, galena) + qtz ± mica. The edges of qtz and tour crystals exhibit evidence of chemical disequilibrium with the cal + py + qtz ± mica assemblage, and may represent either a strong reaction between cal + py–rich fluids that filled open spaces and fractures within already-formed tour + qtz, or the complete replacement/alteration of a pre-existing mineral (K-feldspar?) into cal + py + qtz ± mica.

P-T conditions of tour + qtz formation is constrained by qtz fluid inclusion microthermometry data and application of the intersecting isochor method, which indicate trapping conditions from 438-500°C with pressures from 3.1 to 3.5 kbar (equivalent to 12 to 13.5 km depth assuming lithostatic pressure). Calculated O stable isotope signature of qtz source fluid composition (+11.0 to +12.0‰) is in agreement with either a primary magmatic water or metamorphic water source. Age determination of the py via Re-Os method is forthcoming.