MINERALOGICAL DIVERSITY OF ABYSSAL PEGMATITES OF THE SOUTHERN ADIRONDACKS, NEW YORK

Over a dozen granitic pegmatites are exposed in the southern Adirondack Highlands; they range in size from small tabular bodies < 1m wide to large irregular bodies ~100 m in diameter. All of the bodies are hosted by upper amphibolite to lower granulite facies biotite hornblende gneisses; none are spatially associated with granitic plutons. The three largest bodies, the Tyrell quarry in the town of Mayfield, the Big Pit in the town of Batchellerville, and the Overlook quarry in the town of Overlook, were all mined for feldspar in the early 20th century. Typical of abyssal pegmatites, all three have relatively simple mineral assemblages. Two of the three are crudely zoned, with quartz cores surrounded by quartz-albite-microcline-biotite-bearing rocks (Tan, 1966).

The three differ markedly in their accessory mineral assemblages, with the Batchellerville pegmatite containing many aluminous minerals (muscovite, hercynite, corundum, chrysoberyl, sillimanite, dumortierite, almandine), and the Mayfield pegmatite containing many phosphate minerals (fluorapatite, autunite, torbernite, chernikovite, monazite, xenotime). While all three contain tourmaline, only the Mayfield and the Batchellerville pegmatites contain beryl. The only other beryllium or boron bearing minerals identified to date are dumortierite and phenakite, both of which are found only in the Batchellerville pegmatite.

Radiometric ages on zircons from the Batchellerville (1098 +/- 28 Ma) and Mayfield (1009 +/- 22 Ma) pegmatites bracket the range of ages attributed to the Ottawan orogeny in the Adirondack Highlands (Lupulescu et al., 2012). However, based upon the lack of deformational features in any of the southern Adirondack pegmatites, all are thought to have been emplaced after the peak of Ottawan metamorphism (< 1040 Ma).

Typical of most abyssal pegmatites, the southern Adirondack pegmatites are thought to have formed through anatectic melting of the surrounding gneisses. The highly variable accessory mineral assemblages seen in these pegmatites most likely reflect local variations in the chemical and mineralogical composition of the host lithologies.