GRUGAN, Chris ('98) (Advisor: James McLelland)
Rocks of the Lyon Mountain suite are well exposed in the open pit mines of the Lyon Mountain Mine in Lyon Mountain, NY. Using field, petrogaphic and chemical analyses, these rocks were found to be identical to rocks of the Lyon Mountain Gneiss that is found throughout the Adirondack Highlands. The origin of the Lyon Mountain Gneiss is highly debated. Using field observations, petrographic and chemical analyses this study will document the intrusive origin of these rocks. The magnetite horizons are contained in a syentitic rock that is clearly cross cut by a magnetite poor, quartz-perthite granite. The quartz-perthite granite is cross cut by a quartz-albite pegmatite that also contains magnetite. Other quartz-albite pegmatites cross cut foliation in surrounding calc-silicates east of Ausable Forks, NY. These unmistakable cross cutting relationships establish an intrusive origin for the highly debated Lyon Mountain granitoids.
Additional studies were conducted to determine the origin of the magnetite horizons at Lyon Mountain mine. Though uncertain at this time, cross cutting relationships demonstrated in the ore body preclude a metasedimentary origin for the ores occurring in a roughly layered ore horizon. Using field and petrographic observations, magnetite formation appears to begin between the emplacement of the syenitic and granitic facies of the Lyon Mountain Gneiss. The emplacement of magnetite appears to continue through and even after the emplacement of the quartz-albite pegmatite. While the later ores most likely originated from deuteric or hydrothermal fluids, the earlier ores are more enigmatic. They may be magmatic in origin or they too may have been derived from deuteric fluids; however, current evidence does not obviate either method.
