CONTROL OF MAGNETITE MINERALIZATION IN THE LYON MOUNTAIN GRANITE GNEISS, HAMMONDVILLE MINING DISTRICT, EASTERN ADIRONDACK HIGHLANDS, NEW YORK

The Lyon Mountain Granite Gneiss (LMG) of the Adirondack massif is commonly associated with large magnetite ± apatite deposits, locally with abundant REE. These deposits were mined for nearly 200 years, but the petrogenetic relationship with the LMG remains ambiguous. The LMG is a ferroan, equigranular leucogranite consisting of varying proportions of perthite, K-feldspar, albite, magnetite, biotite, amphibole, ± pyroxene. Xenoliths of the surrounding rocks occur in abundance within the LMG in varying states of digestion, and can be large screens that continue for 100s of meters. The LMG ranges from massive to well-layered, with local subsolidus foliation and rare, thin mylonite layers. The orientation of layering and overall plutonic structure of the LMG mimic those of the well-developed fabrics in the older surrounding units that likely imparted a major control on LMG emplacement. Massive units commonly host dispersed euhedral mafics lacking a preferred orientation. Petrography reveals that the unit rarely records deformation, indicating that emplacement occurred late in the orogenic cycle. Layered units are characterized by subtle changes in grain size or changes in modal mineralogy. In the area of the historic mining town of Hammondville, varying amounts of magnetite are disseminated throughout the LMG. Massive units lacking visible mafic minerals contain magnetite as the only mafic phase. Magnetite layers are increasingly present near magnetite deposits becoming thicker and more regular toward ore seams. The magnetite deposits are well layered, with major seams ranging from half a meter to several meters thick. Magnetite seams generally follow the local foliation or layering in the LMG, but crosscutting ore seams and apophyses extending into the host rock are not uncommon. A near ubiquitous presence of magnetite throughout the LMG, inferred magmatic layering and seams that parallel this layering indicates that Fe mineralization originally related to LMG magmatism was likely modified during metamorphism or metasomatism. Local sub-solidus deformation of magnetite seams and host LMG suggests ore mineralization occurred late in the tectonic cycle. Continued geochemical and structural work is ongoing to understand the relationship between the evolution of the LMG and magnetite (± REE) mineralization.