MULTIPLE STAGES OF MINERALIZATION OF THE HAMMONDVILLE MAGNETITE-APATITE DEPOSITS

The Hammondville mining district, located in the Eagle Lake quadrangle in the eastern Adirondack Mountains, contains Kiruna-type low-Ti magnetite-apatite deposits hosted in Lyon Mountain granite gneiss (LMG). The origin of this type of iron deposit has remained the subject of debate for decades. Recent 1:12,000-scale mapping of the Hammondville pluton, coupled with whole-rock geochemistry and microscopy, offers new insight into the petrogenesis of these deposits and of the LMG. Based on whole-rock geochemistry, the LMG is a metaluminous to peraluminous leucogranite. Compositions are ferroan and plot in rift and extensional fields on tectonic discrimination diagrams suggesting it was derived from partial melting of preexisting continental crust. Magnetite is the major mafic component in the pluton, commonly concentrated into faint layers that define a weak fabric. The LMG lacks a pervasive deformational fabric except for minor localized shear zones mostly constrained to areas of mineralization. Magnetite deposits occur as thin layers (several cm) within layered LMG, as thick seams (up to a few m) of several varieties and are commonly associated with late crosscutting pegmatites. Mineralization is generally concordant to pluton wide layering. Magnetite seams are typically composed of Mag, Qz, Cpx, Ab, ±Amp, ± Bt, ±Ap, ±Zrn. Zones of thin seams variably interlayered with sodic altered LMG are typically located adjacent to major seams. Texturally early seams display magmatic features such as appendages that may be sheared; others appear to bulge into and deform adjacent layering of the host LMG. However, a lack of subsolidus fabric suggests that the LMG was not fully crystallized. Magnetite mineralization apparently formed over a protracted period of time and by several mechanisms. We suggest early magnetite may have been introduced as an iron-rich magma locally co-mingling with LMG forming parallel layering. Later Fe-rich magma locally cut magmatic fabrics and deformed existing layering that was still a crystal mush. The intrusion of crosscutting pegmatite magmas remobilized magnetite and may have supplied fluids related to the albitization of the host granite and alteration of preexisting ore bodies as much as 20 to 60 million years after LMG plutonism.