PETROLOGY AND GEOCHEMISTRY OF MAFIC META-IGNEOUS DIKES WITHIN THE BALMAT ZN DEPOSIT, GOUVERNEUR, NY

This study petrographically and geochemically characterizes a suite of meta-igneous dikes within the Balmat Zn deposit.  Predominantly hosted by mid-Proterozoic Grenville series siliceous dolomitic marbles, the Balmat deposit's metamorphism and deformation culminated with the ~1.1Ga Grenville orogeny.

Dikes studied here are composed of plagioclase, phlogopite, biotite, scapolite, and epidote with minor clinopyroxene (diopside, augite), pyrrhotite, pyrite, calcite, quartz, vesuvianite, dravite, clintonite, chlorite, and rare quartz xenocrysts (embayed with pyroxene reaction rims suggesting they reacted with the magma as it was emplaced).  Plagioclase An content could not be determined due to grain deformation and small grain size (<200μm).  Most clinopyroxene is altered to epidote. Dikes are fine-grained, brown to black in color, weakly foliated, and range in thickness from 5cm to 1m; color variations are due to the presence of dravite, biotite, and/or phlogopite; brown facies are phlogopite-richer and contain fine-grained disseminated pyrite.  Larger boudinaged blocks of dike rock may contain more pristine, black, biotite-richer cores that are mantled by irregular, diffuse, rinds of lighter brown phlogopite-pyrite rock.  The dikes cross-cut all host rock types and recrystallized (relict) bedding.

Seven samples of dike rock were collected in situ from five different locations within the Balmat Zn deposit (Mud Pond ore body, MP; car shop, CS; and Sylvia Lake ore body, Sl) and analyzed via reflected and transmitted light microscopy and electron microprobe.  Samples 21-SL, MP-3415 and MP-3464 are black; MP-2664-A, MP-2664-B, 3100-CS-A, and 3100-CS-B are brown.  All samples (except 21-SL, MP-3415) are weakly foliated and contain macroscopic plagioclase and mica.  Few metamorphic textures are observed except for weak foliation exhibited by some micas.  Due to the extreme metamorphic conditions that affected the dike material, including retrograde reactions and chemical metasomatic equilibration with host rocks, whole rock data are unable to definitively identify the dike rock igneous protolith.  Based on field relationships, the presence and composition of augite, and the overall mafic nature of the sample material, the dike rock protolith was probably basaltic in composition.