CONTAMINATION OF THE SOUTH MOUNTAIN BATHOLITH BY SULFIDES FROM THE COUNTRY ROCKS

Abstract

Sulfide minerals (Po + Py + Ccp >> Sp + Gn + Apy) are common constituents of the Meguma Supergroup (MSG) country rocks, and are also common in the marginal facies of the South Mountain Batholith (SMB), Nova Scotia, relative to the interior of the batholith. The distribution and relative modal proportions of sulfides in the granites near the contact indicate that they are probably contaminants from the MSG country rocks. We trace the textural and chemical changes that occur in Py, Po, and Ccp from the country rocks through their xenoliths to the granite during their assimilation in the granite. Pyrrhotite in the country rock ranges from large polycrystalline porphyroblasts with many inclusions to small individual grains free of inclusions, all with low concentrations of Co and Ni. Chalcopyrite in the country rocks occurs mainly as anhedral inclusions in pyrrhotite, and as small isolated grains in the silicate matrix. Pyrite in the country rocks occurs as euhedral cubes or as anhedral cores to pyrrhotite grains. Relative to their occurrences in the MSG country rocks, characteristics of sulfide grains in the granitic rocks include generally smaller grain-sizes, Po grains with fewer silicate and oxide inclusions, development of some Po grains with an inclusion-rich core and an inclusion-free rim, Po grains that have a lower Fe:S ratio and contain higher concentrations of Co (up to 0.17 wt%) and Ni (up to 0.41 wt%), local increases in the Ccp:Po ratio, better development of crystal faces on Ccp inclusions in Po, development of micro-dropper or silicate-melt-displacing textures, and a general absence of Py except for its association with Ccp in microgranitoid enclaves. The grain-size reduction is consistent with disintegration of polycrystalline MSG sulfide grains, either in the solid state or in a partially melted state, as well as dissolution in the SMB magma. The higher concentrations of Co and Ni in the SMB pyrrhotite are consistent with cation exchange with the granitic melt. Most of the remaining characteristics are consistent with partial melting of the multiphase MSG sulfide blebs related to their incorporation in the SMB magma. From known phase-relations in the system Cu–Fe–S, we deduce that temperatures of the SMB magma exceeded the temperature of beginning of melting of the composition-dependent multiphase MSG sulfide aggregates, and thus at least some of them underwent partial melting. Other more refractory grains, which have undergone only textural (annealing, ductile deformation) and chemical modification in the solid state, are simply modified xenocrysts.