Hugh Samson


B.Sc. (Honours) Thesis

Origin of Sulfides in the Contact Granodiorites of the South Mountain Batholith, Nova Scotia

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Late in the Acadian Orogeny in southwestern Nova Scotia, the late Devonian South Mountain Batholith (SMB) intruded the Cambro Ordovician meta sandstones and siltstones of the Meguma Terrane. Whole rock and mineral chemical data for major elements (A/CNK) and radiogenic isotopes (Sr Nd) show that the SMB evolved by a combination of assimilation of the Meguma country rock and fractional crystallization of its magmas (AFC processes). The purpose of this investigation is to test the AFC model using textural and compositional variations in sulfide minerals from Meguma, xenolith, and SMB rock samples. The dominant sulfide phases in the Meguma, xenoliths, and SMB are pyrite and pyrrhotite, with common occurrences of chalcopyrite, and rare occurrences of sphalerite and galena. Meguma rocks distal from the contact with the SMB are dominated by anhedral, 0.25 0.5 mm pyrrhotite grains. Anhedral, 0.25 1.0 mm pyrite grains occur commonly and anhedral, < 0.25 mm chalcopyrite grains occur rarely as inclusions in pyrite and pyrrhotite. Meguma samples proximal to the SMB contact contain large, 0.5 5.0 mm anhedral grains of pyrrhotite as the dominant phase, with small to medium sized, 0.25 1.0 mm, < 0.25 1.0 mm subhedral pyrite and < 0.25 mm, anhedral chalcopyrite grains occurring as inclusions in pyrrhotite. Sulfides from xenoliths near the contact of the SMB are transitional in texture and composition. Pyrrhotite is the dominant phase, occurring in large sulfide bands up to 75 mm in length, and 15 mm in width. Common inclusions of anhedral, 0.5 5.0 mm pyrite grains, and 0.25 0.5 mm chalcopyrite grains occur within xenolith samples. The SMB granodiorites contain sulfide globules exhibiting a bleb texture, which are dominated by 0.25 5.0 mm, euhedral to anhedral pyrite grains. Subhedral to anhedral, 0.25 1.5 mm chalcopyrites commonly coexist with pyrite. Distinct changes in the abundance, size, shape, and inter granular relationships exist between sulfide minerals from the Meguma country rock, across the contact in xenoliths, and into the SMB. Chemical compositions of the sulfide minerals show trace amounts of As, Cu, Pb, Zn, and Ni. In SMB samples, the chalcophile trace elements Cu and Ni are preferentially concentrated in pyrite and pyrrhotite. Many of the sulfides in the SMB that occur within xenolithic remnants appear to be texturally and chemically modified equivalents of the Meguma sulfides. The concentration of chalcophile trace elements in SMB sulfide samples, as well as bleb texture associated with SMB sulfides suggest the presence of an immiscible sulfide liquid in the SMB. Models proposed for the origin of sulfides in the SMB include: primary magmatic crystallization from the silicate magma; xenocrystic from the Meguma; initially dissolved from the Meguma, then followed by crystallization from the SMB magma; and formation of immiscible droplets of Meguma sulfides that crystallized as the SMB silicate magma cooled. The latter model, involving formation of an immiscible sulphide liquid, can account for most of the textural and chemical observations, but is difficult to reconcile with known phase relations in the Cu Fe S system.

Pages: 87
Supervisor: D. Barrie Clarke