ABUNDANCE AND SPECIATION OF GOLD IN MASSIVE SULFIDES OF THE BATHURST MINING CAMP, NEW BRUNSWICK, CANADA

Abstract

Massive sulfide deposits of Zn–Pb–Cu–Ag type in the Bathurst Mining Camp, New Brunswick, are hosted within a Middle Ordovician bimodal volcanic and sedimentary sequence that has undergone complex polyphase deformation and associated regional metamorphism to the lower- to upper-greenschist grade. These factors are partly responsible for the present geometry and textural modification of these hydrothermal deposits, originally formed on the seafloor. Despite the importance of heterogeneous ductile deformation, some primary features are evident, in particular fine-grained colloform pyrite and base- and precious-metal zonation within many of these deposits. The average Au content (bulk instrumental neutron-activation analyses; n = 215) of massive sulfides from 43 deposits in the Bathurst Mining Camp is 0.85 ppm, with values as high as 6.86 ppm. Positive correlations of Au content with Ag (r′ = 0.53), As (r′ = 0.57) and Sb = (r′ = 0.65) suggest that Au is mainly associated with arsenian pyrite and a distal suite of elements (Au + Sb + As ∠ Ag). The submicroscopic Au contents of pyrite and arsenopyrite in eight deposits with elevated bulk Au contents were investigated using a Cameca IMS–4f secondary-ion mass spectrometer. Results suggest that arsenian pyrite is the most important host for Au in massive sulfides of the Bathurst Mining Camp, with an average Au content of 9.1 ppm, and values reaching 42.9 ppm. Arsenopyrite was found to contain much less Au, with an average of 2.7 ppm. Recrystallization of sulfides during greenschist-facies metamorphism has resulted in pyrite morphologies with variable Au contents. Invisible Au was in part released and adsorbed as submicroscopic inclusions on As-rich surfaces of pyrite and on arsenopyrite.