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 preciousmetal 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.

The coupled geochemistry of Au and As in pyrite from hydrothermal ore deposits

“…These differences are most likely a reflection of a more complex evolution of pyrite in orogenic Au deposits, where up to 6 generations of pyrite are reported. In these systems, Aubearing, coarse pyrite precipitated from hydrothermal fluids , and its trace-elements composition was modified under high-temperature conditions (300-400 o C), with at least partial re-equilibration with Au-bearing fluids (e.g., Mumin et al 1994;McClenaghan et al, 2004;Morey et al 2008;Sung et al, 2009). In the distal parts of orogenic Au deposits framboid relics are observed suggesting that pyrite grew at the expense of an earlier, finegrained seed under fluid-assisted conditions ).…”

“…Gold and arsenic incorporation into pyrite is strongly coupled in pyrite from many different types of ore deposits, including Carlin-type Au (e.g., Cline, 2001;Emsbo et al, 2003;Reich et al, 2005;Muntean et al, 2011), porphyry Cu, Cu-Au, and epithermal Au (e.g., Cook and Chryssouliss, 1990;Deditius et al, 2008;Reich et al, 2013), orogenic Au (e.g., Morey et al, 2008;Large et al, 2007), volcanogenic massive sulphide (VHMS) (Huston et al, 1995;McClenaghan et al, 2004), iron-oxide Cu-Au (IOCG) (Foster et al, 2007) and…”

Analysis of Au-Ag Mineralization in the Caribou Base-Metal VMS Deposit, New Brunswick; Examination of Micro-Scale Inter- and Intra-Sulphide Distribution and Its Relation to Geometallurgy

“…Stockwork mineralization is present in the footwall of these deposits along with large alteration-induced halos formed from the circulation of hydrothermal fluids [21,22,24]. Massive sulphide bodies in the BMC are generally hosted above stockwork mineralization and vent zones and are composed of fine-grained pyrite, sphalerite, galena, chalcopyrite and pyrrhotite with minor arsenopyrite, marcasite and tetrahedrite [8]. At the base of these lenses, also referred to as the basal sulphide facies, mineralization has generally been metasomatically altered by high temperature hydrothermal fluids resulting in an assemblage of pyrite, pyrrhotite and chalcopyrite [8].…”

“…Peak metamorphism conditions ranged from 325-425 °C and from 4-6 kbar [31][32][33]. During these conditions, recrystallization increased the grain size of some sulphide grains, including porphyroblasts of pyrite, marcasite and arsenopyrite [8]. …”

“…Massive sulphide bodies in the BMC are generally hosted above stockwork mineralization and vent zones and are composed of fine-grained pyrite, sphalerite, galena, chalcopyrite and pyrrhotite with minor arsenopyrite, marcasite and tetrahedrite [8]. At the base of these lenses, also referred to as the basal sulphide facies, mineralization has generally been metasomatically altered by high temperature hydrothermal fluids resulting in an assemblage of pyrite, pyrrhotite and chalcopyrite [8]. Above the basal sulphide facies are Pb-and Zn-rich lenses that exhibit sulphide layering [25,26].…”