BASE METAL DEPOSITS OF THE SILURO-DEVONIAN TOBIQUE CHALEURS BELT, WEST-CENTRAL NEW BRUNSWICK, CANADA

A bimodal volcano- sedimentary succession was erupted / deposited in a trans-tensional basin (fore-deep) that formed in response to oblique docking of the Miramichi Terrane with the North American Craton in the Early Silurian. This succession was subsequently intruded by small Upper Silurian to Middle Devonian felsic and mafic bodies. Zn-Pb deposits occur intermittently along the full length of a north easterly trending belt from Nash Creek in the north to Sewell Brook, Shingle Gulch, and Mount Costigan in the central part, to Gravel Hill in the south.

All of the deposits are hosted by Lower Devonian felsic volcanic units of the coeval Tobique and Dalhousie groups. Most are spatially associated with known or inferred faults. Only the Gravel Hill deposit has been directly linked to a hypabyssal intrusion. The host rocks are commonly substantially enriched in K2O (up to 15 wt. %), which is attributed to low-T seawater alteration. Genetically related hydrothermal alteration tends to be restricted to the immediate vicinity of mineralization, and commonly includes open-space quartz and quartz-carbonate vein fillings. Chlorite occurs with some of the higher grade zones at Sewell Brook and Gravel Hill, whereas quartz breccias are common at Mount Costigan.

Sulphide mineralization consists of veins, and replacements in zones of enhanced permeability. Only one deposit has massive sulphide lenses, which probably formed as replacement rather than as stratiform exhalative mineralization. The sulphide mineralogy is very simple, i.e. pyrite » sphalerite > galena > chalcopyrite, with Zn:Pb ³ 2:1, With some exceptions these deposits have very low Cu, but over short intervals Ag and Au may reach > 15 g/t and 2-3 g/t, respectively. d³⁴S isotope analyses from two deposits in the central part of the belt have yielded a wide range in values (-1.5 through +11‰ avg. » 6 ‰) but they are low relative to the global sea water curve. Limited fluid inclusion data suggest low temperature (Th=140 °C uncorrected for pressure) and low salinities (6-8 wt. % NaCl eq.) for the mineralizing fluids at the Shingle Gulch deposit.