GEOCHEMICAL AND GEOPHYSICAL CHARACTERISTICS OF THE LATE ORDOVICIAN BOLAND BROOK FORMATION, NORTHWESTERN NEW BRUNSWICK: IMPLICATIONS FOR SEISMIC STUDIES AND NI - CO EXPLORATION

Exploration drilling for Ni and Co in northwestern New Brunswick has provided a unique opportunity to evaluate the geochemical and geophysical properties of the Boland Brook Formation (BBF) to a vertical depth of 1000 metres. Twelve (12) samples were analysed for whole-rock and trace-element geochemistry, seven (7) samples for dS³⁴_CDT of pyrite and eleven (11) samples for three geophysical parameters (density, compressional wave velocity, and magnetic susceptibility).

The BBF is a fine-grained siliciclastic unit that occupies a basal position within the Grog Brook Group. The unit comprises dark grey, noncalcareous siltstone interbedded with moderately calcareous sandstone and minor carbonaceous laminae. The Ritchie Brook Member (RBM), a 10 to 15 metre thick carbonaceous unit, occurs at the top of the formation and was thought to be related to local Ni/Co geochemical anomalies.

The geochemical profile shows that Ni, Co, Cr and Cu vary between 130-208 ppm, 23-31 ppm, 150-246 ppm and 34-50 ppm, respectively. The RBM is slightly depleted in Ni and Cr relative to the rest of the formation; unusual, given that the organic carbon content of the RBM is more than twice that of the bulk of the formation. The dS³⁴(pyrite) values range from -0.85 to -9.16 ‰ for disseminated pyrite indicating a bacteriogenic origin (probably syn-sedimentary).

The geophysical parameters are consistent throughout the profile. Magnetic susceptibilities are low, varying between 0.01 and 0.02.  The density (ρ) and velocity (Vp) values cluster close to 2.8 g/cc and 6.0 km/s, respectively, at a confining pressure of 200 MPa with impedances (ρ x Vp) between 16 and 17.5. These data indicate that the BBF is “seismically transparent” and, therefore, any seismic survey conducted in the area would easily detect the presence of intrusions or sulphide deposits.