Volcanic groups of the Central Mobile Belt of the Newfoundland Appalachians have previously been subdivided into "early" and "late" arc sequences, separated in time by a quiescent Caradocian stage defined in some areas by fossil-bearing black shales.New U–Pb zircon ages of [Formula: see text] and 473 ± 2 Ma for rhyolites of the Buchans and Roberts Arm groups, respectively, show them to be correlative early Ordovician sequences. These ages serve to refute both the previous Rb–Sr whole-rock isochron ages of 447 Ma and the idea that these groups were "late arc" sequences. These new ages corroborate evidence from late Arenig – early Llanvirn conodonts in the Buchans Group and calibrate this fossil occurrence.A new U–Pb zircon age of 479 ± 3 Ma from plagiogranite of the Mansfield Cove Complex immediately west of the Roberts Arm Group shows that this plutonic body is only slightly older than the adjacent volcanic rocks and not Hadrynian as previously supposed. Local field relationships suggest this body may represent part of a disrupted ophiolite. It is coeval with the ophiolitic Annieopsquotch Complex along a strike to the south and may form part of a belt of rocks derived from early Ordovician ocean floor that is discontinuously exposed along the western boundary of the Buchans – Roberts Arm Belt.Zircons from rhyolite at the northeast termination of the Tulks Hill volcanics, part of the extensive Victoria Lake Group, give an age of [Formula: see text]. This dated sequence contains limestone previously dated as Llanvirn–Llandeilo by conodonts. This part of the group is therefore younger than the Buchans Group, and the designations "early" and "late" arc are not appropriate. The thrusting that juxtaposed these groups is no longer constrained to be of Silurian age but could have been middle to late Ordovician. Precambrian zircons included in the Victoria Lake Group rhyolite could have been incorporated from associated sedimentary rocks and suggest that the group may have formed in a tectonic setting transitional between oceanic and continental and received detritus from several sources.

Sixteen kilometres of high resolution Vibroseis reflection seismic data have been acquired in the vicinity of the former Buchans mine. Direct identification of the cause of several reflectors is possible because the geology is tightly constrained by underground workings and drill holes both of which locally exceed 1 km depth. Many of the mine-scale thrust faults are imaged as reflectors but conformable and intrusive contacts generally responded poorly. A significant shallow-dipping thrust, the Powerline Fault, is recognized below the orebodies and traced throughout the Buchans area, primarily as a result of the seismic survey. It truncates ore stratigraphy and forms the floor thrust of a large duplex–stack, which hosts all the orebodies. Its presence has negative implications for exploration in the immediate mine area. Several lines of evidence suggest that this fault has a significant component of out-of-sequence movement. A strong reflector 4.5 km below Buchans is correlated with the surface expression of the Victoria River Delta Fault, an important regional structure, newly recognized southeast of Red Indian Lake. This shallow, north-dipping sole thrust forms the structural base of the Buchans Group and brings it above a younger fossiliferous Llanvirn volcanic sequence. This fault is not itself the Red Indian Line but is one of a series of faults that collectively effect substantial geological contrasts in central Newfoundland. The seismic survey was a cost-efficient means of gaining knowledge of Buchans structure, which might otherwise have been acquired at much higher cost and over a longer period of time.

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A novel application of the tensor controlled source audio‐magnetotelluric (CSAMT) method was part of a multidisciplinary geophysical study of an existing mine site at Buchans, Newfoundland. The orthogonal components of the horizontal electromagnetic fields used for magnetotelluric and CSAMT interpretation of the earth’s conductivity structure were found to be inappropriate at Buchans because of strong scattering in the electric fields. Instead, the length of the major axes of the electric and magnetic field polarization ellipses and the vertical magnetic field were used as data. The data from two bipole sources demonstrate that the bulk response of the earth in the vicinity of Buchans is predominantly one‐dimensional (1-D). These data were inverted to layered earth models with a first‐order correction for electric field distortions. The parameter space considered during the inversion was contracted substantially by incorporating the vertical magnetic field data and by using depths to interfaces as determined by reflection seismic data. The model resulting from the inversions is essentially a two‐layered earth with an increase in resistivity between 1000–1400 m depth. The contrast in the electrical properties is interpreted to be coincident with the Powerline Fault, a floor thrust of a duplex structure with significant out‐of‐sequence movement. Hence, the thrusting may have caused the emplacement of older fractured, and locally mineralized rocks over younger more competent (resistive) ones.

Conodonts have been recovered from carbonate clasts within breccia units in the Buchans Group of central Newfoundland. The assemblage is characterized by specimens of "Cordylodus" horridus Barnes and Poplawski and Histiodella holodentata Ethington and Clark. These and other species recovered suggest a Whiterockian (latest Arenig–early Llanvirn) age for the clasts. This is somewhat older than the whole-rock Rb–Sr age of 447 ± 18 Ma on volcanics of the Buchans Group. The carbonate clasts are considered to be derived from the area of deposition of the Buchans Group rather than from some exotic source. This interpretation is based on knowledge of local tectonics, inferred paleogeogaphy, and geographic distribution of the conodont assemblage. All other clasts in the breccia units are considered of local origin. If the conodonts are of local Buchans Group origin then they indicate a Whiterockian age for the unit. In view of this, current regional models for the Central Volcanic Belt may require substantial revision. A Whiterockian age for the Buchans Group is consistent with the observation that all significant volcanogenic massive sulphides of the Canadian Appalachians are of Caradoc or older Ordovician age. It is suggested that the Buchans Group is thrust southeastwards over the Victoria Lake Group, which is a probable distal facies equivalent.