Geochemistry of Ordovician black shales at Meductic, southern Miramichi Highlands, New Brunswick

Abstract

The geochemistry of black shales is useful for identifying geological events such as anoxia, for predicting the proximity of submarine hydrothermal springs and associated mineral deposits, and for indicating the existence of mineral deposits in precursor terrains. In the Miramichi Highlands of New Brunswick, large massive sulfide deposits occur in a mid-Ordovician (Llanvimian) sequence of felsic volcanic and sedimentary rocks of the Tetagouche Group, disconformably overlying relatively monotonous quartzose wacke and grey-green shale of the Miramichi Group. Black shales commonly occur at the boundary between the Miramichi and Tetagouche groups, and in some areas such as at Meductic in the southern Miramichi Highlands, they display high heavy metal values in common with Scandinavian-Caledonide black shales. Like the massive sulphide deposits, the Meductic metalliferous black shales were deposited in a back-arc basin associated with an island-arc system formed during the closing of the proto-Atlantic Ocean and the onset of the Taconian orogeny.The average Meductic black shale, as deduced from the Bright Eye Brook Formation at the base of the Tetagouche Group, shows high values of V and Ba, moderate enrichment in Nb, Th, Pb, Ti, and Cr, and depletion in Sr, Cu and Ni compared to black shale USGS standard SDO-1. Total organic carbon is well above the minimum value for a carbonaceous shale, the highest value (2.35%) corresponding to highest V (3016 ppm) and highest Ba (16,173 ppm). CaO, Fe2C>3 and MnO contents are low, and SiC^, K^O and P2O5 contents are relatively high compared to the standard. Chondrite-normalized REE patterns show elevated LREE, fairly flat HREE signatures, a negative Eu anomaly and a slightly positive Ce anomaly. These features are characteristic of marine volcanological detrital input rather than a hydrothermal regime. Correlation of high V with Ba indicates that conditions were favourable for deposition of both metals. It is postulated that early deposition in an oxic environment (higher than normal MnO) was succeeded by gradual stagnation (increasing levels of V). Concomitant enrichment in Ba under anoxic conditions was linked to a regime of elevated heat flow. This scenario is compatible with the record of sea level fluctuations during the early Palaeozoic.L'analyse g£ochimique des schistes noirs permet de d6celer des phdnomenes gdologiques comme l'anoxie, qui annonce la proximite de sources hydrothermales sous-marines et de gTtes mindraux connexes et qui rdv61e l'existence de gTtes mineraux dans des terrains pr6curseurs. Dans les hautes terres de Miramichi, au NouveauBrunswick, on releve des gisements de sulfures massifs importants dans une sequence de l'Ordovicien moyen (Llanvirnien) de roches sedimentaires et volcanofelsiques du groupe de Tetagouche qui recouvrent de fafon discordante des wackes quartzeuses relativement monotones et des schistes gris-vert du groupe de Miramichi. On trouve couramment des schistes noirs le long de la frontidre sdparant les groupes de Miramichi et de Tetagouch...

Stratigraphy and geochemistry of Ordovician volcanic rocks of the Eel River area, west-central New Brunswick

“…2, 2 km southeast of Meductic (Fig. 1), into overlying iron formation and volcanic rocks of the Meductic Group (Hennessy and Mossman 1996) suggests a conformable relationship, in contrast to the unconformity observed between the Miramichi and Tetagouche groups in the central and northern parts of the Miramichi terrane. Venugopal (1978Venugopal ( , 1979 informally referred to Ordovician felsic and mafic volcanic rocks in the Eel River area as the Pocomoonshine Volcanics and overlying Ordovician sedimentary rocks as the Belle Lake Slates.…”

“…However, the lithologic components of the two units are quite distinct; the former is largely mudstone whereas the latter is mainly feldspathic wacke. Moreover, the mudstone and shale of the Bright Eye Brook Formation have a unique trace-element geochemical signature that distinguishes them from shale of the Patrick Brook Formation (Fyffe and Pickerill 1993;Fyffe 1994;Hennessy and Mossman 1996). These differences justify removing the Bright Eye Brook Formation and conformably underlying Baskahegan Lake Formation from their previous assignment to the Miramichi Group and placing them in the newly established Woodstock Group.…”