GEOCHEMISTRY OF ORDOVICIAN BLACK SHALES AT MEDUCTIC, SOUTHERN MIRAMICHI HIGHLANDS, NEW-BRUNSWICK

The geochemistry of black shales is useful for identifying geological events such as anoxia, for predicting the proximity of submarine hydro thermal springs and associated mineral deposits, and for indicating th e existence of mineral deposits in precursor terrains. In the Miramich i Highlands of New Brunswick, large massive sulfide deposits occur in a mid-Ordovician (Llanvirnian) sequence of felsic volcanic and sedimen tary rocks of the Tetagouche Group, disconformably overlying relativel y monotonous quartzose wacke and grey-green shale of the Miramichi Gro up. Black shales commonly occur at the boundary between the Miramichi and Tetagouche groups, and in some areas such as at Meductic in the so uthern Miramichi Highlands, they display high heavy metal values in co mmon with Scandinavian-Caledonide black shales. Like the massive sulph ide deposits, the Meductic metalliferous black shales were deposited i n a back-are basin associated with an island-are 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 va lues of V and Ba, moderate enrichment in Nb, Th, Pb, Ti, and Cr, and d epletion in Sr, Cu and Ni compared to black shale USGS standard SDO-1. Total organic carbon is well above the minimum value for a carbonaceo us shale, the highest value (2.35%) corresponding to highest V (3016 p pm) and highest Ba (16,173 ppm). CaO, Fe2O3 and MnO contents are low, and SiO2, K2O and P2O5 contents are relatively high compared to the st andard. Chondrite-normalized REE patterns show elevated LREE, fairly f lat HREE signatures, a negative Eu anomaly and a slightly positive Ce anomaly. These features are characteristic of marine volcanological de trital input rather than a hydrothermal regime. Correlation of high V with Ba indicates that conditions were favourable for deposition of bo th metals. It is postulated that early deposition in an oxic environme nt (higher than normal MnO) was succeeded by gradual stagnation (incre asing levels of V). Concomitant enrichment in Ba under anoxic conditio ns was linked to a regime of elevated heat flow. This scenario is comp atible with the record of sea level fluctuations during the early Pala eozoic.