STRATIGRAPHY OF THE GOLDENVILLE GROUP-HALIFAX GROUP TRANSITION (GHT) OF THE MEGUMA SUPERGROUP AT CARIBOU GOLD DISTRICT (DRILLCORE LL81-5A), NOV A SCOTIA
Burns, Craig Gordon
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Drillcore LL81-5A, recovered from Caribou gold district, Halifax County, Nova Scotia, by Sherritt Gordon Mines Ltd. in 1980, is a 625.40 m-long continuous section of the CambroOrdovician Meguma Supergroup, representing 500 stratigraphic metres of the transition between the sandy Goldenville Group and the overlying younger, shaly, Halifax Group (GHT). This core presents an unusually long and continuous section of the upper part of the GHT, which appears to exert significant control over trace metal concentration in the Meguma Supergroup. The purpose of this study is to characterize the geology of drillcore LL81-5A, which should promote a better local and regional understanding of the GHT. Three conformable stratigraphic units are noted in LL81-5A. In order of decreasing age and depth, these are: Unit A (36 stratigraphic metres)- fining upward sequences of massive, carbonaterich metawackes and silty slates; Unit B ( 113 m stratigraphic metres)- interbedded manganiferous, chloritic, pyrrhotitic meta-argillites and metawackes and Unit C (3 51 stratigraphic metres)- fining upward sequences of parallel- and ripple cross-laminated, pyrrhotitic, locally carbonate-rich metasiltstones and carbonaceous slates similar to base-cut-out Bouma sequences, with locally abundant soft-sediment deformation structures. These units are equivalent to undivided Goldenville Group lithologies and Moshers Island and Cunard formations of the lower Halifax Group, respectively. Unit B meta-argillites host a trace element and trace metal assemblage that includes Ba, Pb, Zn, Ni, As and Mo, and are significantly enriched in MnO relative to other lithologies studied in LL81-5A. Unit C slates contain a trace element and metal assemblage of Ba, Rb, Sr, V, Cr, Sc, Cs, La, Ce and U and are interpreted as metalliferous black slates. Unit A metawackes are depleted in most trace elements and metals relative to the other main lithologies studied, whereas Unit C metasiltstones contain average relative concentrations of most trace elements and metals. Decreasing grain size, fining-upward, generally sharply-bounded sedimentary sequences, many with an upward succession of sedimentary structures indicative of decreasing water energy, along with an increasing carbonaceous material and trace element and trace metal influence through the GHT in LL81-5A, implies deposition of these sediments from turbidity currents within a progressively anoxic, coarse sediment-starved basin.