CHEMOSTRATIGRAPHY AND DEPOSITIONAL ENVIRONMENT OF AN ORDOVICIAN SEDIMENTARY SECTION ACROSS THE MIRAMICHI GROUP TETAGOUCHE GROUP CONTACT, NORTHEASTERN NEW-BRUNSWICK

A thick section of Ordovician sedimentary rocks underlies and overlies felsic to mafic volcanic rocks of the Tetagouche Group, Bathurst Mini ng Camp. The dark grey quartzose slates and siltstones of the Patrick Brook Formation (Miramichi Group) occur below the volcanic rocks, wher eas the dark grey to black slates and siltstones of the Boucher Brook Formation (Tetagouche Group) are intercalated with the volcanic rocks and overlie the sequence. The Miramichi-Tetagouche contact represents the interpreted Gander-Dunnage boundary in northeastern New Brunswick. Distinguishing between these two similar formations is important for stratigraphic and geotectonic interpretations of the Bathurst Mining C amp and for exploration in these sequences. The geochemical compositio n of a semi-conformable section of rocks from the Boucher Brook (Middl e to Late? Ordovician) and Patrick Brook (Early to Middle Ordovician) formations was determined to identify geochemical chemostratigraphic d iscriminants, as well as to determine the depositional environment in which these were deposited. The high Al2O3 and distinctly higher high- field-strength elements (LREE, Th, HREE, and Y) in the Patrick Brook r ocks are characteristic of mature sedimentary rocks and indicate inten se chemical weathering (tropical environment) in the source regions, w hich is consistent with their compositional similarity to Avalon-deriv ed shales analogous to Gander Zone sedimentary rocks. The Boucher Broo k slates and siltstones are immature sedimentary rocks based on the pr eservation of albite and the less coherent trace-element systematics t o phyllosilicate indices (Al2O3 and K2O). The Boucher Brook Formation is probably derived from the associated volcanic rocks. The higher Mn and Fe and positive Ce/Ce anomaly in some Boucher Brook Formation com pared to the Patrick Brook rocks indicate that the Boucher Brook rocks in this section were deposited in a transitional anoxic/ oxic environ ment. The Patrick Brook rocks that immediately precede felsic volcanis m and formation of massive sulphide deposits are highly reduced based on C and S contents, which is consistent with the sulphur isotope data . Moderately heavy delta(34)S values are indicative of SO42- reduction to H2S under anoxic conditions, which is significant in the formation and preservation of massive sulphides in the basal Tetagouche sequenc e.