Variations in style of volcanism and massive sulfide deposition in Early to Middle Ordovician island-arc sequences of the Newfoundland Central Mobile Belt

Abstract

The Newfoundland Central mobile belt records the Early Paleozoic birth, development, and destruction of the Iapetus ocean. Following opening of the ocean in the Cambrian, a compressional plate margin developed in the Early Ordovician and an island arc was formed above an east-dipping subduction zone. Volcanism was active at this plate margin from late Tremadocian to early Caradocian, at which time subduction ceased and the products of the volcanism were blanketed by argillite and flysch.The pre-Caradocian island-arc volcanic sequences of the Central mobile belt are host to numerous volcanogenic massive sulfide occurrences. Both the volcanic rocks and the associated massive sulfide occurrences show distinct contrasts across the volcanic belt. In the northern part of the belt (Notre Dame Bay area), submarine, dominantly mafic, volcanic rocks host pyritic Cu-Zn deposits associated with local felsic domes. These sequences pass southward into the Victoria River area, where volcanic rocks are characterized by laterally extensive felsic pyroelastic sheets which host polymetallic Zn-Pb-Cu deposits. In southern Newfoundland (the Hermitage flexure area), partially emergent, preponderantly felsic volcanic rocks host Pb-Zn-Ag massive sulfide mineralization formed during the waning stages of volcanism.Lead isotope ratios in massive sulfide deposits in the Hermitage flexure area are consistently more radiogenic than in deposits hosted by more mafic volcanic sequences to the north. This, coupled with geologic evidence, suggests that these sequences formed under the influence of sialic crustal material. It is inferred that the Early-Middle Ordovician island-arc volcanism in the Newfoundland Central mobile belt occurred in both oceanic and continental margin settings. The compositions and geologic environments of the associated massive sulfide deposits reflect these variations in tectonostratigraphic settings.Time and space relationships within the volcanic arc during the interval of active volcanism are not entirely clear. All Early to Middle Ordovician volcanic rocks in central Newfoundland may have formed from contemporaneous volcanic activity. If this is the case, simultaneous volcanism must have occurred both in oceanic environments and at or near the continental margin. Alternatively, the volcanism may have been slightly diachronous with an early episode of ensimatic mafic volcanism followed by migration of the volcanic centers toward the continental margin.The massive sulfide deposits of the pre-Caradocian island arc e. xhibit lead isotope ratios which are distinctly more radiogenic than those in the Buchans, Roberts Arm, and postCaradocian Cutwell Groups. This appears to support geologically based inferences that the latter sequences are not part of the main phase of island-arc activity in central Newfoundland.

Geochemistry of Ordovician volcanic rocks of the Tetagouche Group of southwestern New Brunswick

“…The stratigraphy of the upper part of the Tetagouche Group better correlates with the Dunnage Zone (Fyffe, 1982;van Staal, 1987). The Ordovician sequences of the Dunnage Zone include volcanic arc suites (e.g., Strong, 1977;Swinden and Thorpe, 1984) as well as within-plate basalts and MORB (e.g., Jenner and Fryer, 1980;Wasowski and Jacobi, 1985) similar to those of the Tetagouche Group.…”

Oceanic Pb-isotopic sources of Proterozoic and Paleozoic volcanogenic massive sulfide deposits on Prince of Wales Island and vicinity, southeastern Alaska

“…7) include the mainly Carboniferous Besshi-type (oceanic-mantle related) deposits in Japan (Sato and Sasaki, 1980), the Devonian Shasta deposits related to a primitive island arc in California (Doe and others, 1985), and the Lower Paleozoic (mostly Ordovician through Silurian) deposits related to primitive island arcs and arc rifting in Newfoundland and the Eastern Townships of Quebec (Swinden and Thorpe, 1984;Cumming and Krstic, 1987;Swinden, 1996;Pollock and Wilton, 2001). The Paleozoic massive sulfide deposits, which are among those with the most primitive Pb-isotopic compositions, have been used to estimate the composition of cogenetic Devonian (Shasta) and Ordovician (Newfoundland) mantle ( fig.…”

“…Such a close link to older source rocks is particularly evident for the Niblack deposit, which notably is hosted by felsic metavolcanic and tuffaceous metasedimentary rocks that resemble the lithologic sequence in the Wales Group (Gehrels and Berg, 1992;Maas and others, 1995). Detrital zircons dated by U-Pb Pb ratios for Ordovician mantle from primitive intraoceanic arcs in Newfoundland (Swinden and Thorpe, 1984;Swinden, 1996) and for Devonian mantle from a primitive island arc associated with massive sulfide deposits in the Klamath Mountains, Calif. (Shasta; Doe and others, 1985). Fields for pelagic sediment from the Pacific Ocean (Marianas arc, Meijer, 1976); for basalts from the Marianas arc, the Hawaiian plume (Koolau caldera), and the Pacific and Indian Oceans (estimated age, 550 m.y., using average crustal Pb-evolution curve of Stacey and Kramers, 1975;see fig.…”

The Lake Bond deposit: superimposed volcanogenic and synorogenic base and precious metal mineralization in the Robert's Arm Group, central Newfoundland

“…Base and precious metal contents and ratios vary widely among the deposits, and the relative contents of the various metals is closely related to the composition of volcanic rocks in the underlying volcanic sequences. Copper, zinc and gold are most abundant in deposits with dominantly mafic and ultramafic substrates whereas increasing abundances of felsic volcanic rocks in the substrate are correlated with relative enrichments in Pb and Ag (Swinden and Kean, 1984;Swinden and Thorpe, 1984;Swinden etal., 1988).…”