Natural Resources

THE TULKS VOLCANIC BELT, VICTORIA LAKE SUPERGROUP, CENTRAL NEWFOUNDLAND - GEOLOGY, TECTONIC SETTING AND VOLCANOGENIC MASSIVE SULPHIDE MINERALIZATION

J.G. Hinchey

St. John's, Newfoundland, 2011

Abstract

The Tulks Volcanic Belt (TVB), of the Victoria Lake supergroup, central Newfoundland, is dominated by quartz ± feldspar porphyritic felsic volcaniclastic rocks and lesser amounts of mafic volcanic rocks and intercalated sedimentary rocks. The belt has traditionally been viewed as a single stratigraphic sequence of ca. 498 Ma age, but recent geochronological studies imply that it may be composite and include rocks as young as 453 Ma. These rocks developed in volcano-sedimentary basins developed in active volcanic arcs on the peri- Gondwanan margin of the Iapetus Ocean.

The belt is host to five important clusters of VMS deposits. From south to north, these include the Boomerang, Tulks Hill, Tulks East, Daniels Pond and Bobbys Pond deposits, and a number of smaller sulphide prospects. The major VMS deposits are hosted by felsic volcanic, pyroclastic and volcaniclastic rocks. Mineralizing styles vary from ‘classic’ exhalative-type mineralization developed on the seafloor, to ‘replacement’ style mineralization developed in a sub-seafloor environment. There are also mineralized debris flow breccias, suggesting potential for transported ores. As such, the TVB contains a continuum of VMS deposit types.

Volcanogenic massive sulphide deposits in the TVB are interpreted to have formed in volcanic, volcaniclastic and sediment-rich basins as tectonomagmatic conditions changed from convergent (e.g., active-arc environment) to extensional (e.g., back-arc or arc-rift) environments. The change from compressional to extensional regimes would allow for active rifting, conduit formation, and high levels of focused heat flow, which are ideal conditions for the development of large and productive hydrothermal systems. The deposits display mineralization, alteration, and textural characteristics indicative of both bimodal-siliciclastic type deposits, and deposits classified as ‘hybrid bimodal felsic VMS-epithermal’ deposits. These two types of deposits dominate the southern and northern parts of the belt, respectively. The latter are intermediate to ‘classic’ VMS-type deposits and epithermal-type mineralization of the type generally associated with subaerial volcanism. It is suggested that VMS deposits in the northern part of the belt formed in a higher standing portion of the basin in relatively shallow-water conditions, proximal to vents and active magmatic systems that may have supplied some fluid input. This is in contrast to a deeper water, more distal environment envisioned for the deposits in the southern part of the belt.

To clarify their tectonostratigraphic affinity within the Victoria Lake supergroup and to better understand these mineralizing environments, U–Pb geochronology, trace-element lithogeochemistry and Sm/Nd isotopic geochemistry have been applied to the host rocks of all of the major VMS deposits in the TVB. However, the absence of zircon in many sampled volcanic rocks complicated these geochronological studies, and the exact age of host sequences to some deposits remains unknown. A subvolcanic porphyry from the Tulks Hill deposit, dated previously at 498 +6/-4 Ma, provides a minimum age for the nearby Tulks Hill and Tulks East deposits; and was interpreted to be the age of all the TVB rocks. Two new U–Pb zircon ages were obtained; one from the felsic tuff that hosts mineralization at the Boomerang deposit and the other from a felsic dyke interpreted to be broadly synvolcanic. The combined TIMS and SHRIMP data for these two samples indicate an identical U–Pb age of 491 ± 3 Ma. This date is younger than the 498 +6/-4 age from Tulks Hill, although the errors do overlap at their older and younger limits, respectively. Inheritance patterns in the Boomerang samples suggest the presence of older crustal material having Cambrian (514–510 Ma) ages, akin to those reported from the Tally Pond group, an older sequence within the Victoria Lake supergroup. The new geochronological results suggest that VMS mineralization in the Tulks area and at Boomerang may represent temporally discrete events, despite some apparent similarities. The age determined for the Boomerang deposit is closer to (but not identical with) a U–Pb date of 488 ± 3 Ma, obtained some 30 km to the southwest of the Boomerang deposit, from the Pats Pond group. This suggests that the younger sequence of rocks may be regionally extensive, as proposed by other workers, and implies that it may have potential elsewhere for VMS mineralization similar to the Boomerang deposit.

The comparison of lithogeochemical patterns from the major deposits is complicated by the effects of hydrothermal alteration near the VMS mineralizing environments. Nevertheless, examination of immobile trace-element signatures suggests that the host sequences to the deposits, with the exception of the Daniels Pond deposit, cannot be easily distinguished on the basis of their geochemistry. The volcanic and pyroclastic rocks are all broadly arc-related, and show a mixture of calc-alkaline and tholeiitic signatures that perhaps record the construction and later rifting of individual arc sequences. Nd-isotope signatures from felsic rocks in the Boomerang area, the Pats Pond group, and one sample from the Bobbys Pond deposit are higher (εNd of +3.8 to +5.5) than those from the Tulks Hill, Tulks East, and Daniels Pond areas (εNd of around +2 to +3). Although not a straightforward correlation, as the volcanic rocks that host the Tulks East and Tulks Hill deposits also locally contain higher εNd values of between +4 to +5, the data may support a link between the Boomerang deposit area and the Pats Pond group; representing a possibly younger package of mineralized rocks. The new results, when taken with the results to date, suggest that the tract of rocks known as the TVB includes rocks of more than one age, but of generally similar geochemistry and tectonic setting.

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