Geology and paleotectonic history of the Tally Pond Group, Dunnage Zone, Newfoundland Appalachians: an integrated geochemical, geochronological, metallogenic and isotopic study of a Cambrian island arc along the peri-Gondwanan margin of Iapetus

The Cambro-Ordovician Victoria Lake Supergroup lies within the Exploits Subzone of the Newfoundland Appalachians and consists of felsic volcanic rocks with lesser amounts of mafic pillow lava, mafic and felsic pyroclastic rocks, chert, greywacke and shale. The group is a composite and structurally complex assemblage of volcanic, volcaniclastic, and epiclastic rocks which formed in a variety of island-arc, rifted arc, back-arc and mature-arc settings. It is divisible into several separate volcanic terranes that include the Tulks and Tally Pond belts. -- The Tally Pond Group comprises Cambrian island-arc felsic pyroclastic rocks with intercalated mafic volcanic rocks and epiclastic volcanic and sedimentary rocks. The group hosts numerous volcanogenic massive sulphide deposits including the Duck Pond and Boundary deposits, the largest undeveloped VMS deposits in the Victoria Lake Supergroup. In the area of the Duck Pond deposit these rocks form two structurally juxtaposed sequences, the Upper block and the Mineralized block which form a structural window through an overthrust package of Ordovician sedimentary rocks. -- 1:50 000 scale mapping and geochemical studies in the Tally Pond area have resulted in new interpretations of the local geology and a redefinition of the Tally Pond belt. The Tally Pond belt is now elevated to group status, composed of four distinct rock formations comprising Cambrian island arc felsic pyroclastic rocks with intercalated mafic volcanic rocks and epiclastic volcanic and sedimentary rocks. -- The oldest rocks in the study area are arc plutonic rocks of the Crippleback Lake Quartz Monzonite which forms the original basement to theTally Pond Group. The Lake Ambrose Formation is a sequence of dominantly mafic volcanic rocks comprised of vesicular and amygdaloidal, generally pillowed, flows and mafic to andesitic tuff, agglomerate and breccia that were unconformably deposited upon the Crippleback Lake Quartz Monzonite. The mafic volcanic rocks are intercalated with felsic volcanic rocks of the Boundary Brook Formation that consist of flow banded and massive rhyolite, felsic breccia, lapilli tuffs and quartz porphyry. Both of these rock units were intruded by small stocks and dykes of quartz porphyritic rhyodacite that may be coeval with the volcanic rocks in places. -- An extensive unit of black shale melange is in tectonic contact with the volcanic rocks of the Tally Pond Group. The melange consists of volcanic and sedimentary clasts set in a matrix of fine-grained black shale. The melange unit is also in contact with a volcaniclastic and epiclastic sequence of sedimentary rocks, the Bumt Pond Formation. This unit is dominated by greywacke and conglomerate containing volcanic detritus interpreted to be derived from the adjacent and underlying volcanic rocks. Dykes, stocks, and small plutons of medium-grained gabbroic-dioritic rocks intrude all of the rocks of the Tally Pond Group. -- The youngest rocks in the study area are conglomerates and coarse-grained sandstones of the Rogerson Lake Conglomerate. The unit was deposited during the Silurian and contains volcanic clasts from the underlying volcanic sequences of the Tulks belt and Tally Pond Group. -- Geochemical analysis indicates that the volcanic rocks of the Tally Pond belt are bimodal, with geochemical affinities consistent with a volcanic arc paleotectonic environment. Mafic rocks vary from sub-alkalic basalts to basaltic-andesites, consistently exhibit an arc signature, and are depleted arc tholeiites with moderate LREE enrichments. The felsic rocks are rhyolite to rhyodacite, are variably LREE-enriched island arc rocks, and are tholeiitic in nature; however, some have transitional to slight calc-alkaline affinities. Subalkalic gabbro and diorite intrusions are transitional in nature and exhibit LREE enrichment relative to the MREE and HREE. The altered footwall felsic rocks that lie beneath the Duck Pond VMS deposit exhibit REE depletions of different magnitudes; Eu being depleted in all of the altered samples to varying degrees. The hanging wall felsic rocks do not define a fractionation trend with the mafic varieties and the two sequences are not genetically related. -- The U-Pb geochronological data for a quartz crystal tuff indicate that the volcanic succession of the Boundary Brook Formation is confined to an age of 509 Ma. These data coupled with field relationships, confirm that the main episode of felsic volcanism in the Tally Pond Group is Middle Cambrian. The 465 Ma age for the Harpoon Gabbro indicates that the mafic intrusions in the Tally Pond Group are Ordovician and therefore much older than previously thought, as they were considered to be Silurian-Devonian. This Ordovician age represents the youngest magmatism recognized in the Tally Pond Group and is comparable to similar Arenig-Llanvim ages from the Red Indian Lake area. The age of the Harpoon Gabbro temporally correlates with other Early and Mid- Ordovician arc sequences in the Exploits Subzone, namely the Wild Bight and Exploits groups. -- Chemical compositions of the alteration minerals chlorite, sericite and carbonate are quite variable in the samples analyzed from the Duck Pond deposit. Chlorites from the Duck Pond deposit contain a broad range of Fe/(Fe+Mg) ratios and atomic Si concentrations. Those from the chaotic carbonate alteration zone have a restricted Si content and have Fe/(Fe+Mg) ratios typical of high Mg chlorites. Chlorites from the feeder pipe to the Duck Pond deposit contain the highest atomic Si amounts and have intermediate Fe and Mg contents. The most ferroan-rich chlorites are those from the altered rhyolite rocks of the hanging-wall to the Duck Pond deposit. Sericites from the Duck Pond deposit are all classified as muscovite with little variation in composition. Carbonates from the Duck Pond deposit are dominantly dolomite with small compositional variations between the samples from the different alteration zones. -- Geochronological data from the Rogerson Lake Conglomerate indicate that the age spectra in these rocks is dominated by Paleozoic zircons with minor Mesoproterozoic input. A large majority (-50%) of zircons from samples of the Rogerson Lake Conglomerate have Paleozoic ages of between 420 and 550 Ma; however the majority of these grains have ages in the 490-540 Ma range. These ages correspond well with the ages of Exploits arc/backarc volcanic sequences in the Victoria Lake Supergroup that are unconformably beneath the Rogerson Lake Conglomerate. The conglomerate detritus also contains zircon populations that are Ordovician, approximately 440-480 Ma. The source of these grains is most likely the adjacent rocks of the Notre Dame arc. -- A minor quantity of zircons from Laurentian basement were identified in the Rogerson Lake Conglomerate. Neoproterozoic age groups (890, 1030 and 1250 Ma) correspond with rocks of the Grenville Orogen, while the Middle Mesoproterozoic ages (ca. 1500) are correlated with basement gneisses of the Grenville Orogen that are currently exposed in westem Newfoundland. The high proportion of Paleozoic zircons relative to Proterozoic grains is presumably the result of Middle Ordovician exhumation of the Notre Dame arc and its subsequent collision and accretion to Laurentia. -- Lead isotope data from the Tally Pond belt show a very small variation in ²⁰⁶Pb/²⁰⁴ Pb ratios and are relatively more radiogenic than other VMS deposits in Newfoundland. The lead isotope data for the Tally Pond belt define three groups, 1) a primitive group; 2) a slightly more radiogenic group; and 3) a much more radiogenic group. The relatively more radiogenic lead ratios are found in the deep sections of the Upper Duck lens and may indicate the influence of a continental lead source in the initial hydrothermal ore system, followed by introduction of more mantle-derived lead when the fluid system became better developed. The high levels of the Upper Duck lens contain less radiogenic lead, which is almost certainly derived from a mantle source. Comparison with VMS deposits elsewhere in the Dunnage Zone indicate that there are two general groups of deposits: a primitive group in the Notre Dame Subzone, and a relatively more radiogenic group represented by deposits in the Exploits Subzone. Deposits in the Notre Dame Subzone were influenced by lead that evolved from the Laurentian margin while those in the Exploits Subzone appear to have been influenced by Gondwanan continental crust. -- Sulphides for the Tally Pond belt in the area surrounding the Duck Pond and Boundary deposits are characterized by a wide range of δ³⁴S values (-17 to +13), representing sulphur from a variety of potential sources. Sulphide minerals hosted by mafic volcanic rocks from the Upper block of the Duck Pond Deposit, from the South Moose Pond zone, and from a mafic intrusion in the Upper block contain sulphur isotope values that fall within the range of igneous rocks with a mantle origin. This suggests that the sulphides in the mafic volcanic rocks were derived from reduced sulphur from a deep-seated magmatic source. -- Sulphide minerals from the Mineralized block of the Duck Pond Deposit consist of pyrite, chalcopyrite, and sphalerite and are hosted by altered rhyodacitic flows, intensely chlorite altered feeder zones, and the massive sulphide lens. The felsic ash-tuff hosted Boundary Deposit has sulphur isotope ratios (+10 to +13) that are slightly higher than those of the Duck Pond Deposit (+5 to +11). Both the Duck Pond and Boundary deposits have sulphur isotope ratios that are higher than the normal mantle value for igneous rocks and show a shift towards a seawater sulphate value. The sulphur in these deposits is most likely derived from inorganic high-temperature reduction of seawater sulphate. -- The lowest δ³⁴S values (-17 to -13) are from sediment-hosted pyrite in the Upper block of the Duck Pond Deposit, while slightly higher δ³⁴S values of-7 and -8 [per thousand] are from the North Moose Pond area, located northeast of the Duck Pond Deposit. The sulphur in these sedimentary-hosted samples are interpreted to result from biogenic reduction of seawater sulphate