For this issue of ‘The Deeper Feature’ Dr. Sam Rice tours us through the gold geology of Newfoundland and why it has become such a hotspot for exploration and mining in 2018.
The island of Newfoundland is situated at the north-eastern end of the Silurian Appalachian mountain belt which, prior to the opening of the North Atlantic Ocean at around 65 Ma was continuous with the Caledonides in Europe. The island straddles the suture zone of the Iapetus Ocean which was subducted prior to collision of the Avalonian microcontinent with Laurentia in Silurian times, around 440 million years ago. The island’s geology is divided into several zones that represent the major components of this ancient continental collision zone.
In the northwest of Newfoundland, the rocks belong to the Humber Zone and represent the ancient continental margin of Laurentia. The rocks of Central Newfoundland belong to the Dunnage Zone and comprise ophiolites, volcanic arcs and subduction-accretion complexes; the vestiges of the Iapetus Ocean. These rocks structurally overlie the Cambrian-Ordovician sedimentary rocks of the Gander Zone, situated to the south-east, that were deposited on the eastern margin of the Iapetus Ocean. To the southeast of the Gander Zone lie Proterozoic Gondwanan basement rocks of the Avalon Zone.
These terranes were brought together around 440 Ma by accretion and collision during the Appalachian-Caledonian orogeny and were subsequently deformed by sinistral transpression and intruded by Devonian calc-alkaline mafic to granitic zoned plutons.
Avalon Zone hosts gold mineralization that is Neoproterozoic in age and, therefore, pre-dates the Appalachian tectonic events. These are well-preserved high- and low-sulphidation epithermal systems that are amongst the oldest in the world. The best known of these deposits is the Hope Brook Au-Cu mine owned by First Mining; estimated to contain 954 000 oz gold. Between 1987 and 1997 Hope Brook produced 752 163 ounces of gold. The Hope Brook deposit is hosted by a Late Proterozoic volcanic-sedimentary succession intruded by altered quartz feldspar porphyry dikes and sills and is similar to other major deposits of the Appalachian Belt, for example, the Haile gold deposit in South Carolina.
Two stages of Neoproterozoic gold mineralization are recognized in the Avalon Zone; an earlier (635-620 Ma) phase of high and low sulfidation epithermal and Au-Cu porphyry mineralization; and a younger (590-560 Ma) high sulfidation Au-Cu event. The gold mineralization is the result of volcanism during cycles of rifting, subduction and orogeny long before the early Palaeozoic opening and closing of the Iapetus Ocean.
Most gold deposits in Newfoundland, however, are found within the Lower Palaeozoic rocks of the Dunnage Zone and are of Early Palaeozoic age, most probably Early Devonian. The Dunnage Zone comprises a deformed assemblage of ophiolitic, arc and back-arc rocks plus volcaniclastic and epiclastic sedimentary rocks which represent Early to Middle Palaeozoic oceanic lithosphere of Iapetus. The Dunnage Zone is divided into the Notre Dame and Exploits sub-zones. These are divided by a fault zone called the Red Indian Line, interpreted to mark the position of the ancient Iapetus suture. The Notre Dame Subzone to the north and the Exploits Subzone to the south of the suture formed on opposite sides of the Iapetus Ocean.
The Notre Dame Subzone includes the Fleur De Lys Supergroup, broadly equivalent to the Dalradian rocks in Scotland and Ireland. Gold deposits in the Notre Dame sub-zone include Point Rousse, Green Bay and Cape Ray. These are structurally-controlled orogenic type deposits. The Point Rousse Project, operated by Anaconda Mining is situated within the Betts Cove ophiolite and its volcano-sedimentary cover sequence. Anaconda’s Pine Cove open pit mine has produced in excess of 118,028 ounces of gold since 2010. The Green Bay Project, owned by Maritime Resources, includes the Hammerdown and Orion deposits. Between 200 and 2004 Hammerdown produced 143 000 ounces of gold. The property is estimated to contain in excess of 400 000 ounces of gold in the Measured and Indicated categories and in excess of 600,000 ounces in the Inferred category at a 3 g/t cut-off grade. At Green Bay auriferous sulphide veins are focussed along the contacts of felsic porphyry dykes within deformed steeply dipping metavolcanic and metavolcaniclastic sedimentary rocks. Gold also occurs at Cape Ray within boudinaged quartz veins in a graphitic shear zone.
There are numerous gold occurrences and prospects in the Lower Palaeozoic units south of the Red Indian Line in the Exploits Subzone. These include Moosehead, Golden Promise and Clarks Brook.
Exploits Subzone exhibits two different sedimentary successions separated by the NE-SW trending Dog Bay Line; a long-lived, reactivated basin-bounding fault. East of the Dog Bay Line is the Davidsville Group, composed of Cambro-Ordovician siliciclastic and volcaniclastic rocks which unconformably overlie ophiolitic rocks of the Gander River Complex ophiolitic rocks and pass stratigraphically upwards into Late Ordovician deep-marine graphitic and sulphidic shale and chert.
The Davidsville Group is intruded by the Mount Peyton Plutonic Complex which comprises an outer and older mafic rim dated at 424 Ma and a younger granitic core. The age, composition and setting of the intrusion is equivalent to the zoned calc-alkaline plutons in the Caledonides in Britain and Ireland (Rice et al., 2018, Ore Geology Reviews). By analogy with Scottish mineralization the Mount Peyton Pluton is likely to have contributed heat and fluids and possibly sulphur and metals necessary for gold mobilization and concentration.
West of the Dog Bay Line, Cambro-Ordovician calc-alkaline volcanic rocks of the Victoria Lake Supergroup pass upwards into deep marine sandstone and shale followed by Late Ordovician black graphitic and sulphidic shales. These rocks are conformably overlain by deep to shallow marine conglomerates and sandstones of the Badger Group. These pass gradationally upwards into Silurian terrigenous arkosic sandstones of the Botwood Group.
The Moosehead prospect, owned by Sokomon Iron, is situated within the Silurian volcanic rocks. Drill intersections up to 170.3 g/t Au over 1.53 metres plus quartz float with grades up to 442 g/t Au are recorded. Furthermore, 278 g/t Au was intercepted over 0.50 metres at a depth of 257 m. Several other companies, such as Tasca Resources, have followed this up by acquiring ground along this trend.
Gold mineralization at Moosehead is hosted by a series of sigmoidal veins related to E-W trending shear zones within ankerite-altered green-grey sandstones and mudstones and is, therefore, typical of Phanerozoic ‘slate-belt’ type orogenic gold. However, textures and mineralogy are indicative of low-sulphidation epithermal conditions at shallow depths (<1 km) and may be comparable to the Southern Uplands in the Scottish Caledonides.
At the Golden Promise property, gold-bearing veins are recorded over a wide area. The central vein system, the Jaclyn Main Zone, is hosted within Lower Ordovician metasedimentary rocks of the Victoria Lake Supergroup and overlying Caradocian shale. Historic drill intercepts in the Jaclyn Main Zone reach 93.71 g/t Au over 1.25 m true thickness.
The prospective veins are predominantly hosted by gently NW dipping interbedded greywacke sandstones and mudstones. Veins are subvertical and strike 070° to 090° and are estimated to reach up to 4 m true thickness. The veins contain visible gold grains up to 3mm diameter and are composed of vuggy milky white quartz indicating an extensional structural setting. The host rocks exhibit pale green silica-sericite-carbonate alteration typical of turbidite-hosted (slate belt) orogenic gold.
Wilding Lake and Valentine Lake, within the Exploits Subzone are associated with the structural contact between the Dunnage Zone and the Gander Zone. At Wilding Lake gold occurs in structurally-controlled zones of disseminated sulphide and stringers within altered feldspar porphyry intrusions. The gold mineralisation is associated with red ochre and altered feldspar porphyry. Gold at Wilding Lake also occurs along the faulted contact between gabbro and conglomerate units.
Most of the occurrences exhibit similarities with typical orogenic type lode gold in turbidite/slate belts; they formed relatively deep (around 5 km), are structurally controlled and associated with shear-zones and folds. They exhibit wallrock alteration with disseminated pyrite-arsenopyrite mineralization. However, some deposits appear to have formed at very shallow depths, <1 km, in a low-sulphidation epithermal environment.
These deposits exhibit fine-grained silicification, massive and banded chalcedonic veining, a range of veining textures and cockade breccias e.g. Moosehead and Beaver Brook. Silurian fossils within brecciated host rocks indicate that mineralisation in the Dunnage Zone is Silurian or younger in age and it seems likely that gold mineralisation was closely related to late orogenic, post-subduction magmatism.
It has been suggested that the Silurian rocks of the Exploits Subzone, e.g. the Botwood Formation, have the potential to host Carlin-type gold. This model would require depths intermediate between epithermal and orogenic types described above and is supported by the occurrence of jasperoid due to partial dissolution and silicification of semi-calcareous rocks of the Indian Islands Group. The Carlin model is also supported meteoric fluid inclusions and a pyrite trace element suite that includes Au-Sb-As-Pb-Ba.
Newfoundland has many promising prospects deserving of further exploration. As a whole, the island has great potential for high grade gold deposits because it exhibits all four components of a mineralization system. The system is likely to have been metasomatically fertilized during the long history of Proterozoic and Early Palaeozoic subduction. Devonian post-subduction magmatism combined with transpression-transtension provides a mechanism and a favourable lithospheric architecture for the effective transfer of heat energy heat and mass from depth. Furthermore, a relatively low degree of crustal thickening during Appalachian orogenesis has resulted in preservation of shallow-formed epithermal deposits.
Understanding Newfoundland’s ore geology in the context of soft continental collision and post-subduction magmatism could therefore also benefit exploration and discovery in other Phanerozoic orogens globally.
Dr. Samuel Rice is an exploration geologist and researcher in tectonics with 10 years of experience in the resources sector and expertise in structural geology, tectonics, mineral deposits and exploration. Sam holds a PhD from the University of Edinburgh on the development of the Tethyan suture zone in Turkey. Samuel explored for oil and gas and researched the Mesozoic and Cenozoic tectonic development of the Black Sea Basin and the Caucasus mountains at CASP, Cambridge as well as spending time in Mali, Ghana, Zimbabwe, Mozambique and the British Isles with gold exploration companies.
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