Ancient terranes - modern analogs for mining development
Modern analogs could help develop mines in some of the world’s oldest geological terranes. Michael Forrest finds out more from Glen Kuntz, CEO of Canadian exploration company Mega Precious Metals Inc.
The classification of mineral deposits is a useful method for describing the origin of the ore and its host environment. Most classifications are related to the method of mineral formation, which in turn defines how that fits, along with the host rock, in the global plate tectonic framework. Each classification has implications for the type and size of mineral resource that might be expected in each setting, which has implications for exploration technique and, ultimately, the mining technology used.
Classification is based on the minerals found with the ore body, its structural setting, and the local and regional geology. Its aim is to draw parallels with other known ore bodies and mines so that the entire process, from exploration through to development and raising finance, has a framework with purpose and direction. For example, the porphyry copper deposits on the western cordilleras of the Americas are directly related to the subduction of oceanic plates beneath the continents, a large-scale event leading to large-scale deposits. Other mineralisation may be related to sedimentary controls, such as the lead–zinc deposits of the Mississippi Valley, USA.
However, devising a classification or model relies on the assumption that present-day geological processes, such as subduction, occurred in the past. Tracing geological events throughout the Phanerozoic (current geological eon, covering around 542 million years) has the benefit of a fossil record to establish time–process relationships. However, the Palaeozoic and Archean (more than 595Ma) do not have this advantage and it is recognised that the earth’s thermal regime was different to that of the Phanerozoic. Past research suggested that the plate tectonic regime might not extend into the early part of earth’s history, an assumption based on the driving force of temperature gradient between the mantle and crust being greater than it is today, as the earth has been cooling since its planetary formation. More recent advances in geological reconstructions indicate that plate tectonics have indeed been operating in deep geological time, but their size and formation or destruction may have been on different scales, reflecting higher heat flow.
‘Exploring in Archean terranes has challenges that relate to interpretation of the style of mineralisation that involves a disciplined systematic exploration approach to outline mineralisation,’ says Glen Kuntz, CEO of exploration company Mega Precious Metals Inc, in Ontario, Canada. By understanding the analogues of these ancient systems, new light can be shed on their origins and exploration potential.
One of the most extensive areas of Archean shield can be found in northern Canada and over the past two decades, significant, gold, diamond and base-metal mineralisation have been discovered. The region can be divided into a number of cratons, divided by ductile and structural shear zones, within which a wide variety of mineralisation can be found. Kuntz explains that his company has a number of projects in northern Canada, from the Monument Bay gold property in the northern Superior craton, to the more westerly Blue Caribou copper deposit in Nunavut within the Slave Lake Craton, some 1,700km distant. ‘Although both of these deposits are hosted in rocks dated around 3,000 million years (3.0Ga) and are components of the Canadian shield, their origin is very different and that has implications for exploration,’ he says.
The company’s objective is to look for mineralised corridors and at Monument Bay, their most advanced project, they believe they have found one. It spans the Ontario–Manitoba border and is hosted in the Stull Lake greenstone belt of the northern Superior craton. Gold occurrences are hosted within sedimentary and volcanic rocks that are enveloped in granitoid rocks and associated with three east– west trending shear zones, a maximum of 10km apart and extending over 30km. Of particular note is the Twin Lakes deposit, which has a total resource of 2.78 million ounces of gold (all categories measured, indicated and inferred). Although lode gold deposits are well known through the Archean, the Monument Bay deposits have a number of unusual characteristics that have analogs across the world. Three types of mineralisation are found within the Twin Lakes deposit, predominantly hosted in quartz–feldspar–porphyry host rocks, locally mineralised smoky quartz veins, quartz–carbonate–tourmaline veins, and quartz–carbonate–albite scheelite (tungsten carbonate) veins, which form the majority of the gold resources.
Finding a vein
Although scheelite is not uncommon in vein lode gold deposits, the scheelite at Twin Lakes appears to be the controlling influence of the mineralisation. In fact, the gold grade can be estimated by the scheelite content, while tungsten in soils is an excellent gold indicator element. There are a number of Archean gold scheelite deposits worldwide, including the Hollinger and Dome mines at Timmins, Ontario, and Mt Charlotte in Western Australia. Phanerozoic examples include Macrae’s mine New Zealand. All are in similar structural, lithological and geochemical settings.
‘It is not only vein-lode type mineralisation that can be found across the time boundary of the Archean and Phaneroic,’ says Kuntz, whose company has been exploring in the Nunavut Province at the Blue Caribou property Kitikmeot Region, Nunavut, some 450km northeast from Yellowknife, Northwest Territories. The underlying geology is the 3Ga Slave craton part of the Canadian Shield, a complex of granitoid, volcanic and sedimentary sequences. The Slave craton has seen intense exploration activity that resulted in the development of a number of diamond mines in the 1990s, and latterly of massive sulphide and gold deposits. A defining feature is a thick lithospheric keel reaching the diamond stability field at the same time as charting extensive rifting, compressional deformation, magmatism and metamorphism. ‘At the Blue Caribou, the mineralisation can be described as high-grade quartz brecciated vein with elevated sulphide values,’ says Kuntz. These include values of up to 20.5% copper, 780g/t molybdenum, 1.57g/t gold and 355g/t silver. Mega Precious Minerals Inc has completed sufficient drilling to satisfy an inferred estimate of 2.78Mt of more than 72,000t contained copper, 1,575t molybdenum, 2.61Moz silver and 16,500oz gold, over a limited area and depth using a 1% copper cut-off grade. There is a strong correlation between silver and copper values but a less robust correlation with gold and copper, and a poor copper–molybdenum association.
The intensive exploration of the Canadian shield over the past decades has resulted in greater understanding of ancient geological terranes, while introducing a few geological enigmas. The occurrence of diamonds requires deep, strong cratons, yet parts of these cratons host deposits classified with more active geology. According to Kuntz, the mineral assemblage at Blue Caribou has a distinct similarity to porphyry copper–molybdenum deposits that are found in much later deposits – notably in Andean South America – with metal ratios of copper much greater than gold, and silver much greater than gold. The most striking difference, however, is the presence of structurally controlled large quartz veins instead of a pervasive quartz sulphide stock work.
‘The correlation of styles of mineralisation from more modern to ancient geological terranes can be seen at Blue Caribou and Monument Bay, confirming similar processes into deep geological time,’ says Kuntz. ‘However, given the enormity of time, we still have some way to go to a complete understanding of these mineralising events.’