Yukon consists of valley-filling basalt lava flows and cinder cones. Ne Ch'e Ddhawa, a cinder cone away from the connection of the Yukon and Pelly rivers formed between 0.8 and one million years ago when this area lied beneath the vast Cordilleran Ice Sheet. The youngest volcano, Volcano Mountain just north of the junction of the Yukon and Pelly rivers, formed in past 10,000 years (Holocene), producing lava flows that remain unvegetated and appear to be only a few hundred years old. However, dating of sediments in a lake impounded by the lava flows indicated that the youngest lava flows could not be younger than mid-Holocene and could be early Holocene or older. Therefore, the most recent activity in the Fort Selkirk volcanic field is unknown. The lava flows from Volcano Mountain are unusual because they originate much deeper in the Earth's mantle than the more common basaltic lava flows found throughout the Yukon and are very uncommon in the geological record. This lava, known as olivine nephelinite, is also unusual because it contains small, angular to rounded fragments of rock called nodules. Economic geology Greenstone belts The predominantly volcanic Archean and Proterozoic greenstone belts throughout Canada are important for estimating Canada's mineral potential. Consequently, geologists study greenstone belts to understand the volcanoes and the environment in which they erupted, and to provide a working model for mineral exploration. The 1,904‑ to 1,864‑million-year-old Flin Flon greenstone belt of central Manitoba and east-central Saskatchewan is one of the largest Paleoproterozoic age volcanogenic massive sulfide ore deposits in the world, containing 27 copper-zinc-(gold) deposits from which more than 183 million tonnes of sulfide ore have been mined. The 2,575‑million-year-old Yellowknife greenstone belt in the Northwest Territories is the host for world-class gold deposits with total production of 15 million ounces of gold. In the Archean Hope Bay greenstone belt of western Nunavut, three large gold deposits have been known as Doris, Boston and Madrid, while the 2,677‑million-year-old Abitibi greenstone belt of Ontario and Quebec is the second most prolific gold producing area on Earth; the most prolific gold producing area is the Witwatersrand hill range in South Africa. Intrusions Other magmatic formations, such as dike swarms and sills, are known to contain base and precious metal deposits. The 2,500- to 2,450-million-year-old Matachewan dike swarm of eastern Ontario hosts the 2,491- to 2,475-million-year-old long East Bull Lake Intrusion and associated intrusions. The 2,217-