Was Rossland built on a dormant volcano?

On Episode 2 of Talk Like A Local, Tourism Rossland asked, "Was Rossland built on a dormant Volcano?" Check out the episode here:

Now.. for the answer!

300 Million Years Ago:

The Rossland geological story can best be placed some 300 million years ago. In the Pennsylvanian Period of the Paleozoic Era, the “Mount Roberts Formation,” was laid down as mud and lime sediments in a marine basin on the bottom of an ancient, semi-tropical sea.

Some of our volcanic specimens on display.

Some of our volcanic specimens on display.

Geologists theorize that about 200 million years ago, this region was subjected to “extension” - the earth’s crust being stretched. This enormous stress on the earth’s crust caused large bodies of rocks to break, also known as “block faulting.” In the Rossland area, a fault-bounded section of the sea bottom was elevated to form an “undersea tectonic high.”

190 million years ago, stratovolcanic islands and resulting sediments formed on the earth’s surface.

160 million years ago, Ancestral North American slid westward, driven by a spreading ridge in the middle of the Atlantic.

Over millions of years, mini-plates and volcanic island chains were amalgamated onto the westward moving plate. The immense forces caused parts of the accreted terranes to buckle into mountains and other parts to sink and melt.

The Elise Formation

This formation lies on top of the Archibald Formation and is dominantly volcanic. After the Archibald Formation was laid down, extension cracks in the earth’s crust opened up and allowed a rush of molten magma to reach the sea floor to feed rapidly growing volcanic islands. During its active life, it created piles of rock like the Elise about 3000 metres (9000 feet) thick on the sea floor.

These terrane collections are called “docking” events and British Columbia is made up of several of them. The docking collision caused parts of the new terrane to get buried beneath the on-rushing North American plate. Once the terranes reach a depth of 80km, they melt and magma forces its way up to form plutons and larger bodies of intrusive rock. It is hypothesized that these plutons and large intrusive rocks were metal rich, and some of the fluids at great depth and temperature formed the gold-copper veins in the intrusive “Rossland Monzonite” and the volcanic Elise Formation host rocks.

Over the next 150 million years, a combination of tectonic uplift and erosion has exposed these mineralized rocks on the surface.

Our mineral-rich rock is due to volcanic rock, but we are NOT located within a dormant volcano/caldera!

Gold! Silver! Molybdenum?

Molybdenum is a silvery white, very hard, transition metal. It was often confused with graphite and lead ore but has one of the highest melting points of all pure elements. The principal molybdenum deposits on the western and southern slopes of Red Mountain were known from the early days of the Rossland camp when gold-copper ore was being mined nearby. It was present in the gold-copper mineralisation in the War Eagle and the Centre Star mines but at that time had no commercial value. When it did come into demand, ti was mined from 1966 to 1972 from open pits on the west and southwest slopes of Red Mountain mainly on the Coxey, Golden Queen, Novelty, Giant, St. Elmo, and Mountain View mines.

The mineralisation of the Rossland Mines is hosted by Early Jurassic-aged rocks of the Rossland Volcanic Group, part of the Quesnellia Terrance. The Rossland Monzonite, a small body of intrusive rock (a “pluton”) aged-dated at 167 million years, is associated with the gold-copper mineralisation of the Rossland Mines and may have been partly responsible for its emplacement. The molybdenum mineralisation mined separately in the late 1960s is about 4 million years younger, possibly associated with a younger monzonite, the Rainy Day Pluton. Both the Rossland Monzonite and the Rainy Day Pluton, as well as many other instrucive events, were generated by the massic forces of the tecctonic collision when the Quesnellia Terrane “docked” onto the western edge of Ancestral North America.