An ancient mountain-building event
The La Ronge Greenstone Belt is the geological remnant of a violent, formative period in Earth's history. It is a major feature within the Trans-Hudson Orogen, a vast mountain belt created between 1.9 and 1.8 billion years ago when ancient continents collided. This continental collision, which helped form the core of North America, was similar in scale to the processes forming the Himalayas today. The La Ronge belt itself is a Paleoproterozoic volcanic island arc, a chain of volcanoes that formed in an ancient ocean called the Manikewan Ocean.
The rocks that make up the belt were once submarine lava flows, volcanic sediments and magma chambers. They range from basaltic to rhyolitic in composition. During the Trans-Hudson orogeny, these volcanic rocks were crushed, folded, and cooked at temperatures reaching the middle amphibolite facies of metamorphism. This intense heat and pressure transformed the original volcanic rocks, creating new green-colored minerals like chlorite and epidote, which give the "greenstone" belt its name. The entire process involved the opening of an ocean basin, the formation of volcanic arcs through subduction, and the final continental collision in a Wilson Cycle.
Gold from the ocean floor
The gold deposits of the La Ronge belt come from its volcanic past. Many of the gold occurrences are volcanogenic massive sulfide (VMS) deposits. These formed on the seafloor of the Manikewan Ocean at hydrothermal vents, similar to the "black smokers" found in modern oceans. Super-heated water, rich in dissolved metals, erupted from cracks in the oceanic crust and deposited minerals like pyrite, chalcopyrite, sphalerite, and gold as it mixed with the cold seawater.
Later, during the continental collision around 1.8 billion years ago, these mineral deposits were deformed and remobilized. Hot fluids flowed through large-scale cracks and shear zones, concentrating the gold into the high-grade quartz veins that are mined today. Prominent past and present mining operations, including the Seabee, Santoy, and Star Lake mines, target these structurally controlled gold deposits. The gold is often found alongside other metals such as copper, zinc, silver, and molybdenum. Geologists prospecting in the region look for these ancient shear zones, which can be traced for kilometers, to find new deposits.