#mineralization

Ocean crust is made by volcanic activity, which pulls elements out of the mantle that are otherwise hard to reach and concentrates them. Rifting generates volcanic rocks and also brings heat near the surface and that heat can drive convection. If a little water finds its way down into the core of a rift zone, it will be heated and want to rise back up into the ocean due to its lower density. As this water rises upwards and is expelled, new cold water sinks down to take its place, creating a convection cell. As the hot water moves through the rocks, it dissolves some of the soluble portion of the rock, including elements like copper and nickel, along with sulfur. When that hot water reaches the ocean, it cools off and also interacts with any dissolved oxygen in the water. Those dissolved elements – the copper, the sulfur, and so on – they precipitate out rapidly, leaving a black trail in the water and often forming vertical columns called black smokers. If an area hosts hydrothermal vents for long enough, those elements build up into a massive sulfide deposit; some of them are even found in bowl shaped areas or depressions where the minerals can accumulate easily. On occasion those sulfide deposits get caught up in mountain building and pushed up onto a continent where we can find them. This is a section of a drill core at the Potter Mine in Ontario, Canada, which taps 2.7-billion-year-old rocks. These rocks were once an ancient seafloor, but they have been folded and twisted and metamorphosed, bringing them up to a level where they can be mined. The shiny material in this drill core is the mineral chalcopyrite, a copper-iron-sulfide mineral and the main target of the mine. The dark layers are also sulfides – these are mostly pyrrhotite, an iron-sulfide mineral, with a bit of sphalerite mixed in (zinc sulfide). These sulfides once formed in flat layers, but they have been folded and now sit almost vertical so that this core slices right along the layering. -JBB Image credit: James St. John https://flic.kr/p/2eJQ49o