The late precambrian is like a puzzle. There are difficulties in determining ages of things and sequences of events due to the fact that without fossils, its impossible in most cases to do radiocarbon dating. There are, however, three distinct ages of rocks. The oldest are quartz sands, which suggest a widespread sea. Then the volcanic rocks, the time period during which escaping gasses formed bubbles in which the now highly desirable Minnesota agates and thomsonites were formed. After that came a period of erosion and sedimentary deposition of sandstones. The area known as the Duluth Complex (because it is so complex) has many colorful minerals that sound like they would be of interest to the collector. I don't know if there are any collecting areas there, though.