I was hoping you might know! No, it's in limestone - in Castleton, albeit not a bit regularly seen. It's in Pit Top Passage, high above the Bottomless Pit in Speedwell, which is mainly phreatic, developed on a fizzling-out vein - this is in one wall of the passage, and there's a tiny remnant of a vein just above it, so that may be a clue. It's the only occurrence up there (or anywhere that I've seen anyway), and seems to have been exposed by the miners. The colours are wonderful.