VOLCANOGENIC MASSIVE SULFIDE
Volcanogenic massive sulfide (VMS) deposits appear to have formed at ancient seafloor hydrothermal vents (“black smoker” environments) in or near mid-ocean ridges (see modern example; see another example). VMS deposits principally contain various sulfide minerals, and can be economically significant.
The rock shown below is from the Flambeau VMS Deposit in Wisconsin, USA. It is dominated by pyrite and some chalcopyrite, but may contain small amounts of chalcocite, bornite, and other sulfides. It’s rich enough in copper and copper-iron sulfides to qualify as a copper ore. This deposit was mined out for its copper in the 1990s. The ores here also have a little finely-disseminated gold.
The Flambeau VMS Deposit is Paleoproterozoic in age. The sulfides are hosted in metamorphosed volcanic rocks (metadacites & metarhyodacites) of the Pembine-Wausau Terrane. This terrane was a volcanic island arc (analagous to modern-day Japan or the Aleutian Islands) that collided with the southern margin of the Superior Craton during the Penokean Orogeny (1.85-1.86 billion years ago).
Locality: Flambeau Mine, just south of Ladysmith, central Rusk County, northwestern Wisconsin, USA. 45º 26’ 28” North, 91º 07’ 01” West.
Volcanogenic massive sulfide (VMS) (5.3 cm across at its widest) from the Precambrian of Wisconsin’s Flambeau Mine, USA.