VOLCANOGENIC MASSIVE SULFIDE
Volcanogenic massive sulfide (VMS) deposits appear to
have formed at ancient seafloor hydrothermal vents (Òblack smokerÓ
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. 45¼ 26Õ 28Ó North, 91¼ 07Õ 01Ó West.
Volcanogenic massive sulfide (VMS) (5.3 cm across at its widest) from the
Precambrian of Wisconsin's Flambeau Mine, USA.