English: Gossan is a porous rock rich in iron hydroxy-oxide minerals. It forms by oxidative weathering of various sulfide deposits - in this case, a volcanogenic massive sulfide (VMS) deposit in Wisconsin.

VMS deposits appear to have formed at ancient seafloor hydrothermal vents (“black smoker” environments) in or near mid-ocean ridges. These deposits principally contain various sulfide minerals and can be economically significant.

The gossan seen here is decomposed from sulfide-rich rocks of Wisconsin's Flambeau VMS Deposit. Unweathered rocks in the area have pyrite, chalcopyrite, chalcocite, bornite, and other sulfides - many of these are copper ores. The Flambeau Deposit was mined out for its copper in the 1990s. The rocks 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 to 1.86 billion years ago).

The small metallic orange-colored patches and tiny specks in this sample are native copper, so the rock can be called a cupriferous gossan.

Locality: Flambeau Mine, just south of the town of Ladysmith, central Rusk County, northwestern Wisconsin, USA (45º 26’ 28” North latitude, 91º 07’ 01” West longitude)

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See info. at:

en.wikipedia.org/wiki/Gossan