DESCRIPTIVE MODEL OF SEDIMENT-HOSTED Cu

MODEL 30b

By Dennis P. Cox

APPROXIMATE SYNONYM Sandstone Cu, includes Cu-shale (Lindsey, 1982).

DESCRIPTION Stratabound, disseminated copper sulfides in reduced beds of red-bed sequences.

GENERAL REFERENCES Tourtelot and Vine (1976), Gustafson and Williams (1981).

GEOLOGICAL ENVIRONMENT

Rock Types Red-bed sequence containing green or gray shale, siltstone, and sandstone. Thinly laminated carbonate and evaporite beds. Local channel conglomerate. Some deposits in thinly laminated silty dolomite.

Textures Algal mat structures, mudcracks, crossbedding and scour-and-fill structures. Fossil wood in channels.

Age Range Middle Proterozoic and Permian and early Mesozoic. Other Phanerozoic ages possible.

Depositional Environment Epicontinental shallow-marine basin near paleo-equator. Sabkhas. High evaporation rate. Sediments highly permeable.

Tectonic Setting(s) Intracontinental rift or aulacogen--failed arm of triple junction of plate spreading. Passive continental margin. Major growth faults.

Associated Deposit Types Halite, sylvite, gypsum, anhydrite. Sandstone uranium, basalt copper, and Kipushi Cu-Pb-Zn.

DEPOSIT DESCRIPTION

Mineralogy Chalcocite and other Cu2S minerals + pyrite ± bornite ± native silver. Cu2S replacement of early fine-grained pyrite is common. Deposits may be zoned with centers of chalcocite ± bornite, rims of chalcopyrite, and peripheral galena + sphalerite. Some deposits contain carrollite and Co-pyrite and Ge minerals.

Texture/Structure Fine disseminated, stratabound, locally stratiform. Framboidal or colloform pyrite. Cu minerals replace pyrite and cluster around carbonaceous clots or fragments.

Alteration Green, white, or gray (reduced) color in red beds. Regionally metamorphosed red beds may have purple color.

Ore Controls Reducing low-pH environment such as fossil wood, algal mat. Abundant biogenic sulfur. Pyritic sediments. Petroleum in paleoaquifers. High permeability of footwall sediments is critical. Boundaries between oxidized and reduced sediments.

Weathering Surface exposures may be completely leached. Secondary chalcocite enrichment down dip is common.

Geochemical Signature Cu, Ag, Pb, Zn (Mo, V, U) (Co, Ge). Au is low. Weak radioactivity in some deposits.

EXAMPLES

Kupferschiefer, GRMY (Wedepohl, 1971)

White Pine, USMI (Brown, 1971)

Western Montana (Belt), USMT (Harrison, 1972, 1982)

Kamoto, ZIRE (Bartholome and others, 1976)