DESCRIPTIVE MODEL OF HOMESTAKE Au

By Byron R. Berger

Model 36b

APPROXIMATE SYNONYMS Volcanogenic gold, iron-formation hosted Au, Archean lode gold.

DESCRIPTION Stratabound to stratiform gold deposits in iron-rich chemical sediments in Archean metavolcanic terrane.

Rock Types Regionally metamorphosed mafic and felsic metavolcanic rocks, komatiites, and volcaniclastic sediments interlayered with banded iron-formation. Intruded by felsic plutonic rocks and locally by quartz porphyry, and syenite porphyry.

Age Range Mainly Archean.

Depositional Environment Controversial: submarine host-spring activity related to volcanism, or later hydrothermal activity related to intrusive rocks.

Tectonic Setting(s) Archean greenstone belts. Commonly near regional division or "break" between predominantly metavolcanic and predominantly metasedimentary rocks. Greenschist-facies metamorphism.

Associated Deposit Types Kuroko massive sulfide deposits, Algoma Fe, low-sulfide gold-quartz veins.

DEPOSIT DESCRIPTION

Mineralogy Native gold + pyrite + pyrrhotite arsenopyrite magnetite sphalerite chalcopyrite. May contain minor tetrahedrite + scheelite + wolframite + molybdenite fluorite stibnite. Realgar at Hemlo deposit. Some deposits show zoning from proximal pyrrhotite magnetite to distal arsenopyrite.

Texture/Structure Narrow thinly laminated beds, veins, or lenses, overlying stringers (stockworks).

Alteration Host rocks contain quartz + siderite and (or) ankerite + tourmaline + chlorite + magnetite in mafic volcanic terranes. Chromian mica and chlorite particularly around veins and stockworks. Banded oxide-facies iron-formation replaced by pyrite or pyrrhotite.

Ore Controls Bedded ores in Fe-rich siliceous or carbonate-rich chemical sediments with vein and stockworks in feeder zones to these sediments, often interlayered with flow rocks. Beds may be cut by concordant or sharply discordant quartz-carbonate veins with gold.

Weathering Gossans from oxide and carbonate iron-formation.

Geochemical Signature Au + Fe + As + B + Sb (+ platinum-group metals in mafic volcanic terranes). Bi, Hg, and minor Cu-Pb-Zn-Ag-Mo.

EXAMPLES

Homestake USSD (Rye and Rye, 1974)

Passagem, BRZL (Fleischer and Routhier, 1973)

Dome Mine, CNON (Fryer and others, 1979)

Agnico Eagle, CNQB (Barnett and others, 1982)

Vubachikwe, ZIMB (Fripp, 1976)