DESCRIPTIVE MODEL OF BESSHI MASSIVE SULFIDE

MODEL 24b

By Dennis P. Cox

APPROXIMATE SYNONYM Besshi type, Kieslager.

DESCRIPTION Thin, sheetlike bodies of massive to well-laminated pyrite, pyrrhotite, and chalcopyrite within thinly laminated clastic sediments and mafic tuffs.

GENERAL REFERENCES Klau and Large (1980), Fox (1984).

GEOLOGICAL ENVIRONMENT

Rock Types Clastic terrigenous sedimentary rocks and tholeiitic to andesitic tuff and breccia. Locally, black shale, oxide-facies iron formation, and red chert.

Textures Thinly laminated clastic rocks. All known examples are in strongly deformed metamorphic terrane. Rocks are quartzose and mafic schist.

Age Range Mainly Paleozoic and Mesozoic.

Depositional Environment Uncertain. Possibly deposition by submarine hot springs related to basaltic volcanism. Ores may be localized within permeable sediments and fractured volcanic rocks in anoxic marine basins.

Tectonic Setting(s) Uncertain. Possibly rifted basin in island arc or back arc. Possibly spreading ridge underlying terrigenous sediment at continental slope.

Associated Deposit Types None known.

DEPOSIT DESCRIPTION

Mineralogy Pyrite + pyrrhotite + chalcopyrite + sphalerite magnetite valleriite galena bornite tetrahedrite cobaltite cubanite stannite molybdenite. Quartz, carbonate, albite, white mica, chlorite, amphibole, and tourmaline.

Texture/Structure Fine-grained, massive to thinly laminated ore with colloform and framboidal pyrite. Breccia or stringer ore. Cross-cutting veins contain chalcopyrite, pyrite, calcite or galena, sphalerite, calcite.

Alteration Difficult to recognize because of metamorphism. Chloritization of adjacent rocks is noted in some deposits.

Ore Controls Uncertain. Deposits are thin, but laterally extensive and tend to cluster in en echelon pattern.

Weathering Gossan.

Geochemical Signature Cu, Zn, Co, Ag, Ni, Cr, Co/Ni >1.0, Au up to 4 ppm, Ag up to 60 ppm.

EXAMPLES

Besshi, JAPN (Kanehira and Tatsumi, 1970)

Motoyasu, JAPN (Yui, 1983)

Kieslager, ASTR (Derkman and Klemm, 1977)

Raul, PERU (Ripley and Ohmoto, 1977)