Besshi-type volcanogenic sulphide deposits a review

CIM Bulletin, Vol. 77, No. 864, 1984

U.S. FOX, Senior Geologist, Teck Explorations Limited, Toronto, Ontario

Besshi-type Cu-Zn (-Ag-Au±Co) deposits are tabular, stratiform volcanogenic sulphide bodies which are usually associated with mafic volcanic rocks of oceanic or intra-plate geochemistry, but which can also be found locally with ultra-mafic volcanics or subvo/canic mafic intrusions. A thick sequence of continental/y-derived clastic sediment usually encloses both the mafic volcanics and the ore. Besshi-type deposits form subaqueously, in epicratonic rifting environments, more-or-less proximally, and as a result of hydrother-mal convection in their subjacent mafic volcanic - clastic sedimentary piles. They belong to a rift-related meta/logenic spectrum that includes sediment-hosted and Cyprus-type base metal deposits, and may differ genetically from these mainly with respect to rate and degree of crustal extension. Modern analogues to these deposits are found in the submarine volcanogenic sinters of the Gulf of California rift system. The associated lithologies and tectonic setting of Besshi-type deposits clearly distinguish them from Kuroko-type ores. This difference also seems to be reflected in their lead isotopic character. For example, leads from Besshi-type deposits in the Sanbagawa belt of Japan and from the Blue Ridge belt of southeastern U.S.A. lie on extended anomalous lead lines and have low slopes in 207pb/204 pb - 206Pb/204 pb space (Sato and Sasaki, 1980; LeHuray, 1982), wheras those from most Kuroko-type mining districts exhibit only narrow isotopic ranges and lie near or on the orogene growth curve (e.g., Franklin et al., 1981). Moreover, where Besshi- and Kuroko-type deposits are found in the same orogene, the former often occur earlier in the tectonic cycle than the latter (e.g., Japan, the Norwegian Caledonides, the southern Appalachians and south-central Finland). Besshi-type deposits are not confined to any particular part of the world, or to any geological epoch. Moreover, in many mineralized belts, Besshi-type deposits are the predominant volcanogenic ore type. On a world-wide scale, deposits of this type therefore constitute an important base metal resource.

Keywords: Mineral exploration, Besshi-type deposits, Massive sulphides, Sanbagawa belt, Blue Ridge belt, Trondheim region, Outokumpu region, Mafic volcanics, Continentally-derived sediment, Lead isotopes, Epicratonic rift environment, Guaymas basin