Besshi-type VMS deposits (Part I)

Volcanogenic massive sulphide (VMS) deposits of the Besshi type are named after deposits on the southern Japanese island of Shikoku. These deposits generally contain lower base metal concentrations than other volcanogenic massive sulphides and are viewed as low-grade copper deposits; the zinc grade is typically too low to be mined economically. These deposits do, however, often contain cobalt, which can be of economic grade, and some have economically recoverable precious metals. Metal zoning is usually poorly developed in Besshi deposits.

The sulphide mineralization consists predominantly of iron sulphides (pyrite and/or pyrrhotite), with lesser chalcopyrite; sphalerite may or may not be present. Unlike other VMS styles, however, Besshi sulphides can contain a highly varied and complex mineralogy, including magnetite, arsenopyrite, galena, bornite, tetrahedrite-tennantite, cobaltite, stannite and molybdenite. Quartz, carbonate, albite, sericite, chlorite, amphibole and tourmaline can be found as gangue minerals in the deposits. Unlike the Kuroko-type VMS deposits, there is typically no barite present in Besshi deposits, though other chemical sedimentary rocks (carbonates or iron-oxide beds) may be associated.

The Besshi massive sulphide deposits are thin, stratiform and tabular. The massive sulphides may be finely or coarsely layered, or massive; the sulphide lenses are usually just several metres thick but can extend for several kilometres. The deposits are typically deformed and metamorphosed, and highly deformed ones are almost linear in shape. Deformation can obscure the deposit’s feeder systems, but one deposit widely classified as a Besshi type, namely the Windy Craggy deposit in British Columbia, has a well-defined feeder/stockwork of extensively chlorite-quartz altered wall rock cut by sulphide veins. Crosscutting the massive sulphide horizons, there may be veins of recrystallized pyrite and/or chalcopyrite, opened and filled when the horizons were deformed.

A chlorite alteration halo has developed in the country rock surrounding the sulphide horizons, which may be a relic of pre-deformational alteration. Other alteration minerals that may show up in the host rocks of Besshi deposits are quartz, carbonate, pyrite, sericite and graphite.

The sulphide horizons generally occur in thick sequences of marine sedimentary rocks, ranging from black shale to arkose to greywacke. The clastic hosts themselves are generally finely laminated sedimentary rocks that resemble turbidites (sediments deposited on the ocean floor through seafloor slumping). There can also be volcanic tuffaceous interlayers. The clastic sediments are typically graphitic.

There are usually no felsic volcanic rocks present, though thin layers of basalt are often present in the sequence of sediments. The basalts have a tholeiitic composition (with pyroxenes, plagioclase feldspar and olivine, which have a high iron content relative to their sodium and potassium content).

The host rocks to these deposits can be metamorphosed such that the sedimentary rocks have become schists, quartzites, metacherts and/or pelites, and the basalts can be amphibolites.

Their host rocks, mineralogy and chemistry place Besshi deposits along a continuum between the copper-zinc massive sulphides and the sedimentary-exhalative deposits. Although Besshi deposits are modeled as VMS-types, with the massive sulphides forming from the exhalation of hydrothermal fluids on to the seafloor, there is some debate as to the mechanism of deposition. The deposits have been characterized as the products of: seafloor accumulation in the form of sulphide chimneys and the like (that is, “black smokers”); hydrothermal brine pools that formed on the seafloor after exhalation; and replacement of clastic sedimentary rocks by sulphur-bearing hydrothermal fluids that flowed upwards in a convection cell system but did not actually exhale on the seafloor.

— The author is a professor of geology at Memorial University in St. John’s, Nfld.