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The Main Contact Tuff, in the vicinity of Millenbach mine, Noranda, Quebec, is an extensive, although discontinuous, ore-related volcanic exhalative metalliferous sediment, or "tuffaceous exhalite," of Archean age. It was formed by the variable contribution of two constituents: (1) exhalite (chemical) composed mainly of pyrite, quartz, sphalerite, pyrrhotite, and chalcopyrite and (2) tuff (clastic) composed of quartz, chlorite, and sericite.Tuffaceous exhalites such as the Main Contact Tuff and the similar tetsusekiei of the Japanese Kuroko deposits (Miocene age) indicate a fossil hydrothermal system that may or may not have produced economic concentrations of metallic sulfides. The Main Contact Tuff displays cryptic variations, which provide exploration guides at different scales: (1) the Fe/[Fe + Mg] ratios of chlorite decrease from 0.63 to 0.32 over a distance of 0.3 km approaching ore from the north, and from 0.72 to 0.32 over 1.5 km from the south; (2) the FeO/[FeO + MgO] (sulfide-free) ratios of whole-rock specimens decrease from about 0.8 to 0.3 approaching the ore; (3) ilmenite is replaced in the ore zone by rutile and (or) sphene; and (4) the most manganiferous ilmenite is found close to ore. On the other hand, trace elements of exhalative origin (e.g., Ag, Co) in the Main Contact Tuff appear not to provide useful vectors towards ore. They are concentrated relative to normal pelagic rocks, but their distributions with respect to ore are complicated by multiple input sources and postexhalative redistributions of elements.

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