Tuffaceous exhalites as exploration guides for volcanogenic massive sulphide deposits

Abstract

Abstract Ancient hydrothermal activity on the sea floor not only resulted in economic accumulations of copper, lead and zinc sulphides but, in some volcanogenic massive sulphide (v.m.s.) districts, it also produced an associated, distinctive, thin, iron- and silica-rich sediment with geochemical and mineralogical anomalies which can provide stratigraphically-controlled lithogeochemical exploration guides. The distinctive rock type is referred to in Japan, as tetsusekiei (literally iron-quartz) and, in North America, as ferruginous chert, graphitic chert, cherty tuff, tuffite, sulphide iron formation, etc. The term we prefer is “tuffaceous exhalite” which denotes a rock consisting of a mixture of two very different components: (1) tuff which is commonly chemically and texturally indistinguishable from that of the indigenous environment; and (2) chemical contributions from the exhalative plume in the form primarily of silica, iron (oxide and/or sulphide), sulphur and various trace elements, and graphite or kerogen of uncertain origin. Of the two components, the latter is expected to reflect best any chemical or isotopic anomalies in sea water adjacent to metalliferous hydrothermal discharge, and therefore to provide the more useful lithogeochemical information in the search for v.m.s. deposits. On this basis, we have carefully separated the two components, either physically or statistically, in our analyses in order to avoid the problem caused by their variable ratios from sample to sample, which might mask real trends in the distribution of trace elements. In some cases, however, we have found that chemical interaction between the two components during penecontemporaneous alteration or subsequent metamorphism precluded the clean separation we were seeking. We have studied the following areas of tuffaceous exhalites of contrasting age and metamorphism as part of an ongoing research program on lithogeochemical exploration for v.m.s. deposits: 1. (1) Sturgeon and Savant Lakes, NW Ontario, Archean age, low grade metamorphism; 2. (2) the iron formation hosting Willroy no. 4 orebody, Manitouwadge district, Ontario, Archean age, high-grade metamorphism; 3. (3) the Main Contact Tuff, Millenbach mine, Noranda, Quebec, Archean age, low-grade metamorphism; 4. (4) Austin Brook iron formation, Bathurst district, New Brunswick, Ordovician age, moderate metamorphism; and 5. (5) tetsusekiei of Fukazawa mine, Hokuroku district, Japan, Miocene age (13Ma), not metamorphosed.