EDITORIAL & OPINION — GEOLOGY 101 — Sedex massive sulphide deposits, Part 2

Sedementary exhalative deposits grade between 4% and 30% combined lead and zinc, with tonnages of up to 200 million tonnes. The giant deposits at Sullivan, in British Columbia, contained 170 million tonnes of ore with 5.5% zinc and 5.8% lead; Mt. Isa in Australia contains 125 million tonnes grading 6% zinc and 7% lead; Broken Hill in Australia contained 300 million tonnes grading 12% zinc and 13% lead; and Red Dog in Alaska has 77 million tonnes with 17.1% zinc and 5% lead.

Unlike volcanic-related massive sulphide deposits, sedex deposits contain no copper, though they do have significant amounts of lead, compared with most (but not all) Mississippi Valley-type deposits. Besides lead and zinc, sedex deposits also produce silver.

The targets of first-phase exploration are usually the large sedimentary basins in which these deposits tend to appear. The basins range in age from 300 million to 1.8 billion years. Sedex deposits generally occur in smaller, fault-bounded sub-basins within a larger basin. Follow-up targets include horizons that are the stratigraphic equivalents of known deposits, and mineralized veins and stockworks that may have acted as feeder zones.

Sedimentary fill within prospective basins would include sulphur-rich shale-argillite clastic sedimentary rocks, which are interlayered with chemical sedimentary rocks, including chert, carbonate (calcite, siderite and ankerite) and barite.

Other prospective exploration targets in the search for sedex deposits are fault-bounded sub-basins, since hydrothermal exhalations were controlled by fluid movement along these faults. Synsedimentary faults can be identified by the presence of synsedimentary fault breccias, which are composed of sedimentary fragments cemented by more sedimentary material.

Because the sulphide horizons are large and considerably more conductive and denser than the host sedimentary rocks, geophysics can often locate a deposit. Such geophysical exploration often includes airborne and ground surveys for magnetic, gravity and electromagnetic properties, as well as ground-based induced-polarization surveys.

Another initial exploration technique could be regional geochemical surveys for enhanced lead, zinc and barium in regions underlain by suitable sedimentary rocks.

Vent regions have geochemical halos in lead, zinc and silver, the values of which increase toward the vent. Therefore, if a regional geochemical survey detects enhanced concentrations of lead, zinc and silver, follow-up surveys for these metals could be used to track down a vent and, hence, possible massive sulphides.

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