geology 101

Banded iron formation-hosted gold deposits, Part 2

Banded iron formation-hosted gold deposits are important in terms of Canadian and U.S. gold production, as illustrated by mines such as the Lupin and Musselwhite in Canada and the Homestake in South Dakota.

In general, gold deposits in banded iron formations (BIFs) contain from 0.1 to 100 million tonnes of ore grading between 4 and 30 grams gold per tonne. The Homestake mine, a world-class example of this deposit type, has produced over 1,180 tonnes of gold from 118 million tonnes of ore since operations began in 1876; remaining reserves at the end of 1996 were over 21.5 million tonnes of ore grading 6.72 grams gold.

Lupin has over 9 million tonnes of ore grading 10 to 11 grams gold. The gold is relatively pure, with moderate to low silver content of generally less than 6 grams. The gold ore is mined in a similar manner to that of mesothermal lode gold deposits, with emphasis on veins or sulphide-rich portions of the BIFs.

Since the veins and BIFs are frequently narrow units, mining is typically an underground operation, but there is some production from open pits. The bulk ore is crushed, then fed through a processing and refining plant akin to those in use at archetypal mesothermal lode gold operations.

As BIF-hosted gold deposits are restricted to greenstone belt terranes in Archean to Early Proterozoic shield areas, exploration would be directed towards regions such as the Superior and Slave provinces of the Canadian Shield. The main points in both variations to the genetic model for these deposits are that deformation either provided permeable pathways for the gold-bearing ore fluids along faults, or caused remobilization of pre-existing gold accumulations, essentially enriching and upgrading gold concentrations. Exploration would focus on highly deformed, structurally complicated portions of BIFs within greenstone belts, especially where regional fault-shear systems cut through.

The dominant structural style of the deformation manifested at most gold-bearing BIFs is folding; hence contorted fold zones in a BIF would also be a favorable exploration target. Though deformation is strongly developed in these deposits, metamorphic grade usually does not exceed greenschist facies.

Exploration should further zero in on portions of BIFs that are sulphide facies or on areas with sulphide alteration overprinting oxide facies BIF.

Since BIFs account for less than 5% of the area of greenstone belts, exploration would first be directed towards locating these sedimentary rocks within the greenstone belt piles. Such exploration would be aided by airborne and ground geophysical surveys over the greenstone belts, since the greatly elevated metal contents of the host rocks make them very electrically conductive and thus discernable by electromagnetic surveys.

Also, the rocks’ magnetite (plus pyrrhotite) contents make them readily detectable by magnetic surveys. Induced polarization surveys would also be very advantageous in detailed exploration for, and mapping of, these conductive host rocks. Regional geochemical surveys for iron formation and elevated concentrations of gold, iron, arsenic, bismuth

and antimony could also prove effective in exploration.

— The author is a professor of geology at Memorial University in St.

John’s, Nfld.