RESEARCH CHIBOUGAMAU NUGGETS

You may be in for a big surprise if your next drilling project in the Canadian Shield hits a pocket of “salty” brine. That is, you may very well find gold nuggets in that brine. One such nugget, containing an estimated 190 oz of gold, was found along with many smaller ones in brine solutions at the Copper Rand copper/gold mine in the Chibougamau mining district of Quebec last year. The sample is about 8 inches in length and contains about 74 oz of gold — a grade equivalent to 12,400 oz of gold per ton] Values like this greatly enhance overall ore grades in the mine. Average grades at the Copper Rand mine in 1987 were 0.124 oz gold per ton and 1.45% copper, from 348,000 tons of ore. Pleasant Surprise

The discovery of gold nuggets in the brine came quite unexpectedly. Routine exploration drilling at the mine regularly intercepted “salty” brine solutions, beginning as early as the late 1960s. Little attention was paid to them, however, until a co- operative fluid inclusion study identified copper and gold coexisting with the sodium chloride, calcium chloride and methane-rich fluids. The chemical similarities between the ore fluid and the salty brine prompted sampling of the brine in 1979. Subsequent sampling from the vugs, exposed during mining operations, showed the vein structures associated with these brine-filled vugs had consistently high gold values. The study was conducted by Prof Jayanta Guha of the University of Quebec at Chicoutimi.

So far, two areas of the Copper Rand mine have yielded gold in the form of nuggets. The gold is always found in close association with vugs containing brine and methane, according to Guha, and is related to structures that formed late in the history of shearing of the host rocks.

It is possible, Guha hypothesises, that similar gold enrichment may be associated with other salty brine solutions reported in the Canadian Shield. The chemical composition of the vug fluids at the Copper Rand mine shows that they are sodium- and calcium- rich brine solutions. A comparison with saline groundwater samples from mines in Yellowknife, N.W.T., and Sudbury, Ont., as well as brine from the Centennial mine in Michigan, shows an overall similarity. Gold mines in the Yellowknife area also contain brine and methane, and, Guha suggests, the relationship between high-grade gold occurrences and brine solutions should be investigated more closely as a potential exploration tool.

The rocks in the Copper Rand mine area belong to the Chibougamau- Matagami Archean greenstone belt and comprise volcanic and volcano- sedimentary sequences with mafic to ultra-mafic sills and felsic intrusions (See geological map). The Archean rocks are overlain by sedimentary units of the Proterozoic Chibougamau Formation and are intruded by dikes of Proterozoic age. The ore zones lie within shear zones and are hosted by anorthosite of the Dore Lake Complex.

A systematic evaluation of calcium chloride, sodium chloride and methane-bearing vugs in the Copper Rand mine indicates that zones of anomalously high gold concentration are spatially associated with these vugs.

These zones occur in late structures and the gold mineralization in them has a completely different character from that found in most of the deposit. The characteristic feature of these high-grade gold zones is the phenomenal concentration of visible gold which occurs along with quartz, carbonate and chlorite in very limited areas. The term “nugget” is used for this secondary gold even though the visible gold does not have the rounded appearance of a typical nugget. Finely dispersed native gold grains associated with pyrite and chalcopyrite are found in most portions of the deposit. This primary gold is rarely visible. Later Evolution

Based on structure, lithology, fluid inclusion, and isotope data of the Copper Rand deposit, Guha suggests the gold nuggets formed after the initial mineralizing period. The primary gold deposits formed during shearing in the Proterozoic Age, but it is difficult to imagine the sealing and conservation of vugs through a period of major deformation. Therefore, it is more plausible that the vugs that are preserved and the associated nugget gold formed late in the evolution of the shear zone.

One of the late structures that contained the most important find of gold nuggets is the HW-1 fault. This fault formed after the primary ore lenses, which have been truncated by the fault. The fault zone is characterized by tightly folded bands of fine-grained, blackish-brown material consisting of chlorite and tourmaline, bands and stringers of quartz-carbonate and also bands of chlorite. Also, halite has been observed within the fault zone.

The presence of halite within the fault zone shows that a saline brine must have been present when the nugget gold was forming during a late stage in the evolution of the shear zone. But the source of the saline brine in the vugs is still unknown. It is now known that the origin of the primary ore fluid is the formation waters related to the Proterozoic sedimentary basin that covered the Archean rocks. Guha suggests that the vugs may contain trapped residual ore fluids which evolved through exchange, or by the addition of unmodified formation waters, or by mixture of both processes. REFERENCES Guha, J. and Kanwar, R., Vug Brines-Fluid Inclusions: A Key to the Understanding of Secondary Gold Enrichment Processes and the Evolution of Deep Brines in the Canadian Shield; in Saline Water and Gases in Crystalline Rocks. Edited by P. Fritz and S. K. Frape. Geological Association of Canada Special Paper 33, 1987. p. 95-101. * 03Joyce Musial is a Toronto-based consulting geologist and freelance writer. — 30 —