Searching for Signatures: Evidence for the magmatic origins of gold ** Tired of looking for gold deposits in the heavily explored camps of the Precambrian Shield? Well, consider moving outside the nation’s greenstone belts and step into completely new Precambrian terrain.
One mining operation in Canada — the Renabie mine — has done this successfully in the past, but now Prof Edward Spooner and a group of graduate student geologists at the University of Toronto have produced some strong scientific evidence that lends rational support to the idea and which could lead to the discovery of new deposits. Their work, recently featured in a cover story in Nature, an international scientific journal, suggests gold exploration should be targeted in and around igneous intrusions which are outside the confines of metamorphosed volcanic belts, as well as those which occur within them, such as the Lamaque, Magino and Belmoral deposits. In Spooner’s view, gold deposits may have originated from magmatic fluids and, therefore, we can direct our exploration activities to these areas and not immediately eliminate them from consideration.
Spooner’s group at the U of T has produced geochemical evidence from the Hollinger-McIntyre gold-quartz vein system in Timmins, Ont. (now the Timmins and Schumacher operation of Pamour Inc.) and from the gold- and tungsten-enriched molybdenite mineralization in a granodiorite stock at Mink Lake in northwestern Ontario. The evidence supports the idea that Archean gold-quartz vein fluids are of magmatic origin.
This fairly academic research has been supported by Ontario Geoscience research grants from the Ontario Ministry of Northern Development and Mines, the Ontario Geological Survey, Noranda Exploration and the federal Natural Sciences and Engineering Research Council (nserc). It has led one of Spooner’s graduate students to examine the Renabie gold deposit, 80 km northeast of Wawa, Ont. This area contains several complex vein systems which contain several of the elements typical of Archean gold mineralization — in this case carbonate, pyrite, low copper-lead-zinc sulphides, tellurium, molybdenum, boron and bismuth — but which are actually located outside a greenstone belt. Also, they do not appear to connect with greenstone belt lithologies at depth.
One objective of this work is to evaluate the direct implications for exploration geologists. This will involve testing intrusion/gold relationships in and outside greenstone belts in the Superior and Slave provinces. It will also involve examining the geochemistry of the intrusions in order to test whether or not there are identifiable trace element “signatures” which could help mineral explorationists pinpoint sites for gold exploration.
This research, which is supported by a $225,000, 3-year grant from the nserc, will be geared to determining the chemical composition of the magma, or magmas, from which the Archean gold quartz vein systems may have formed in order to test for the existence of trace element signatures.
Such signatures, if they can be identified, will be of considerable use to geologists involved in routine property evaluation and, particularly, in exploration for Renabie-type gold deposits in intrusive terraines outside Archean greenstone belts. It is also possible that vlf or helicopter-borne electromagnetic systems might be able to detect large Renabie-type vein structures in the same way as the rich Hishikari deposit in Japan (1.5 million tonnes at 80 g gold and 40 g silver per tonne) was discovered by Scintrex in late 1981.
If Spooner’s research proves applicable, gold explorationists could have thousands of square kilometres of new hunting ground. We may be re-aligning our compasses and moving about 5 km or possibly more outside of all greenstone belts in future gold exploration efforts.
Spooner’s research group started working on gold in 1983 when graduate student Peter Wood spent a summer mapping surface pits and selected scheelite-rich underground stopes in the Hollinger-McIntyre vein system. Wood has since been joined by graduate students David Burrows, who is studying copper-gold-silver-molybdenum orebodies at the McIntyre and Lamaque mines; Nicholas Callan, who is studying the Renabie area; and Cornel de Ronde, who is studying gold mineralization in the 3.5-billion- year-old Barberton greenstone belt, in South Africa.
Archean gold-quartz vein/shear zone systems constitute one of the most important types of gold ore deposits; however, their origin is controversial. Thus, geochemical data from the Hollinger-McIntyre gold- quartz vein system was studied and compared with data from the Mink Lake gold- and tungsten-enriched molybdenite mineralized system which has been shown with reasonable confidence to be magmatically derived. The fluid inclusion and light stable isotope properties were studied and presented by the U of T researchers in Nature as evidence of the magmatic origin theory.
The Hollinger-McIntyre vein system in Timmins is the second largest known Archean gold-quartz vein shear zone deposit in the world (the largest is the Golden Mile in western Australia). Out of total production and reserves to date (994.7 tonnes of gold), 62% of the gold occurs in the Hollinger deposit alone, making it an excellent example of the Archean gold-quartz type of vein deposit.
The composition of carbon isotopes in the vein and wallrock alteration carbonate in the Hollinger deposit was used as a source tracer since fluid inclusion work shows that it is a major component of Archean gold-quartz vein fluids. Changes in the amount of carbon 13 relative to carbon 12 (two isotopes of carbon) found in rock samples taken from the Hollinger define a delta value range from — 4.8 parts per 1,000 (ppt) to — 0.1 ppt with an arithmetic mean of — 3.0 ppt +-1.5 ppt. Data for vein and wall rock alteration carbonate are statistically identical at — 2.8 ppt, +-1.8 ppt and — 3.3 ppt +-1.3 ppt. This suggests the two rock types are cogenetic. Values for a further 14 other gold mineralized locations in the Timmins area are statistically identical. They can therefore be pooled and compared with the carbon isotope composition of reference magmatically derived carbonate associated with gold- and tungsten-enriched molybdenite mineralization at Mink Lake (see diagram, left).
The Mink Lake granodiorite intrusion, which covers an area 1.5×3.25 km, is about 110 km northeast of Red Lake, Ont. It is an unmetamorphosed, completely post-tectonic stock in the Archean Uchi Subprovince. It is a sodic granodiorite in composition and contains a 350×1,000-m zone of molybdenite mineralization and associated carbonate metasomatism. The molybdenite mineralization and carbonate alteration, which are enriched in gold (up to 0.6 g per tonne) and tungsten, are considered to be magmatically derived for a number of geological and geochemical reasons (discussed by Burrows in 1984).
By determining the carbon isotope compositions of various types of carbonate at Mink Lake, the researchers have defined a narrow reference range in carbon 13 delta value from — 3.9 ppt to — 2.7 ppt. This range is completely enclosed within the the data pooled from the Timmins area. The arithmetic means of the two data sets are also statistically identical. Since the carbonate in the Archean Mink Lake stock is magmatically derived, the researchers have concluded that the Timmins area deposits could also be of magmatic derivation.
This conclusion also suggests a magmatic origin for the largest known Archean gold-quartz vein/shear zone deposit in the world, Australia’s Golden Mile, since carbon 13 delta values are — 3.4 ppt +-0.4 ppt.
In additon to the carbon 13 isotope data, the researchers have also studied the oxygen and hydrogen isotope compositions and fluid inclusion properties of the Hollinger and Mink Lake systems. The estimated fluid oxygen 18 delta values from carbonate, quartz and scheelite from Hollinger are similar to Mink Lake values; the measured fluid inclusion hydrogen isotope compositions for Hollinger lie exactly within the ma
gmatic range; and the fluid inclusion studies show that mineralization was deposited by water/carbon dioxide fluids undergoing phase separation and that the fluids from both systems were characterized by a fairly pure carbon dioxide component and by a relatively low salinity aqueous component.
With the geochemical evidence behind him, Spooner is now evaluating geological and geochemical evidence to test the hypothesis that there is more gold to be found in and around igneous intrusions located outside, as well as within greenstone belts. At this stage, the U of T group is concentrating on the Renabie mine, a producing mine outside a greenstone belt, and on the Lamaque stock-hosted mineralization, in Val d’Or, Que.
The Renabie mine, northeast of Wawa, has produced about four million tonnes of ore grading 6.6 g gold and 2.1 g silver per tonne since the 1940s and reserves to date are 1.5 million tonnes grading 6 g gold per tonne. In addition to the Renabie mine, two nearby vein systems — the Braminco and Nudulama — have produced high- silica fluxing ore containing 4.8 to 6.6 g gold per tonne for the Kidd Creek smelter in Timmins over the past two years.
The most important geological point about the gold deposits in the Renabie area is that, usually, they are not contained within a greenstone belt. The 1.7×2.6-km area, lies to the east of a steep, westerly-dipping greenstone contact. The Renabie, C zone, Nudulama and Zone 2 veins do not plunge to intersect the greenstone belt metavolcanics at depth. This is particulary clear for the Nudulama zone, which lies 1.7 km away from the contact and which pinches out to a minor vein 425 m below surface. Greenstone material does not lie to the east of the Renabie area. This region comprises the Wawa domal intrusive terrain and, further to the east, the Kapuskasing granulites.
The immediate host rocks in the Renabie area consist of intrusive gneissose trondhjemite along the greenstone contact (400-800 m wide) and a younger and more extensive, variably foliated mafic tonalite with abundant leucocratic pegmatite dikes. In 1986 surface and underground mapping by graduate student Callan in close co-operation with Renabie’s mine geologist, Chester Moore, showed that mineralization occupies different orientations of dilated shear zone structures, that mineralization started after shear zone initiation, and that mineralization occurred during shear zone deformation.
The ore bodies consist of large, composite pods of quartz vein material up to 12 m wide with strike lengths up to 70 m and down-plunge extensions of as much as 1,500 m. The quartz vein material presents a “sheeted” appearance because of the abundance of wall rock foliae often spaced at 4-cm intervals or more. The mineralogy consists dominantly of quartz vein material with strained muscovite- carbonate-pyrite alteration selvages. Gold appears to have a general spatial association with pyrite and, particularly, galena and tellurides, which is being studied. Base metal sulphides such as galena, chalcopyrite and bornite do occur and anhydrite is relatively frequent in occurrence. Tellurium and other elements typical of Archean gold systems such as boron (tourmaline), molybdenum, and bismuth are present as minor associates. REFERENCES Burrows, D. R. (1984): Geology and Geochemistry of Molybdenite Mineralization within an Archean Granodiorite Intrusion, Mink Lake, northwestern Ontario. Unpublished M.Sc. thesis, University of Toronto, 262p. Burrows, D. R., P. C. Wood and E. T. C. Spooner (1986): Carbon Isotope Evidence for a Magmatic Origin for Archean Gold-quartz Vein Ore Deposits. Nature, Vol. 321, p. 851-854. Burrows, D. R., P. C. Wood, A. V. Thomas and E. T. C. Spooner (1986): A Magmatic Orgin for Archean Gold-quartz Vein Mineralization; geological, igneous geochemical, fluid inclusion and light stable isotope evidence from Au and W enriched MoS2003 mineralization in the Mink Lake granodiorite stock, northwestern Ontario, and from the Hollinger Au deposit, Timmins, northern Ontario, Geocongress ’86, Johannesburg, Extended Abstracts, p. 283-286. Williams, N. (1986): Archean Gold. Exploring for Magmatic Origins, Nature, Vol. 321, p. 812. Wood, P. C., D. R. Burrows and E. T. C. Spooner (1986): Au-quartz Vein and Intrusion-hosted Cu-Au-Ag-Mo Mineralization, Hollinger- McIntyre Mines, Timmins, Ontario, Ontario Geological Survey, Miscellaneous Paper 130, p. 115-137. Wood, P. C., D. R. Burrows, A. V. Thomas and E. T. C. Spooner (1986): The Hollinger McIntyre Au-quartz Vein System, Timmins, Ontario, Canada; Geologic Characteristics, Fluid Properties and Light Stable Isotope Geochemistry, Proceedings of Gold ’86 Symposium, Toronto, p. 56-80. Joyce Musial is a Toronto-based geologist and freelance writer.
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