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Geology of the Red Lake Mine, Ontario, Canada: the Richest Grade Gold Mine in the World
By Dr. Fuzail Siddiqui
Principal Consultant, Mineral Resources Research
61-Finchley Circle, Markham
Ontario, Canada L3R 8S2
Introduction
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A superb gold specimen from the Red Lake Mine, Balmertown, Ontario, Canada. The siliceous minerals were etched off of the top portion of this specimen to thoroughly reveal the gold. The bottom portion of the specimen was left natural to give an idea of how the gold was emplace. Photo by David K. Joyce.
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How many of us are aware that the richest grade gold mine in the entire world is right here in Ontario? Current reserves grade over 2 ounces per ton, the highest in the world!
Red Lake mine's sudden rise to fame had its beginning in 1995 when Goldcorp Inc., the new owners of the Red Lake mine, announced the discovery of a new high grade zone (HGZ) at depth. In 2000, taking a step unprecedented in the history of exploration, Goldcorp made their more than 18,000 borehole database available to the geological community. They challenged geologists everywhere 'to examine the data, to pick out important clues that might lead to the discovery of another high-grade ore body within their Red Lake mine property'. After an analysis of available data and information the writer submitted an exploration proposal that qualified for one of the 25 semi-finalist awards of the 'Challenge' competition. Here is a summary of what I learnt about the fascinating geology of this mine.
Regional Geology and Structure
The Red Lake mine is located in the northwest corner of the province of Ontario, Canada on the Red Lake Greenstone Belt (RLGB) that consists of a typical Archean sequence of metavolcanic and metasedimentary rocks. Gold occurs in the Balmer assemblage (2992-2958 my), which occupies the core and constitutes about 50 percent of the RLGB. The Balmer consists of basaltic tholeiite and komatiite lava flows with intercalated magnetite-quartz iron formations (MNDM 1999). The gold deposits of Red Lake area are an example of the 'Archean Lode Gold' deposits model (Roberts 1987).
The RLGB is traversed by large-scale deformation zones, which predominantly have either a NE-SW or a NW-SE trend. The Red Lake mine (and the adjacent Campbell mine of Placer Dome Inc.) is located inside one of these zones- the NW-SE trending Cochenour-Gullrock deformation zone. The area has been subjected to greenschist and amphibolite grade regional metamorphism. Also evident is hydrothermal alteration that accompanies the deformation zones and widens at the intersection of deformation zones. Gold deposits and occurrences are almost entirely confined to the deformed/altered areas.
A thick leaf of gold passes right through the matrix of mostly quartz with a little pyrrhotite in it. As well there appears to be pyrite and chalcopyrite on the gold, itself. Photo by David K. Joyce.
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Local Geology
Basaltic tholeiite flows are the dominant rock type in the Red Lake mine and host most of the vein type native gold mineralisation. Ultramafic flows of basaltic komatiite are host to replacement type sulphide mineralisation. Ultramafic flows of peridotitic komatiite have no significant mineralisation but sulphide mineralisation may occur near lithological contacts. The calc-alkalic felsic volcanic and sedimentary package, that surrounds the mine to the east and north, is barren. HGZ appears mainly in basaltic tholeiite and has an en echelon geometry associated with a complex interfingering of ultramafic rocks.
Local Structure
Overall structure of the Red Lake mine consists of shallow to steep plunging syncline-anticline-syncline fold train. The folds, which belong to the regional F2 folding event, plunge locally to the west. Most ore zones are associated with shear zones and faulting that are roughly parallel to the axial plane of the F2 folding and the related regional S2 foliation. In the Red Lake mine the S2 foliation strikes 110-140° and dips 65-85° to the SW. Areas with repeated brecciation are most favourable for high-grade mineralisation.
Structural Control of Mineralisation
Gold zones follow three basic structural trends:
- Foliation parallel veins (NW trending) in otherwise homogenous basaltic rock. Azimuth 135°, dip 60-72° SW.
- Oblique veins (N to NE trending) wider elongate structures with generally less strike extent and approximate dip of 45° W. Faults control geometry and extent of these veins, in particular faults that juxtapose different lithologies.
- High-grade zones with a pod or pipe like geometry. Tend to be controlled by lithological contacts or by intersection of faults with lithological contacts. Azimuth 115°, dip 85° S.
A system of EW to NE trending faults also occurs which is generally poorly mineralized. These may be related to the pre-mineralisation F1 folding event. When these faults cut a NW trending vein, large mineralized zones occur that are visible at the scale of mining stopes. A combination of the above 1, 2 and 3 styles creates complex vein arrays, including NS and EW components. These arrays are most common near high angle mafic-ultramafic contacts. The spatial association of the HGZ with the high angle mafic-ultramafic contacts has been noted at Goldcorp and has led to the belief that enhanced dilatancy occurred in these environments and that the dilatancy was sustained over long periods.
That the location of the ore is structurally controlled is undeniable. However there is controversy about the sequence of major tectonic events that shaped the final positioning of the ore in space. Evidence presented by SRK (1999) appears convincing that "the entire spectrum of ore body types and shapes at the Red Lake mine can be neatly explained by a single shear zone model." Also that, "the cospatial development of ductile deformation fabrics, hydrothermal brecciation and tension veining, all attest to a complex polyphase history of plastic deformation accompanied by episodic periods of brittle failure and fluid flow. Shearing does not appear to be a ground preparation for later deposition of ore in open spaces created. Rather shear and veining are a continuous process. The continuity of vein formation throughout the history of the shear zones implies fluid circulation or the availability of fluids over a long period of time. It is also apparent that shear zones that host the gold deposits do not involve large displacement."
Alteration
Addition of Si, K, S, CO2, As, Sb, Tl, Sn, Se, (Zn+Hg) and Rb together with loss of P typifies alteration associated with gold mineralisation. Most of the alteration is confined to the immediate vicinity of the mineralisation and therefore cannot be used as a guide for exploration (Penczak 1999). Rb and As are anomalous outside the mineralisation and may be used as indicators. Tourmaline occurs specifically in the HGZ but it is not clear from the available data if the associated Boron is anomalous outside the HGZ vein system.
Ore Types and Mineralogy
Generally there are three types of mineralisation in the Red Lake area:
- Vein ore
- Disseminated Sulphide ore (DSO)
- Replacement ore
Vein Ore: Quartz-carbonate veins containing fine-grained, disseminated arsenopyrite ± pyrite and native gold plus replacement silica. At depth pyrrhotite occurs instead of pyrite.
DSO: 5 to 10 percent finely disseminated gold bearing pyrrhotite and pyrite mineralisation (e.g. East South C or ESC type ore). Also found in combination with replacement type ore within the DSO areas (lower ESC ore), as well as on footwall of HGZ.
Replacement Ore: Mineralogically variable but may have up to 30 percent fine acicular arsenopyrite. Indicates that the area was soaked in gold bearing SiO2 (± arsenic and sulphur) solutions.
This is an impressive and showy specimen of native gold associated with berthierite, the rare sulphosalt, in sheared quartz and altered volcanics. The main body of the specimen is gold laced rock and quartz that has a very thick (8 mm or so) "plate" of gold sticking up out of the back of it in a very aesthetic manner. Photo by David K. Joyce.
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Solubility of Gold
Chloride gold complexes are stable in strong brines. However chloride poor fluid inclusions, generally found in Archean lode gold deposits, indicate some other mechanism for solubilisation of gold. Neutral solutions with low total sulphur are capable of transporting gold in comparatively high concentrations as the thio-complex - Au(HS)2. Gold thio-complexes have been shown to dominate transport of gold in geothermal fields in New Zealand (Robert 1987). Hodgson et.al. (1982) discussed the data on stability of base metal thio-complexes and concluded that in alkali solutions, with low sulphur content, base metals will not be as soluble as gold. This provides a possible mechanism for generation of gold rich but low base metal bearing hydrothermal solutions. In the absence of direct fluid inclusion data this writer believes that gold was transported as thio-complex in the Red Lake deposit, in line with the general Archean lode gold model characteristics.
Precipitation of Gold
The following mechanisms can lead to precipitation of gold being transported as a dissolved thio-complex (Robert 1987):
- Oxidation of fluids.
- Change of pH (due perhaps to loss of CO2 by reaction with host rock or by pressure decrease).
- A decrease of temperature.
- A reduction in the activity of reduced sulphur by reaction with the host rock to precipitate iron sulphide.
It is believed that one or a combination of the above mechanisms is responsible for localizing gold mineralisation in the HGZ.
Gold Assays
A unimodal lognormal distribution and a straight-line lognormal probability plot of the normalized uncut drill assays from the HGZ indicate that the gold assays belong to a single population. This points to a single coherent overall genetic process that culminated in ore deposition.
Discussion
The above facts and opinions are gleaned from the texts and illustrations supplied in 'the Challenge CD' and from additional relevant literature consulted by the writer. What follows are the results of the writer's analysis (including a close scrutiny of the plans and sections in GEMCOM) of what caused the formation of the HGZ.
The following simplified sequence of significant events is believed to emerge from a study of available information:
- Formation of RLGB by extrusion of lava flows in submarine environment. Most flows are basaltic tholeiites with minor localised ultramafic and rhyolitic flows. The last two represent the two extremes of differentiation of the source magma. The gold brought up with the extremely hot Archean magma was leached out from the interior of the flows towards the rim, by contact with sea water, and probably fixed in sulphide bearing interflow sediments.
- Following burial, much of the brine was squeezed out of the lava-sediment pile.
- Pre-mineralisation D1 deformation, due to compression with pure (non-rotational) shear resulted in F1 folds with NE-SW trending axial planes but without major ductile deformation and related axial plane foliation.
- Regional metamorphism generated CO2-H2O hydrothermal fluids by dehydration and degassing. These fluids were driven out, mainly upwards, along the dynamic pressure/temperature gradients.
- Coeval with regional metamorphism, the D2 deformation set in due to compression with simple (rotational) shear and resulted in the F2 folds. A syncline-anticline-syncline F2 fold train dominates the structure in the Goldcorp Red Lake mine property. F2 fold axes are oriented NW-SE with axial plane dipping steeply SW. D2 deformation created a system of S2 foliation and faults parallel to the axial plane and also caused tension gashes perpendicular to the fold axis along fold hinges (see oblique veins described above in Structural Control of Mineralisation).
- These penetrative fabrics served as channel/s for the migration of gold bearing fluids in waves through ductile shear and dilatational as well as episodic brittle fractures. (The extreme north and south portions of the Red Lake mine property are outside the major deformation zones and are considered barren).
- Gold preferentially precipitated in the iron rich basaltic tholeiite and very much less so in the iron poor ultramafics due to gradual reduction in the fugacity of reduced sulphur, in the gold bearing fluids, by fluid/rock reaction to form iron sulphides.
- Gold precipitated from the fluids, in the fractures traversed, in areas where changes in sulphur fugacity, pH, temperature and pressure combined to render the gold thio-complex solution most unstable.
Based on the above the writer believes that the HGZ occupies the nose of a tight syncline where not only a number of fracture systems converge but also basaltic horizons, chemically favourable for the deposition of gold, have been repeated due to folding.
A gorgeous specimen of native gold with a very thick plate of gold protruding from the top of the specimen and leaning out to the right. The gold plate portion has had the quartz removed from it while the base of the specimen has been left natural. Photo by David K. Joyce.
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Specimen Recovery
Fortunately for the Rockhounds specimens from this mine, containing a variety of shapes, sizes and concentrations of visible gold, are available for collectors because the owners have taken the unusually progressive step of preserving native gold specimens. For a detailed listing of available specimens, together with a description of the procedure of specimen recovery, the readers are referred to www.davidkjoyceminerals.com.
References
- Hodgson, C. J. et. al., 1982, A review of the geological characteristics of 'gold only' deposits in the Superior Province of the Canadian Shield, CIMM Special, Volume 24, p.211-228
- Ministry of Northern Development and Mines (MNDM), 1999, Geology of Red Lake District. Available from Internet: http://www.mndm.gov.on.ca.
- Penczak, R. S., 1999, Geochemistry of alteration around some high-grade zones and geochemical characteristics of altered rocks at the Red Lake mine. Goldcorp Inc., Red Lake Mine Division internal report.
- Robert, R. G., 1987, Archean Lode Gold Deposits, Geoscience Canada v. 14, No. 1.
- SRK Consultant Report, 1999, Red Lake Structural Study, Report No. 2CG006.00
About the Author
Fuzail Siddiqui received his M.Sc. (Geology) from the UNESCO funded Geology Department at Panjab University, Lahore, Pakistan. He received his Ph.D. (Mineralogy) from King's College, London, UK. Fuzail started his career as a University teacher of mineralogy/petrology and economic geology in Pakistan. He carried out regional mapping in virgin territories, and discovered the first recorded occurrence of a nepheline syenite-carbonatite complex in the Himalayas. For 25 years, he worked in the Zambian copper mines, in progressively more important positions, up to the second highest position in Geology. There he had a diversified career, as a process mineralogist, mine chemist, computer geologist cum system administrator and finally a corporate executive, involved in privatization of the Zambian mines. Since coming to Canada in 1998 he operates his independent consultancy service 'MINERAL RESOURCES RESEARCH' from Markham, Ontario. Currently he is focused on providing 3D geological modeling and mineral resource exploration and marketing consultancy.Fuzail has served on the council of the Walker Mineralogical Club, Toronto. He is one of the winners of the 'Challenge' the global web-based gold exploration competition launched by Goldcorp Inc. of Toronto in March 2000. This article is based on his award-winning exploration proposal.
He can be contacted in the following ways:
By mail: Dr Fuzail Siddiqui, Principal Consultant,
Mineral Resources Research (MRR),
61 Finchley Circle, Markham, Ontario,
Canada L3R 8S2
By Phone and Fax: (905) 947 9377
By email: siddiquf@yahoo.com
Copyright © 2003 Fuzail Siddiqui
E-mail: siddiquf@yahoo.com
Reprint instructions:
This article may not be copied, distributed or reprinted in any form without permission from the author. To contact the author, please use the e-mail address provided. If you are unable to contact the author, please contact the Canadian Rockhound. Authorized reprints must acknowledge the author and the Canadian Rockhound, and include the website URL address of the Canadian Rockhound.
Photographs of gold specimens were kindly provided by David K. Joyce (www.davidkjoyceminerals.com).
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How to cite this article:
SIDDIQUI, Fuzail. Geology of the Red Lake Mine, Ontario, Canada: the Richest Grade Gold Mine in the World. Canadian Rockhound [online]. 2003, vol. 7, no. 1. Available from World Wide Web: <http://www.canadianrockhound.com/2003/01/cr0307101_redlake.html>.
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