Canadian Rockhound: Native Gold and Pegmatite Minerals from the Bird River-Bisset Region of Manitoba

A variety of interesting and unusual minerals are found in the Canadian Shield. The Shield is a vast region of exposed Precambrian rock extending from Québec through Ontario, into northern Manitoba and Saskatchewan, and into the northeastern corner of Alberta. It extends northwards into the Northwest Territories and the newly created Nunavut Territory. The shield is composed of crustal fragments that were formed somewhere between 4.0 and 2.5 billion years ago. Much of the exposed rock in the shield region is igneous and metamorphic. The shield also shows evidence of past cycles of mountain building, continental collision and volcanism.

Such activity occurred in Saskatchewan and Manitoba between 1.8 and 1.9 billion years ago, creating the right conditions for the formation of the minerals found in these two provinces today. The Bird River - Bisset region in southeastern Manitoba is of interest to the collector, as it is more easily accessible and contains a variety of other interesting and sometimes rare minerals.

This article is a compilation of articles recently published by the Mineral Society of Manitoba in their newsletter, The Mineral Vein. While it is not possible to list and describe all the minerals found in the region, we will introduce the collector to some of the important minerals and ores found. Here we describe native gold, alluaudite, amesite, laurite, and cernyite. The last four are pegmatite minerals from the Bird River-Bisset region of Manitoba. Before collecting, you should be aware that permission is required to collect at some of the localities.

Native Gold

Native Gold, or "Free Gold" as it is colloquially known, is found in either flakes or veinlets of pure metal or as an alloy with silver called electrum. Native gold is very often the first thing that most people think of when the word "mineral" is mentioned.

Native gold in Manitoba occurs in quartz veins as native metal. Other gold containing minerals are very rare to non existent in the province.

Hand size samples of Native gold are not common due to the very high price commanded for this material. Most gold mine dumps will have been scoured thoroughly for any visible gold of reasonable size. All former gold mines in the province are held under claim by various mining companies and/or individuals. Permission to collect must be sought from these claim holders.

A very large high grade gold sample from the San Antonio Mine, now the Harmony Gold Mine, Bissett, Manitoba is part of the collection of the National Museum of Natural Science, Ottawa. The sample is 24 cm x 20 cm x 5 cm in size. A photograph of this sample is on the cover of the recent Manitoba Energy and Mines publication "Gold Deposits of Manitoba" (1996).

This publication lays out the major occurrences of gold in Manitoba. There are many mines, shafts and prospects for gold in the Bissett area. These are described therein in detail including location maps. A second major location for gold is the Snow Lake area where the New Britannia Mine is currently active. A third location is the Lynn Lake area.

Native gold in a specimen of any significant size is a valuable addition to a mineral collection. It occasionally occurs as octahedral crystals but these have not been found in any Manitoba mines.

Alluaudite

Approximately (Na,Ca)Fe⁺²(Mn,Fe⁺²,Fe⁺³,Mg)₂(PO₄)₃

Alluaudite is a primary phosphate mineral that generally occurs as embedded crystals in complex, phosphate-rich pegmatites. The Alluaudite group is so complex that the formula is only an approximation; there are several substitutions for sodium, calcium, iron and manganese that would require a detailed chemical analyses for virtually every specimen.

The Tanco Mine at Bernic Lake, Manitoba is a complex, spodumene/tantalum-bearing pegmatite, approximately 100 kilometers northeast of Winnipeg. It is actively mined for ceramic grade Spodumene, Pollucite for the production of cesium formate, and Montebrasite as a lithium phosphate concentrate for ceramic applications. Tanco also produces Tantalite, seasonally, from the reprocessing of dump material. Phosphate is a major impurity in the ceramic grade spodumene so its occurrence at Tanco must be monitored closely.

The Tanco Mine has a phosphate mineralogy composed mainly of Apatite, Montebrasite/Amblygonite, and Lithiophilite. Minor amounts of other phosphates such as Alluaudite and Lithiophosphate occur sporadically.

Alluaudite at the Tanco Mine occurs as 0.5 cm. dark brown embedded crystalline aggregates. The aggregates are arranged in 2 cm. to 3 cm. dendritic form in coarse grained flesh-coloured potassium feldspar, associated with very coarse grained (5 cm plus) black tourmaline from the wall zone of the Tanco Mine. The exact location within the Tanco Mine is not known as the Alluaudite samples were found on the dump.

Collecting at the Tanco Mine can only be done with permission. If you wish to collect please contact Mr. P. Vanstone, Chief Geologist, Tantalum Mining Corporation, Lac du Bonnet, Manitoba.

Amesite

Mg₂Al (Si,Al)O₅ (OH)₄
A member of the Kaolinite-Serpentine Group

The Bird River Sill is a layered mafic-ultramafic sill located in southeast Manitoba, approximately 100 kilometers northeast of Winnipeg and 30 kilometers east of Lac du Bonnet. It is one of North America's largest sources of Chromite.

Much of the "Serpentine" in the Chromite bearing parts of the Bird River Sill is actually Amesite- Serpentine with a high aluminum content. The assemblage that occurs at the top of the Upper Main Seam is a Chromite-Amesite-Hydrogrossular. The Hydrogrossular is also unusual in that it has a high (44 mole%) content of the hydrous analogue of Uvarovite.

Amesite is not a common mineral and appears to be restricted to areas of layered mafic/ultramafic complexes where the ultramafic (Mg) and mafic (Al) units are in contact. The Chromite assemblage in the Bird River Sill is restricted to an area close to the contact between the Peridotite (ultramafic) and Gabbro (mafic).

Amesite in this case occurs as grey-green fine grained masses with very fine grained to random fibrous slick surfaces. The slick surfaces are often coloured apple green. There is ~ 5% Chrome in the Amesite so this may be the colouring material for the slick surfaces. Fibrous crystals of the Amesite to 2 cm in length exist as a re-crystallization feature along some of the fracture surfaces, especially in the "Government Pit" at the junction of the access road and the Chrome layers.

The Amesite is the major gangue mineral for the chromite deposit, which has a minimum of 5 million tonnes outlined. The deposit will be bulk sampled by Gossan Resources Limited in the near future, so new exposures of the Amesite-Serpentine will be uncovered.

Permission for collecting must be obtained from Mr. Jim Campbell, P.Eng. of Gossan Resources Ltd., 52 Donald Street, Winnipeg, Manitoba, tel: (204) 943-1990.

Laurite

(Ru,Ir,Os)S₂
A platinum group mineral

Laurite is a ruthenium, osmium and iridium sulphide mineral that occurs as fine to very fine grains in deposits of platinum group minerals. Platinum group minerals occur as placer and hard rock deposits.

Platinum group minerals and chromite are found concentrated in the mafic/ultramafic Bird River Sill, east of Lac du Bonnet, 75 miles northeast of Winnipeg. The highest concentration of both occurs close to the boundary of the mafic (gabbro) and ultramafic (peridotite) layers within the Bird River Sill. The chromite is deposited in two main stratabound layers, the Upper Main and Lower Main Seams, as well as several thinner layers.

Platinum group minerals, especially Laurite, occur within the Lower Main Seam as equant inclusions within chromite grains, generally 2 to 3 microns in size. The inclusions occur at random within the Lower Main Seam and are common enough that most hand size samples of chromite will contain a few grains of Laurite. The inclusions of Laurite appear to be primary as they do not occur along fractures in the chromite.

The lack of any change in grain size of minerals within the Bird River Sill indicates that there is little chance of finding Laurite or any of the other platinum group minerals on a larger scale.

The very high reflectance of most platinum group minerals will allow even very small grains of the minerals to be visible under a microscope if they are exposed in a fractured chromite grain. However, it will probably require preparation of a polished section to determine the presence of Laurite in any given sample.

Permission to collect at this locality must be obtained from Mr. Jim Campbell, Gossan Resources Limited, 52 Donald Street, Winnipeg, Manitoba, tel: (204) 943-1990.

Cernyite

Cu₂CdSnS₄

Cernyite is a copper cadmium tin sulphide and a member of the Stannite group of minerals. Cernyite was first described in 1979 from the Tanco Mine and the Hugo pegmatite in South Dakota. It was named in honour of Dr. P. Cerny of the University of Manitoba, a noted world authority on rare-element pegmatites.

Cernyite is part of a complex suite of sulphides and sulphosalts which form a minor component at the Tanco Mine. In most cases these minerals are very fine grained and intergrown on a micron scale.

Cerny is steel grey in colour with a metallic lustre with a black streak. It is intergrown with Kesterite at Tanco. The two minerals are physically similar and instruments such as a reflected light microscope or X-ray diffraction must be used to tell them apart. It would be difficult to tell if Cernyite existed in an untested hand sample.

Sulphide minerals in rare-element pegmatites such as Tanco are often mineralogically complex. The very slow cooling of pegmatites and their unusual composition allow for the growth of phases such as Cernyite that are found nowhere else.

Conclusion

Tanco is such a complex of different minerals that there are probably more new minerals to be found in even so well studied a deposit. The University of Manitoba has ongoing research programs at Tanco that may bring new phases to light. More information about the minerals from Bird River-Bisset can be obtained by contacting the University of Manitoba Geological Sciences department, or by contacting the Mineral Society of Manitoba, c.o. The Manitoba Museum of Man and Nature, 190 Rupert Ave., Winnipeg, Manitoba, Canada, R3B 0N2.

This article may not be copied, distributed or reprinted in any form without the author's permission. 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, original source and the Canadian Rockhound, and include the website URL address of the Canadian Rockhound.

This article contains content previously published in several issues of The Mineral Vein, the official newsletter of the Mineral Society of Manitoba. Reprinted in the Canadian Rockhound with permission.