Atlin hydromagnesite deposits, Atlin Mining Division, British Columbia, Canadai

Regional Level Types
Atlin hydromagnesite deposits	Deposit
Atlin Mining Division	Division
British Columbia	Province
Canada	Country

The Atlin hydromagnesite deposits are located immediately east of Atlin, British Columbia, in the Atlin Mining Division. Atlin is a small unincorporated community with a rich mining history dating back to the days of the Klondike gold rush (Dickinson and Smith, 1995).
There is a good description of the deposit on the British Columbia “Minfile” site, current to 2013, part of which is quoted below:
“Accumulations of hydromagnesite are located within topographic lows immediately east of Atlin. In addition to the two main bodies, a number of small, isolated patches of hydromagnesite occur along the lakeshore in the vicinity of Atlin.
The largest deposit is about 7.29 hectares with an average depth of 81 centimetres and it has several smaller satellite bodies. It is located northeast of Atlin, north of the airfield road and lies in a slight depression which opens northwest to a swampy area. Bedrock geology comprises eclogite and mantle tectonites of the upper Mississippian to Permian Cache Creek Complex, and basaltic volcanic rocks of the upper Mississippian to Permian Nakina Formation (Cache Creek Complex).
Glaciofluvial materials underlie the deposit and the basal contact with the underlying clay-like soil and grit is sharp with no evidence of gradation. Near the base of the deposit however, the hydromagnesite may be more porous and is traversed by irregular vein-like films of glassy hydromagnesite. The surface is slightly raised and hummocky and is crosscut by cracks and fractures up to 3 centimetres wide and one metre deep. The bodies are relatively barren of vegetation and have slightly irregular but sharply defined boundaries.
The hydromagnesite is white, powdery and remarkably uniform in texture and composition with no evidence of bedding or structure. The white surface color assumes a yellow tinge after a depth of about 30 centimetres although this colour disappears with exposure to air. The hydromagnesite becomes quite plastic, like clay, when wet.”
Giles Peatfield comments:
This group of hydromagnesite occurrences at Atlin has been described in more or less detail by: Gwillim (1901); Robertson (1905); Young, (1916); Cummings, (1940); and Grant, (1987). A caution here is that much of the information in the later references is simply a repeat of data from earlier papers.
There has been no serious commercial production from the deposits; Robertson (1905) reported that the claim owner “. . . dug from the surface exposures some 200 tons of this material, which was sacked and shipped to San Francisco, Cal. as an experimental lot.” Young (1916) reported that in 1915 a shipment of 500 tons was made to Vancouver, British Columbia. It seems that the cost of transportation was the principal problem for possible exploitation. Closer to home, Dickinson and Smith provided some interesting information regarding possible mining of the hydromagnesite. “Since the market for magnesite [sic] always remained small, the easiest to mine mineral in the district was never exploited. Atlinites, however, were practiced at making use of the materials at hand, and they found several good uses for magnesite. In a wet stage it resembles plaster of paris, and sets as hard, so it was used for chinking log cabins. It was also suitable for glazing windows. A less obvious use was as a wood preservative. Early fence posts set in holes containing a couple of shovels full of magnesite mixed with the backfill, never seemed to rot.”
There are a large number of analyses of the Atlin hydromagnesite. As examples, I have chosen to list an average of the major components of five (very similar) analyses reported by Cummings (1940) as follows: MgO - 42.08; CaO - 0.33; CO2 - 35.80; total H2O - 18.97. For comparison, Traill (1983), quoting Reinecke (1920) who in turn was quoting Young (1915) gave the following figures for 8 analyses by N. L. Turner: MgO - 41.10; CaO - 0.90; CO2 - 35.39; total H2O - 19.06. And finally, the figures given by Palache, Berman and Frondel (1951) for theoretical hydromagnesite are: MgO - 44.14; CaO - n/a; CO2 - 36.14; total H2O - 19.72.

Comments on the Minerals Reported:
Calcite: Cummings (1940) implied, although he did not specifically state, that calcite might be present. Hoffmann (1903) reported that underlying the hydromagnesite deposits was a layer of “calcareous tufa” which one assumes would have calcite (or possibly aragonite?).
Hydromagnesite: It is noteworthy that this was first reported here by Hoffmann (1900), who wrote that “Another series of some three or more deposits of hydromagnesite has more recently been discovered by Mr. J. C. Gwillim . . . on the east side of Atlin Lake, in Cassiar District.”
Magnesite: Cummings (1940) reported that in microscopic examination he identified minute crystals of magnesite. Hoffmann (1900) noted that “Large exposures of a more or less pure magnesite rock, as also of serpentinized rocks occur, it may be observed, in the immediate vicinity of these deposits.”

Giles Peatfield
BASc. (Geological Engineering) University of British Columbia 1966.
PhD Queen's University at Kingston 1978.
Worked for Texas Gulf Sulphur / Texasgulf Inc. / Kidd Creek Mines - 1966 to 1985.
Vancouver based consultant 1982 to retirement in 2016

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Link to British Columbia Minfile:	104N 079

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References