Spherulitic Alkali Rhyolite Dikes, Atsutla Range, Jennings River Map-Area, Atlin Mining Division, British Columbia, Canadai

Regional Level Types
Spherulitic Alkali Rhyolite Dikes	Occurrence
Atsutla Range	- not defined -
Jennings River Map-Area	- not defined -
Atlin Mining Division	Division
British Columbia	Province
Canada	Country

Latitude 059º 13' North; Longitude: 131º 14' West. Note that this is a general reference point for two occurrence areas separated by about 10 kilometres. More specifically, the “A” occurrence of Mathews and Watson (1953) is at about 59º 13’ 31” North; 131º 09' 39” West while their “B” occurrence is at about 59º 12’ 19” North; 131º 19' 36” West. The area is 175 kilometres south-west of Watson Lake and 270 kilometres south-east of Whitehorse, Yukon and 113 kilometres north-west of Dease Lake, British Columbia.
Giles Peatfield comments:
In the course of regional mapping in the Tuya-Teslin area of northern British Columbia during 1943, Watson and Mathews (1944) discovered, at two separate locations in the Atsutla Range, spherulitic alkali rhyolite dikes. The mineralogy of the dikes had some unusual characteristics, leading to a detailed study (Mathews and Watson, 1953).
Mathews and Watson (1953) wrote that “One dike cuts typical granitic rock in the central part of the [Glundebery] batholith on the north fork of Nazcha Creek (locality A, . . .). This dike, which is dark greenish grey and aphanitic, has a width of 3 feet, a vertical dip, and an exposed length of about 20 feet. Its margins have closely-spaced flow layers parallel to the walls; a few inches toward the center of the dike the flow layers are distinctly contorted. These zones of contorted flow layers grade, in turn, into a central zone marked by discrete to coalescent dark blue to almost black spherulites, 3 to 5 mm. in diameter. The flow layers become progressively less distinct toward the center of the zone and in places are marked only by lines of spherulites.”
A second occurrence was described by Mathews and Watson (1953) as follows: “Several dikes, which are moderately to steeply dipping and up to 20 feet wide, cut the wall rocks of the Glundebery batholith on the west side of Glundebery Valley (locality B, . . .). These dikes have pale grey finely crystalline cores and white to buff microcrystalline margins that contain more or less well defined dark fibrous spherulites up to 6 mm. in diameter.”
I have not been able to find a definitive age for the dike rocks. The volcanic and sedimentary strata at Mathews and Watson’s (1953) Occurrence “B” were mapped by Gabrielse (1969) as part of the Kedahda Formation of Carboniferous (?) and Permian age. More recent work, reported by Zagorevski et al. (2018), has shown on the basis of paleontology and U-Pb zircon dating to be Triassic. There are two K-Ar dates available for the Glundebery Batholith rocks: Wanless et al. (1968) reported an age of 79 ± 11 Ma on hornblende for a sample collected near Blackfly Lake, about 12 kilometres east of Watson and Mathew’s (1953) Occurrence “A”; Wanless et al. (1972) reported an age of 74 ± 4 Ma for hornblende for a sample collected near Occurrence “A”. All that can be said at this time is that the dikes are younger than the Glundebery Batholith, perhaps latest Cretaceous or possibly younger.
Giles Peatfield Comments on the Mineral Reported:
Note that these notes refer to minerals found within the spherulitic rhyolite dikes.
Amphibole group: Mathews and Watson (1953) reported riebeckite as stout to acicular prisms, described as follows: “The riebeckite forms stout to acicular prisms with parallel extinction, negative elongation, low birefringence, and refractive indices of about 1.68. The pleochroism is distinct with X dark blue; Y yellow green; and Z pale yellow. The needles show well-defined amphibole cross-sections. Some needles in the spherulites of the Nazcha valley dike cannot be identified with certainty because of their exceedingly small size, but they show the same pleochroism as riebeckite found elsewhere in the dike. Evidence presented later in this paper suggests that the riebeckite, as well as acmite [see note below for pyroxene group], may contain zirconium.” Dr. J. Douglas Scott has suggested (in an email dated 15 March 2024) that based on the pleochroism values “. . . the material is from somewhere along the Riebeckite - Magnesio-riebeckie join rather than end-member [sensu stricto] riebeckite.” – refer to Anthony et al. (2001) for details regarding pleochroism of riebeckite and related species.
Feldspar group: Mathews and Watson (1953) wrote that “Phenocrysts were seen in a conspicuously spherulitic zone in one of the Glundebery Valley dikes [occurrence B] but not elsewhere. They consist of euhedral to highly corroded quartz grains of β habit up to 0.5 mm. in diameter, and euhedral feldspar crystals up to 1.5 mm. long, composed of fine grid-like microperthitic intergrowths of potash and soda feldspar . . . . The feldspar phenocrysts are markedly similar to those in the groundmass of much of the granite of the Glundebery batholith.” Further, Mathews and Watson (1953) wrote that “Where the feldspars are fine-grained and clouded with minute impurities, stain tests . . . had to be used to distinguish the potash-rich and plagioclase-rich areas. As shown by the chemical analyses . . . , little or no calcium is available for anorthite and the plagioclase, therefore, approximates pure albite. The optical properties of the plagioclase in the coarser non-spherulitic rhyolite indicate this same composition.”
Goethite: Mathews and Watson (1953) wrote that “Black opaque grains, goethite, and hematite are common as alteration products of riebeckite needles. In one section of non-spherulitic rhyolite, however, black opaque grains and goethite occur together as interstitial grains up to 0.4 mm. in diameter making up about 5 per cent of the rock and distributed fairly uniformly throughout it.”
Hematite: See note above for goethite.
Pyroxene group: Mathews and Watson (1953) described acmite, which is now known to be aegirine. Their description of the mineral is somewhat complex. They wrote that “Acmite [aegirine] occurs chiefly as small equant grains and stout prisms disseminated throughout the spherulites of the Nazcha Valley dike [occurrence A] and it is also present as stout radiating fibers in a few interspherulitic parts of this rock. It has a pale yellow color, scarcely noticeable pleochroism, high birefringence, and refractive indices ranging from 1.76 to more than 1.79. The prisms have negative elongation and parallel extinction. Although the indices of the acmite (?) psuedomorphous after riebeckite have not been determined, its other optical properties correspond to those given above.” They further wrote that “One of the two common alteration products of the riebeckite is a pale yellowish pseudomorph with distinctly higher birefringence but the same parallel extinction and negative elongation. Scattered needles of riebeckite within a single bundle may be replaced by this yellowish mineral and adjacent needles may remain completely unaltered. A few needles are composed of riebeckite at the base and of the yellowish mineral at the distal end, the two parts being separated by a clearly defined boundary cutting obliquely across the crystal. This alteration product has been identified tentatively as acmite.” Traill (1983) mentioned this occurrence for aegirine, making a general comment that “Aegirine is a rare soda pyroxene found in alkali rocks such as nepheline syenite, soda granite soda aplite, and phonolite. Aegirite is a synonym. The name, acmite, is used by some people to characterize the yellow-brown variety. Others have referred to sharply pointed crystals as acmite, and bluntly terminated crystals as aegirine.”
Quartz: See note above for feldspar group.
Zircon: Mathews and Watson (1953) noted that “Zircon occurs as fine crystals adjacent to the black opaque grains, goethite, and hematite after riebeckite but it was not seen in the vicinity of unaltered riebeckite.”
Giles Peatfield Comments on the Rock Types Reported:
These rock names refer to the dikes and to the surrounding rocks, as reported by Watson and Mathews (1944), Gabrielse (1969) and others.

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.
Consultant 1985 to 2016 59

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References