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File Created: 06-Oct-1987 by Henry W. Marsden (HWM)
Last Edit:  07-Feb-2020 by George Owsiacki (GO)

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NMI 104P12 Cr1
Name ANVIL CHROMITE, ICE LAKE CHROMITE, BLUE RIVER ULTRAMAFITE, MYST METALS Mining Division Liard
BCGS Map 104O060
Status Prospect NTS Map 104O09E
Latitude 059º 33' 27'' UTM 09 (NAD 83)
Longitude 130º 00' 00'' Northing 6602558
Easting 443481
Commodities Chromium, Nickel Deposit Types M03 : Podiform chromite
Tectonic Belt Omineca Terrane Cassiar, Slide Mountain
Capsule Geology

The Ice Lake chromite showings are located adjacent to Ice Lake, 28 kilometres north-northwest of the town site of Cassiar. The showings were discovered by Geological Survey Branch geologists during regional mapping of the area in 1987. The hostrocks are dunite of the Permian Blue River Ultramafic Complex, part of the Tournaisian (lower Early Mississippian)-Permian(?) Sylvester allochthon (Fieldwork 1987). The showings have not been evaluated for their platinum group metals potential and examination of the chromite has only been cursory in nature.

The Blue River ultramafite is a thrust slice of the lower part of an ophiolite exposed in the core of the McDame Synclinorium. It is structurally overlain by a crudely layered gabbro that is correlative with the Zus Mountain gabbro to the east. The ultramafite consists of serpentinized and rodingitized dunite and peridotite sheet dikes. These rocks are composed of greater than 90 per cent olivine (dunite) and up to 10 per cent orthopyroxene and clinopyroxene (harzburgite to wehrlite). Intruded into this body are fine grained gabbro dikes. Chromite is ubiquitous throughout but only as disseminations of up to 1 per cent of the rock.

Alteration of the ultramafite varies from moderate to intense and four alteration types are present. The first and most extensive is serpentinization. This is best developed around the margins and base of the ultramafite but is present throughout the body. The serpentinite is usually quite sheared with the greatest intensity of deformation developed along the base and edges of the body. Non-rotated or slightly rotated blocks of serpentinite and peridotite are present in the sheared zones. The second most common type of alteration is secondary amphibole, tremolite, that is found scattered throughout the dunite. The third type of alteration is talc developed from retrograde metamorphism of the dunite. Talc alteration is most commonly developed within 305 to 457 metres of the adjacent Early Cretaceous Cassiar granitic intrusions. Large talc crystal rosettes and masses form zones of 10 to 30 percent of the rock, frequently overprinting the tremolite alteration. The fourth major type of alteration is regenerated dunite. This dunite contains a more forsteritic olivine (Fo 92-94) than the primary dunite (Fo 88-92). The regenerated olivine grains are up to 3 millimetres in diameter with rims of serpentinite. This alteration is readily identified in the field by its 'spotty' appearance. Locally rodingite zones, often tabular masses up to 100 metres in length and occasionally as discrete pods 0.5 metre across, occur in the dunite.

The Blue River ultramafite occupies the core of the McDame Synclinorium. The ultramafite consists of two, thin skin thrust sheets in the upper part of Division II of the Sylvester allochthon as defined by Nelson (Fieldwork 1987). The ultramafite is considered the lower part of an ophiolite sequence. The complex has been complicated by pre-thrust, transform fault serpentinization and serpentinite diapiric intrusion (Saleeby, J.B., 1979, 1984). The lower thrust layer consists of the dunite-peridotite body. This appears to be soled by a serpentinized breccia-shear zone. Large blocks of dunite, gabbro and pelagic sediments are included in this zone and it is postulated to be a major sole thrust (Nelson, J.L., personal communication, 1990). The upper thrust slice is the gabbro unit which is only of local extent due to erosion. The complex rests on a stack of pelagic sediments, limestone, chert, basalt breccia and flows, diabase and diorite. The structural setting and position leads to the correlation between the Blue River Ultramafite and the Zus Mountain Ultramafite where a more complete section is preserved (Fieldwork 1987).

The mineralization at Ice Lake consists of two pods of aggregate chromite in dunite, found during regional mapping. On the north side of the lake, one pod is 1 by 20 metres and contains 50 per cent chromite. The second pod is at the southeast end of the lake and consists of a 1 square metre pod containing 50 per cent chromite (Bradford, J., 1987, unpublished field notes). To the southeast in a small gully, between the heads of Heazelwood and Claimjumper creeks, is a zone of massive chromitite boulders. It is believed that the boulders are local to their source (Nelson, J.L., personal communication, 1990). No chemical analyses of the chromitites have been made and there has been no work done on the showings since their discovery.

In 2015, a rock sample (tag number A21559) from a chromite-bearing harzburgite assayed 0.28 per cent chromium and 0.22 per cent nickel (Assessment Report 35654).

In 2015, Minfocus Exploration Corp. conducted limited prospecting and geological mapping which included the collection of specimens of the major hostrock types for multi-element assays (including nickel, platinum and palladium) and/or a limited petrographic study. The Myst Metals project area covers four mineral occurrences: Blue River Nickel (104P 001), Wolfe (104P 055), Anvil Chromite (104P 100), and Ice Lake (104O 018).

Bibliography
EMPR ASS RPT 35654
EMPR FIELDWORK *1987, pp. 233-243, 245-248
EMPR OF 1988-10; 1996-11
EMPR MP MAP 1992-11
EMPR BULL 83
GSC MEM 319
GSC MAP 1110A
GSC OF 2779

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