Fundamental Minerals

Sialic Minerals

Feldspar

Feldspathoid

Quartz

Femic Minerals

Accessory Minerals

Very Rare Minerals

Secondary Minerals

Texture

Alteration Products

Glass and related Texture

Volcanic Rocks

Komatiites

Lamprophyres

komatiite, Alexo (Canada)

The Abitibi greenstone belt is located within the southeastern portion of the Superior Province (Fig.1), the world’s largest exposed Archean craton, which is composed of amalgamated Mesoarchean gneissic units and Neoarchean granite-greenstone terranes. The Abitibi greenstone belt straddles the Ontario-Quebec border, covering an area that is approximately 500 km from east to west and 350 km from north to south. The belt is bound to the north by the Opatica Subprovince, a dominantly meta-plutonic domain. To the southeast, the Abitibi greenstone belt is bounded by the Pontiac Subprovince, which is dominated by metasedimentary schist and paragneiss derived from turbiditic graywacke and minor conglomerate. The Grenville Province is located to the east and largely consists of high-grade metamorphic rocks. To the west, the Abitibi greenstone belt is bounded by the 500-km-long north northeast-trending Kapuskasing structural zone that juxtaposes granulite facies metamorphic rocks with lower grade metamorphic rocks of the Abitibi greenstone belt.

Fig.1: Generalized geological map of the Abitibi greenstone belt showing the distribution of volcanic, intrusive, and siliciclastic rocks. The map also highlights the location of major ore deposits. Modified from Thurston et al., 2008.

The Abitibi greenstone belt is primarily composed of laterally extensive submarine volcanic rocks, which formed diachronously over a 100 m.y. period between 2795 and 2695 Ma. These volcanic rocks belong to six supracrustal assemblages, which have been delineated according to age, rock type, and geochemical characteristics (Thurston et al., 2008). In the southern Abitibi greenstone belt, mafic volcanic rocks form approximately 90% of the outcrop area, with felsic volcanic rocks accounting for most of the remainder outcrop areas (Goodwin, 1977). Komatiites represent a small, but important part of many of the volcanic successions in the Abitibi. Several models have been proposed to explain the distribution of rock types in the Abitibi. Some workers argue for an allochthonous origin where each assemblage represents an accreted Terrane. Others envision that the various volcanic assemblages formed autochthonously perhaps on an oceanic plateau. In either scenario, both plume and arc-related volcanism may have played important roles in the formation of these submarine assemblages.

Alexo Komatiite

The Alexo deposit occurs within the 2.720-2.710 Ma Kidd-Munro assemblage of the Abitibi greenstone belt, in the northeast part of Dundonald Township, 45 km northeast of the city of Timmins, Ontario (Fig. 2).

Fig.2: Geological map of the southern Abitibi greenstone. LLCfz = Larder Lake-Cadillac fault zone; PDfz = Porcupine-Destor fault zone. Modified from Thurston et al., 2008.

The petrology of the Alexo komatiite flow and an underlying komatiitic basalt flow were studied by Arndt (1977, 1986) and Barnes et al. (1983). The age of the flow was determined by Dupre et al. (1984), using Pb-Pb and Sm-Nd methods, to be 2690 ± 15 and 2752 ± 87 Ma, respectively.

The flow is ~16 m thick (Fig. 3), with an uppermost ultramafic hyaloclastite breccia (~2 m thick), a thick spinifex layer (~9 m), and an underlying olivine cumulate layer (~5 m thick). Immediately below the hyaloclastite there is a thin, discontinuous chill zone. This rock, which is made up of 1-2% small euhedral olivine phenocrysts in a groundmass of very fine skeletal olivine grains and altered glass, was taken by Arndt (1986) and Nisbet et al. (1993) to represent the highly magnesian (28% MgO) liquid from which the flow differentiated. Underlying spinifex rocks vary widely in composition as a result of olivine crystallization and accumulation. The spinifex zone is subdivided in three layers (Fig.3):

1. Random spinifex,
2. Chevron spinifex
3. Plate spinifex.

In the lower part of the plate spinifex zone olivine blades exceed 50 cm in length and lie in a matrix of relatively coarse, zoned, acicular grains of pigeonite-augite and plagioclase. The B1 layer, an irregular band rich in originally horizontal tabular skeletal olivine grains, lies between the spinifex and cumulate parts of the flow. The cumulate itself contains up to 80% of close-packed and overgrown, polyhedral to skeletal olivine grains, again in matrix of pyroxene and glass.

Despite a generally high level of textural and mineralogical preservation, the flow has been affected by various types of alteration which occurred at different times and affected different parts of the flow to different degrees:

(1) In the flow-top breccia all phases are completely chloritized.
(2) Small amounts of sulfide have been added to samples near the flow top; pyrite and pyrrhotite to the breccia and pentlandite and pyrrhotite to the uppermost spinifex-textured facies.
(3) The entire flow has been hydrated, a process manifested in the alteration of olivine to antigorite plus magnetite, the alteration of pyroxene to tremolite, and the transformation of glass to an assemblage of fine-grained chlorite and titanite.
(4) Samples from the lower part of the spinifex layer and the B1 layer in the eastern part of the outcrop are rodingitized. In this type of alteration, olivine is completely replaced by chlorite in spinifex lavas or by an assemblage of chrysotile plus brucite and magnetite in the cumulate layer.

Fig.3: Sketch map of part of the Alexo area showing the komatiite flow. Modified from Lahaye, Y., & Arndt, N. (1996).

Panoramic view of Komatiite thin section from Alexo, Canada. Image by Andy Tindle (Virtual Microscope). PPL image, field of view = 2.5cm.

Panoramic view of Komatiite thin section from Alexo, Canada. Image by Andy Tindle (Virtual Microscope). XPL image, field of view = 2.5cm.

Sample kindly given to me by Michael C. Lesher (Laurentian University, Canada).

Bibliography

• Lahaye, Y., & Arndt, N. (1996). Alteration of a komatiite flow from Alexo, Ontario, Canada. Journal of Petrology, 37(6), 1261-1284.
• Houlé, M. G., Lesher, C. M., & Davis, P. C. (2012). Thermomechanical erosion at the Alexo Mine, Abitibi greenstone belt, Ontario: implications for the genesis of komatiite-associated Ni–Cu–(PGE) mineralization. Mineralium Deposita, 47(1-2), 105-128.

Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Alexo Komatiite, Canada. PPL image, 1x (Field of view = 9mm)	Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Alexo Komatiite, Canada. PPL image, 1x (Field of view = 9mm)	Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Alexo Komatiite, Canada. PPL image, 1x (Field of view = 9mm)
Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Alexo Komatiite, Canada. PPL image, 1x (Field of view = 9mm)	Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Alexo Komatiite, Canada. PPL image, 1x (Field of view = 9mm)	Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Alexo Komatiite, Canada. PPL image, 1x (Field of view = 9mm)
Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Alexo Komatiite, Canada. PPL image, 1x (Field of view = 9mm)	Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Alexo Komatiite, Canada. PPL image, 1x (Field of view = 9mm)	Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Alexo Komatiite, Canada. PPL image, 1x (Field of view = 9mm)
Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Alexo Komatiite, Canada. PPL image, 2x (Field of view = 7mm)	Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Alexo Komatiite, Canada. XPL image, 2x (Field of view = 7mm)	Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Alexo Komatiite, Canada. PPL image, 2x (Field of view = 7mm)
Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Alexo Komatiite, Canada. PPL image, 2x (Field of view = 7mm)	Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Alexo Komatiite, Canada. XPL image, 2x (Field of view = 7mm)	Skeletal olivine crystals (Serpentinized), regularlyarranged, in a groundmass of skeletal Pyroxenes. Alexo Komatiite, Canada. PPL image, 2x (Field of view = 7mm)
Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Alexo Komatiite, Canada. PPL image, 2x (Field of view = 7mm)	Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Alexo Komatiite, Canada. PPL image, 2x (Field of view = 7mm)	Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Alexo Komatiite, Canada. PPL image, 2x (Field of view = 7mm)
Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Alexo Komatiite, Canada. XPL image, 2x (Field of view = 7mm)	Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Alexo Komatiite, Canada. PPL image, 2x (Field of view = 7mm)	Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Alexo Komatiite, Canada. XPL image, 2x (Field of view = 7mm)
Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Alexo Komatiite, Canada. PPL image, 2x (Field of view = 7mm)	Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Alexo Komatiite, Canada. PPL image, 2x (Field of view = 7mm)	Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Alexo Komatiite, Canada. XPL image, 2x (Field of view = 7mm)
Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Alexo Komatiite, Canada. PPL image, 2x (Field of view = 7mm)	Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Alexo Komatiite, Canada. PPL image, 2x (Field of view = 7mm)	Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Alexo Komatiite, Canada. PPL image, 2x (Field of view = 7mm)
Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Alexo Komatiite, Canada. PPL image, 2x (Field of view = 7mm)	Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Alexo Komatiite, Canada. XPL image, 2x (Field of view = 7mm)	Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Alexo Komatiite, Canada. PPL image, 2x (Field of view = 7mm)
Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Alexo Komatiite, Canada. PPL image, 2x (Field of view = 7mm)	Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Alexo Komatiite, Canada. PPL image, 2x (Field of view = 7mm)	Skeletal olivine crystals (partially Serpentinized) in a groundmass of skeletal Pyroxenes. Alexo Komatiite, Canada. PPL image, 2x (Field of view = 7mm)
Skeletal olivine crystals (partially Serpentinized) in a groundmass of skeletal Pyroxenes. Alexo Komatiite, Canada. XPL image, 2x (Field of view = 7mm)	Skeletal olivine crystals (partially Serpentinized) in a groundmass of skeletal Pyroxenes. Alexo Komatiite, Canada. PPL image, 2x (Field of view = 7mm)	Skeletal olivine crystals (partially Serpentinized) in a groundmass of skeletal Pyroxenes. Alexo Komatiite, Canada. XPL image, 2x (Field of view = 7mm)
Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Alexo Komatiite, Canada. PPL image, 2x (Field of view = 7mm)	Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Alexo Komatiite, Canada. PPL image, 2x (Field of view = 7mm)	Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Alexo Komatiite, Canada. PPL image, 2x (Field of view = 7mm)
Skeletal olivine crystals (partially Serpentinized) in a groundmass of skeletal Pyroxenes. Alexo Komatiite, Canada. XPL image, 2x (Field of view = 7mm)	Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Alexo Komatiite, Canada. PPL image, 2x (Field of view = 7mm)	Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Alexo Komatiite, Canada. XPL image, 2x (Field of view = 7mm)
Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Alexo Komatiite, Canada. PPL image, 2x (Field of view = 7mm)	Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Alexo Komatiite, Canada. XPL image, 2x (Field of view = 7mm)	Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Alexo Komatiite, Canada. PPL image, 2x (Field of view = 7mm)
Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Alexo Komatiite, Canada. PPL image, 2x (Field of view = 7mm)	Skeletal Pyroxenes. Alexo Komatiite, Canada. XPL image, 2x (Field of view = 7mm)	Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Alexo Komatiite, Canada. PPL image, 2x (Field of view = 7mm)
Plumose Skeletal Pyroxenes between olivine crystals. Alexo Komatiite, Canada. PPL image, 2x (Field of view = 7mm)	Plumose Skeletal Pyroxenes between olivine crystals. Alexo Komatiite, Canada. XPL image, 2x (Field of view = 7mm)	Skeletal olivine crystals (Serpentinized) in a groundmass of skeletal Pyroxenes. Alexo Komatiite, Canada. PPL image, 2x (Field of view = 7mm)
Skeletal Pyroxenes and Skeletal Magneitite. komatiite di Alexo, Canada. PPL image, 10x (Field of view = 2mm)	Skeletal Pyroxenes and Skeletal Magneitite. komatiite di Alexo, Canada. PPL image, 10x (Field of view = 2mm)	Skeletal Pyroxenes and Skeletal Magneitite. komatiite di Alexo, Canada. PPL image, 10x (Field of view = 2mm)
Skeletal Pyroxenes and Skeletal Magneitite. komatiite di Alexo, Canada. PPL image, 10x (Field of view = 2mm)	Skeletal Pyroxenes and Skeletal Magneitite. komatiite di Alexo, Canada. PPL image, 10x (Field of view = 2mm)	Skeletal Pyroxenes and Skeletal Magneitite. komatiite di Alexo, Canada. PPL image, 10x (Field of view = 2mm)
Plumose Skeletal Pyroxenes. Alexo Komatiite, Canada. PPL image, 2x (Field of view = 7mm)	Plumose Skeletal Pyroxenes. Alexo Komatiite, Canada. PPL image, 2x (Field of view = 7mm)	Plumose Skeletal Pyroxenes. Alexo Komatiite, Canada. XPL image, 2x (Field of view = 7mm)
Plumose Skeletal Pyroxenes. Alexo Komatiite, Canada. PPL image, 2x (Field of view = 7mm)	Plumose Skeletal Pyroxenes. Alexo Komatiite, Canada. XPL image, 2x (Field of view = 7mm)	Skeletal Pyroxenes and Skeletal Magneitite. komatiite di Alexo, Canada. PPL image, 10x (Field of view = 2mm)
Skeletal Pyroxenes between olivine crystals. Alexo Komatiite, Canada. PPL image, 10x (Field of view = 2mm)	Skeletal Pyroxenes between olivine crystals. Alexo Komatiite, Canada. XPL image, 10x (Field of view = 2mm)	Skeletal Pyroxene. Alexo Komatiite, Canada. PPL image, 10x (Field of view = 2mm)
Skeletal Olivine (spinal column). Alexo Komatiite, Canada. PPL image, 10x (Field of view = 2mm)	Skeletal Olivine (spinal column). Alexo Komatiite, Canada. XPL image, 10x (Field of view = 2mm)	Skeletal Pyroxenes between olivine crystals. Alexo Komatiite, Canada. PPL image, 10x (Field of view = 2mm)
Skeletal Pyroxenes between olivine crystals. Alexo Komatiite, Canada. XPL image, 10x (Field of view = 2mm)	Skeletal Pyroxenes between olivine crystals. Alexo Komatiite, Canada. XPL image, 10x (Field of view = 2mm)	Skeletal Pyroxenes between olivine crystals. Alexo Komatiite, Canada. PPL image, 10x (Field of view = 2mm)