"Rock of the Month # 230, posted for August 2020" ---

Let's start with a few terms, in minimally-technical definitions:

• Granite, a plutonic (relatively deep-seated) igneous rock that crystallized in the Earth's crust, generally cooling slowly enough that many crystals have time to grow relatively large (a few mm or cm). Composed essentially of quartz, feldspar(s), and a mineral that is often darker, such as a mica or amphibole, plus minor ("accessory") minerals that may comprise a few percent of the rock.
• Leucogranite, simply a granite with a very low percentage of darker minerals (such as biotite mica or the amphibole hornblende), resulting in a pale ("leucocratic") rock.
• Pegmatite, a very coarse grained variant of plutonic igneous rock, generally formed by crystallization very late in the cooling of a body of molten rock (magma). The late stages of solidification of a granite mass occur at lower temperatures, and often a higher proportion of volatile components (such as water, lithium or boron). Elements that have failed to find a home in the major granite minerals may form unusual minerals at this stage. Examples of pegmatite minerals include the borosilicate tourmaline family, beryl (the family of gemstones including emerald) and the lithium mica lepidolite. The term is often used in a qualitative sense: granites which are coarse, with many cm-scale crystals, are often termed pegmatitic.
• S-type granite. A more technical term from petrology (the study of rocks), referring to granites that are thought to have formed in part by the melting of pre-existing sediments. Pegmatites often occur as minor intrusions of irregular form, or as segregations in the walls and apices of magma chambers. Some of the S-type granites may form by the anatexis (partial melting, in metamorphism) of metasediments. They may be enriched in oxide minerals of metals such as tin, tantalum and niobium, in beryl and many other minerals (Kissin et al., 1986).

These samples (Fig. 1) and related outcrops (Fig. 2) are in a district that was long ignored by mineral exploration, perceived to be limited to the deep crustal rocks of the Quetico belt, largely metasediments and granitic intrusives, locally cut or overlain by diabase dykes and sills of the younger Mid-Proterozoic Midcontinent Rift. This changed with the drill confirmation of a mineralized ultramafic intrusion beneath Current Lake (see a sample of drill core here). The December 2006 discovery by Magma Metals Ltd. led to exploration and further discoveries in the belt, which runs east-west to the north of the city of Thunder Bay. The mineralized bodies are of Keweenawan (mid-Proterozoic) age, a little older than the more extensive, voluminous and prominent diabase sheets (sills), which form local beauty spots such as the Sleeping Giant on the Sibley Peninsula, a landmark of the north shore of Lake Superior.

Mineralogically, the typical leucogranite is quite simple, with the variable proportions of quartz, alkali and sodic plagioclase feldspars, amd muscovite. They are accordingly non-magnetic, though some units have precipitated coarse magnetite crystals (Fig. 2), and may accordingly form prominent magnetic highs on geophysical maps. Typical leucogranites are not appreciably magnetic (magnetic susceptibility <0.02x10^-3 SI units, less than metasediments (circa 0.2x10^-3 SI units), cf. magnetite-bearing diabase (circa 10x10^-3 SI units) and peridotites. These quartzose rocks are, at least in hand specimen, typically quite massive and undeformed, indicating a relatively late date of emplacement in the host rocks. Being hard and massive, they often appear in the boreal forest as small rounded knobs, often with bare summits, in the landscape which is for the most part gently rolling, except where diabase sills form vertical cliffs. The granite knobs are typically 10-100 metres in scale, and quite smoothly rounded.

The belt is named for Quetico, a settlement and large, eponymous park west of Thunder Bay, just north of the border with Minnesota. The silty to sandy quartzose sediment known as greywacke is the most abundant rock type. The sedimentary wedges were derived from volcanic arcs to north and south (the Wabigoon and Wawa subprovinces, respectively), and the belt is now a 10-100 km -wide, 1200-km-long, stratigraphically north-facing pile of "monotonous metagreywacke". Thermal relaxation in the 20 million years following accretion of the southern arc resulting in melting and the formation of granite suites, including leucogranite and pegmatite (Percival, 1989; Percival and Williams, 1989). The Quetico subprovince of the Precambrian Canadian Shield is composed largely of metamorphosed silts and sands. These metasediments were largely derived from (and deposited during and after) the volcanic climax in the neighbouring Wawa, Wabigoon and Abitibi subprovinces, 2700-2690 Ma. Amphibolite facies regional metamorphism, migmatite formation and granite intrusion (with late pegmatites) occurred 2670-2650 Ma (Percival and Sullivan, 1988; Williams, 1991). Migmatites (Sawyer, 2008) are the partially melted and deformed remnants of pre-existing rocks, a complex class of metamorphic rocks that have seen higher temperatures of metamorphism than more common metamorphic rocks such as schist and gneiss, both common in Precambrian crustal rocks.

Fig. 2: Quetico leucogranites often contain accessory minerals, which may include almandine-spessartine garnets (left) and/or magnetite (right), both of which occur as equant crystals that may achieve cm dimensions. These two outcrops are from the same circa 100 km² district as the samples in Figure 1, accessible beside Highway 527 which cuts south-north across the structural grain of the Quetico belt, or alongside or near the network of logging roads that run east and west from the highway. Photos: June 2008 and August 2009.

Exploration in the 1990s compared garnet crystals panned from local streams and glacial sediments with those in leucogranites. Chemical and textural similarity indicated that one was derived by erosion of the other. Many of the garnets were clearly zoned in optical terms, rich in Mn (the Mn-Al garnet spessartine, Mn₃Al₂(SiO₄)₃, transitional to the common Al-Fe garnet almandine, Fe²⁺₃Al₂(SiO₄)₃) with yellow cores somewhat enriched in Mn and Ca (and perhaps in Y too) and colourless rims with higher Fe and Mg (Wilson, unpublished work, 1995).

References

Kissin,SA, Zayachkivsky,B and Branscombe,LA (1986) Genesis of pegmatites in the Quetico gneiss belt of northwestern Ontario - granitoids, "barren" pegmatites, and metasediments, with additional data on rare-element pegmatites, from the Georgia Lake pegmatite field. OGS Misc.Pap. 130, 65-78.

Percival,JA (1989) A regional perspective of the Quetico metasedimentary belt, Superior province, Canada. Can.J.Earth Sci. 26, 677-693.

Percival,JA and Sullivan,RW (1988) Age constraints on the evolution of the Quetico Belt, Superior province, Ontario. GSC Paper 88-2 (Radiogenic Age and Isotopic Studies: Report 2), 155pp., 97-107.

Percival,JA and Williams,HR (1989) Late Archean Quetico accretionary complex, Superior province, Canada. Geology 17, 23-25.

Sawyer,EW (2008) Atlas of Migmatites. Canadian Mineralogist Spec.Publ. 9, 371pp.

Williams,HR (1991) Quetico subprovince. In `Geology of Ontario' (Thurston,PC, Williams,HR, Sutcliffe,RH and Stott,GM editors), OGS Spec.Vol. 4, part 1, 709pp., 382-403.