Calcitic marble, York River, Bancroft, Grenville province of Ontario

Calcitic phlogopite-tremolite-diopside marble

--- York River zone, Bancroft, Ontario, Canada

Figure 1. Hand specimen, a small, fresh-broken piece of the marble, collected from outcrop beside the York River, just east of the small town of Bancroft. Coarse calcite is clearly visible, plus lesser white to pale brown silicate grains.

"Rock of the Month #204, posted for June 2018" ---

MARBLE is one of the small number of rocks that are almost universally known, even and especially by novelists, few of whom are evidently geologists! The most commonly-recognized names of distinct rock types or rock "clans" include granite, marble and basalt, and perhaps limestone and sandstone, chalk or flint, though occasionally a writer will fixate on a less-common variant, such as travertine.... This month's subject is the 204^th in this series, and marks 17 years'-worth of specimens, from the most mundane to the vanishingly rare.

Marble is a distinctive crystalline rock, formed by the metamorphism, under heat and a variable degree of pressure, of carbonate sedimentary rocks such as limestone and dolostone. The formation of marble involves recrystallization of typically very fine-grained ("mud") calcareous sedimentary rock to form much coarser carbonate crystals. The sample shown here was collected in situ from the left bank of the York River, with the blessing of property owners Jules and Bonnie. At first glance it is pure white crystalline calcite (Fig. 1). A polished thin section was prepared and examined, revealing the presence of additional minerals (Table 1, Figs. 2-5). The principal mineral is calcite, in crystals up to at least 7x5 mm in section. The microscope reveals prisms of calcium-rich silicates, diopside and tremolite, which are abundant but, like calcite, are white to colourless in appearance, containing little iron, which would impart a darker, often greenish hue. There are also flakes of the mica, phlogopite, which are pale brown in hand specimen. The diopside was later partially altered to a brownish silicate, identified tentatively as a member of the epidote family. The marble also contains a trace of pyrite, "fool's gold", common iron sulphide.

The sample described here appears largely unstrained, but marbles are not immune from deformation in shear zones, forming mylonites (rocks in which the minerals of platy to prismatic forms are typically rotated into parallel alignment) by grain size reduction of calcite and other minerals such as graphite (Carlson et al., 1990; van der Pluijm, 1991). The latter author found that calcite grain size ranges from several mm in the protolith to 0.05-0.2 mm in mylonite to <0.03 mm in ultramylonite. The York River marble's calcite is not always free of deformation: curving planes cutting some of the crystals attest to a later (post-metamorphic) episode of stress, resulting in nascent fractures decorated with tiny bubbles ("secondary fluid inclusions) seldom larger than 0.005 mm in diameter.

Belts of carbonate rocks, all now marbles, occur throughout the Central Metasedimentary Belt of the Grenville province, and in Ontario are exposed at many localities from the Ottawa River west to Georgian Bay of Lake Huron. The individual marble units may be either calcitic or dolomitic in their bulk composition.

From an industrial minerals standpoint, Grant et al. (1989) reviewed the carbonate metasediments over a large area of the Grenville, from Burleigh Falls and Wilberforce east past Bancroft to Eganville, Renfrew and Arnprior, and (further south) Eldorado, Madoc, Plevna and Ompah, east to Carleton Place, Perth and Westport, Perth Road, Kingston and Gananoque.

Table 1. Modal mineralogy (visual estimates)

Table 1. Modal mineralogy

Mineral	Vol.%	Idealised mineral formula	Class
Calcite	_78	CaCO₃	Carbonate
Diopside	_15	CaMgSi₂O₆	Clinopyroxene
Tremolite	__5	Ca₂(Mg>>Fe)₅Si₈O₂₂(OH)₂	Amphibole
Phlogopite	__2	KMg₃(Si₃Al)O₁₀(OH)₂	Mica
Epidote (?)	Trace	Ca₂(Al₂Fe^III)[O\|OH\|SiO₄\|Si₂O₇]	Epidote
Pyrite	Trace	FeS₂	Sulphide

Figures 2-3. Left: coarse calcite plus minor diopside. At lower right is a small flake of phlogopite mica, showing distorted first-order interference colours. Right: twinning in calcite. The carbonate has recrystallized to produce a fabric of typical 120-degree grain-boundary triple junctions. As mentioned in the text, some later deformation is evident in some calcite crystals in the rock, with curved fractures studded with minute fluid inclusions. Photomicrographs are stacks of 10 digital images, long-axis field of view 1.7 mm, 50X nominal magnification, in crossed-polarized, transmitted-light (XP-TL) illumination.

The Wider Picture

Suites of unusual rock types and mineral assemblages are found in a belt running for circa 160 km in a WSW-ENE direction, including the town of Bancroft, which soon became a focus for mining of diverse commodities. The Grenville subprovince of the Precambrian Canadian shield is noted for many old mines, most of which worked so-called industrial minerals, such as calcite, feldspar, apatite (phosphate), micas, graphite, wollastonite, vermiculite, barite, and useful rocks such as marble, granite and nepheline syenite. There were also mines, small by modern standards, for various metals, including gold, uranium, lead and zinc, some of which contained traces of other elements, such as arsenic and mercury.

Bancroft remains - in the context of mineral collecting - the "Mineral Capital of Ontario" to this day, with a well-attended rock, mineral, fossil and jewellery show held at the start of August each year, the Bancroft Gemboree: the 55th annual meeting happens on 2-5 August, 2018.

The York River zone includes, besides the featured marble, more exotic skarns and nepheline-bearing lithologies, described and catalogued in a string of helpful publications (e.g., Hewitt and James, 1955; Hewitt, 1969; Anon, 1982; Masson, 1982; Sabina, 1986; Fouts, 1998; Lumbers and Vertolli, 1998). Besides the mineralogy mentioned here, Grenville marbles may also contain a range of other minerals, such as dolomite, graphite, scapolite, sphene (titanite), apatite, forsterite (magnesian olivine), chondrodite, spinel and talc.

Mineralogist and professional collector Louis Moyd was one of many scientists to be intrigued by these odd mineral assemblages, which often include the feldspathoid silicate group, most notably nepheline and sodalite, as well as industrial-grade corundum (Al₂O₃ - formerly much in demand as an abrasive, not a gemstone, due to the Mohs hardness of 9). Much of Moyd's work focused on these nepheline- and corundum- bearing lithologies. He argued that the intrusion of granitic melts into silica-poor Grenville terrain triggered release of CO₂ from dolomitic marbles, promoting the alteration of dark gneisses and generation of aluminous, silica-poor, calcic rocks. These rocks are often but not always foliated, and include a range of gneisses, massive crystalline rocks of syenitic composition, and pegmatites (Moyd, 1946, 1949). Moyd's postulated fluids effected mineralogical changes in the host rocks, such as the formation of silicates in dolomitic marbles.

It has also been argued (Gittins, 1961, 1967) that some of the nepheline-bearing rocks are igneous rocks that retain original igneous textures, while the gneisses are of two kinds: metamorphosed igneous rocks and metasomatic (nephelinized) rocks. Moyd argued that many of the exotic rocks, to which earlier petrographers assigned curious and obscure names *, were not true igneous rocks formed from a primary magma. His well-expressed argument contributed to a conversation over petrogenesis that continues to this day.

* One of these, dungannonite, named for a local township, may be considered an anorthosite with accessory nepheline and corundum (Casselman, 1988, p.90). Corundum occurrences in the area are noted only where a marble, a nepheline syenite and a granite all lie in close proximity (ibid., p.104). Corundum is coarser, and therefore more prominent, near intrusive contacts (as an aside, supervised by W.T. Jolly, Anne Casselman's work is surely the most elaborate BSc thesis I have ever seen! - GCW).

Figures 4-5. Left: phlogopite flake in calcite host. Right: Tremolite crystals (left) and a rounded diopside grain (upper right) in calcite. Note the typical 120-degree intersecting cleavage planes in the basal section of the amphibole, tremolite. Photomicrographs are stacks of 10 digital images, long-axis field of view 1.7 mm, 50X nominal magnification (except left, the mica, 100X and 0.88 mm), in XP-TL illumination.

Adams and Barlow (1910) noted a very close association between granite, nepheline syenite, and "crystalline limestone" (marble). Although "there appears to be the germ of limestone syntexis in their thinking they certainly were not championing this mode of origin" (Gittins, 1967). Adams and Barlow noted that nepheline syenites generally occur along the borders of granite intrusions in the marbles. A study of drill core, from a well-known nepheline syenite pegmatite in the area, led to the conclusion that the pegmatite might have formed by partial melting of the enclosing gneisses, but could not be derived directly from typical nepheline syenite magma (Reeve and Anderson, 1972). Later work indicated that Na metasomatism (nephelinization), due to exchange of Na for Ca in plagioclase feldspar, is unlikely to have generated enough Na to produce much nepheline in these rocks, arguing against a metasomatic origin (Anderson and Cermignani, 1990, 1991).

See also illustrated examples of Grenville corundum and marbles.

References, in chronological order, n=19

Adams,FD and Barlow,AE (1910) Geology of the Haliburton and Bancroft areas, province of Ontario. GSC Memoir 6, 419pp. plus 2 maps.

Moyd,L (1946) Petrology of the nepheline and corundum-bearing rocks of southeastern Ontario. Amer.Mineral. 31, 201.

Moyd,L (1949) Petrology of the nepheline and corundum rocks of south-eastern Ontario. Amer.Mineral. 34, 736-751.

Hewitt,DF and James,W (1955) Geology of Dungannon and Mayo townships. ODM Ann.Rep. 64 part 8, 65pp. plus map 1955-8.

Gittins,J (1961) Nephelinization in the Haliburton-Bancroft district, Ontario, Canada. J.Geol. 69, 291-308.

Gittins,J (1967) Nepheline rocks and petrological problems of the Haliburton-Bancroft area. GAC/MAC Field Trip Guidebook, Kingston, 346pp., 31-57.

Hewitt,DF (1969) Geology and Scenery, Peterborough, Bancroft and Madoc Area. ODM Geological Guide Book 3, 114pp.

Reeve,EJ and Anderson,GM (1972) A chemical study of the Golding-Keene pegmatite and adjacent gneisses, York River area, Ontario. Can.Mineral. 11, 578-579.

Anon (1982) Bancroft's Guide to Rockhounding. Bancroft and District Chamber of Commerce, Bancroft, Ontario, 17pp.

Masson,SL (1982) Geology and mineral deposits of the Bancroft area, Eastern Part, southern Ontario. OGS Map P2524, 1:10,000 scale.

Sabina,AP (1986) Rocks and Minerals for the Collector: Bancroft - Parry Sound area and Southern Ontario. GSC Misc.Rep. 39, 182pp.

Casselman,AE (1988) A Petrological Study of the Type `Dungannonite' Corundum Locality, Egan Chute, York River area, Bancroft. BSc Thesis, Brock University, 110pp. plus 2 maps.

Grant,WT, Papertzian,VC and Kingston,PW (1989) Geochemistry of Grenville marble in southeastern Ontario. OGS MDC 28, 266pp. plus 1:250,000 scale map.

Anderson,GM and Cermignani,C (1990) Mineralogical constraints on the metasomatic origin of the York River nepheline gneisses, Bancroft, Ontario. GAC/MAC Prog.w.Abs. 15, 2, Vancouver.

Carlson,KA, van der Pluijm,BA and Hanmer,S (1990) Marble mylonites of the Bancroft shear zone: evidence for extension in the Canadian Grenville. BGSA 102, 174-181.

Anderson,GM and Cermignani,C 1991) Mineralogical and thermodynamic constraints on the metasomatic origin of the York River nepheline gneisses, Bancroft, Ontario. Can.Mineral. 29, 965-980.

Van der Pluijm,BA (1991) Marble mylonites in the Bancroft shear zone, Ontario, Canada: microstructures and deformation mechanisms. J.Struct.Geol. 13, 1125-1135.

Fouts,C (1998) Bancroft & District Mineral Collecting Guidebook. Bancroft & District Chamber of Commerce, 50pp.

Lumbers,SB and Vertolli,VM (1998) Geology of the Bancroft Area. OGS map P3385 (1:50,000 scale).

Graham Wilson, 05-07,16 July 2018

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