"Rock of the Month # 58, posted April 2006" ---

Marbles are granular, finely- to coarsely-crystalline metamorphic rocks formed by the recrystallization of pre-existing carbonate-rich rocks, generally sedimentary limestones and dolostones. The choice of this sample was harder than you might think: a short list of ten was considered, winnowed from 49 marble samples from the Grenville province in Ontario, listed in Turnstone's reference catalogue. The featured marble (top) is a deformed, banded example from the York River skarn zone east of Bancroft. The "runners-up", which serve to hint at the diversity of marbles in the Central Metasedimentary Belt of the Grenville province, are (middle) a salmon-coloured marble from Faraday township, also near Bancroft, and (base) a dazzling white marble from Lanark county. The coin is a Canadian quarter, diameter 23 mm. Additional notes on each marble follow.....

Notes on 3 examples of Grenville marbles from southeast Ontario

All three show intense effervescence in dilute hydrochloric acid, consistent with a calcite-dominant composition, and are not appreciably magnetic.

2156

Banded marble from the York River skarn zone, a well-known rockhound locality east of Bancroft in Dungannon township, Hastings county. The unusual local geology, with close conjunction of granitic rocks, nepheline-bearing gneisses and pegmatites, and marbles has attracted interest for a century. This mineral occurrence is on the east bank of the York River, opposite another locality, the Goulding-Keene quarry. Hogarth et al. (1972, pp.62- 63) ascribe a multi-stage origin to the zone, concluding with late- stage ductile deformation and mylonitization. This is an attractive 17x14x6-cm slab of banded marble, with abundant greenish-black masses of secondary minerals (possibly brucite nodules with tochilinite?) and widespread traces of fine-grained (grain size 1 mm or less) orangey (?) grossular (Al,Ca)-rich garnet. The bands and clots of dark pyroxene occur in a very fine-grained, pale greyish, apparently massive calcitic matrix. The York River skarn zone, reportedly of dolomitic composition, is noted for specimens of garnet, diopside, vesuvianite (idocrase) and wollastonite, and other minerals, over 30 species in total (Sabina, 1986, pp.16-17; Fouts, 1998, p.22). The brucite association is said to include an obscure hydrated Mg-Fe carbonate, brugnatellite. Rocks like this really deserve a thin section, currently unavailable!

2173

Orange marble from the Hastings Road West quarry (the lower and more southerly of two contiguous sites, perhaps synonymous with the Barker quarry) in Faraday township, near Bancroft, in Hastings county. A very attractive orange marble, containing green to greenish white inclusions and bands of finer-grained material, probably largely calcitic carbonate of a delicate salmon hue, grain size 1-2 mm, with patches enriched in pale to dark green amphiboles. Size of sample is 15x9x5 cm. See also Kingston and Papertzian (1994).

2092

White calcitic marble from the Tatlock marble quarry, Darling township, Lanark county. This is an active quarry, the source of feed for the important OMYA plant near Perth (see, e.g., Sangster et al., 2001; 2005). The raw material (marble) from this site is generally very pure indeed: a little pyrite is present occasionally. Very pure white calcitic marble, granular and massive, quite coarse-grained (2-7 mm). This sample is 12x9x4.5 cm.

The building stones of the region have been used in the past for a wide range of construction, decorative and landscaping purposes. Several local stones, including some Grenville marbles from the Bancroft area and Quebec, were employed in the rebuilding of the Parliament Buildings in Ottawa in the decade following the fire of 1916 which destroyed the original Centre Block (Lawrence, 2001). Fine-grained white dolomitic marble of the former McMillan quarry in Dungannon township was used in government buildings in both Ottawa and Toronto. Marble and granite quarries still operate today, and surely have potential for future sales. Ontario architects and interior designers import facing stone, tile and other rock from around the world: from Europe, India. Latin America, the Far East and elsewhere. Very specific requirements may demand imports from overseas, but there is certainly scope for a major revival of "home-grown" building stones!

The best single source of information on building-stone resources in Ontario lies with the provincial government (Ministry of Northern Development and Mines Resident Geologists' offices, and the Ontario Geological Survey). Building stone properties were reviewed by Verschuren et al. (1985). A survey of limestone (and some marble) resources by DMBW&OGS (1989) includes descriptions of marble quarries in the Madoc area, plus Tatlock, Haley Station and other districts. Many marbles are described in detail by Grant et al. (1989), and a few more by LeBaron et al. (1990). There is also an extensive literature of regional memoirs, mineral-locality descriptions and mineralogical research articles, all of variable relevance to the marbles. Some 15 percent of >2,800 MINLIB bibliographic records on the Grenville province concern, or at least mention, marble.

Postscript: identifying a mystery rock:

A "mystery rock" was proffered in Campbellford in October 2021. It is shown in the above photo. Along the way, here are some observations that led to the identification: note that the quantitative density and magnetic measurements are not required to reach a conclusion:

• Size: fits nicely in the hand, almost 12 x 6.5 x 4 cm
• Weight: 399.02 grams
• Heft: feels pleasingly solid
• Aspect: Banded, with parallel bands circa 3-8 mm thick
• Texture: appears finely granular, grain size roughly 1 mm
• Acid reaction: fairly rapid effervescence in 10% hydrochloric acid
• Mineralogy: appears to be largely calcite, with abundant small flakes of pale brown phlogopite mica
• Fossils: none seen
• Specific gravity: 2.68
• Magnetism: not appreciably magnetic but instrumentally measured value of magnetic susceptibility is around 0.475x10^-3 SI units (logΧ = 2.52 (10^-9 m³/kg)), more than typical sediments, probably due to presence of traces of the iron oxide magnetite.
• Conclusions: A carbonate rock. The banding, presence of mica, relatively solid feel and (though not diagnostic) absence of fossils are consistent with a calcitic marble rather than the local limestone. This would be a Grenville marble. The rock would have been deposited as limy mud on a carbonate shelf, in a warm, fairly shallow sea, perhaps 1200 million years ago, and then deeply buried, squeezed and metamorphosed (developing the banded fabric, mica flakes and inferred magnetite grains) sometime in the following 300 million years. In the geologically recent past, it was brought tens to 100 or more km from the north and east during the Laurentian glaciation. Maybe not exactly local, but a regional representative, of a kind mostly found, eastwards of Peterborough, north of highway 7! A small piece such as this could be part of the regional deposit known as glacial till, or originally mounded into a subglacial landform such as a drumlin or esker, both of which can be seen in Trent Hills and west past Rice Lake to Peterborough. This piece was picked up in a garden off Raglan Street, within 100 metres or so from the place where a 12-tonne boulder was dug up along Grand Road, beside the Trent river (see Gabbro erratic boulder).

References

DERRY, MICHENER, BOOTH and WAHL and ONTARIO GEOLOGICAL SURVEY (1989) Limestone Industries of Ontario, Volume II - Limestone Industries and Resources of Eastern and Northern Ontario. Ontario MNR Report, 196pp.

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

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.

HOGARTH,DD, MOYD,L, ROSE,ER and STEACY,HR (1972) Classic Mineral Collecting Localities in Ontario and Quebec. Int.Geol.Congress 24, Montreal, Excursion Guidebook A47-C47, reprinted as GSC Misc.Pap. 37, 79pp.

KINGSTON,PW and PAPERTZIAN,VC (1994) New dimension-stone quarry developments in eastern Ontario. OGS Misc.Pap. 163, 62-65.

LAWRENCE,DE (2001) Building stones of Canada's federal parliament buildings. Geoscience Canada 28 no.1, 13-30.

LEBARON,PS, VERSCHUREN,CP, PAPERTZIAN,VC and KINGSTON,PW (1990) Building Stone Potential in Eastern Ontario. OGS MDC 30, 368pp.

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

SANGSTER,PJ, PAPERTZIAN,VC and LAIDLAW,DA (2001) Southern Ontario regional resident geologist program - 2000 (southeast district). OGS OFR 6052 part 1, 35pp.

SANGSTER,PJ, PAPERTZIAN,VC, STEELE,KG, LEE,CR, BARUA,M, LAIDLAW,DA and CARTER,TR (2005) Report of Activities, 2004. OGS OFR 6152, 77pp.

VERSCHUREN,CP, VAN HAAFTEN,S and KINGSTON,PW (1985) Building stones of eastern Ontario, southern Ontario. OGS OFR 5556, 116pp.