Precambrian metagabbro

--- Cordova Mines, southeast Ontario, Canada

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Figure 1. Ten short lengths of drill core from the Cordova Mines gold deposit, selected from the Drill Core Library at the Ontario Ministry of Northern Development and Mines facility in Tweed (MNDM, 1989). The core was drilled for Lasir Gold in January 1988. Most of the samples are medium to coarse-grained metagabbro, dominated by plagioclase feldspar and dark green clinopyroxene which has been uralitized, i.e., converted to secondary amphiboles such as actinolite in regional metamorphism. Much of the gabbro is essentially massive, if recrystallized, but some intervals are foliated and locally sheared. Other minerals include magnetite and ilmenite, epidote and carbonates.

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Figures 2-3. Views of polished thin sections in transmitted light. On the left: massive metagabbro sample 2. On the centre and right: pale, sheared, recrystallized (and thus finer-grained) samples 5 and (far right) 8. Each slice is some 4 cm high.

"Rock of the Month #149, posted for November 2013" ---

Metagabbro of the relatively small (circa 30 km2), subcircular outcrop of the Cordova pluton is exposed around the village of Cordova Mines, just north of Highway 7 east of Peterborough, Ontario. A small gold mine operated here intermittently between 1892 and 1940, the largest producer amongst numerous small gold mines developed in the region since the 1860s. The significance of these mines is historic rather than economic.

Local Geology

The Cordova Mines area lies on the Canadian shield a few km north of the unconformably-overlying northern margin of the Michigan basin, with its Paleozoic basin-filling sediments. In a study of 12 metagabbros in the Madoc-Bancroft area, the greenschist facies Cordova metagabbro recorded the lowest temperatures, the Boulter metagabbro the highest (Jwad, 1992). This is consistent with the Cordova pluton's setting in the Hastings basin, which displays perhaps the lowest metamorphic grades in the Grenville province in Ontario. Half of Belmont township is underlain by Paleozoic rocks, the other half by Grenville metavolcanic -dominated strata. In the latter, there are at least three mafic to felsic volcanic cycles (Bartlett et al., 1980; Bartlett and Moore, 1981). The host gabbro of the gold-bearing structures also contains iron oxide mineralization, known as the Belmont or Ledyard magnetite deposit, 800 m south of Cordova Mines in Belmont township (Rose, 1958, pp.18-20). The Cordova gabbro is also of interest for its building stone potential (LeBaron et al., 1990). The Cordova gabbro is dated at 1242±3 Ma (Davis and Bartlett, 1988). Thomas and Cherry (1981) distinguished four facies of gabbro: pegmatitic, fine-grained, foliated (layered) marginal gabbro, and minor anorthosite pods. The gold is associated with pyrite-quartz-ankerite hydrothermal alteration.

Petrography and Physical Properties

Samples 5-6, 8-10 show variable visual signs of shearing, while the other five samples are relatively massive (undeformed). All samples except No.7 show traces of sulphide (pyrite) on broken surfaces, at nominal levels of 0.1 to 0.7 volume percent. A polished thin section was prepared from samples 2, 5 and 8 to compare textures and mineralogy. The polished sections reveal abundant Fe-Ti oxides plus disseminated and shear-related sulphides such as pyrite, pyrrhotite and chalcopyrite.

The specific gravity values measured on core samples lie within the narrow range 2.91 to 3.22, the low and high values related to sheared sample 8 and massive sample 4, respectively. The uncorrected magnetic susceptibility ranges from 0.7 to 6.0x10-3 SI units, except for sheared and magnetite-rich sample 5 (18.2x10-3 SI units). This last is the only appreciably magnetic sample, though scattered magnetite crystals may attract a pen magnet in other lengths of the core. Carbonates, as indicated by reaction in 10% HCl, were noted in ankeritic shears (sample 8) and a calcite veinlet (sample 9). No SW/LW UV fluorescence was noted in any sample.

Systematic Mineralogy

The three thin sections shown here contain at least 14 discrete mineral species. A routine study affirms that the relatively undeformed gabbro is composed of amphiboles (hornblende, and a later generation of overgrowths on the coarse hornblende derived from magmatic pyroxene) and plagioclase feldspar (turbid with alteration to fine-grained epidote and/or zoisite), plus abundant columnar apatite, iron-titanium oxides such as ilmenite, and chlorite (penninite). Accessory phases include pyrrhotite, pyrite, chalcopyrite and hematite. Progressive deformation sees the introduction of increasing proportions of quartz, calcite and pyrite, coarser epidote crystals, and minor magnetite, combined with increasing amounts of chlorite and biotite mica, and a reduction in apatite content. Chalcopyrite, principal host of copper in the gold ore, is widespread but mostly very fine-grained (0.01 mm). On the edge of the most strongly sheared zone, coarser (0.1 mm) chalcopyrite occurs with the chlorite and biotite, calcite and quartz. The detailed study of Thomas (1985) also reports galena and ankerite, and (as in this instance) did not find arsenopyrite, a sulphide common in other Grenville gold deposits.

No information is available about the mineralogical association of the gold recovered during the mining years, but it seems likely that both Au and Ag occurred largely as native gold in sheared rock, intergrown with and included within quartz, carbonate, sheet silicates and pyrite. Other minerals reported here include orthoclase K-feldspar, as well as secondary titanite (sphene: Wilson, 1994).

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Figures 4-5. Photomicrographs of (on the left) massive metagabbro sample 2 and (right) sheared, recrystallized (and thus finer-grained) sample 5. Transmitted plane-polarized and (right) cross-polarized light, nominal magnification 50X, long-axis field of view circa 1.7 mm. The mineralogy of the massive gabbro is dominated by deep green secondary amphiboles, turbid plagioclase feldspar (much altered to epidote-family granular material), apatite and ilmenite. The sheared rock is dominated by deformed plagioclase, plus an alteration assemblage of quartz, chlorite and calcite, with accessory opaque magnetite.

Economic Geology

The gold rush in the region commenced with a small find at nearby Eldorado in 1866 (Boyce, 1992). The Cordova Mines deposit is quite well-documented, though by modern standards for hard-rock mines the historic production was very small. There are numerous descriptions and more than 60 articles, maps and books that include the area (see, e.g., Hopkins, 1924; Bruce, 1933; Carter, 1936; Kindle, 1936; Malczak et al., 1985; Thomas, 1985; Easton et al., 1986; LeBaron, 1991).

Mineralization was found in shear zones on the margins of the gabbroic pluton. The original discovery of gold at surface was made in 1890. Carter (1936, pp.63-64) observed that gold is localized along veins and shear zones in gabbro, the most important orebodies having chimney-like form, occurring at intersections of two veins. The Cordova mine produced 235 oz gold in 1892-1903. The nearby Ledyard occurrence (scarcely a mine, though such it was) produced another 13 ounces gold in the 1890s. Between 1892 and 1940 the total metal recovery at Cordova was 22,744 oz (708.3 kg) Au and 687 oz Ag from 120,670 tons of ore milled (average grade 0.189 oz/ton or 6.47 g/T: Satterly, 1943, pp.36-40). The metal was mostly won in 1912-1917, cf. the nearby Deloro mine's 10,360 oz (322.2 kg) production in 1897-1902 (Boyce, 1992). The ore was found entirely within, and near the margin of, the gabbro mass. The major minerals are labradorite and hornblende. Pegmatitic diorite and anorthosite occur with the gabbro/diorite, which is also cut by aplite dykes. Two injections of similar basic compositions are inferred. Near the west contact shear zones (1 to >40' wide, averaging 6' [0.30 to 12.2 m wide, averaging 1.83 m]) cut the intrusion, which is altered to biotite and chlorite, affirming Carter's earlier recognition of structural control on the mineralization. Pyrite is the main sulphide. Other minerals include minor pyrrhotite, plus a gangue of quartz, carbonate, plagioclase and orthoclase, The veins are often banded, especially along shear directions.

The gold at Cordova is found in four east-trending shear zones (and a fifth at the Ledyard mine). During the mining days, high-grade ore was indicated by a relatively blue colour indicating sulphide-included quartz, the white quartz being barren (Thomas, 1985, p.58).

Brown (1998) recounts the numerous small mines that for a period were connected to the outside by branch railway lines, including the Blairton, Coe Hill and Marmoraton iron mines. The spur line to Cordova Mines closed in 1941.

The old mine was inactive for many years, but in 1988 a drill program (that produced the core described herein) explored the main vein once again: carbonate alteration around quartz-carbonate veins on the flank of the Cordova gabbro mass. The host structure is a carbonatized shear zone up to 15 m wide and 700 m long. Magnetite is common in the host schist, which grades out into undeformed metagabbro (Davies and Whitehead, 2001). Modest exploration has occurred in the region since, for gold in the 1980s and 1990s (continued to this day by a dedicated few prospectors, more than companies). Some exploration for nickel and associated metals such as copper and PGE (platinum group elements) has been conducted in the past decade, a cyclic exercise repeated at intervals at least since the 1950s (Wilson, 1994). Some of the work has been made public, including the results of airborne geophysical surveys (e.g., Ontario Geological Survey, 2010a,b). Interest in the site, in addition to any exploration for new resources, factors in the 70,000 tons of mine tailings grading 0.04 oz/ton and underground reserves of roughly 150,000 tons grading 0.16 oz/ton (Narain and Burkart, 1986).

Sulphide-related gold in the Grenville province is typically associated with high arsenic values. The arsenic (mostly in the form of arsenopyrite, FeAsS) rendered much of the ores refractory, impeding production, and subsequently posing environmental hazard, since in the early days of mining the modern health and safety regulations lay far in the future. The nearby Deloro site, home to five old, small gold mines, is one of the major contaminated industrial sites in the province. Some of the arsenic no doubt came from the original late 19th century mining operations, but the bulk of the contamination (arsenic and low-level radioactivity) resulted from processing of arsenical silver-cobalt ores and uranium ores that were shipped to the site for treatment from northern mines in the first half of the 20th century.


This work was proposed by Francis Manns and the late Sivenas Prokopis. Full details in Turnstone G.S.L. Report 2013-10P. Peter LeBaron of the Tweed MNDM office assisted with core recovery. Anne Hammond prepared the polished thin sections. S.G. determinations by Lisa Winegarden.

Selected references in chronological order

Hopkins,PE (1924) Ontario gold deposits: their character, distribution and productiveness. ODM Ann.Rep. 30 part 2, 73pp., 1921, 2nd edition.

Bruce,E (1933) Mineral Deposits of the Canadian Shield. Macmillan Company of Canada Ltd, Toronto, 428pp.

Carter,OF (1936) The structural features of the gold deposits of Ontario. MA Thesis, University of Toronto, 73pp.

Kindle,ED (1936) Gold Occurrences of Ontario East of Lake Superior. GSC Memoir 192, 167pp.

Satterly,J (1943) Mineral occurrences in the Haliburton area. ODM Ann.Rep. 52 part 2, 106pp.

Rose,ER (1958) Iron deposits of eastern Ontario and adjacent Quebec. GSC Bull. 45, 120pp. plus 5 maps.

Bartlett,JR, Moore,JM and Murray,MJ (1980) Belmont and southern Methuen townships, Peterborough county. OGS Misc.Pap. 96, 92-95.

Bartlett,JR and Moore,JM (1981) Marmora, Belmont and southern Methuen townships, Peterborough and Hastings counties. OGS Misc.Pap. 100, 73-76.

Thomas,PB and Cherry,ME (1981) The geology of the Cordova gabbro and its associated gold deposits. OGS Misc.Pap. 100, 251-253.

Malczak,J, Carter,TR and Springer,JS (1985) Base metal, molybdenum, and precious metal deposits of the Madoc - Sharbot Lake area, southeastern Ontario. OGS OFR 5548, 374pp. plus map P2737.

Thomas,PB (1985) The Nature of Gold Deposits in Shear Zones in the Cordova Gabbro, Grenville province, Ontario. MSc Thesis, University of Ottawa, xi+156pp. plus map.

Easton,RM, Carter,TR and Springer,JS (1986) Mineral deposits of the Central Metasedimentary belt, Grenville province, Ontario and Quebec. GAC/MAC Field Trip Guide Book 3, 55pp.

Narain,M and Burkart,J (1986) Central regional geologist area, central region. OGS Misc.Pap. 128, 312-322.

Davis,DW and Bartlett,JR (1988) Geochronology of the Belmont Lake metavolcanic complex and implications for crustal development in the central metasedimentary belt, Grenville Province, Ontario. CJES 25, 1751-1759.

MNDM (1989) Tweed Drill Core Library Catalogue. Ontario Ministry of Northern Development and Mines, 20pp.

LeBaron,PS, Verschuren,CP, Papertzian,VC and Kingston,PW (1990) Building Stone Potential in Eastern Ontario. OGS MDC 30, 368pp.

LeBaron,PS (1991) Exploration for gold in southeastern Ontario, 1980-1990. OGS OFR 5808, 147pp. plus 1:125,000 scale map.

Boyce,G (1992) Eldorado: Ontario's First Gold Rush. Natural Heritage / Natural History Inc., Toronto, 160pp.

Jwad,N (1992) Evaluation of Amphibole Relationships in the Metagabbros of the Madoc-Bancroft Area, Southeastern Ontario. MSc Thesis, Department of Geology, University of Windsor, 155pp.

Wilson,GC (1994) Mafic-Ultramafic Intrusions, Base-Metal Sulphides, and Platinum Group Element Potential of the Grenville Province in Southeastern Ontario. OGS OFR 5880, 196pp.

Brown,R (1998) Ghost Railways of Ontario. Polar Bear Press, Toronto, 224pp.

Davies,JF and Whitehead,RE (2001) CO2, alkalies and REE systematics in hydrothermally altered gabbro hosting the Cordova gold-bearing veins, Ontario. Explor.Min.Geol. 10, 321-328.

Ontario Geological Survey (2010a) Airborne magnetic and electromagnetic surveys: colour-filled contours of the residual magnetic field and electromagnetic anomalies - Bancroft area. OGS map 60153, 1:20,000 scale.

Ontario Geological Survey (2010b) Airborne magnetic and electromagnetic surveys: shaded colour image of the second vertical derivative of the residual magnetic field and Keating coefficients - Bancroft area. OGS map 60168, 1:20,000 scale.

Graham Wilson, 12-17 October 2013, updated 16-18 November and 06 December 2013.

View a glacial erratic of this same metagabbro

Examine a "meteorwrong", a smaller boulder of this same rock (short and extended versions available)

See an example of a "coronite", a more intensely metamorphosed gabbro from the Muskoka district.

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