"Rock of the Month # 279, posted for September 2024" ---

Fuchsitic quartz-vein breccia

from the former Kenilworth gold mine, Timmins, Ontario, Canada. The mineral assemblage quartz- ankerite- mariposite (fuchsite, Cr-bearing white mica) is found in association with gold mineralization in many gold camps around the world. The gold values may largely occur in associated lithologies, but this "QAM" style of hydrothermal alteration of mafic, chromium-rich host rocks is a definite prospecting clue. See also an example from Western Australia.

The district near the Kenilworth and DeSantis mines contains mafic and ultramafic rocks on the regional Porcupine-Destor fault zone (Carlson, 1967). The Kenilworth (formerly Naybob or Hayden) mine is an erstwhile underground gold mine along the western extension of the storied Timmins / Porcupine mining camp in northeast Ontario. Ogden township displays carbonatized schists, porphyries, quartz veins, and green Cr micas, as well as occurrences of native gold, base-metal sulphides and tourmaline. The Hayden mine site was described by Hawley (1927, pp.24-25). Hayden Gold Mines was incorporated in 1912 and in 1915 sank a shaft near an exposed 15-foot (4.57-metre) -wide quartz vein with "partly assimilated inclusions of quartz porphyry", located just south of the shaft. By 1922 a 2-compartment shaft had been sunk to the 500-foot level. Over the years, some gold was produced, as summarized in Ontario Mineral Inventory file MDI42A06NW00022. In 1963, resources were summarized as 600,000 tons grading 0.20 oz/ton gold (south zone) plus a total of 138,500 tons grading up to 0.25 oz/ton (north zone). Episodic gold procuction was reported between 1932 and 1964, peaking in 1939-1942, with 412,237 ounces gold and 40,796 ounces silver recovered in 1940. If we assume that all the Au and Ag recovery was from native gold, that would imply an average gold composition of 910 fine (i.e., 91 wt% Au and 9 wt.% Ag). This is probably close to the true value, even if a little gold that was recovered was originally hosted within associated minerals, such as pyrite, arsenopyrite or chalcopyrite.

At the time of the 1985 visit, the mine stood idle, an unworked resource (EMR Canada, 1986). The metasediments and metavolcanics of the Timmins West district were eventually surveyed anew (Vaillancourt, 2000), including massive rhyolite and breccia, tuff, komatiitic flows, banded iron formation, clastic sediments and felsic porphyries. Spinifex-textured komatiitic flows are the likely source of Cr, generating the green micas during alteration.

A suite of samples was recovered from the mine tips beside the old headframe in 1985. The area was visited in a sampling program with the goal of investigating the occurrence of gold in lithologies enriched in reduced carbonaceous matter (Wilson and Rucklidge, 1987). A large slab (sample 732, Fig. 2) displays complex folding of a pink (?) sheared porphyry sill, with apparent chilled margins on either side, within a laminated pyritic, carbonaceous shale or siltstone. The siltstone consists of quartz, muscovite and pyrite lenses, plus minor to accessory reduced carbonaceous matter, carbonate, chalcopyrite and pyrrhotite. Porphyroblasts of pyrite occur in both porphyry and host metasediment. Drill logs from 1948 speak of "carbonaceous tuff", "acid tuff" and "greenstone" (Wilson, 1985).

Fig. 2: large sawn and polished slice showing evident felsic porphyry unit with pale margins, within a strongly foliated carbonaceous, pyritic host schist. Sample 732, collected on 20 September 1985. Not appreciably magnetic, magnetic susceptibility 0.02-0.07 (porphyry) and <0.01 (host black schist) x10^-3 SI units.

Samples 737 and 738 are variably foliated green schistose rocks (Fig. 3). Sample 738 (Wilson, 1988) is a foliated, bright green lithology, with bulk mineralogy similar to the featured sample: ankeritic carbonate, quartz and (thin-section modal estimate) 7% green mica, plus traces of pyrite and Fe oxides. These rocks appears to have a mafic-ultramafic (basaltic to komatiitic) protolith, the tectonic foliation cut by superposed brittle quartz-ankerite or quartz-calcite veinlets. The green mica is concentrated along the foliation.

Fig. 3: A green, foliated schist cut by quartz-carbonate veining, shown in mirror image in an offcut slice. The pearly white carbonate, intergrown with greyish-white quartz, does not react appreciably with 10% hydrochloric acid, more consistent with iron-rich ankerite or siderite than with dolomite or calcite. Sample 737, collected on 20 September 1985. Not appreciably magnetic, magnetic susceptibility 0.2-0.3x10^-3 SI units.

REFERENCES

Carlson,HD (1967) The Geology of Ogden, Deloro and Shaw townships, District of Cochrane. ODM OFR 5012, 138pp.

Energy,Mines and Resources Canada (1986) Canadian Mineral Deposits Not Being Mined in 1986. EMR Mineral Bull. MR 213, 375pp.

Hawley,JE (1927) Geology of Ogden, Bristol and Carscallen townships, Cochrane district, Ontario. ODM Ann.Rep. 35 part 6, 1-36.

Ontario Mineral Inventory file MDI42A06NW00022. Last revised 14 April 2023.

Vaillancourt,C (2000) New geological mapping and compilation in the Timmins West area - Bristol and Odgen townships. In `Summary of Field Work and Other Activities 2000' (Ayer,JA et al., editors), OGS OFR 6032, chapter 4, 11pp.

Wilson,GC (1985) Untitled (carbonaceous gold ores of the Hoyle Pond, Owl Creek and Kenilworth mines, Timmins area, Ontario: and a carbonaceous siltstone from the Terrace Bay area). Technical report, 29pp., 30 December.

Wilson,GC (1988) Untitled (five samples from gold deposits in the Timmins and Larder Lake areas of Ontario, and a pyritic breccia from Colorado). Technical report, 7pp., 01 December.

Wilson,GC and Rucklidge,JC (1987) Geology, geochemistry, and economic significance of carbonaceous host rocks in gold deposits of the Timmins area. In `Geoscience Research Grant Program, Summary of Research 1986-1987', OGS Misc.Pap. 136, 241pp., 66-76.

DRAFT version, Graham Wilson, 06-12 August 2024

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