Gold Skarn

--- Dividend-Lakeview mine, Osoyoos, southern British Columbia, Canada


[186 kb] [161 kb]

Figure 1. Left: small hand specimen sawn in two, showing bands rich in tarnished metallic sulphides (in one case arsenopyrite, in the other, pyrite). A quartz vein cuts the calcitic host rock, a silicified marble, which contains disseminated and foliation-related sulphides. The sample was recovered in situ from a metre-scale marble lens in metavolcanic host rocks. Right: the corresponding offcut rock slice and polished thin section. The sample is banded: in the centre, the vein of coarse, highly strained quartz. On one (top, at right) side is a band of granular pyrite, with deep green chlorite and both disseminated and late veinlet-hosted calcite, cutting the quartz vein. On the opposite side of the quartz vein is a band rich in arsenopyrite (to 1.0 mm) with lesser chalcopyrite (to 0.3 mm) and fractured magnetite (to 0.25 mm). The arsenopyrite is sometimes fractured or crushed, and the chalcopyrite occurs as blebs that may be attached to the arsenopyrite, and in veinlets that penetrate the host sulphide. The gangue (matrix) to the ore minerals is largely deformed calcite crystals and fibrous pressure shadows of quartz (Fig. 3). These textures indicate a significant level of shear-related deformation in all the mineral assemblages.

At least four arsenopyrite crystals were found to host tiny (at most 0.02 mm) grains of native gold. The sample assayed 34.1 ppm Au (0.99 troy oz/short ton), 33.1 ppm Ag, 1.31% As, 0.39% Cu, 344 ppm Pb, 215 ppm Bi, 98 ppm W, 97 ppm Co and 1.37 ppm Te.


"Rock of the Month #153, posted for March 2014" ---

Gold skarn

is a style of mineralization commonly found within the same plate-tectonic settings as porphyry deposits, in British Columbia, Irian Jaya (West Papua) and elsewhere. These skarns are commonly enriched in gold, silver, bismuth, tellurium, arsenic and other metals and metalloids. Because a skarn is developed by contact metamorphism in vicinity to an intrusive body of magma, such recrystallization and new mineral deposition typically occurs on and near lithological contacts. Skarns in B.C. are developed in and around Hedley, Greenwood, Osoyoos and Rossland in the south, Texada Island, Zeballos on Vancouver Island, and elsewhere (Ettlinger and Ray, 1988, 1989; Wilson et al., 1990). The principal metal of economic interest may be gold, or iron, or copper. Ettlinger and Ray (1989) made a detailed review of precious metals in skarns across British Columbia, and found that gold and silver are enriched in at least 126 out of more than 350 skarn occurrences recorded in the province. Within the 49 sites with recorded production, 82% of the gold and 57% of the silver were produced by just two: the Nickel Plate and Phoenix mines (near Hedley and Greenwood, respectively).

The Dividend-Lakeview property displays gold skarn associated with sulphidic marble lenses within a sheared metavolcanic succession of the late Permian Kobau Group. Major zones of extensive hydrothermal alteration, principally garnetites, occur to the west of the structurally -controlled Dividend pit. A suite of 37 samples assayed up to 42.5 ppm gold and 68.8 ppm silver (1.24 oz/T Au and 2.01 oz/T Ag), 1.31 wt.% As, 1.30% Cu and 607 ppb Bi (Wilson, 1990). Native gold is hosted by arsenopyrite. Samples in the old mine workings display such minerals as chalcopyrite and pyrrhotite, epidote, chlorite, calcite and zoned crystals of garnet. The Dividend claims are underlain by altered limestones in contact with basic volcanics within the contact aureole of a major granitic body.

The site is scarcely 1 km north of the International (Washington, U.S.A.) boundary on the 49th parallel. The claims are on the lower eastern slope of Kruger Mountain, in a 3 km2 area 365 to 760 m above sea level. They lie above, and some 3 km west of the small town of Osoyoos. The region is littered with the remains of small mines, former smelting sites and ghost towns (Basque, 1992). More recently, a form of liquid gold has been more significant to the local economy. Elements of the local climate, geology and geomorphology favour the production of wines in the broader Okanagan region (Bowen et al., 2005). The geology of the southern Okanagan, including the relation to water resources and vineyards, is further explained in Roed and Fulton (2011).

The local geology includes metasediments and metavolcanics of the Paleozoic (Permian) Anarchist Group, which is overlain by the late-Permian Kobau Group, intruded on their north side by the Mesozoic Osoyoos batholith. The mineralization is associated with shearing, as seen in Figure 3, and may be of Mesozoic or even Tertiary age (Daly, 1912; Tully, 1981). The strata of the Anarchist Group (named for Anarchist Mountain) include quartzite, mica schist, chlorite schist, limestone and greenstone. An altered diorite may be related to the metavolcanics, which vary from massive to schistose. Granodiorite to quartz diorite of the Osoyoos batholith displays granitic and gneissic textures (Cockfield, 1935). Locally, Osoyoos diorite is intruded by later granite containing molybdenite (McKechnie, 1964). Mapping by Haynes (1984) found a greenstone unit (a highly sheared (?) andesite flow, with coarser dioritic to gabbroic facies); a small occurrence of very fine-grained aphanitic basalt; recrystallized limestone, found under the Dividend workings; and a massive band of garnetite, found in the Dividend workings and elsewhere. The skarn hosts sulphide lenses 0.5-1.5 m wide. The garnetite zones have formed by alteration of limestone, localized at limestone- volcanic contacts (Pasieka, 1987). The geology of the "Boundary District" examined by Daly on his transect was re-examined by Peatfield (1978), from the southern Okanagan south to the Eocene mineralization of the Republic graben in northern Washington. Carboniferous-Permian collision and subduction resulted in development of an island arc sequence (the Anarchist Group) east of oceanic crust (the Cache Creek Group). Concerning Triassic skarn Cu deposits (ibid., pp.185-199), Peatfield suggested that stratabound aggregates of calc-silicate rocks without obvious connection to igneous rocks may be less "skarns" than metamorphosed stratabound carbonate-hosted Cu deposits.


[31 kb] [34 kb] [30 kb]

Figure 2. Three views of the near-surface workings that clearly followed small controlling structures and pockets of ore within the host rocks. The rusty weathered surfaces are characteristic of many ore deposits, especially those with abundant, rapidly-oxidizing iron sulphides such as pyrrhotite.

Gold was first reported in the area in 1894 (Tully, 1981) and the Dividend-Lakeview property was first explored in 1901, with little success. Small, irregular but high-grade gold mineralization was noted (Anon, 1914). It is reported (Anon, 1933, p.A134) that 15 tons of ore and concentrates were shipped from this mine on one occasion, of average grade about 2.5 oz/T gold. Total historic production was almost 20,000 ounces of gold from roughly 100,000 tons of ore (Pasieka, 1987). This seems quite believable, given the aprons of broken rock that descend the steep slopes from the complex shallow diggings that form a jagged notch in the hill crest. The Dividend ore body, comprising skarns, quartz veins, and quartz-calcite veins in shears, appears to be mined out (Hainsworth, 1983). Further mineralization exists at depth, no doubt (gold values have been returned by drilling) but may be too modest to be exploited in a modern economic context.

Pyrrhotite is the most abundant sulphide ore mineral in the skarns, which also carry magnetite, chalcopyrite, arsenopyrite and pyrite. Pyrrhotite may carry only low Au values, whereas arsenopyrite was thought to be gold-rich (Cockfield, 1935), a finding confirmed repeatedly thereafter (e.g., Wilson et al., 1990).


[127 kb] [164 kb]

Figure 3. Two photomicrographs of the same subject: small crystals of white arsenopyrite with tiny, irregular blebs of yellow chalcopyrite in a zone of small fibrous quartz pressure shadows. Nominal magnification 50X, long-axis field of view 1.7 mm, plane -polarized reflected light (left) and crossed-polarized transmitted light (right).

References

Anon (1914) Osoyoos. B.C. Min.Mines and Petrol.Res. Ann.Rep. for 1913, K172-K174.

Anon (1933) Osoyoos mining division. B.C. Min.Mines and Petrol.Res. Ann.Rep. for 1932, A134-A139.

Basque,G (1992) Ghost Towns and Mining Camps of the Boundary Country. Sunfire Publications Ltd, Langley, B.C., 152pp.

Bowen,PA, Bogdanoff,CP, Estergaard,BF, Marsh,SG, Usher,KB, Smith,CAS and Frank,G (2005) Geology and wine 10: use of geographic information system technology to assess viticulture performance in the Okanagan and Similkameen valleys, British Columbia. Geoscience Canada 32, 161-176.

Cockfield,WE (1935) Lode gold deposits of Fairview Camp, Camp McKinney, and Vidette Lake area, and the Dividend-Lakeview property near Osoyoos, B.C. GSC Memoir 179, 38pp.

Daly,RA (1912) Geology of the Forty-Ninth Parallel. GSC Map 84A, 1:62,500 scale.

Ettlinger,AD and Ray,GE (1988) Gold-enriched skarn deposits of British Columbia. In `Geological Fieldwork 1987', BC MEMPR Paper 1988-1, 560pp., 263-279.

Ettlinger,AD and Ray,GE (1989) Precious Metal Enriched Skarns in British Columbia: an Overview and Geological Study. BC MEMPR Paper 1989-3, 128pp.

Hainsworth,WG (1983) Report on the Dividend-Osoyoos Claims, Osoyoos Mining Division, Osoyoos, B.C. W.G. Hainsworth and Associates Ltd, Vancouver, Report for Golden Dividend Resources Corp., Vancouver, 20pp.

Haynes,L (1984) Dividend Group. BC MEMPR Assessment File 11924, 16pp.

McKechnie,ND (1964) Gem, Dividend-Lakeview. B.C.Min.Mines and Petrol.Res. Ann.Rep. for 1963, 65-67.

Pasieka,CT (1987) A Property Report on the Dividend Group of Claims, South Osoyoos Area, Osoyoos Mining Division, British Columbia. Consultant's report for Golden Dividend Resources Corporation, 9pp.

Peatfield,GR (1978) Geologic History and Metallogeny of the `Boundary District', Southern British Columbia and Northern Washington. PhD Thesis, Queen's University, 250pp. plus appendices.

Roed,MA and Fulton,RJ (editors) (2011) Okanagan Geology South: Geologic Highlights of the South Okanagan, British Columbia. Okanagan Geology Committee, Sandhill Book Marketing Ltd, Kelowna, BC, 238pp.

Tully,DW (1981) Report on the 1980 Program of Diamond Drilling, Lakeview Claim Group. BC MEMPR Assessment File 9180, a Report for Rideau Resources Corporation, Vancouver, 16pp. plus appendices.

Wilson,GC (1990) Geology of the Dividend-Lakeview Claim Group, Osoyoos, B.C. TGSL Report for Golden Dividend Resources Corporation, Vancouver, 48pp.

Wilson,GC, Rucklidge,JC and Kilius,LR (1990) Sulfide gold content of skarn mineralization at Rossland, British Columbia. Econ.Geol. 85, 1252-1259.

Graham Wilson, 01,08-09,24 February 2014.

Visit the Turnstone "Rock of the Month" Archives!