VEIN-TYPE DEPOSITS
by TOM SCHROETER, Senior Geologist, Ministry of Energy and Mines
A vein-type deposit is a fairly well defined zone of mineralization, usually inclined and discordant, which is typically narrow compared to its length and depth. Most vein deposits occur in fault or fissure openings or in shear zones within country rock.
A vein deposit is sometimes referred to as a (metalliferous) lode deposit. A great many valuable ore minerals, such as native gold or silver or metal sulphides, are deposited along with gangue minerals, mainly quartz and/or calcite, in a vein structure.
A vein system is a group of discrete veins with similar characteristics and usually related to the same structure.
Mode of Formation
As hot (hydrothermal) fluids rise towards the surface from cooling intrusive rocks (magma charged with water, various acids, and metals in small amounts) through fractures, faults, brecciated rocks, porous layers and other channels (i.e. like a plumbing system), they cool or react chemically with the country rock. Some form ore deposits if the fluids are directed through a structure where the temperature, pressure and other chemical conditions are favourable for the precipitation and deposition of ore minerals. The fluids also react with the rocks they are passing through to produce an alteration zone with distinctive, new minerals.
The presence of intrusive rocks and alteration associated with them provide important guides to prospecting ground for seasoned prospectors. Deposits are often controlled by the physical characteristics of the country rocks. For example, in the Bridge River gold camp, good fissure veins occur in igneous rocks whereas they are poorly developed in sedimentary rocks and serpentine. In the Sheep Creek gold camp, large quartz veins exist in quartzite, whereas in argillite the veins are very narrow. The igneous rocks and quartzites fracture readily while the "softer" rocks do not tend to hold open spaces
Characteristics
Vein deposits include most gold mines, many large silver mines and a few copper and lead-zinc mines. Many examples are shown on Fig. 1.
Veins commonly consist of quartz (sometimes of several varieties such as amethystine or chalcedony) usually occurring as interlocking crystals in a variety of sizes or as finely laminated bands parallel to the walls of the vein. Minor amounts of sulphide minerals and other gangue minerals such as calcite and various clay minerals often occur; gold is rarely visible. Veins range in thickness from a few centimetres to 4 metres, the average mining width being around 1.2 metres (e.g. at Bridge River). They can be several hundreds of metres long and extend to depths in excess of 1,500 metres. Mineralization commonly occurs in shoots within the vein structures. These may be up to 150 metres in strike length, 30 metres in width and greater than 250 metres vertical.
Many outcrops of good looking veins are barren of gold or other ore minerals, but rich ore shoots may occur unexposed on surface, either down dip or along strike. Therefore, geochemical pathfinders are required. These include arsenic, antimony, or mercury which may be enriched in the rocks adjacent to the gold ore, either within the vein structure or in adjacent country rocks, producing a "halo".
Grades of gold historically have been in the 13.7 to 17.1 g/tonne range with cut-off around 8.6 g/tonne. Many more recently developed deposits have larger tonnages and lower grades and can be mined economically thanks to more efficient mining and milling methods. Mining requires edits, drifts, shafts and narrow slopes. If a vein system occurs near the surface it may be possible to mine by open pit methods which would greatly reduce mining costs.
Mineral Associations
- gold with pyrrhotite, e.g. Scottie Gold
- gold with arsenopyrite, e.g. Rossland
- gold with pyrite, e.g. Surf Inlet
- gold with chalcopyrite, e.g. Willa
- gold with minor sulphides - classic 'free gold', Bridge River, Toodoggone and Blackdome
- silver with galena and galena-sphalerite, e.g. Slocan District
- silver with tetrahedrite or other copper - antimony or copper-arsenic sulphides, e.g. Equity Silver
- chalcopyrite, e.g. Churchill Copper, Davis Keays
Lindgren's Classification (1920-30)
Hydrothermal deposits were broadly grouped into three types whose mineralogy and mode of occurrence indicated different conditions of origin:
- Hypothermal - fairly high temperatures (300-500ฐC) and generally at considerable depths (several km) including porphyry copper type deposits. (Not discussed further here).
- Mesothermal - moderate temperatures (200-300ฐC) and pressures, (approximately 1-5 km depth).
- Epithermal - comparatively low temperatures (50-200ฐC). The three types grade into one another.
- sulphides include chalcopyrite, sphalerite, galena, tetrahedrite, bornite and chalcocite.
- gangue includes quartz, carbonates (calcite, siderite, rhodochrosite) and pyrite.
- most show abundant replacement phenomena.
- some associated with ultramefic rocks including listwanites (fuchsite or mariposite (green mica) bearing altered varieties).
- ribbon structures parallel to vein walls.
- includes 'porphyry' copper type deposits.
- extensive alteration zones with varying amounts of sericite, quartz, calcite, doIomite, pyrite, orthoclase, chlorite and clay minerals.
- closely related to igneous rocks, both spatially and genetically.
Epithermal Characteristics
- deposited normally within 1,000 m (3,000 ft.) of surface; average 350 metres.
- form as vein fillings, irregular branching fissures, stockworks or breccia pipes.
- open space fillings are common and include vugs, drusy cavities, cockscomb textures, crustifications, and symmetrical banding (generally conspicuous).
- colIoidal eextures are characteristic implying free circulation of fluids.
- repeated cycles of mineralization are evident, including rebrecciation and multistage banding.
- in older rocks, these deposits have usually been removed by erosion unless preserved by down faulting, etc.
- majority of deposits are Tertiary in age (esp. SW USA), however, some are much older, e.g. Toodoggone deposits are early Jurassic (approximately 180 Ma).
- wallrock alteration is typically widespread and conspicuous, esp. chlorite, sericite, alunite, zeolites, adularia, silica, pyrite and calcite.
- Ore mineralogy includes: sulfantimonides and sulfarsenides (polybasite, stephanite, pearceite, pyrargyrite, proustite and others), gold and silver tellurides (sylvanite, calaverite and hessite), stibnite, argentite (acanthite), cinnabar, native mercury, electrum, native gold, native silver, selenides and minor galena, sphalerite and chalcopyrite.
- Gangue mineralogy includes quartz, amethyst, chalcedony, adularia, calcite, rhodochrosite, barite, fluorite and hematite.
- striking analogies to modern hot springs. Often so diluted with ground water that mineral content is quite low (typical
- striking analogies to modern hot springs. Often so diluted with ground water that mineral content is quite low (typical sinters); however, some do contain sulphides and free gold, e.g. Steamboat Springs, Nevada.
- deposits are formed in extensional tectonic settings with local normal faulting large scale volcanic collapse structures.
- veins are never uniformly mineralized along strike. generally less than 20% of the total vein is mineralized.
- vertical zoning is common (see Fig. 1).
- andesites are more common country rocks.
- economically, deposits are attractive because they have a high unit value of precious metals (esp. 'bonanza' types) with generally low or no base metals. Commonly reserves include tonnages less than 1 million tonnes but with good grades (>17 g/tonne gold). They have a relatively short but productive mine life, providing a quick payback and high rates of return on modest amounts of invested capital.
Alteration of Vein Minerals
Sulphide minerals oxidize readily to sulphates, many of which are soluble in water. The result is that weathered outcrops contain no sulphide, i.e. a gossan whereby the metalliferous material has been removed in solution and redeposited at greater depths. If the zone of groundwater is reached a phenomenon called secondary enrichment may occur.
Silicification is the key alteration associated with internal mineralization flanked on one or both sides by argillic (clay minerals) alteration and an outer extensive propyllitic (chlorite, calcite, epidote, pyrite) alteration.
Other Vein Deposits
- asbestos, e.g. Cassiar Asbestos
- 'saddle' veins - on crests' of anticlines and domes, e.g. Sheep Creek
- calcite veins - important sources of silver at Cobalt, Ontario
- shear zone veins - often long, linear belts such as Bralorne-Pioneer system and Yellowknife, N.W.T.
- A suitable fracture or plumbing system must be identified, i.e. tectonic terrane.
- A zone of high silica + clays + pyrite may indicate a vein system at depth, i.e. represents a good; drill target.
- Trace element geochemistry provides pathfinders to mineralization, esp. arsenic, antimony, mercury, thallium and selenium.
- Detailed mapping of alteration both on the hanging-wall and footwall to indicate possible direction to mineralization.
- Basic indentification of 'ore' and gangue mineralogy both in the field and in the laboratory (assay, X-ray, etc.).
Barr, D.A., 1-980, Gold in the Canadian Cordillera: Canadian Institute of Mining and Metallurgy Bulletin v. 73, n. 818, p. 59-76.
Berger, B.R., 1982, The geological attributes of Au-Ag-base metal epithermal deposits, 1n Erickson, R.L., compiler, -Characteristics of mineral deposit occurrences: U.S. Geological Survey, Open-File Rep. 82-795, p. 119-126.
Berger, B.R., and Eimon, P.I., 1982, Comparative models of epithermal gold-silver deposits: AIME Preprint 82-13, p. 25.
Buchanan, L.J., 1981, Precious metal deposits associated with volcanic environments in the southwest,
Dickinson, W.R. and Payne, W.D., editors, Relations to tectonics of ore deposits in the southern Cordillera: Arizona Geol. Soc. Digest, v. XIV, p. 237-262.
Colvine, A.C. et al, 1984, An integrated model for the origin of Archean lode gold deposits: Ontario Geological Survey, Open File Rep. 5524, p. 98.
Ney, C.S., 1975, Mining and Prospecting Notes in Prospecting and Mining School, Notes for Prospectors, B.C. and Yukon Chamber of Mines, p. 31.
Panteleyev, A., 1986, A Canadian Cordilleran model for epithermal gold-silver deposits, in Geoscience Canada: Geological Association of Canada, in press.
Schroeter, T.G. and Panteleyev, A., 1985, Lode gold-silver deposits of the Northern Cordillera: Canadian Institute of Mining and Metallurgy, Spec. Vol., in press.