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
Vein deposits include most gold mines, many large silver mines and
a few copper and lead-zinc mines..
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
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 adits, 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.
- 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
very high temperatures (300-500°C) and generally at
great depths (several km) including porphyry copper type deposits
Mesothermal Type Characteristics
Mesothermal type- moderate temperatures (200-300°C) and pressures,
(approximately 1-5 km depth).
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 ultramafic rocks including listwanites
(fuchsite ormariposite (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
closely related to igneous rocks, both spatially and genetically.
Classic 'examples' include: Motherlode District, California; Coeur d'Alene
District, Idaho; Cassiar District, B.C. and Archean lode gold deposits in
Ontario, Quebec and Manitoba.
Epithermal Type Characteristics
Epithermal - comparatively low temperatures (50-200°C). The three
types grade into one another.
deposited normally within 1,000 m (3,000 ft.) of surface;
average 350 metres.
form as vein fillings, irregular branching fissures, stockworks or
open space fillings are common and include vugs, drusy cavities,
cockscomb textures, crustifications, and symmetrical banding
colIoidal eextures are characteristic implying free circulation of
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
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
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.
Classic examples include: Creede, Colorado; Toodoggone Camp, B.C.;
Blackdome, B.C.; Premier, B.C.; Comstock Lode, Nevada and Pachuca, Mexico.
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
1.A suitable fracture or plumbing system must be identified, i.e.
2.A zone of high silica + clays + pyrite may indicate a vein system
at depth, i.e. represents a good; drill target.
3.Trace element geochemistry provides pathfinders to mineralization,
esp. arsenic, antimony, mercury, thallium and selenium.
4.Detailed mapping of alteration both on the hanging-wall and
footwall to indicate possible direction to mineralization.
5.Basic indentification of 'ore' and gangue mineralogy both in the
field and in the laboratory (assay, X-ray, etc.).
Barr, D.A ., 1980, 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, In 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
MiningSchool, Notes for Prospectors, B.C. and Yukon Chamber of Mines, p.
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.
Epithermal Vein-type Gold Deposits
with a brief look at the factors that affect the viability of a gold
An epithermal gold deposit is one in which the gold mineralization occurs
within 1 to 2 km of surface and is deposited from hot fluids.
The fluids are estimated to range in temperature from less than
100°C to about 300°C and, during the formation of a
deposit, can appear at the surface as hot springs,
similar to those found in Yellowstone National Park (in northwestern
Wyoming, southern Montana and eastern Idaho).
The deposits are most often formed in areas of active volcanism around the
margins of continents.
Two types of fluids LS and HS
Epithermal gold mineralization can be formed from two types of chemically
Ä "low sulphidation" (LS) fluids, which are reduced and have a
near-neutral pH (the measure of the concentration of hydrogen ions)
"high sulphidation" (HS) fluids, which are more oxidized and
Low sulphidation fluid type
LS fluids are a mixture of rainwater that has percolated into the
subsurface and magmatic water (derived from a molten rock source deeper in
the earth) that has risen toward the surface.
Gold is carried in solution and, for LS waters, is deposited when the
water approaches the surface and boils.
High sulphidation fluid type
HS fluids are mainly derived from a magmatic source and deposit gold near
the surface when the solution cools or is diluted by mixing with
The gold in solution may come either directly from the magma source or it
may be leached out of the host volcanic rocks as the fluids travel through
Mode of deposition
In both LS and HS models, fluids travel toward the surface via fractures
in the rock, and mineralization often occurs within these conduits. LS
fluids usually forth large cavity filling veins, or a series of finer
veins, called stockworks, that host the gold.
The hotter. more acidic HS fluids penetrate farther into the host rock,
creating mineralization that may include veins but which is mostly
scattered throughout the rock.
LS deposits can also contain economic quantities of silver, and minor
amounts of lead, zinc and copper, whereas HS systems often produce
economic quantities of copper and some silver.
Other minerals associated with LS systems are quartz (including
chalcedony), carbonate, pyrite, sphalerite and galena, whereas an HS
system contains quartz, alunite, pyrite and copper sulphides such as
Geochemical Exploration- indicator elements
Geochemical exploration for these deposits can result in different
chemical anomalies, depending on the type of mineralization involved.
LS systems tend to be higher in zinc and lead, and lower in copper,
with a high silver-to-gold ratio.
HS systems can be higher in arsenic and copper with a lower
Location and grade
Many countries have epithermal gold deposits, including Japan, Indonesia,
Chile and the western U.S., each of which occupies a portion of the "Rim
of Fire," the area of volcanism that rings the Pacific Ocean from
Southeast Asia to western South America. Epithermal gold is also found in
British Columbia at the Baker mine, in the Toodoggone district, and near
the Taseko River.
Epithermal gold deposits, which contribute significantly to the
world's gold supply, are an important exploration target which must be
evaluated carefully based on the amount of metal they might provide, and
at what cost.
The amount of gold in any type of deposit is calculated based on the
ore's grade (the amount of gold per tonne of rock) and tonnage (total
number of tonnes) available at that grade.
The higher the grade of the material, the lower the tonnage required to
make recovery economical.
A high-grade deposit could have gold values ranging from 10 to more than
150 grams per tonne, whereas a low-grade deposit grades in the range of 1
to 5 grams.
Low-grade deposits may have up to, and possibly more than, 200 million
tonnes of rock, whereas a high-grade deposit is frequently smaller.
Distribution of gold within a deposit
Assay results acquired through drilling are important indicators of a
deposit's grade and tonnage.
High grades over short distances can be as significant as low grades over
longer distances, and both types of deposit can be mined profitably.
Drill results, however, offer only a limited view of a deposit and may be
difficult to reproduce.
For instance, a single drill hole may intersect a high-grade zone in an
otherwise low-grade (high sulphidation type epithermal) deposit, giving
the appearance of a higher grade than actually exists.
Other metals as by-products
Factors other than tonnage and grade come into play in calculating the
economic significance of an epithermal deposit.
For instance, the presence of other metals in the ore can increase the
value of a deposit, and many epithermal deposits contain a significant
silver and/or copper content.
The price of gold
The price of gold (and other metals) is also an important condition in
economic evaluation, as low prices may render small or low grade deposits
A deposit is only "economic" when:
you can find a buyer
the selling price gives you a good profit plus covering the costs of
mining, extraction, recovery, refining, taxes, environmental
this situation is projected to continue for the life of the mine
Some deposits are only "economic" over short periods of time as world
demand and political situations change.
This is why there is some urgency to get an "economic" deposit on-line as
soon as possible.
What is profitable today may be "uneconomic" in the future.
The life of a mine is short so investors expect to get a large return
early in the life of the mine.
You can't compare mining to the long term life cycle in other business
Location of deposit
Many epithermal deposits occur in remote regions of under-developed
countries, and the construction of infrastructure, such as roads and
mills, may be necessary before deposits can be mined.
These expenses increase the cost of a mining operation and must be taken
into consideration when calculating the economics of a deposit.
Mining and processing methods
Mining and processing methods are also important in determining economics.
Since epithermal deposits are often formed at depths of less than 2 km
(closer if erosion of overlying material has resulted), many are amenable
to relatively less expensive open-pit mining methods.
Deeper deposits that can be exploited only through underground methods are
Finally, recovery methods for epithermal gold deposits can entail either
conventional milling or cyanide leaching.The cost of both procedures can
increase if gold is contained in minerals that are difficult to process,
such as arsenopyrite.