Mineral deposits are concentrations of minerals and, as such, is a geological term. Whether a mineral deposit is also an ore deposit depends on economics. "Ore" is therefore an economic term.
Ore deposits can be classified according to a number of different criteria, such as
There are basically five genetic processes that lead to the concentration of minerals
It is probable that more mineral deposits have been formed by deposition from hydrothermal (literally - hot water) solutions than any other process. Much research has therefore focused on understanding the types of fluids that produce the deposits and where they are likely to form. Mineralisation occurs deep underground where it can not be seen. By the time the deposit is exposed through the agents of uplift and erosion, the hydrothermal solutions that carried the metals are no longer present. Nevertheless, many details of the processes of deposition are now understood, although much research still needs to be done.
The principle ingredient of hydrothermal solutions is water. Pure water, however, can not dissolve metals. Hydrothermal solutions are always brines, containing dissolved salts such as NaCl, KCl, CaSO4 and CaCl2. The range in salinity varies from that of seawater (around 3.5 wt %) to about ten times the salinity of seawater. Such brines are capable of dissolving small amounts of elements such as Au, Ag, Cu, Pb and Zn. High temperatures increase the effectiveness of the brines to dissolve metals.
The waters of hydrothermal solutions can be derived from magmatic sources, meteoric (ground water) sources and seawater. Some hydrothermal solutions may also be produced by regional metamorphism.
During wet partial melting, the water that causes the melting is released when the magma solidifies. This water carries with it soluble constituents such as NaCl, as well as elements such as Au, Ag, Cu, Pb, Zn, Hg, and Mo that do not easily enter into the common minerals (e.g. quartz, feldspar) by ionic substitution.
Meteoric and seawater can also form hydrothermal solutions if they are heated sufficiently and a convection system is generated. The source of this heat is magmatic intrusions, so magma is a key ingredient in the generation of hydrothermal mineral deposits. Hydrothermal mineral deposits are thus associated with convergent and divergent plate boundaries.
Hot brines can hold in solution greater concentrations of metals than cold brines. As a hydrothermal solution moves upwards, it cools and the dissolved minerals precipitate out of solution. To be effective in generating sufficient mineralisation to form ore bodies, the process must be continuous over a large period of time, so a convection cell is required to maintain a constant precipitation.
If the upward movement is slow, the precipitation of the minerals would be spread over a wide area and may not be sufficiently concentrated to form an ore body. Sudden cooling, caused by rapid movement of the fluid into porous layers such as volcanic tephra or into open fractures such as veins and brecciated rocks, leads to rapid cooling and the rapid precipitation of minerals over a limited region.
Boiling, rapid pressure decrease, reactions with adjacent rock types, and mixing with seawater can also cause rapid precipitation and the concentration of mineral deposits.
The temperature and composition of the fluid can be determined by the study of fluid inclusions. These are small pockets of fluids that were trapped within the crystal structure of minerals such as quartz that crystallised at the same time as the mineral deposits. When the samples are cooled, salts precipitate out of solution, allowing the composition of the solution to be determined. The fluid inclusions also contain vapour pockets. Heating these until the vapour dissolves can give an indication of the minimum temperature of the solution. The actual temperature can not be much higher than this, as expansion would cause the fracturing of the mineral. Phase diagrams can show which minerals will precipitate out of solution at specific pressure-temperature conditions, so determining the temperature of the fluid is a powerful exploration tool.
Note that it is important to determine that the fluid inclusions are primary and formed at the same time as the mineral, rather than secondary, forming in fractures at a later time. Inclusions that mimic the growth form of the crystal are likely to be primary, and trapped at the edge of the crystal while it was growing, whereas nclusions forming a straight line through the crystal may be secondary.
In 1962, oil/gas drilling struck a 350°C brine at 1.5 km depth. As the brine flowed upwards and cooled, it deposited a siliceous scale. Over a period of 3 months, some 8 tons were precipitated, containing 20 wt % Cu and 8 wt % Ag. This was the first unambiguous evidence that mineral deposits can be formed from hydrothermal fluids.
In 1964, oceanographers discovered a sries of hot, dense brines at the bottom of the Red Sea. The higher density of the brines (i.e. increased sanility) means that they remain at the bottom of the sea, despite being hot. The sediments at the bottom of these pools contain ore minerals such as chalcopyrite, sphalerite and galena. The Red Sea is a stratabound mineral deposit in the making.
In 1978, deep-sea submarines on the East Pacific Rise, at 21°N, found 300°C hot springs emerging in plumes along the oceanic ridge, 2500 m below sea level. Minerals precipitated out of the solution as soon as it emerged, and around the vents was a blanket of sulphide minerals. This is the modern analogue of volcanogenic massive sulphide (VMS) deposits.
At divergent boundaries, water from the ocean floor flows through fractures in the oceanic crust. The waters are heated by the nearby magma source, producing a seawater convection cell which reacts with neighbouring rocks to leach out metals. These dissolved metals are transported to the ocean floor where they mix with cold bottom waters. The sudden decrease in temperature causes the minerals to precipitate from solution and they are incorporated into sediments deposited along the ocean ridge system.
Figure 1: Circulation of fluids and precipitation of mineral deposits at divergent boundaries.(from Chernicoff, fig. 20-23a, pg 590)
Discharge is focused along fault or fracture systems. Exploration of the ocean floor using submersible craft discovered plumes of hot waters (360°C) expelled along the oceanic ridge. These "black smokers" are modern day analogues to fossil VMS deposits. They have been observed over the past several years forming in deep submarine trenches off the Pacific Coast of North America. The plumes contain dissolved minerals such as manganese, iron, copper and zinc sulphides, and the mineral deposits they produced were termed massive sulphides. The precipitating minerals make the water coming from tall vents resemble smoke rising from a chimney when viewed in the lights of the submersible craft (the floor of the ocean is normally in complete darkness) and the hydrothermal vents have been named "black smokers" because of this appearance. Scientists discovered populations of previously unknown organisms living near the vents.
Sedimentary structures in the massive component of the deposits may result from mechanical reworking and downslope transportation of sulphide ores after initial deposition. Underlying alteration and stringer mineralization result from the interaction of hot discharging fluids with the footwall rocks.
The ore-mineral constituents in VMS deposits are derived from the igneous rocks of the crust. Pyroxene is a major component of these rocks, and as Cu and Zn are present in pyroxenes in minor amounts through atomic substitution, the resulting fluids are rich in these elements. Most VMS deposits are Cu-Zn rich as a result. In contenental crust, Pb is a common trace element, so VMS deposits associated with andesitic and rhyolitic volcanism near subduction zones are Zn-Pb-Cu deposits.
Volcanic-associated massive sulphide (VMS) deposits occur throughout the world and throughout the geological time column in virtually every tectonic domain that has submarine volcanic rocks as an important constituent. VMS deposits are major sources of Cu and Zn and contain significant quantities of Au, Ag, Pb, Se, Cd, Bi, Sn as well as minor amounts of other metals.
As a group, VMS deposits consist of massive accumulations of sulphide minerals (more than 60% sulphide minerals) which occur in lens-like or tabular bodies parallel to the volcanic stratigraphy or bedding.
They are usually underlain by a footwall stockwork of vein and stringer sulphide mineralization and hydrothermal alteration. They may occur in any rock type, but the predominant hosts are volcanic rocks and fine-grained, clay-rich sediments. The deposits consist of ubiquitous iron sulphide (pyrite, pyrrhotite) with chalcopyrite, sphalerite, and galena as the principal economic minerals. Barite and cherty silica are common gangue accessory minerals.
VMS deposits tend to occur in districts. Up to two dozen deposits might be clustered in an area of a few tens of square kilometres. Known VMS districts are good hunting grounds for new discoveries. Deposits within a specific district tend to have similar metal ratios and a fairly narrow range in composition. In any given district, deposits will tend to range in size from less than one million tonnes to several tens of millions of tonnes, with most deposits at the small end of the range and only a few large deposits.
The distribution of metals and sulphide types is commonly zoned on the scale of an individual lens and in clusters of lenses.
Cu is usually high relative to Zn + Pb in the core of the pipe and in the spine of the massive sulphides.The ratio of Zn + Pb to Cu increases around the outside of the pipe and towards the upper part and margins of the massive zone.
Au and Ag usually are highest in the fringe areas. Barite also tends to occur at fringes. Proportions of Zn, Pb and Ba also tend to increase in lenses peripheral to the center of the deposit, both laterally and vertically (up-strastigraphy).
Pyrrhotite + magnetite may occur in the core zone with pyrite usually becoming dominant at the fringes.
Figure 2: Essential characteristics of an idealised volcanogenic massive sulphide deposit.
Hydrothermal deposits also form on land when metal-rich fluids are expelled from magma chambers. These fluids form veins and may contain concentrations of economic minerals. One of the last minerals to form during the cooling of a magma chamber is quartz. Quartz is precipitated in veins from quartz-rich fluids expelled from magma chambers or from fluids formed during metamorphism and often form associations with gold deposits.
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.
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, good fissure veins may occur in igneous rocks whereas they are poorly developed in sedimentary rocks and serpentine. Large quartz veins exist in quartzite, whereas in mudstones 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 amethyst 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 m. 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 may be associated with pyrrhotite, arsenopyrite, pyrite, chalcopyrite and with minor sulphides - the classic 'free gold'.
Silver is commonly associated with galena and galena-sphalerite, tetrahedrite or other copper minerals, antimony or copper-arsenic sulphides and chalcopyrite.
Based on notes from Porphyry Copper Deposits by W.J. McMillan
for "The Prospecting School on the Web" of the B.C. & Yukon Chamber of
McMillan, W.J. and Panteleyev, A., (1988) Porphyry Copper Deposits, in Roberts, R.G. and Sheahan, P.A. Ore Deposit Models, Geoscience Canada, Ottawa.
The major products from porphyry copper deposits are copper and molybdenum or copper and gold.
The term porphyry copper now includes engineering as well as geological considerations; it refers to large, relatively low grade, intrusion-related deposits that can be mined using mass mining techniques.
Geologically, the deposits occur close to or in granitic intrusive rocks that are porphyritic in texture.
There are usually several episodes of intrusive activity, so, are commonly associated with swarms of dykes and intrusive breccias. The country rocks can be any kind of rock, and often there are wide zones of closely fractured and altered rock surrounding the intrusions.
This country rock alteration is distinctive and changes as you approach mineralization. Where sulphide mineralization occurs, surface weathering often produces rusty-stained bleached zones from which the metals have been leached; if conditions are right, these may redeposit near the water table to form an enriched zone of secondary mineralization.
No single model can adquately portray the alteration and mineralization processes that have produced the wide variety of porphyry copper deposits. However, volatile-enriched magmas emplaced in highly permeable rock are ore-forming processes that can be described in a series of models that represent successive stages in an evolving process.
Various factors, such as magma type, volatile content, the number, size, timing and depth of emplacement of mineralizing porphyry plutons, variations in country rock composition and fracturing, all combine to ensure a wide variety of detail. As well, the rate of fluid mixing, density contrasts in the fluids, and pressure and temperature gradients influence the end result. Different depths of erosion alone can produce a wide range in appearances even in the same deposit.
End-member models of hydrothermal regimes attempt to show contrasting conditions for orthomagmatic systems dominated by magmatic (waters derived from molten rock) and convective systems, dominated by meteoric waters (usually groundwater). The convecting fluids transfer metals and other elements, and heat from the magma into the country rock and redistribute elements in the convective system. The two models represent end-members of a continuum. The fundamental difference between them is the source and flowpath of the hydrothermal fluids.
Volatiles and metal are concentrated during crystallisation of the magma, then break through the crystallised carapace as hydrothermal fluids in the post-mgmatic stage. The initial wave of escaping fluids fractures the country rock that creates a crackle zone and plumbing system that controls the travel paths of subsequent hydrothermal fluids and localises alteration and mineralisation. Further cracking results from magmatic pressures, boiling and hydrofraturing.
The fluid is mostly meteoric or seawater. Thermally driven convective cells are initiated by emplacement of the magma. The permeability of the country rock is increased by the intrusive events to allow convective circulation. the convective fluids concentrates ore and gangue minerals near the intrusion.
Magmatic intrusion generates an ascending hydrothermal plume. Magmatic
component constitutes up to 95% of the hydrothermal fluid Permeable country rocks are the primary source of fluids. Magmatic
fluiuds may be only 5% of the hydrothermal fluids. Salinity is high, ranginng from 15 wt % to 60 wt %. Salinity is low, generally less than 15 wt %. Multiple episodes of boiling, caused by repeated self-sealing and
refracturing of the rocks. Boiling is localised and of limited duration. Fluid temperatures 400°C - 650°C, persisting over long periods of
time. Fluid temperatures may briefly reach 450°C, but quickly drop to around
250°C. The lower temperatures are then maintained for a long
time. Pervasive alteration and mineralisation form a series of shells around
the core of the intrusion. Alteration and mineralisation are both pervasive and fracture
controlled. Metals and sulphur are derived from the magma and are concentrated in
residual fluids. Metals and sulphur are scavenged from the enclosing rocks by convective
Magmatic intrusion generates an ascending hydrothermal plume. Magmatic component constitutes up to 95% of the hydrothermal fluid
Permeable country rocks are the primary source of fluids. Magmatic fluiuds may be only 5% of the hydrothermal fluids.
Salinity is high, ranginng from 15 wt % to 60 wt %.
Salinity is low, generally less than 15 wt %.
Multiple episodes of boiling, caused by repeated self-sealing and refracturing of the rocks.
Boiling is localised and of limited duration.
Fluid temperatures 400°C - 650°C, persisting over long periods of time.
Fluid temperatures may briefly reach 450°C, but quickly drop to around 250°C. The lower temperatures are then maintained for a long time.
Pervasive alteration and mineralisation form a series of shells around the core of the intrusion.
Alteration and mineralisation are both pervasive and fracture controlled.
Metals and sulphur are derived from the magma and are concentrated in residual fluids.
Metals and sulphur are scavenged from the enclosing rocks by convective ground waters.
Figure 3: Model of hydrothermal systems with orthomagmatic and convective fluid flow patterns.
Porphyry copper provinces seem to coincide, worldwide, with orogenic belts. This remarkable association is clearest in Circum-Pacific Mesozoic to Cenozoic deposits but is also apparent in North American, Australian and Soviet Paleozoic deposits within the orogenic belts.
Porphyry deposits occur in two main settings within the orogenic belts; in island arcs and at continental margins. Deposits of Cenozoic and, to a lesser extent, Mesozoic age predominate. Those of Paleozoic age are uncommon and only a few Precambrian deposits with characteristics similar to porphyry coppers have been described. Deformation and metamorphism of the older deposits commonly obscure primary features, hence they are difficult to recognise.
Porphyry copper deposits comprise three broad types:
Strong alteration zones develop in and around granitic rocks with related porphyry deposits. If the alkali to hydrogen ratio is low, feldspars, micas and other silicates become unstable and hydrolysis occurs, releasing cations and driving the hydrothermal system toward equilibrium.
Four alteration types are common
Weak hydrolysis. Quartz and alkali feldspar are stable, but plagioclase and mafic minerals react with the fluid to form albitised (Na) plagioclase, chlorite, epidote, carbonate and montmorillonite.
More intense hydrolysis. Characterised by quartz, kaolinite and chlorite.
Quartz and sericite (fine muscovite), commonly accompanied by pyrite.
High temperature alteration by concentrated hydrothermal fluids. All rock constituents ar unstable. Alteration assemblages of quartz (commonly resorbed), K-feldspar, biotite and intermediate plagioclase.
As a generallised model, these alteration assemblages form distinct zones around the intrusion, with a shell of potassic alteration grading outward through a shell of cream or green quartz and sericite (phyllic), white, chalky clay (argillic) and then greenish chlorite, epidote, sodic plagioclase and carbonate (propylitic) alteration zones into unaltered country rock. In reality, the complete sequence is rarely developed or preserved, and assemblages are strongly influenced by the composition of the host rocks. Often there is early development of a wide area of secondary biotite that gives the rock a distinctive brownish colour.
Stockworks of veins with many cross-cutting relationships demonstrate that multiple episodes of fracturing and sealing occur. In general, alteration types are potassic and propylitic, then phyllic and finally argillic.
Original sulphide minerals in these deposits are pyrite, chalcopyrite, bornite and molybdenite. Gold is often in native found as tiny blobs along borders of sulphide crystals. Most of the sulphides occur in veins or plastered on fractures; most are intergrown with quartz or sericite. In many cases, the deposits have a central very low grade zone enclosed by 'shells' dominated by bornite, then chalcopyrite, and finally pyrite, which may be up to 15% of the rock. Molybdenite distribution is variable, Radial fracture zones outside the pyrite halo may contain lead-zinc veins with gold and silver values.
Mineralization in porphyry deposits is mostly on fractures or in alteration zones adjacent to fractures, so ground preparation or development of a 'plumbing system' is vitally important and grades are best where the rocks are closely fractured. Porphyry-type mineral deposits result when large amounts of hot water that carry small amounts of metals pass through permeable rocks and deposit the metals.
Intrusions associated with, porphyry copper deposits are generally emplaced as crystal-liquid mixtures at less than 4 km depth and usually only 1-2 km. they are porphyritic, reflecting rapid chilling. Porphyry dykes are ubiquitous and many breccia bodies refect an explosive escape of volatiles. Several periods of brecciation occur.
Intense hydrothermal alteration accompanies and affects the breccias and dykes. In such cases it can be difficult to distinguish porphyry dykes from similar host rocks and, in some cases, even to recognise breccia bodies.
1.Dykes and granitic rocks with porphyritic textures.
2.Breccia zones with angular or locally rounded fragments; look for sulphides between fragments or in fragments.
3.Epidote and chlorite alteration.
4.Quartz and sericite alteration.
5.Secondary biotite alteration - especially if partly bleached and altered.
6.Fractures coated by sulphides, or quartz veins with sulphides. To make ore, fractures must be closely spaced; generally grades are better where there are several orientations (directions).
If you are doing geochemical soil or stream silt sampling, copper is the best pathfinder element.
The deposits can be huge; worldwide, some are more than two billion tonnes.
Epithermal gold deposits form in hydrothermal systems related to volcanic activity. These systems, while active, discharge to the surface as hot springs or fumaroles. Thus, the study of active hydrothermal systems provides information on hydrothermal processes that are related to metal transport and deposition. In turn, this information can be used to predict how gold deposits form, and where to find them.
Epithermal gold deposits occur largely in volcano-plutonic arcs (island arcs as well as continental arcs) associated with subduction zones, with ages similar to those of volcanism. The deposits form at shallow depth, of <1 km - 2km, and are hosted mainly by volcanic rocks. Epithermal deposits occur as small vein systems (less than a million tonnes in size), but with good grades.
Epithermal deposits form from dilute (< 5 wt % NaCl) waters that undergo boiling or effervescent degassing, fluid mixing and oxidation at temperatures in the range of 200 - 300°C. Boiling and mixing of fluids appears to be the most important cooling mechanisms.
Skarns are generally thought of as being the result of contact metamorphism of impure limestone.
However, although the majority contain at least some limestone, skarns can form during regional or contact metamorphism and from a variety of metasomatic processes involving fluids of magmatic, metamorphic, meteoric, and/or marine origin. They are found adjacent to plutons, along faults and major shear zones, in shallow geothermal systems, on the bottom of the seafloor, and at lower crustal depths in deeply buried metamorphic terrains. What links these diverse environments, and what defines a rock as skarn, is the mineralogy. This mineralogy includes a wide variety of calc-silicate and associated minerals but usually is dominated by garnet and pyroxene.
Skarns can be subdivided according to several criteria, the most common being their mineralogy and their enclosing rock types. Exoskarns are skarns developed in the sedimentary rocks surrounding the themal source (pluton). Endoskarns are those developed within the igneous intrusion. Magnesian and calcic skarn can be used to describe the dominant composition of the original rock and resulting skarn minerals. Such terms can be combined, as in the case of a magnesian exoskarn which contains forsterite-diopside skarn formed from dolostone.
The vast majority of skarn deposits are associated with magmatic arcs related to subduction beneath continental crust.
A descriptive skarn classification can be based on the dominant economic minerals.
The largest skarn deposits, with many over 500 milliion tonnes. They are mined for their magnetite. Minor amounts of Ni, Cu, Co and Au may be present, but typically only Fe is recovered. They are dominantly magnetite, with only minor silicate gangue.
Most gold skarns are associated with relatively mafic diorite - granodiorite plutons and dyke/sill complexes. Some large Fe or Cu skarns have Au in the distal zones. There is the potential that other skarn types have undiscovered precious metals if the entire system has not been explored.
These are found in association with calc-alkaline plutons in major orogenic belts. They are associated with coarse grained, equigranular batholiths (with pegmatite and aplite dykes), surrounded by high temperature metamorphic aureoles. This is indicative of a deep environment.
These are the world's most abundant type and are particularly common in orogenic zones related to subduction both in continental and oceanic settings. Most are associated with porphyritic plutons with co-genetic volcanic rocks, stockwork veining, brittle fracturing, brecciation and intense hydrothermal aleteration. These features are all indicative of a relatively shallow environment. The largest copper skarns can exceed 1 billion tonnes and are associated with porphyry copper deposits.
Most occur in continental settings associated either with subduction or rifting. They are also mined for lead and silver, and are high grade. They form in the distal zone to associated igneous rocks.
Most are associated with leucocratic (lacking ferromagnesian minerals) granites and form high graade, small deposits. other metals are also commonly associated, the most common being Mo-W-Cu skarns.
These are almost exclusively associated with high silica granites generated by partial melting of continental crust. Greisen alteration by fluorine produces a characteristic yellowish mica.
More on skarns can be found in the Web page Skarn Deposits, by Dr L.D. Meinert