7 Formation of ore deposits

Authors

xxx

Published

October 5, 2026

7.1 Learning objective

How and where are ores formed from which non-ferrous metals such as copper and precious metals such as silver are extracted? In this unit you will gain a general understanding of important ore formation processes and learn in which tectonic settings the respective processes take place.

7.2 Prior knowledge

Readers should have a basic understanding of general geology and be familiar with the basics of plate tectonics.

7.3 Outline

This unit consists of text with two interactive illustrations, which visualise the most important aspects of the text.

7.4 Learning content

This unit is based on the following questions:

How can ore deposits be formed? What processes play a role in this? What types of ore deposits exist and what tectonic settings play a role in their formation?

There will be an introductory text in which the different tectonic settings that can exist at active plate boundaries will be briefly presented. It will also show which ores are primarily formed in which tectonic settings. To deepen and consolidate the content conveyed in the text, it is presented again in the form of an interactive graphic in which all these tectonic settings are shown. You can click on the individual settings and an info box appears with information on the respective tectonic setting.

Vein deposits and their weathering processes are described in the detail, as the importance of interaction with the environment (alteration by fluids and atmosphere) and the individual phases of formation can be clearly and concisely explained using this example.

7.4.1 Tectonic Settings

7.4.1.1 Oceanic Ridge

The oceanic ridge is a divergent plate boundary or so-called constructive plate boundary, where oceanic plates are drifting apart and new oceanic crust is formed.

An important deposit type formed here are volcanic massive sulphides (VMS) of the Cyprus type. These contain copper and copper-zinc rich ores with enrichment of gold. The main ores of this deposit type are chalcopyrite ($\mathrm{CuFeS_2}$), pyrite ($\mathrm{FeS_2}$), pyrrothite $\mathrm{Fe_1-x_S}$, sphalerite $\mathrm{ZnS}$ and sulphide minerals of the tetrahedrite group. VMS deposits are formed by hydrothermal solutions on the ocean floor. Cold seawater penetrates through cracks several kilometers deep through the oceanic crust and is heated to a temperature of 400°C - 500°C, dissolving metals from the surrounding rock of the oceanic crust. The metal-rich and heated water rises and is shot back into the ocean. The dissolved metals precipitate as sulphides, accumulate around the vents (a so-called black smoker) and gradually grow to chimneys.

Sedimentary exhalative ore deposits (SEDEX), are closely related to VMS deposits and are often formed at spreading centres, but can also occur at passive continental margins. SEDEX deposits are mainly formed in clastic sediments and are deposited on the seabed by the escape of hot, metal-bearing, non-magmatic hydrothermal solutions along fault systems. The ores mostly are stratiform, with a thin band of sulphide minerals alternating with marine sediments. Metals found in these ore bodies are zinc, lead, silver and copper.

7.4.1.2 Ocean Floor

There are several types of deposits on the ocean floor, and they are mainly formed by the precipitation of minerals from sea water. These include, for example, ferromanganese nodules and ferromanganese crusts as well as deep-sea muds, all of them with iron and manganese as their main metals and rich in, e.g., rare earth elements.

At present, ferromanganese nodules are not economically relevant. There are several projects (for example the international project ‘MiningImpact’) that are looking into the impact that deep sea mining of ferromanganese nodules could have on the ecosystem.

7.4.1.3 Magmatic Island Arc

A magmatic island arc or intra-oceanic arc occurs when one oceanic crust subducts beneath another oceanic crust. The resulting volcanism favours the formation of porphyritic copper deposits with sulphide ores. Porphyry copper deposits associated with island arc volcanism are generally characterised by a higher gold content compared to deposits formed at magmatic arcs where an oceanic crust subducts beneath a continental crust. These are characterised by a higher molybdenum content (see hotspot: ‘magmatic arc’).

Another type of deposit that can form in this environment are skarn systems. skarn systems. Skarns are generated by metasomatic processes during contact metamorphism between two adjacent lithologic units. While skarns can form in many different rock types, they mostly occur in carbonate rocks and are typically generated by the infiltration of fluids emanating from a proximal magmatic intrusion, leading to replacement of the carbonate rocks. Skarns are characterised by the presence of calcium silicate minerals. They generally are rich in garnet and pyroxene, and can furthermore contain wollastonite, actinolite, magnetite or hematite and epidote. Skarns are formed from silica-rich fluids that are enriched in incompatible elements. They are economically relevant sources of e.g. tin, tungsten, manganese, copper, gold, zinc, lead, nickel, molybdenum and iron.

7.4.1.4 Back-arc Basin

If the oceanic crust is subducted at a steep angle below the continental crust, the earth’s crust can stretch behind the magmatic area as seen from the subduction zone, causing it to sink. This back-arc basin can be caused by so-called slab roll-back or by the fact that part of the oceanic crust beneath the back-arc area breaks off and hot mantle material rises. The expansion in the back-arc area results from this process, also known as slab break-off, due to heating and thermal bulging of the crust. Hydrothermal processes can lead to the formation of polymetallic sulphides.

If hot mantle material continues to rise into the crust underlying the back-arc area, a spreading centre is created where new oceanic crust is formed, as at the oceanic ridge. As the hot newly formed crust comes into contact with the cold water, hydrothermal fields can also form in the back-arc basin as described in the hot spot ‘Oceanic ridge’.

7.4.1.5 Granitic Plutons in Continental Crust

When magma intrusions in the continental crust cool, large bodies of rock (plutons) form that lie about five to ten kilometers below the Earth’s surface. The heat emanating from the plutons leads to metamorphic overprinting of the surrounding rock of the continental crust. This contact or regional metamorphism causes, for example, the formation of skarns due to reaction of ore-forming fluids with carbonate-containing rocks. As a pluton rises, the overlying rock is mechanically stressed and a system of veins and cracks is formed. The circulation of metal-bearing fluids can cause metal ores to be deposited in the veins. In the roof area of the plutons, greisen can form in the late stages of pluton solidification. Their parageneses contain high amounts of quartz ($\mathrm{SiO_2}$) and muscovite ($\mathrm{KAl_2(AlSi_3O_{10})(OH)_2}$) due to transformation of the feldspars by ascending fluids and gases (pneumatolysis). The formation of tin-, tungsten-, lithium-, fluor-bearing ores are associated with such greisens. For example tin greisens can form.

Pegmatites form under similar conditions as greisens from the fluid-rich remaining melt, which is enriched in volatiles and incompatible trace elements. Pegmatites are characterised by coarse to giant mineral grains (several centimetres to several metres). They are economically important as they can contain deposits of rare earth elements, for example.

7.4.1.6 Magmatic Arc

A magmatic or volcanic arc occurs at active continental margins, also known as destructive plate boundaries. The older and denser oceanic plate pushes under the less dense continental crust. Between the subducting oceanic crust and the continental crust, a depression is formed along the subduction zone, the deep-sea trench. Due to the large mechanical forces and friction caused by the subduction, the sediments on the subducting plate (oceanic plate) can be planed off and attached to the continental crust. Sediments can also be deposited from the continental margin. This wedge-shaped block of sediment is known as an accretionary wedge. The subducting oceanic crust is partially melted. The melted material rises in the continental crust. When the molten material cools within the crust, plutons are formed. When the magma chamber continues to be supplied with material, the melt rises and volcanoes align along the subduction zone. By scraping off the oceanic crust and input of sedimentary material from the continental margin, a so-called accretionary wedge can form, which is part of the arc-trench system. On such active continental margins, copper-gold skarn systems form, which are associated with porphyritic deposits.

7.4.2 Secondary Ore Formation

When the ores of a deposit come into contact with the atmosphere, the ore body is affected by oxygen and meteoric water. The primary minerals are transformed and form secondary ore. Prominent examples are porphyritic Cu deposits. Depending on the penetration depth of the weathering into the ore body, minerals such as goethite ($\mathrm{α-Fe^3+^O(OH)}$), malachite ($\mathrm{Cu_2(CO_3)(OH)_2}$) and covellite ($\mathrm{CuS}$) are formed. The weathering scheme of a sulphide ore deposit is explained in detail in the interactive hotspot picture.

The term “secondary ore deposits” refers to any local concentration of minerals formed by weathering and sedimentation. They are not directly derived from tectonic settings, however, they depend on primary ores as the source of secondary enrichment. Major processes are mechanical or chemical weathering.

One example for a formation are (paleo-) placer deposits. They are secondary mineral accumulations containing precious metals and heavy minerals that have been liberated from primary deposits by mechanical weathering, transported, and re-deposited.

Another example is the formation of laterites. Laterites are residual deposits that are formed by strong chemical weathering of rocks and are mainly found in the tropics and subtropics. These soils are generally referred to as laterites, which can be very variable in their chemical composition, depending on the source rock of the laterite. If the source rocks e.g. are ultramafic rocks, the laterites are rich in chromium and/or nickel sometimes worth mining.

The third important secondary deposit type are carbonate-hosted lead-zinc ore deposits, also knows as Mississippi Valley Type (MVT) Zn-Pb ore deposits. The exact formation process of this deposit type is still unclear. Their formation is caused by the dissolution of lead, zinc, and additional elements in low temperature hydrothermal fluids upon contact with sulphur and often hydrocarbon. Because the sulphur-containing solutions are acidic, they dissolve the surrounding carbonate rocks and replace them with massive accumulation of sulphide ores, predominantly galena ($\mathrm{PbS}$), sphalerite ($\mathrm{ZnS}$), marcasite (orthorhombic $\mathrm{FeS_2}$), and pyrite (cubic $\mathrm{FeS_2}$).

7.5 Self check

Now you can provide answers to the following questions:

What distinct ore zones exist in a weathering vein?
Which ore types form in secondary processes?
What is hydrothermal ore formation?
Which tectonic parameters influence ore formation?
In which ore deposit types copper, lead-silver or tin occur, respectively?
Which are the most important ore minerals for copper, lead, silver and tin?

7.6 Further reading

To delve deeper into this topic, we recommend the following resources…

Robb, L., 2020, Introduction to ore-forming processes, John Wiley & Sons.
Fontboté, L., Kouzmanov, K., Chiaradia, M., and Pokrovski, G. S., 2017, Sulfide Minerals in Hydrothermal Deposits, Elements, 13(2), 97–103.
Hannington, M. D., 2014, 13.18 - Volcanogenic Massive Sulfide Deposits, In Treatise on Geochemistry (Second Edition) (eds. H. D. Holland, and K. K. Turekian), 463–88, Elsevier, Oxford.
Kissin, S. A., and Mango, H., 2014, 13.16 - Silver Vein Deposits, In Treatise on Geochemistry (Second Edition) (eds. H. D. Holland, and K. K. Turekian), 425–32, Elsevier, Oxford.
Leach, D. L., Bradley, D. C., Huston, D., Pisarevsky, S. A., Taylor, R. D., and Gardoll, S. J., 2010, Sediment-Hosted Lead-Zinc Deposits in Earth History, Economic Geology, 105(3), 593–625.
Pirjano, S., 2016, A classification of mineral systems, overview of plate tectonic margins and examples of ore deposits associated with convergent margins, Gondwana research, 13: 44-62. (https://doi.org/10.1016/j.gr.2015.08.013)

--- title: Formation of ore deposits author: - name: xxx corresponding: true orcid: 0000-0001-2345-6789 email: a@b.c - name: xxx orcid: 0000-0001-2345-6789 --- ## Learning objective How and where are ores formed from which non-ferrous metals such as copper and precious metals such as silver are extracted? In this unit you will gain a general understanding of important ore formation processes and learn in which tectonic settings the respective processes take place. ## Prior knowledge Readers should have a basic understanding of general geology and be familiar with the basics of plate tectonics. ## Outline This unit consists of text with two interactive illustrations, which visualise the most important aspects of the text. ## Learning content This unit is based on the following questions: How can ore deposits be formed? What processes play a role in this? What types of ore deposits exist and what tectonic settings play a role in their formation? There will be an introductory text in which the different tectonic settings that can exist at active plate boundaries will be briefly presented. It will also show which ores are primarily formed in which tectonic settings. To deepen and consolidate the content conveyed in the text, it is presented again in the form of an interactive graphic in which all these tectonic settings are shown. You can click on the individual settings and an info box appears with information on the respective tectonic setting. Vein deposits and their weathering processes are described in the detail, as the importance of interaction with the environment (alteration by fluids and atmosphere) and the individual phases of formation can be clearly and concisely explained using this example. <iframe src="/H5P/Tectonic__Settings.html" width="100%" height="380px" ></iframe> ### Tectonic Settings #### Oceanic Ridge The oceanic ridge is a divergent plate boundary or so-called constructive plate boundary, where oceanic plates are drifting apart and new oceanic crust is formed. An important deposit type formed here are volcanic massive sulphides (VMS) of the Cyprus type. These contain copper and copper-zinc rich ores with enrichment of gold. The main ores of this deposit type are chalcopyrite ($\mathrm{CuFeS_2}$), pyrite ($\mathrm{FeS_2}$), pyrrothite $\mathrm{Fe_1-x_S}$, sphalerite $\mathrm{ZnS}$ and sulphide minerals of the tetrahedrite group. VMS deposits are formed by hydrothermal solutions on the ocean floor. Cold seawater penetrates through cracks several kilometers deep through the oceanic crust and is heated to a temperature of 400°C - 500°C, dissolving metals from the surrounding rock of the oceanic crust. The metal-rich and heated water rises and is shot back into the ocean. The dissolved metals precipitate as sulphides, accumulate around the vents (a so-called black smoker) and gradually grow to chimneys. Sedimentary exhalative ore deposits (SEDEX), are closely related to VMS deposits and are often formed at spreading centres, but can also occur at passive continental margins. SEDEX deposits are mainly formed in clastic sediments and are deposited on the seabed by the escape of hot, metal-bearing, non-magmatic hydrothermal solutions along fault systems. The ores mostly are stratiform, with a thin band of sulphide minerals alternating with marine sediments. Metals found in these ore bodies are zinc, lead, silver and copper. #### Ocean Floor There are several types of deposits on the ocean floor, and they are mainly formed by the precipitation of minerals from sea water. These include, for example, ferromanganese nodules and ferromanganese crusts as well as deep-sea muds, all of them with iron and manganese as their main metals and rich in, e.g., rare earth elements. At present, ferromanganese nodules are not economically relevant. There are several projects (for example the international project 'MiningImpact') that are looking into the impact that deep sea mining of ferromanganese nodules could have on the ecosystem. #### Magmatic Island Arc A magmatic island arc or intra-oceanic arc occurs when one oceanic crust subducts beneath another oceanic crust. The resulting volcanism favours the formation of porphyritic copper deposits with sulphide ores. Porphyry copper deposits associated with island arc volcanism are generally characterised by a higher gold content compared to deposits formed at magmatic arcs where an oceanic crust subducts beneath a continental crust. These are characterised by a higher molybdenum content (see hotspot: 'magmatic arc'). Another type of deposit that can form in this environment are [skarn systems.](https://www.mindat.org/glossary/skarn) skarn systems. Skarns are generated by metasomatic processes during contact metamorphism between two adjacent lithologic units. While skarns can form in many different rock types, they mostly occur in carbonate rocks and are typically generated by the infiltration of fluids emanating from a proximal magmatic intrusion, leading to replacement of the carbonate rocks. Skarns are characterised by the presence of calcium silicate minerals. They generally are rich in garnet and pyroxene, and can furthermore contain wollastonite, actinolite, magnetite or hematite and epidote. Skarns are formed from silica-rich fluids that are enriched in incompatible elements. They are economically relevant sources of e.g. tin, tungsten, manganese, copper, gold, zinc, lead, nickel, molybdenum and iron. #### Back-arc Basin If the oceanic crust is subducted at a steep angle below the continental crust, the earth's crust can stretch behind the magmatic area as seen from the subduction zone, causing it to sink. This back-arc basin can be caused by so-called slab roll-back or by the fact that part of the oceanic crust beneath the back-arc area breaks off and hot mantle material rises. The expansion in the back-arc area results from this process, also known as slab break-off, due to heating and thermal bulging of the crust. Hydrothermal processes can lead to the formation of polymetallic sulphides. If hot mantle material continues to rise into the crust underlying the back-arc area, a spreading centre is created where new oceanic crust is formed, as at the oceanic ridge. As the hot newly formed crust comes into contact with the cold water, hydrothermal fields can also form in the back-arc basin as described in the hot spot 'Oceanic ridge'. #### Granitic Plutons in Continental Crust When magma intrusions in the continental crust cool, large bodies of rock (plutons) form that lie about five to ten kilometers below the Earth's surface. The heat emanating from the plutons leads to metamorphic overprinting of the surrounding rock of the continental crust. This contact or regional metamorphism causes, for example, the formation of skarns due to reaction of ore-forming fluids with carbonate-containing rocks. As a pluton rises, the overlying rock is mechanically stressed and a system of veins and cracks is formed. The circulation of metal-bearing fluids can cause metal ores to be deposited in the veins. In the roof area of the plutons, [greisen](https://www.mindat.org/glossary/greisen) can form in the late stages of pluton solidification. Their parageneses contain high amounts of [quartz](https://www.mindat.org/min-3337.html) ($\mathrm{SiO_2}$) and [muscovite](https://www.mindat.org/min-2815.html) ($\mathrm{KAl_2(AlSi_3O_{10})(OH)_2}$) due to transformation of the [feldspars](https://www.mindat.org/min-1624.html) by ascending fluids and gases [(pneumatolysis)](https://www.mindat.org/glossary/pneumatolysis). The formation of tin-, tungsten-, lithium-, fluor-bearing ores are associated with such greisens. For example tin greisens can form. [Pegmatites](https://www.mindat.org/glossary/pegmatite) form under similar conditions as greisens from the fluid-rich remaining melt, which is enriched in volatiles and incompatible trace elements. Pegmatites are characterised by coarse to giant mineral grains (several centimetres to several metres). They are economically important as they can contain deposits of rare earth elements, for example. #### Magmatic Arc A magmatic or volcanic arc occurs at active continental margins, also known as destructive plate boundaries. The older and denser oceanic plate pushes under the less dense continental crust. Between the subducting oceanic crust and the continental crust, a depression is formed along the subduction zone, the deep-sea trench. Due to the large mechanical forces and friction caused by the subduction, the sediments on the subducting plate (oceanic plate) can be planed off and attached to the continental crust. Sediments can also be deposited from the continental margin. This wedge-shaped block of sediment is known as an accretionary wedge. The subducting oceanic crust is partially melted. The melted material rises in the continental crust. When the molten material cools within the crust, plutons are formed. When the magma chamber continues to be supplied with material, the melt rises and volcanoes align along the subduction zone. By scraping off the oceanic crust and input of sedimentary material from the continental margin, a so-called accretionary wedge can form, which is part of the arc-trench system. On such active continental margins, copper-gold skarn systems form, which are associated with porphyritic deposits. ### Secondary Ore Formation <iframe src="/H5P/Minerals_hydrothermal-Cu.html" width="100%" height="380px" ></iframe> When the ores of a deposit come into contact with the atmosphere, the ore body is affected by oxygen and meteoric water. The primary minerals are transformed and form secondary ore. Prominent examples are porphyritic Cu deposits. Depending on the penetration depth of the weathering into the ore body, minerals such as [goethite](https://www.mindat.org/min-1719.html) ($\mathrm{α-Fe^3+^O(OH)}$), [malachite](https://www.mindat.org/min-2550.html) ($\mathrm{Cu_2(CO_3)(OH)_2}$) and [covellite](https://www.mindat.org/min-1144.html) ($\mathrm{CuS}$) are formed. The weathering scheme of a sulphide ore deposit is explained in detail in the [interactive hotspot picture](http://file/Users/greifelttim/Desktop/Orga/TERRA%20LID/EDU_Pilot_GLOBALID/Location%20of%20Ore%20Minerals/Location%20of%20ore%20minerals%20in%20copper%20ore%20deposits%20New.html). The term ["secondary ore deposits"](https://www.mindat.org/glossary/secondary_mineral_deposit) refers to any local concentration of minerals formed by weathering and sedimentation. They are not directly derived from tectonic settings, however, they depend on primary ores as the source of secondary enrichment. Major processes are mechanical or chemical weathering. One example for a formation are (paleo-) [placer](https://www.mindat.org/glossary/placer) deposits. They are secondary mineral accumulations containing precious metals and heavy minerals that have been liberated from primary deposits by mechanical weathering, transported, and re-deposited. Another example is the formation of [laterites](https://www.mindat.org/glossary/laterite). Laterites are residual deposits that are formed by strong chemical weathering of rocks and are mainly found in the tropics and subtropics. These soils are generally referred to as laterites, which can be very variable in their chemical composition, depending on the source rock of the laterite. If the source rocks e.g. are ultramafic rocks, the laterites are rich in chromium and/or nickel sometimes worth mining. The third important secondary deposit type are carbonate-hosted lead-zinc ore deposits, also knows as [Mississippi Valley Type](https://en.wikipedia.org/wiki/Carbonate-hosted_lead-zinc_ore_deposits) (MVT) Zn-Pb ore deposits. The exact formation process of this deposit type is still unclear. Their formation is caused by the dissolution of lead, zinc, and additional elements in low temperature hydrothermal fluids upon contact with sulphur and often hydrocarbon. Because the sulphur-containing solutions are acidic, they dissolve the surrounding carbonate rocks and replace them with massive accumulation of sulphide ores, predominantly [galena](https://www.mindat.org/min-1641.html) ($\mathrm{PbS}$), sphalerite ($\mathrm{ZnS}$), [marcasite](https://www.mindat.org/min-2571.html) (orthorhombic $\mathrm{FeS_2}$), and pyrite (cubic $\mathrm{FeS_2}$). ## Self check Now you can provide answers to the following questions: * What distinct ore zones exist in a weathering vein? * Which ore types form in secondary processes? * What is hydrothermal ore formation? * Which tectonic parameters influence ore formation? * In which ore deposit types copper, lead-silver or tin occur, respectively? * Which are the most important ore minerals for copper, lead, silver and tin? ## Further reading To delve deeper into this topic, we recommend the following resources... * Robb, L., 2020, Introduction to ore-forming processes, John Wiley & Sons. * Fontboté, L., Kouzmanov, K., Chiaradia, M., and Pokrovski, G. S., 2017, Sulfide Minerals in Hydrothermal Deposits, Elements, 13(2), 97–103. * Hannington, M. D., 2014, 13.18 - Volcanogenic Massive Sulfide Deposits, In Treatise on Geochemistry (Second Edition) (eds. H. D. Holland, and K. K. Turekian), 463–88, Elsevier, Oxford. * Kissin, S. A., and Mango, H., 2014, 13.16 - Silver Vein Deposits, In Treatise on Geochemistry (Second Edition) (eds. H. D. Holland, and K. K. Turekian), 425–32, Elsevier, Oxford. * Leach, D. L., Bradley, D. C., Huston, D., Pisarevsky, S. A., Taylor, R. D., and Gardoll, S. J., 2010, Sediment-Hosted Lead-Zinc Deposits in Earth History, Economic Geology, 105(3), 593–625. * Pirjano, S., 2016, A classification of mineral systems, overview of plate tectonic margins and examples of ore deposits associated with convergent margins, Gondwana research, 13: 44-62. [(https://doi.org/10.1016/j.gr.2015.08.013)](https://doi.org/10.1016/j.gr.2015.08.013)