Hydrothermal mineral deposits are accumulations of valuable minerals which formed from hot waters circulating in Earth's crust through fractures. They eventually produce metallic-rich fluids concentrated in a selected volume of rock, which become supersaturated and then precipitate ore minerals. In some occurrences, minerals can be extracted for a profit by mining. Discovery of mineral deposits consumes considerable time and resources and only about one in every one thousand prospects explored by companies are eventually developed into a mine. A mineral deposit is any geologically significant concentration of an economically useful rock or mineral present in a specified area. The presence of a known but unexploited mineral deposit implies a lack of evidence for profitable extraction.
Hydrothermal mineral deposits are divided into six main subcategories: porphyry, skarn, volcanogenic massive sulfide (VMS), sedimentary exhalative (SEDEX), and epithermal and Mississippi Valley-type (MVT) deposits. Each hydrothermal mineral deposit has different distinct structures, ages, sizes, grades, geological formation, characteristics and, most importantly, value. Their names derive from their formation, geographical location or distinctive features.
Generally, porphyry-type mineral deposits form in hydrothermal fluid circulation systems developed around felsic to intermediate magma chambers and/or cooling plutons. However, they did not precipitate directly from the magma. While, a skarn deposit is an assemblage of ore and calc-silicate minerals, formed by metasomatic replacement of carbonate rocks in the contact aureole of a pluton. Volcanogenic massive sulfide deposits form when mafic magma at depth, (perhaps a few kilometers beneath the surface), acts as a heat source, causing convective circulation of seawater through the oceanic crust. The hydrothermal fluid leaches metals as it descends and precipitates minerals as it rises. Sedimentary exhalative deposits, also called sedex deposits, are lead-zinc sulfide deposits formed in intracratonic sedimentary basins by the submarine venting of hydrothermal fluids. These deposits are typically hosted in shale. Hydrothermal epithermal deposits consist of geological veins or groups of closely spaced geological veins. Finally, Mississippi Valley-type (MVT) are hosted in limestone or dolomite that was deposited in a shallow marine environment in a tectonically stable intraplate environment. As expected in such an environment, volcanic rocks, folding and regional metamorphism are absent as a general rule. MVT deposits commonly lie in close proximity to evaporites.
Background A mineral ore deposit is the volume of rock that can be mined at a profit. Therefore, there are many variants that can define whether a mineral deposit is profitable or not, such as price, tonnage, or location. Mineral commodities can be classified as metals or non-metals. Metals refer to elements of the periodic table which include base, ferrous, minor fissionable and precious metals. On the other hand, non-metals refer to industrial minerals such as gypsum, diamonds, oil, coal and aggregate. Hydrothermal deposits of economically valuable and recoverable minerals are generally considered scarce, meaning such deposits are very small relative to the total area of earth's surface.
Each of these deposit types is usually considered to represent a distinctive deposit group with common features and in a similar four-dimensional geodynamic context. The formation of deposits of a particular type can vary in time and location, but different deposit types may also form synchronously, but spatially separated within the same broad orogen.
Hydrothermal mineral deposits play a key role in nearly all modern industrial activities. + Mineral Deposit Classification Igneous Ore minerals are precipitated directly from a magma. Sedimentary Ore minerals are concentrated or formed by sedimentary processes. Metamorphic Ore minerals are formed during metamorphism. Hydrothermal Ore minerals are precipitated by a hydrothermal solution percolating through intergranular spaces and along bedding planes and fractures in the host rocks. According to some authors, the hydrothermal solutions can have four origins, although any single volume of hydrothermal solution is commonly a mixture of two or more types:
Deuteric fluid derived from magma at a late stage of crystallization Metamorphic fluid derived by progressive removal of hydrothermal fluids during regional metamorphism Meteoric water descending from the surface Fluid formed by the degassing of the core? and mantle Basinal brines are also considered as a possible source of hydrothermal fluids. The fluids are thought to be connate water expelled from sediments with compactions and tectonic forces.
Ore minerals can form at the same time and from the same processes as the host rock, also termed as syngenetic, they can form slightly after the formation of the host rock, perhaps during weathering or compaction, also termed as diagenetic, or they can form much later than the host rock or epigenetic. Host rock is the rock surrounding the ore deposit. Hydrothermal deposit Subcategory Known abbreviation Formation Principal metals Host rocks Porphyry Epigenetic Cu, Mo, Au The ore is spatially associated with one or more high-level intrusions of felsic to intermediate composition such as granite, granodiorite or diorite. Skarn Epigenetic Cu, Mo, Ag, Au A skarn deposit is an assemblage of ore and calc-silicate minerals, formed by metasomatic replacement of carbonate rocks in the contact aureole of a pluton. Volcanogenic massive sulphide VMS Syngenetic Cu, Zn, Pb The host rocks are mainly volcanic, with the felsic volcanic rocks pointing to a convergent setting (island arc or orogenic belt). Sedimentary exhalative SEDEX Syngenetic Zn, Pb These deposits are commonly stratiform and are typically hosted in shale. Epithermal Epigenetic Au, Ag The host rocks can be sheared muscovite granite, small plutons. Mississippi Valley-type MVT Epigenetic Pb, Zn MVT deposits are hosted in carbonate rocks, whereas sedex deposits are found within marine shales
Porphyry Ore Deposits
Porphyry deposits account for most of the copper and molybdenum world production, 60 and 95 percent of its supply respectively.
Porphyry-type ore deposits form in hydrothermal fluid circulation systems developed above and around high-level, subvolcanic felsic to intermediate magma chambers and/or cooling plutons. The ore is temporally and genetically related to the intrusions, but did not precipitate directly from the magma.
Formation Porphyry mineral deposits are formed when two plate tectonic plates collide in an advanced subduction zone, then cools off reacting with existing rocks and finally forming a copper deposit. The level of displacement is usually shallow at less than two kilometers below surface in an active volcanic area.
An example for a typical arc-island porphyry deposit is described as follows:
The formation starts during early volcanism on the seafloor above a subduction zone in an oceanic-oceanic collision zone Then as the magma crystallizes, volatiles such as water, carbon dioxide and sulfur dioxide increase in concentration in the liquid phase of the magma. Eventually, at a very late stage of crystallization, the volatile concentration becomes so great that a separate hydrothermal fluid phase separated from the silicate magma. As the amount of hydrothermal fluid increases, vapour pressure increases. At some point, vapour pressure exceeds the strength of the overlying roof rocks and a volcanic explosion takes place fracturing the overlying rock. The sudden reduction in confining pressure on the remaining magma leads to instantaneous vigorous boiling of the magma as more and more volatiles separate. Consequently, the closing of the fractures in the roof rocks by precipitation of minerals allow confining pressure to increase once again. As time passed, increasingly felsic magmas rise up into the core of the volcano. Some of these later magmas probably erupt on the surface, forming new layers of volcanic rocks that will later be removed by erosion.
Finally, volcanic activity ceased and erosion removed the upper portions of the volcano and exposed the intrusive rocks and stockwork mineralization that used to lie within.
Porphyry characteristics Age Average age of 13 million years, continental and oceanic arcs of Tertiary and Quaternary age. Size Amongst the largest in the world, especially porphyry-type deposits. Location The 25 biggest porphyry deposits are found in the southwest Pacific and South America. Host rocks Ore is associated with one or more subvolcanic intrusions of felsic to intermediate composition such as granite, granodiorite or diorite. Economic metals In island arc settings where the host plutons are typically andesitic in composition, the elements of economic interest are mainly copper and gold. In contrast, those that occur in continental orogenic belts are typically rhyolitic in composition and carry copper, molybdenum and gold, and in some cases tin and/or tungsten. Grade Commonly low in grade and have relatively low dollar value Fractures Ore minerals are generally confined to small veinlets and less common larger veins that formed as fracture fillings in the host rocks. Hydrothermal breccia can often occur, sometimes in the form of pebble dikes. Hydrothermal alteration The wallrock on both sides of each veinlet is typically altered to varying degrees. The primary silicate minerals such as feldspar and amphibole are replaced by hydrothermal minerals stable at temperatures of about 400 °C or less such as chlorite, epidote, muscovite and quartz.
Alteration assemblages typically include proximal potassic, intermediate phyllic or QSP, and more distal propylitic and argillic alteration.Taylor, R.D., Hammarstrom, J.M., Piatak, N.M., and Seal II, R.R., 2012, Arc-related porphyry molybdenum deposit model: Chapter D in Mineral deposit models for resource assessment: U.S. Geological Survey Scientific Investigations Report USGS Numbered Series 2010-5070-D, http://pubs.er.usgs.gov/publication/sir20105070D
Where veinlets are close together, the zones of alteration around each veinlet overlap, making the whole rock hydrothermally altered. Mining activity Bingham Mine, The Chuquicamata deposit, El Teniente deposit, Henderson Mine
Skarn Mineral Deposits Skarn Mineral Deposits tend to be small in size but high in mineral grade. Therefore, it is a balance and challenge to find a profitable skarn orebody.
Geologically speaking, a skarn deposit is an assemblage of ore and calc-silicate minerals, formed by metasomatic replacement of carbonate rocks in the contact aureole of a pluton. Typical calc-silicate minerals are garnet, epidote, pyroxene, chlorite, amphibole and quartz – magnesian minerals dominate if dolomite is replaced whereas calcic minerals dominate where limestone is replaced.
Skarn deposits are of economic interest, since they are the source of numerous metals as well as minerals of industrial application.
Formation Skarn formation, as illustrated in the figure on the right, can be explained in three stages:
Intrusion of a felsic to intermediate magma body rich in volatiles. Contact metamorphism and minor metasomatism, skarn formation, occurs in favorable locations. Continued crystallization of the magma and widespread release of volatiles as a hydrothermal fluid which causes widespread skarn formation and localized brecciation. Characterized by decreasing temperatures and hydrothermal activity, during which sulfide deposition occurs in veins and retrograde alteration is common.