Mineral Deposoits From Hydrothermal Fluids
Alpine Fissure-type veins
Alpine Fissure-type veins are localized mineral deposits hosted by competent (hard) lithologies, such as tuff or sandstone beds, igneous sills and dykes. They are particularly conspicuous where such rock units occur within deformed sequences of less competent (soft) rocks such as mudstones, and are formed as a consequence of the response of the competent rocks to the immense compressive forces that prevail during the deformation. During deformation, incompetent rocks are literally squashed, often into complex folds: however, a competent bed cannot accommodate the stress in this way and instead it fractures repeatedly due to its inherent brittleness. Alpine Fissure-type veins are the mineralized infillings of the fractures.
The very localized hydrothermal systems responsible for the deposition of the minerals in Alpine Fissure-type veins are capable of dissolving rock-forming minerals, including relatively immobile substances like titanium oxides. Compressive pressure-solution mechanisms are thought to be involved with the dissolution process. In this relatively high-pressure environment (due to the compressive forces that prevail during this type of deformation), the sudden creation of an open, brittle fracture forms a localized low-pressure zone (i.e. a space) into which any hydrothermal fluids in the immediate area will immediately migrate and precipitate minerals.
Alpine-Fissure-type veins are common in the deformed Lower Palaeozoic rocks of North Wales. Some very fine crystallized mineral specimens are known from these veins: apart from the dominant mineral, quartz, these include albite, clinozoisite, anatase, brookite, synchysite, monazite, xenotime, axinite and various sulphides. The brookite crystals formerly obtained from the now closed locality of Prenteg, near Tremadog in Snowdonia, are among the world’s finest examples of the mineral.
Amygdale infill and veins in volcanic rocks 
Hot fluids, either resulting from volatiles in the magma, or from the heating and circulation of fluids in the rocks into which a magma intrudes, can dissolve chemical elements from both the lava and the country rock. As the circulating fluids cool they precipitate low-temperature mineral assemblages.
In other areas such low-temperature mineralization, deposited from hydrothermal systems associated with lavas and near-surface intrusions, are the source of finely-crystallized zeolite specimens. However in Wales this style of mineralization is poorly represented.
Eruptive volcanic rocks and shallow intrusions are common in the Lower Palaeozoic rocks of Wales, but the original amygdale-minerals have generally been altered by subsequent low-grade regional metamorphism. The only primary amygdale mineralization in Wales is restricted to a small number of dykes of Tertiary age which outcrop across parts of northern Gwynedd and Anglesey. Typical minerals include, analcime, calcite and prehnite.
Copper-dolomite association 
The copper-dolomite association consists of saddle dolomite (with its noticeably curved crystals) in association with major chalcopyrite and minor pyrite, forming replacement deposits in carbonate rocks. It occupies faults, bedding planes, joints and vugs within extensively dedolomitized limestones.
This style of hydrothermal mineralization is thought to be derived from residual fluids formed during deposition of large sedimentary-exhalative Pb-Zn deposits, such as those of the Irish Midlands. Such fluids are magnesium-rich and sulphur-poor, and they readily precipitate saddle dolomite when they encounter carbonate rocks high in the Earth's crust. Copper is scavenged during the migration of the fluid from source rocks such as oxidized continental sandstones or volcanics.
The Great Orme copper deposits near Llandudno in North Wales are the best UK example of this class of mineral deposits. The mineralization, which in the upper parts of the mines has undergone heavy supergene alteration, consists of veins, bedding-plane ‘flats’ and irregular void infills in dedolomitized Carboniferous Limestone. Irregular, highly vuggy deposits are characteristic. The Great Orme ‘ore’ typically shows, when not oxidized, saddle-dolomite with its characteristic curved crystals forming crusts on limestone and studded with chalcopyrite crystals. Although calcite is common at Great Orme, it is entirely of supergene origin.
Epithermal polymetallic veins & pipes
Epithermal mineralization is emplaced at low-temperatures (50-200°C) in environments close to the surface at depths of less than 1 km. It often contains exotic elements such as mercury, antimony, silver and gold. Several sub-types are recognized globally, of which two are known from Wales, restricted to two areas: firstly in the upper part of the Coed-y-Brenin porphyry-copper system (Fe-Ag-Au) and secondly at Deganwy in North Wales (Sb-Pb).
At the shallow levels in the Earth's crust where epithermal deposits are formed, abrupt changes in physical and chemical conditions result in mineral precipitation and often intense hydrothermal alteration of the host-rocks. Factors that influence the conditions prevailing in this environment, and which therefore dictate the character of the mineralization, include the local geology (and hence permeability and reactivity of host rocks) and the pressure and temperature of the hydrothermal fluid (water at temperatures in excess of 100°C can remain liquid under high pressure but as it nears a low-pressure environment it boils suddenly and often explosively). The hydrothermal fluids may be of residual magmatic origin, but commonly they are generated when groundwaters are heated by a body of molten rock, such as a sub-volcanic magma-chamber.
Epithermal deposits occur commonly as pipe-like zones in which the rocks are brecciated and highly altered. Veins can also occur, especially along fault-zones: however such vein mineralization is typically discontinuous. Texturally, minerals often show banding and open-space growths into vugs.
In volcanic districts, epithermal systems are common and they frequently reach the surface, whence the boiling hydrothermal fluids erupt as geysers and fumaroles. Many old epithermal mineral deposits represent the fossilised ‘roots’ of ancient fumarolic systems. Because such things are near-surface features, erosion often removes them in due course, which is why old epithermal mineral deposits are relatively uncommon worldwide. Most of them are of Mesozoic age or younger.
Limestone hosted hematite deposits 
This distinctive class of mineralization consists of low-temperature, iron oxide-dominated mineral assemblages occurring along faults, in karstic voids and as replacement bodies in Carboniferous Limestone. Occurrences are characterized by an often irregular geometry, with pipe or lens-like ore bodies a common development. They generally have a massive form, but with vuggy areas containing well-crystallized paragenetically late minerals. Hematite, goethite, quartz, calcite, dolomite and baryte are the commonest minerals present, while, characteristically, the wallrocks are extensively dolomitized.
Although there are similarities in the mode of formation to Mississippi Valley type deposits, the means by which the hydrothermal fluids were generated and also the fluid geochemistry differ strongly in this case. It is generally accepted that the fluids were formed from low-temperature neutral to alkaline brines derived from Triassic Sabhka (salt-flat) environments: the brines, rich in iron leached from Triassic red-beds, are thought to have interacted with warmer fluids originating from greater depths. This is an unusual type of mineral deposit with very few examples known outside of the UK. The most extensively developed UK example is the West Cumbria iron-mining district, but there are important examples in the Llantrisant-Taff's Well area of South Wales, including the iron-ores worked at Llanharry and Mwyndy mines.
Mesothermal polymetallic veins
Mesothermal literally means medium-temperature, and refers to hydrothermal mineral deposits formed at between 200-300°C. A number of mineralization types fall within this division but an important class, so far as Wales is concerned, is that of polymetallic veins.
Polymetallic veins are common in the Lower Palaeozoic rocks of Wales, with examples known from Pembrokeshire, throughout Central Wales into Snowdonia and on Anglesey.
Typically, they occupy faults, fissure-fractures and shear-zones. The mineralization may extend right along such structures, or may be developed intermittently along them. Repeated movements and pulses of mineralizing activity are typical. Dips may vary considerably and a common feature is steep dips in competent rocks (e.g. sandstones, dolerite sills) and less steep dips in incompetent rocks (e.g. shales, mudstones). Wallrocks are commonly altered, with a bleached appearance.
Polymetallic veins carry copper, lead, zinc, silver and gold (all of economic importance), arsenic, antimony, iron and a wide-ranging suite of accessory metals, often occurring as rare sulphides, arsenides or tellurides. They may form by a number of processes, of which two are important in Wales:
• Metamorphism: When thick sequences of sedimentary and volcanic rocks are buried to greater and greater depths they are subjected to increasing pressures and temperatures, resulting in low-grade metamorphism. This liberates significant amounts of water from hydrated minerals, such as clays, as they recrystallize. This water can then migrate as a hydrothermal fluid along suitable pathways through the rocks, such as faults and shear-zones, where minerals are deposited. Some of the best Welsh examples of such veins are those in the Dolgellau Gold-belt, hosted by Middle to Upper Cambrian sedimentary rocks and intrusions, and formed prior to the Caledonian deformation that uplifted the Welsh Basin in Devonian times. The veins occupy fault-fractures with strike lengths of several kilometres and typically reveal ribbonlike textures as in the example illustrated above. Metamorphic reactions causing widescale dewatering of deeply-buried rocks is believed to have been the mechanism which generated the hydrothermal fluids.
• Igneous activity: large-scale igneous centres, such as the caldera that dominated Snowdonia during Middle Ordovician times, are capable of generating hydrothermal fluids which form when groundwaters are heated at depth by a magma-chamber. The hot waters then rise up through the volcanic and surrounding sedimentary rocks in a process known as hydrothermal convection, again finding fracture-systems along which to migrate and precipitate minerals. The copper-rich veins of the Snowdon district were developed within a conjugate fracture-system related to the apical graben of the Snowdon Caldera, and are again pre-tectonic with respect to the end-Caledonian deformation. 
Article Sources: http://www.museumwales.ac.uk/en/834/