Mineral Deposoits From Hydrothermal Fluids
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, analcim
