Geology

Geology determines where mineral deposits occur, the form they take underground and often the way a mine must be developed to extract them. These records explain the geological processes and structures that shaped the mines in the archive.

Deposit Types

Record	What it is	Mines	Commodities
Evaporite Deposits	A layered sequence of different salt minerals left behind as an isolated body of seawater slowly evaporates — not a single deposit type but an ordered stack of several, each mineral precipitating out in a predictable…	20	Gypsum, Potash, Salt
Greisen and Pegmatite Deposits	A zone of intensely altered granite, converted by high-temperature fluids escaping from the granite's own crystallising magma into a coarse, quartz-and-mica rock hosting tin and tungsten ore — the deposit type behind…	8	Tin, Tungsten
Iron Replacement Deposits	An iron ore body formed not by filling an open fracture, like a vein, but by fluids chemically dissolving away limestone and depositing solid iron oxide in its place — the process behind West Cumbria's exceptionally…	16	Iron
Placer Deposits	A concentration of valuable mineral formed not underground but at the surface, where moving water sorts loose rock and sediment by weight, leaving dense minerals like gold behind while lighter material is carried away.	10	Coprolite, Gold, Tin
Podiform Deposits	A pod-shaped, isolated concentration of ore — most often chromite — enclosed within a body of serpentinised ultramafic rock, with no continuous vein or seam connecting one pod to the next.	2	Chromite
Sedimentary Beds and Seams	A layer of economically useful material formed as part of ordinary sedimentary rock deposition — laid down in more or less continuous sheets over a wide area, rather than concentrated along a narrow fracture the way a…	631	Alum, Chalk, Coal, Iron, Jet, Oil Shale, Salt
Sedimentary Ironstones	Sedimentary ironstones are iron ores laid down as sediment: iron-rich beds and bands of nodules lying within an ordinary sequence of shales, sandstones or coal seams.	19	Iron, Ironstone
Veins and Lodes	A sheet-like body of mineral-bearing rock filling a crack or fracture in the surrounding country rock — the single most common shape taken by Britain's historic metal deposits, and the shape that dictated how the great…	243	Antimony, Arsenic, Calcite, Cobalt, Copper, Gold, Lead, Silver, Tin, Uranium, Witherite, Zinc

Formation Processes

Record	What it is	Mines	Commodities
Coal Formation and Rank	Coalification is the slow change of buried plant matter into coal, and rank is the measure of how far that change has gone: from peat through lignite and sub-bituminous coal to bituminous coal and, at the top,…	14	Anthracite, Coal
Hydrothermal Deposits	Mineral deposits formed when hot, chemically active water — carrying dissolved metals picked up from the surrounding rock — flows through fractures and cracks, then deposits that dissolved material as solid minerals…	112	Antimony, Arsenic, Barite, Calcite, Cobalt, Copper, Fluorspar, Gold, Lead, Silver, Tin, Uranium, Witherite, Zinc
Kaolinization and China Clay Deposits	The alteration of granite's feldspar minerals into soft, white clay (kaolinite), leaving the quartz and mica largely untouched — the process behind Cornwall's china clay industry, one of the largest clay-mining…	4	China Clay
Laterite and Residual Weathering Deposits	An ore body formed not by anything being added to a rock, but by long chemical weathering taking almost everything else away — leaving the least soluble elements (iron and aluminium oxides, in Britain's case)…	1	Bauxite, Iron
Slate and Regional Metamorphism	A fine-grained metamorphic rock, formed when mudstone or shale is subjected to intense pressure during mountain-building, that splits cleanly into thin, durable sheets — the roofing material behind one of Britain's…	35	Slate

Structural Controls

Record	What it is	Mines	Commodities
Anticlines and Synclines	Folds are bends in layered rock. An anticline is an arch, with the oldest beds in its core; a syncline is a trough, with the youngest beds in its core.	17	Coal
Concealed Coalfields	A concealed coalfield is one whose Coal Measures do not reach the surface.	19	Coal
Dykes and Sills	A dyke is a sheet of igneous rock that cuts across older beds; a sill is a sheet forced in between them, parallel to the bedding.	15	Coal, Whinstone
Faults and Fractures	A break in rock along which the two sides have moved relative to each other (a fault) or simply cracked without necessarily moving (a fracture or joint) — not itself a source of valuable material, but one of the most…	51
Strike and Dip	Strike is the compass bearing of the horizontal line formed where a tilted rock layer, mineral vein or fault plane intersects a horizontal surface; dip is the angle that same plane inclines downward away from the strike…	15