This is the report of a geology trip by the Bristol U3A Geology Group to the Providence iron mine at Long Ashton.
New Providence Iron Mine
Ashton Hill Iron Mine No.2
ST 535 709
New Providence Mine
“Mine below Providence Mine. Part choked entrance in pit. 19C red ochre mine. Fine passage with deads and pit props leads to Red Rift with bedding chamber and pool. 2m active micro gour slope, cave pearls and calcited twigs in main passage.”
is mainly Hematite, Fe2
and Red Ochre
When the mine was worked out, many miners went to work in the nearby Durnford Limestone Quarry.
Iron in the form of hematite and earthy red ochre was mined at Providence Iron Mine, in a field known as the Iron Plantation. Yields varied from 600-3000 tons of ore per annum between 1858 and 1878. Mining continued here until the First World War. Reports from cave explorers in the 1950s refer to an enormous main rift (ST 5350 7093) leading to a partially choked adit entrance (ST 5370 7070) and further workings below. The Miners Rest on Providence originated as a cottage, owned by the BEAMES family, where miners could obtain refreshment.
Providence mine also produced Baryte – Barium Sulphate – Ba SO4
The iron minerals here are in veins in the Hotwells Carboniferous Limestone. This known as part of the Bilbao Supergene Mineralisation . There is a lot of info on this on the Internet. ( Google Bilbao Supergene Mineralisation for more info )The crustal extension in the Early Permian ( C. 290 Ma) to the Late Jurassic ( C. 150Ma ) created rift basins. This was caused by the crustal relaxation after the Variscan Orogeny, These created more stable platforms between the rift basins. The basins subsided and infilled rapidly with sediment. Basin waters squeezed out onto platforms via tension structures thus allowing mineralised hot waters to flow up into the Triassic sediments, leaving mineral deposits. New Providence Iron mine is in one of these giving rise to localised iron ore sediments.
There is another vein of iron ore shown on the BGS map. Sheet 264. but we saw no sign on the surface that this deposit had been worked.
Unlike the lead and zinc ores, many of the iron ore deposits are secondary deposits. Intense weathering of the iron pyrite-rich Coal Measures, and other iron bearing rocks during Permian and Triassic times released the iron into the groundwater. The iron was subsequently redeposited as many thin discontinuous veins of haematite or pyrite, within the Carboniferous Limestone and the Dolomitic Conglomerate, and especially along the unconformity between the two. Many of these pyrite veins have now been altered to form limonite or ochre. Ochre also occurs infilling cavities in the Carboniferous Limestone and Dolomitic Conglomerate, or as a replacement ore-body, where metal-rich ground-waters have chemically replaced the host rock with iron ore.
There are several oxides of iron and each has several polymorphs so iron is a complicated subject.
the most common isotope of iron
. About 91.754% of all iron is iron-56. … This means that as the Universe ages, more matter is
converted into extremely tightly bound nuclei, such as 56Fe.
Iron is a “special” element because of its nuclear binding energy
. The idea is that when you fuse two light elements together, you get a heavier element plus energy. You can do this up to iron.
Similarly, if you have a heavy element that undergoes fission and splits into two lighter elements, you also release energy. Down to iron.
The physical reason for this has to do with the balance between nuclear forces
and the electromagnetic force.
Due to the way these energies work, and because iron is thus thought of as the most stable, if you want to get energy from fusion or fission, your best bet is to use atoms that are farthest away from iron — very light (like hydrogen) or very heavy (like uranium).
As a side note, this is also why Type 2 supernovae
happen — the star can no longer gain energy from fusion because it can’t fuse past iron, so the outward pressure from energy generation stops and the star collapses.
This will happen to the sun soon – as the sun runs out of hydrogen and starts producing heavier elements until it gets to iron and then fusion will stop and the sun will collapse – in about 5 billion years time.
Haematite – Also spelled Hematite.
This mineral is one of the most important ores of iron. It can vary in colour from metallic grey to bright red. It is a form of ferric oxide Fe2
.. It is the oldest oxide of iron ever to have formed on the earth. Its occurrence is widespread in rocks and soils. It is harder than pure iron. It has been used throughout history as a pigment.
It occurs in several forms.
Botryoidal or kidney ore, magnetite, iron rose and specularite
Is a hydroxide of iron that has also been used as a pigment – brown ochre. Its chemical formula is (FeO(OH). It contains iron of ferric form.Its main use is as iron ore and is also the source mineral for yellow ochre. Colour is yellowish to dark brown and black.
Most often in botryoidal, reniform, or stalactitic aggregates of radiating crystals or ball-like crystals. Also grainy, in veins, concretionary, oolitic, and in earthy masses. It
often assumes the shape of other minerals forming a pseudomorph in place of the original mineral or as a coating above it. It is the main component of rust and bog iron ore. It forms prismatic needle-like crystals ( Needle iron ore ) acicular.
This a hydrated version of Goethite. It is a component of rust and is yellow to brown in colour. Mined as yellow ochre. eg Winford quarries.
Iron sulfide FeS2
– fool’s gold. Often found in anoxic, shallow seas. Easily oxidised so specimens often decay.
Iron carbonate FeCO3
. 48% iron so a valuable iron ore
Glossary for 17th January 2019
Botryoidal / Reniform
Texture or mineral habit is one in which the mineral has a globular external form resembling a bunch of grapes as derived from the Greek botruoeidēs. This is a common form for many minerals, particularly haematite, the classically recognized shape.
Honeycomb pattern of limonite
(a mixture of hydrous iron and manganese oxide minerals) that remains in the cavity after a sulfide mineral
grain has dissolved. The boxwork may be spongelike, triangular, pyramidal, diamondlike, or irregular in shape and may be coloured various shades of ochre and orange through dark brown. The colour and shape of the boxwork can sometimes be used to identify the dissolved sulfide minerals
Refers to a coating of fine crystals
on a rock fracture surface, vein
or within a vug
A form of Calcrete where iron is emplaced instead of calcium
is a process of differential weathering and erosion by which an exposed well-jointed coarse-grained rock such as granite or gneiss, develops a corrugated surface of flutes; especially the formation of small-scale ridges and depressions by wave action.
secondary formation within sedimentary
and volcanic rocks
. Geodes are hollow, vaguely circular rocks, in which masses of mineral
matter (which may include crystals) are secluded. The crystals are formed by the filling of vesicles
in volcanic and sub-volcanic
rocks by minerals deposited from hydrothermal fluids
; or by the dissolution of syn-genetic concretions and partial filling by the same, or other minerals precipitated from water, groundwater
or hydrothermal fluids.
Rust-coloured oxide and hydroxide minerals of iron and manganese
that cap an ore
deposit. Gossans form by the oxidation of the sulfide minerals in an ore deposit and they thus may be used as clues to the existence of subsurface ore deposits. especially if distinctive boxworks are present.
In addition to hydrous oxides of iron and manganese, gold and silver in the native (natural, nearly pure) state and various sulfate, carbonate, and silicate minerals can occur in gossans. The hydrous oxide minerals occur as the residuum when sulfide minerals are dissolved from the outcrops; they are either indigenous
(i.e., fixed at the site of the original sulfide mineral) or transported. Indigenous hydrous oxides indicate the presence of copper, whereas transported hydrous oxides indicate its absence or its presence in very low proportion to iron and manganese.
In mineralogy, a pseudomorph
is a mineral or mineral compound that appears in an atypical form (crystal system), resulting from a substitution process in which the appearance and dimensions remain constant, but the original mineral is replaced by another. The name literally means “false form”.
A sedimentary structure superficially resembling an oscillation ripple mark, and having a concave side that is always oriented toward the top of the bed. Also known as a scallop.
A geologic mountain-building event caused by Late Paleozoic continental collision
(Laurussia) and Gondwana
to form the supercontinent
. It is seen in our area as pressure from the South West towards the North East.
is a small to medium-sized cavity inside rock
. It may be formed through a variety of processes. Most commonly, cracks and fissures opened by tectonic activity (folding
) are partially filled by quartz
, and other secondary minerals
. Open spaces within ancient collapse breccias
are another important source of vugs. Vugs may also form when mineral crystals
or fossils inside a rock matrix are later removed through erosion or dissolution processes, leaving behind irregular voids. The inner surfaces of such vugs are often coated with a crystal druse
. Fine crystals are often found in vugs where the open space allows the free development of external crystal form. The term vug
is not applied to veins and fissures that have become completely filled, but may be applied to any small cavities within such veins. Geodes
are a common vug-formed rock, although that term is usually reserved for more rounded crystal-lined cavities in sedimentary rocks
and ancient lavas
© Richard Kefford 2019 Eorðdraca
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