Peak District Mineral Field
One of the major industries in the Peak District was the extraction of metal bearing minerals, which continues today. But how did they get there in the first place? The mineral deposits are mainly hydrothermal, occurring in veins of varying thickness in the rocks. Mineralisation is exclusive to the Limestone area.
Conventional thinking suggests that mineralisation is related to the vulcanism of Carboniferous times. This theory needs a large mass of magma deeply buried beneath the White Peak. By now, this magma will have solifified but no such mass has ever been found. This however, was the accepted explanation when I studied geology in the 1970s.
However, it is now known the Peak District metalliferous minerals are mesothermic in origin. They were created at moderate depths, pressure and temperature as the mineralogy is galena and sphalerite. In the Castleton area, Blue-John (type of fluorite) and barytes exist alongside galena. This mineralogy suggests an epithermic origin i.e. closer to the surface.
Another mechanism of mineralisation was needed.
Peak District Mineralisation – is the Debate Over?
Debate and research strongly suggets the source of the circulating groundwater was the deep water basins. These basinal areas surrounded the limestone shelf areas.
Take a look at the Castleton mineralisation for an example. This shows how the mechanism works. Hydrothermal solutions circulating through sediments and rocks picked up minerals from the basin clays to the north. These hydrothermal solutions then penetrated the limestones via joints and faults. Minerals such as galena and sphalerite were deposited in fissures along the edge of the St George’s Land shelf.
The Odin mine is one such example of this. It is a fault (slickenslides can be seen in the walls) that was mineralised. There is still some mineralisation on the walls of the fault. The Odin mine was worked in the 13th century. Many of the local rakes and their subsidiary scrins (vertical mineral veins) are wrench faults.
Sedimentary Origins of Mineral Deposits
In places, the mineral deposits have a sedimentary origin. Underground watercourses eroded hydrothermal mineral deposits and deposited the detritus in caves. This provided rich and easy pickings for miners e.g. in the Millclose mine. Such deposits were easy to extract as they were unconsolidated and the caves were large.
Types of Mineral Vein in the Peak District
Miners working the source veins of these deposits had to work harder! The original veins were effectively part of the rock and also generally quite narrow and well consolidated.
Two types of mineral vein are recognised. Firstly, where the junction between mineral and rock is very clear and sharp, it is clear that hydrothermal deposition has taken place. The minerals simply precipitated onto the sides of a joint or other fissure underground.
In the other type of vein, the junction is not clear and the mineral appears to penetrate the rock. Veins of this nature are created by metasomatism, essentially a type of metamorphism. The circulating hot hydrothermal solutions have altered the host rock to form the mineral.
Elements that were not present in the original rock are introduced by the hydrothermal solution. These elements could originate either from the basin or be picked up locally. Hot water under high pressure can travel large distances before conditions are right for precipitation. Groundwater can change significantly during that time.
Separate Sources of Mineralisation
There appear to be two sources of circulating hydrothermal solutions in the Peak District. The majority of mineralisation is lead with some zinc but in the Ecton region, rich copper deposits were mined. Ecton copper mine formed the basis of the fortune of the Dukes of Devonshire and paid for the construction of the grand buildings of Buxton.
From Gangue to Valuable Mineral Resource
Until recent times, the non metal minerals (except Blue-John) were simply regarded as gangue and discarded. These are now needed in large quantities for the paper industry amongst others and are currently mined, both underground and open cast. Also, in the 1960s and 70s, there was a great deal of reworking of spoil heaps in order to recover the fluorite and barytes.