|Introduction | History
of lead mining | History
of zinc mining | Lead
ore & mines
Zinc ore & mines | Iron
ore, ochre & mines | Coal mining
Zinc ore and mines
The lead, zinc and copper veins on Mendip were deposited by hot
mineralising fluids (typically between 50 and 150 ° C)
rising up from depth and depositing various minerals as they
cooled. The source of the fluids were the deep sedimentary basins
either side of the Mendips. As the Carboniferous, Triassic and
Jurassic sediments in these basins were buried, compacted and
heated over time, some of the water in the rock was forced out,
along with any dissolved metals and migrated into the neighbouring
Carboniferous Limestone. Here the change in chemistry, temperature
and pressure led to the deposition of various minerals including
lead, zinc, and locally copper.
of zinc ore
Zinc ore occurs in two types of deposit: as primary zinc ore in thin
veins known as rakes, or a secondary deposit formed by weathering
of the primary mineral veins. Zinc ore is most commonly found as
zinc carbonate (ZnCO3), known as calamine or smithsonite.
It generally occurs as rounded, crystalline crusts or granular, honeycombed
masses that have a vitreous or pearly luster and are typically dirty
brown or grey in colour. Locally it was known as ‘dry bone’ ore,
having a cellular to spongy appearance reminiscent of dry bone.
Calamine is actually a secondary mineral, found principally in the
oxidized zone of the zinc-bearing ore deposits. It is derived
from the alteration of the primary zinc sulphide (ZnS) mineral sphalerite.
This is generally a dark grey or black, highly lustrous mineral,
but can vary in appearance. Both occur on Mendip.
Its chief use was to make brass by mixing with copper.
The primary zinc ore, sphalerite is typically found in thin veins
cutting through the rock. In these veins, the ore occurs as either
thin layers encrusting on the walls of the vein, or as thin bands,
pockets or crystals within the vein. The veins were always associated
with other waste minerals known as ‘gangue’, usually
calcite (CaCO3), pyrite (FeS2) or barytes (BaSO4).
Many of these veins were very thin, sometimes only a few centimetres
wide, and often pinched and swelled along their length, sometimes
forming complex anastomosing networks with other veins.
In the upper parts of the vein, percolating oxygenated groundwaters
cause alteration and breakdown of the primary sulphide minerals.
This chemical reaction was enhanced by the presence of pyrite, which
when oxidized produces sulphuric acid. The zinc liberated by the
oxidation of sphalerite reacted with the carbonate from the host
rock to form calamine. This process often destroyed the original
vein, instead creating a series of pockets or vugs infilled with
Old spoil tips and shafts creating ‘gruffy ground’,
Shipham, formed by intensive mining. Photo. Elaine Burt
Zinc was principally mined around Shipham and Rowberrow, where abundant
evidence of the mining activity can be seen in the form of ‘gruffy
ground’. This hummocky landscape is formed by the numerous
shafts and trial pits excavated by the miners. The principal ore
mined around Shipham was calamine, the sphalerite only being of
secondary importance. Several old mines are still accessible to
cavers. Calamine was also locally mined elsewhere, particularly
The main use of calamine was in the brass industry, where it was
mixed with copper to form brass, but it was also sold to pharmacists
for its medicinal properties. Today calamine lotion is still available
as a soothing and mild antiseptic lotion for use on irritating skin