Parys Underground Group

MYNYDD PARYS MINERALS

This page shows examples of the types of mineral found at parys mountain.
Please note the site is within an SSSI and specimen collection is not allowed.

The minerals were laid down underwater in Black Smokers. These are volcanic vents that spewed out very acidic hot water laden with the chemical building blocks from which minerals are formed. This hot water was rich in sulphur from the volcanic activity. Most of the minerals that were formed contain sulphur.

As the acidic hot water, laden with metals and sulphur, sank down onto the sea floor, it cooled and became pH neutral. In cool waters the chemicals arranged themselves into crystals of minerals. Over time this happened again and again until the whole of the ancient sea floor was covered with minerals.

This former sea floor is now part of the geology of north east Anglesey. It was buried beneath later rock formations. Mountain building movements uplifted the rocks and they were exposed to weathering and erosion. The minerals that originally formed on the ancient sea floor are the minerals that were mined at Mynydd Parys.

Those original Black Smoker minerals are:

FeS2 pyrite Pyrite is the commonest of the sulphide minerals and is far more widespread than its orthorhombic dimorph, marcasite. It occurs in many geological settings, including sedimentary, igneous and metamorphic rocks of all ages and in a great range of hydrothermal mineral deposits. It is also abundant in many placer deposits. Because of its widespread occurrence, associated minerals are highly diverse. Pyrite is readily identified by its cubic crystal shape when well-formed. In polished section it often shows high relief against other sulphides due to its greater hardness: it polishes well when fresh and has a relatively bright appearance. The instability of pyrite (pyrite-decay) is well-known to curators of both mineral and fossil collections. Pyrite can react with air and moisture, the reaction creating an acidic solution of iron which then goes on to attack the remaining pyrite plus other more stable sulphides such as galena and sphalerite.
PbS galena – ore of lead Galena is an important component of the 'bluestone' ore, where it occurs intimately intergrown with sphalerite, chalcopyrite and numerous minor phases
CuFeS2 chalcopyrite – ore of copper chalcopyrite was the principal source of copper at this mine and is also a significant component of the 'bluestone' reserves identified by recent drilling (Greenly, 1919; Pointon & Ixer, 1980). The ore was deposited from hot fluids exhaling onto the sea-bed, much in the same way as is observed today in underwater 'black smokers'. Associated minerals include pyrite, galena, sphalerite and a host of minor species.
ZnFeS sphalerite – ore of zinc sphalerite is an abundant component of the 'bluestone' ore at this locality. Two generations have been identified (Pointon & Ixer, 1980) - the first crowded with chalcopyrite inclusions, the second inclusion-free. Associated minerals are pyrite, chalcopyrite and galena. The presence of significant sphalerite deposits drove much of the recent exploration at the site.
Copper – rare in native form significant occurrences were reported by Lentin (1800) in the near-surface zone of the orebody, while Greenly (1919) noted the presence of leaflike or mosslike aggregates in gossan in the Great Lode.
SiO2 – quartz quartz is present in one form or another everywhere in Wales: it is true to say that virtually every square metre of the surface of the Welsh landscape has some quartz present, and in some places (e.g. parts of Snowdonia) it is so abundant in veins as to give rock outcrops a white-streaked appearance.

The chemical elements that make up these metals were loose as ions in the scalding acid water. They combined to form the minerals like this:

2Fe3+ + 3S2- = 3FeS2

Gradually, over time these minerals have weathered and reacted with rainwater to form other, secondary, minerals. The chemicals that make up these other minerals are all found in those first, or primary minerals, that were formed on the ancient sea floor. Some of these secondary minerals, were first discovered by geologists on Anglesey.

The minerals that are made after weathering and reaction with rainwater are a combination of the minerals that were already there. These are the secondary minerals that you can find at Mynydd Parys :

PbSO4 Anglesite Anglesite occurs in the oxidation zones of lead-bearing ore deposits. It forms early during the galena alteration process, in relatively acidic conditions where fluid circulation is restricted. In more open systems, carbonate ions contributed by dissolved CO2 in rainwater raise the pH beyond that where anglesite is stable and the much commoner lead carbonate, cerussite, forms instead. Similar micro-environments occur within mine spoil and anglesite is not uncommon as a post-mining mineral, often in association with covellite and linarite. The name anglesite is derived from Anglesey, the island in North Wales upon which the mineral was first discovered - at Parys Mountain.
CaSO4 Gypsum gypsum is a common sulphate mineral typically formed as an evaporite mineral particularly by the evaporation of lakes in arid regions. Gypsum also forms as thin bladed crystals on the walls of caves and mines as the variety selenite.
Cu2(CO3)(OH)2 Malachite malachite is a common secondary mineral found in the oxidized zone of copper-bearing orebodies.
Cu2O Cuprite cuprite is mentioned by Greenly (1919), although no specific details are recorded.
CaMg(CO3)2 Dolomite some well-formed crystals of ferroan dolomite were reported by Southwood & Bevins (1995) from dumps derived from recent underground workings in the Morfa-Ddu area of the site.
Cu3(CO3)2(OH)2 Azurite azurite typically forms in the oxidized zones of copper-bearing hydrothermal mineral deposits
FeO(OH) Goethite goethite is a common weathering product of iron-bearing minerals in oxygenated environments and can be an important component of iron ore derived from weathered iron mineralization. Goethite is also a primary precipitate in hydrothermal, marine, and bog environments upon oxidation of reduced iron-bearing waters.
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