The mineralisation of Mynydd Parys extends roughly 3 km NNE-SSW in a band 1km wide. It is associated with an ancient volcanic event ( late Ordovician ca 480 my BP). Involving the extrusion of mostly silicic - rhyolites/dacite lavas and the ejection of ashes. These deposits grade laterally into the shallow water volcanic sediments which include silceous sinter and cherts and also host intrusive rhyolites and later dolerites. This volcanic sequence overlies the Parys shales and is in turn overlain by later silurian shales. These beds appear to have been compressed into a steep trough shaped structure trending NE-SW and tilted over to the SE. The axis being exposed at the ends of the Great opencast. The region is also traversed by the steep NNW-SSE cross faults and to the north there are older Precambrian schists of the Mona complex,brought up by the Carmel head and Corwas thrust faults.
The primary mineralisation comprised pyrite (FeS2) which can be seen in the slump structures in the exposures at the centre of the Great open cast,indicating formation on the sea floor. This was followed by a phase dominated by Chalcopyrite (CuFeS2) and then by one dominated by the intimate mixture of sphalerite (ZnS) and Galena (PbS) with only minor chalcopyrite. This is know as "Bluestone"
These are believed to have formed from exhalations on the sea floor analogous to the black smokers seen in other ocean today. The ore deposit is thus thought to be of the "Kuroko" type and as such is unique in Britain.
A secondary phase remobilisation occurred during the later Caledonian metamorphism and has been dated to be 360 my BP. As a result of of this unique geology several Sites of Special Scientific Interest ( SSSI ) have been established on the mountain.
Weathering products are dominated by colourful red/yellow Fe3+ hydrous oxides but also include a diverse range of sulphate materials. Amongst these are abundant jarosite (KFe(SO4)(OH) and anglesite (PbSO4) for which Mynydd Parys is the world type locality.
The minerals , Pisanite ( [(Fe2+,Cu)SO4.7H2O] , Antlerite [Cu3(SO4)(OH)4] Basaluminite [Al4(SO4)(OH)10.5H2O] and Anglesite a lead sulphate are also present and are generally rare elsewhere in the UK.
The present weathering in intense , due to the very high acidity generated by the oxidation of pyrite and other sulphide minerals to form sulphuric acid. Some pools have been recorded as having a pH of as low as 2.
This extreme chemistry has resulted in a remarkable flora and fauna. Eight sits have been established as SSSI on the basis of their unusual lichen communities and there are also unique liverworts and mosses.