This extract is taken from a field guide prepared for a visit on 21 April 98 to Parys Mountain as part of the Symposium on Magmatism and Mineralization in Arcs and Ocean Basins Geoscience 98 Biennial Conference at Keele University, U.K.
Parys General background
The Parys Mountain deposit is located about 3 kilometres from the north coast of Anglesey, in northwestern Wales, within a sequence of Ordovician? to Silurian volcanic rocks and shales (Figure 1). Basement rocks include the Monian Supergroup of late Precambrian age. The deposit was an historic source of copper in Britain over a century (1768-1904), although most production took place during the first 50 years of this period. The ore was recovered from a series of open pits (Figure 2), and also underground workings which extended below the open pits to a maximum of 150 metres below the surface. The originally mined deposits were viewed as vein systems or lodes within Ordovician felsic volcanic and shaley rocks (Greenly, 1919).
No major mining or exploration operations were carried out from 1904 to 1961, when the first drilling programs were begun. From 1961 to 1990, drilling to depths well below the old pits and workings, and in particular to their west, was carried out mainly by CIGOL, Parys Mountain Mines, Intermine, Cominco, Imperial Metals and Anglesey Mining, and an geological picture of the deeper volcanic and sedimentary sequence began to emerge (Hawkins, 1966; Westhead, 1991; Tyler and Charter, 1997). It was during this period of exploration that stratiform lenses of Cu-Zn-Pb-rich massive sulfide mineralization were discovered in the subsurface to the west of the open pits, and also down dip to the north, near the lower contact between a thick sequence of felsic volcanic rocks and underlying shales (e.g. Figures 3 and 4). These zones of sulfide mineralization have been termed the Chapel, Engine, Deep Engine, South Central and North Central Zones.
Various aspects of the massive sulfide deposits and the stratigraphy and structure of their host rocks have been studied over the last two decades (Thanusuthipitak, 1974; Pointon, 1979; Pointon and Ixer, 1980; Southwood, 1982, 1984; Westhead, 1993). During this period, it was generally accepted that the disposition of the main geological units on the property was the result of folding of an initial sequence of mudstone overlain by rhyolite and then shales, into a large anticline-syncline structure with an east-west strike and a moderate dip to the north, with the north limb overturned to the south. The occurrence of Silurian shales flanked by volcanic rocks which were assumed to be Ordovican, based on limited graptolite evidence, was taken as providing support for a major synclinal axis (Westhead, 1991). The age of the Silurian shales, which are exposed in the open pits, has been determined from graptolite studies (Greenly, 1919).
From 1985 to 1990, Anglesey Mining plc carried out further drilling (22 holes totalling over 8000 m) in the western part of the property, and sunk the Morris shaft to 300 m depth in the Engine Zone. Underground drilling in 1990 from the 280 m level intersected several zones of massive sulfides near the lower contact between a thick sequence of felsic volcanic rocks and underlying shales of presumed Ordovician age (e.g. Figures 3 and 4). These are known as the Engine and Chapel Zones (first discovered by Cominco in the 1977-81 period). The White Rock Zone, which is dominantly silica with occurrences of semi-massive sulfides, also is present in this area but its stratigraphic relations with the other Zones are uncertain due to underground faulting. In 1990, mineable reserves in the Engine and Chapel Zones were estimated as 1.41 Mt grading 1.99% Cu, 3.42% Pb, 6.65% Zn, 99 g/t Ag and 0.79 g/t Au; with reserves in the White Rock Zone of 0.84 Mt grading 0.49% Cu, 3.43% Pb, 6.72% Zn, 78 g/t Ag and 0.66 g/t Au (Charter, 1995). These form part of an overall estimated geological reserve of 6.45 Mt at similar grades.