V.—The Leadhills-Wanlockhead Lead and Zinc Deposits

Abstract

SynopsisThe Leadhills–Wanlockhead mining field is situated within a synclinorial belt of greywackes, bounded to the north and south by anticlinoria in which lower stratigraphical elements are exposed. The universal strike is N.E.–S.W. The southern margin of the northern anticlinorium is delimited by a strike thrust fault, inclined to the north-west. The shear zone caused by this thrust is considered to be the principal structural feature governing the localization of the ore deposits.The complex N.E.–S.W. folding and faulting, imposed during the Caledonian orogeny, is crossed by a series of intersecting joints, whose average strike is N.N.W. The majority of the mineral veins also trend N.N.W. The formation of these discordant structural features under Caledonian stress is demonstrated by the presence of N.N.W. trending Caledonian dykes.Sinistrai movement along the joint pattern, ascribed to a reorientation of the “Caledonian” stress towards an “Hercynian” direction, resulted in the formation of open spaces on the more north-westerly trending members of the joint system in the greywacke belt. This feature is considered to be the secondary structural control responsible for the localization of the ore deposits.Fifty-seven minerals were identified from the deposits. Fifteen of these minerals had not been previously recorded from the locality, including (1) a new chromian mineral; (2) a new variety, chromian leadhillite; (3) a mineral previously recorded only as an artificial product, lead hydroxyapatite; (4) phœnicochroite, not previously confirmed in the British Isles.Two periods of mineralization were distinguished. The first consisted of quartz veins with which are associated small amounts of gold, pyrite and muscovite, tentatively assigned to the Caledonian orogeny, and the second comprised the lead-zinc mineralization. The paragenetic relationships of the primary minerals of the lead-zinc mineralization indicate two generations of sulphides; the second generation is accounted for by reprecipitation of elements derived from the replacement of the first generation by late stage quartz. A study of the distribution of elements through the paragenesis suggests that some elements were derived from other than a magmatic source, and that contamination has probably played a considerable rôle in the control of the character of the gangue minerals.Evidence that the mineralizing solutions had a deep-seated origin is provided by the mineral zones and the geochemical character of the deposit. Emplacement of the minerals took place at a temperature of the order of 143°−281° C, and a depth of the order of 2000–4000 feet below the surface.The Leadhills–Wanlockhead deposits are related to other lead-zinc deposits in Britain. On the basis of geochemical assemblage and the relation to igneous activity, it is concluded that the deposits were probably derived from the top of the tholeiitic crustal layer and the base of the granitic crustal layer, and were genetically associated with the Hercynian orogeny.