The source and significance of argon isotopes in fluid inclusions from areas of mineralization

Argon isotopes in fluid inclusions in quartz veins associated with granite-hosted tungsten mineralization in the southwest and north of England have been investigated in detail by the 40Ar-39Ar technique. The natural argon is present as a number of discrete components which can be identified through correlations with 39Ar, 38Ar and 37Ar induced by neutron bombardment of potassium, chlorine and calcium. The potassium-correlated component arises principally from in situ decay of potassium in solid phases in the inclusions. In the case of the Hemerdon tungsten deposit of southwest England the phases responsible are small (∝25 μm) captive authigenic micas which are shown to have been deposited from a fluid 268±20 Ma ago, shortly after the emplacement of the host granite. The chlorine-correlated component is present in the brines which constitute the fluid phase of the inclusions. The argon in these hydrothermal fluids is made up in part of ''parentless'' or ''excess'' 40Ar, leached from surrounding crustal rocks, and in part of dissolved ancient atmospheric argon. (orig./WB)