Helium and Argon in Ore Fluids: So What?

Abstract

The isotopic composition of He and Ar trapped in crustal fluids and fluid inclusions carries a signature of the source of the fluids and the processes which have affected them. There are three distinct sources of the noble gases; atmosphere, crust and mantle. Atmospheric gases carried in groundwaters are characterised by the presence of 36Ar and 4~ and minor amounts of helium (3He/4He = 1.39 x 10 _6 = Ra). The crust is a source of radiogenic 4~ and 4He, very low levels of3He (-0.01-0.05 Ra) and no 36Ar. The prime signature of the mantle is helium with a high 3He content (e.g. 3He/4HeMoRB ~ 8 Ra), together with 4~ and, in most situations, negligible 36Ar (4~ ~ 40,000). Fluids trapped in ore minerals frequently carry all three components (Stuart et al., 1995). We have investigated He and Ar isotopes, sometimes in combination with halogens, trapped in ore fluids from a variety of areas and types of mineralisation. Our aims have been to see what qualitative aspects of the mineralisation processes are reflected in noble gas patterns and to what extent these can be used to resolve conflicting interpretations, what aspects can be understood in a quantitative way, and whether these can be used, for example, to model the large scale features of specific ore deposits. In this report we contrast the He and Ar isotope observations from two widely separated porphyry-Cu deposits (PCD) with those from an MVT deposit. The Machangqing PCD deposit in West Yunnan province, China, is part of the Ailaoshan-Jinshajiang alkali-porphyry intrusive zone of Eocene age formed in an extensional setting. The mantle derived magmas intrude lower Ordovician microelastic rocks and Devonian limestone and appear to have assimilated some crustal material. The Sungun PCD is located in NW Iran and is associated with a series of calc-alkaline hypabyssal igneous intrusions of Oligo-Miocene age. The magma was emplaced in a continental margin tectonic setting related to the subduction and assimilation of the Persian plate during the major Alpine orogenic phase. The Sungun porphyries occur as stocks and dykes intruding a series of Oligocene dacitic breccias, tufts and trachyandesitic lavas, Eocene arenaceous-argiUaceous rocks, and UpperCretaceous limestones. The He and Ar compositions of gases released by crushing sulphides from the two deposits are shown in Figs. 1 and 2. 4~ is the only isotope present in high abundance in all components and consequently this is the only method of representing all the data, end members and processes in a single figure. Mixtures of three end members define a mixing plane, while elemental fractionation of He and Ar drive compositions radially from, and perpendicular to, the ASW axis. In both figures the data lie on 3component mixing planes. The three end members are fluids enriched respectively in atmosphere (ASW), mantle helium and argon ( label led EARLY) and crustal helium (labelled LATE). The 3He/4He ratios, 1.2 Ra (Machangqing) and 2.25 Ra