Disturbed 40Ar 39Ar systematics in hydrothermal biotite and hornblende at the Scotia gold mine, Western Australia: Evidence for argon loss associated with post-mineralisation fluid movement

Abstract

Homblende and biotite that formed during gold mineralisation at the Scotia mine, Western Australia, have erratic 40Ar 39Ar release spectra and total gas ages that are ∼200–900 million year younger than the ca. 2600–2620 Ma minimum age of gold mineralisation, as given by 40Ar 39Ar plateau (muscovite) ages of crosscutting pegmatite dykes. Analysed hornblendes are dominated by magnesio hornblende but also contain small domains of ferro-actinolitic hornblende, actinolitic hornblende, and actinolite. Biotite also appears to be substantially altered to chlorite along cleavage planes. Relatively young apparent ages and high K/Ca ratios of argon released from hornblendes at temperatures less than ∼1000°C are interpreted to be the result of degassing of contaminant biotite. However, this cannot totally explain the young ages of hornblendes. Gas fractions released at furnace temperatures above 1000 C, where the effect of biotite degassing is demonstrably negligible, still have apparent ages that are ∼200–900 million years younger than the age of muscovite from post-gold pegmatite dykes. The close proximity of disturbed hydrothermal hornblende samples to apparently undisturbed pegmatite muscovite samples (less than a few metres in some cases) is difficult to reconcile with argon loss in hydrothermal hornblende being the product of thermally-driven volume diffusion. Given a suitable thermal history, argon loss could occur preferentially in hornblendes if (1) the closure (for slow cooling) and blocking (for reheating) temperatures of hydrothermal hornblendes were lower than published estimates, as has been observed in structurally complex metamorphic hornblendes and/or (2) the closure and blocking temperature of pegmatite muscovite were higher than commonly estimated. However, neither of these interpretations can easily explain the large variation in hornblende ages. It is instead suggested that argon loss occurred during mineral-fluid interaction during movement of a retrograde fluid along the mineralised lode structures and that this occurred at ambient temperatures below the blocking temperature of pegmatite muscovite. There is abundant geological evidence for the passage of such a fluid at the Scotia mine, including the presence of numerous late brittle fractures containing retrogressive low-temperature mineral assemblages. Late fluid movement is probably related to Proterozoic orogenic activity along the nearby southeastern margin of the Yilgarn Craton. The difference in argon systematics between hydrothermal minerals and pegmatite muscovite is largely ascribed to the relatively low permeability of the more massive pegmatite dykes (with respect to ore zones) preventing fluid egress to muscovite samples. The variations in ages of hydrothermal minerals are probably related to the extent of fluid/mineral interaction as this is a function of parameters, such as fluid/rock ratio, fluid P-T-X conditions, permeability, and mineral microstructures that may vary on short time and length scales. Recognition of possible argon loss in hornblende via fluid interaction is important for the interpretation of 40Ar- 39Ar systematics in environments, such as many hydrothermal ore deposits, where minerals may be exposed to fluids after crystallisation.