A carboniferous 40 / 39 Ar amphibole emplacement age for the Au-bearing Sams Creek alkali-feldspar granite dike, west Nelson, New Zealand

Abstract

A peralkaline alkali-feldspar granite dike associated with Au-sulfide mineralisation at Sams Creek has previously yielded inconsistent Triassic K-Ar ages, and has thwarted attempts at U-Pb zircon dating. We report here a disturbed 40Ar/39Ar step heating spectrum and both single and multi-grain laser fusion analyses of amphibole, from which we interpret an age of 319 ± 8 Ma (MSWD 0.05) for dike emplacement. Although the sample is a complex mix of arfvedsonite and late-stage riebeckite of possibly widely disparate ages, the age gradient is best explained by significant argon loss in a relatively low metamorphic grade setting from amphiboles, which are less retentive of radiogenic argon than most common amphiboles. The youngest step heating and laser ages of c.≤180 Ma indicates that disturbance of the Ar system occurred in the Late Jurassic to Early Cretaceous, probably due to the thermal effects of the c. 170-105 Ma Median Batholith and satellite plutons. The c. 319 Ma emplacement age is consistent with correlation to the widely distributed Foulwind Suite (294-320 Ma) of peralkaline and A-type granites. The age helps to constrain the timing of at least some of the movement on the Devil River Fault which crosscuts the dike, and leads to the suggestion that north-south folding and greenschist facies metamorphism which affects both dike and host rocks may be related to mid-Jurassic to Early Cretaceous subduction.