Direct dating of overprinting fluid systems in the Martabe epithermal gold deposit using highly retentive alunite

Abstract

Abstract. The Martabe deposits in Sumatra, Indonesia formed in a shallow crustal epithermal environment (200–350 °C) associated with mafic intrusions, usually recognised in domes, adjacent to an active right-lateral wrench system. Ten samples containing alunite were collected for high-resolution geochronology, to determine if overprinting fluid systems could be recognised by dating alunite separates. The heating time for each step was chosen to ensure reasonable uniformity in terms of the incremental percentage of 39Ar gas release during each of many steps, allowing age spectra to be analysed using the method of asymptotes and limits. Several distinct growth events could be recognised. In addition, each sample was subjected to ultra-high-vacuum (UHV) furnace step-heating, and 39Ar diffusion experiments conducted at the same time as 40Ar/39Ar geochronology, to determine the argon retentivity of the mineral grains being analysed. The heating schedule ensured Arrhenius data uniformly populated the inverse temperature axis, with sufficient detail to allow the application of the Fundamental Asymmetry Principle (FAP) during analysis of the Arrhenius spectrum. Results show activation energies between 370–660 kJ/mol. Application of Dodson’s recursion determines that closure temperatures would range from 400–560 °C for a cooling rate of 20 °C/Ma, which is higher than any possible temperature to be expected in the natural system. This gives confidence that the ages represent growth during periods of active fluid movement and alteration, since the deposit formed at temperatures < 200 °C at a depth of < 2 km. We conclude that gold in the Purnama pit was the result of fluid rock interactions during very short-lived mineral growth episodes at ~ 2.25 and ~2.00 Ma.

Complex 40Ar ∕ 39Ar age spectra from low-grade metamorphic rocks: resolving the input of detrital and metamorphic components in a case study from the Delamerian Orogen

“…Result tables for each step heating experiment are given in Reid and Forster (2022). Weighted mean ages from selected steps are calculated in the case of three or more steps forming a coherent grouping based on Pearson's chi statistic to assess whether the scatter remains within the 95 % confidence limit and calculated using the software eArgon (see Muston et al, 2021, for detailed description of statistical methods).…”

“…In this respect, we utilise a similar methodology to Dallmeyer et al (1988), who analysed whole rock slate and phyllite samples via furnace step heating. However, here we apply a care-ful heating schedule for our ultrahigh vacuum (UHV) step heating experiments, utilising a larger number of increments in the step-heating schedule to carefully resolve any variations in age within the sample (thereby applying methods similar to Forster and Lister, 2009;Muston et al, 2021).…”

Complex 40Ar/39Ar age spectra from low metamorphic grade rocks: resolving the input of detrital and metamorphic components in a case study from the Delamerian Orogen

“…Kula et al, 2010;Clauer, 2013) or, in the case of single minerals or grains, as a result of different gas domains having a range of diffusion properties (e.g. Lovera et al, 1989;Muston et al, 2021). As discussed by Forster and Lister (2004), the upper asymptote of a staircase age spectrum is the minimum age for crystallisation of the sample and conversely, the lower asymptote is a maximum age for either a less retentive domain within the sample, or a geological resetting event (Fig.…”