Preprints
https://doi.org/10.5194/gchron-2021-25
https://doi.org/10.5194/gchron-2021-25

  13 Sep 2021

13 Sep 2021

Review status: this preprint is currently under review for the journal GChron.

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

Jack Muston1, Marnie Forster2, Davood Vasegh2, Conrad Alderton3, Shawn Crispin3, and Gordon Lister4 Jack Muston et al.
  • 1AngloGoldAshanti, Perth, 2601 Australia
  • 2Structure Tectonics Team, Research School of Earth Sciences, Australian National University, Canberra, 2601 Australia
  • 3PT Agincourt Resources, Martabe Mine, Sumatra, Indonesia
  • 4Sustainable Minerals Institute, WH Bryan Mining and Geology Research Centre, The University of Queensland, Brisbane 4068, Australia

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.

Jack Muston et al.

Status: open (until 27 Oct 2021)

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Jack Muston et al.

Jack Muston et al.

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Short summary
This paper outlines a method that helps mineral explore for gold. When can see the alteration caused by the mineralising fluids, it is possible to use geochemical methods to vector towards the deposit. But such methods failed at Martabe. We decided to work out why. Our results showed that there were several distinct and separate very short-lived alteration events. The answer might be that vectoring is possible only when samples are compared from the same mineralising system.