Abstract

<strong class="journal-contentHeaderColor">Abstract.</strong> The Martabe gold deposits in Sumatra formed in a shallow crustal epithermal environment associated with intermediate mafic intrusions adjacent to an active right-lateral wrench system. Gas/fluid temperatures reached 200–350 <span class="inline-formula"><sup>∘</sup></span>C. The structural geology suggests episodic switches in stress orientations during a Plio-Pleistocene seismotectonic evolution. Different mineralisation events may have been associated with oscillations in this earthquake cycle, so samples containing alunite were collected for <span class="inline-formula"><sup>40</sup></span>Ar <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M3" display="inline" overflow="scroll" dspmath="mathml"><mo>/</mo></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="8pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="e653eaf840568ee76bb20ba3bf368ae0"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="gchron-5-153-2023-ie00001.svg" width="8pt" height="14pt" src="gchron-5-153-2023-ie00001.png"/></svg:svg></span></span> <span class="inline-formula"><sup>39</sup></span>Ar geochronology to constrain the timing. <span class="inline-formula"><sup>39</sup></span>Ar diffusion experiments were performed to constrain variation in argon retentivity. The age spectra were produced by incremental step-heating with heating times chosen so similar percentages of <span class="inline-formula"><sup>39</sup></span>Ar gas release occurred during as many steps as possible. This ensured the detail necessary for analysis of the complex morphology of these spectra by applying the method of asymptotes and limits, which enabled recognition of different growth events of alunite in overprinting fluid systems. It was possible to provide estimates as to the frequency of individual events and their duration. The heating schedule also ensured that Arrhenius data populated the inverse temperature axis with sufficient detail to allow modelling. Activation energies were between 370–660 kJ mol<span class="inline-formula"><sup>−1</sup></span>. Application of Dodson's recursion determined closure temperatures that range from 400–560 <span class="inline-formula"><sup>∘</sup></span>C for a cooling rate of 100 <span class="inline-formula"><sup>∘</sup></span>C Ma<span class="inline-formula"><sup>−1</sup></span>. Such estimates are higher than any temperature to be expected in the natural system, giving confidence that the ages represent the timing of growth during periods of active fluid movement and alteration: a hypothesis confirmed by modelling age spectra using the MacArgon program. We conclude that gold in the Purnama pit resulted from overprinting fluid rock interactions during very short mineralisation episodes at <span class="inline-formula">∼2.25</span> and <span class="inline-formula">∼2.00</span> Ma.

Highlights

  • 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 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

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Summary

Introduction

Sumatra has a number of Neogene epithermal deposits of varying size and grade. The southern extent of 30 the island has a number of low-sulphidation epithermal deposits (e.g., Mangani mine, Lebong district), while the north has predominately high-sulphidation epithermal deposits (e.g., Martabe mine, Meluak deposit, and Miwah mine). The mineral alunite [KAl2(SO4)2(OH)6] is of particular interest as 55 it is a potassium bearing mineral that has been shown to be a useful 40Ar/39Ar geochronometer for dating alteration systems (e.g., Arribas et al 1995, 2011) It commonly forms in porphyry and epithermal gold systems when hot, highly acidic fluids interact with and alter potassium-feldspars. The temperature was immediately reduced, and the sample left for a minimum of 12 hours pumping away unwanted gases Such long periods of cleaning under UHV conditions prior to measurement minimised the effect of contaminants, especially in the first few steps where low retentivity diffusion domains release their gas. Corrections are done by interpolation, but in general the blanks are so low that this is not essential

Age spectra from resistance-furnace step-heating experiments
Ultra-high-vacuum (UHV) 39Ar diffusion experiments
Arrhenius data and closure temperature estimates
Probability Curve Analysis of Frequently Measured Ages (FMAs)
The role of the plateau in age spectrum
Dealing with the natural complexity of an age spectrum
What causes a complex age spectrum?
The benefits in using the method of asymptotes and limits
Comparison with laser methods
Recognising an inverse isochron on a York Plot
Using Arrhenius data to estimate argon retentivity
Findings
Timing mineralisation episodes in the Martabe mine
Conclusion

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