Direct dating of overprinting fluid systems in the Martabe epithermal gold deposit using highly retentive alunite
<p>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&g...
| Main Authors: | , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2023-04-01
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| Series: | Geochronology |
| Online Access: | https://gchron.copernicus.org/articles/5/153/2023/gchron-5-153-2023.pdf |
| Summary: | <p>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.</p> |
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| ISSN: | 2628-3697 2628-3719 |