Geochemical and Age Constraints on Devonian Black Shales and Limestones in Myanmar: Insights into Gold Source Rocks

Charles Makoundi; Jordan Sheppard; Khin Zaw; Tin Aung Myint; Naing Win

doi:10.63335/j.hp.2026.0049

Abstract

Pyrite trace element chemistry, lithogeochemistry, and U–Pb dating of detrital zircon were carried out on the Devonian black shales and limestones, Northern Shan State, Myanmar. The findings indicate that the Upper to Middle Devonian Padaukpin Formation is enriched in CaO and show a similar content in SiO₂, TiO₂, Al₂O₃, Fe₂O₃, and K₂O compared to Post-Archean Australian Shale (PAAS). The Middle Devonian Wetwun Formation is depleted in SiO₂, TiO₂, Al₂O₃, Fe₂O₃, and K₂O but enriched in CaO relative to PAAS. The Lower Devonian Zebingyi Formation is enriched in CaO and strongly depleted in SiO₂, TiO₂, Al₂O₃, Fe₂O₃, and K₂O compared to PAAS. Total organic carbon (TOC) remains high in the Lower Devonian between 5% and 9.7% (mean = 8%) but drops in the Middle Devonian to value of 2.5% to 3.2% (mean = 2.9%), only to rise again in the Middle to Upper Devonian to 9.7%. Using trace element (TE) and TOC%, the contents of trace elements Mo, As, Zn, Cu, V, and Ni are inversely proportional to the TOC content in the Upper and Middle Devonian. Pyrite trace element chemistry shows that the beginning of the Middle Devonian, in the Lower Wetwun black shale, is marked by elevated levels of the gold (up to 1.1 ppm Au), which corresponding to a peak in oxygen content in the atmosphere (up to 28.7 wt% O₂) in oxic to anoxic redox conditions. The evidence suggests that the Wetwun shales are likely gold source rocks. The research outcome indicates a limited dataset of age dating pointing to three maximum depositional ages of 399, 480, and 537 Ma. There are a few zircon age peaks ranging from over 400 Ma up to 2500 Ma, suggesting a discernible connection with Gondwana; however, further work is required to confirm this relationship.

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