192-6 Origin of Salt Diapir Brimstone: Dancing on the Edge of Occam’s Razor

Session: Twenty-Seven Years of Advances in Understanding Salt-Sediment Interaction: A Legacy of Katherine A. Giles (Posters)

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Abstract:

Sulfur as an element is common in salt diapirs because sulfate minerals, i.e., gypsum (CaSO₄·2H₂O) and anhydrite (CaSO₄), are usually precipitated during the formation of evaporite deposits. Salt diapirism can lead to the concentration of these lithologies in the crest of the diapirs, whereby water undersaturated with respect to halite removes this major component of the salt body, leaving a residual caprock. Due to the properties of salt bodies acting as seals and the upturning of minibasin strata toward the diapir, migration of hydrocarbons into such caprocks have often been observed – the famous example being the Spindletop gusher that triggered the Texas oil boom in 1901.

Such reservoirs are often associated with considerable accumulations of native (elemental, zero-valent) sulfur – referred to as Brimstone in alchemy, often used as an allusion to hell. Indeed, the devil is in the details when it comes to the process that leads to the formation of these massive native sulfur accumulations found at some salt diapirs. Two opposing explanations have been put forward: i) native sulfur production by a two-step process that combines reduction of sulfate-sulfur to sulfide and subsequent oxidation of the sulfide to zero-valent sulfur, and ii) direct reduction of native sulfur to zero-valent sulfur without an oxidative step. The former is the commonly accepted interpretation, mainly because the latter process remains elusive, i.e. the biochemical mechanisms only exist in theory but have not been demonstrated experimentally. As such, the decision which interpretation of native sulfur generation at salt diapirs to accept hinges on the law of parsimony (Occam’s Razor) instead of scientific facts.

Two sulfide minerals, hauerite (MnS₂) and pyrrhotite (Fe_(1-x)S, with x = 0 to 0.125) that are found in association with the native sulfur deposits may shed new light on this issue. What is remarkable about these minerals is that they form under tight thermodynamic constraints – namely the competition for manganese between sulfide (MnS₂) and carbonate (e.g., rhodochrosite, MnCO₃) minerals and the competition for sulfide between pyrite (FeS₂) and pyrrhotite. We present how the presence of these minerals informs the understanding of the sulfur redox-chemistry in a setting where hydrocarbons and methane react with sulfate minerals.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-9713

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