In situ geochemical characterization of pyrite crystals in burial dolomites of St. George Group carbonates

Abstract

SIMS was used to measure the δ34S of pyrite disseminated in burial dolomite matrix of St. George Group at three localities in western Newfoundland. At Main Brook, δ34S values for the pyrite grains show wide variation (-15 to +20 ‰ (n = 20), but are mostly negative. Combined with a paucity of two-phase fluid inclusions in the host burial dolomite, the depleted δ34S values suggest that the pyrite is a by-product of BSR. Predominantly negative δ34S values was also obtained for sampled pyrite at Daniel’s Harbour and Port au Choix, however relatively high homogenization temperatures (>100oC) of two-phase fluid inclusions in the host dolomites is incompatible with a BSR process for pyrite formation. More so, Daniel’s Harbour is a site for main stage sulfide mineralization (MVT system), hosted in similarly burial dolomite and that has been previously constrained to be associated with TSR. A relative proximity of the currently studied pyrite samples to this MVT system deposit is thus inconsistent with an in-situ BSR for these pyrites. The analyzed pyrite grains are encased in bitumen, post-date burial dolomite and pre-date late stage saddle dolomite. Thus, the depleted δ34S signature was likely inherited from migrated hydrocarbons in the reservoir. Incursion of an initial pulse of hot sulfate-rich brine into the formation can cause thermal cracking of hydrocarbons, thereby releasing its low δ34S. Subsequently, the main stage sulfide mineralization (Mississippi Valley Type deposit) occurred via in-situ TSR. These findings have implication for the paragenetic history of sulfide minerals precipitated during MVT-type mineralization.