In situ thermochemical sulfate reduction during ore formation at the Itxaspe Zn-(Pb) MVT occurrence (Basque-Cantabrian basin, Northern Spain)

Organic matter is thought to play a role in the genesis of many Mississippi Valley-type (MVT) deposits, acting as a reducing agent during thermochemical sulfate reduction (TSR). Although TSR is an extremely slow reaction t low temperatures (<100ºC), under favorable conditions it may supply the necessary reduced sulfur during ore formation. To test this hypothesis, the Itxaspe Zn-(Pb) MVT occurrence in the Basque-Cantabrian basin (Northern Spain) was studied. Sphalerite, the main ore phase, is generally found disseminated in Urgonian (Lower Cretaceous) carbonates, and in close relationship with solid bitumen. The bitumen source rock was very likely deposited in a marine marginal setting. Differences in composition of the bitumen samples are attributed to a fractionation during hydrocarbon expulsion and/or migration. The fluids involved in ore deposition were low temperature (Th ~130ºC), Na-Ca-Cl-(K-Mg)-type brines (salinities ~12.5 equiv. mass % NaCl). The origin of brine solutes (including sulfate) is related to the dissolution of Mesozoic evaporite units, although the contribution of evaporated seawater brines cannot be ruled out. The temperatures of ore deposition, the close relationship between the bitumen and ore phases, the presence of aromatic sulfur-bearing compounds and the d34S of sulfides and sulfates are consistent with an in situ TSR during ore formation in the Itxaspe Zn-(Pb) occurrence. Therefore, at least for small mineralizations like Itxaspe, our conclusion is that the necessary reduced sulfur can be supplied by TSR during ore genesis at the site of metal deposition

In situ thermochemical sulfate reduction during ore formation at the Itxaspe Zn-(Pb) MVT occurrence (Basque-Cantabrian basin, Northern Spain)

Organic matter is thought to play a role in the genesis of many Mississippi Valley-type (MVT) deposits, actino as a reducing agent during thermochemical sulfate reduction (TSR). Although TSR is an extremely slow reaction t low temperatures (lt;100ºC), under favorable conditions it may supply the necessary reduced sulfur during ore formation. To test this hypothesis, the Itxaspe Zn-(Pb) MVT occurrence in the Basque-Cantabrian basin (Northern Spain) was studied. Sphalerite, the main ore phase, is generally found disseminated in Urgonian (Lower Cretaceous) carbonates, and in close relationship with solid bitumen. The bitumen source rock was very likely deposited in a marine marginal setting. Differences in composition of the bitumen samples are attributed to a fractionation during hydrocarbon expulsion and/or migration. The fluids involved in ore deposition were low temperature (Th ~130ºC), Na-Ca-Cl-(K-Mg)-type brines (salinities ~12.5 equiv. mass % NaCl). The origin of brine solutes (including sulfate) is related to the dissolution of Mesozoic evaporite units, although the contribution of evaporated seawater brines cannot be ruled out. The temperatures of ore deposition, the close relationship between the bitumen and ore phases, the presence of aromatic sulfur-bearing compounds and the d34S of sulfides and sulfates are consistent with an in situ TSR during ore formation in the Itxaspe Zn-(Pb) occurrence. Therefore, at least for small mineralizations like Itxaspe, our conclusion is that the necessary reduced sulfur can be supplied by TSR during ore genesis at the site of metal deposition

Dégradation et lessivage des hydrocarbures de la formation ordovicienne de Thumb Mountain encaissant le gîte Zn–Pb de Polaris (Territoires du Nord-Ouest, Canada)

Carbonate rocks of the Ordovician Thumb Mountain Formation, host to the Zn–Pb Polaris deposit, contain hydrocarbons that show types of alteration classically attributed to biodegradation and water washing. The hydrocarbons of the upper part of this formation and of the overlying Irene Bay Formation indicate alterations due to water washing only. The hydrocarbons of the impermeable shales of the overlying Cape Phillips Formation display indices of only higher thermal maturity than the underlying units. Contrary to classical concepts of hydrocarbon biodegradation, n-alkanes, cyclohexylalkanes, even isoprenoids and perhaps also steranes seem to have been degraded simultaneously and not successively in this sequence. This alteration process is mainly responsible for a log-linear decrease of the amounts of n-alkanes and cyclohexylalkanes with increasing depth. The severe and uniform alteration of aromatic hydrocarbons throughout the interval, which is opposite to the progressive alteration of associated n-alkanes, can be attributed solely to water washing. This conclusion necessitates a reconsideration of previous interpretations attributing the loss of short-side chain substituted polyaromatic compounds to microbial activity. Hopanes, tri-and tetra–cyclic terpanes as well as aromatic steroids and hopanoids seem to have been unaffected by the alteration phenomena. The increase in the degree of alteration of the hydrocarbons with increasing depth implies that the responsible migrating fluid circulated per ascensum. </jats:p

In situ thermochemical sulfate reduction during ore formation at the Itxaspe Zn-(Pb) MVT occurrence (Basque-Cantabrian basin, Northern Spain)

Organic matter is thought to play a role in the genesis of many Mississippi Valley-type (MVT) deposits, acting as a reducing agent during thermochemical sulfate reduction (TSR). Although TSR is an extremely slow reaction t low temperatures (<100ºC), under favorable conditions it may supply the necessary reduced sulfur during ore formation. To test this hypothesis, the Itxaspe Zn-(Pb) MVT occurrence in the Basque-Cantabrian basin (Northern Spain) was studied. Sphalerite, the main ore phase, is generally found disseminated in Urgonian (Lower Cretaceous) carbonates, and in close relationship with solid bitumen. The bitumen source rock was very likely deposited in a marine marginal setting. Differences in composition of the bitumen samples are attributed to a fractionation during hydrocarbon expulsion and/or migration. The fluids involved in ore deposition were low temperature (Th ~130ºC), Na-Ca-Cl-(K-Mg)-type brines (salinities ~12.5 equiv. mass % NaCl). The origin of brine solutes (including sulfate) is related to the dissolution of Mesozoic evaporite units, although the contribution of evaporated seawater brines cannot be ruled out. The temperatures of ore deposition, the close relationship between the bitumen and ore phases, the presence of aromatic sulfur-bearing compounds and the d34S of sulfides and sulfates are consistent with an in situ TSR during ore formation in the Itxaspe Zn-(Pb) occurrence. Therefore, at least for small mineralizations like Itxaspe, our conclusion is that the necessary reduced sulfur can be supplied by TSR during ore genesis at the site of metal deposition