Characterization of Dolomitizing Fluids in the Carboniferous of the Cantabrian Zone (NW Spain): A Fluid-Inclusion Study with Cryo-Raman Spectroscopy

Abstract

A late diagenetic dolomitization pervasively affected Carboniferous carbonates of the Variscan Cantabrian Zone (NW Spain). The process generated replacive and void-filling dolomite phases, spatially related with various calcite cements. The nature of the diagenetic fluids has been investigated by cryo-Raman spectroscopy, i.e., a combination of Raman spectroscopy and low-temperature microthermometry, which reveals in great accuracy the salinity and the major types of dissolved cations and anions in single fluid inclusions. Fluid properties obtained only from microthermometry are distinctively different. The study demonstrates that this improved analytical method of fluid inclusions is a valuable contribution for the interpretation of fluids in dolomite research. In primary fluid inclusions of the Cantabrian dolomites, hydrohalite and two unknown salt hydrates, one of which resembles MgCl 2 ?12H 2 O, were detected by cryo-Raman spectroscopy, whereas only hydrohalite appeared in the calcite primary inclusions. The presence of CaCl 2 hydrates is suspected only from low eutectic temperatures. Dolomite formed from an evolving fluid, as reflected by the highly variable equivalent Na/Ca ratios calculated, at high and nearly constant total salinities. The approximately constant Na/Ca ratio in the first calcite cement reflects a homogeneous source for the salts. Cryo-Raman spectroscopy reveals that different cooling procedures may induce the formation of different phase assemblages within the same fluid inclusion in both dolomite and calcite. Consequently, fluid inclusions display different melting behaviors, corresponding to different values of calculated salinities. Salinity calculation only from microthermometry may lead to an underestimation of true salinities. According to an assumed hydrostatic geothermal gradient, maximum formation conditions for the dolomites are 150 6 30uC and 40 6 10 MPa, corresponding to a depth of 3.9 6 1.0 km. The first calcite formed at 130 6 20uC and 35 6 5 MPa, corresponding to a depth of 3.4 6 0.6 km. The minimum T-P of formation is defined by the homogenization conditions. CHARACTERIZATION OF DOLOMITIZING FLUIDS 1307 J S R

Late Dolomitization in Basinal Limestones of the Southern Apennines Fold and Thrust Belt (Italy)

“…The field, petrographic and geochemical characteristics of the investigated dolostone bodies are adequate to describe them as non-planar, saddle, burial, epigenetic or high temperature dolomites (Hewett, 1928;Beales and Jackson, 1966;Radke and Mathis, 1980;Anderson and Macqueen, 1982;Boni et al, 2000b;Gasparrini et al, 2006a). All these terms found in literature commonly refer to dolostone bodies consisting of grey to black replacive, coarse dolomite containing variously shaped cavities lined by void-filling, white sparry dolomite.…”

“…These mechanisms may occur at different stages in the diagenetic evolution of carbonate rocks, ranging from processes acting soon after deposition to late-stage, epigenetic processes operating well after lithification and exhumation (Warren, 2000;Machel, 2004). Among the late-stage models of dolomitization, replacement by warm to hot solutions in the frontal parts of orogenic belts has been increasingly recognized in recent years (Qing and Mountjoy, 1994;Boni et al, 2000a;Swennen et al, 2003;Gasparrini et al, 2006a). Originally, this model was introduced to explain the peculiar vuggy dolomites, often displaying zebra textures and saddle-shaped crystals, associated with many MVT-type mineralizations of North America (Jackson and Beales, 1967;Anderson and Macqueen, 1982;Machel et al, 1996).…”

Dissolution profile of dolomite in chloric acid solution: The effect of chloric acid concentration and pulp density

“…The calculation molar ratio of both magnesium and calcium carbonates indicates that there are free calcium carbonate mineral beside dolomite mineral. In nature, dolomite is formed through the dolomization process, in which sea water containing magnesium salt invades hot rock that arisen from the melting some part of earth crust [13,14]. Obviously, in dolomite from Gresik, there are some parts of calcium carbonate that were not replaced by magnesium, which still remains as calcium carbonate.…”

Geochemical characterization of an organic-rich carbonate from the Grosmont Formation, Alberta, Canada

“…4). Measured Mg/Ca ratios in dolomite can elucidate dolomitization processes (Fuchtbauer, 1972;Sass and Katz, 1982;Morrow, 1990;Usdowski, 1994;Gasparrini et al, 2006;Kaczmarek and Sibley, 2011). Kaczmarek and Sibley (2011) suggest that stoichiometric dolomite forms from high Mg/Ca solutions.…”