GeoResources - Large-Scale Hydrothermal Dolomitisation in the SW-Cantabrian Zone (NW Spain): Processes and Origin of the Dolomitising Fluids


	Large-Scale Hydrothermal Dolomitisation in the Southwestern Cantabrian Zone (NW Spain): Causes and Controls of the Process and Origin of the Dolomitising ﬂuids
	Abstract





	Scientists Marta Gasparrini, formerly University of Heidelberg Thilo Bechstädt, GeoResources and University of Heidelberg Maria Boni, GeoResources and University of Heidelberg Ronald Bakker, University of Leoben

	Abstract Large-scale, massive late diagenetic burial dolomitization affected Paleozoic carbonates of the southwestern Variscan Cantabrian Zone (Spain). These dolomites are most prominent and laterally and vertically extensive in Upper Carboniferous carbonates. The dolomites from the Cantabrian Zone are excellently exposed and represent an interesting outcrop analogue for hydrocarbon reservoirs elsewhere. In comparable geotectonic settings of the European Variscan Orogen large masses of similar dolomites could have formed if Mg-rich brines were available. Two dolomite phases occur, replacive and void-filling dolomites. These formed by a common fluid in evolution, which first replaced the precursor carbonates generating new porosity, and then precipitated dolomite cements in an almost isothermal and isochemical process. These two dolomite phases were followed by calcite cementation. The process of dolomitization was controlled by rock anisotropies and was due to the circulation of hypersaline and hydrothermal marine-derived brines, as suggested by the dominance of Na and Mg in the fluids and crush-leach data, showing sea-water specific relations of indicative ions. Field evidence and dating of mineral phases filling porosity in the dolomites indicate that the dolomitization process post-dated the compressive phase of the Variscan deformation and occurred in Early Permian time during post-thrusting orocline formation in an extensional setting. Lithospheric delamination-induced extensional tectonics and crustal thinning provided the heat required to trigger fluid convection and dolomite precipitation. Fluid circulation was facilitated by the reactivation of highly permeable Variscan fault zones. Early Permian seawater evaporated to a high degree and marine-derived formation waters stored within the Palaeozoic succession are proposed to be the fluids responsible for dolomitization. The nature of the diagenetic fluids has been investigated by cryo-Raman spectroscopy, 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. This improved analytical method 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 MgCl2.12H2O, were detected by cryo-Raman spectroscopy, whereas only hydrohalite appeared in the calcite primary inclusions. The presence of CaCl2 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 ±30˚C and 40±10 MPa, corresponding to a depth of 3.9±1.0 km. The first calcite formed at 130±20˚C and 35±5 MPa, corresponding to a depth of 3.4±0.6 km. The minimum T–P of formation is defined by the homogenization conditions.

	Publications Gasparrini, M., Bechstädt, T. & Boni, M. (2006): Massive hydrothermal dolomite in the southwestern Cantabrian Zone (Spain) and its relation to the late Variscan evolution.- Marine and Petroleum Geology, 23, 543-568. Gasparrini, M., Bakker, R.J. & Bechstädt T. (2006): Characterisation of dolomitising fluids in the Carboniferous of the Cantabrian Zone (NW Spain): a fluid inclusion study with cryo-Raman spectroscopy.- J. Sediment. Res., 76, 1304-1322.

	Funding Organizations German Research Fund (DFG) German Academic Exchange Fund (DAAD)

GeoResources - Large-Scale Hydrothermal Dolomitisation in the SW-Cantabrian Zone (NW Spain): Processes and Origin of the Dolomitising Fluids

Large-Scale Hydrothermal Dolomitisation in the Southwestern Cantabrian Zone (NW Spain): Causes and Controls of the Process and Origin of the Dolomitising ﬂuids