Mississippi Valley-Type, Sedex, and Iron Deposits in Lower Cretaceous Rocks of the Basque-Cantabrian Basin, Northern Spain
F. Velasco, J.M. Herrero, P.P. Gil, L. Alvarez and I. Yusta
11 Hydrothermal Fluids and Genetic Model: Discussion The ore deposits of the Basque-Cantabrian zone are located near the margins of the platforms, supporting the idea of a strong tectonic control. These fractures have probably favoured the circulation of solutions, acting as a feeder of ascending hydrothermal fluids. The particular hydrology of each locality as weil as the, in part, structurally controlled sedimentological and palaeogeographic features determine the development of different morphological types of ore deposits (replacement, open-space filling, disseminated, bedded, massive). The frequent spatial association of ore deposits of different morphologies in a single mineralized assemblage is consistent with this hypothesis. The nature of carbonate sediment and the porosity determine the ability to accept replaceme nt mineralizations.
On the basis of texturaI and mineralogical studies a multi-stage diagenetic origin can be envisaged, responsible for the replacement phenomena and the massive precipitation of sulphides, especially during the latest stages. Dissolution of carbonate host rocks, indicating an acidic medium, is shown by different replacement textures induding pseudomorphs. Successive removals and re-equilibrium implied important volume changes, accompanied by a strong increase in porosity. In these environments, diagenetic crystallization rhythmites and collapse breccias develop, cemented with white sparry carbonates. In pi aces , this mineralizing environment was probably located at the seafloor surface, where the conditions were very dose to those typical of exhalative sedimentary phenomena, as is the case in the southern part of the Troya mine.
Wall rocks next to the orebodies locally exhibit minor responses to the action of ore fluids. Along synsedimentary faults the rocks show an increase in grade of alteration assemblages, induding dolomite, iron carbonates and/or silicification. These hydrothermal alterations do not extend more than a few metres away from the ore contacts. Data of carbon and oxygen isotopes seem to corroborate that an important part of the hydrothermal alteration, associated with the mineralizing processes, took place in environments that evolved towards increasing temperatures, as the burial diagenesis advanced.
Fluid indusion data, morphology, mineralogy, textures of the minerals and the geochemical data are arguments in favour of a hydro thermal origin with circulation at a moderate temperature (120-230°C) and with medium-high salinity arising from sediment dewatering during burial stages. The complexity of the hydrothermal systems suggests mixing of two chemically distinct brines: a surficial meteoric water and a deeper and saturated basinal brine. Deeper saline hot basinal brines, able to leach and transport the met als from the underlying basal units, circulated in zones where 250°C was locally exceeded. These temperatures are consistent with anchizone paragenesis in the central part of the basin (Arostegui et al. 1991). The other fluids had a moderate temperature, were less saline and oxidizing, and had a possible marine meteoric origin (meteoric-phreatic pore solutions).
The geothermometric measures, which indicate that 200°C was exceeded in the basin during mineralization, correspond to depths of 4000m. If these calculations are correct and if we assign anormal residence time to the sediments and do not generalize the existence of anomalous geothermal environments, the approximate age for the mineralizing event would correspond to the last upper Aptian (Clansayesian) to lower Albian, contemporary with the most intense pulsation of the rifting and most anomalous geothermal environment.
Lead isotope analyses indicate that the metals were leached from underlying rocks (basement and detrital rocks derived from it as the VI and V 2 Wealden units). This is mainly supported by the relative homogeneity of the lead isotope ratios and the dearly more radiogenic values than Palaeozoic Sedex deposits described by Pesquera et al. (1985) and Pesquera and Velasco (1989).
Metal base sulphide and iron mineralizations in the Basque-Cantabrian basin are in most cases epigenetic, developed after deposition of the host carbonates, but some syngenetic ores are deposited at the seafloor surface (e.g. Troya). Even if they show a great diversity in morphologies and mineral parageneses, both types of ores formed as a result of the same type of discharge of ore solutions during Lower Cretaceous. Their diversities reflect the local geological setting and the variations in chemical composition and source of the ore solutions.
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