The geochemistry of the south Portuguese zone, Spain and Portugal
Mass balance calculations illustrate that the incorporation of silicate within sulphide analysis has a variable impact on the REE profiles, depending upon the degree of contamination and the REE characteristics of the silicate phases. Manganiferous Chert Deposits Manganiferous cherts are dominantly biogenic, produced from radiolarian tests, with only minor participation of hydrothermal and hydrogenous sources. Reduced detrital input is necessary for a thriving radiolarian population, and as the depositional basins were compartmentalised and separated by small ridges, this suggests that the surface area of sediment source areas was diminished. Absence of radiolarian tests is attributed to recrystallisation, as tests initially comprise amorphous silica. Chert REE and Nd-isotope geochemistry is variable and this is attributed to the presence or absence of detrital and/or volcanic material, which overprints the initial hydrothermal characteristics. This variation in signature is dominantly controlled by the spatial location of the chert, within the depositional basin., Genetically, both the massive sulphides and chert deposits are independent, the cherts being biogenic and sulphides being hydrothermal. Additionally, both may be precipitated concurrently. It has been previously suggested that sulphides were preserved and oxidation hindered, by the presence of siliceous gel cap. This is not necessary if bottom-hugging brines were present. This factor explains why sulphides are preserved in the absence of a capping horizon.