MINERALOGY AND PARENTAL BRINE EVOLUTION IN THE PEDRO DE VALDIVIA NITRATE DEPOSIT, ANTOFAGASTA, CHILE

The Chilean nitrate deposits have been formed by a complex paragenesis of saline minerals that infill the porosity and open spaces of differ ent country rocks. The study of the mineral infilling sequences in the rock porosities at the Pedro de Valdivia deposit has enabled to gain information about the evolution of the parental brines of the saline ( nitrate bearing) paragenesis. This evolution is revealed by the precip itation of a sequence of progressively more soluble minerals: silicate s (zeolites); calcite; Ca-, Na-, (K-) and Mg-sulphates; Na- and KMg-ni trate-sulphates; nitratine; and small amounts of iodates, Na- and KMg- iodate-sulphates, chromates, berates and perchlorates. Two specific tr ends have been distinguished in the general sequence. A Na- trend char acterised by the association glauberite-darapskite-(hectorfloresite), and a KMg-Na- trend related to alite-humberstonite-niter-(fuenzalidait e).Parental brines of the deposit were related to the phreatic system from which saline associations precipitate. Regional data drive the au thors to consider as Oligocene-Miocene (but older that 6 Ma) the age o f the mineralisation. The main physiographic features were similar to the recent ones in this period, the climate being more humid. The gene sis of the parental brines of the Chilean nitrate deposits is assumed to include thermal processes and recycling by leaching of previous sal ts formed in the arid environment of the Atacama Desert. Evaporation h as also been involved in their genesis. It is the on ly mechanism effe ctive enough to saturate the brines in very soluble minerals. Neverthe less, the nitrate ores cannot be considered as true evaporites owing t o their arrangement infilling veins land porosities) and the almost co mplete absence of displacive growths las nodules or enterolithes) and other typical structures of evaporites.