The present investigation consisted of a geochemical study of the medium and upper layars of the Pedra do Fogo Formation aiming at correlating its elemental distribution pattern with the sedimentary environment where that formation was deposited. Pelitic material with carbonate content below 30% was sampled in the three different outcrops of the Pedra do Fogo Formation for mineralogical and chemical analyses. Illite and dolomite, with subordinate amounts of smectite, calcite, quartz and K-feldspar were determined by X-ray diffraction techniques as the constituents of the mineral assemblages. Trace element (B, Ba, Co, Cr, Cu, Ga, Li, Mn, Ni, P, V, Pb, Sr, Rb and Zn), CO2 and K concentrations were determined by emission spectrography, atomic absorption spectrometry and colorimetry. B, Ga, K, Rb and V were found to be preferentially fixed in illite. Co, Cr, Cu, Na and Pb show only a partial preference for this mineral, but it is likely that the presence of iron oxides also controls their distribution. Mn is dominantly fixed in the carbonate phases, while Li, O, Sr and Zn have no special tendency for fixation in either minerals. However, Li and Zn distribution might be partly controlled by the smectite content of the samples. Likewise P and Sr distributions seem to be heavily conditioned by the salinity variations that occurred during the deposition of those sedimentary layers. Most analyse'd elements have concentrations below the respecti.ve world average found for shales. Botb the presence of carbonate and specially the low salinity of the environment probably account for that difference. The interpretation of the chemical data based on B-V, Ga-B, B-K2O and B-Ga-Rb diagrams and on B content indicates a wite range of variation for the medium salinity during the deposition of the Pedra do Fogo rocles. Accordingly, the observed sequence is composed of intercalations of marine and fresh water sediments, the latter being dominant. The marine intercalations are more frequent in the bottom of the sampled stratigraphic section and become progressivily rarer towards the top layers which were deposited in a typically fresh water environment. The different parpmeters used in the chemical interpretation of the depositional environment of the Pedra do Fogo Formation are internally consistent and agree very well with the geological and paleontological information that has been gathered by other authors.
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