Resumo

DI PASQUO, Mercedes  e  CASTRO, Liliana. Niveles fosfáticos en los depósitos del Cámbrico superior en el área de San Isidro, Precordillera de la provincia de Mendoza, Argentina. Rev. Asoc. Argent. Sedimentol. [online]. 1994, vol.1, n.2, pp.91-102. ISSN 1853-6360.

Upper Cambrian phosphatic layers intercalated in an about 20 meters thick sequence, are located in the right margin of San Isidro Creek (San Isidro area), 20 Km west of Mendoza City (Fig. 1). This paper represents the first description of an Argentinian phosphogenic event of Upper Cambrian age, which correlates with one of the World's major Phanerozoic phosphogenic episodes. Leanza et al. (1986) documented Argentine phosphorite episodicity and palaeolatitudinal distributions that coincide with most global phosphogenic events. However, although they recognized the Cambrian as a major phosphogenic period, no accumulation of phosphorite were identified in the study area. The first purpose was to establish several favorable areas for prospecting phosphate based on the analyses of lithological facies and palaeoenviroments and the use of palaeomagnetical data. One of these areas was the Upper Cambrian Precordillera of Mendoza Province, which was located approximately around 20° paleolatitudinal position (Valencio et al., 1980). The second objective was to propose a genesis for the prospected phosphates and the relationship with other known deposits in the world. Finally, we try to contribute with new data for palaeogeographic model reconstructions. The results of microfacies analyses undertaken by di Pasquo (1993) are consistent with Bordonaro (1985) who had defined the depositional environment of this deposits as a "condensed section type" of continental slope facies. According to di Pasquo (l993), this unit consists of dark brown laminated and rhythmic claystones (LCI), mudstones (LC2) containing pyrite and highly carbonized organic matter and subordinated wackstones (LC3). The layers are composed mainly of clay minerals (illite-smectite), calcite and quartz with subordinate phosphate minerals. The latter are concentrated chiefly in the phosphorite layer (LC2), but also occur in small amounts in the intercalated limestones (Fig. 2). Phosphates occur as coated grains, phosphatized grains and peloids of different sizes, from several mm to a fraction of mm in diameter. Bioclasts (LC2) consist of well preserved trilobite tests which show an incipient phosphatization and phosphatic brachiopoda tests (Lingula sp?). The green claystone facies (LC4) is composed mainly of illite and quartz, and contains no carbonate. Silicaspheres (replaced radiolarians'?), trilobite fragments, and conodonts are the principal bioclastic particles. This layers is intercalated between mudstones. The phosphate is composed of isotropic carbonate fluorapatite associated with anisotropic francolite. The latter seems to occur as a replacement of bioclasts and carbonate debris. The results of the chemical analyses of the phosphatic levels suggest that there are essentially two groups of major oxides: those which are located predominantly within the apatite lattice (CaO, Na2O, P2O5 and CO2), and those located in other minerals, mainly of chemical (CaO, CO2) or detrital origin (SiO2, TiO2, Al2O3, Fe2O3, MnO, MgO, K2O). Clay minerals chiefly, and occasionally feldspars host the bulk of K2O, SiO2 and Al2O3, whereas the main source of CaO and CO2 is the micrite-sparite present in the recrystallized matrix which probably precipitated prior to the phosphatization of the layer. SEM analyses indicate that the phosphatic particles in the bulk samples are composed of Si, Ti, Al, Ca, K, P, S and Cl. The general results for the deposition of this marine sequence are summarized as follows: (l) Organic-rich laminated mudstones and claystones of this deposit together with typical cosmopolitan fauna suggest a low energy and low oxygen conditions that accumulated mainly on a gentle low angle slope (< 2°) and outer shelf. Siliceous claystones with planktonic fauna that indicate a deeper environment, suggest at least moderate fluctuations of the sea level during the Late Cambrian (di Pasquo, 1993; Fig. 2). (2) The black shale-chert-phosphorite assemblage observed, is generally taken to indicate a marine upwelling environment (Cook & Shergold, 1986). (3) Petrographic evidences indicate that the phosphatization of the mudstone would have occurred during the early diagenesis by replacement of carbonate ion by phosphate ion in the sediment layer from Cambrian sea-waters enriched in phosphorus, as proposed by Cook et al. (1990). This replacement is associated with a low rate of marine clastic sedimentation, which allows chemical or biochemical sedimentation to become dominant during some intervals. (4) The abundance of phosphorites around the Proterozoic-Phanerozoic transition, and the implication that this is the result of a major phosphogenic episode (or episodes), is consistent with a major change in the phosphate content of the ocean at this time. The circulation of deep phosphorus-rich ocean waters into the shallow photic zone and a sea-level rise at this same time may also have been a contributing factor. The phosphatization of some levels of the mudstones facies (LC2) have not occur at the beginning of a transgression, but at a time when the sea depth in the shelf zone reaches a sufficient depth, in comparison with Eganov's results (Eganov, fide Cook & Shergold, 1979). (5) According to Cook (personal communication), the Precordilleran occurrences appear to be similar to northern Australian deposits where fossil fragments and the presence of siliceous intervals and chert nodules in the underlying levels are common features. On the other hand, the symmetric distribution of depositional facies, the existence of common fauna during Cambrian together with paleogeographical reconstruction based on paleomagnetic data are features that give the idea that both Appalachian and Precordilleran basin were close during the eoPalaeozoic times (Sellés Martínez, 1988; Dalla Salda et al., 1992a, l992b, 1993; Dalziel, 1991, 1993; Ramos et al., 1993, and Benedetto, 1993). With this in mind, we would suggest that the phosphorite occurrences in the Precordillera must have been linked with those in the Appalachian (Mathieu, W., 1893 y Mathieu, G., 1903, fide Christie & Sheldon, 1986), although the information on the latter deposits is sparse. This would be other element to take into account in the relation between Gondwana and Laurentia. (6) Middle and Upper Cambrian phosphate deposits are relatively minor compared to those in the lower Cambrian (Cook, 1992). The occurrences studied to date are uneconomic, but they may serve as lithologic guide for major deposits of Cambrian age in Argentina.

Palavras-chave : Phosphatic levels; Cambrian; Precordillera; Mendoza.

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