The occurrence of intermediate schorl-dravite and alkali-deficient, Cr-(V)bearing tourmalines in the volcanic-sedimentary sequence of the Serra do Itaberaba Group - SP

THE OCCURRENCE OF INTERMEDIATE SCHORL-DRAVITE AND ALKALI-DEFICIENT, Cr-(V)BEARING TOURMALINES IN THE VOLCANIC-SEDIMENTARY SEQUENCE OF THE SERRA DO ITABERABA GROUP - SP

GIANNA M. GARDA ¹, PAULO BELJAVSKIS ¹, CAETANO JULIANI ¹ AND DAILTO SILVA ²

¹Instituto de Geociências, Universidade de São Paulo, São Paulo.

²Instituto de Geociências, Universidade Estadual de Campinas, Campinas, SP.

Presented by ANTONIO C. ROCHA-CAMPOS

In the Morro da Pedra Preta Formation, metamorphosed volcanic-sedimentary sequence of the Serra do Itaberaba Group (northeast of São Paulo City - SP), tourmalines occur in tourmalinite, metachert, iron formation, metasediments, calc-silicate and metabasic/intermediate rocks, and hosted by quartz veins. In Tapera Grande and Quartzite areas, intermediate schorl-dravite compositions predominate. Under the microscope, these tourmalines stand out for their color, zoning (alternating light and dark greenish blue tints from core to rim), and strong pleochroism (dark blue to light brown). In general, the rims are much darker than the cores, variations due to varying Fe and Mg contents.

A second generation of light green tourmalines also occurs in quartz veins of Quartzite. These are alkali-deficient, Cr- and V-bearing tourmalines with higher Mg# than those of the schorl-dravite series. The occupancy of the X-site, according to X-ray fluorescence data, is d Ca_0.33Na_0.15, thus corresponding to foitite, considered as an alkali-deficient schorl.

Raman studies also discriminated two groups of tourmalines, one belonging to the buerguerite-schorl series and the other to the dravite-buerguerite-uvite series.

Stable isotope data allowed to define sediment and hydrothermal waters as fluid sources, ruling out the association of the tourmalines with e.g. the Brasiliano granitoid bodies found in the area. d¹⁸O compositions for tourmalines (+12 per mil) and host metachert and quartz veins (+13 per mil) are very similar, showing fluid equilibration during (re)crystallization of quartz and tourmaline.

The presence of at least two distinct groups of tourmalines indicates distinct environments and timing for tourmaline generation. In Tapera Grande, tourmalines were formed in a submarine exhalative-sedimentary environment. Their composition was not strongly affected by medium-grade metamorphism.

In Quartzite, tourmaline compositions reflect that of the country rock, once fluid percolation along Sertãozinho fault and associated fractures caused leaching of Cr (and V) and the crystallization of alkali-deficient, Cr-(V-)bearing tourmalines in veins, together with quartz. The heat source for mineralizing fluids must have been a granitoid body (Pau Pedra) south of Tapera Grande and intermediate to acid pipes in the Quartzito area. These fluids were also responsible for distinct types of mineralization, characterized in Tapera Grande by the assemblage gold-pyrite-pyrrhotite-chalcopyrite, and in Quartzite by electrum-pyrite-chalcopyrite-sphalerite-galena-scheelite-molibdenite. — (December 14, 2001) .

Publication Dates

• Publication in this collection
  09 Oct 2002
• Date of issue
  Sept 2002