Geochemistry of tourmalines associated with iron formation and quartz veins of the Morro da Pedra Preta Formation, Serra do Itaberaba Group (São Paulo, Brazil)

Gianna M Garda,Dailto Silva,Paulo Beljavskis,Caetano Juliani

doi:10.1590/s0001-37652003000200008

Abstract

Tourmalines of intermediate schorl-dravite composition occur in iron formation (including metachert and tourmalinites), metasediments, calc-silicate and metabasic/intermediate rocks of the Morro da Pedra Preta Formation, a volcanic-sedimentary sequence of the Serra do Itaberaba Group (northeast of São Paulo City, southeastern Brazil). The Morro da Pedra Preta Formation is crosscut by quartz veins that contain both intermediate schorl-dravite and an alkali-deficient, Cr-(V-)bearing tourmaline, in which the occupancy of the X-site is ϑ0.51Ca0.33Na0.15, characterizing it as intermediate to foitite and magnesiofoitite end-members. Mg# values for this tourmaline are higher than those for intermediate schorl-dravite. Raman spectroscopy also confirms the presence of two groups of tourmalines. Stable isotope data indicate sediment waters as fluid sources, rather than fluids from magmatic/post-magmatic sources. Delta18O compositions for tourmalines, host metachert, and quartz veins are similar, showing that fluid equilibration occurred during crystallization of both quartz and tourmaline. Syngenetic, intermediate schorl-dravite tourmalines were formed under submarine, sedimentary-exhalative conditions; amphibolite-grade metamorphism did not strongly affect their compositions. Younger tourmalines of compositions intermediate to foitite and magnesiofoitite reflect the composition of the host rocks of quartz veins, due to fluid percolation along faults and fractures that caused leaching of Cr (and V) and the crystallization of these alkali-deficient, Cr-(V-)bearing tourmalines.

Highlights

1997”, held in the Czech Republic in June 1997
Tourmaline is a common mineral in the Serra do Itaberaba Group (State of São Paulo, southeastern Brazil), in the basal volcanic-sedimentary sequence named Morro da Pedra Preta Formation (Juliani 1993), which was explored for gold in the 16th and 17th centuries
A wide range of tourmaline compositions is identified in the Morro da Pedra Preta Formation, from intermediate schorl-dravite with varying Al contents, intermediate schorldravite with varying Mg and Fe contents, and Cr(-V)-bearing foitite-magnesiofoitite, which is characterized by high Mg contents and X-site deficiency (Quartzito quartz veins)

Summary

Introduction

1997”, held in the Czech Republic in June 1997. Frank C. Tourmaline is a common mineral in the Serra do Itaberaba Group (State of São Paulo, southeastern Brazil), in the basal volcanic-sedimentary sequence named Morro da Pedra Preta Formation (Juliani 1993), which was explored for gold in the 16th and 17th centuries. Scanning electron microscopy revealed gold disseminated in tourmalinites and associated rocks in the Tapera Grande prospect, and alkali-deficient tourmalines associated with mineralized and barren quartz veins in the Quartzito prospect, both areas found in the Morro da Pedra Preta Formation. The association of distinct tourmaline compositions with distinct types of mineralization in the Morro da Pedra Preta Formation led the authors to further investigate the subject, once rock-forming minerals such as amphibole, garnet, staurolite, and sillimanite (among others) proved to be useful for petrogenetic studies of the Serra do Itaberaba Group, but not for mineralization

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