دانلود رایگان ترجمه مقاله محدودیت های زمین شناسی و ایزوتوپیک بر روی تکامل فلززایی ذخایر رسوبی – الزویر 1999

محدودیت های زمین شناسی و ایزوتوپی در تکامل فلززایی ذخایر روی-مس رسوبی برزیل

Geological and Isotopic Constraints on the Metallogenic Evolution of the Proterozoic SedimentHosted Pb-Zn (Ag) Deposits of Brazil

Abstract

Integrated studies of seven Proterozoic sediment-hosted, Pb-Zn-Ag sulfide deposits of Brazil, permit the estimation of the age of the hosting sequence and the mineralization, the nature of the sulfur and metal sources, the temperature range of sulfide formation and the environment of deposition. These deposits can be classified into three groups, according to their ages. (a) Archean to Paleoproterozoic: the Boquira deposit, in Bahia state, consists of stratiform massive and disseminated sulfides hosted by parametamorphic sequences of grunnerite-cummingtonite+magnetite that represent a silicate facies of the Boquira Formation (BF). Lead isotope data of galena samples indicate a time span between 2.7 and 2.5 Ga for ore formation, in agreement with the stratigraphic position of the BF. The relatively heavy sulfur isotope compositions for the disseminated and stratiform sulfides (+8.3 to +12.8 ‰ CDT)suggest a sedimentary source for the sulfur. (b) Paleo to Mesoproterozoic: stratiform and stratabound sulfides in association with growth faults are present in the Canoas mine (Ribeira, in Paraná state) and in the Caboclo mineralization (Bahia state). They are hosted by calcsilicates and amphibolites in the Canoas deposit, whereas in the Caboclo area the mineralization is associated with hydrothermally altered dolarenites at the base of the 1.2 Ga Caboclo Formation. The interpreted Pb-Pb age of the Canoas mineralization is coeval with the 1.7 Ga host rocks. Sulfur isotopic data for Canoas sulfides (+1.2 to +16 ‰ CDT) suggest a sea water source for the sulfur. The range between −21.1 and +8.8 ‰ CDT for the Caboclo sulfides could suggest the action of bacterial reduction of seawater sulfates, but this interpretation is not conclusive. (c) Neoproterozoic: stratiform and stratabound sulfide deposits formed during the complex diagenetic history of the host carbonate rocks from the Morro Agudo (Bambui Group), Irecê and Nova Redenção (Una Group), yield heavy sulfur isotope values (+18.9 to +39.4 ‰ CDT). The uniform heavy isotope composition of the barites from these deposits (+25.1 to +40.9 ‰) reflect their origin from Neoproterozoic seawater sulfates. The late-stage, and most important, metallic concentrations represent sulfur scavenged from pre-existing sulfides or from direct reduction of evaporitic sulfate minerals. Lead isotope data from the Bambui Group suggest focused fluid circulation from diverse Proterozoic sediment sources, that probably was responsible for metal transport to the site of sulfide precipitation. (d) Late Proterozoic to Early Paleozoic: lead-zinc sulfides (+pyrite and chalcopyrite) of Santa Maria deposits, in Rio Grande do Sul, form the matrix of arkosic sandstones and conglomerates, and are closely associated with regional faults forming graben structures. Intermediate volcanic rocks are intercalated with the basal siliciclastic members. Lead isotope age of the mineralization (0.59 Ga) is coeval with the host rocks. Sulfur isotopic values between −3.6 and +4.1 are compatible with a deep source for the sulfur. Geological, petrographic and isotopic data of the deposits studied suggest that they were formed during periods of extensional tectonics. Growth faults or reactivated basement structures probably were responsible for localized circulation of metal-bearing fluids within the sedimentary sequences. Sulfides were formed by the reduction of sedimentary sulfates in most cases. Linear structures are important controls for sulfide concentration in these Proterozoic basins.

Introduction

The Proterozoic Eon is characterized, on almost every continent, by its relatively higher abundance of base metal sulfide deposits. World-class lead-zinc and/or copper deposits are present in the Proterozoic sedimentary basins of most continental areas, such as: Broken Hill, McArthur and Mount Isa-Century districts, in Australia; Sullivan, in Canada; Gamsberg and Zambia-Congo copperbelt, in the African continent These deposits contain huge rcscrvcs, with total mctnl content exceeding 250 million tonncs, In contrast, such dcpos~ls hcive not been discovered in tlic South American coiilincnl until now In Brdl, representing almost half of the entire continentLil med, there are no large basc-mctal sulfide deposits despitc the wicleapread occurrciicc of Protero7oic seditnentary basins coveiing iiioie than 500,000 kin? of its territory. Except for the Morro AgudoVazante mines, in the state of Minas Gerais (2 to 3 million tonnes of Zn+Pb metal content), there are only small deposits and occurrences distributed in these Proterozoic basins. The main question at this point is: why are no worldclass Proterozoic sediment-hosted sulfide deposits present on the South American continent? The answer to this question may be the inadequate investment in mineral exploration by m1111ng compmics md/or lack of satkfactoi y nietallogeiitc models that co~il~l bc cmployed lui explordtion plmning. 117 any casr, there 15 A C~SC’I~~LI~ miuiig KeuluKi4s from the ncademlc ins11 lrilioiis L~~i~l iniiiiiig coiiiptuiies conccrnmg thc obsencr of adeqiintcx rntoi iiiiltioi~ tl~t curilcl bc usrd for constrticliiig consislcnl ~iict~illogcnic ~noclels In this paper we disc~ss llic pic-limin,iiy icsulls of oiii on-going mvcstigntton of seven An-I %-AX scdi md-lioslccl sulfide deposits in Urazil, covering a timc sp.iii fiorii approximately 2.5 Ga to 0.57 Ga. These deposits are: Boquira, Caboclo, Ire& and Nova Redensgo, in the state of Bahia; Morro Agudo, in Minas Gerais state; Canoas in Paranh and Santa Maria, in Rio Grande do Sul state (Fig. 1). In these deposits geological, petrographical and geochemical data, along with radiogenic and stable isotope signatures and fluid inclusions studies, are being employed to interpret their geotectonic setting and trace the metallogenic evolution. It is our firm belief that an integrated and adequate interpretation of these data should certainly open new perspectives for base-metal exploration in the Proterozoic sedimentary basins of Brazil. All the deposits studied have been described earlier and mapped by the authors and/or by others. The deposits of Boquira, Caboclo, Ire&, Nova Redensgo and Morro Agudo are being studied since 1987, by the Research Group on “Metallogenesis and Mineral Deposits Modeling”, from the CNPq (the National Research Council of Brazil) and the University of Bahia, Brazil, in collaboration with other national and international research centers. They have also been mapped respectively by Fleischer (1976), ConceiqBo Filho and Silva11984), Monteiro et al. (1987), Moraes Filho and Leal (1990) and Dardenne (1978a). Though the deposits of Canoas and Santa Maria have been visited and sampled by two of the authors, the main research work is being carried out, respectively, at the Universities of SBo Paulo and Estadual Paulista at Rio Claro (Tassinari et al., 1990; Daitx, 1996) and at the University of Rio Grande do Sul (Badi, 1983; Remus et al., 1997). Much of the information on these two deposits are taken from the works cited above. Many Brazilian mining companies and institutions, such as Companhia Baiana de Pesquisa Mineral (CBPM), Companhia de Pesquisa de Recursos Minerais (CPRM), Companhia Mineira de Metais (CMM), Companhia Brasileira do Cobre (CBC), Plumbum Minerasso e Metalurgia and Departamento Nacional da ProduqBo Mineral (DNPM), also contributed to the geological characterization of the deposits studied.