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Jane Nobre-Lopes
Serviço Geológico do Brasil (SGB), Av. Pasteur, 404, Urca, 22290-240, Rio de Janeiro - RJ - jnobre@rj.cprm.gov.br
Abstract Zn-Ag mineral deposits at Januária region, as well as most of the Pb-Zn mineralization scattered in the middle São Francisco valley, occur in a regionally extensive stratigraphic marker of brecciated dolostones belonging to the Bambuí Group (Sete Lagoas Formation), a platform cover developed over the São Francisco Craton.The Sete Lagoas Fm. is informally divided into eight members, grouped into three main shallowing-upward successions, interpreted as a parasequence set of a progradational stacking pattern. The increase in pelitic sediments upwards in the overlying Serra de Santa Helena Fm. shut down the carbonate platform. The carbonate rocks has undergone diagenetic alteration in subaerial, submarine and subsurface environments. Subsurface diagenesis resulted in the most important rock modifications. The dolostones that host mineralization consist of replacement dolomites (early and late) and dolomite cements. Except for one type of dolomite cement that lacks features of subaerial or submarine diagenesis, all the dolomites yield d18O values lighter than the limestones suggesting modification under elevated temperature, mostly under burial conditions. 87Sr/86Sr ratios of all dolomites are more radiogenic than those calculated for Neoproterozoic seawater. The brecciated dolostone is interpreted as resulting from repeated dissolution, dolomite cementation and pre-ore dissolution/collapse brecciation of the replacement dolostones by warm brines during burial. The ore-bearing breccias were also related to warm subsurface brines. The timing of dolomitization, as well as the emplacement of mineral deposits, is poorly constrained but occurred sometime after shallow burial, during the Neoproterozoic or younger, depending on the tectonics affecting the area. Palavras chave: carbonate rocks, sedimentology, diagenesis, dolomitization, mineral depositsSmall Zn-Ag and Pb-Zn mineral deposits occur scattered in the middle São Francisco river valley. They are hosted in a regionally extensive stratigraphic marker of brecciated dolostones that belongs to the Sete Lagoas Fm., which is the basal unit of the Bambuí Group (Figure 1). The origin of the Zn-Ag deposits of Januária is controversial, whether singenetic and related to a subaerial exposure (e.g. Dardenne 1979; Lopes 1979; Beurlen 1973), or epigenetic and MVT-like (Robertson, 1963). In order to solve this question we investigated the relationship between mineral deposits, diagenetic processes and the regional evolution of the sedimentary basin. As mineralization is always related to brecciated dolostones, the investigation of the origin of the breccias was also of main importance. Breccias are supposed to be of hydrothermal origin or related to subaerial exposure. The role of tectonics affecting the evolution of the basin, as well as the emplacement of the mineral deposits, though considered, is outside the scope of this report. The carbonate rocks outcropping in the middle São Francisco valley were deposited on the stable portion of the São Francisco Craton (Alkmim et al. 1993) and are unaffected by the bordering Brasiliano fold belts (750-730 Ma, according to Cordani et al. 2000). These carbonates are interpreted to represent an extensive carbonate platform affected by growth faults active during sedimentation. The Januária region had a lower subsidence rate compared to the neighbouring areas (Nobre-Lopes, 2002). Our stratigraphic interpretation is similar to that of previous workers. This research, however, attempted a detailed sedimentological study of the carbonate rocks of the Sete Lagoas Fm. that is informally divided into eight members (from bottom to top) and grouped into three main shallowing-upward successions, named respectively basal, intermediate and upper, (Nobre-Lopes, 2002). The Basal Succession is made of argillaceous lime mudstone (Member 1) and calcirudite (Member 2) with interbeds of low-angle cross-bedded calcissiltite. It is interpreted as recording a prograding interval deposited on a low-energy platform, or shallow shelf cut by tidal channels and sporadically affected by storms. The Intermediate Succession starts with flat laminated lime mudstone with interbeds of low-angle cross-bedded calcissiltite that grades
upwards into cross-bedded dolomitic calcarenite with prominent reactivations surfaces. Dolomitic calcarenites are mostly intrapseudo-ooidal packstones to grainstones (Dolomitic calcarenite member 3). The basal contact of the Dolostone member 4 is marked by an abrupt change in the character of sedimentations, from rippled-bedding (Member 3) to planar cross-bedding with current ripples interbeds. Allochems, wherever observable, are micritic remains, pseudo-ooids and well rounded silicified grains withouth internal texture preservation. The Stromatolite dolostone member 5, stratigraphically above, is composed of stromatolitic bioherms of different sizes and non-stromatolitic dolostone, mostly finely laminated dolomudstone. The lower limit of the Ooid-intraclast dolostone member 6 is frequently difficult to define because strongly affected by dissolution/collapse brecciation. The upper limit is a regionally extensive surface capped by sediments of the Dolomudstone member 7. The original layers of the Member 6 are mostly destroyed by dissolution/collapse brecciation but remains of low-angle planar cross-bedding and trough cross-bedding are still visible. Grainstone to packstone dominate over packstone to wackestone; the main allochems are remains of ooids and intraclasts. The Dolomudstone member 7 composes the uppermost part of the Intermediate Succession having widespread desiccation cracks and tepees on the upermost units. It is made mostly of dolomudstone with interbeds of ooidal grainstones to packstones and intrapackstones to wackestones; stromatolite biostromes are also present. The overall interpretation of the Intermediate Succession is that it represents a shallowing-upwards succession from muddy to sandier sediments deposited in offshore through a sandier shoreface with stromatolite reefal barrier, lagoonal and beach to tidal flat environments; subaerial exposure of the carbonate platform ended the intermediate shallowing-upwards succession. The Uppermost succession is made of dolomudstones with thin interbeds of intraclast grainstones to packstones and stromatolite bioherms (Laminated Dolomudstone member 8). It is interpreted as representing a series of low-energy prograding tidal flat cycles recording environments from shallow subtidal to high-intertida/supratidal. The three main successions are interpreted as a parasequence set of a progradational stacking pattern. The increase in pelitic sediments upwards in the overlying Serra de Santa Helena shut down the carbonate platform.
Diagenesis, dolomitization and min-eral emplacement The carbonate rocks of the Sete Lagoas Fm.
have undergone diagenetic alteration in subaerial, submarine and subsurface
environments. Subaerial diagenetic features include desiccation cracks
(members 2, 7 and 8), tepees, vadose cements (member 7) and small-scale
dissolution vugs (member 5); submarine diagenetic environment is represented
by isopachous fibrous cement around allochems (Member 6). Subsurface
burial diagenesis resulted in the most important modifications in
these carbonate rocks and includes compaction, dissolution, dolomitization,
brecciation, precipitation of sulfide and silicate ore minerals, late
calcite cement, fluorite and bitumen. The dolomite cements, VcCD and SD are closely associated with each other, occur in cavities and fractures in MCD/CCD and McCD; crosscut the limits between members 6 and 7. SD occurs in rhombohedral (pink or pale gray saddle dolomite) and saddle forms (white saddle dolomite); white SD is later than rhombohedral ones. VfCD affects the all above described dolomites, except possibly the white SD. The dolomites, basal limestones and late calcite cement (LCC) were analysed for C/O isotopes, as well as to Sr isotopes. Samples of the basal limestones withouth significant diagenetic overprints were analysed in order to use them as a reference for estimating the isotopic signature of the Neoproterozoic seawater of the Sete Lagoas Fm., to compare with post-deposicional diagenetic changes. The d18O values (PDB) range from –6.11 to –6.56‰ (mean= -6.39) and d13C range from 0.26 to 0.58‰ (mean= 0,42‰). The 87Sr/86Sr ratio of seawater during deposition of the Sete Lagoas Fm. is estimated to have been between 0.7076 and 0.7079. The heaviest d18O values obtained in early replacement dolomites (McCD) are -4.63‰, which is within the range of values for dolomites that would precipitated from the Sete Lagoas Fm. seawater, or slighly modified seawater. The 87Sr/86Sr ratios range from 0.7085 to 0.7094, higher than the estimated isotopic signature from Neoproterozoic Sete Lagoas Fm. seawater. This suggests that their original isotopic signature were partly modified by later more radiogenic diagenetic fluids. For MCD/CCD the heaviest d18O value obtained is -6.94‰, which is slighly depleted compared to the Neoproterozoic seawater. The 87Sr/86Sr ratios range from 0.7112 and 0.7118 and are more radiogenic than the estimated seawater and early replacement dolomites from the Sete Lagoas Fm.. The present data do not provide an unequivocal conclusion about the origin of MCD/CCD; they could result from previous dolomites that were neomorphosed or formed during burial (as suggested by relationship with stylolites) from more radiogenic fluids. Dolomite cements, VcCD/SD have d18O values ranging from –8.40 to –9.62‰; these are the most depleted values of all the dolomites. 87Sr/86 Sr ratios are similar to MCD/CCD, thus more radiogenic than the estimated for Neoproterozoic seawater. A few fluid inclusion measurements in SD suggest entrapment temperature above 230ºC. Also SD is frequently considered as to be hydrothermal in origin (Berger & Davies, 1999) though with some restriction (Machel, 2002). Thus VcCD/SD are interpreted as being formed in subsurface during burial by warm diagenetic (hydrothermal?) fluids. VfCD cements all types of breccia fragments and is closely related to mineralization. It also acts as cement and internal sediment displaying lamination and or normal grading. The heaviest d18O values of -4.48‰ are similar to those of early replacements dolomites, however the 87Sr/86Sr values are not compatible and are very radiogenic ranging from 0.7113 to 0.7105. There are no diagenetic features or field relationship indicationg that VfCD precipitated directly from seawater. Thus, VfCD is interpreted as resulting from chemical or chemically induced mechanical disaggregation by subsurface brines. All described dolomites occur in the brecciated dolostone level and dissolution occurred in several phases resulting in repeated dissolution/collapse brecciation. Furthermore, large-scale dissolution seems closely associated with the presence of dolomite cements, especially rhombohedral SD (more frequent near mineralized areas) and VcCD (common in barren areas) because these are the first cements to fill vugs and breccias, reinforcing that dissolution occurred mostly in subsurface. Dissolution related to subaerial exposure are minor and differ from subsurface in at least three main aspects: dissolution vugs related to meteoric waters are small, areally restricted and the infilling material are only fine dolomitized sediment. Thus repeated dissolution and dolomite cementation occurred during burial, under the action of warm subsurface fluids, affecting the replacement dolostones and resulting in pre-ore dissolution/collapse brecciation. The ore-bearing dissolution/collapse breccias also are related to subsurface warm fluids and interpreted to result from selective sulfide replacement in acidic fluids of pre-ore collapse breccia (see Qing and Mountjoy, 1994 for additional mechanisms). The timming of dolomitization as well as the emplacement of mineral deposits is poorly constrained because the paucity of data but occurred sometime after shallow burial. The main control in epigenetic base metal ore deposits in carbonate rocks without magmatic affiliation was ground preparation related to tectonics and dissolution that allowed the mineralizing fluid to flow over large areas at different time periods. Therefore, the mineral deposit emplacement was possibly related to the evolution of the Brasiliano Cycle (compressional model) or would be related to extensional tectonics. In this case, the age of emplacement cannot be constrained because the lack of geochronological data.
Acknowledgments This report is part of my PhD thesis presented at Instituto de Geociências of UNICAMP on August 2002. Advisors: Prof. Dr. Job Jesus Batista (UNICAMP) and Prof. Dr. Eric W. Mountjoy (McGill University, Canada) to whom I am very grateful. C/O isotopes were analysed at the G. G. Hatch Laboratories, University of Ottawa, Canada. Sr isotopes were analysed at the Laboratório de Geologia Isotópica (PARA-ISO), Universidade Federal do Pará. Fluid inclusions were analysed at the Department of Earth and Planetary Sciences, McGill University, Canada.
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