TRANSITIONAL PYROCLASTIC, VOLCANIC-EXHALATIVE ROCKS TO IRON ORES IN THE CAUÊ FORMATION, TAMANDUÁ AND CAPITÃO DO MATO MINES: AN OVERVIEW  OF METALLOGENETIC AND TECTONIC ASPECTS   

 

Suckau, V.E.¹; Suita, M.T.F²; Zapparolli, A.C.¹; Spier, C.A.¹; Ribeiro, D.T.¹

 

1.        Minerações Brasileiras Reunidas (MBR) - Águas Claras, Av. de Ligação, 3580, 34000-000, Nova Lima, MG - vsk@mbr.com.br, cbs@mbr.com.br, tmt@mbr.com.br

2.        Departamento   de Geologia, DEGEO, Escola de Minas, UFOP, Ouro Preto, MG - suita@degeo.ufop.br

 

 

Abstract

 

The Tamanduá and Capitão do Mato iron deposits are located in the eastern limb of the Moeda syncline, Iron Quadrangle, a NS megasyncline, in the meridional segment of the São Francisco Craton, 18 km southeast from Belo Horizonte. The mines are exploited by MBR a subsidiary company of the CAEMI group controlled by Companhia Vale do Rio Doce (CVRD). The main iron orebodies of the Cauê Formation occur as NW/SE- striking lenses in synformal structures superimposed and sub-parallel to the eastern limb of the Moeda syncline. The Tamanduá Deposit is about 1,800 m long, 300 to 500 m across, with ore up to 500 m deep, while the Capitão do Mato deposit is about 2,500 m long, 350 m across, with ore depth up to 250 m. The friable rich itabirites and the iron-rich orebodies of the Cauê Formation are limited and transitional to sequences of felsic to mafic-ultramafic (quartz-chlorite to Mg-chlorite schists), which are characterized as representative of a volcaniclastic sedimentary sequence, such as pyroclastic, volcanic breccias and possible exhalites (manganiferous iron argillaceous formations) that occur interbedded, interfingered and concordant with the itabirite sequence. For a better understanding of the genesis and control of the iron deposits, a geochemical, petrological and metallogenic study has been done on these rocks and associated ores, which are extremely altered and deformed, hindering the identification of their mineralogy, textures, structures and original geochemistry. Despite all this, these rocks are identified as volcanic pyroclastic and mafic and felsic extrusives (basalts and dacites?).

 

Key-words: Cauê mine, iron ore, volcaniclastic rocks.

 

The itabirites of the Cauê Formation (to the bottom of the Itabira Group) are oxidized, metamorphosed and heterogeneously deformed BIFs containing discontinuous orebodies, which vary in scale from a few centimetres to hundreds of meters. Multiple processes obliterated partially or totally the mineralogy, texture and structure of the original sediments, making difficult to identify primary characteristics, especially at the high-strain domains in the Iron Quadrangule (IQ).

     Iron ores have been formed throughout the IQ by the BIF enrichment. The genesis of the iron ores hosted within the Cauê Formation has been the subject of debate since the beginning of the 20^th century, and continues to be the focus of studies by several authors (e.g., Dorr, 1964; and see  Spier et al., 2003 for general references). The precise time of mineralization is also a source of controversy. Dorr (1964), for instance, postulated a syn-metamorphic and deformational mineralization. But there is much

questioning of this by other authors (e.g., Rosière, 1981; Taylor et. al., 2001; Rosière & Rios, 2002 in Spier et al. 2003).

     Along the deposits, the contact between the Cauê Fm. and the phyllites of the Batatal Fm. can be gradational, abrupt and sheared. Similarly to what occurs in the Tamanduá mine, synformal isoclinal folds control the ore morphology of the Capitão do Mato Mine up to its eastern limits. In the Tamanduá and Capitão do Mato Mines, the ore is surrounded by impermeable lithologies: (1) phyllites of the Batatal Fm. along the eastern flank of the Tamanduá synform while in the Capitão do Mato Mine the Batatal Fm. occurs in the north/northeastern  flank of the Capitão do Mato synform; (2) metavolcanic rocks which occur along the western limb of the Tamanduá synform and along the S/SW limb of the Capitão do Mato synform; (3) a set of intrusive basic dykes with decametric to more than 100 m thickness, striking NE/SW with 40º-70º E-SE dips. These intrusives truncate all lithologies of the Caraça Group in both deposits delimiting the synforms to NW and SE, where the main and thick iron ore is confined. Basic intrusive sills belonging to the metavolcanic sequence and the iron ore pack with main direction (NW-SE) were also mapped. Silva (1992) characterized these intrusive as metagabbros (tremolite-actinolite-zoisite-clinozoisite schists to more massive variations) and dated them at 906 Ma (U/Pb in zircon and baddeleyite). Both the dykes and the sills had their preferential route facilitated during the intrusion by pre-existing structures, that filled open spaces and the emplacement of the metavolcanic rocks followed the same old structures.

     The genesis and the late enrichment of the iron formation which generated the Tamanduá and Capitão do Mato mines have conditionings that are related to the tectonic evolution, metamorphism, metasomatism and hydrothermalism, supergenic enrichment and finally weathering processes related to the collapse structures (Ribeiro, 2003). These complex interrelationships have as a primordial basis the genesis, the depositional environment that was probably anomalously richer in iron and related to a restricted submarine extrusive volcanic basin, which made possible and conditioned the final enrichment of the deposit. These processes associated with mechanical factors such as differential pressure and porosity were also factors of control and enrichment of the originally deposited BIF.

     The iron ores that are mined out are friable and compact hematites (in the Tamanduá  and Capitão do Mato mines, respectively). The Tamanduá orebodies are limited and transitional to a pyroclastic mafic (-ultramafic?) to felsic sequence. Metavolcanic lenses and layers occur with gradational contacts and interbedded (showing an unequivocal syn-sedimentary deposition) with iron-manganiferous rich, friable and argillaceous itabirites (exhalites?), with compact hematite, and argillaceous (millimetric to centimetric) bands showing amygdaloidal or vesicular textures (Figures 1 and 2) now filled either by kaolin or quartz. Their thickness vary from few centimetres up to 50 m thick or more, with lateral variations. These rocks are deeply weathered until approximately 400 m deep. The volcanosedimentary sequence is intruded by the basic dykes when may occur severe hydrothermalism (sericitization, carbonatization, etc.).

     Description of outcrops and petrographic studies of drill-hole samples show a low greenschist facies (Fe-chlorite zone) metamorphosed mafic (quartz-chlorite to Mg-chlorite schists, with clinochlore, illite, vermiculite, epidote, pyrite, pyrrhotite and chalcopyrite, besides elongated retangular ripiform plagioclase associated to shards filled and surrounded by iron oxide/hydroxide (Figures 2 and 3) to felsic (tuffs, breccias and agglomerates, mostly dacitic terms) sequence, with pyroclastic meta-agglomerates, breccias, tuffs and ignimbrites, (Figure 3).  Eroded gulf quartz are also present (Figure 4), amygdules or vesicles filled by quartz or kaolin (Figure 5), besides pyroclastic euhedral quartz associated to shards and ignimbrite clasts.

     The whole sequence was deformed and has the same structural inheritance (syn-tectonic) of the Cauê Formation. The presence of interlayered felsic to mafic(ultramafic?) rock types suggests an extensional explosive bimodal volcanism at least in one of the extrusive phases, with tuffs, breccias and volcanic bombs.

     Besides these petrographic studies, twenty-nine whole rock samples were analysed for oxides and trace elements (REE) from those, nineteen samples have a volcanic affiliation. Their composition was mostly established by their “immobile” (Ti, Nb, Y, Zr, etc.) elements.

Figure 1. Spheroidal vesicles developed over a contact with an iron manganiferous  rock from the Capitão do Mato Mine.

 

The present data show that these samples, in petrogenetic and tectonic diagrams (Figures 6, 7, and 8), are mostly positioned in the field of tholeiitic intraplate basalts transitional to MORB and less to IAT/spreading center islands types. The findings that the samples belong to only one group are not conclusive as other

graphs show samples spreading to other fields.These aspects may result from intracontinental and/or oceanic hydrothermalism, several stages of regional metamorphism, supergenic processes and weathering.

 

CONCLUSIONS

      We may suggest that the sequence of enrichment events of the deposits had as its first origin an anomalous iron-rich source related to a hydrothermal circulation of sea water in an associated extensional tholeiitic mafic to felsic volcanism, which also was the source of iron for the deposit at the bottom of the sea, in a restricted transitional continental to marine basin. The superposition of thermo-tectonic events over this environment, led to various oxidizing and leaching pulses which acted as concentrators of mineralization, probably under the presence of several origin fluids. The evidence indicates that the formation of the enriched ore may have developed in different temperatures, and mechanical factors may have also acted as controllers in the formation and concentration of iron-rich ore bodies. The exogenous processes were only possible where there were lithological-structural traps and conditioners associated with the impermeable country rocks that allowed the replacement and leaching of silica. Thus, besides acting as a source of iron, the sedimentary-volcanic sequence served also as a pipeline and a barrier for the mineralizing fluids.

    The presence of metavolcanic rocks delineating the main iron orebody in a specific and favorable tectonic environment is of major importance because it breaks the genetic paradigms previously accepted for the deposition of the Cauê Formation. It creates the possibility for the existence of similar deposits in the context of such formation, and brings new evidence for the iron exploration, its metallogeny, and tectonic evolution in the I.Q.

 

 

Figure 2. Photomicrograph of ripiform plagioclases in fine grained chlorite/clinochlore matrix (500x) from a Tamanduá Mine (TM) sample.

 

 

Figure 3. Photomicrograph of shards (having the form of or suggesting a spider) filled with iron hydroxides in a fine grained chlorite/clinochlore matrix (1000x) from a  TM sample.

 

Figure 4. Photomicrograph of a grain of “gulf quartz” (1000x) from a TM sample.

 

 

Figure 5.   Photomicrograph   of   spheroidal    amygdule/

vesicle filled by quartz. Note the dark rim and the flow texture (500x) from a  TM sample.

 

 Figure 6. Ti  x Cr graphic showing distribution of most of TM samples in the MORB  field .

 

Figure 7. Ti  x Zr graphic showing distribution of the TM samples in the transition of volcanic (mostly) intra-plate to MORB and volcanic island arc fields

Figure 8.  AI (Alkaline Index) x Al₂O₃ graphic showing most of the TM samples in the tholeiitic field.

 

REFERENCES

Dorr II, J.V.N. 1964. Supergene iron ores of Minas Gerais, Brazil. Economic Geology, v. 59:1203-1240.

Ribeiro, D.T. 2003. Enriquecimento supergênico de Formações Ferríferas Bandadas: Estruturas de colapso e desordem. Departamento de Geologia,  Universidade Federal do Rio de Janeiro, Doctoral Thesis.

Rosière, C.A. 1980. Strukturelle and texturelle untersuchen in der eisenerzlagerstaette “Pico de Itabira” bei Itabirito, Minas Gerais, Brasilien, Clausthaler Geowissenschaftliche Dissertationen 9, Clausthal Zellerfeld, , 302 p.

Rosière, C.A., Rios, F.J. 2004. The origin of hematite in high grade iron ores based on infrared microscopy and fluid inclusion studies: the example of the Conceição mine, Quadrilátero Ferrífero, Brazil. Economic Geology 99: 611-624;

Silva A. M. 1992. Geologia e petroquímica dos enxames de diques máficos do Quadrilátero Ferrífero e Espinhaço Meridional, MG. Instituto de Geociências, Universidade de Brasília. MSc Thesis, 117 p.

Suckau, V.E., Costa, T.A.V., Suita, M.T.F., Oliveira, D.M., Ferreira Filho, F.A. 2004. Aspectos estruturais, petrológicos e metalogenéticos da mina de ferro do Tamanduá, MBR, Quadrilátero Ferrífero, MG. XLII Congresso Brasileiro de Geologia, XLII, Anais (CD-ROM), Araxá, Outubro 2004.

Spier, C.A, Oliveira, S.M.B., Rosière, C.A. 2003. Geology and geochemistry of the Águas Claras and Pico iron mines, Quadrilátero Ferrífero, Minas Gerais, Brasil. Mineralium Deposita, v. 38: 751-774.

Taylor D., Dalastra, H.J., Harding, A.E., Broadbent G.C., Barley, M.E. 2001. Genesis of high-grade hematite orebodies of the Hamersley Province, Western Australia, Economic Geology, v.  96:837-873.