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Title: Geology and base-metal mineralization associated with Archaean iron-formation in the Pojuca Corpo Quatro deposit, Carajás, Brazil
Author: Winter, Caroline Jane
ISNI:       0000 0001 3570 9245
Awarding Body: University of Southampton
Current Institution: University of Southampton
Date of Award: 1994
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The Pojuca Corpo Quatro deposit is host to 58Mt of Cu (at a grade of 0.9%) and 8.5Mt of Zn (1%), and is one of several base-metal (±Au) orebodies hosted within Archaean iron-formation on the northwestern edge of the Serra dos Carajá Belt, in the eastern Amazonian Craton of ParáState, Brazil. The near-vertical, SW-dipping strata, within which the Corpo Quatro (C4) deposit is hosted, is known as the Corpo Quatro Formation, and forms part of the regionally extensive IgarapéPojuca Group, dated at 2759 Ma. A study of the Corpo Quatro Formation, which is composed of a sequence of interbedded volcanic and sedimentary strata, has revealed that the sequence was originally developed as a result of crustal thinning in an intra-continental rifting environment, in which intermittent volcanic activity was accompanied by the deposition of iron-formation and clastic sedimentary rocks. The volcanic activity is characterized by thick units of basaltic- to basaltic andesite affinity, which are comparable with those found in other lithological groups of the Carajá Belt (IgarapéSalobo Group and Gr~ao ParáGroup). Enveloped within the Corpo Quatro basalts, the sedimentary rocks display increased clastic input up sequence, from magnetite-rich iron-formation at the base to quartz-rich pelites at the top. The iron-formation (Rochas Bandadas) at the base of the sedimentary sequence displays mineralogical meso-banding typical of BIF, and grades upwards into silicate (amphibole)-rich iron-formation. This in turn grades into a quartz-rich unit (Rochas Fragmentos), which was formed as a result of mixing of clastic sediments derived from iron-formation and basaltic rock end members. This increase in detrital input is represented by significant enrichment in the relatively immobile elements Al, Ti, Zr, V and Cr, which were introduced via heavy minerals and clays. The host rocks of the Corpo Quatro Formation underwent an episode of regional metamorphism which reached ~5-7kbar at upper greenschist to lower amphibolite facies (staurolite-andalusite zone), and was followed by Na-Ca-metasomatism which accompanied two closely timed stages ofsulphide deposition generated by post-peak metamorphic fluids. The C4 base-metal mineralization occurs mostly in gruneriterich iron-formation as stratiform and fracture-fill sulphides, and in sulphide-quartz veins which cross-cut the metamorphic sequence. The fluids responsible for mineralization have been identified as Na-Cadominant, with the metals probably transported as chloride complexes. The initial stage of mineralization (pyrrhotite-chalcopyrite ± sphalerite) was deposited by Na-Ca-(K)-rich aqueous fluids ofmoderate salinity (4-34 wt.% NaCl eq.). It is believed that mineralization was lithologically controlled, and resulted from sulphidation of magnetite to produce pyrrhotite in the Rochas Bandadas and subsequent replacement of the pyrrhotite by the copper- and zinc-bearing sulphide minerals. Na-Ca-metasomatism of Fe-rich amphiboles in the Rochas Bandadas and albitization of the metabasalts resulted in further deposition of sulphide minerals at temperatures between 280 and 470°C. The second stage of ore mineralization was strongly related to the deposition of quartz-biotite veins, and was generated at slightly lower temperatures (280 to 420"C) by high salinity Na-Ca-K-Fe-rich fluids (15-41 wt.% NaCl eq.). Wallrock-fluid interaction caused retrograde alteration of garnet and hornblende to biotite in the Rochas Fragmentos, which resulted in the deposition of ore minerals (chalcopyrite, sphalerite, molybdenite, gold and nickel-bearing phases) at reaction sites, triggered by the increase in cation content of the high salinity fluids. Retrograde metamorphism continued after the deposition of base-metal mineralization at Pojuca C4, with the local development of chlorite-zone assemblages in narrow (up to 3m-wide) shear zones and the deposition of late calcite-bearing veins.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Geology