Geologia e petrologia do prospecto GT-34 : evidência de metassomatismo de alta temperatura e baixa fO2, província mineral Carajás, Brasil

AUTOR(ES)
DATA DE PUBLICAÇÃO

2008

RESUMO

The GT-34 Prospect in the Carajás Mineral Province is located within older gneissmigmatite terrains (Xingu Complex). Sulfide mineralization is hosted by irregular bodies of brecciated rocks and/or orthopyroxene-amphibole bearing rocks outcropping among gneissic rocks. Distribution of sulfide-rich intervals form an irregular NE trend of about 1.5 km-long and up to 500 meters deep. Breccias include highly heterogeneous fragmental rocks resulting from alteration of gneisses. Sulfide-rich zones are closely associated with rocks consisting mainly of orthopyroxene (orthopyroxenitite) and amphibole, considered to form by metasomatism. Sulfide-rich intersections may be up to dozens of meters-thick or restricted to few centimeters-thick veins. Brecciation of orthopyroxenitites by sulfide-bearing veins is observed throughout the GT-34 Prospect. In these zones orthopyroxene is brecciated and partially replaced by an assemblage of sulfides associated with variable proportions of apatite, scapolite and hornblende. Enrichment in sulfides occurs in discrete veins, in veining systems developing net textured rocks or in semi-massive brecciated zones where partially altered fragments of orthopyroxenitites frequently occur. Apatite is ubiquitous and abundant (up to 25 vol. %) in brecciated semi-massive sulfides or sulfide-rich veins. Sulfide minerals consist mainly of pyrrhotite with associated pyrite, chalcopyrite and pentlandite. The abundance of apatite in the sulfide-rich samples results in their high P contents (up to 7.9 wt. % and several values between 1-5 wt. %) and good correlation with S values. Sulfide-rich zones are enriched in Fe, reflecting the abundance of Fe-bearing sulfides but metasomatic rocks closely associated with the sulfiderich zones are not Fe-enriched. Higher Mg contents (>3 wt. % Mg) characterizes the orthopyroxene-rich rocks (orthopyroxenitites). These rocks are closely associated with sulfiderich zones but not neccessarily their host rocks. Compositions of orthopyroxene crystals from orthopyroxenitites collected in different portions of the GT-34 Prospect are very similar. En contents for orthopyroxene from the GT-34 vary from 68.0 to 77.5 %. Variation in En content shows no correlation with contents of TiO2, Cr2O3, CaO and Al2O3. When compared to orthopyroxene with similar En content of orthopyroxenites from mafic-ultramafic layered intrusions, orthopyroxene crystals from the GT- 34 Prospect show lower contents for TiO2, Cr2O3, CaO and Al2O3. Orthopyroxenitites have extremely low Cr2O3 (<0.01 wt. %; or 22 to 71 ppm Cr) and TiO2 (0.03 to 0.14 wt. %) contents, also indicating distinctive compositional features when compared to orthopyroxenites of magmatic origin. These compositional features support the interpretation that orthopyroxene crystals (and orthopyroxenitites) in the GT-34 Prospect are not magmatic, being originated by metasomatic processes. Sulfide-rich rocks (S >10 wt. %) have high total Fe2O3 (27.10 to 44.71 wt. %) and P2O5 (4.20 to 20.92 wt. %) contents, reflecting their abundance in Fe-sulfides (pyrrhotite, pyrite, chalcopyrite and pentlandite) and apatite. Compositional trends for orthopyroxenitites, sulfidebearing orthopyroxenitites and sulfide-rich rocks suggest that sulfide-bearing rocks result from mild to extensive replacement of orthopyroxene-bearing rocks. REE contents in sulfide-bearing rocks are correlated with the abundance of apatite. This correlation is illustrated by the plot of Ce versus P2O5 contents for orthopyroxenitites, sulfide-bearing orthopyroxenitites and sulfide-rich rocks, indicating the progressive replacement of orthopyroxenitites by apatite-sulfide rich zones. Interpretation of available data of the GT-34 Prospect suggests that high-temperature orthopyroxene-bearing metasomatic replacement bodies (Phase 1) developed within gneissic country rocks, followed by a late event of veining, brecciation and sulfide mineralization (Phase 2). Geological conditions appropriated to sustain the extensive high-temperature metasomatic system described in the GT-34 prospect (P >0.5 Kb), suggest temperatures over 700C for the crystallization of orthopyroxene. The second Phase in the GT-34 system consists of brecciation and veining of orthopyroxenitites and host gneisses, together with sulfide-apatite mineralization. This process involved the crystallization of significant amount of sulfides and apatite, thus promoting the concentration of a diverse range of elements (e.g. P, F, S, REE, Fe, Cu, Ni). The close spatial association suggests a genetic link between early alterarion (Phase 1) and later sulfidization (Phase 2). However, relative enrichment and depletion of elements are distinctively different in these two events indicating that fluids associated with metasomatism, and/or physical conditions prevailing during alteration, were highly different during these events. Sulfide-rich zones have abundant Fe-bearing sulfides (pyrrhotite, pyrite, chalcopyrite, pentlandite) and no associated oxides. Crystallization of pyrrhotite and pyrite without Fe-oxides (magnetite or hematite) indicates conditions of high sulfur fugacity (fS2) and low oxygen fugacity (fO2). The temperature of crystallization of sulfide-rich zones is constrained by associated hornblende, suggesting temperatures above 500C, and the lack of orthopyroxene (e.g. T <700C). Characteristics of the GT-34 Prospect suggest that metasomatism occurred under higher temperature and lower fO2 conditions, compared to Cu-Au deposits in Carajás. These features possibly indicate that metasomatism and sulfidization of the GT-34 Prospect represent deep zones (e.g. deeper crustal level) of the regional 2.5 Ga Cu-Au ore-forming system of Carajás.

ASSUNTO(S)

carajás metassomatismo alteração iocg litogeoquímica apatita geologia

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