Abstract
The Cristalino deposit, located in the Serra do Rabo region (Pará State, Brazil), is related to a hydrothermal system in which two major alteration stages could be distinguished most likely with the involvement of a hyper-saline fluid. The first stage (410-650 °C) is characterized by a distal sodic metasomatism that produced almost pure chessboard albite, minor schorlitic tourmaline and REE-rich minerals (allanite-Ce, monazite). It was followed by a pervasive calcic-ferric alteration that generated abundant actinolite (XMg = 0.87–0.69, Cl up to 0.59 wt %) in addition to Ce-allanite and magnetite associated with sulfide disseminations and breccia-like bodies composed of chalcopyrite-pyrite-magnetite-Au (early ore association). Locally, Fe-edenite (XMg = 0.67–0.42, Cl up to 2.94 wt %) replaced calcic-ferric assemblages within restrict sodic-calcic alteration halos. From 410° down to 220 °C, the previous alteration assemblages were overprinted by the hydrothermal products of the second stage. Potassic (K-feldspar, minor biotite) and propylitic (epidote, chlorite, calcite) alterations came into play successively. K-feldspar is practically stoichiometric, but it contains some impurities, notably BaO (up to 1.21 wt %). Chlorite shows the greatest compositional variation among all minerals and its composition seems to have been particularly controlled by the type of host rock, chemistry of the hydrothermal fluid and temperature. Both chamosite and clinoclore (XFe = 0.37–0.80) are present, the former being more common. Chlorine contents are in general <0.02 wt % and a little more significant in chlorites that replaced chessboard albite (up to 0.06 wt %). Their formation temperature ranges from 220° to 360 °C, with infill chlorites showing the highest values. The late ore association (chalcopyrite ± Au ± hematite ± pyrite) is contemporaneous with the potassic and propylitic alterations and bears evidence that the Cristalino system evolved to the final stages with increase in oxygen fugacity. Comparatively to other IOCG deposits from the southern sector of the Carajás domain, especially the Sossego and Visconde deposits, two varieties of amphibole (mainly actinolite and edenite) have been formed, but the Cristalino chlorites, despite some overlapping, present very distinct populations not yet described in the other two deposits, confirming the diversity of IOCG systems in Carajás.
Published Version
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