Contexto geológico e origem da mineralização sulfetada estratiforme de São Martim, SW do Cinturão Araguaia, Pará

Abstract

The São Martim Prospect lies on the southwestern portion of the Araguaia Belt and is characterized by sulfide concentrations hosted by a sedimentary sequence that belongs to the Couto de Magalhães Formation. The sulfides occur mainly in black diamictites and rudites as thin stratiform layers, clasts and disseminations. Pyrite is the dominant sulfide, being normally associated with different amounts of chalcopyrite, sphalerite, galena, apatite, monazite, xenotime, allanite, barite, anhydrite, calcite, albite, stilpnomelane and chlorite. Lead isotopic compositions of the different pyrite varieties are very homogeneous, indicating that the metals are derived from a common source. This homogeneity precluded an isochronic Pb age to be obtained for the mineralization, but most Pb-Pb model ages for pyrite yielded values between 716 and 616 Ma which are consistent with the Neoproterozoic evolution of the Araguaia Belt. Microthermometric data on fluid inclusions trapped in calcite crystals from th e mineralized zones reveal a fluid composition comparable to the NaCl-CaCl 2 -H 2 O system and salinities that vary from 11 to >23wt.% NaCl. This saline fluid might represent exhalative emanations or connate water that flowed through evaporitic layers. Homogenization temperatures mostly fall in the 90-110°C range which is compatible with the presence of kerogen in the carbonaceous matter of the black diamictite matrix and with a diagenetic environment. The proposed genetic model comprises an autochthonous phase I (exahalative sulfides), an allochthonous phase (sulfide clasts and sulfide-bearing lithic clasts resulting from the dismantling of preexisting exahalites) and an autochthnous phase II (diagenetic sulfides with some exahalative contribution). Comparison with the Kupferschiefer-type deposits points to promising perspectives of finding similar environments in the Araguaia belt.