On the origin of the Neoproterozoic Peresopolis graphite deposit, Paraguay Belt, Brazil

摘要

The Peresopolis graphite deposit is located northeast of Brasilandia Town in Mato Grosso State (Brazil). It consists of an 1800 m long, 200 m wide low-crystallinity graphite-bearing tabular layer that trends ENE and dips 65 degrees ESE. The deposit is hosted in carbonaceous phyllites, which along with basal metadiamictites and upper metarenites make up the upper unit (Coxipo Formation) of the Cuiaba Group in the late Cryogenian to Cambrian Paraguay Belt (ca. 650-500Ma). The carbonaceous phyllites show a mineral assemblage consisting mostly of graphite-quartz-muscovite-albite and pyrite and dolomite to a lesser extent; alteration minerals include tosudite and kaolinite. XRD analysis confirmed the gangue material and defined the graphite as low-order crystallinity. Carbon isotope data for graphite ore returned a light and very restricted range of delta C-13(org) between 29 and -28 parts per thousand suggesting organic matter as the source of carbon. One hundred and sixty measurements of Raman graphite spectrum returned a well-fit between full width at half maximum parameter (FWHM) which allowed its use as a geothermometer. Resulting temperatures are in the range between 285 and 300 degrees C +/- 30 degrees C, indicating low-to very-low metamorphic conditions for transformation of organic matter into amorphous graphite. The deposition of the organic matter should have taken place in an outer slope of a glaciomarine system and its transformation into the ore occurred because of deformation and low-grade metamorphism related to the development of the Neoproterozoic Brasiliano/Pan-African Orogeny (850-500Ma). (C) 2018 Elsevier Ltd. All rights reserved.