The Metallogenesis of the Mesoproterozoic Morro Agudo Zn-Pb Sulfide District and Vazante-Paracatu Mineral Belt: Evidence from Mineralogy, Geochemistry, and Sulfide Mineral Chemistry

Abstract

The Vazante-Paracatu Mineral Belt is a Proterozoic carbonate-siliciclastic basin with two distinct styles of base metal mineralization (sulfide and silicate). The Lapa and Morro do Calcário formations host Zn-sulfide deposits (Morro Agudo, Ambrósia, Fagundes) and occurrences in the northern part of the belt, and the Serra do Poço Verde Formation hosts hypogene Zn-silicate deposits (Vazante and North Extension) and occurrences in the southern part. Mineralization at the Morro Agudo district is stratigraphically controlled within dolomite lenses and structurally controlled by a steep normal fault. The genesis of the Morro Agudo district has been interpreted as MVT, SEDEX, or Irish-type. The Vazante Group dolomitic rocks experienced multiple generations of hydrothermal carbonate precipitation, silicification, and overprinting by several generations of pyrite, sphalerite, and galena. Mineral chemistry of sphalerite reveals mixing of Fe and Cd-rich endmembers, and the earlier generations are enriched in Co, Cu, Ge, Hg, Pb, and Tl, with lower Mn and Fe compared to the last generation. Fluid temperatures calculated from trace element composition range from 82–320°C (mean ~200–250°C), which is consistent with previous fluid inclusion studies. Galena mineral chemistry indicates low Cu and Mn and high Ag, Cd, and Se contents in the initial galena phase. Low Fe, In, and Mn in sphalerite are similar to compositions of other global MVT deposits, and the high estimated temperatures of formation suggest a higher geothermal gradient in the Neoproterozoic or high advective heat flow during the Brasiliano Orogeny. The Vazante-Paracatu district is characterized by As-Ag-Cd-Cu-Fe-Hg-In-Mn-S-Sb-Se-Tl±Ge-Mo-Re-Te, with Zn-silicate ore more enriched in Ag, Cu, In and Se and poorer in Mn and Tl than Zn-sulfide ore. Geochemistry of the siliciclastic units reveals that the Serra do Garrote Formation has the highest contents of ore-related elements and has potential to be the major contributor of metals during hydrothermal fluid flow. Host rocks of the sulfide ore and the overlying phyllite-bearing Canastra Group may have contributed to the local enrichments of some metals. Ore fluid temperatures are similar throughout the Vazante Group, suggesting an extensive, circulating hydrothermal, metalliferous fluid event with distinct precipitation mechanisms for Zn-silicate and Zn-sulfide ore.