Geology, petrography, and mineral chemistry of the Vazante non-sulfide and Ambrósia and Fagundes sulfide-rich carbonate-hosted Zn–(Pb) deposits, Minas Gerais, Brazil

• Published in: Ore geology reviews Vol. 28; no. 2; pp. 201 - 234
• Main Authors: Soares Monteiro, Lena Virgínia, Bettencourt, Jorge Silva, Juliani, Caetano, de Oliveira, Tolentino Flávio
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01.02.2006
• Subjects: Carbonate-hosted Zn–Pb deposit; Non-sulfide zinc deposit; Vazante deposit; Willemite; Vazante deposit; Willemite; Carbonate-hosted Zn–Pb deposit; Non-sulfide zinc deposit
• ISSN: 0169-1368; 1872-7360
• DOI: 10.1016/j.oregeorev.2005.03.005

The Vazante–Paracatu region represents the most important Zn district known in Brazil and includes the Vazante hypogene non-sulfide Zn deposit composed mainly of willemite (Zn 2SiO 4) and sphalerite-rich carbonate-hosted Zn–(Pb) deposits. Fagundes is a stratabound deposit, characterized by strong silicification, dolomitization and a Fe-rich carbonate alteration halo. The main ore is represented by rhythmically banded, colloform, and zoned pyrite, sphalerite and galena, related to wall rock dissolution and sulfide infilling, which probably occurs late during the burial diagenesis. Later veins and breccia ore types reflect epigenetic mobilization, related to brittle–ductile structures. The Ambrósia mineralization is mainly fault-controlled and related to brecciated dolomite, which is tectonically imbricated with black shales and slates. Typical features include host rock recrystallization, minor silicification, baroque dolomite and ankerite formation. Pyrite, marcasite, sphalerite and minor galena occur in brecciated comb-veins and veinlets, which overprint stylolites and tectonic fractures, suggesting an epigenetic origin for the ore. The Vazante deposit differs from all other deposits of the district due to the presence of willemitic ore, which is composed of willemite, dolomite, siderite, quartz, hematite, Zn-rich chlorite, barite, franklinite and zincite. The willemitic ore occurs tectonically imbricate to small sulfide ore bodies, which comprises sphalerite and galena, metabasites and hydrothermalized dolomites within the Vazante Shear Zone. The relationships between willemite formation and the development of mylonitic structures suggest that willemitic mineralization and deformation are synchronous episodes closely related to the Vazante Shear Zone. Low Zn/Cd ratios in sphalerite from Vazante (64 to 98) and Fagundes colloform (96 to 244) and zoned sphalerite (89 to 305) could reflect the regional role of mineralizing fluids with similar low Zn/Cd ratios and low contents of reduced sulfur (∑S red). High Ge (up to 2200 ppm) and the low Fe, Cu, Mn and Ag contents in late-diagenetic Fagundes sphalerite might suggest that this metal-bearing fluid could be derived from the underlying basin fill, which comprise clastic sediments and organic matter-rich pelitic sequences. Systematic relationships among sphalerite composition and paragenetic evolution of the Fagundes and Ambrósia deposits suggest that progressive fluid mixing processes were important for the genesis of the sulfide-rich deposits in the district. These mixing processes possibly involved the hot metal-bearing fluid with low contents of reduced sulfur and moderate-temperature, highly saline fluids, which could represent an important sulfur supply. The predominance of the highly saline brines in later epigenetic mineralization episodes might be related to episodic migration of hydrothermal fluids mainly derived from reduced shale sequences during the Brasiliano compressive events. The small variation in chemical and sulfur isotopic composition of the Vazante sphalerite could imply that the high-temperature metal-bearing fluid with low Zn/Cd ratios could represent a minor reservoir of reduced sulfur, which permitted only subordinate sphalerite precipitation in the Vazante deposit. The lack of reduced shale sequences above the Vazante deposit could represent a limiting factor for H 2S supply. Additionally, overall mixture between this hot sulfur-deficient metal-bearing fluid and meteoric fluids channeled to the Vazante Shear Zone favor the high fO 2/S 2 conditions responsible for the stability of the Vazante willemitic assemblage.