Styles of hydrothermal alteration and gold mineralizations associated with the Nova Lima Group, Quadrilátero Ferrífero: Part II, the Archaean mesothermal gold-bearing hydrothermal system

Abstract

Structurally hosted, mesothermal, lode-gold deposits in metamorphic terranes constitute a single, coherent group of epigenetic precious met al deposits. Many mesothermal gold deposits in the Quadrilatero Ferrifero (QF) region of Minas Gerais, Brazil are encompassed in the sub-amphibolite to greenschist facies group of Archean, greenstone belt deposits. In general, the hydrothermal alteration of these deposits characterizes a core of Fe-carbonates, normally with micas and albite, grading out into calcite and chlorite. Quartz veining and sulfidation are variably developed, with gold residing mainly within sulfides. Gold mineralizing fluids were CO2-H2O-rich, of relatively low salinity, with a near neutral to slightly alkaline pH and mainly reducing. In the QF, changes in the molar fraction Xco2, a(K+/H+) and a(Na+/H+) controlled the alteration assemblages of metamafic and meta-ultramafic rocks. Chlorite dominates the incipiente, and carbonate the advanced alteration assemblages, suggesting that a H2O-rich evolved from an original, higer CO2:H2O-ratio fluid, typical of the more advanced stages of alteration. Changes in ais and Xco2 controlled Al-poor banded iron formation (BIF) carbonate- and sulfide-alterations. The capacity of BIF mineral assemblage to buffer the incoming fluid fO2 determined 1) the evolving fluid ais (and Xco2), and thereby 2) the sulfide paragenesis in physical space, in chemical space and over geological time. Variations in ais and Pfiuid can also be deduced from the gold-associated alteration in BIF. Shear zones are occupied mainly by pyrrhotite, whereas pyrite + arsenopyrite occur as bedding-replacement zones in BIF. The incipient (buffered) and more advanced (unbuffered) stages of alteration reflect the gradual compositional evolution of a relatively uniform, external fluid through interaction with wallrocks, thus reflecting varying fluid versus rock ratios. The hydrothermal alteration and gold precipitation were roughly isothermal, in keeping with temperature and pressure calculations for deposits in other parts of the world. Suggested slight variations inf02, pH and ais in the QF deposits probably played an important role in gold deposition.