Styles of Hydrothermal Alteration And
Total Page:16
File Type:pdf, Size:1020Kb
Revista Brasileira de Geociências 28(3):355-366, setembro de 1998 STYLES OF HYDROTHERMAL ALTERATION AND GOLD MINERALIZATION ASSOCIATED WITH THE NOVA LIMA GROUP OF THE QUADRILÁTERO FERRÍFERO: PART II, THE ARCHEAN MESOTHERMAL GOLD-BEARING HYDROTHERMAL SYSTEM LYDIA MARIA LOBATO*& FREDERICO WALLACE REIS VIEIRA** RESUMO ESTILOS DE ALTERAÇÃO HIDROTERMAL E MINERALIZAÇÕES AURÍFERAS ASSOCIADAS COM O GRUPO NOVA LIMA DO QUARILÁTERO FERRÍFERO: PARTE II, O SISTEMA HIDROTERMAL AURÍFERO MESOTERMAL ARQUEANO Depósitos mesotermais, estruturalmente controlados do tipo lode-gold ocorrem em terrenos metamórficos e constituem um grupo específico de sistemas epigenéticos de met als preciosos. Muitos depósitos mesotermais de ouro no Quadrilátero Ferrífero (QF), Minas Gerais, Brasil, são englobados no grupo pertencente aos greenstone belts do Arqueano e encaixados em rochas típicas de associações metamórficas das fácies xisto-verde e sub-anfibolito. Em geral, a alteração hidrotermal dos mesmos caracteriza uma zona central de carbonates de ferro, normalmente com albita e micas, gradando para calcita e clorita. Venulações quartzosas e sulfetação também desenvolvem-se, com ouro incluído principalmente nos sulfetos. Os fluidos mineralizantes eram ricos em CÓ2-H2O, de salinidade relativamente baixa, de pH próximo do neutro a algo alcalino e de caráter geralmente redutor. No caso do QF, variações na fração molar Xco2 e nas atividades de K+/H+ e Na+/H+ controlaram as paragêneses de alteração das rochas metamáficas e meta-ultramáficas. Clorita domina a alteração incipiente e carbonates a avançada, sugerindo que um fluido dominado por H2O evoluiu de fluido original com valores maiores da razão CO2:H2O, típico dos estágios mais avançados da alteração hidrotermal. Mudanças nos parâmetros ais and Xco2 controlaram a alteração carbonática e a sulfetação de formação ferrífera bandada (BIF), rocha pobre em Al. A capacidade dos minerais da fí/Fem tamponar a/32 do fluido infiltrante determinou a 1) aΣS (e Xco2) do fluido em evolução, e assim 2) associação de sulfetos no espaço físico, no espaço químico e no tempo geológico. Variações na aΣS e Pfluido também podem ser deduzidas pela alteração da BIF, à qual associa-se a deposição de ouro. Zonas de cisalhamento são ocupadas principalmente por pirrotita, enquanto pirita + arsenopirita ocorrem substituindo o bandamento da BIF. Os estágios de alteração incipiente (tamponado) e mais avançados (não tamponados) refletem a gradual evolução composicional de um fluido original externo, relativamente uniforme, através da interação com encaixantes, assim refletindo variações na razão fluido versus rocha. A alteração hidrotermal e precipitação de ouro associada foram praticamente isotérmicas, estando de acordo com cálculos de temperatura e pressão para diversos depósitos em outras partes do mundo. As pequenas variações em fO2, pH e ais, sugeridas para os depósitos no QF, tiveram provavelmente um importante papel na deposição do ouro. Palavras-chave: ouro, Quadrilátero Ferrífero, condições na alteração hidrotermal 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 Quadrilátero Ferrífero (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. Keywords: gold, Quadrilátero Ferrífero, hydrothermal alteration conditions INTRODUCTION The objective of this paper is dual. Firstly, and the physical conditions of ore formation (Reed 1997). Predictive we intend to provide a thorough review of the critical knowledge base insights regarding orebody geometry and grade can thereby be in- regarding hydrothermal alteration and mineralization processes asso- ferred. ciated with Archean, mesothermal gold deposits. Secondly, this knowl- The usefulness of hydrothermal alteration, both as a guide to ore edge base is applied to the context of mesothermal, lode-gold miner- and as an indicator of the character of solutions associated with ore alizations in the Quadrilátero Ferrífero, Minas Gerais, Brazil. Thereby, deposition, has been recognized for a long time (Rose and Burt 1979). a state-of-the-art integration of hydrothermal alteration and gold min- The significance of wallrock alteration was initially pointed out in the eralization conditions in the Quadrilátero Ferrífero is presented, as well late 1940's by way of the recognition of the association of ore with as a critical evaluation of the generalized, gold-mineralizing, fluid zones of serialization, silicification and chloritization. conditions. Since then, the improvements in analytical techniques have broad- By far the greatest number of mineral deposits has been formed by ened the knowledge on the subject via studies in the fields of lithogeo- precipitation from hydrothermal solutions (Skinner and Barton 1973). chemistry, isotope geochemistry, and fluid inclusions, among others. Hydrothermal ore deposits form because a hot, aqueous solution The fields of isotope and fluid inclusions studies have been of utmost carrying met als and other components precipitates minerals in ore traps importance in unraveling fluid-rock interaction in fossil and recent due to physico-chemical disequilibrium between rocks and fluid (Skin- geothermal systems and sea-floor deposits. They have also established ner 1997). Fluids travelling through rocks result in mineral reactions the role that magmatic, meteoric and metamorphic fluids play in ore that accommodate such severe disequilibrium, generating new - hy- deposit models (Appleyard and Guha 1991). drothermal - mineral associations. Hydrothermal alteration assemblages display zonal patterns that Hydrothermal alteration is a chemical replacement of original min- reflect, from the ore zone to the least altered host rock, progressive erals in a rock by new minerals where a hydrothermal fluid delivers rock buffering. Hence, alteration haloes provide a much larger explo- the chemical reactants and removes the aqueous reaction products. It ration target than the deposits themselves, and are regularly used by is a function of variations of temperature, pressure and, most impor- most exploration companies in their search for ore (c.f. abstracts in tantly, fluid composition (Reed 1997, Rose and Burt 1979). This Walton and Jambor 1998). chemical interaction between wallrocks and hydrothermal solutions is The study of hydrothermal alteration associated with gold deposi- certainly a major factor in the precipitation of many ores. An under- tion in Archean, mesothermal deposits has received a lot of attention standing of hydrothermal alteration is therefore of value, because it in the literature in the last ten years. Efforts to focus on the alteration provides insights into the chemical attributes and origins of ore fluids assemblages rather than on the vein and ore material alone are certainly * Instituto de Geociências, CPMTC, Universidade Federal de Minas Gerais, Cx. Postal 719, Av. Antônio Carlos 6627, Pampulha, [email protected], 31270.901, Belo Horizonte, MG ** Mineração Morro Velho Ltda., Fazenda Rapunha s/n, dopl @bismail.com.br, 34000.000, Nova Lima, MG 356 Revista Brasileira de Geociências, Volume 28,1998 adding to the understanding of hydrothermal systems related to equilibrium with the fluids that were traversing them (Lindgren 1895 mesothermal gold mineralizations (McCuaig and Kerrich 1994). and Knopf 1929, in William-Jones et al. 1994). The literature on the subject is abundant. The reader is invited to consult the reviews by Roberts (1988), Robert et al. (1991), Boyle ARCHAEAN MESOTHERMAL GOLD DEPOSITS AND AS-