Gold mineralization within the Witwatersrand Basin, South Africa: evidence for a modified placer origin, and the role of the Vredefort impact event
C. L. Hayward, W. U. Reimold, R. L. Gibson, L. J. Robb, 2005. "Gold mineralization within the Witwatersrand Basin, South Africa: evidence for a modified placer origin, and the role of the Vredefort impact event", Mineral Deposits and Earth Evolution, I. McDonald, A. J. Boyce, I. B. Butler, R. J. Herrington, D. A. Polya
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The chemical composition of gold within the Archaean metasedimentary rocks of the Witwatersrand Supergroup displays significant heterogeneity at the micro-, meso-and regional scales. A detailed electron microbeam analytical and petrological study of the main auriferous horizons in the Central Rand Group throughout the Witwatersrand Basin indicates that gold has been remobilized late in the paragenetic sequence over distances of less than centimetres. Contemporaneous chlorite formation was strongly rock-buffered. Gold mobilization occurred under fluid-poor conditions at temperatures that did not exceed 350 °C. Widespread circulation of mineralizing fluids within the Central Rand Group is not supported by the gold and chlorite chemical data. Brittle deformation that affects most of the paragenetic sequence of the Central Rand Group late in its post-depositional history is followed by sequences of mineral growth and dissolution that appear throughout the Central Rand Group and have consistent textural relationships with gold. The consistent location within the paragenetic sequence, the wide regional and stratigraphic extent of the brittle deformation, together with mineral chemical and petrological data suggest that the Vredefort Impact Event (2.02 Ga) was the cause of this late deformation, and that post-impact fluid-poor metamorphism resulted in crystallization of a significant proportion of the gold on and within mineral grains that were deformed during this event.
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Mineral Deposits and Earth Evolution
Mineral deposits are not only primary sources of wealth generation, but also act as windows through which to view the evolution and interrelationships of the Earth system.
Deposits formed throughout the last 3.8 billion years of the Earth’s history preserve key evidence with which to test fundamental questions about the evolution of the Earth. These include: the nature of early magmatic and tectonic processes, supercontinent reconstructions, the state of the atmosphere and hydrosphere with time, and the emergence and development of life. The interlinking processes that form mineral deposits have always sat at the heart of the Earth system and the potential for using deposits as tools to understand that evolving system over geological time is increasingly recognized. This volume contains research aimed both at understanding the origins of mineral deposits and at using mineral deposits as tools to explore different long-term Earth processes.