A Giant Mesoarchean Crustal Gold-Enrichment Episode: Possible Causes and Consequences for Exploration
Hartwig E. Frimmel, 2014. "A Giant Mesoarchean Crustal Gold-Enrichment Episode: Possible Causes and Consequences for Exploration", Building Exploration Capability for the 21st Century, Karen D. Kelley, Howard C. Golden
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Comparison of conglomerate-hosted, Witwatersrand-type gold deposits and/or occurrences worldwide reveals that this deposit type is by no means unique to the Kaapvaal craton but common to most Archean and/or Paleoproterozoic cratons. The age of the variably mineralized fluvial to fluvio-deltaic conglomerates ranges from 3.1 to 1.9 Ga. They were deposited in tectonic settings ranging from continental rifts to passive margins and synorogenic foreland basins, and all of them are paleoplacers. Although several of them show evidence of local mobilization of ore components by postdepositional hydrothermal fluids, purely epigenetic hydrothermal models fail to explain the geometry of the orebodies as well as available lithogeochemical, mineral chemical, and isotope data. Conglomerates older than 2.4 Ga are characterized by an abundance of detrital (and secondary) pyrite, and in most cases also detrital uraninite, whereas most of the younger examples (<2.2 Ga) contain Fe oxides instead. A common denominator of Witwatersrand-type deposits is the stratigraphic position above erosional unconformities adjacent to an Archean to Paleoproterozoic hinterland. The Witwatersrand deposits themselves differ from all other examples of this type by a gold endowment that is two to three orders of magnitude greater, an abundance of gold-rich “carbon” seams that reflect former microbial mats, a scarcity of gold nuggets, and orders of magnitude higher Os contents in the gold.
For the Witwatersrand gold, a genetic model is proposed that involves the following requirements: (1) an anomalous mantle domain as the ultimate source, strongly enriched in siderophile elements, caused by inhomogeneous mixing with cosmic material that was added during intense meteorite bombardment of the Hadean to Paleoarchean Earth, plume-like ascent of relics from inefficient core formation, or plumes from the core-mantle boundary; (2) elevated gold extraction into juvenile crust when mantle temperature reached its maximum in the Mesoarchean; (3) several orders of magnitude higher run-off of gold from the Mesoarchean land surface due to intense weathering under an aggressive, reducing atmosphere and high gold solubility in coeval river water; (4) trapping of gold from river water on the surface of local photosynthesizing microbial (cyanobacterial) mats; and (5) reworking of these mats into erosion channels during flooding events (and by eolian deflation) and redeposition of gold as placer particles. Postdepositional hydrothermal and/or metamorphic overprints explain why much of the gold is now located in texturally late positions but had little significance on the macroscale distribution of the gold. Elsewhere in the world, a less fertile hinterland and/or less reworking of older sediments led to correspondingly lower gold endowment. Most of the Archean sedimentary rocks were affected by crustal reworking in the course of later tectonic overprints. The multitude of fluids and melts involved in these reworking processes gave rise to the great variety of gold deposit types known in post-Archean crustal sections.
The probability of discovering a new supergiant cluster of Witwatersrand-type deposits is considered very low. However, considerable potential exists for finding new smaller economic deposits of this type in Mesoarchean to Paleoproterozoic fluvial to fluviodeltaic basal conglomerates, deposited especially in foreland basins next to Mesoarchean hinterland and/or auriferous sediment successions that could be reworked.
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Earth’s near-surface mineralogy has diversified over more than 4.5 b.y. from no more than a dozen preplanetary refractory mineral species (what have been referred to as “ur-minerals” by Hazen et al., 2008) to ~5,000 species (based on the list of minerals approved by the International Mineralogical Association; http://rruff.info/ima). This dramatic diversification is a consequence of three principal physical, chemical, and biological processes: (1) element selection and concentration (primarily through planetary differentiation and fluidrock interactions); (2) an expanded range of mineral-forming environments (including temperature, pressure, redox, and activities of volatile species); and (3) the influence of the biosphere. Earth’s history can be divided into three eras and ten stages of “mineral evolution” (Table 1; Hazen et al., 2008), each of which has seen significant changes in the planet’s near-surface mineralogy, including increases in the number of mineral species; shifts in the distribution of those species; systematic changes in major, minor, and trace element and isotopic compositions of minerals; and the appearance of new mineral grain sizes, textures, and/or morphologies. Initial treatments of mineral evolution, first in Russia (e.g., Zhabin, 1979; Yushkin, 1982) and subsequently in greater detail by our group (Hazen et al., 2008, 2009, 2011, 2013a, b; Hazen and Ferry, 2010; Hazen, 2013), focused on key events in Earth history. The 10 stages we suggested are Earth’s accretion and differentiation (stages 1, 2, and 3), petrologic innovations (e.g., the stage 4 initiation of granite magmatism), modes of tectonism (stage 5 and the commencement of plate tectonics), biological transitions (origins of life, oxygenic photosynthesis, and the terrestrial biosphere in stages 6, 7, and 10, respectively), and associated environmental changes in oceans and atmosphere (stage 8 “intermediate ocean” and stage 9 “snowball/hothouse Earth” episodes). These 10 stages of mineral evolution provide a useful conceptual framework for considering Earth’s changing mineralogy through time, and episodes of metallization are often associated with specific stages of mineral evolution (Table 1). For example, the formation of complex pegmatites with Be, Li, Cs, and Sn mineralization could not have occurred prior to stage 4 granitization. Similarly, the appearance of large-scale volcanogenic sulfide deposits may postdate the initiation of modern-style subduction (stage 5). The origins and evolution of life also played central roles; for example, redox-mediated ore deposits of elements such as U, Mo, and Cu occurred only after the Great Oxidation Event (stage 7), and major Hg deposition is associated with the rise of the terrestrial biosphere (stage 10; Hazen et al., 2012).