Deposit Footprints and Mineral System Science
As mineral exploration moves into regions dominated by transported cover, conventional techniques (e.g., lag gravel) may not be applicable and thus, increasingly, there is a need for new, innovative approaches. To develop these approaches, potential mechanisms that transfer metals from buried mineral deposits through cover to the surface need to be identified. This paper presents an overview of some of the experimental and field trials conducted in Australia as part of an industry-supported three-year CSIRO/AMIRA project. The objective was to define vadose zone processes that might form elemental anomalies at surface over buried deposits in semiarid and arid terrains, and to compare methods that detect these anomalies. Studies were conducted at seven sites representing orogenic Au, volcanogenic massive sulfide (VMS; Cu-Zn-Ag), and magmatic Ni mineralization with transported cover ranging in thickness from 2 to 30 m. Three vertical metal migration mechanisms are important in vadose environments: (1) biological, (2) gaseous, and (3) capillary. An integrated approach, combining different mechanisms with the nature and evolution of transported regolith and climatic settings, was considered to obtain the best prediction of metal transfer. Upward element transfer by vegetation (Acacia aneura and Eucalyptus spp.) occurs in areas of transported cover up to 30 m thick, but not in environments which lack supergene enrichment and have hypersaline acid groundwater. Microbial populations are different in soil over mineral deposits than in those from background sites. Metals, detected by gas collectors, are transferred to surface as gases. Soil pit experiments show that strong geochemical anomalies can form rapidly (over 7 months) through 2 m of transported cover, and assist in understanding the genesis of natural geochemical anomalies. Seasonal variations suggest that migration of elements from source to surface may vary in time and intensity. Anomaly formation in the pit experiments is an episodic process largely driven by capillarity, in which batches of metals in water-soluble form are translocated. Soil-forming processes may form false anomalies and the data need to be interpreted with care.
Figures & Tables
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).