Innovations in Exploration Technology and Exploration Targeting
Muon geotomography, a novel geophysical exploration and imaging technology, uses cosmic rays to create three-dimensional (3-D) images of subsurface density distributions. The first controlled field test confirming the capability of muon geotomography for imaging a dense orebody in a complex geologic environment was conducted at the Price volcanic-hosted massive sulfide (VHMS) deposit, Vancouver Island, British Columbia, Canada. The semimassive and massive polymetallic mineralization of the Price deposit is situated in a Paleozoic stratigraphic package of rocks known as the Sicker Group including the Price, Myra, Thelwood, and Flower Ridge Formations, indicative of volcanic rocks formed in a rifted oceanic island-arc system. The field application involved placing a sensor with an active area of 1 m2 beneath the massive sulfide orebody in an underground tunnel for exposures of about two weeks at several locations. Muon flux data were inverted to recover a 3-D density image of the deposit. The inverted data were in good agreement with drill core data. However, some distortions of the image were observed due to the limitations imposed by the available tunnel which restricted the angular views available to the sensors. Muon geotomography works best when sensors are placed such that they can view the region under study from a range of different angles. The demonstrated ability to perform accurate forward model simulations makes the sensitivity of the technique predictable for specific survey situations. The results demonstrate the potential of muon geotomography for identification and characterization of orebodies located in complex geologic environments. Three-dimensional images from muon geotomography surveys may be used to guide drilling operations toward regions of high-density contrast, thereby significantly reducing costs and environmental impact associated with locating orebodies.
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).