The Mineral System Concept: The Key to Exploration Targeting
T. Campbell McCuaig, Jon M. A. Hronsky, 2014. "The Mineral System Concept: The Key to Exploration Targeting", Building Exploration Capability for the 21st Century, Karen D. Kelley, Howard C. Golden
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To aid conceptual targeting, the past two decades have seen the emergence of the mineral systems concept, whereby ore deposits are viewed as small-scale expressions of a range of earth processes that take place at different temporal and spatial scales. The mineral systems approach has been spurred by three main drivers: the recognition of patterns of mineralization in increasingly available large geoscience datasets; advances in geographic information system (GIS) technologies to spatially query these datasets; and marked advances in understanding the evolution of earth systems and geodynamics that provide context for mineralization patterns. An understanding of mineral systems and the scaledependent processes that form them is important for guiding exploration strategies and further research efforts.
Giant ore deposits are zones of focused mass and energy flux. Advances in understanding of the physics of complex systems—self organized critical systems—leads to a new understanding of how fluid flow is organized in the crust and how high-quality orebodies are formed. Key elements for exploration targeting include understanding and mapping threshold barriers to fluid flow that form extreme pressure gradients, and mapping the transient exit pathways in which orebodies form.
It is proposed that all mineral systems comprise four critical elements that must combine in nested scales in space and time. These include whole lithosphere architecture, transient favorable geodynamics, fertility, and preservation of the primary depositional zone. Giant mineral deposits have an association with large, longlived deeply penetrating and steeply dipping structures that commonly juxtapose distinctly different basement domains. These structures are vertically accretive in nature, often having limited or subtle expressions at or above the level of ore deposition.
Three transient geodynamic scenarios are recognized that are common to many mineral systems: anomalous compression, initial stages of extension, and switches in the prevailing far-field stress. In each of these scenarios, “threshold barriers” are established which produce extreme energy and fluid/magma pressure gradients that trigger self-organized critical behavior and ore formation.
Fertility is defined as the tendency for a particular geologic region or time period to be better endowed than otherwise equivalent geologic regions. Fertility comprises four major components: secular Earth evolution (variations in the Earth’s atmosphere-hydrosphere-biosphere-lithosphere through geologic history that result in formation of deposits), lithospheric enrichment, geodynamic context, and paleolatitude (in specific mineral systems).
The primary depositional zone is usually within the upper 10 km of the Earth’s surface, where large P-T-X gradients can be established over short distances and time scales. The variable preservation of this zone through subsequent orogeny explains the secular distribution of many ore deposit types.
The mineral system approach has advantages in exploration targeting compared to approaches that use deposit models. Emphasizing common ore-forming processes, it links many large ore systems (e.g., VMS-epithermal, porphyry-orogenic gold) that are currently considered disparate deposit models and relates these ore systems in a predictable way to their large-scale geodynamic context. Moreover, it focuses mineral exploration strategies on incorporating primary datasets that can map the critical elements of mineral systems at a variety of scales, and particularly the regional to camp scales needed to make exploration decisions.
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Building Exploration Capability for the 21st Century
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).