Fractionation and the Magma-Metal Series Approach

The technique that has led to our successful contributions in resource discoveries arose through the employment of the magma-metal series classification. The magma-metal series classification presents a logical description of a geologic process known as fractional differentiation. Fractional differentiation of chemical mass occurs from galactic and earth macro-scales to molecular and atomic nano- and phantom- scales respectively. Fractional differentiation of gas, liquid, and solid state chemical mass has been occurring since the beginning of time at the point source of the BIG BANG currently estimated to have happened about 13 to 14 billion years ago. Earth scale fractional differentiation of earth scale compositional shells began about 4.56 Ga during gravitational consolidation of a hot cosmic dust in a ring about ??? from a solar mass referred to by subsequent human biological fractionates as the ‘sun’.

A current compositionally zoned earth model based on about 21 years of developing the magma- metal series model observational framework is shown in Figure 2. The outermost layer (from about 100 to 1000 km) is broadly referred to as the asthenosphere. The Asthenosphere can be more finely divided into about eight chemically distinct layers that are the ultimate source of many of the magma-metal series that are associated with various kinds of metal deposits. Indeed, it was the metal composition of a multitude of magma-related metal deposits that led to the identification of the various chemically distinct magma types and their inferred sources in the asthenosphere mantle, more rigid lithosphere shell above the asthenosphere, and thin, brittle oceanic and continental crusts which comprise the outermost fractionally differentiated ring immediately beneath the earth’s surface.

In conformance with the universe and earth scale differentiation patterns, terrestrial mineral deposits represent solid-state concentrated element mass precipitated from higher energy, liquid- state, hydrothermal fluids which in turn were fractionated or unmixed from higher energy magmatic sources derived from higher energy source region in the layered earth model described above. The metallic component of these mineral deposits is typically the most sought after in terms of human utilization. In a similar “vein”, petroleum resources represent liquid-state fractionates unmixed from higher energy supercritical state hydrothermal waters produced during hydrous metamorphism of hydrocarbon-stable peridotite sources in the earth’s mantle (see hydrothermal hydrocarbon paper in Appendix 4). The process of energy and material transfer from the inner to outer planet thus takes places through the agency of magmatic, hydrothermal, differentiates.

As these volatile differentiates migrate from their higher energy sources at depth to lower pressure emplacement sites in the upper earth system, the differentiates progressively lose energy during various depressurizations. The geography of mis-chemical fractionation leaves a chemical trail that can ultimately be traced from where the volatile plumes exit into more dispersed chemical environments at the lithosphere/hydrosphere and the lithosphere/atmosphere interface to their ultimate sources in the deep crust or underlying mantle. The chemical trail exhibits a sequence of chemical changes along its path. If that sequence involves magma, it is referred to as a magma-metal series (based on the metal composition of volatile differentiation products). Interaction of a magma-metal series with volatile products in the crust during its ascent produces subtle changes in the composition of primary magmatic sources and leads to a number of variations in the final products of differentiation. In this way a considerable number of magma- metal series have been formed. MagmaChem has systematically cataloged these various magma- metal series over a twenty-five year period (see below).

The recent recognition of crustal scale hydro-carbon-stable hydrothermal volatile systems has added an important new mechanism in addition to magma that can transfer energy from the inner to the outer planet. The empirical calibration of these various hydrothermal hydrocarbon plumes represents the ‘cutting edge’ of the ‘research’ component at MagmaChem. Particularly intriguing is the use of metal chemistry in these systems to identify ultimate peridotite sources at the earth scale and to design drill targets for hydrocarbon on a reservoir scale.

The magma-metal series was originally discovered on an intuitive basis in 1976 and has continuously expanded and refined in the context of a mineral exploration consultancy named MagmaChem from 1983 to the present. The point of application of the MagmaChem consultancy was resource exploration and discovery as discussed previously. The more refined whole-earth model which progressively arose from its application was and is a matter of academic discussion and debate. That debate is not the point of this narrative which rather, is to prove the earth model through what now appears to be a much more reduced risk and time efficient process of resource exploration and discovery. In addition to metals the approach now also has incorporated petroleum geology (see Hydrothermal Hydrocarbons paper).

The magma-metal series classification is comprised of a seven-level bivariate logical system that subdivides igneous rocks into some 194 magma-metal series (see Figure 3). Economically favorable magma-metal series models are relatively few (91 out of 442 rock system models identified as of March 2002).

Many metal occurrences associated with unfavorable, high risk series have been prospected by the client base without success. Even more commonly, certain metals like copper have been prospected in occurrences fundamentally biased for other metals (especially those with lead, zinc, and silver). A more detailed account of this metallogenic natural history is provided in an accompanying power point (see MagmaChem for Investors PowerPoint). In the above context it is important to point out that all of the resource occurrences on both contributions lists (see Successful Contributions List and Untested or Partially Tested Discovery List) have been filtered and have been assigned with mostly ‘probable to definite certainty’ to models that have giant- sized pedigree for a given metal. These properties include BIG BAR’s Troy Ranch and Yuma King properties.

Once a magma fractionally differentiates from its source region, it travels along a fracture system from the high-pressure high energy source region to the low-pressure lower energy trap site in the earth’s upper crust (see Figure 4). The type of crustal volatiles the magma encounters, and the size and volatile content of the conduit system exert strong controls on specific compositional serial characteristics (especially the oxidation state of any assimilated supercritical state crustal volatiles) and ultimate size of the hydrothermal fluid releases in the upper crust.

Where a gabbro magma acquired copious amounts of crustal water, the magma series will crystallize a mineral named hornblende which requires at least four weight percent water to be stable. Hornblende stable magma series are invariably associated with many kinds of productive porphyry metal systems and are present in most of the MagmaChem discovery contributions. Magmas associated with the Troy Ranch and Yuma King properties acquired enough oxidized volatiles to trigger large releases of copper-bearing fluids upon hornblende-stable crystallization of the Stage 2.5 granodiorite (Troy Ranch) and the Stage 3 monzonite system (Yuma King) in the upper crust.

During the diffusive assimilation of supercritical water the oxidation state of a magma is commonly adjusted to and equilibrated with that of the surrounding crust. This oxidative equilibrium is achieved through a process of diffusive hydrogen and electron exchange between the incoming mafic mantle-derived magma and the surrounding crustal wallrocks. Most commonly the exchange is an oxidative one whereby hydrogen and electrons are transferred from the more reduced electron-rich mafic magma to a more oxidized electron-poor wallrock environment.

Also, during the implosion and diffusion of supercritical crustal volatiles (mainly water) into the gabbro magma, a partial isotopic equilibration also takes place. The imploded crustal volatiles also contain isotopic compositions (for example, the Rb-Sr, Sm-Nd, and U-Pb isotope systems) that reflect what is typically a more radiogenic daughter isotope enriched crustal lithologic environment. Depending on the solubilities of the isotopes into the supercritical crustal water, and their radiogenic generation rates in the crustal wallrock, the crustal isotope signature is variably incorporated into the magma. In terms of the original mantle signature in a given rock, the Rb-Sr system generally displays about a 8 to 15% crustal component, the Sm-Nd about 35 to 55% crustal signature, whereas the U-Pb isotopic system shows an 80 to 90% crustal component. The strong incorporation of a radiogenic Pb crustal component strongly impacts magmas that travel through a water-bearing potassium and uranium-rich crust where radiogenic Pb is generated in exponentially higher quantities compared to its generation in uranium-poor mantle sources regions. The high solubility of Pb in aqueous media compared to other elements like rubidium ultimately allows the incorporation of a much larger lead component into the supercritical crustal water which ultimately is assimilated by the incoming mafic magma.

To a person unfamiliar with geology, the above narrative probably appears highly academic. However, the above data provides information that resolves a long standing paradox about magmatism versus crust as sources of metal. The result is a much more predictive and precise source-based exploration tool that distinguishes the magma-metal series approach from more traditional, more general, and ultimately, less specific and predictive approaches found in the literature. The paradox arises from a huge empirical database of magma chemistry (aluminum content, alkalinity, and oxidation state) that relates metal contents at specific metal deposit types (for example, gold and copper deposits) to spatially and temporally associated magma with specific magma petrochemistry. However, the isotopic data suggests a strong crustal contribution to the magma. In the case of magma ultimately derived from a mantle source, the question naturally arises, how much of the metal component reflects assimilation of a crustal metal source versus an original metal source in the mantle.

Mass balance of all the above paradoxical data leads to the conclusion that the mantle source (in the case of aluminum-poor mafic metaluminous magma differentiation sequences) is the source of at least 99% of the metal in a given metal deposit. Aluminum content and alkalinity determine overall metal affinity (for example, copper and gold in metaluminous calc-alkalic magma series) whereas oxidation state determines more specific metal affinities within a given magma-series (for example, gold is reduced, high-ferrous, calc-alkalic, metaluminous magma series versus oxidized, ferric, calc-alkalic metaluminous magma series).

The above conclusion that magmas are the ultimate source of hydrothermal fluid releases for various metals based on their very specific petrochemical characteristics has fundamental practical implications for exploration. Specifically, one does not focus on broadly defined, crustally-defined petrographic provinces where a given geographic cluster of metals may be present. Rather, one focuses on plutons of specific petrochemical affinity that tie to specific metal occurrences. In such a way, one isolates specific copper occurrences that may have giant- sized characteristics versus copper occurrences that are much less prospective. It is important to point out that the copper occurrences at Troy Ranch and Yuma King have been through the petrochemical screening process. The screening indicates that the copper occurrences at Troy Ranch and Yuma King have giant-sized prospectivity.

Once the mafic parent magma arrives in the upper crust it undergoes a pressure-driven fractional differentiation sequence as it travels through the conduit system in the upper crust. Depressurizations induce unmixing of the mafic parent magma into lower volume batches of increasingly less dense and more viscous felsic or granitic magmas. The result is typically a mafic to felsic fractional differentiation sequence composed typically (in the case of a mafic gabbro parent) of 4 to 5 batches of magma or rock systems. Those rock systems compositionally reflect the original chemistry of their mafic parent.

Details of hornblende-stable fractional differentiation sequences associated with the productive porphyry Cu(Mo-Ag) deposits are shown in Figure 5. Productive porphyry copper fluids are co- eval with Stage 3 biotite granodiorite porphyry rocks but are derived from copper enriched fluids fractionated from crystallization of Stage 2.5 hornblende-biotite granodiorite rock systems.

Copper occurrences associated with Stage 2 earlier rocks are not productive as are most copper occurrences associated with the arsenic-rich Stage 4 fluid release from Stage 3 biotite granodiorite quartz porphyry sources (see Figure 5). The above differentiation sequence forms the basis of pluton vectoring which allows one to vector geographically for the most commercial portion of a given porphyry metal sequence.

The process of fractionation does not stop with crystal-liquid fractionation in the pluton rock system. Hydrothermal differention continues the series with mineral assemblage deposition that accompanies fractionation of the hydrothermal fluids once they leave their pluton source. This phenomenon is known in the mineral exploration business as alteration, mineral, and/or metal zoning and is shown in graphical detail in Figure 6 which is an expansion of the Figure 5 narrative. As a fluid leaves its source, it deposits a suite of minerals at various sites of depressurization. The specific mineral assemblage deposits generally depend mainly on the initial composition of the fluid (most concentrated elements generally get deposited first). Geographic analysis of the geochemical patterns is referred to as a metal dispersion study.

As with the more district scale pluton vectoring, alteration/mineral/metal zoning can be used to identify mineral deposit scale drill targets. For example, in the favorable Morenci/Chuquicamata type fractional differentiation sequence, one prospects for chalcopyrite and/or bornite bearing sulfide assemblages associated with the transition from the potassic (k-feldspar and/or phlogopite) alteration zone and the quartz-sericite pyrite zone (see Figure 6). At Troy Ranch the drill holes will be aimed at biotitic alteration zones associated with chalcopyrite sulfide concentrations (analogous to the giant diabase orebody at the nearby Ray copper deposit).

The role of structure and tectonics in the emplacement of a productive magma-metal series cannot be understated. One of the revolutionary concepts employed in the MagmaChem exploration approach has been that the fractional differentiation pattern alluded to above occurs within a geologic setting that is actively being strained by a regional stress field at the time a given magma-metal series is being emplaced. This dynamic approach contrasts markedly with more classic approaches where the traps (both stratigraphic and structural [intersections between two faults or areas of high fracture density are favorites]) are passively filled up with metallizing solutions. The MagmaChem approach recognized that magma-metal series migrate from high- pressure high energy environments to low-energy/low pressure environments where depressurization at the trap site induces extensive depressurization where the main solute components of the hydrothermal brines are typically deposited first. The structural paths of fractional differentiation sequences for selected porphyry copper related magma-metal series are shown in Figure 7, the use of petroleum system terminology (such as ‘conduit’ and ‘trap’ is not accidental here. As it turns out, petroleum systems and metal systems are two aspects of the same process: fractionation and deposition from a hydrothermal medium.

To develop drill targets, it is necessary to constrain how the magma-metal fluids moved. In this sense, it is necessary to understand the three-dimensional nature of the conduit/trap system (using detailed mapping and high resolution geophysics like gravity or magnetics) and how the conduit system was moving during the fluid introduction (using detailed kinematic observations obtained during field mapping or analysis of high resolution geophysics). Utilization of such observations allows predictive and specific three-dimensional construction of the kinds and sizes of low-pressure traps that contain the appropriate chemistry predicted by pluton vectoring and metal dispersion studies. These more precise tools ultimately allow predictive drill hole scale targeting that has greatly improved discovery efficiency