Jeremy P. Richards

Postsubduction porphyry Cu-Au and epithermal Au deposits: Products of remelting of subduction-modified lithosphere

Porphyry Cu ± Mo ± Au and some epithermal Au deposits are formed from hydrothermal fluids exsolved from cooling, water-rich, calc-alkaline magmas emplaced in volcanoplutonic arcs above subduction zones. These magmas originate by partial melting of the metasomatized asthenospheric mantle wedge. However, there is increasing evidence for the existence of a suite of porphyry Cu-Au and epithermal Au deposits related to magmas generated after subduction beneath the arc has ceased. Associated magmas tend to be mildly alkaline, relatively sulfur poor, and emplaced as isolated complexes rather than in voluminous volcanoplutonic arcs. They are likely formed by remelting of previously subduction-modified arc lithosphere, triggered by postsubduction lithospheric thickening, lithospheric extension, or mantle lithosphere delamination. Metasomatized mantle lithosphere or hydrous lower crustal cumulates residual from first-stage arc magmatism contain small amounts of chalcophile and siderophile element–rich sulfides, and constitute a fertile source for hydrous, Au-rich, but relatively sulfur-poor magmas during later remelting. The recognition that porphyry Cu-Au and related epithermal Au systems can also form in postsubduction and collisional tectonic settings expands the range of geological environments and geographical terranes that are prospective for such deposits.

Crustal lineament control on magmatism and mineralization in northwestern Argentina: geological, geophysical, and remote sensing evidence

Abstract The relationship between long-lived deep crustal lineaments and the locations of magmatic centers and associated mineral deposits has been investigated in the Puna region of northwestern Argentina, through the analysis of regional aeromagnetic surveys, Landsat images, and geological information. The good exposure and excellent preservation of basement and supracrustal geology in this region makes it particularly suitable for such a study. At a regional scale, several contrasting magnetic domains are recognized, which correlate with crustal geology. Two basement domains are separated by a NNE-trending boundary, which is believed to correlate with a Paleozoic suture zone between the Pampia (to the southeast) and Arequipa–Antofalla terranes (to the northwest). Locally overlying these basement terranes is the Cenozoic magmatic domain, which is best developed in the N–S-trending volcanic arc at the western edge of the Puna (the Cordillera Occidental). In addition, four southeast-trending volcanic zones extend for several hundred kilometers across the Puna. Many important mineral deposits and areas of hydrothermal alteration are associated with these volcanic breakouts, and we have selected three such areas for more detailed study: Bajo de la Alumbrera (Argentinas largest porphyry copper deposit), Cerro Galan (the largest ignimbrite caldera in Argentina, with associated hydrothermal alteration zones), and El Queva (a historic polymetallic district located within a major volcanic range). A comparison of lineament maps generated from aeromagnetic and Landsat TM images reveals broad correlation between these different remote sensing techniques, which respectively highlight subsurface magnetic and surface geological features. In addition, the locations of magmatic and hydrothermal centers can be related to the interpreted structural framework, and are seen to occur near the intersections of major lineament zones. It is suggested that in three dimensions, such intersection zones form trans-lithospheric columns of low strength and high permeability during transpressional or transtensional tectonic strain, and may thereby serve as conduits for magma ascent to the shallow crust. Pooling of large volumes of deeply derived magma in shallow crustal magma chambers may then result in voluminous devolatilization and the formation of hydrothermal mineral deposits. It is important to note that in this model, structural intersections serve as facilitators for magma ascent and volatile exsolution, but do not in themselves cause this process—other factors such as magma supply rate and tectonic stress are essential primary ingredients, and local magmatic and volcanic processes affect the ultimate potential for ore formation. Nevertheless, we suggest that lineament analysis provides a valuable framework for guiding the early stages of mineral exploration; other regional and local geological considerations must then be applied to identify priority targets within this framework.

A rapid method for REE and trace-element analysis using laser sampling ICP-MS on direct fusion whole-rock glasses

Abstract A new method of producing glass beads from whole-rock powders has been tested for use in laser sampling-inductively coupled plasma-mass spectrometry (LS-ICP-MS). Glass beads are produced in a tungsten strip heater cell, under an Ar atmosphere of 275 kPa to suppress loss of volatile elements such as Li, B, K and Na, and to minimize oxidation. A suite of whole-rock powders comprising six international standard reference materials, SY 2 , SY 3 , MRG- 1 , BIR- 1 , G- 2 and RGM- 1 , was selected for this test, to provide a wide range of major- and trace-element compositions. Electron microprobe major-element analyses and backscattered electron imaging were used to evaluate homogeneity of the beads. Results for all the international standards were within ± 15% of recommended values, and most elements are within ± 5%. Trace-element analyses were also carried out using nebulizer mode ICP-MS, to help in defining the optimum conditions for producing glasses. Mounts of glass beads were then analyzed on a Perkin Elmer Sciex® Elan 5000 Laser Sampling ICP-MS for 43 major, minor and trace elements, including the rare-earth elements (REEs). The Nd:YAG laser was operated in Q -switched mode, with a 240-ms delay, at 144 mJ, and 1.5-min laser ablation, of which the last minute was counting time on ICP-MS. Analysis of the reference materials as unknowns, using NBS- 612 glass as a standard for calibration and Al as an internal standard yielded 25 elements within ± 20% of recommended values, and precision of between ± 5% and ± 15% (relative standard deviation, RSD). This group of elements includes Ca, Co, Sr, Y, Cs, Ba, all 14 REEs, and Zr, Nb, Hf, Ta, Th and U. The latter group are difficult to analyze using other methods, and may be unstable in solution. Detection limits for the suite range from 0.02 to 0.3 ppm. Ablation efficiencies for the glasses were assessed by measuring the mass of material ablated, and it was found that the REE content, as well as Fe 2+ content, may strongly affect the absorption of Nd:YAG radiation, and thus the ablation efficiency of a material. This study documents a low contamination method of creating acceptably homogeneous glasses for solid state analyses, and may have wider application in the analysis of highly refractory materials as glasses, when produced with minimum quantities of fluxes.

Intraplate-type magmatism in a continent-island-arc collision zone: Porgera intrusive complex, Papua New Guinea

Cogenetic volatile-rich alkali basalts and gabbros, hawaiites, and mugearites occur in the late Miocene age epizonal Porgera intrusive complex, which is temporally and spatially associated with rich gold-silver mineralization. The least evolved rocks show enrichments in light rare earth elements ((La/Yb){sub cn} = 15-19) and other incompatible elements (e.g., Ba/La {approx} 8-10, La/Nb {approx} 0.6-0.7, Sr/Nd {approx} 25) characteristic of intraplate alkalic basalts and have isotopic compositions ({sup 87}Sr/{sup 86}Sr {approx} 0.7035, {epsilon}{sub Nd} {approx} +6, {sup 206}Pb/{sup 204}Pb {approx} 18.66, {sup 207}Pb/{sup 204}Pb {approx} 15.56, {sup 298}Pb/{sup 204}Pb {approx} 38.55) consistent with derivation from a time-averaged depleted mantle source. The Porgera intrusive complex was emplaced at 6 Ma in Jurassic-Cretaceous shelf-facies sedimentary rocks near the edge of the Australasian plate. Intrusion appears to have occurred in a back-arc environment during subduction of an oceanic microplate segment on two sides, beneath the continental margin and an island arc. We suggest that this unusual tectonic setting promoted partial melting of asthenospheric source materials that were perhaps modified by deep (>150 km) subduction processes to form alkalic magmas with intraplate character.

Magmatic-hydrothermal processes within an evolving Earth: Iron oxide-copper-gold and porphyry Cu ± Mo ± Au deposits

Iron oxide-copper-gold (IOCG) deposits formed by magmatic-hydrothermal fluids (MH-IOCG) share many similarities with, but have important differences from, porphyry Cu ± Mo ± Au (porphyry) deposits: MH-IOCG deposits predominantly occur in Precambrian rocks, are Fe oxide rich, and have volumetrically extensive high-temperature alteration zones, whereas porphyry deposits occur almost exclusively in Phanerozoic rocks, are Fe sulfide rich, and have narrower high-temperature alteration zones. We propose that these deposit types are linked by common subduction-modified magmatic sources, but that secular changes in oceanic sulfate content and geothermal gradients at the end of the Precambrian caused a transition from the predominance of S-poor arc magmas and associated S-poor MH-IOCG systems, to S-rich arc magmas and associated S-rich porphyry deposits in the Phanerozoic. Phanerozoic MH-IOCG and rare Precambrian porphyry deposits are explained by local or periodic fluctuations in oceanic oxidation state and sulfate content, or remobilization of previously subduction-modified lithosphere in post-subduction tectonic settings.

Sources of metals in the Porgera gold deposit, Papua New Guinea: Evidence from alteration, isotope, and noble metal geochemistry

The Porgera gold deposit is spatially and temporally associated with the Late Miocene, mafic, alkalic, epizonal Porgera Intrusive Complex (PIC), located in the highlands of Papua New Guinea (PNG). The highlands region marks the site of a Tertiary age continent-island-arc collision zone, located on the northeastern edge of the Australasian craton. The PIC was emplaced within continental crust near the Lagaip Fault Zone, which represents an Oligocene suture between the craton and volcano-sedimentary rocks of the Sepik terrane. Magmatism at Porgera probably occurred in response to the Late Miocene elimination of an oceanic microplate, and subsequent Early Pliocene collision between the craton margin and an arc system located on the Bismarck Sea plate. Gold mineralization occurred within 1 Ma of the time of magmatism. Metasomatism accompanying early disseminated Au mineralization in igneous host rocks resulted in additions of K, Rb, Mn, S, and CO2, and depletions of Fe, Mg, Ca, Na, Ba, and Sr; rare-earth and high-field-strength elements remained largely immobile. Pervasive development of illite-K-feldspar-quartz-carbonate alteration assemblages suggests alteration by mildly acidic, 200 to 350°C fluids, at high water/ rock ratios. Strontium and lead isotopic compositions of minerals from early base-metal sulphide veins associated with K-metasomatism, and later quartz-roscoelite veins carrying abundant free gold and tellurides, are remarkably uniform (e.g., 87Sr86Sr = 0.70745 ± 0.00044 [n = 10], 207Pb204Pb = 15.603 ± 0.004 [n = 15]). These compositions fall between those of unaltered igneous and sedimentary host rocks, and specifically sedimentary rocks from the Jurassic Om Formation which underlies the deposit (igneous rocks: 87Sr86Sr ≈ 0.7035, 207Pb204Pb ≈ 15.560; Om Formation: 87Sr86Sr |t~ 0.7153, 207Pb204Pb ≈ 15.636). It is therefore suggested that the hydrothermal fluids acquired their Sr and Pb isotopic signatures by interaction with, or direct derivation from, a plutonic root of the PIC and host sedimentary rocks of the Om Formation. It is likely that Au was also derived from one or both of these two sources. Concentrations of Au in unaltered igneous and sedimentary rocks from Porgera (≤10 ppb Au) do not indicate that either lithology represents a significantly enriched protore, although Au and platinum-group element (PGE) abundances in the igneous rocks suggest a mild primary magmatic enrichment of Au relative to the PGE (average [Au/(Pt + Pd)]mantlenormalized = 14.0 ± 6.5 [n = 8]). Evidence that the Porgera magmas were rich in volatiles permits speculation that Au may have been concentrated in a magmatic fluid phase, but alternative possibilities such as derivation of Au by hydrothermal leaching of solidified igneous materials or sedimentary rocks cannot be excluded at this time.

Petrology and geochemistry of alkalic intrusives at the Porgera gold deposit, Papua New Guinea

Abstract The geochemistry of the Middle to Late Miocene mafic intrusive complex at Porgera in the Papua New Guinea Highlands, has been investigated because of its spatial association and possible metallogenic relationship to a major gold deposit. Whole-rock major and trace-element data, combined with electron microprobe analyses of mineral phases, indicate classification of the intrusives as a volatile-rich, alkali basalt→hawaiite →mugearite fractionation suite. Enrichments in incompatible elements including Nb (up to 82 ppm) and the LREE (La/Lu)cn = 13−15) are consistent with an intra-plate, alkaline affinity for the parental magma, and derivation from an enriched garnet lherzolite source in the subcontinental lithosphere. Partial melting probably occurred in response to Middle Miocene uplift of the edge of the Australian craton during collision with an island-arc system, while associated trans-lithospheric faulting (e.g., the Lagaip Fault Zone) may have provided conduits for the ascent of magmas. The numerous small stocks and dykes which make up the intrusive complex were emplaced at high crustal levels, and solidified rapidly with little in situ fractionation. They appear to have been ejected at different times from a large, buried magma chamber, after variable degrees of fractionation of the parental magma. Most of the exposed intrusions are porphyritic: olivine + clinopyroxene ± hornblende phenocrysts occur in alkali basalts/gabbros, whereas hornblende + clinopyroxene + plagioclase occur in more evolved hawaiites and mugearites; apatite and spinel (Cr-rich to Ti-rich magnetite) occur as accessory phenocrysts in most samples. High Fe3+/Fe2+ ratios in whole rocks (average=0.8) and minerals suggest crystallization under high fO2 conditions. Miarolitic cavities and vesicles are present in most samples, and suggest that a volatile phase was exsolved from the magma during or after high-level emplacement. Compositions of interstitial and miarolitic minerals indicate late-stage enrichments of Fe, Ti and alkalis in residual melts, and evolution of a moderately oxidized, NaCl-bearing aqueous fluid. The mafic, alkaline nature and high fO2 of the magmas, combined with evidence for extensive fractionation and the evolution of a volatile phase, support theories of a relationship between magmatism and the introduction of gold at Porgera. Parts of the deposit show similarities to other gold occurrences associated with alkaline magmatism (e.g., the Emperor mine, Fiji, and Cripple Creek, Colorado). However, the close association of gold mineralization with structural features rather than specific igneous intrusions still permits the tenure of models involving remobilization of gold by nonmagmatic hydrothermal fluids, or the introduction of gold by deeply circulating fluids ascending along the same faults which controlled emplacement of the alkaline magmas.

Characteristics of late Cenozoic volcanism along the Archibarca lineament from Cerro Llullaillaco to Corrida de Cori, northwest Argentina

Abstract The Archibarca lineament is one of several NW–SE-trending transverse lineaments that cut across the Central Andes of Argentina and Chile. Central Andean, Late Miocene–Quaternary subduction-related volcanism is mainly restricted to a ∼50-km-wide arc forming the approximately N–S axis of the Cordillera, but extends along the transverse lineaments for up to 200 km to the SE. Lineaments are interpreted to be deep-seated, long-lived basement structures or anisotropies, which can control the localization of magmatism and, in some cases, magmatic–hydrothermal ore deposits (e.g., the Escondida porphyry Cu deposit, Chile). As a first step towards exploring the regional-scale controls on magmatism and related mineralization exerted by such structures, the styles of volcanism and near-surface hydrothermal activity along a segment of the Archibarca lineament in the Puna of northwest Argentina are described here. Volcanic structures have been mapped and sampled along a 50-km transect from Cerro Llullaillaco, a large medium-K dacitic Quaternary stratovolcano, to Corrida de Cori, a range of Pliocene–Pleistocene high-K andesitic vents. Apart from a southeastward increase in K content and the predominance of dacitic lavas at Cerro Llullaillaco, the geochemical affinity of late Cenozoic volcanic rocks varies little in time or space. This uniformity extends further SE to Cerro Galan, where published data closely match the results from the study area. In detail, trace element compositions reveal the localized (in both time and space) effects of crustal contamination (recognized as Th>10 ppm), and depth of fractionation (1/Yb>0.7 ppm−1, reflecting garnet residue). Explosive volcanic rocks such as ignimbrites show the strongest indications of crustal contamination, whereas the Cerro Llullaillaco dacite lavas mostly record significant garnet fractionation. Other lavas from the Llullaillaco area, including one flow from Cerro Llullaillaco, do not show garnet control, suggesting that different batches of magma stalled and fractionated at different levels in a thick (∼60-km) crust prior to eruption. The youngest volcanism in the Corrida de Cori area is represented by olivine–phyric basaltic andesite cinder cones and flows. The ascent of these relatively primitive magmas appears to have been controlled by late Quaternary normal faults, which directly tapped deeply derived melts. The Corrida de Cori volcanic range has experienced intense fumarolic alteration with deposition of abundant sulfate and native sulfur (previously mined at Mina Julia). Deeper levels of hydrothermal alteration have been sampled by an ignimbrite erupted from Cerro Escorial, which, among other lithic clasts, contains numerous fragments of vein quartz. Fluid inclusions in this quartz record evidence for a boiling, high-salinity fluid, which may represent a link between a high temperature magmatic–hydrothermal system at depth (i.e., a porphyry-type system) and shallow-level fumarolic activity. An ignimbrite erupted from Cerro Escorial preserves textures such as internal wave forms between flow units and surface wave morphologies at its distal limits that indicate flow as a series of dense turbulent pulses, which interdigitated and interfered with one another. Lithic lag breccias occur near the base of the flow proximal to the vent, but no air-fall deposits are preserved, probably due to transport of ash far from the vent by strong, high-altitude winds.

Petrology and geochemistry of calc-alkaline volcanic and subvolcanic rocks, Dalli porphyry copper–gold deposit, Markazi Province, Iran

Early Miocene igneous rocks associated with the Dalli porphyry ore body are exposed within the Urumieh-Dokhtar Magmatic Arc (UDMA). The Dalli porphyry Cu–Au deposit is hosted by subduction-related subvolcanic plutons with chemical composition from diorite to granodiorite, which intruded andesitic and dacitic volcanic rocks and a variety of sedimentary sequences. 40Ar/39Ar age data indicate a minimum emplacement age of ∼21 million years for a potasically altered porphyritic diorite that hosts the porphyry system. The deposit has a proven reserve of 8 million tonnes of rock containing 0.75 g/t Au and 0.5% Cu. Chondrite-normalized rare earth element (REE) patterns for the subvolcanic rocks are characterized by light REE enrichments [(La/Sm) n  = 2.57–6.40] and flat to gently upward-sloping profiles from middle to heavy REEs [(Dy/Yb) n  = 0.99–2.78; (Gd/Yb) n  = 1.37–3.54], with no significant Eu anomalies. These characteristics are generated by the fractionation of amphibole and the suppression of plagioclase crystallization from hydrous calc-alkaline magmas. In normalized multi-element diagrams, all analysed rocks are characterized by enrichments in large ion lithophile elements and depletions in high field strength elements, and display typical features of subduction-related calc-alkaline magmas. We used igneous mineral compositions to constrain the conditions of crystallization and emplacement. Biotite compositions plot above the nickel–nickel oxide (NNO) buffer and close to oxygen fugacity values defined by the hematite–magnetite (HM) buffer, indicating oxidizing conditions during crystallization. Assuming a minimum crystallization temperature of 775°C, the oxygen (fO2) and water (fH2O) fugacities are estimated to be 10−10.3 bars (∼ΔNNO+4) and ≤748 bars, respectively, during the crystallization of biotite phenocrysts. The temperature and pressure conditions, estimated from temperature–corrected Al-in-hornblende barometry and amphibole-plagioclase thermometry, suggest that the hornblende phenocrysts in Dalli rocks crystallized at around 780 ± 20°C and 3.8 ± 0.4 kbar. An alternative method using the calcic amphibole thermobarometer indicates that the Dalli magmas were, on average, characterized by an H2O content of 4.3 wt.%, a relatively high oxygen fugacity of 10−11.0 bars (ΔNNO+1.3), and a hornblende phenocryst crystallization temperature of 880 ± 68°C and pressure of 2.6 ± 1.7 kbar.

Mining, Society and a Sustainable World

The Role of Mining in Developed and Developing Economies.- The Resource Curse: A Modern Myth?.- Extractive Economies, Growth, and the Poor.- The Challenge of Mineral Wealth: Using Resource Endowments to Foster Sustainable Development.- The Role of Mining in the Economies of Developing Countries: Time for a New Approach.- A Hierarchy of Natural Resources with Respect to Sustainable Development-A Basis for a Natural Resources Efficiency Indicator.- Mining and Sustainable Development.- Sustainable Energy and Mineral Resource Extraction and Consumption-Can a Viable Biosphere Be Preserved?.- Sustainable Development and Mining-An Exploratory Examination of the Roles of Government and Industry.- Natural Resource-Based Sustainable Development Using a Cluster Approach.- Mining and the Environment.- Improving Environmental Performance in the Minerals Supply Chain Using a Life-Cycle Approach: The Role of Fuel and Lubricant Suppliers in Enabling Sustainable Development.- Global Trends in Mine Reclamation and Closure Regulation.- Environmental Liability in the Mining Sector: Prospects for Sustainable Development in the Democratic Republic of the Congo.- Mining and Societal Issues.- Applications of Stakeholder Engagement and Eco-Efficiency as Enablers of Corporate Responsibility in the Australian Mining and Minerals Processing Industry.- Socio-Economic Impacts of the Nanisivik and Polaris Mines, Nunavut, Canada.- Assessing the Socio-Economic Impacts of Mining: Case Study of the Landau Colliery, South Africa.- Public Policy Processes and Sustainability in the Minerals and Energy Industries.- The Extractive Industries Transparency Initiative: Panacea or White Elephant for Sub-Saharan Africa?.