Magma Pathways Research Articles

Dynamics of explosive volcanism at Fuego volcano imaged with very long period seismicity

[1] The current activity at Fuego volcano, Guatemala, is characterized by frequent explosions, intermittent lava flows, and persistent degassing. Seismic and infrasound data of explosive activity recorded over 19 days in January 2009 on a temporary broadband network show that explosive intensity is highly variable. However, the seismic expression of the largest explosions includes similar very long period (VLP) earthquakes for each explosion. Waveform modeling in the 10–30 s band constrains the source centroid to a position 300 m below and 300 m west of the summit crater. The calculated moment tensor indicates a volumetric source, which is modeled as a near-vertical dike feeding a SW dipping (35°) sill. The sill is the dominant component, and its projection to the surface nearly intersects the summit crater. The deformation history of the sill is interpreted as (1) an initial inflation that is due to pressurization, followed by (2) a rapid deflation as overpressure is explosively released, and finally (3) a reinflation as fresh magma flows into the sill and degasses. This cycle is attributed to the development of a brittle plug at the top of the magma column and effective sealing of degassing pathways that is due to degassing crystallization as water exsolves and the magma is undercooled. Data from apparent tilt, SO2 concentrations, and infrasound records support this interpretation. The VLP source location suggests the magma pathway may be migrating westward, which has hazard implications for populations living on the flanks of Fuego.

Microstructural evidence for magma confluence and reusage of magma pathways: implications for magma hybridization, Karakoram Shear Zone in NW India

In the Karakoram Shear Zone, Ladakh, NW India, Miocene leucogranitic dykes form an extensive, varied and complex network, linking an anatectic terrane exposed in the Pangong Range, with leucogranites of the Karakoram Batholith. Mineral paragenesis of the heterogeneous anatectic source rocks suggests melting has resulted from water influx into rocks at upper amphibolite facies conditions, and microstructures suggest anatexis was contemporaneous with shearing. The network is characterized by continuous and interconnected dykes, with only rare cross-cutting relationships, forming swarms and chaotic injection complexes where magmatic rocks cover up to 50% of the outcrop area. Despite this volume of magma, the system did not lose continuity, suggesting that it did not flow en masse and that the magma network was not all liquid simultaneously. Leucogranites in this network, including leucosomes in migmatites, carry an isotopic signature intermediate between the two main anatectic rocks in the source, suggesting efficient homogenization of the magmatic products. Here, we describe a number of microscopic features of these magmatic rocks which suggests that several pulses of magma used the same pathways giving rise to textural and chemical disequilibrium features. These include: (i) narrow, tortuous corridors of fine-grained minerals cutting across or lining the boundaries of larger grains, interpreted to be remnants of magma-filled cracks cutting across a pre-existing magmatic rock; (ii) corrosion of early formed grains at the contact with fine-grained material; (iii) compositional zoning of early formed plagioclase and K-feldspar grains and quartz overgrowths documented by cathodoluminescence imaging; (iv) incipient development of rapakivi and anti-rapakivi textures, and (iv) different crystallographic preferred orientation of early formed quartz and fine-grained quartz. Mapping of the fine-grained corridors interpreted to represent late melt channels reveal an interlinked network broadly following the S-C fabric defined by pre-existing magmatic grains. We conclude that early formed dykes provided a pathway exploited intermittently or continuously by new magma batches. New influxes of magma opened narrow channels and migrated through a microscopic network following predominantly grain boundaries along an S-C fabric related to syn-magmatic shearing. A mixed isotopic signature resulted not from the mixing of magmas, but from the micro-scale interaction between new magma batches and previously crystallized magmatic rocks, through local equilibration.

Setting the scene for self-destruction: From sheet intrusions to the structural evolution of rifted stratovolcanoes

We investigate the processes of growth and consumption of rifted stratovolcanoes to better understand their morphostructural and geological evolution in relation to sheet intrusions. Field data collected from four rifted volcanoes, located in Italy and Chile, and representing structural end members, are integrated with scaled physical models and numerical three-dimensional (3D) modeling. Due to preferential along-rift magma pathways, volcanoes grow perpendicular to rift strike mostly by intrusions, and parallel to the rift mostly by effusions. These constructive processes are partially or fully balanced by destructive processes that occur both perpendicular to the rift and along the rift zone, and that are initiated by the sheet intrusions. Field data indicate that sheets and/or flank eruptions increase in number approaching the summit part of a cone. Since dyking within a volcano causes a magmatic driving overpressure and lateral displacement/deformation of the cone flank, this displacement is assumed to be greater where intrusions have the highest frequency. Lateral deformation can also lead to flank failure along a deep-seated slip surface. Symmetrical rifts may produce deep-seated collapses in opposing directions, while asymmetrical rifts can preferentially lead to destruction on the tectonically downthrown side. If the collapse depression is refilled by new volcanism, the cone attains a new critical height (and mass), setting the scene for renewed failure. This sequence inhibits the cone from widening normal to the rift zone. The field examples studied, plus analog and numerical modeling, also indicate that dyking within the rift zone destabilizes the volcano flanks along the rift: where dyke tip stresses approach the slope, surface landsliding might be induced. This in turn produces debuttressing above the dyke, favoring effusive eruptions and lavas flowing within the landslide depression. In this case a balance can occur between new lava outpourings and gravity mass removal, or the volcano can grow along the rift if the emitted magma volume is higher than the collapsed material. From a mechanical point of view, the different behavior of volcanic slopes, both parallel to and perpendicular to dyke strike, is a function of magma forces and the cumulative dyke-induced displacement. The combination of along-rift-strike landsliding and normal-to-rift deep-seated large-scale lateral collapse, characteristically produces in map view four main zones of “consumption” and “rebuilding” on the volcano. These alternate with four triangular sectors where the volcano slopes are relatively more stable. When volcanic activity ends, cone consumption is concentrated along the rift and is favored by fault slip or channeled erosion.

The late Neoproterozoic Frog Lake hornblende gabbro pluton, Avalon Terrane of Nova Scotia: evidence for the origins of appinites

The late Neoproterozoic Frog Lake pluton, in the Avalon terrane of the Cobequid Highlands, Nova Scotia, consists predominantly of hornblende gabbro. It shows petrographic similarities to water-rich mafic intrusions known as appinites that are present in some collisional orogens. This study aims to further understanding of the origin of appinitic intrusions. In the field, the main hornblende gabbro was intruded between screens of metasedimentary country rock that is of upper greenschist metamorphic grade. The contacts appear to have been pathways for magma of gabbroic, tonalitic–granodioritic, and granitic composition that carried enclaves of gabbroic lithologies. Some of these magmas had a high volatile content, resulting in abundance of hydrous mineral phases, pegmatites, and diffuse felsic segregations. These varied rocks in the contact zones experienced progressive shear resulting in syn-magmatic deformation. Low-Ti hornblende gabbros have trace-element abundances similar to subduction-related low-K mafic rocks, including some enrichment in large-ion lithophile elements and marked relative depletion in Nb and Y. High-Ti hornblende gabbros and pyroxene–mica gabbro show more alkaline characteristics, with higher amounts of Nb, Y, P2O5, and high-field-strength elements. Tonalite and granite veins are geochemically similar to volcanic-arc granite. Comparison with appinites in the literature suggests that the Frog Lake pluton represents a deeper structural level than most appinites. The Frog Lake appinites were part of the feeder system to back-arc volcanic rocks of the Jeffers Group. Comparison with other appinites also leads to the conclusion that there is not a single type of “appinitic magma”: different appinitic plutons range in composition from low-K calc-alkaline to shoshonitic. The essential characteristic is a water-rich mafic magma. Appinites occur in settings undergoing crustal-scale strike-slip shear, where the faults allow rapid rise of mafic magma to shallow crustal levels.

P-wave velocity structure beneath Asama Volcano, Japan, inferred from active source seismic experiment

An active seismic survey with unprecedented density of seismometers found a high velocity zone to the west of the summit of Asama Volcano, Japan. The high velocity zone coincides with an area of magma-filled crack (dike) intrusion associated with the 2004 eruptions inferred from precise earthquake relocations and ground deformation modeling. It also coincides with an area of high resistivity surrounded by low resistivity, indicating that the solidification of magma due to repeating intrusions is responsible for the high velocity. This thus endorses the magma pathway previously speculated by seismic and geodetic observations. These findings demonstrate that dense seismic exploration combined with geophysical monitoring is an effective way to understand the dynamics of volcanic eruptions.

A multiparameter approach to volcano monitoring based on 4D analyses of seismo‐volcanic and acoustic signals: The 2008 Mt. Etna eruption

Volcanic tremor and low frequency events, together with infrasound signals, can represent important precursory phenomena of eruptive activity because of their strict relationship with eruptive mechanisms and with fluid flows through the volcano's feeding system. Important variations of these seismo‐volcanic and infrasound signals, recorded at Mt. Etna volcano, occurred both in the medium‐ and short‐term before the eruption, that took place on 13 May 2008. The most significant changes were observed in the frequency content and location of LP events, as well as in volcanic tremor location, that allowed us to track the magma pathway feeding the 2008 eruptive activity. The infrasound showed three different families of events linked to the activity of the three active vents: North‐East Crater, South‐East crater and the eruptive fissure. The seismic and infrasonic variations reported, corroborated by ground deformations variations, help to develop a quantitative prediction and early‐warning system for effusive and/or explosive eruptions.

Granitization and magmatic replacement in the contact aureole of the Yurchik gabbronorite massif (Ganal Ridge, Kamchatka)

It is shown that formation of high-temperature granulite-like rocks in the contact aureole of the Yurchik gabbronorite intrusion of the Ganal Ridge in East Kamchatka was caused by contact metamorphism, metasomatism, and local melting of the primary sedimentary-volcanogenic rocks of the Vakhtalkinskaya Sequence of the Ganal Group. The temperature in the inner part of the aureole reached 700–800°C and caused transformation of basic rocks into two-pyroxene-plagioclase, clinopyroxene-amphibole-plagioclase, and amphibole-plagioclase rocks, while sedimentary rocks were replaced by garnet-biotite and garnet-cordierite-biotite hornfelses. Locally, basic volcanic hornfelses were subjected to metasomatic alteration with the formation of bodies of biotite-orthopyroxene-plagioclase metasomatites. In the zones of the most intense fluid filtration, the metasomatites experienced local magmatic replacement resulting in the formation of biotite-orthopyroxene-plagioclase ± garnet migmatite veinlets and patches. Bodies of garnet enderbites were formed after sedimentary interlayers at temperatures of 700–800°C and a lithostatic pressure of 3.2–4.8 kbar. The comparison of the chemical composition of the Vakhtalkinskaya basic volcanics and the products of their transformation indicates that, in terms of chemistry, the metasomatic alterations and magmatic replacement correspond to siliceous-alkaline metasomatism (granitization) causing a subsequent and uneven influx of Si, Al, Na, K, Rb, Ba, Zr, Nb, and Cl and removal of Fe, Mg, Mn, Ca, and some trace elements (Cr, Co, Ti, Y, and S). The processes of metamorphism and metasomatism were presumably provoked by highly mineralized mantle fluids that were filtered through magmatic channels that served as pathways for gabbroid magma.

Shallow magma pathway geometry at Mt. Etna volcano

A fundamental goal of volcano seismology is to understand the dynamics of active magmatic systems in order to assess eruptive behavior and the associated hazard. Imaging of magma conduits, quantification of magma transport, and investigation of long‐period seismic sources, together with their temporal variations, are crucial for the comprehension of eruption‐triggering mechanisms. At Mt. Etna volcano, several intense episodes of tremor activity were recorded during 2007, in association with strombolian activity and/or intense fire fountaining episodes occurring from the South East Crater (SEC). The locations of the tremor sources and of the long‐period seismic events are used here to constrain both the area and the depth range of magma degassing, highlighting the geometry of the shallow conduits feeding SEC. The imaged conduits consist of two connected resonating dike‐like bodies, NNW‐SSE and NW–SE oriented, extending from sea level to the surface. In addition, we show how tremor, long‐period (LP), and very‐long‐period (VLP) event locations and signatures reflect pressure fluctuations in the plumbing system associated with the ascent/discharge of gas‐rich magma linked to the lava fountains. The evidence here reported, also corroborated by ground deformation variations, can help develop a better prediction and early warning system for those eruptions (effusive or explosive) that apparently manifest no clear precursors.

Subsurface structure across the axis of the Tongariro Volcanic Centre, New Zealand

The relationship between structure and volcanism in the Tongariro Volcanic Centre, New Zealand, is largely masked by a mantle of young volcanic deposits. Here we report the results of an integrated geophysical investigation (using gravity, multi-level aeromagnetic and magnetotelluric methods) of subsurface deposits and structures in the upper 1–2 km across the axis of the Tongariro Volcanic Centre. Modelling of these data across the Tama Lakes saddle shows that the outcropping volcanic deposits are up to 800 m thick, underlain by Tertiary sediments (of a few 10's to a few 100 m in thickness) and in turn lying above a basement of probable Mesozoic greywacke. Basement faulting is shown to be concentrated in the centre of the rift, which is 18 km wide at this location, but no vertical offset is resolved at the rift axis. Vertical displacements on basement faults of 250–300 m are modelled giving a minimum total basement subsidence of 650 m. A 5 km-wide, deep low resistivity zone occurs at the axis of the rift which is interpreted as either resulting from extensive fracturing and/or hydrothermal alteration within the basement. Steep-sided volcanic bodies with a high proportion of lavas/dykes coincide with the Waihi fault and the rift axis. Coincidence with the Waihi Fault suggests that this fault system may have provided magma pathways to the surface and a focus for dyke emplacement, which could have contributed to rift extension. The lack of offset at the rift axis may reflect the juvenile nature of faulting at this location, which is consistent with the notion of a migration of faulting towards the centre of the graben, alternatively, rifting may have been entirely accommodated by dyke emplacement.

Regional-scale aeromagnetic survey of the south-west of Algeria: A tool for area selection for diamond exploration

The main objective of the most regional-scale aeromagnetic surveys is to assist in mineral development through improved geologic mapping. The aeromagnetic survey of the south-west of Algeria is used for diamond potential evaluation and target-area selection. This region of Algeria, forming part of the west African Craton, has until very recently been under-explored for diamonds. This paper is a contribution to area selection for primary diamond deposits based on a conceptual model for kimberlitic occurrences. According to the proposed exploration model, the emplacement of diamondiferous kimberlites is controlled by three principal criteria: (1) existence of a sufficiently thick lithosphere, favourable to diamond genesis and preservation, (2) presence of major tectonic trends and lineaments that could act as pathways for kimberlitic magma, the principal diamond-bearing lithology, and (3) the recognition of magnetic anomalies related to mafic–ultramafic intrusives, signs of deeply-rooted magma. Interpretation of the aeromagnetic data, using energy spectral analysis and different data-enhancement filters, has permitted elucidation of all three criteria. Their combination provides a final assessment for three possible areas as targets for primary diamond deposits emplacement.

Frontiers in Textural and Microgeochemical Analysis

Igneous rocks display a large and varied range of textures and compositions, reflecting complex magma pathways, differentiation processes and cooling histories. Integrating geochemical analyses with petrographic and textural information allows us to gain valuable insights into the details of the magma system: the rock9s texture serves as a window into the crystallization history of the magma, while the compositions of the components (crystals and glass) document the conditions and pathways of evolution. In this issue of Elements we take a look at the frontiers in igneous petrology, focussing on the state of the art in textural and microgeochemical analysis and on how we can use some of the latest approaches to unravel the complexities of the magmatic system.

Petrological variation of large‐volume felsic magmas from Hakkoda‐Towada caldera cluster: Implications for the origin of high‐K felsic magmas in the Northeast Japan Arc

Abstract The Hakkoda‐Towada caldera cluster (HTCC) is a typical Late Cenozoic caldera cluster located in the northern part of the Northeast Japan Arc. The HTCC consists of five caldera volcanoes, active between 3.5 Ma and present time. The felsic magmas can be classified into high‐K (HK‐) type and medium‐ to low‐K (MLK‐) type based on their whole‐rock chemistry. The HK‐type magmas are characterized by higher K2O and Rb contents and higher 87Sr/86Sr ratios than MLK‐type magmas. Both magmas cannot be derived from fractional crystallization of any basaltic magma in the HTCC. Assimilation‐fractional crystallization model calculations show that crustal assimilation is necessary for producing the felsic magmas, and HK‐type magmas are produced by higher degree of crustal assimilation with fractional crystallization than MLK‐type magmas. Although MLK‐type magmas were erupted throughout HTCC activity, HK‐type magmas were erupted only during the initial stage. The temporal variations of magma types suggest the large contribution of crustal components in the initial stage. A major volcanic hiatus of 3 my before the HTCC activity suggests a relatively cold crust in the initial stage. The cold crust probably promoted crustal assimilation and fractional crystallization, and caused the initial generation of HK‐type magmas. Subsequently, the repeated supply of mantle‐derived magmas raised temperature in the crust and formed relatively stable magma pathways. Such a later system produced MLK‐type magmas with lesser crustal components. The MLK‐type magmas are common and HK‐type magmas are exceptional during the Pliocene–Quaternary volcanism in the Northeast Japan Arc. This fact suggests that exceptional conditions are necessary for the production of HK‐type magmas. A relatively cold crust caused by a long volcanic hiatus (several million years) is considered as one of the probable conditions. Intensive crustal assimilation and fractional crystallization promoted by the cold crust may be necessary for the generation of highly evolved HK‐type felsic magmas.

Comparative lithogeochemistry of komatiites in the Norseman-Wiluna and Abitibi Greenstone Belts, and implications for nickel sulfide targeting

This paper uses large sets of whole-rock geochemical data to make comparisons between the komatiitic rocks of Abitibi (AGB) and Norseman-Wiluna (NWGB) greenstone terranes. The NWGB komatiite suite has a much higher proportion of highly olivine-enriched cumulates than the AGB suite, as indicated by data-density distributions on plots of MgO vs FeO and MgO vs Cr, although average compositions of the komatiite magmas in the two belts were not significantly different. NWGB komatiites appear generally more contaminated, on the basis of various ratios of strongly to moderately incompatible low-mobility trace elements. Both factors which are likely contributors to the much higher Ni sulfide resource endowment of the NWGB. The combination of high degrees of contamination and presence of olivine adcumulates in the NWGB attests to the presence of exceptionally high-intensity, prolonged eruptions, capable of forming long-lived entrenched magma pathways, represented by highly olivine-enriched cumulates, and capable of melting substrates to form orebodies. This is in contrast with more episodic, lower volume eruptions in the AGB. The contrast is interpreted as the result of crustal structure and tectonic setting, rather then the size and intensity of mantle plume sources.

Volcanic systems and calderas in the Vatnajökull region, central Iceland: Constraints on crustal structure from gravity data

Abstract A Bouguer anomaly map is presented of southern central Iceland, including the western part of Vatnajokull and adjacent areas. A complete Bouguer reduction for both ice surface and bedrock topography is carried out for the glaciated regions. Parts of the volcanic systems of Vonarskarð-Hagongur, Barðarbunga-Veiðivotn, Grimsvotn-Laki, and to a lesser extent Kverkfjoll, show up as distinct features on the gravity map. The large central volcanoes with calderas: Vonarskarð, Barðarbunga, Kverkfjoll and Grimsvotn, are associated with 15–20 mGal gravity highs caused by high density bodies in the uppermost 5 km of the crust. Each of these bodies is thought to be composed of several hundred km3 of gabbros that have probably accumulated over the lifetime of the volcano. The Skaftarkatlar subglacial geothermal areas are not associated with major anomalous bodies in the upper crust. The central volcanoes of Vonarskarð and Hagongur belong to the same volcanic system; this also applies to Barðarbunga and Hamarinn, and Grimsvotn and Þorðarhyrna. None of the smaller of the two volcanoes sharing a system (Hagongur, Hamarinn and Þorðarhyrna) is associated with distinct gravity anomalies and clear caldera structures have not been identified. However, ridges in the gravity field extend between each pair of central volcanoes, indicating that they are connected by dense dyke swarms. This suggests that when two central volcanoes share the same system, one becomes the main pathway for magma, forming a long-lived crustal magma chamber, a caldera and large volume basic intrusive bodies in the upper crust. Short residence times of magma in the crust beneath these centres favour essentially basaltic volcanism. In the case of the second, auxillary central volcano, magma supply is limited and occurs only sporadically. This setting may lead to longer residence times of magma in the smaller central volcanoes, favouring evolution of the magma and occasional eruption of rhyolites. The eastern margin of the Eastern Volcanic Zone is marked by a NE–SW lineation in the gravity field, probably caused by accumulation of low density, subglacially erupted volcanics within the volcanic zone. This lineation lies 5–10 km to the east of Grimsvotn.

Changing depths of magma fractionation and stagnation during the evolution of an oceanic island volcano: La Palma (Canary Islands)

La Palma (Canary Islands) represents an oceanic island volcano with an active rift zone, inferred to have formed during the last 800 ka following southward growth of the former radial–symmetrical stratovolcano Taburiente. We carried out clinopyroxene–melt thermobarometry and microthermometry of fluid inclusions to reconstruct the evolution of the magma plumbing systems over time and to understand the genetic relationship between Taburiente and the presently active Cumbre Vieja rift zone. Clinopyroxene–melt equilibria of phenocryst rims and glassy groundmass indicate pressures of 0.60–1.04 GPa (∼19–34 km depth) for Taburiente, 0.47–1.17 GPa (16–40 km) for the former Cumbre Nueva rift arm of Taburiente, and 0.50–0.78 GPa (16–26 km) for Bejenado volcano that formed after collapse of the Cumbre Nueva rift. These pressures are interpreted to reflect depths of magma storage and major crystal fractionation. CO 2-dominated fluid inclusions hosted by clinopyroxenes and olivines indicate pressures of formation or re-equilibration within an overall range of 0.25–0.61 GPa (∼8–19 km depth). Respective frequency maxima are at 0.41–0.50 GPa for Taburiente dunite xenoliths, 0.26–0.43 GPa for Cumbre Nueva ankaramites, and 0.26–0.32 GPa for Bejenado cumulate xenoliths. These pressures are interpreted to reflect levels of temporary magma stagnation during ascent. Our data show that the magma pathways during all volcanic phases including the presently active Cumbre Vieja rift [Klügel, A., Hansteen, T.H., Galipp, K., 2005. Magma storage and underplating beneath Cumbre Vieja volcano, La Palma (Canary Islands). Earth and Planetary Science Letters 236, 211–226] are characterized by two distinct storage levels: a system of prolonged storage within the upper mantle, and a system of short-term stagnation within the lower crust or near to the Moho. Both the mantle and crustal storage systems show a migration to shallower levels from 1.0 Ma to present, probably as a result of changing thermomechanical properties of the mantle and crust and possibly stoping. Our combined barometric data and field observations suggest that the extinct Taburiente/Cumbre Nueva and the active Cumbre Vieja represent two distinct volcanoes with separate magma plumbing systems. In this case, the present rift configuration does not reflect continuous growth of the Taburiente shield volcano during the last 800 ka. None of the La Palma volcanoes shows any indicators of a long-lived shallow magma reservoir where magmas fractionate and from which rift zones emanate, which is an important difference to Hawaiian shield volcanoes characterized by shallow subcaldera magma chambers.

Transport network and flow mechanism of shallow ore-bearing magma in Tongling ore cluster area

Abundant studies revealed that shallow intrusions of the Yanshanian epoch resulted in the mass mineralization of the Tongling region. Various evidences showed there existed a concealed magma chamber at −10 km depth in the middle part of this region during Yanshanian epoch, from which the ore-forming magma was generated and then transported to the superficial layer. Yet the transport network and flow mechanism of the shallow ore-bearing magma, the key problem associated with ore-forming process, was relatively little focused on. Integrate analysis of structural mechanics, statistical fractal and geological facts suggested that NE trending high-angle fold-related thrust faults and the tessellated basement ones served as the main pathways for the shallow magma’s transporting, moreover, the saddle void spaces among adjacent strata in the folds upon this fault system provided the place for magma’s emplacement. So the folds in the upper part and faults in the lower part of the upper crust constituted the fluid’s transport and emplacement network. During the deformation of geologic body with multi-layer structure, the layers in the upper part tended to fold when received the jacking stress from the lower part, while the lower one inclined to fault undergoing loads of the upper part. And the producing probability of this structure assemblage was highly increased in the condition, such as in the Tongling area, that the mechanic rigidity of the lower layers was stronger than that of the upper ones. For the pre-existence of fluid-conducting network, the top magma with high volatile in the magma chamber transported rapidly to the superficial layer in dyking pattern, located in the void spaces of folds, filled and reconstructed them. The sudden drop of pressure caused the fluid unmixing from the magma and mass ore-forming elements concentration. Pulse activity of the dyking may be the principal reason why magmatic bodies in the Tongling area were spatially-temporally concomitant and limited flux in chemical compositions.

The Role of Preexisting Geologic Architecture in the Formation of Giant Porphyry-Related Cu Au Deposits: Examples from New Guinea and Chile

Development of giant porphyry-related copper and/or gold deposits in New Guinea and northern and central Chile occurred during Tertiary magmatic events that overprinted earlier extensional tectonic settings. The earlier tectonic settings consisted of a Mesozoic passive margin in New Guinea, a Jurassic-Cretaceous back-arc basin in northern Chile, and an Oligocene intra-arc basin in central Chile. The preexisting architecture of the basement rocks, the fault systems, and the stratigraphic packages associated with these settings played a strong role in controlling development of the giant deposits. Although the extensional tectonic settings in these terranes were different, they share a number of elements that were used during formation of the Tertiary ore deposits. These were the presence of coupled systems of deeply detached, listric synsedimentary faults and steep transverse faults, and the presence of a relatively flat-lying heterogeneous volcano-sedimentary rock package. During Tertiary collision the deeply detached listric faults were inverted and strongly focused uplift, exhumation, and associated overpressuring-induced failure and fluid flow. In addition, steep transverse faults were activated to form wrench systems, with attendant steep, deeply tapping pathways for magma and/or fluid associated with dilatant jogs, flexures, or fault intersections. Ore deposits are commonly located in the hanging wall of the thrust faults. Under compression associated with collision, the competent units of flat-lying stratigraphic packages formed plates overlying the folded, weaker units underneath and separated by a detachment or otherwise faulted contact. Examples of the competent upper units include the Darai/Mendi Limestone or equivalents in New Guinea and the lavas of the Farellones Formation in central Chile. These plates appear to have impeded magma ascent and formed a cap on a magma and/or fluid system pressurized by collision and orogenesis. This geometry provided an ideal location for focusing of magmas and magmatic hydrothermal fluids. From these three example terranes a set of exploration indicators have been constructed to highlight belts that are prospective for giant porphyry Cu systems. These indicators include (1) the presence of magmatic arcs that migrated into preexisting extensional architectures; (2) coupled systems of deeply detached, listric synsedimentary faults and steep transverse faults; (3) rigid basement blocks that could form regional stress field perturbations; and (4) fold belts with a major plate (to 50 km wide) of competent, relatively undeformed or gently folded stratigraphy overlying more complexly folded and faulted parts of the sequence.

Komatiites and nickel sulphide orebodies of the Black Swan area, Yilgarn Craton, Western Australia. 1. Petrology and volcanology of host rocks

The Black Swan Succession is a bimodal association of dacitic and komatiitic volcanic rocks located about 50 km NNE of Kalgoorlie, within the 2.7-Ga Eastern Goldfields greenstone province of the Yilgarn Craton. The komatiite stratigraphy comprises a steep dipping, east facing package about 700 m in maximum thickness and about 2.5 km in strike length (Fig. 1), which hosts a number of economically exploitable Ni sulphide orebodies including the Silver Swan massive ore shoot (approximately half a million tonnes at about 10.5% Ni). The sequence can be subdivided into a Lower Felsic Unit, comprising coherent and autobrecciated facies of multiple dacite lava flows; an upper Eastern and lower Western Ultramafic Unit, each showing marked lateral facies variation, and an Upper Felsic Unit coeval with the Eastern Ultramafic Unit. The komatiite sequence has been metamorphosed at sub-greenschist facies in the presence of high proportions of CO2-rich fluid, giving rise to pervasive talc–carbonate and talc–carbonate–quartz assemblages, with extensive preservation of pseudomorphed igneous textures. Cores of lizardite serpentinite are present in the thickest parts of the ultramafic succession. The degree of penetrative deformation is generally very low, and original stratigraphic relationships are largely intact in much of the sequence. The Eastern Ultramafic Unit and Western Ultramafic Unit are interpreted as components of a single large komatiite flow field, representing overlapping stages in the emplacement of a series of distributory lava pathways and flanking sheet flows. The Western Ultramafic Unit which hosts the bulk of the high-grade massive and disseminated ores is a sequence dominated by coarse-grained olivine cumulates, 2 km wide and up to 500 m thick, with major magma pathways represented by thick, homogenous olivine mesocumulate piles at its northern and southern ends: respectively 400 and 200 m thick. The sequence between the two major pathways consists of olivine orthocumulates (oOC) with minor spinifex-textured intervals. The Unit is capped by a persistent spinifex-textured crust less than 1 m thick, and is locally vesicular. The Eastern Ultramafic Unit contains the Black Swan Cumulate Zone, a 500-m thick sequence of very coarse-grained hopper-textured, locally vesicular oOC containing disseminated sulphides in its lower 200 m. The zone is flanked to the north and south by complexly interdigitated sequence of highly irregular, spinifex-capped, olivine cumulate-rich flow lobes between 1 and 100 m thick, and dacitic lavas and tuffs. The complexity of the 3-D spatial relationship of these units suggests a combination of simultaneous eruption of dacite and komatiite, combined with thermal or thermomechanical erosion. The Eastern and Western Units are interpreted as the result of more or less continuous prolonged eruption of olivine charged komatiite lava, which developed localised thermo-mechanical erosion channels in the dacitic substrate. Komatiite and dacite eruption was synchronous, giving rise to complex interdigitation and extensive contamination and hybridisation.

A multidisciplinary approach to detect active pathways for magma migration and eruption at Mt. Etna (Sicily, Italy) before the 2001 and 2002–2003 eruptions

Two strong flank eruptions occurred in July–August 2001 and from late October 2002 to late January 2003 at Mt. Etna volcano. The two eruptions mainly involved the upper southern flank of the volcano, a particularly active area during the last 30 years, damaging several tourist facilities and threatening some villages. The composite eruptive activity on the upper southern flank of Mt. Etna during 2001–2003 has confirmed “a posteriori” the results of a multidisciplinary study, started well before its occurrence by combining geological, seismic and geochemical data gathered in this part of the volcano. We were able, in fact, to highlight fractured zones likely to be re-activated in the near future in this area, where the largest majority of eruptive fissures in the recent past opened along N120° to N180° ranging directions. The spatial distribution of earthquake epicentres during the period June 30th 2000–June 30th 2001 showed the greatest frequency in a sector compatible with both the direction of the main fissures of the pre-2001 period and that of the 2001 and 2002 lateral eruptions. Soil CO 2 and soil temperature surveys carried out in the studied area during the last 3 years have revealed anomalous release of magmatic fluids (mainly CO 2 and water vapour) along some NNW–SSE-trending volcano-tectonic structures of the area even during inter-eruptive periods, indicating persistent convective hydrothermal systems at shallow depth connected with the main feeder conduits of Etna. The temporal changes in both seismic and geochemical data from June 30th, 2000 to June 30th, 2001 were compared with the evolution of volcanic activity. The comparison allowed to recognize at least two sequences of anomalous signals (August to December 2000 and April to June 2001), likely related to episodes of step-like magma ascent towards the surface, as indicated by the following eruptive episodes. The N120° to N180° structural directions are in accord with one of the main structural lines affecting eastern Sicily; they would be important pathways for magma uprise to the surface that will keep on feeding the eruptive activity of Etna in the near future. This study also pointed out the high instability of the southern slope of Etna, a sector where the potential hazard by lava flow invasion will remain high also in the near future.

Characteristics of deep (≥13 km) Hawaiian earthquakes and Hawaiian earthquakes west of 155.55°W

High precision relocation of earthquakes recorded by the Hawaiian Volcano Observatory (HVO) seismic network provides new information on the characteristics of seismic faulting at this oceanic hot spot. Using waveform cross correlation, we have measured correlation coefficients and travel time differences for a set of 14,605 deep (≥13 km) earthquakes recorded from 1988 to 1998. We find that about half of the analyzed earthquakes are in similar event clusters that delineate fault zones in the lower crust and upper mantle. We suggest that much of this deep seismicity reflects rupture in the brittle lithosphere away from the magma pathways, although at Kilauea the stresses from magma movement may additionally help trigger mantle earthquakes on preexisting faults in regions with high differential ambient stresses. Focal mechanisms of similar event clusters throughout Hawaii display characteristic patterns and appear consistent with the hypothesis that deep earthquakes on preexisting faults reflect the stresses due to volcano loading and flexure. We also present the results of applying cross correlation analyses and relocation to ∼7000 earthquakes at all depths located west of 155.55°W and recorded from 1988 to 1998. The pattern of relocated earthquakes at the Kealakekua fault zone is consistent with the presence of a low‐angle detachment on the west flank of Mauna Loa.