U–Pb,Re–Os,and <Superscript>40</Superscript>Ar/<Superscript>39</Superscript>Ar geochronology of the Nambija Au-skarn and Pangui porphyry Cu deposits,Ecuador: implications for the Jurassic metallogenic belt of the Northern Andes

New U–Pb, Re–Os, and ⁴⁰Ar/³⁹Ar dates are presented for magmatic and hydrothermal mineral phases in skarn- and porphyry-related ores from the Nambija and Pangui districts of the Subandean zone, southeastern Ecuador. Nambija has been one of the main gold-producing centers of Ecuador since the 1980s due to exceptionally high-grade ores (average 15 g/t, but frequently up to 300 g/t Au). Pangui is a recently discovered porphyry Cu–Mo district. The geology of the Subandean zone in southeastern Ecuador is dominated by the I-type, subduction-related, Jurassic Zamora batholith, which intrudes Triassic volcanosedimentary rocks. The Zamora batholith is in turn cut by porphyritic stocks, which are commonly associated with skarn formation and/or porphyry-style mineralization. High precision U–Pb and Re–Os ages for porphyritic stocks (U–Pb, zircon), associated prograde skarn (U–Pb, hydrothermal titanite), and retrograde stage skarn (Re–Os, molybdenite from veins postdating gold deposition) of the Nambija district are all indistinguishable from each other within error (145 Ma) and indicate a Late Jurassic age for the gold mineralization. Previously, gold mineralization at Nambija was considered to be Early Tertiary based on K–Ar ages obtained on various hydrothermal minerals. The new Jurassic age for the Nambija district is slightly younger than the ⁴⁰Ar/³⁹Ar and Re–Os ages for magmatic–hydrothermal minerals from the Pangui district, which range between 157 and 152 Ma. Mineralization at Nambija and Pangui is associated with porphyritic stocks that represent the last known episodes of a long-lived Jurassic arc magmatism (∼190 to 145 Ma). A Jurassic age for mineralization at Nambija and Pangui suggests that the Northern Andean Jurassic metallogenic belt, which starts in Colombia at 3° N, extends down to 5° S in Ecuador. It also adds a new mineralization style (Au-skarn) to the metal endowment of this belt. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Reply to the comments on “Xenoliths in ultrapotassic volcanic rocks in the Lhasa block: direct evidence for crust–mantle mixing and metamorphism in the deep crust”

Stepanov et al. (Contrib Mineral Petrol, 2017) question our conclusion that the UPVs in southern Tibet were derived by partial melting of an old, metasomatized subcontinental lithospheric mantle (SCLM) of the subducted Indian plate. Instead, they propose that these ultrapotassic volcanic rocks (UPVs) are shoshonitic and were generated in two steps: direct melting of crustal rocks first, and then the melts interacted with mantle peridotite. However, the trace element, isotopic, thermal, structural, and seismic evidence is consistent with the xenolith evidence (Wang et al in Contrib Mineral Petrol 172:62, 2016) for hybridisation of ascending Indian subcontinental lithospheric mantle-derived UPV magmas with the deep, isotopically unevolved, Tibetan crust. This necessitates a model whereby partial melting of subducting Indian SCLM generates the UPV suite of southern Tibet.     

Supply‐limited bedform patterns and scaling downstream of a gravel–sand transition

A distinct suite of sand bedforms has been observed to occur in laboratory flows with limited sand supply. As sand supply to the bed progressively increases one observes sand ribbons, discrete barchans and, eventually, channel spanning dunes; but there are relatively few observations of this sequence from natural river channels. Furthermore, there are few observations of transitions from limited sand supply to abundant supply in the field. Bedforms developed under limited, but increasing, sand supply downstream of the abrupt gravel–sand transition in the Fraser River, British Columbia, are examined using multi‐beam swath‐bathymetry obtained at high flow. This is an ideal location to study supply‐limited bedforms because, due to a break in river slope, sand transitions from washload upstream of the gravel–sand transition to bed material load downstream. Immediately downstream, barchanoid and isolated dunes are observed. Most of the bedform field has gaps in the troughs, consistent with sand moving over a flat immobile or weakly mobile gravel bed. Linear, alongstream bedform fields (trains of transverse dunes formed on locally thick, linear deposits of sand) exhibit characteristics of sand ribbons with superimposed bedforms. Further downstream, channel spanning dunes develop where the bed is composed entirely of sand. Depth scaling of the dunes does not emerge in this data set. Only where the channel has accumulated abundant sand on the bed do the dunes exhibit scaling congruent with previous data compilations. The observations suggest that sediment supply plays an important, but often overlooked, role in bedform scaling in rivers.     

Mikrotax:Developing a Genuinely Effective Platform for Palaeontological Geoinformatics

正Planktonic microfossils have played an enormously important role in modem earth science,notably including industrial hydrocarbon exploration and academic Deep Sea Drilling,providing essential data for age models and rich suites of palaeoceanographic data.Conversely this broad application has resulted in the development of detailed knowledge of planktonic microfossils and in stimulating study of them.Over 50 years of     

Topic: Perception or reality – waste, landfill and the environment

When Einstein was asked to define the environment he gave the excellent answer 'everything that isn't me'. This article is about Einstein's environment, and its protection.     

Variation of effective seismic velocities of minerals with pressure and its use in velocity prediction

summary . Regression analyses of P -wave velocity measurements on rocks at variable confining pressure yield'effective'mineral velocities which can be used in conjunction with geological data to predict the variation of velocity with depth in major units of continental crustal layering. Such prediction equations are more reliable than those based on density, mean atomic weight, or single-crystal data from the literature, because they take into direct account the effects of cracks on velocity. This is particularly important in the upper crust where many cracks are likely to be partly open: a by-product of the technique is the statistical relationship of cracks and alteration to minerals present in the samples. The technique is demonstrated in measurements of V_p to 3 kbar in rocks sampled in the vicinity of the Lewisian Units Seismic Traverse, in north-west Scotland.     

Reference Minerals for the Microanalysis of Light Elements

The quantitative determination of light element concentrations in geological specimens represents a major analytical challenge as the electron probe is generally not suited to this task. With the development of new in situ analytical techniques, and in particular the increasing use of secondary ion mass spectrometry, the routine determination of Li, Be and B contents has become a realistic goal. However, a major obstacle to the development of this research field is the critical dependence of SIMS on the availability of well characterized, homogeneous reference materials that are closely matched in matrix (composition and structure) to the sample being studied. Here we report the first results from a suite of large, gem crystals which cover a broad spectrum of minerals in which light elements are major constituents. We have characterized these materials using both in situ and wet chemical techniques. The samples described here are intended for distribution to geochemical laboratories active in the study of light elements. Further work is needed before reference values for these materials can be finalized, but the availability of this suite of materials represents a major step toward the routine analysis of the light element contents of geological specimens.     

Place,exile and identity: the contemporary experience of Palestinians in Sydney

Statelessness, displacement and exile, far from being particular moments in history, continue to affect the lives of Palestinians living in Sydney, in the maintenance of national identity and community formation. The impacts that displacement and subsequent ‘statelessness’ have had on the ways in which Palestinians maintain a sense of national identity have similarities with those of other ‘displaced’ peoples. Without the recognition of national boundaries to secure national sovereignty (Palestine) and accommodate the continuation of a shared and inherited history, the most significant element through which Palestinian identity is kept ‘alive’ is the one thing that all Palestinians share: a communal sense of injustice which involves being denied the right to live in, or return to, their homeland. Statelessness, nationalism and the now ‘imagined community’ of Palestine are more crucial for Palestinians living in Sydney than more primordial elements of identity, such as language or local community. Statelessness and exile in Australia have constructed a particular sense of Palestinian nationalism. The continuity of Palestinian identity as a primarily political phenomenon is a situation analogous to that of other diasporic refugee communities.     

Buried glacier ice in southern Iceland and its wider significance

Geo-electrical resistivity surveys have been carried out at recently deglaciated sites in front of three glaciers in southern Iceland: Skeiðarájökull, Hrútárjökull, and Virkisjökull. The results show the presence of old glacier ice beneath debris mantles of various thickness. We conclude that buried glacier ice has survived for at least 50 years at Virkisjökull and Hrútárjökull, and probably for over 200 years at Skeiðarájökull. Additional data from a further site have identified a discontinuous ice core within 18th-century jökulhlaup deposits. Photographic and lichenometric evidence show that the overlying debris has been relatively stable, and hence melting of the ice at all four sites is proceeding slowly due to the heat-shielding properties of the overburden. The geomorphic implications are pertinent when considering the potential longevity of buried ice. The possible implications for dating techniques, such as lichenometry, radiocarbon dating and cosmogenic surface-exposure dating are also important, as long-term readjustments of surface forms may lead to dating inaccuracy. Finally, it is recognised that landscape development in areas of stagnant ice topography may post-date initial deglaciation by a considerable degree.     

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

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 Galán, 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.