Controls on supergene enrichment of porphyry copper deposits in the Central Andes: A review and discussion

The Central Andes host some of the world’s largest porphyry copper deposits. The economic viability of these deposits is dependent on the size and quality of their supergene enrichment blanket. Published models that have strongly influenced exploration policy suggest that supergene enrichment ceased at 14 Ma due to an increase in aridity. Here we discuss these models using published geochronological, geomorphological and geological data. Geochronological data indicate that supergene oxidation and enrichment has been active between 17 and 27°S across the forearc of northern Chile and southern Peru from 44 to 6 Ma, and on the Bolivian Altiplano and Eastern Cordillera of Argentina from 11 Ma to present. There is evidence for cessation at 20, 14 and 6 Ma. However, a major problem is that as more geochronological data become available the age ranges and periods of enrichment increase. This suggests that the full spectrum of enrichment ages may not have been sampled. The relationship between supergene enrichment and the age of regional pediplain surface development is not well constrained. Only in two areas have surfaces related to enrichment been directly dated (southern Peru and south of 26°S in Chile) and suggest formation post 14 Ma. Sedimentological data indicate that a fluctuating arid/semi-arid climate prevailed across the Atacama Desert until between 4 and 3 Ma, climatic conditions that are thought to be favourable for supergene enrichment. The balance between uplift, erosion, burial and sufficient water supply to promote enrichment is complex. This suggests that a simple model for controlling supergene enrichment is unlikely to be widely applicable in northern Chile. General models that involve climatic desiccation at 14 Ma related to rainshadow development and/or the presence of an ancestral cold-upwelling Humboldt Current are not supported by the available geological evidence. The integration of disparate sedimentological, geomorphological and supergene age data will be required to fully understand the controls on and distribution of supergene oxidation and enrichment in the Central Andes.     

Accuracy assessment of QuickBird stereo imagery

The Geographical Survey Institute of Japan has recently carried out an evaluation of the metric performance of QuickBird stereo satellite imagery. This paper describes the accuracy assessment of the sensor orientation and geopositioning phases of the study, the aim of which was twofold. First, it was desired to confirm the metric potential of QuickBird imagery for 1:25 000 scale topographic mapping. Second, a determination was to be made of the accuracy attainable from the Basic image product. The techniques of rational functions and affine bundle adjustment were employed, the former with bias compensation. The results obtained both reassert the high precision of the rational functions approach and cast doubt upon the applicability of the 3D affine model for accurate geopositioning from QuickBird imagery.     

Visibility Analysis In Vision Metrology Network Design

The aim of this paper is to present a method whereby accuracy enhancement of an existing photogrammetric network is achieved through the automatic selection of additional camera stations. The determination of the positions of these 'accuracy fulfilment' camera stations is based upon what has been termed 'visibility uncertainty prediction modelling' of visibility constraints derived from the existing network geometry. Following a review of vision constraints in network design, the concepts of visibility uncertainty prediction and visibility uncertainty spheres are introduced. These provide a mechanism to predict the visibility of current object target points for the new accuracy fulfilment images. This in turn aids in network design improvement. The visibility uncertainty modelling is then illustrated for two close range photogrammetric network configurations, for which the test results demonstrate that the proposed model can reliably predict target visibility with an overall certainty of 75%.     

Acoustic and petrophysical relationships in low-shale sandstone reservoir rocks

This paper describes the measurements of the acoustic and petrophysical properties of two suites of low‐shale sandstone samples from North Sea hydrocarbon reservoirs, under simulated reservoir conditions. The acoustic velocities and quality factors of the samples, saturated with different pore fluids (brine, dead oil and kerosene), were measured at a frequency of about 0.8 MHz and over a range of pressures from 5 MPa to 40 MPa. The compressional‐wave velocity is strongly correlated with the shear‐wave velocity in this suite of rocks. The ratio V_P/V_S varies significantly with change of both pore‐fluid type and differential pressure, confirming the usefulness of this parameter for seismic monitoring of producing reservoirs. The results of quality factor measurements were compared with predictions from Biot‐flow and squirt‐flow loss mechanisms. The results suggested that the dominating loss in these samples is due to squirt‐flow of fluid between the pores of various geometries. The contribution of the Biot‐flow loss mechanism to the total loss is negligible. The compressional‐wave quality factor was shown to be inversely correlated with rock permeability, suggesting the possibility of using attenuation as a permeability indicator tool in low‐shale, high‐porosity sandstone reservoirs.     

Policies to reduce nitrogen loss to water from agriculture in the United Kingdom

Water supply companies in England face increasing difficulty and costs in meeting the 50 mg l⁻¹ nitrate limit in drinking water from groundwater sources. Some surface water abstractions also exceed the limit, but usually only in late autumn. Nitrogen-limited eutrophication is not a significant problem of estuary or coastal waters in the UK. Local problems are under investigation but the cause is rarely clear. Agricultural land is recognized as the main source of nitrate reaching ground and surface waters in the UK. Considerable research has been carried out on nitrate and farming systems. There is a substantial, ongoing government funded research programme. A significant remaining problem is the ability to relate losses from agriculture to nitrate in abstracted water or entering tidal waters. Much work remains to be done to improve our predictive ability.

In 1989, the UK government set up a Pilot Nitrate Scheme. This involved ten Nitrate Sensitive Areas. These were groundwater catchments in which farmers were invited on a voluntary basis to enter into 5 year contracts with the Ministry of Agriculture, Fisheries and Food (MAFF). The basis of the agreements are that the farmer will carry out a range of modifications to current farming practice, thus reducing nitrate loss. There are also options for converting current arable to extensive grass. These areas will initially run for 5 years. In addition nine Nitrate Advisory Areas were targeted for intensive advice.

In 1991, MAFF published its Code of Good Agricultural Practice for the Protection of Water. This provides guidance to farmers on the agreed balance between agricultural production and environmental protection of water which all farmers should follow. It includes a section on nitrate.

In 1989, the UK government set up a Pilot Nitrate Scheme. This involved ten Nitrate Sensitive Areas. These were groundwater catchments in which farmers were invited on a voluntary basis to enter into 5 year contracts with the Ministry of Agriculture, Fisheries and Food (MAFF). The basis of the agreement is that the farmer will carry out a range of modifications to current farming practice, thus reducing nitrate loss. There are also options for converting current arable to extensive grass. These areas will initially run for 5 years. In addition nine Nitrate Advisory Areas were targeted for intensive advice.     

Structural controls and genesis of epithermal gold-bearing breccias at the Lebong Tandai mine, Western Sumatra, Indonesia

Lebong Tandai is a low-sulphidation, volcanic-hosted epithermal gold deposit of Neogene age, located within the foothills of the Barisan Mountains, Sumatra. To date, the mine has produced approximately 40 tonnes of gold and 400 tonnes of silver. The mineralisation is exclusively in the form of tabular quartz-cemented breccias bodies which are localised along faults. The breccias comprise angular to sub-rounded clasts of the wallrocks and earlier barren breccias cemented by banded or massive quartz, and in many instances, the clasts are supported within the quartz cement.The sulphide minerals occur as either a single cockade band around the clasts in the breccia, or as polymineralic aggregates disseminated throughout the breccia cement. The main precious-metal-bearing phase is electrum, with silver-sulphosalts and silver-tellurides also present. Highly variable concentrations of pyrite, sphalerite, galena and chalcopyrite are associated with the precious-metal phases.With the exception of two minor lodes, the mineralised breccias are localised along strike-slip faults which display changes in orientation indicative of D-, R- and P-shears and T-fractures, with individual segments ranging from a few metres to a few hundred metres in length. Two strike-slip fault systems are recognised, one sinistral, trending east-west and the other dextral, trending northwest, the latter of which is parallel to the Sumatran Fault System. The majority of gold and silver production is from breccias localised along faults formed during the sinistral tectonism. The breccias are believed to have been generated during compressional reactivation of the east-west sinistral strike-slip faults in response to the subduction of the Indian-Australian plate beneath Sumatra. Supralithostatic fluid pressures are a necessary pre-requisite for such reactivation, and the sudden drop in fluid pressure during reactivation is thought to have resulted in both the formation of the breccias by hydraulic fracturing, and the deposition of amorphous silica, precious metals and base metal sulphides. High rates of fluid flow subsequent to fracturing are thought to have led to fluidisation of the breccia clasts and abrasion to their current morphologies.Microthermometry of fluid inclusions in sphalerite indicates that the mineralising fluids were of low salinity, less than 3 wt% NaCl_equivalent, and that mineralisation took place at temperatures of 260–280°C. Variations of salinity and homogenisation temperature due to boiling are poorly developed, although if boiling occurred, the metalliferous minerals would have been deposited early in the boiling process before the fluid had cooled appreciably.     

Paleogeographic and paleotectonic response to magmatic processes: a case history from the Archean sequence in the Chibougamau area,Quebec

The Chibougamau area, Québec, is characteristic of the internal zone of the Archean Abitibi Orogenic Belt. The paleogeographic, paleotectonic and magmatic history of the Archean sequence in the Chibougamau area is subdivided into three stages.In the first stage a submarine volcanic chain formed mainly by the effusion of submarine lava flows composed of primitive, potash-poor, tholeiitic basalt. The volcanic chain gradually grew to sea level. In the second stage, volcanic islands emerged and grew. Mainly pyroclastic eruptions of strongly differentiated, calc-alkaline andesite and dacite concentrated on the volcanic islands, whereas effusion of basalt continued at first in the surrounding basin. A felsic volcaniclastic apron was deposited around the volcanic islands. In the third stage, the volcanic islands were uplifted and were eroded to the level of their subvolcanic plutons. The debris derived from this volcanic-plutonic terrain was deposited in downfaulted marine and continental basins. The contemporaneous volcanism was shoshonitic.The first paleogeographic stage is interpreted as the growth of an immature island arc. During the second stage, the island arc became mature and its crust was thickened by accretion of plutonic material. The third stage is a period of back-arc extension.

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Zusammenfassung Das Gebiet von Chibougamau, Québec, ist characteristisch für die interne Zone des Archaischen Abitibi Orogens. Man kann seine paleogeographische, paleotectonische und magmatische Geschichte in drei Phasen gliedern.Eine submarine Vulkankette formte sich in der ersten Phase, hauptsächlich durch Effusion von submarinen Lavaergüssen aus primitivem, kaliarmen, tholeiitischem Basalt. Die Vulkankette wuchs langsam bis zum Meeresspiegel. Vulkanische Inseln bildeten sich und wuchsen während der zweiten paleogeographischen Phase. Vorwiegend pyroklastische Eruptionen von stark differenzierten, kalk-alkalischem Andesit und Dazit konzentrierten sich mehr und mehr auf den Inselvulkanen, während die Effusion von Basalt zunächst in den Becken noch stattfand. Ein Mantel aus felsitischen vulkanoklastischen Gesteinen wurde um die Inselvulkane abgelagert. Die dritte Phase begann mit einer Hebung der Inselvulkane und mit ihrer Erosion bis zum Niveau ihrer subvulkanischen Plutone. Der Detritus dieses vulkanisch-plutonischen Geländes wurde in marinen und kontinentalen Verwerfungsbecken abgelagert. Der gleichalte Vulkanismus ist shoshonitisch.Wir deuten die erste paleogeographische Phase als Wachstumsphase eines primitven Inselbogens. Während der zweiten Phase reifte der Inselbogen und seine Kruste verdickte sich durch Akkretion plutonischen Materials. Die dritte Phase ist eine Periode der Dehnung im Hinterland eines Inselbogens.

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Résumé La région de Chibougamau, Québec, est caractéristique de la zone interne de la ceinture orogénique archéenne de l'Abitibi. Son évolution paléogéographique, paléotectonique et magmatique se subdivise en trois phases.Lors de la première phase paléogéographique, une chaîne sous-marine de volcans se formait, essentiellement par l'émission de coulées de lave composée de basalte primitif, hypopotassique, tholéiitique. Graduellement cette chaîne volcanique s'élevait jusqu'au niveau de la mer. A la phase suivante, des îles volcaniques émergeaient et croissaient. Des éruptions essentiellement pyroclastiques d'andésites et de dacites calco-alcalines et fortement différenciées se concentraient sur les îles tandis que l'effusion de laves basaltiques continuaient dans le bassin. Un manteau de roches volcaniclastiques felsiques se déposait autour des îles volcaniques. Lors de la troisième phase, les îles volcaniques furent soulevées et furent érodées jusqu'au niveau des masses plutoniques sub-volcaniques. Le débris de ce terrain volcano-plutonique fut déposé dans des bassins de faille marins et continentaux. Des shoshonites dominaient le volcanisme contemporain.Nous interprétons la première phase paléogéographique comme une phase de croissance d'un arc insulaire immature. Lors de la deuxième phase, 1'arc insulaire devenait mature et sa croûte s'epaissît par accrétion de matériel plutoni-que. Enfin, la troisième phase est une période d'extension en arrière d'un arc insulaire.

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BrO formation in volcanic plumes

Volcanoes have only recently been recognized as a potentially major source of reactive bromine species to the atmosphere, following from the detection of bromine monoxide (BrO) in the plume emitted by Soufrière Hills Volcano, Montserrat. However, BrO is not expected to be emitted in significant quantity from magma, presenting a puzzle regarding its formation. We report here new field measurements of the tropospheric plume emitted by Mt. Etna, Italy, which provide the first direct evidence of fast oxidation of halogen species in a volcanic plume, and lead to an explanation of how BrO is generated from magmatic HBr emissions. We show that the timescale of BrO formation (a few minutes after emission into the atmosphere) is consistent with rapid heterogeneous halogen chemistry involving sulphate aerosol in the plume. The model highlights considerable complexity to the oxidative chemistry of volcanic plumes.     

Nature, diversity of deposit types and metallogenic relations of South China

The South China Region is rich in mineral resources and has a wide diversity of deposit types. The region has undergone multiple tectonic and magmatic events and related metallogenic processes throughout the earth history. These tectonic and metallogenic processes were responsible for the formation of the diverse styles of base and precious metal deposits in South China making it one of the resource-rich regions in the world. During the Proterozoic, the South China Craton was characterised by rifting of continental margin before eruption of submarine volcanics and development of platform carbonate rocks, and the formation of VHMS, stratabound copper and MVT deposits. The Phanerozoic metallogeny of South China was related to opening and closing of the Tethyan Ocean involving multiple orogenies by subduction, back-arc rifting, arc–continent collision and post-collisional extension during the Indosinian (Triassic), Yanshanian (Jurassic to Cretaceous) and Himalayan (Tertiary) Orogenies. The Late Palaeozoic was a productive metallogenic period for South China resulting from break-up and rifting of Gondwana. Significant stratabound base and precious metal deposits were formed during the Devonian and Carboniferous (e.g., Fankou and Dabaoshan deposits). These Late Palaeozoic SEDEX-style deposits have been often overprinted by skarn systems associated with Yanshanian magmatism (e.g., Chengmenshan, Dongguashan and Qixiashan). A number of Late Palaeozoic to Early Mesozoic VHMS deposits also developed in the Sanjiang fold belt in the western part of South China (e.g., Laochang and Gacun).South China has significant sedimentary rock-hosted Carlin-like deposits, which occur in the Devonian- to Triassic-aged accretionary wedge or rift basins at the margin of the South China Craton. They are present in a region at the junction of Yunnan, Guizhou, and Guangxi Provinces called the ‘Southern Golden Triangle’, and are also present in NW Sichuan, Gansu and Shaanxi, in an area known as the ‘Northern Golden Triangle’ of China. These deposits are mostly epigenetic hydrothermal micron-disseminated gold deposits with associated As, Hg, Sb + Tl mineralisation similar to Carlin-type deposits in USA. The important deposits in the Southern Golden Triangle are Jinfeng (Lannigou), Zimudang, Getang, Yata and Banqi in Guizhou Province, and the Jinya and Gaolong deposits in Guangxi District. The most important deposits in the Northern Golden Triangle are the Dongbeizhai and Qiaoqiaoshang deposits.Many porphyry-related polymetallic copper–lead–zinc and gold skarn deposits occur in South China. These deposits are related to Indosinian (Triassic) and Yanshanian (Jurassic to Cretaceous) magmatism associated with collision of the South China and North China Cratons and westward subduction of the Palaeo-Pacific Plate. Most of these deposits are distributed along the Lower to Middle Yangtze River metallogenic belt. The most significant deposits are Tonglushan, Jilongshan, Fengshandong, Shitouzui and Jiguanzui. Au–(Ag–Mo)-rich porphyry-related Cu–Fe skarn deposits are also present (Chengmenshan and Wushan in Jiangxi Province and Xinqiao, Mashan-Tianmashan, Shizishan and Huangshilaoshan in Anhui Province). The South China fold belt extending from Fujian to Zhejiang Provinces is characterised by well-developed Yanshanian intrusive to subvolcanic rocks associated with porphyry to epithermal type mineralisation and mesothermal vein deposits. The largest porphyry copper deposit in China, Dexing, occurs in Jiangxi Province and is hosted by Yanshanian granodiorite. The high-sulphidation epithermal system occurs at the Zijinshan district in Fujian Province and epithermal to mesothermal vein-type deposits are also found in the Zhejiang Province (e.g., Zhilingtou). Part of Shandong Province is located at the northern margin of the South China Craton and the province has unique world class granite-hosted orogenic gold deposits. Occurrences of Pt–Pd–Ni–Cu–Co are found in Permian-aged Emeishan continental flood basalt (ECFB) in South China (Jinbaoshan and Baimazhai in Yunnan Province and Yangliuping in Sichuan Province). South China also has major vein-type tungsten–tin–bismuth–beryllium–sulphide and REE deposits associated with Yanshanian magmatism (e.g., Shizhuyuan and Xihuashan), important world class stratabound base metal–tin deposits (Dachang deposit), and the large antimony deposits (Xikuangshan and Woxi). During the Himalayan Orogeny, many giant deposits were formed in South China including the recently emerging Yulong and Gangdese porphyry copper belts in Tibet and the Ailaoshan orogenic gold deposits in Yunnan.