bluuebunny/crossref_metadata_2025 huggingface dataset:
The effects of longwall coal mining on overlying aquifers
Authors: Booth Colin J.
Published: April 2008

The hydrogeological effects of longwall mines are vertically zoned. The heavily fractured strata immediately above the mine dewater, but they are typically overlain by a zone of low permeability that prevents shallower aquifers from draining to the mine. However, shallow bedrock aquifers experience head changes caused by fracturing during subsidence. New fracture void space takes up water, causing large head drops especially in confined aquifers. Increased fracture permeability affects heads because upper aquifers in high relief areas lose water through fractured aquitards to lower aquifers, and because the higher permeabilities lower hydraulic gradients and up-gradient heads, and increase down-gradient discharge. In addition, a secondary drawdown spreads out laterally through transmissive aquifers from the potentiometric low in the subsiding zone. After undermining, water levels may recover due to closure of fractures and to recharge flowing back into the affected area. Studies at two active longwall mines in Pennsylvanian coal measures in Illinois support the conceptual model, with variations. Unconsolidated, unconfined aquifers were not significantly affected by mining. At one site, heads in a moderately transmissive sandstone declined due to mining but recovered fully afterwards. Increased permeability led to enhanced well yields, but water quality deteriorated, probably because of oxidation and mobilization of
in situ
sulphides during the unconfined and recovery phases. At the other site, heads in a poorly transmissive sandstone fell rapidly during subsidence and did not recover; hydrogeological responses varied at the site scale due to variations in bedrock-drift continuity. Predictions and monitoring schemes can be guided by the general conceptual model, but must consider local hydrogeological variations. Effects in shallow aquifers not in direct contact with the mine can be simulated using readily available flow models.
Contaminant source characterization of the San José Mine, Oruro, Bolivia
Authors: Banks David , Holden Wayne , Aguilar Eduardo , Mendez Carlos , Koller David , Andia Zelmar , Rodriguez Javier , Sæther Ola Magne , Torrico Armando , Veneros Renato , Flores Jorge
Published: April 2008

The San José Mine is a mothballed Ag-Sn mine near Oruro on the Bolivian Altiplano. A groundwater risk assessment has been carried out considering: (i) the current mine water pumping operation; (ii) potential future mine flooding; and (iii) mine waste leachate, at risk sources. Mine flooding rates have been simulated using two models (MIFIM and MODFLOW), with input data based on the observed water inflow distribution and calculated mine volumes. Mine water chemistry has been characterized by field analyses. Transport of contaminants in groundwater in the Quaternary sedimentary aquifer complex surrounding the mine has been assessed by empirical data and hydraulic-geochemical modelling using MODFLOW and MPATH. Empirical and modelled data suggest that no risk is (or will be) posed to Oruro’s public water supply wellfields at Challapamapa. Continued pumping and discharge of mine water poses a potential risk to surface water recipients and private groundwater abstractions located alongside these.
Hydrogeological and geochemical interactions of adjoining mercury and coal mine spoil heaps in the Morgao catchment (Mieres, NW Spain)
Authors: Loredo J. , Ordóñez A. , Pendás F.
Published: April 2008

Asturias, NW Spain, has a long coal and metal mining tradition including mining for mercury, which was intermittently exploited from Roman times up to 1974. In the valley of the Morgao Stream, there is a spoil heap from the Los Rueldos mercury mine immediately uphill from another one from the Morgao coal washery, both situated less than 3 km away from the town of Mieres (30,000 inhabitants). At the Los Rueldos site, arsenic is abundant in the form of As-rich pyrite. Total arsenic concentrations in representative samples from the spoil heap range from 4746 to 62 196 mg kg
−1
. Readily leachable Pb and Zn are respectively present at average concentrations of 3680 and 45 mg kg
−1
. Leachate from this spoil heap drains on to the El Batán coal spoil heap, together with acid drainage from old underground workings of the Los Rueldos Mine, which has very low pH (2.5), with up to 2900 mg 1
−1
SO
4
, and between 5.3 and 8.3 mg 1
−1
As. The total flow of polluted water from the Los Rueldos site that flows into the Morgao Stream is estimated to average 3200 m
3
year
−1
(corresponding to only 1% of the total flow from the Morgao drainage basin). Analysis of the Morgao Stream downstream of both spoil heaps clearly shows the results of the dilution effects, with the As content being lowered by more than 95%.
Hydrogeological and geochemical consequences of the abandonment of Frazer’s Grove carbonate hosted Pb/Zn fluorspar mine, north Pennines, UK
Authors: Johnson K. L. , Younger P. L.
Published: April 2008

The problems associated with predicting where mine water will emerge and what the quality will be in a post-closure situation are recognised world-wide. The closure of Frazer’s Grove, a fluorspar mine in the North Pennines in the UK has given the opportunity to study in detail the relationship between rising groundwater and the strata/mineworkings through which it is rising. Detailed sampling and surveys both above and underground were carried out before, during and after rebound. During the rebound phase the mine water was stratified. Since the mine water emerged in August 1999, stratification has broken up and reestablished itself twice to date. The possible causes of the break-up of stratification are examined with the aid of hydrogeochemical data and geophysical techniques. The main contaminants present in the mine water are zinc, manganese, iron and sulphate. A general exponential decrease in dissolved metal concentration in the mine water is seen with time. The hydrogeochemical data also establishes the origin of the contamination in the mine water discharge with zinc and manganese originating from Frazer’s Grove mine itself and iron from several sources. The Frazer’s Grove mine investigation provides insight into water quality and its likely development with time in abandoned mines.
Pb isotope variations in Archaean time and possible links to the sources of certain Mesozoic-Recent basalts
Authors: Luais Béatrice , Hawkesworth Chris J.
Published: April 2008

The present-day Pb isotope ratios of mafic and felsic rocks, feldspars and ore samples (galenas) from Archaean cratons ranging in age from 3.8–3.7 Ga to 2.6–2.7 Ga have been used to investigate their initial
207
Pb/
204
Pb ratios, and μ
1
ratios (source U/Pb ratios). Two broad trends of initial
207
Pb/
204
Pb evolution are observed, with samples from Greenland, Abitibi and SW India tending to have lower initial
207
Pb/
204
Pb ratios through time than those from eastern India, Australia and southern Africa (Kaapvaal and Zimbabwe). The calculated μ
1
values are in the range of 7–9 and they tend to be constant for a given craton through time, with Zimbabwe as a notable exception, for both mafic and felsic rock types. It follows that each craton is characterized by specific μ
1
values, or initial
207
Pb/
204
Pb ratios; μ
1
for Greenland is 7.37 ± 0.27, and they are higher for Abitibi at 7.57 ± 0.13, which are similar to SW India, where μ
1
= 7.57 ± 0.13. In Scotland μ
1
= 7.94 ± 0.152, in Finland μ
1
is 8.03 ± 0.08, in eastern India it is 8.03 ± 0.1, and for Australia μ
1
= 8.07 ± 0.14, and there tend to be higher values of 8 ± 0.2 and 8.32 ± 0.29 in the Kaapvaal and Zimbabwe, respectively. Overall, there is therefore a strong provinciality of μ
1
values in the Archaean areas, and unexpectedly the high μ
1
Archaean terranes are close to the areas of high μ
1
Mesozoic continental flood basalts (CFB), and the DUPAL anomaly in recent oceanic basalts. The cratons with high μ
1
values are in the southern hemisphere near the 130–190 Ma CFB from the Gondwana supercontinent, and the maximum DUPAL anomalies at latitudes 30–45°S south of Africa and 0–15°S south of India. The variations in μ
1
are based on
207
Pb/
204
Pb ratios, and so they primarily reflect events that took place early in Earth history. In one model, the high μ
1
values were initially generated at the core-mantle boundary, through processes linked to core formation and late accretion events, and they were mobilized in plumes responsible for at least some of the early Archaean mafic crust. Preservation of domains characterized by different U/Pb ratios is inferred to have been at relatively shallow levels, as they are preserved in both felsic and mafic rocks, and they appear to have survived for long periods of time, despite the effects of mantle convection and plate motion. Recycling of early Archaean lithosphere in the mantle by delamination might perpetuate these anomalies through time in a given area, perhaps even contributing to the modern DUPAL Pb anomalies. Delamination rather than subduction as the process of lithosphere recycling might explain why such regional domains are not observed in the kappa (κ
1
: source Th/U) ratios of these Archaean samples.
Stable isotopes in the Archaean Belingwe belt, Zimbabwe: evidence for a diverse microbial mat ecology
Authors: Grassineau N. V. , Nisbet E. G. , Fowler C. M. R. , Bickle M. J. , Lowry D. , Chapman H. J. , Mattey D. P. , Abell P. , Yong J. , Martin A.
Published: April 2008

Sulphide-rich sediments, stromatolitic limestones and tidal-flat deposits in the late Archaean (2.7 Ga) Manjeri and Cheshire Formations, Belingwe greenstone belt, Zimbabwe show evidence for complex and extensive prokaryotic mat communities, including (1) shallow-water coastal sulphur mats; (2) mats, probably in somewhat deeper water; (3) nearby stromatolites that lived by oxygenic photosynthesis in shallow coastal settings. Petrological and geochemical (rare earth element; REE) evidence, coupled with high-resolution stable isotope results, identifies several complex interdependent metabolic consortia of bacteria and archaea. These microbial consortia would have exchanged nutrients and products both locally within prokaryotic mats and more widely via the waters of the Belingwe basin. This isotopic, sedimentological and REE evidence for a complex ecology of bacteria and archaea is consistent with metabolic inferences from rRNA phylogeny and is direct evidence that a diverse prokaryotic community, managing carbon on a global scale, had evolved by the late Archaean.
Chitinozoa and Nd isotope stratigraphy of the Ordovician rocks in the Ebbe Anticline, NW Germany
Authors: Samuelsson J. , Gerdes A. , Koch L. , Servais T. , Verniers J.
Published: April 2008

Strongly tectonized Ordovician rocks appear in the Ebbe Anticline (Rheinisches Schiefergebirge), West Germany. These fine-grained detrital rocks of the Herscheider Schichten are divided into the Plettenberger Bänderschiefer. Unterer (Kiesberter) Tonschiefer, (Rahlenberger) Grauwackenschiefer, and the Oberer (Solinger) Tonschiefer. The scope of this investigation was to improve the dating of the entire Ordovician succession. but especially the Oberer (Solinger) Tonschiefer. We used chitinozoans. Palaeozoic microfossils of high biostratigraphic value, and Nd isotopes, which previously have been used for correlation and terrane affinity analysis. Chitinozoan preservation is poor, but some taxa could be confidentially identified to the species level. The ɛNd(t) values obtained from the Ordovician succession range from -8.0 to -9.2. Joint evaluation of chitinozoan and Nd isotope data together with previously known age-ranges suggest the following ages for the Herscheider Schichten: Plettenberger Bänderschiefer (early Abereiddian, earliest Llanvirn), Unterer (Kiesberter) Tonschiefer (early to mid Abereiddian, early Llanvirn). (Rahlenberger) Grauwackenschiefer (Aurelucian, earliest Caradoc), and Oberer (Solinger) Tonschiefer (late Caradoc). The Ebbe ɛNd(t) values are most readily compared with ɛNd(t) values from Avalonia. and we therefore support the inclusion of the Ordovician rocks of the Ebbe Anticline in that palaeocontinent.
Late Oligocene and early Miocene glacimarine sedimentation in the SW Ross Sea, Antarctica: the record from offshore drilling
Authors: Hambrey Michael J. , Barrett Peter J. , Powell Ross D.
Published: April 2008

Recent offshore drilling in the SW Ross Sea has recovered three cores with a cumulative thickness of 1500 m of Oligocene-early Miocene strata. Together with data from earlier drilling, notably from the CIROS-1 drillhole 70 km to the south (702 m of core), the cores record shallow marine glacigenic sedimentation at the margin of a rift basin bordering a mountain range that was breached by outlet glaciers from an ice sheet. This paper focuses on the late Oligocene-early Miocene record, which preserves sedimentary facies that represent warmer glacier ice and climatic conditions than are evident in the Antarctic today. Strata were cored near the South Victoria Land coast, and sedimentary facies include diamictite, conglomerate, breccia, sandstone, siltstone, mudstone and rhythmite. Facies associations, combined with seismic stratigraphic data, indicate an alternating proximal and distal marine record of glacigenic sedimentation, including phases of glacier grounding and variable degrees of iceberg rafting. Reworking by gravity-flow processes and near-shore submarine currents is also evident. These facies, together with evidence for periglacial vegetation, provide evidence of a late Oligocene-early Miocene climatic regime resembling that in the high-Arctic today. Thus, the climate was transitional between cooltemperate conditions of early Oligocene and cold-polar conditions of Quaternary time.
Contrasting glacial sedimentation processes and sea-level changes in two adjacent basins on the Pacific margin of Canada
Authors: Barrie J. Vaughn , Conway Kim W.
Published: April 2008

During the late Wisconsin Fraser Glaciation on the Pacific Margin of Canada, ice moved offshore from the Coast Mountains of the Canadian Cordillera and south into the Strait of Georgia, reaching a maximum extent at about 14 000
14
C
bp
. Most of the strait was ice-free by 11 300
14
C
bp
. Deglaciation was very rapid with regional downwasting and widespread stagnation. This resulted in a stratigraphy of thick till (30–60 m), overlain by ice-proximal glacimarine sediments and a thin and discontinuous ice-distal glacimarine unit. Glaciation of Queen Charlotte Basin reached a maximum sometime after 21 000
14
C
bp
. Deglaciation in this region began sometime after 16 000 to 15 000
14
C
bp
and ice had retreated fully onto mainland British Columbia by 13 500
14
C
bp
. Deglaciation was rapid, with the eastward retreat of an ice shelf. This resulted in a stratigraphy of a till up to 50 m in thickness, usually turbated by iceberg scour and overlain in some areas by thin, ice-proximal glacimarine sediments and much thicker (20 m) widespread ice-distal glacimarine sediments. A significant difference between these two regions is the deglacial relative sea-level history. Rapid regression of the outer Queen Charlotte Islands shelf occurred between approximately 14 600 and 12 500
14
C
bp
, primarily due to rapid isostatic rebound and contemporaneous with deglaciation of the continental shelf. Sea-level reached a maximum lowstand of greater than 150 m and remained low until approximately 12 400
14
C
bp
. In Georgia Basin, sea-level was at a relative high stand of 50 to 200 m during initial deglaciation, falling to between 0 to 50 m below present sometime after 10 000
14
C
bp
. We suggest that rapid emergence on the northern margin of the outer shelf was due to forebulge effects. Further, the very limited extent of glacial ice on the Queen Charlotte Islands and the exposure to the open Pacific forced the retreat of the Cordilleran ice-sheet margin eastwards thereby resulting in dominantly ice-distal glacimarine sedimentation. In contrast, the initial relative sea-level highstand during deglaciation between the Vancouver Island and Cordilleran glaciers in the Strait of Georgia resulted in significant ice-proximal deposition and limited ice-distal glacimarine deposition.
Developing high-resolution chronologies in glacimarine sediments: examples from southeastern Alaska
Authors: Jaeger John M.
Published: April 2008

Glacial systems release sediment to the marine environment over a range of time periods, including short-term seasonal, tidal, and diurnal scales. Often, the sedimentary record providing the highest temporal resolutions of short-term processes is found in relatively inaccessible areas such as proximal to glacier termini or below floating ice sheets. To assess the importance of short-term glacial and oceanographic processes in creating glacimarine strata, it is necessary to evaluate strata production over similar time scales. Time-series coring, sediment traps, or bathymetric profiles may be difficult to perform in these harsh settings. Coupling the observations of sedimentary structures seen in core X-ray radiographs with sound chronologies allows for the evaluation of short-term glacimarine sedimentation. For seasonal time-scales, the highly particle-reactive radioisotope
234
Th (24 day half-life) can be used to measure the rates of strata production under both steady and non-steady sediment deposition. To create appropriate age-depth relationships, two approaches are used: a rigorous and exacting mathematical model developed from a steady-state transport-reaction equation; and the less exacting CIC (constant initial concentration) and CRS (constant rate of supply or constant flux) point transformations. These methods are used for developing chronologies of cores collected in 1995 in Icy Bay, Alaska at 4, 12, and 32 km from the tidewater terminus of the Guyot Glacier. Examples are given of the boundary conditions that must be satisfied to use each approach in developing age-depth relationships. For the two cores collected closest to the terminus, age-depth relationships can be generated using all three approaches, whereas the presence of bioturbation in the most ice-distal sample severely complicates matters. The high (∼0.5 cm d
−1
) but non-steady sedimentation rates at the ice-proximal station create a high-resolution sedimentary record. The controls on sedimentation at this location are evaluated by examining sedimentary structures (alternating light and dark laminae and beds) seen in digitized X-ray radiographs. The CRS method is used to convert depth to time in X-ray radiographs. The down-core pixel intensities of the grey-scale X-ray radiograph positives are used to create a time series of sedimentation reflected in proxy data (the bulk density of the sediments). By using Blackman-Tukey spectral analyses and relatively new wavelet techniques to evaluate periodic processes, sedimentation at this location can be related to seasonal variability in meltwater production, fortnightly tidal influences on particle settling rates, and episodic precipitation-induced sediment deposition.
Retreat signature of a polar ice stream: sub-glacial geomorphic features and sediments from the Ross Sea, Antarctica
Authors: Shipp Stephanie S. , Wellner Julia S. , Anderson John B.
Published: April 2008

Three research expeditions to the Ross Sea, Antarctica resulted in collection of a dataset of more than 270 km of side-scan and chirp-sonar data, more than 330 km of swath bathymetry and 3.5 kHz data, and 24 cores within a glacially-carved trough. The former ice-stream flow path is divided into six zones, covering a distance of approximately 370 km, distinguished by unique stratigraphic signatures and geomorphic features. An erosional surface with thin, patchy lodgement till characterizes Zone 1. This region is interpreted as having experienced relatively high basal shear stress conditions. Zones 2, 3, and 4 are characterized by an erosional surface and thin, time-transgressive subglacial and grounding-line proximal deposits that include back-stepping moraines, flutes, transverse moraines, and corrugation moraines. These zones represent the transition between erosional and depositional regimes under the expanded LGM ice sheet; material eroded from the inner shelf was transported toward the outer shelf, possibly as a thin deforming till layer. The two outer zones are depositional and include maximum grounding-line (Zone 5) and pro-glacial deposits that were overridden subsequently by the ice sheet (Zone 6). Surface features include mega-scale glacial lineations, corrugation moraines, and iceberg furrows. Ice in these zones is interpreted as having experienced relatively lower basal shear stress, an extensional regime, and faster flow. This advance may have destabilized the ice sheet, initiating local draw-down and production of icebergs that furrowed the sea floor. Corrugation moraines are thought to represent annual retreat moraines, constraining the retreat rate of the ice sheet across the continental shelf to a consistent 40 to 100 m a
−1
.
The timing and location of major ore deposits in an evolving orogen: the geodynamic context
Authors: Blundell Derek J.
Published: April 2008

Although it is possible to identify the potential controls on mineralization, the problem remains to identify the critical factors. Very large mineral deposits are rare occurrences in the geological record and are likely to have resulted from the combination of an unusual set of circumstances. When attempting to understand the mineralization processes that occurred to form a major ore deposit in the geological past, especially the reasons why the deposit formed at a particular time and location within an evolving orogenic system, it is instructive to look at mineralization in modern, active subduction complexes. There it is possible to measure and quantify the rates at which both tectonic and mineralizing processes occur. In a complex subduction system, regions of extension develop. For example, subduction hinge retreat is a process that creates extension and generates heat from the upwelling of hot asthenosphere ahead of the retreating slab, producing partial melting, magmatism and associated mineralization. Seismic tomography not only images mantle as it is now, but subduction slab anomalies can be interpreted in terms of the past history of subduction. This can be used to test tectonic plate reconstructions. Tectonic and magmatic events occur rapidly and are of short duration so that many are ephemeral and will not be preserved. Furthermore, they can be diachronous as is the case with the lithospheric slab tear clockwise around the Carpathian Arc during the Neogene.

If the tectonic setting is paramount in determining the onset of the mineralization process and generation of mineralizing fluids, the fluid transport system that localizes the mineralization in space and time and concentrates the metal charge is the key to finding when and where the ore deposits occur. Fault and fracture networks in the crust provide various mechanisms for the localized expulsion of fluid in pulses of short duration. Excess surface water flow following large earthquakes in the Basin and Range region of USA offers a modern analogue to quantify fluid flow related to extensional faulting. Evidence from the Woodlark basin, east of Papua New Guinea, suggests that similar conditions pertain in the oceanic environment. Whilst there are limits to the use of regions of active tectonism as modern analogues to explain the mineralization of ancient orogenic systems, they do provide the best opportunity to understand the mechanisms of mineral processes and the controls on the location and timing of major ore deposits.
Auriferous arsenopyrite-pyrite and stibnite mineralization from the Siflitz-Guginock area (Austria): indications for hydrothermal activity during Tertiary oblique terrane accretion in the Eastern Alps
Authors: Amann G. , Paar W. H. , Neubauer F. , Daxner G.
Published: April 2008

Polymetamorphic schists and marbles of the Austroalpine Kreuzeck-Goldeck Complex are the host to auriferous arsenopyrite-pyrite as well as stibnite mineralization. In the Siflitz-Guginock area both types of mineralization are closely related spatially, but restricted to different lithologies. The auriferous arsenopyrite-pyrite mineralization is either disseminated or bound to quartz veins and strongly silicified fault-zones hosted in phyllites to (garnet-) micaschists. Similar disseminations within marbles are of minor importance. A stockwork-like mineralization of stibnite-filled fractures with weak metasomatic replacement is limited to marbles.

Both types of mineralization in the Kreuzeck-Goldeck Complex are intimately related to roughly east-west-trending semi-ductile to brittle strike-slip faults which formed during orogen-parallel wrenching. Semi-ductile to brittle kinematic indicators point to dextral, as well as sinistral, modes of fault movements, coeval with the formation of pyrite-arsenopyrite-bearing quartz veins, as well as the intrusion of Oligocene lamprophyre dykes. Mineralizing fluids are suggested to be derived from devolatilization of the subducted Penninic upper crust. Fluid ascent and ore precipitation is controlled by a transpressive strike-slip regime related to oblique terrane accretion during the Late Eocene to Oligocene.

Subsequent development of a Late Oligocene-Early Miocene pure shear regime with contraction trending (N)NE-(S)SW led to the development of conjugate NW-SE dextral and NE-SW sinistral brittle strike-slip faults, and overprinting by ESE-WNW oriented extension. These later events are related to the formation of auriferous mineralization from within the metamorphic core complex of the Tauern Window to the north of the Kreuzeck-Goldeck Complex, hence implying a significant change in spatial, temporal and structural control of Tertiary auriferous mineralization in the Eastern Alps.
40Ar/39Ar geochronology of magmatism and hydrothermal activity of the Madjarovo base-precious metal ore district, eastern Rhodopes, Bulgaria
Authors: Marchev Peter , Singer Brad
Published: April 2008

The Madjarovo volcanic complex and ore district comprise alteration styles from potassium silicate, advanced argillic and sericite alteration to adularia-sericite alteration/mineralization with a close and unambiguous spatial relationship to specific magmatic events. New40Ar/39Ar laser fusion and incremental heating experiments on nine sanidine, biotite, adularia, K-feldspar, and alunite samples constrain the ages and time span of lavas and tephras comprising the complex and their relationship to the hydrothermal activity. These results demonstrate that high-K calc-alkaline to shoshonitic volcanic activity beganc.32.7 Ma and terminatedc.500 ka later with the extrusion of quartz latite lavas at 32.2 Ma. The final stage of volcanism was accompanied by intrusion of compositionally similar monzonite stocks and trachytic dykes (c.32.2–32.1 Ma) and associated barren advanced argillic and sericite alteration (lithocap) and adularia-sericite base/precious metal vein mineralization. A probable thermal event atc.12–13 Ma disturbed the ages of alunite and sericite-bearing alteration at low stratigraphic levels. However, field relations combined with a plateau age of 32.1 ± 0.2 Ma from adularia in low-sulphidation veins that cross-cut lithocap indicate that hydrothermal activity, including base- and precious-metal vein deposition, was coeval with the youngest magmatic activity.
Discrimination criteria for assigning ore deposits located in the Dinaridic Palaeozoic-Triassic formations to Variscan or Alpidic metallogeny
Authors: Jurković Ivan , Palinkaš Ladislav A.
Published: April 2008

Among the ore deposits located in the Palaeozoic-Triassic formations of the Dinarides, siderite (± ankerite) and barite mineralization hosted by Carboniferous and Lower Permian sediments as well as polymetallic deposits in the Mid-Bosnian Schist Mts (MB), caused a long standing dispute concerning their metallogenic affiliation.
1.05–1.01 Ga Sveconorwegian metamorphism and deformation of the supracrustal sequence at Sæsvatn, South Norway: Re-Os dating of Cu-Mo mineral occurrences
Authors: Stein H. J. , Bingen B.
Published: April 2008

Re-Os dating of a suite of nine molybdenite samples from two small Cu-Mo mineral occurrences in the epidote-amphibolite facies Sæsvatn supracrustal sequence provides a temporal record of Sveconorwegian metamorphism and deformation. The sequence is situated in the western allochthonous lithotectonic domain of the Sveconorwegian orogen, the Rogaland-Hardangervidda terrane of south Norway. Onset of metamorphism occurred at about 1047 ± 2 Ma, as recorded in small gash veinlets, followed by a deformational peak at about 1032 ± 2 Ma, as recorded in mineralized breccia and mineralized metagabbro comprising a ductile shear zone constituting the Langvatn deposit. Deformation waned significantly by about 1017 ± 2 Ma, based on mineralization hosted in a brittle fault zone within stratigraphically higher metabasalts exposed at the Kobbernuten deposit. The origin of the mineralization at both deposits is most probably metamorphic with ore constituents provided by metasomatism of hosting basalt and gabbro. A metamorphic origin is supported by an array of Re-Os ages that can be related to structural features and the stratigraphic sequence, the absence of plutons related in time and space, the confinement of ore occurrences to mafic sequences in a bimodal supracrustal package that includes rhyolites and clastic units, and clear evidence for Cu mobility in mafic units. The ‘main’ Sveconorwegian orogenic event, probably a continent-continent collision involving imbrication, stacking, and burial of terranes took place at about 1.05 Ga and thereafter. Peak deformation in the Sæsvatn supracrustal sequence in the western part of the Sveconorwegian orogen (South Norway) may correlate with thermal metamorphism in the Idefjorden terrane in the eastern part of the orogen (SW Sweden), with a timing of about 1.03 Ga for both regions. The results of this study indicate that comparatively low grade domains in the orogen (greenschist to epidote-amphibolite facies), corresponding to upper crust, were deformed in a ductile fashion at about 1.03 Ga and were affected by brittle deformation as early as 1.025–1.015 Ga. In the high-grade domains (amphibolite to granulite facies), corresponding to middle and lower crust, ductile deformation is younger, beginning at about 1.025 Ga and persisting until 0.97 Ga.
Fluorine in orthoamphibole dominated Zn-Cu-Pb deposits: examples from Finland and Australia
Authors: Rajavuori Leena , Kriegsman Leo M.
Published: April 2008

Volcanogenic massive sulphide (VMS) deposits commonly occur within much larger fluorine (F)-bearing hydrothermal systems, where cordierite-orthoamphibole rocks are characteristic pathfinder assemblages. Here we report whole-rock and mineral F contents for orthoamphibole bearing rocks and associated rock types from Zn-Cu-Pb deposits in Finland (Orijärvi, Iilijärvi, Pyhäsalmi, Mullikkoräme) and central Australia (Oonagalabi). Textural and mineralogical data suggest that F influx predates peak metamorphism in these deposits.

The Mullikkoräme whole-rock data show positive correlations between F and X
Mg
= molar Mg/(Mg + Fe) and between F and elements of relatively low mobility (Ti, Al, Mg) and negative correlations between Fe and these elements. This suggests that iron was leached from silicate rocks by F-bearing fluids and was transferred to the Fe-rich sulphide ore. When normalized to immobile elements, F correlates positively with total metal content (Cu + Pb + Zn + Fe), consistent with the commonly observed increase of F content towards ore bodies.

Combining all microprobe data, hydrous minerals show the following order of decreasing F/(F + OH) ratio when coexisting: apatite > chondrodite > biotite > gedrite > (hornblende, muscovite, anthophyllite) > chlorite. The low- to medium-grade Finnish samples (Mullikkoräme: 500–560 °C at 1–3 kbar; Orijärvi: 550–650 °C at 3–5 kbar; Pyhäsalmi: 600–700 °C at 5–7 kbar) contain mainly F-poor anthophyllite, whereas F-rich gedrite is dominant in the higher grade Oonagalabi deposit (750–800 °C, 8–9 kbar). Temperature seems to have a significant, but X
Mg
a negligible influence on F partitioning between biotite and orthoamphiboles.
Continental growth in the Proterozoic: a global perspective
Authors: Windley Brian F.
Published: April 2008

During Proterozoic time, growth of the continents took place by the addition of mantlederived, juvenile material to pre-existing continental blocks. This accretion took place largely within three tectonic environments: (1) most importantly, in accretionary orogens such as the Birimian, the Baltic Shield, the Arabian-Nubian Shield and the early Altaids in Central Asia-these orogens grew largely by the accretion of island arcs, oceanic plateaus, accretionary prisms and ophiolites; (2) in the juvenile parts of collisional orogens as in the Trans-Hudson and Grenville; (3) within supercontinents that underwent rifting and breakup, giving rise to continental flood basalts and mafic dyke swarms. In addition to plate tectonics, the role of plume tectonics is increasingly emphasized as a fundamental process in Earth evolution. A mantle superplume may increase the oceanic spreading rate, the subduction rate and thus the island-arc production rate. It may also be responsible for the formation of a supercontinent, thus preserving the juvenile parts of collisional orogens, and it may be instrumental in the fragmentation of a supercontinent, giving rise to juvenile continental flood basalts. The balance between these processes is still poorly understood, as are calculated growth rates of Proterozoic crust.
Proterozoic-Cambrian history of Dronning Maud Land in the context of Gondwana assembly
Authors: Bauer W. , Thomas R. J. , Jacobs J.
Published: April 2008

Dronning Maud Land contains a fragment of an Archaean craton covered by sedimentary and magmatic rocks of Mesoproterozoic age, surrounded by a Late Mesoproterozoic metamorphic belt. Tectonothermal events at the end of the Mesoproterozoic and in Late Neoproterozoic-Cambrian times (Pan-African) have been proved within the metamorphic belt. In western Dronning Maud Land a juvenile Mesoproterozoic basement was accreted to the craton atc.1.1 Ga. Mesoproterozoic rocks were also detected by zircon SHRIMP dating of gneisses in central Dronning Maud Land, followed by a long hiatus for which geochronological data are lacking, an amphibolite to granulite facies metamorphism and syntectonic granitoid emplacement of Pan-African age have been dated. During this orogeny older structures were completely overprinted in a sinistral tranpressive deformation regime, leading to the mainly coast-parallel tectonic structures of the East Antarctic Orogen. Putting Antarctica back in its Gondwana position, the East Antarctic Orogen continues northward in East Africa as the East African Orogen, whereas a connection to the marginal Ross Orogen at the Pacific margin of East Antarctica is suggested along the Shackleton Range. The East Antarctic-East African Orogen resulted from closure of the Mozambique Ocean and collision of West and East Gondwana, i.e. western Dronning Maud Land was part of West Gondwana. During this collision the lithospheric mantle probably delaminated, allowing the asthenosphere to underplate the continental crust and producing heat for the voluminous, typically anhydrous, Pan-African granitoids of central Dronning Maud Land.
A review of the evolution of the Mozambique Belt and implications for the amalgamation and dispersal of Rodinia and Gondwana
Authors: Grantham G. H. , Maboko M. , Eglington B. M.
Published: April 2008

Geochronological, isotopic, lithological and structural data from the Mozambique Belt, and its extensions in Antarctica, Sri Lanka, India and Madagascar, are summarized and reviewed within a Gondwana framework. Much of the southern Mozambique Belt is dominated by Rodinian-Grenvillian-age juvenile magmatism and crustal genesis, with a strong metamorphic overprint during the Pan-African. Magmatism at
c.
800 Ma, possibly related to Rodinian fragmentation, is restricted to the Zambesi Belt, a few examples in northern Mozambique, southern Malawi, Tanzania, Madagascar and the Rayner Complex in Antarctica. Significantly, no crust of this age is recognized in western Dronning Maud Land. Amalgamation of Gondwana initially involved the closure of the Mozambique Ocean between the Tanzanian-Congo Cratons and the Madagascar-India-Enderby Cratons forming the East African Orogeny between
c.
580 and
c.
800 Ma. This was followed by the
c.
500–580 Ma transpressional sinistral collision of the combined Kalahari-East Antarctic Cratons and associated Grenville-age belts in the south, with the northern block comprising the combined Tanzanian-Congo-Madagascar-India-Enderby Block.
Proterozoic geochronology and tectonic evolution of southern Africa
Authors: Hanson R. E.
Published: April 2008

The Precambrian foundation of southern Africa consists of Archaean cratonic nuclei surrounded by belts formed during three separate Proterozoic orogenic episodes. Crust that formed or was reworked at 2.05–1.8 Ga is represented by orogenic belts (e.g. Kheis-Magondi, Ubendian-Usagaran) that partly wrap the ancient craton margins or form extensive basement in younger belts. Orogenic belts formed at 1.35–1.0 Ga record arc magmatism and collisional events during assembly of the Rodinia supercontinent. The Namaqua-Natal Belt defines a major convergent plate boundary active at this time along the southern margin of the Archaean Kaapvaal Craton, and the western part of this belt is inferred to link with a largely buried, NE-trending orogen present in the Kalahari region. Orogenesis in the same time frame farther north is recorded by the Kibaran and Irumide Belts, which are separated by the Palaeoproterozoic Bangweulu Block but are inferred to have undergone a linked tectonic evolution. East of the Irumide Belt, extensive Mesoproterozoic arc crust occurs in Malawi and Mozambique, although strong Neoproterozoic overprinting of the arc rocks makes their original relations unclear. Breakup of Rodinia is signalled by widespread Neoproterozoic alkaline and bimodal magmatism associated with rift zones which, in some cases, evolved into major ocean basins. Subsequent amalgamation of Gondwana led to collisional orogenesis culminating at 575–505 Ma in the Mozambique and West Congo-Kaoko-Gariep-Saldania Belts along the present eastern and western margins of southern Africa. Coeval deformation in the interconnecting, transcontinental Damara-Lufilian-Zambezi Orogen may reflect destruction of linked rifts and narrow ocean basins driven by farfield stresses from the collisional plate margins.
Middle Jurassic to Cretaceous extensional tectonics and sedimentation in the eastern external zone of the Betic Cordillera
Authors: Fernández-Fernández E. , Jabaloy A. , González-Lodeiro F.
Published: April 2008

In the External Zones of the eastern Betic Cordillera, two sets of Mesozoic highangle normal faults can be observed, one with ENE-WSW strikes and the other with N-S strikes. Both sets of faults generate half-grabens and grabens, infilled with wedge-shaped and lens-shaped formations deposited during the Late Jurassic to Early Cretaceous. The relationships of these formations indicate progressive tilting of the hanging walls during deposition of the rocks. The largest basins are related to the ENE-WSW faults.

The rocks of Middle Jurassic age, which predate the faulting stage, are shallow-marine oolitic limestones. The Lower Cretaceous Fardes Formation shows evidence of deposits closer to the carbonate compensation depth (CCD). This evidence indicates that normal faulting was related to very significant thinning of the continental crust.

Palaeomagnetic studies in the area demonstrate the existence during the Miocene of clockwise and counterclockwise rotations with vertical axes. Restoring the faults to their original orientation, the present-day ENE-WSW faults and their main basins had original N-S strikes, while the N-S faults originally had WNW-ESE strikes.

This extensional stage occurred at the same time as the rifting of Iberia and North America, the opening of the Gulf of Biscay and the aborted rifting of the Iberian chain.
Evolution of the southern margin of the Donbas (Ukraine) from Devonian to Early Carboniferous times
Authors: Mccann T. , Saintot A. , Chalot-Prat F. , Kitchka A. , Fokin P. , Alekseev A.
Published: April 2008

A Devonian-Early Carboniferous succession comprising thick clastic and carbonate sediments with interbedded volcanics was examined along the southern margin of the Donbas fold belt, Ukraine. Following initial rifting and subsidence, a continental (fluvial, lacustrine) succession was established. This first phase of synrift activity (Eifelian) was accompanied by the extrusion of basalts. In Late Givetian-early Famennian times, half-graben development was pronounced and a series of E-W-trending half-grabens were formed, along with coeval volcanic activity. Subsequent basin subsidence led to the establishment of a broad marine carbonate platform across the region (Late Famennian-Tournaisian-early Viséan). Renewed uplift led to partial exposure and karstification of the platform. This was accompanied by trachyte intrusion and extrusion. The overlying chert-rich unit was probably deposited under lacustrine conditions, although a shelf environment has also been suggested. Renewed tectonic activity along the main basin-bounding fault resulted in the synsedimentary deformation of this unit.
Precise tracing of exhumation and provenance using
40
Ar/
39
Ar geochronology of detrital white mica: the example of the Central Alps
Authors: Von Eynatten H. , Wijbrans J. R.
Published: April 2008

Single-grain
40
Ar/
39
Ar dating of detrital white mica from Oligocene to Miocene (31–13 Ma) sediments of the North Alpine Foreland Basin in Switzerland reveals three prominent age clusters indicating cooling of the source rocks below 350–420°C in Carboniferous, Early Permian, and Tertiary times. Precise calibration of sedimentation age throughout the study area enables the thermal evolution of the hinterland in space and time to be precisely traced. Palaeozoic mica ages are documented in all samples and are used as additional provenance indicators. Tertiary mica ages are restricted to sediments younger than 21 Ma, and are only found in central and western drainage systems. Tertiary micas document progressively increasing average cooling rates up to 34–41°C/Ma in the source area (Lepontine Dome), between 21 Ma and 14 Ma. The observed cooling rates and the time-span for rapid cooling in the source area (between 19 and 14 Ma) agree with thermal models derived from currently exposed rocks of the Lepontine metamorphic dome. This study proves that detrital mica geochronology is a robust tool for deciphering the thermal histories of ancient orogens which are no longer exposed today.
A case study of hydrocarbon transport along active faults and production-related stress changes in the Monterey Formation, California
Authors: Chanchani Sneha K. , Zoback Mark D. , Barton Colleen
Published: April 2008

Recent field studies show that critically stressed faults, that is, faults that are close to frictional failure in the current stress field, serve as conduits for fluid flow. Similarly, geological field studies of the low permeability siliceous shales in the Monterey Formation, California, clearly indicate that faults influence hydrocarbon transport. We report here a study of the Antelope Shale, a low permeability siliceous shale hydrocarbon reservoir in the Buena Vista Hills field in the southern San Joaquin Valley to determine the influence of the stress state on the relative hydraulic conductivity of the fractures and faults present in the subsurface. Because production has both lowered reservoir pressure and the horizontal stresses, it was necessary to “restore” the reservoir stress state to initial conditions in order to identify correctly the most highly productive intervals. This analysis demonstrates that prior to production, faults in the reservoir were active in a transitional reverse/strike-slip faulting stress state, consistent with regional tectonics. Initial production rates in the field were 2000 barrels of oil per day, principally from intervals where critically stressed faults were encountered.
From seismic data to core data: an integrated approach to enhance reservoir characterization
Authors: Hesthammer Jonny , Fossen Haakon
Published: April 2008

Integrated structural analyses of seismic and various well data are necessary to optimize hydrocarbon reservoir characterization. However, there are many published examples from the oil and gas industry where single data types are analysed but not integrated. This may lead to erroneous interpretations and drainage strategies. As illustrated by an example from the area around well 34/10-B-12 in the North Sea Gullfaks Field, integrated structural interpretation should typically utilize all available seismic surveys, well log correlation data, dipmeter data and core data. Interpretation of seismic data helps in the understanding of large-scale structural and stratigraphic geometries. Time-lapse (4D) seismic helps to identify changes in reservoir properties caused by injection and production. Well log correlation data are used to document variations in zonation thickness caused by sedimentological or structural changes. Dipmeter data tie observations of bedding orientation from seismic data to subseismic scale. Core data represent the most detailed (millimetre to metre scale) data available and can yield information on rock properties as well as sedimentological and (micro)structural features. Small-scale deformation structures such as deformation bands and fractures can typically be identified and characterized. In addition, it is possible from unorientated cores to find the orientation of bedding and deformation structures. This information is compared to observations from dipmeter data, well log correlation data and seismic data to improve the interpretation.

Well 34/10-B-12 is a hanging wall injector near one of the large-scale faults in the Gullfaks Field. Several 3D seismic surveys are available from the area, as are standard well log data, dipmeter information and cores. Together, the data range from millimetre to kilometre with some overlap between the data types. Through integrated analysis, pitfalls such as interpreting any linear feature on timedip attribute maps as faults has been avoided. Also, a geometric relation between core-scale and seismic-scale faults has been established, and it has been possible to relate small-scale and large-scale structures in a model which is consistent with all the available data.
Microseismic monitoring of geomechanical reservoir processes and fracture-dominated fluid flow
Authors: Jupe Andrew J. , Jones Robert H. , Wilson Stephen A. , Cowles John F.
Published: April 2008

Microearthquakes (microseismic events) are induced during hydrocarbon and geothermal fluid production operations in naturally fractured reservoirs. They typically result from shear-stress release on pre-existing faults and fractures due to production/injection induced perturbations to the effective stress conditions. These stress changes may be due to reservoir depletion, flooding or stimulation operations. Over a number of years it has been shown that microseismic monitoring has the potential to provide valuable time-lapse 3D information on the geomechanical processes taking place within a reservoir. These processes include the distribution of fluid flow and pressure fronts within naturally fractured systems, production-related compaction and the reactivation of faults. With the advent of permanent reservoir monitoring systems (e.g. intelligent wells) microseismic monitoring has the potential to become a practicable means of time-lapse imaging of hydrocarbon reservoir processes remote from production/injection boreholes. This paper illustrates some of the ways in which microseismic monitoring can contribute to the development and management of hydrocarbon reservoirs through the presentation of examples from both hydrocarbon and geothermal reservoirs.