South Devon,UK

Where can you teach the fundamental principles of geology in three short field days? In this feature article, I introduce South Devon as a classic area of geology, useful for introducing basic concepts relevant to school and first‐year degree curricula. That this region remains a catalyst for developing young geologists is especially fitting given that it was here that Henry de la Beche embarked on the first mapping project of what would later become the British Geological Survey. In South Devon, a novice geologist can learn about plate tectonics, the unifying theory of our subject, through a visit to two quarries. At Chipley Quarry, near Newton Abbott, sea‐floor spreading is well illustrated by Devonian pillow‐lavas extruded within the back‐arc margins of the Rheic Ocean. At Burrator Quarry, near Yelverton, the aftermath of continental collision is represented by the north‐verging Variscan fold belt, intruded and thermally metamorphosed by the Dartmoor granite. A trip along the coast to Paignton provides an informative lesson about the distinctive stratigraphic imprint that is created by this plate tectonic cycle. Here, tightly folded Devonian slates are unconformably overlain by near‐horizontal Permian breccia‐conglomerates at Waterside Cove. The ‘abyss of time’ at their junction represents the tens of millions of years involved in the uplift and denudation of Rheic Ocean sediments in the course of the Variscan Orogen. The Devonian and Permian successions of South Devon additionally serve as an excellent training ground for interpreting ancient environments with both marine and terrestrial facies spectacularly represented. Fossils abound in these strata, too, including the stromatoporoid life assemblage at Triangle Point, Torquay, where students have the opportunity to do a virtual scuba‐dive over a Devonian reef. Finally, given current discussions about employability in UK universities, there is even room in an excursion to instill the practical importance of geology: for example, a trip to the scenic Burrator Reservoir on Dartmoor reveals what can go wrong for geotechnical engineers when a major dam is sited on a granite weakened by chemical weathering. In summary, South Devon has it all—a brilliant natural laboratory for training and inspiring the next generation of geologists.     

The stratigraphy of the Chalk Group (Cretaceous) of the Devon coast,UK

An almost continuous layer of Upper Cretaceous deposits up to 1000 m thick was probably deposited across much of SW England. Phases of uplift in the late Cretaceous and early Cenozoic, each of which was followed by extensive erosion and dissolution, resulted in the removal of all except a few outliers of Chalk Group that crop out in east Devon and south Somerset. Those on the Devon coast between Sidmouth and Lyme Regis are some of the best exposed Cenomanian to early Coniacian successions in NW Europe and include the most westerly chalks preserved onshore in England. They form an integral part of the Dorset and East Devon World Heritage Site. In contrast to the Chalk of much of southern England, the older formations in Devon, the Beer Head Limestone, Holywell Nodular Chalk and New Pit Chalk, show marked lateral lithological variations that result from a combination of penecontemporaneous movements on local faults and relatively shallow-water environments close to the western edge of the Chalk depositional basin. The younger parts of the succession, the Lewes Nodular Chalk and Seaford Chalk Formations, comprise chalks that do not appear to have been greatly affected by penecontemporaneous fault movements. These formations include lithological marker beds that have been correlated with marker beds in the Sussex type area. The principal sedimentary breaks in the Devon succession cannot be correlated with confidence with eustatic changes in sea level.     

Permo–Triassic unconformity-related Au-Pd mineralisation, South Devon, UK: new insights and the European perspective

An integrated mineralogical-geochemical study of unconformity-related Au-Pd occurrences within and around the Permo–Triassic basins of southwest England, UK, has confirmed the importance of low temperature (86±13°C), hydrothermal carbonate veins as hosts for the mineralisation. Fluid inclusion data for the carbonate gangue, supported by stable isotope (¹³C and ¹⁸O) and radiogenic (⁸⁷Sr/⁸⁶Sr) data, have identified three principal fluids: (1) a reducing calcic brine [>25 wt% salinity, <0.5 NaCl/(NaCl+CaCl₂)] originating in the sub-unconformity basement and an expression of advanced mineral–fluid interaction; (2) an oxidising sodic brine [~16 wt% salinity, >0.9 NaCl/(NaCl+CaCl₂)] originating in the post-unconformity red beds under evaporitic conditions, and (3) an oxygenated, low salinity groundwater (<3 wt% salinity). The sodic brine is reasoned to be the parent metalliferous fluid and to have acquired its enrichment in Au and Pd by the leaching of immature sediments and intra-rift volcanic rocks within the local Permo–Triassic basins. Metal precipitation is linked to the destabilisation of Au and Pd chloride complexes by either mixing with calcic brines, dilution by groundwaters or interaction with reduced lithologies. This explains the diversity of mineralised settings below and above the unconformity and their affinity with red bed brines. The paucity of sulphide minerals, the development of selenides (as ore minerals and as mineral inclusion in gold grains), the presence of rhodochrosite and manganoan calcites (up to 2.5 wt% Mn in calcite) and the co-precipitation of hematite and manganese oxides are consistent with the overall high oxidation state of the ore fluids. A genetic model is proposed linking Permo–Triassic red beds, the mixing of oxidising and reducing brines, and the development of unconformity-related precious metal mineralisation. Comparison with other European Permo–Triassic basins reveals striking similarities in geological setting, mineralogy and geochemistry with Au, Au-Pd and selenide occurrences in Germany (Tilkerode, Korbach-Goldhausen), Poland (Lubin) and the Czech Republic (Svoboda nad Úpou and Stupná). Though the known Au-Pd occurrences are sub-economic, several predictive criteria are proposed for further exploration.Editorial handling: B. Lehmann     

Cretaceous foraminifera from the Turonian succession at Beer, southeastern Devon, England

During Winter 2000/2001 a major cliff fall occurred at the back of the bay in Beer, southeastern Devon. In the subsequent remedial works the cliff was cleared of loose material prior to stabilisation works. This process provided a suite of samples from previously inaccessible parts of the cliff succession. By adding these samples to earlier suites of samples collected over the last 30 years it has been possible to improve our knowledge of the foraminifera of Turonian age in southwestern England.The planktic foraminifera are, perhaps, the most significant component of the assemblages as many of the taxa recorded in Devon are of southern aspect and are relatively rare in others parts of the U.K. The planktic zonation recognised in this section is, therefore, almost identical with that recorded in Tethyan areas and a precise correlation can be generated. There are relatively large numbers of important taxa such as Helvetoglobotruncana helvetica, Dicarinella imbricata, Marginotruncana sigali, M. pseudolinneiana, M. coronata and M. schneegansi. The benthic foraminifera, by comparison, are relatively rare and are represented by a low diversity assemblage. This is typical of northwestern European chalk successions of Turonian age and is coincident with the highest sea levels of the Cretaceous. In the middle to late Turonian there is a dramatic shallowing event that is recorded world-wide at this level. The assemblage changes towards the top of the accessible succession at Beer record this significant, world-wide event.     

The on-site quality control of armourstone during construction of a rock revetment in south Devon,UK

Summary A programme of on-site quality control of the as-produced properties of rock armour used for the construction of a beach revetment for coastal protection in south Devon is described. The programme was based on guidelines set out in the recently published CIRIA/CUR Manual,The Use of Rock in Coastal and Shoreline Engineering (1991). The results of the study indicate that the quality control of heavy armourstone block weight, grading and shape would be greatly facilitated by the introduction of standard specifications as recommended in the CIRIA/CUR Manual. A completely satisfactory test for the quality control of block integrity has yet to be devised and accepted, although the drop test breakage index described in the Manual provides a starting point. An improved method of heavy armour block weight estimation by cubing-up, based on the volume of an imaginary enclosing rectanguloid multiplied by the rock density and a fractional weight shape factor, has been developed. Using shape factors computed from weighed sample blocks of known density, the accuracy of these block weight estimates is shown to be in the order of ±5% for mean or median values. These fractional shape factors also allow some control of possible undesirable block shapes, such as the rectanguloid or wedge shape.     

Archosaur remains from the Otter Sandstone Formation (Middle Triassic, late Anisian) of Devon, southern UK

A new jaw from the Middle Triassic (Anisian) Otter Sandstone Formation of Devon confirms the existence of a derived archosaur (avesuchian). Numerous isolated teeth and vertebrae had already suggested the presence of archosaurs in the Otter Sandstone Formation, presumed predators on the fauna of temnospondyls, procolophonids, and rhynchosaurs, but the new fossil is the first to show some diagnostic characters. Other elements in the same block as the jaw, but not necessarily from the same animal, include a possible skull or pelvic bone, a slender long bone, a small tooth (perhaps prolacertiform), and two presumed archosaur dermal scutes. An additional scute is present, as well as the probable distal end of a pubis, perhaps from a large poposauroid archosaur like the Anisian-age Bromsgroveia or Arizonasaurus. The jaw and pubis represent animals of very different sizes, some 0.8 m and 3 m long in estimated body length respectively.     

Rates and patterns of contemporary floodplain sedimentation: A case study of the river Culm,Devon, UK

A variety of methids have been used to document contemporary rates of overbank sedimentation along an 11 km reach of the floodplain of the River Culm in Devon, UK. These have included measurements of the conveyance loss of suspended sediment between upstream and downstream measuring stations, the use of sediment traps, and the application of caesium-137 measurements to estimate the rate of accumulation of fine sediment over the past 30 years. The resultant data point to sedimentation rates typically of the order of 1500 g m^–2 year^–1 or 1.5 mm year^–1. Values in excess of 15000 g m^–2 year^–1 or 15 mm year^–1 have been documented in closed depressions and in backwater embayments close to the channel, whereas minimal deposition and even scour may occur along those areas that function as secondary channels during floodplain inundation.     

Gold-Silver vein mineralization at Tyndrum,Scotland

Summary Electrum, hessite, petzite and sylvanite have been recorded from veins at Tyndrum, Scotland. Electron probe micro-analyses have also revealed two un-named Ag-Te-S phases. Fluid inclusion studies suggest that the mineralising fluids responsible for the precious metal mineralization contained 7.0 mol % CO₂ and 7 wt % NaCl. T_H (temperature of homogenisation) determinations were in the range 295°C to 325°C and a depth of vein formation 4 km is indicated. Mineral precipitation was probably caused by cooling and adsorption of gold onto pyrite. ³⁴S values of + 1.8%o for galena from the Au + Ag + Te veins suggest a different (possibly igneous) sulphur source to that producing the Pb +Zn vein mineralization in the Tyndrum area. Although an age of 380 Ma was obtained using K-feldspar in the veins the data are not conclusive. It is argued that the Au + Ag mineralization at Tyndrum is due to hydrothermal activity related to Cu +Mo mineralization associated with the Late Caledonian granites.

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Die Gold-Silber-Gang-Vererzung von Tyndrum, Schottland

Zusammenfassung In der Gang-Vererzung von Tyndrum, Schottland, kommen eine Gold-Silber-Legierung, Hessit, Petzit und Sylvanit vor. Außerdem ergaben Mikrosondenanalysen das Vorhandensein von zwei unbekannten Ag-Te-S Phasen. Die Untersuchungen von Flüssigkeitseinschlüssen zeigen, daß die für die frühe Mineralisation verantwortlichen Lösungen 7.0% Mol.% CO₂ und 7 Gew.% NaCl enthalten haben. Die Homogenisie-rungstemperaturen liegen im Bereich von 295-325°C und weisen auf eine Tiefe der Gangentstehung von 4 km hin. Die Au-Ag-Te Mineralbildung kam vermutlich durch Abkühlung und Adsorption von Gold auf Pyrit zustande. ³⁴S-Werte von + 1.8%0, gemessen an Bleiglanz aus Au-Ag-Te-Gängen deuten eine magmatische Schwefelherkunft an, während die Pb-Zn-Gangmineralisation eine andere Schwefelquelle vermuten läßt. Die Altersdatierung der Gänge von 380 Mio. Jahren läßt sich derzeit noch nicht sinnvoll interpretieren. Es wird angenommen, daß die Au-Ag-Vererzung von Tyndrum durch hydrothermale Aktivität im Zusammenhang mit einer an spätkaledonische Granite gebundenen Cu-Mo-Vererzung entstanden ist.

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Nickel-copper mineralization at Talnotry,Newton Stewart,Scotland

Summary Nickel-copper mineralization occurs near the base of a diorite intrusion close to its contact with hornfelsed Ordovician and Silurian shales and greywackes. The principal ore minerals are nickeline, gersdorffite, pyrrhotine, pentlandite and chalcopyrite with minor amounts of molybdenite, tellurobismutite, gold, sphalerite and argentopentlandite. Pyrite, marcasite, violarite and goethite also occur but are interpreted as later alteration products. Much of the pyrrhotine-rich mineralization at the base of the intrusion is in the form of blebs and interstitial aggregates with amphiboles, plagioclase feldspar, biotite, chlorite and quartz. Chalcopyrite-rich and nickeline-gersdorffite-rich mineralization occurs above this and immediately below unmineralized diorite in the form of patches, lenticular masses and stringers along joints and fractures.Whole rock and ore analyses and electron microprobe data on the silicates, sulphides and sulpharsenides are presented.The unmineralized diorite has low SiO₂ and high MgO contents compared to typical diorites and relatively high Cr, Ni and Ti trace element values. In the mineralized diorite, platinum-group elements occur in very low concentrations in the pyrrhotineand chalcopyrite-rich assemblages but Pt, Pd and Au show significant enrichment in the nickeline-gersdorffite-rich mineralization.A magmatic origin for the mineralization is proposed rather than formation by hydrothermal solutions or metasomatism.

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Die nickel-kupfer vererzung von Talnotry, Newton Stewart, Schottland

Zusammenfassung Die Nickel-Kupfer Vererzung befindet sich im Kontaktbereich einer Dioritintrusion mit hornfelsdurchdrungenen silurischen und ordovizischen Grauwacken. Die am häufigsten auftretenden Erzmineralien sind Nickelin, Gersdorffit, Pyrrhotin, Pentlandit, und Chalcopyrit mit kleineren Mengen von Molybdenit, Tellurwismuth, Gold, Sphalerit und Argentopentlandit. Weiters treten als spätere Umwandlungsprodukte Pyrit, Markasit, Violarit und Goethit auf. Ein großer Teil der Pyrrhotin-reichen Vererzung am Fuße des eingedrungenen Diorits bildet fleckige, lückenfüllende Aggregate zwischen den Silikatphasen (Amphibol, Plagioklas, Biotit, Chlorit, Quarz). Die Chalcopyrit- und Nickelin-Gersdorffit-reiche Vererzung tritt zwischen dem unvererzten Diorit im Hangenden und der Pyrrhotin-reichen Vererzung im Liegenden als linsige, lappenartige Konzentration und entlang von Fugen und Brücken auf.Vollgesteins-Analysen sowie ausgewählte Erzanalysen und Mikrosondenergebnisse der Silikate, Sulfide und Sulfarsenide liegen vor.Im Unterschied zu den typischen Dioriten zeigt der unvererzte Diorit einen niedrigen SiO₂ und hohen MgO Gehalt und relativ hohe Cr, Ni und Ti Spurenelementwerte. Die Platingruppenelemente sind in den Pyrrhotin- und Chalcopyrit-reichen Vererzungen nur schwach vertreten, jedoch sind Pt, Pd und An in der Gersdorffit-reichen Vererzung stark angereichert.Ein magmatischer Ursprung dieser Vererzung, im Gegensatz zu einer metasomatischen Entstehung oder Bildung aus hydrothermalen Lösungen wird vermutet.

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The Middle Triassic (Anisian) Otter Sandstone biota (Devon,UK): review,recent discoveries and ways ahead

The Middle Triassic (Anisian) Otter Sandstone was laid down mostly by braided rivers in a desert environment and is now well exposed along the south-east Devon coast in south-west England, part of the ‘Jurassic Coast’ World Heritage Site. It yields uncommon and generally fragmentary fossils, principally of vertebrates, including fish, temnospondyl amphibians and reptiles such as rhynchosaurs, predatory archosaurs, and small superficially lizard-like forms. These provide important information about a freshwater and terrestrial ecosystem that marks recovery from the end-Permian mass extinction, but pre-dated the appearance of dinosaurs and mammals. The constantly eroding Otter Sandstone exposures continue to reveal new taxa (for example, freshwater sharks). Furthermore, microvertebrate material obtained by sieving bone-bearing levels has the potential to further expand the faunal list. Newly discovered associated and articulated vertebrate remains, including small tetrapods, improve knowledge of whole-body anatomy and facilitate systematic work. Invertebrate burrows and reptile footprints provide information on ecological interactions and detailed bed-by-bed collecting casts light on taphonomic processes and faunal changes over time.     

Structural control on gold mineralization at Walhalla,Victoria

Gold mineralization associated with quartz reefs is related to the structural history of the Early Devonian, Walhalla Group. These reefs are situated in the Walhalla Synclinorium, developed during the Middle to Late Devonian Tabberabberan Orogeny. A pervasive north‐south‐trending axial planar cleavage and two styles of folding were produced during regional east‐west compression. The first are upright, open to close folds with sub‐horizontal fold axes. The second are plunging inclined, close to tight folds with fold axes that plunge steeply to the north and south. An extensional event is associated with the emplacement of the Woods Point Dyke swarm and a set of normal faults that offset all earlier structures. High‐angle reverse faults, which post‐date the folding and the emplacement of the dykes, were utilized as conduits for hydrothermal fluids and preferentially localize mineralization to laminated quartz veins. En echelon vein arrays formed during initial stages of reverse faulting became deformed during prolonged shearing to produce ptygmatic veins. Laminated quartz veins within high‐angle reverse faults contain arsenopyrite and pyrite in vein margins and gold in fractures that cross‐cut continuous quartz crystals. Gold, galena, chalcopyrite and sphalerite may also be deposited adjacent to and within fractured arsenopyrite and pyrite. Late‐stage, cross faults developed in a regime of north‐south compression and post‐date the laminated quartz veins and mineralization.     

The sandstone-hosted stratiform copper mineralization at Mwitapile and its relation to the mineralization at Lufukwe, Lufilian foreland, Democratic Republic of Congo

The Lufilian foreland is a triangular-shaped area located in the SE of the Democratic Republic of Congo and to the NE of the Lufilian arc, which hosts the well-known Central African Copperbelt. The Lufilian foreland recently became an interesting area with several vein-type (e.g., Dikulushi) and stratiform (e.g., Lufukwe and Mwitapile) copper occurrences. The Lufilian foreland stratiform Cu mineralization is, to date, observed in sandstone rock units belonging to the Nguba and Kundelungu Groups (Katanga Supergroup).The Mwitapile sandstone-hosted stratiform Cu prospect is located in the north eastern part of the Lufilian foreland. The host rock for the Cu mineralization is the Sonta Sandstone of the Ngule Subgroup (Kundelungu Group). A combined remote sensing, petrographic and fluid inclusion microthermometric analysis was performed at Mwitapile and compared with similar analysis previously carried out at Lufukwe to present a metallogenic model for the Mwitapile- and Lufukwe-type stratiform copper deposits. Interpretation of ETM+ satellite images for the Mwitapile prospect and the surrounding areas indicate the absence of NE–SW or ENE–WSW faults, similar to those observed controlling the mineralization at Lufukwe. Faults with these orientations are, however, present to the NW, W, SW and E of the Mwitapile prospect. At Mwitapile, the Sonta Sandstone host rock is intensely compacted, arkosic to calcareous with high silica cementation (first generation of authigenic quartz overgrowths). In the Sonta Sandstone, feldspar and calcite are present in disseminated, banded and nodular forms. Intense dissolution of these minerals caused the presence of disseminated rectangular, pipe-like and nodular dissolution cavities. Sulfide mineralization is mainly concentrated in these cavities. The hypogene sulfide minerals consist of two generations of pyrite, chalcopyrite, bornite and chalcocite, separated by a second generation of authigenic quartz overgrowth. The hypogene sulfide minerals are replaced by supergene digenite and covellite. Fluid inclusion microthermometry on the first authigenic quartz phase indicates silica precipitation from an H₂O–NaCl–CaCl₂ fluid with a minimum temperature between 111 and 182 °C and a salinity between 22.0 and 25.5 wt.% CaCl₂ equiv. Microthermometry on the second authigenic quartz overgrowths and in secondary trails related to the mineralization indicate that the mineralizing fluid is characterized by variable temperatures (Th = 120 to 280 °C) and salinities (2.4 to 19.8 wt.% NaCl equiv.) and by a general trend of increasing temperatures with increasing salinities.Comparison between Mwitapile and Lufukwe indicates that the stratiform Cu mineralization in the two deposits is controlled by similar sedimentary, diagenetic and structural factors and likely formed from a similar mineralizing fluid. A post-orogenic timing is proposed for the mineralization in both deposits. The main mineralization controlling factors are grain size, clay and pyrobitumen content, the amount and degree of feldspar and/or calcite dissolution and the presence of NE–SW to ENE–WSW faults. The data support a post-orogenic fluid-mixing model for the Mwitapile- and Lufukwe-type sandstone-hosted stratiform Cu deposits, in which the mineralization is related to the mixing between a Cu-rich hydrothermal fluid, with a temperature up to 280 °C and a maximum salinity of 19.8 wt.% NaCl equiv., with a colder low salinity reducing fluid present in the sandstone host rock. The mineralizing fluid likely migrated upwards to the sandstone source rocks along NE–SW to ENE–WSW orientated faults. At Lufukwe, the highest copper grades at surface outcrops and boreholes were found along and near to these faults. At Mwitapile, where such faults are 2 to 3 km away, the Cu grades are much lower than at Lufukwe. Copper precipitation was possibly promoted by reduction from pre-existing hydrocarbons and non-copper sulfides and by the decrease in fluid salinity and temperature during mixing. Based on this research, new Cu prospects were proposed at Lufukwe and Mwitapile and a set of recommendations for further Cu exploration in the Lufilian foreland is presented.     

Formation of dawsonite mineralization at the Zaozernyi deposit,Belarus

Systems of silty sand-water and clayey rock-water were simulated under open-system conditions with respect to CO₂ and oxygen at 25°C. It was shown that the dawsonite(+ quartz)+kaolinite assemblage at the Zaozernyi deposit was formed by interaction of terrigenous sediments with chloride-sodium solutions containing NaCl >100 g/kg H₂O and saturated in carbonic acid (a partial pressure of CO₂ > 0.5 bar). The modeling results do not confirm the possibility of dawsonite formation after primary bauxites (products of kaolinite weathering crusts), which is presumably related to the inconsistent formation parameters of these minerals during weathering of aluminosilciate rocks.     

Two stages of gold mineralization at Hutti mine, India

The Hutti gold mine is located in a high-angle, NNW–SSE-trending shear zone system, which hosts nine discrete auriferous shear zones (reefs). On a clockwise, retrograde PT path two separate stages of deformation/metamorphism (D₂/M₂ and D₃/M₃) occurred synchronous with two distinct stages of gold mineralization, both of which were associated with different fluid types. Stage 1 mineralization developed during D₂/M₂, where the amphibolite host rocks were altered by a metamorphic fluid with a $ {{\delta }^{{18}}}{{O}_{{{{H}_2}O}}} $ of 7.5–10.1?‰, rich in K, S, As, and Au at pressure and temperature conditions of around 3 kbar and 530?+?20/?30°C, respectively. The stage 1 auriferous shear zones are enveloped by a zoned alteration consisting of a distal biotite–chlorite and proximal biotite–plagioclase assemblage. Subsequently, D₂/M₂ was overprinted by D₃/M₃ deformation and metamorphism at 300–400°C and <2 kbar that formed the stage 2 mineralization. The stage 2 mineralizing fluid which originated from outside the greenstone belt (δ¹⁸O_fluid of 3.2–6.8?‰) was rich in Si, Au, and W. This mineralization stage is distinct by the emplacement of laminated quartz veins central to the shear zone, containing locally visible gold at concentrations of up to 1 kg Au/t. The laminated quartz veins are surrounded by a millimeter-scale chlorite₂–K-feldspar alteration halo, which replaced the stage 1 biotite–plagioclase assemblage. The oxygen isotopic composition of the stage 2 fluid suggests a mixture of a magmatic fluid with an oxygen isotopic composition in the range of 6 to 10?‰ and an isotopically light formation fluid that resulted from fluid–rock interaction in the greenstone pile. The two fluid fluxes at stages 1 and 2 both contributed to the overall gold mineralization; however, it was the second fluid pulse, which gave the Hutti mine its status as the largest gold mine in India. The metamorphic evolution was thereby important for the first stage, whereas the second stage was controlled by tectonism and intrusion of the high-heat production Yellagatti granite that re-established the fluid plumbing and mineralizing system.     

Granite-hosted gold mineralization at Timbarra, northern New South Wales, Australia

The Timbarra gold deposits, located in the southern New England Fold Belt of New South Wales, Australia, represent an economically significant and distinctive member of the intrusion-related class of gold deposits. The five known deposits possess a total identified mineral resource of 16.8 Mt at 0.73 g/t gold, for a total of 396,800 contained ounces. The granites in the Timbarra region form a texturally complex, zoned pluton. The gold deposits are found within the Stanthorpe leucomonzogranite (242 to 238 Ma), which intrudes and forms a core to the more mafic, barren, Bungulla monzogranite (248 to 243 Ma). Gold is disseminated in the roof zone (upper 240 m) of a fractionated, magnetite- and ilmenite-bearing, I-type leucomonzogranite phase of the Stanthorpe body. The entire gold resource occurs in the areally extensive main leucomonzogranite pluton and is hosted by a medium- to coarse-grained granite. Disseminated ore is present in all five deposits, comprises >95% of the overall resource at Timbarra, and occurs predominantly as gently dipping, tabular to lenticular bodies that are conformably constrained beneath a fine-grained aplite carapace and internal aplite layers. The disseminated ore consists of gold-bearing muscovite-chlorite-carbonate alteration and infill of primary miarolitic cavities within massive leucomonzogranite or microgranite, and contains no discernable vein, joint, or fracture control at the outcrop or hand specimen scale. Structurally controlled mineralization forms the remaining 5% of the Timbarra resource, and comprises minor, low-density (0.02 to 0.25 per meter), vein-dikes and quartz-molybdenite veins emplaced along steeply dipping east-southeast, east-northeast, and north-northeast striking cooling joints. Both mineralization styles and alteration share a common paragenetic sequence of mineral precipitation. Quartz, perthitic K-feldspar, minor biotite, and albite are the earliest and most abundant infill minerals and commonly line primary cavities and vein-dikes. Subsequent minerals include coeval arsenopyrite, pyrite, fluorite, and molybdenite. The latest minerals include muscovite, chlorite, gold, calcite, silver-bismuth telluride, lead-bismuth telluride, and rare galena and chalcopyrite. The gold ore has a low total sulfide mineral concentration (Б%). Ore contains elevated concentrations of Bi, Ag, Te, As, Mo, and Sb; gold is strongly correlated with Bi, Ag, and Te, but only weakly with Mo, As, and Sb. Gold grains are generally <1 to 50 µm in size, but rarer grains as large as 1 mm in diameter have been observed. Gold fineness ranges from 950 to 600, and varies both within and between individual grains for a given deposit. The moderately oxidized I-type host granite, low-sulfide (Б%) ores, Au-Bi-Ag-Te geochemical signature, muscovite-chlorite-carbonate alteration assemblage, and low-salinity aqueous and carbonic fluids suggest that Timbarra is part of the newly recognized intrusion-related gold deposit class. Timbarra is distinguished from other intrusion-related gold deposits by the disseminated mineralization style within pervasively altered granite, forming gently dipping, tabular to lenticular ore zones.     

Sulphur isotopic investigation of vein lead-zinc mineralization at Tyndrum,Scotland

The Tyndrum Pb+Zn veins, hosted by late Proterozoic quartzites, were probably generated in the Tournaisian (360 Ma). By determination of sulphur isotopic ratios of vein minerals three aspects of the Tyndrum mineralization were addressed, (i) sulphate sulphur sources; (ii) reduced sulphur source; (iii) isotopic equilibrium in the vein system including geothermometry. Twelve galenas have δ³⁴S values ranging from +3.55 ‰ to +6.38 ‰ (this excludes one value of +11.21 ‰ from a large but nearly barren quartz vein). Other sulphides are enriched or depleted in ³⁴S in the sense expected for isotopic equilibrium although there is no evidence for isotopic equilibrium between the vein minerals. The sulphide sulphur source was probably in the Dalradian metasediments where disseminated pyrite averages +6 ‰. Baryte had δ³⁴S values averaging 14 ‰ and was therefore not in isotopic equilibrium with sulphides: a continental groundwater source is most likely.     

Gold and copper mineralization at the El Indio deposit, Chile

A Middle Tertiary volcanic belt in the High Andes of north-central Chile hosts numerous precious- and base-metal epithermal deposits over its 150 km north-south trend. The El Indio district, believed to be associated with a hydrothermal system in the late stages of development of a volcanic caldera, consists of a series of separate vein systems located in an area of 30 km² which has undergone intense argillic-sericitic-solfataric alteration. The majority of the known gold-copper-silver mineralization occurs within a structural block only 150 by 500 m in surface area, with a recognized vertical extent exceeding 300 m. This block is bounded by two high-angle northeast-trending faults oriented subparallel to the mineralized veins.Hypogene mineralization at El Indio is grouped into two main ore-forming stages: Copper and Gold. The Copper stage is composed chiefly of enargite and pyrite forming massive veins up to 20 m wide, and is accompanied by alteration of the wall rocks to alunite, kaolinite, sericite, pyrite and quartz. The Gold stage consists of vein-filling quartz, pyrite, native gold, tennantite and subordinate amounts of a wide variety of telluride minerals. Associated with this stage is pervasive alteration of the wall rocks to sericite, kaolinite, quartz and minor pyrophyllite. The transition from copper to gold mineralization is marked by the alteration of enargite to tennantite and by minor deposition of sphalerite, galena, huebnerite, chalcopyrite and gold. Mineral stability relations indicate that there was a general decrease in the activity of S₂ accompanied by variations in the activity of Te₂ during the Gold stage.Fluid-inclusion data show homogenization temperatures ranging from about 220 to 280°C, with salinities on the order of 3–4 eq. wt. % NaCl for the Copper stage. The Gold-stage inclusions indicate a similar range in homogenization temperatures, but significantly lower salinities (0.1–1.4 eq. wt. % NaCl). Fluid inclusions of transition minerals show a weak inverse relationship between homogenization temperatures (190–250°C) and salinities (3.4–1.4 eq. wt. % NaCl), which may represent mixing of hotter Gold-stage fluids with cooler late-Copper-stage fluids. No evidence of boiling was found in fluid inclusions, but CO₂ vapor-rich inclusions were identified in wall-rock quartz phenocrysts which pre-date copper and gold mineralization.Mineral stability calculations indicate that given a fairly restricted range of solution compositions, the Copper-, Transition- and Gold-stage minerals at El Indio could have been deposited from a single solution, with constant total dissolved sulfur which underwent reduction through time. Limited sulfur-isotope data indicates that pyrite from the Copper stage was not in isotopic equilibrium with Copper-stage alunite or Transition-stage sphalerite. The sulfur-isotope and fluid-inclusion data indicate that two fluids with comparable temperatures but different compositions flowed through the El Indio system. The earlier fluid deposited copper attended by sericite-alunite-kaolinite alteration, and later epithermal fluids deposited gold with quartz-sericite-kaolinite-pyrite alteration.     

Palladium and gold mineralization in podiform chromitite at Kraubath, Austria

Summary ?We report, for the first time, the occurrence of five palladium-rich, one palladium bearing and two gold-silver minerals from podiform chromitites in the Eastern Alps. Minerals identified include braggite, keithconnite, stibiopalladinite, potarite, mertieite II, Pd-bearing Pt-Fe alloy, native gold and Ag-Au alloy. They occur in heavy mineral concentrates produced from two massive podiform chromitite samples (unaltered and highly altered) of the Kraubath ultramafic massif, Styria, Austria. Distribution patterns of platinum-group elements (PGE) in these chromitites show considerable differences in the behaviour of the less refractory PGE (PPGE-group: Rh, Pt, Pd) compared to the refractory PGE (IPGE-group: Os, Ir, Ru). PPGE are more enriched in chromitite showing pronounced alteration features. The unaltered chromitite displays a negatively sloped chondrite-normalised PGE pattern similar to typical ophiolitic-podiform chromitite. Except for the Pd- and Au-Ag minerals that are generally rare in ophiolites, about 20 other platinum-group minerals (PGM) have been discovered. They include PGE-sulphides (laurite, erlichmanite, kashinite, bowieite, cuproiridsite, cuprorhodsite, unnamed Ir-rich variety of ferrorhodsite, unnamed Ni-Fe-Cu-Rh- and Ni-Fe-Cu-Ir-Rh monosulphides), PGE alloys (Pt-Fe, Ir-Os, Os-Ir and Ru-Os-Ir), PGE-sulpharsenides (irarsite, hollingworthite, platarsite, ruarsite and a number of intermediate species), sperrylite and a Ru-rich oxide (?). Three PGM assemblages have been recognised and attributed to different processes ranging from magmatic to hydrothermal and weathering-related. Pd-rich minerals are characteristic of both chromitite types, although their chemistry and relative proportions vary considerably. Keithconnite, braggite and Pd-bearing ferroan platinum, together with a number of PGE-sulphides (mainly laurite-erlichmanite) and alloys, are typical only of the unaltered podiform chromitite (assemblage I). Euhedral mono- and polyphase PGM grains in the submicron to 100 μm range show features of primary magmatic assemblages. The diversity of PGM in these assemblages is unusual for ophiolitic environments. In assemblage II, laurite-erlichmanite is intergrown with and overgrown by PGE-sulpharsenides; other minerals of assemblage I are missing. Potarite, stibiopalladinite, mertieite II, native gold and Ag-Au alloys, as well as PGE-sulpharsenides, sperrylite and base metal arsenides and sulphides are characteristic for the highly altered chromitite (assemblage III). They occur either interstitial to chromite in association with metamorphic silicates, in chromite rims or along cracks, and are thus interpreted as having formed by remobilization of PGE by hydrothermal processes during polyphase regional metamorphism. Received August 3, 2000;/revised version accepted December 28, 2000