Bedrock Sources Research Articles

Sediment provenance in contractional orogens: The detrital zircon record from modern rivers in the Andean fold-thrust belt and foreland basin of western Argentina

This study analyzes detrital zircon U–Pb age populations from Andean rivers to assess whether active synorogenic sedimentation accurately records proportional contributions from varied bedrock source units across different drainage areas. Samples of modern river sand were collected from west-central Argentina (28–33°S), where the Andes are characterized by active uplift and deposition in diverse contractional provinces, including (1) hinterland, (2) wedge-top, (3) proximal foreland, and (4) distal broken foreland basin settings. Potential controls on sediment provenance were evaluated by comparing river U–Pb age distributions with predicted age spectra generated by a sediment mixing model weighted by relative catchment exposure (outcrop) areas for different source units. Several statistical measures (similarity, likeness, and cross-correlation) are employed to compare how well the area-weighted model predicts modern river age populations. (1) Hinterland basin provenance is influenced by local relief generated along thrust-bounded ranges and high zircon fertility of exposed crystalline basement. (2) Wedge-top (piggyback) basin provenance is controlled by variable lithologic durability among thrust-belt bedrock sources and recycled basin sediments. (3) Proximal foreland (foredeep) basin provenance of rivers and fluvial megafans accurately reflect regional bedrock distributions, with limited effects of zircon fertility and lithologic durability in large (>20,000 km2) second-order drainage systems. (4) In distal broken segments of the foreland basin, regional provenance signatures from thrust-belt and hinterland areas are diluted by local contributions from foreland basement-cored uplifts.

Typomorphism and bedrock sources of placer-forming minerals of the Umyt’ya rare metal–titanium placer (Khanty-Mansi Autonomous District)

The paper reports the results of study of the main placer-forming minerals (ilmenite and zircon) from the Umyt’ya coastal–marine placer in the western part of the Khanty-Mansi Autonomous District, Yugra. Microprobe study of 1450 zircon grains taken from the upper and lower ore bodies of the Umyt’ya placer (29 samples) and their comparison with zircon in magmatic rocks from the adjacent folded framing of the North and Nether-Polar Urals (40 analyses) allowed us to recognize five geochemical types of the mineral: Aurbakh (prevailing in the Umyt’ya placer), Severorudnichnyi, Tagil-Kytlym, Shemur, and unknown source. Crystallomorphological characteristics of the identified geochemical types of zircon are given. According to the microprobe data (300 analyses), ilmenite is subdivided into six groups. Comparison of these groups with the composition of ilmenite from magmatic complexes of the North and Nether-Polar Urals showed that they correspond to the same complexes as zircon. Based on peculiarities of the geological structure of the eastern slope of the North and Nether-Polar Urals, four separate areas were identified, each comprising spatially close magmatic complexes regarded as possible sources for ore minerals of the placers: Shchekur’ya–Khorasyur, Yalpigner–Chistop, Vizhai–Ivdel, and Denezhkin Kamen. Based on additional criteria, the Yalpigner–Chistop area was the main source for the Umyt’ya placer.

The Laurentia – West Greenland connection at 1.9 Ga: New insights from the Rinkian fold belt

U-Pb data from the Rinkian fold Belt, western Greenland, provide new constraints on provenance and timing of deposition of the Karrat Group, the emplacement interval of the Prøven Igneous Complex, and the tectonic setting of west Greenland during the interval 2.03–1.83Ga. U-Pb detrital data establish the entire Karrat Group metasedimentary succession as Paleoproterozoic in age, initiated after ca. 2029Ma (Qeqertarssuaq Formation), with deeper water deposition after 1953±31Ma and 1905±20Ma (Nûkavsak Formation). The detrital age profiles highlight a profound change in source region after ca. 1.95Ga that we attribute to thickening, uplift, and exhumation related to collision of a juvenile magmatic arc with the northwest margin of Rae craton at ca. 1.97–1.95Ga (Thelon Orogen). Accordingly, the Karrat Group is viewed as having initiated as an extensional rift basin(s) that received ca. 3.00–2.95Ga detritus from local basement sources, and which evolved into a deeper water foreland-basin succession derived from the north. This foreland basin was intruded by the Prøven Igneous Complex, for which new U-Pb data establish emplacement between 1.90 and 1.87Ga in a within-(Rae) plate setting. Prøven plutonism is coeval with lower-plate mantle magmatism elsewhere across NE Laurentia which may have been triggered by asthenospheric thinning due to plume-induced extension of lower-plate Rae craton at 1.95–1.92Ga. Our data refute a direct link between the Prøven Igneous Complex and the voluminous 1.86–1.845Ga Cumberland Batholith, Baffin Island, long considered its counterpart.

Hillslope coupled stream morphology, flow conditions, and their effects on detrital sedimentology in Garnet Canyon, Teton Range, Wyoming

Characterizing stream erosion in any steep mountain landscape is arduous, but the challenge level increases when the stream flows through a glaciated catchment frequently modified by hillslope debris. Glacial landforms and stochastic mass wasting in alpine systems may interfere with sediment delivery to downstream sites where detrital sediments are often collected to represent upstream bedrock sources. To use detrital sediments as indicators of erosion, we need to understand potential sediment accumulation in flat glaciated reaches or behind rockfall barriers. This study investigates the stream channel in Garnet Canyon, a glaciated catchment located in the central Teton Range, to describe hillslope coupled channel morphology and the subsequent effects on sediment transport throughout the catchment. Stream cross-section surveys and sediment size measurements of the surface bedload were collected in the field within a glacially flattened segment of Garnet Canyon. Calculations of shear stress conditions allowed evaluation of the importance of mineral densities on potential grain entrainment. The length of the Garnet Canyon stream observed in this study was coupled with hillslope deposits. Critical shear stresses were sufficient to move gravel-sized sediments through all sections when calculated with quartz mineral density and through most sections when applying apatite mineral density. These results verify the application of detrital sediments to evaluate erosion rates or spatial bedrock sources because snowmelt stream flow efficiently moves entrained sediment past glacially reduced slopes and potential talus barriers.

New data on the Lower Mesozoic basal series of the Traras mounts (Tlemcen, northwestern Algeria)

The lower Mesozoic of the Traras mount starts with the so-called Red Series, which lays uncomformably on the folded and granitized Paleozoic basement. This series is overlain by Middle Liassic limestones. On the basis of its lower and upper limits and the lack of dating evidence, former authors attribute a Permian to lower Liassic (Sinemurian) age for this series. Palynological results allow to range it in the upper Triassic probably Norian-Rhetian. The Red Series is studied from five sections that depict important thickness and facies variations from southwest to the northeast, inferring the irregularities of the Hercynian erosional surface. In the Central Traras, this series shows over 130-m-thick micaceous sandy conglomeratic units, deposited within alluvial fans, where the bedrock source corresponds to granite of Nedroma or its equivalent. In the southern and eastern Traras, where the series’ thickness respectively does not exceed 30 m and averages 40–80 m, besides alluvial deposits issued from hercynian basement erosion, it exhibits volcanic evidences interbedded by lacustrine deposits. Vertical and spatial distribution depicts synsedimentary infilling of narrow elongated rift basins, typically half-grabens, bounded on one side by a normal fault or a series of normal faults trending NNE-SSW to NW-SE, inherited from the hercynian event. The study of tectonostratigraphic units permitted to put forward geodynamics of the infilling basins.

Predicting redox‐sensitive contaminant concentrations in groundwater using random forest classification

AbstractMachine learning techniques were applied to a large (n > 10,000) compliance monitoring database to predict the occurrence of several redox‐active constituents in groundwater across a large watershed. Specifically, random forest classification was used to determine the probabilities of detecting elevated concentrations of nitrate, iron, and arsenic in the Fox, Wolf, Peshtigo, and surrounding watersheds in northeastern Wisconsin. Random forest classification is well suited to describe the nonlinear relationships observed among several explanatory variables and the predicted probabilities of elevated concentrations of nitrate, iron, and arsenic. Maps of the probability of elevated nitrate, iron, and arsenic can be used to assess groundwater vulnerability and the vulnerability of streams to contaminants derived from groundwater. Processes responsible for elevated concentrations are elucidated using partial dependence plots. For example, an increase in the probability of elevated iron and arsenic occurred when well depths coincided with the glacial/bedrock interface, suggesting a bedrock source for these constituents. Furthermore, groundwater in contact with Ordovician bedrock has a higher likelihood of elevated iron concentrations, which supports the hypothesis that groundwater liberates iron from a sulfide‐bearing secondary cement horizon of Ordovician age. Application of machine learning techniques to existing compliance monitoring data offers an opportunity to broadly assess aquifer and stream vulnerability at regional and national scales and to better understand geochemical processes responsible for observed conditions.

Controls on modern erosion and the development of the Pearl River drainage in the late Paleogene

The Pearl River and its tributaries drains large areas of southern China and has been the primary source of sediment to the northern continental margin of the South China Sea since its opening. In this study we use a combination of bulk sediment geochemistry, Nd and Sr isotope geochemistry, and single grain zircon U-Pb dating to understand the source of sediment in the modern drainage. We also performed zircon U-Pb dating on Eocene sedimentary rocks sampled by International Ocean Discovery Program (IODP) Expedition 349 in order to constrain the source of sediment to the rift before the Oligocene. A combination of Nd and Sr isotopes shows that the Gui, Liu and Dong Rivers are likely not important sources. Single grain zircon dates emphasize the importance of the westernmost tributaries (Hongshui and Yu Rivers), which drain the highest topography and tectonically active areas, as the primary sediment producers. Our data indicate that climate is not the primary control on erosion patterns and intensities. Zircon dating also shows that the Gui and Liu Rivers are not generating large sediment yields. Discrepancies between these new data and earlier samples make the role of the Dong River hard to determine, although Nd isotopes suggest that it is not dominant. The source of sediment during the Eocene at IODP Site U1435 appears to have been a relatively local basement source, or a regionally restricted river only draining nearby areas of the Cathaysia Block, similar, but not identical, to the modern Dong River. There is no evidence for a large regional river and we exclude sediment transport from the southwest (Indochina). Our data are consistent with small drainage systems dominating the basin until the end of the Oligocene (~24Ma), after which the Pearl River expanded towards its modern state as a result of headwater capture largely towards the west.

Denmark Strait during the Late Glacial Maximum and Marine Isotope Stage 3: Sediment sources and transport processes

The sediment records from north and south of Denmark Strait (cores PS2644 and MD99-2323) and on the Kangerlussuaq Trough Mouth Fan (KTMF) (MD99-2260) are evaluated for the period 12 to 60calkaBP with the goal of evaluating: 1) the relative roles of the Greenland and Iceland Ice Sheets in sediment composition, 2) the processes of sediment supply, and 3) the relationship, if any, between the atmospheric derived ice core records and glacial marine sediment history. These questions were addressed using counts of coarse (>2mm) ice rafted debris (IRD), magnetic susceptibility, grain-size spectra, and sediment mineral composition. A significant difference in these variables occurred between MS3 and MIS2, with the latter showing small to modest abrupt changes in felsic and mafic minerals and in IRD, compared to larger and less variable changes during MIS2. The dominant bedrock source archived in the cores is the Cenozoic basaltic outcrops from East Greenland and Iceland (~60%) but there are frequent pulses of felsic-rich sediments (~17–36%), which coincided with Greenland stadials. There is also a persistent detrital carbonate input. The largest shift in most sediment proxies occurred ~28–30calkaBP reflecting the sustained expansion of the ice sheets. The grain-size spectra on the Snorri Drift (MD99-2323) reflected complex depositional histories involving the supply of poorly sorted glacially derived sediment (sand, silt, and clay) and reworking by bottom currents.

Cambrian detrital zircon signatures of the northern Cordilleran passive margin, Liard area, Canada: evidence of sediment recycling, non-Laurentian ultimate sources, and basement denudation

Detrital zircon U–Pb age probability distributions for the Cambrian Vizer formation (informal) and Mount Roosevelt Formation (middle member) of the northern Canadian Cordilleran passive margin indicate extensive recycling from ∼1.7 to 1.6 Ga Paleoproterozoic sandstones and Proterozoic and Lower Cambrian strata, respectively. The units have minor or no first cycle input from Laurentian basement. The lower part of the Vizer formation contains North American magmatic gap (1610–1490 Ma) detrital zircons and lacks ultimate Grenvillian sourced grains, indicating that the grains were likely sourced from a nearby Mesoproterozoic basin and have an ultimate non-Laurentian source. Detrital zircon U–Pb ages of 670–640 Ma from the middle member of the Mount Roosevelt Formation indicate associated volcanic clasts were locally sourced, and are not of syn-sedimentary Middle Cambrian age. Provenance of these units was indirectly impacted by the Liard Line basement feature. Detrital zircon U–Pb age probability distributions from the northern Canadian Cordilleran passive margin indicate sediments were sourced from the east in the Early Cambrian (Terreneuvian; Vizer formation and correlatives) and the northeast during Early Cambrian (Series 2) deposition of Sekwi Formation and correlative strata. In the early Middle Cambrian, the middle member of the Mount Roosevelt Formation was primarily locally sourced, whereas the upper member was derived from Laurentian basement to the east and southeast. The change from reworked Paleoproterozoic cover in the Terrenuvian to primary basement sources in the Middle Cambrian suggests significant denudation of the basement occurred southeast of the Liard Line.

Native gold from the Inagli Pt–Au placer deposit (the Aldan Shield, Russia): geochemical characteristics and implications for possible bedrock sources

The Inagli alluvial Pt–Au placer deposit in the Republic of Sakha (Yakutia), Russia, is linked to the Inagli massif, one of the several Uralian–Alaskan-type alkaline-ultrabasic complexes in the Aldan Shield. Gold from the placer is heterogeneous in composition and is represented by three types. Type 1 gold is the most abundant and is characterized by simple Au–Ag alloys with 4–34 wt% Ag, low Cu (up to 0.08 wt%) and negligible Hg, Pt, and Pd contents, and silver–tellurium sulfosalts (Ag–Cu–Te–S–As compounds) in the inclusion suite. Silicate inclusions are biotite, K-feldspar, Fe–Mg amphibole, chlorite, plagioclase, Fe–Mg pyroxene, zircon, and titanite. Distinctive features of this gold type are most similar to those derived from low-sulfidation systems linked to iron oxide copper–gold or iron skarn types of mineralization. The bedrock source of type 1 gold could be related with monzonite to syenite intrusions surrounding the Inagli massif. Distinctive features of type 2 gold include a wide discontinuous range of Ag content (1–18 wt%), elevated Cu (up to 0.5 wt%), and occasional Pd (up to 0.3 wt%) levels, non-detectable Pt and Hg contents, and rare inclusions of simple sulfides (digenite, pyrrhotite) and Na amphibole. Type 3 gold is distinguished by a narrow range in Ag content (5–8 wt%), elevated Hg (0.5–1 wt%) contents, negligible Cu, Pt and Pd levels, and Au–Pb compounds + K-feldspar inclusions. Microchemical characteristics of type 2 and type 3 gold are interpreted as suggestive of an alkaline–magmatic-related fluid. Based on the grain morphology and microchemical signatures, potential bedrock sources for both gold types could be related to the numerous alkaline veins and potassic alteration zones within the dunite core. A comparison of the Inagli and the Kondyor placer gold allows to generate distinctive generic signatures for gold from Uralian–Alaskan-type alkaline–ultrabasic complexes in the Aldan Shield.

Biogeochemical reconstructions of life histories as a method to assess regional interactions: Stable oxygen and radiogenic strontium isotopes and Late Intermediate Period mobility on the Central Peruvian Coast

Biogeochemical reconstructions of life histories of mobility offer a means to obtain nuanced information about regional interactions in the past. We test this method using the Late Intermediate Period Ychsma society on the central Peruvian coast as a case study. Archaeological and ethnohistoric evidence indicates that the Rimac and Lurín Valleys inhabited by the Ychsma served as a key regional hub for the religious and administrative activities of the Inca Empire and the Spanish Viceroyalty. The nature of regional interactions prior to Inca imperial influence, however, remains unclear. Well-known historical narratives describe populations from the adjacent Huarochirí highlands defeating coastal Ychsma populations for agricultural land, but archaeological evidence concerning the timing and extent of coastal-highland interactions is debated. Here, we assess the potential for radiogenic strontium and stable oxygen isotopic reconstruction of mobility over the life course to shed light on the regional interactions of coastal Ychsma groups during the Late Intermediate Period. We present 87Sr/86Sr and δ18O results for 61 regional environmental baseline samples and archaeological human tooth enamel and bone samples from 64 individuals buried at Armatambo and Rinconada Alta, in the Rimac Valley. Results reveal a broad isotopic range for baseline and archaeological samples likely due to diversity in water and bedrock sources. Nevertheless, one individual presents a non-local strontium value indicating mobility to an outside region. We discuss the advantages of a life history approach, the complications of using isotopes to assess mobility in the Central Andes, and suggested directions for future research.

Placer gold in the Cretaceous Blue Spur Conglomerate at Waitahuna, southern New Zealand

ABSTRACTBlue Spur Conglomerate is a Cretaceous paleoplacer gold deposit that yielded abundant gold in the 19th century Otago gold rush. A modern gold mine near Waitahuna in southeast Otago has provided new insights into the nature of the conglomerate and a window into some technical, logistical and environmental challenges that faced the historical miners. Most of the clasts in the conglomerate were eroded from fresh basement along the Tuapeka Fault Zone, with derivation from the north or northeast. There are three principal gold morphological groups in the conglomerate: angular nuggety gold (up to 15 mm), commonly intergrown with quartz; rounded ellipsoidal particles (10–500 µm); and extensively flattened and folded flakes (10–500 µm). Most of that gold was derived from nearby basement sources in the Otago Schist. Diagenetic alteration of the conglomerate and subsequent weathering has resulted in enhanced clay contents and friability of the conglomerate clasts, which contributed to elevated suspended sediment loads in mine-processing waters.

Low temperature recrystallisation of alluvial gold in paleoplacer deposits

Detrital gold particles in paleoplacer deposits develop recrystallised rims, with associated expulsion of Ag, leading to the formation of Ag-poor rims which have been recognised in most placer gold particles around the world. Recrystallisation is facilitated by accumulation of strain energy as the gold particles are deformed, particularly on particle margins, during transportation in a fluvial system. The recrystallisation process ensues after sedimentary deposition and can occur at low temperatures (<40°C) over long geological time scales (millions of years). In the Otago placer goldfield of southern New Zealand, paleoplacers of varying ages contain gold with varying transport distances and these display differing degrees of rim formation. Narrow (1–10µm) recrystallised rims with 0–3wt% Ag formed on gold particles that had been transported <10km from their source and preserved in Eocene sediments. Relict, coarse grained (∼100µm) gold particle cores have 3–10wt% Ag, which is representative of the source gold in nearby basement rocks. Gold in the Miocene paleoplacers was recycled from the Eocene deposits and transported >20km from their source. The gold particles now have wider recrystallised rims (up to 100µm), so that some particles have essentially no relict cores preserved. Gold in Cretaceous paleoplacers have wide (∼100µm) recrystallised low-Ag rims, even in locally-derived particles, partly as a result of diagenetic effects not seen in the younger placers. Gold particles in all the paleoplacers have delicate gold overgrowths that are readily removed during recycling, but are replaced by groundwater dissolution and reprecipitation on a time scale of <1Ma. The recrystallisation that leads to Ag-poor rim formation is primarily related to the amount of deformation imposed on particles during sedimentary transport, and is therefore broadly linked to transport distance, but is also partly controlled by the age of the paleoplacer on time scales of tens of millions of years. Gold particles that have been derived directly from basement sources can retain their original composition for long distances (tens to hundreds of kilometres) in a river system, with only minor recrystallised rim development. Gold particles that have been recycled through paleoplacer deposits can lose this link to source composition after relatively short transport distances because of extensive recrystallisation.

Stratigraphic and provenance variations in the early evolution of the Magallanes-Austral foreland basin: Implications for the role of longitudinal versus transverse sediment dispersal during arc-continent collision

Resolving the role of longitudinal versus transverse sediment dispersal in ancient sedimentary basins is paramount for understanding filling history and the timing of source area exhumation. The southern Patagonian Andes provide a unique opportunity for constraining these relationships because Upper Cretaceous shallow- and deep-marine strata that record the longitudinal filling history of the Magallanes-Austral foreland basin are exposed along a 500+ km outcrop belt. New stratigraphic, sedimentologic, and facies analyses of the Cenomanian-aged Lago Viedma Formation indicate a protracted phase of a dominantly shoreface and foreshore depositional setting at the northern end of the basin (Austral basin sector), whereas 200 km to the south, age-equivalent strata of the Punta Barrosa Formation are characterized by southward-flowing deep-water fan systems. New sandstone compositional data from both formations are rich in intermediate volcanic grains and suggest an undissected to transitional volcanic arc source. However, detrital zircon populations from the shallow-marine Lago Viedma Formation are dominated by arc sources (126−75 Ma), whereas deep-water strata of the Punta Barrosa Formation contain much greater abundances of Jurassic (199−161 Ma) and pre-Jurassic (>200 Ma) ages. This indicates that deep-water fan systems were not linked to a shelfal sediment dispersal system by a simple northward point-source model, despite consistent southward-directed paleocurrents in deep-water strata. Time-transgressive provenance variations continue southward (along strike), where lithostratigraphic equivalents in the Ultima Esperanza and Fuegian sectors of the basin contain a mix of arc and pre-Jurassic metamorphic basement sources and a paucity of Jurassic ages. We interpret these along-strike provenance variations to be the result of significant local sediment contributions from transverse sources. The influence of transverse tributaries during the early history of the Magallanes-Austral foreland basin suggests that the diachronous onset of coarse clastic deposition in the basin was likely due to the progressive delivery (and initiation) of more locally derived coarse clastic sediment. We attribute this to southward-progressing thrust-belt development associated with progressive north to south collision (or suturing) of the parautochthonous Patagonian arc with attenuated continental crust of South America.

Temporal and spatial patterns of sediment routing across the southeast margin of the Tibetan Plateau: Insights from detrital zircon

AbstractThe Cenozoic deposits of the Tibetan Plateau's southeastern margin are often cited as part of a continental‐scale river system connecting the Paleo‐Yangtze River with the Paleo‐Red River. Confirming the purported connection and any subsequent drainage reorganization has garnered significant attention and varied proposed ages for reorganization. This study presents detrital zircon U‐Pb ages and paleocurrents in Eocene to Pleistocene sedimentary basin deposits distributed over a broad area of the southeast Tibetan Plateau margin within the area of proposed paleoriver connectivity. When combined with previously published studies, our U‐Pb ages allow examination of the temporal and spatial distributions of provenance throughout the Cenozoic. We identify six key age components of the detrital U‐Pb age distributions and use these to examine the patterns of sediment provenance for different Cenozoic epochs. Detailed analysis of these components shows that provenance for both onshore and offshore deposits is best described by local bedrock sources and provides little to no evidence of regional changes in provenance. This suggests that a stable fluvial system similar to the modern drainage network has existed since the Eocene with no evidence for major provenance‐altering river capture. Paleoflow measurements taken throughout the SE margin further corroborate the results of detrital zircon provenance. The combination of U‐Pb age components and paleocurrent directions does not support a Cenozoic connection between the Paleo‐Yangtze and Paleo‐Red Rivers.

Eastern margin of Tibet supplies most sediment to the Yangtze River

Zircon provenance studies of modern and ancient fluvial systems help reveal the relative contributions and importance of upstream sediment sources. A 2014 study of detrital zircon U-Pb age distributions from the Yangtze River (China) and its tributaries proposed a strong anthropogenic control on sediment flux. Those data, along with other data from the region, were reanalyzed using multiple detrital zircon U-Pb age distribution comparison techniques and a distribution-mixing model to construct an improved and quantitative view of provenance. The variability in the Yangtze River trunk stream U-Pb age distributions is evaluated with respect to trunk-to-trunk stream comparisons, trunk-to-tributary comparisons, and in mixture models that consider tributary and bedrock contributions, the latter using a comprehensive compilation of bedrock source terranes. Uniformity in the zircon age distribution of the Yangtze River trunk stream is established in the upper reaches, downstream of the first bend, and maintained by the left-bank tributaries to its outlet. Whether considering the bedrock source terranes or only the modern Yangtze River sediments, the major source of sediments contributing to Yangtze River is clearly the eastern edge of the Tibetan Plateau (e.g., Songpan Ganze complex, Longmenshan Range), where rock uplift rates are high. The purported increase in anthropogenic impact on sediment yield in the lowlands, at least as viewed through detrital zircon age distributions, is insignificant.

Petrology and geochemistry of clastic sedimentary rocks as evidences for provenance of the Late Palaeozoic Madzaringwe Formation, Tshipise-Pafuri Basin, South Africa

Abstract Petrography and geochemistry (major, trace and rare earth elements) of clastic rocks from the Late Palaeozoic Madzaringwe Formation, in the Tshipise-Pafuri Basin, Northern South Africa, have been investigated to understand their provenance. Sandstone petrography and detrital modes indicates that the Late Palaeozoic succession was derived from craton interior and recycled orogen provenance. Sandstones in the Madzaringwe Formation are sub-arkosic to sub-litharenite. The sediments may represent a recycled to craton interior provenance. The geochemical data of major elements show that sandstone and shales have the same source. The study of paleoweathering conditions based on modal composition, chemical index of alteration (CIA) and A-CN-K (Al2O3-CaO+Na2O-K2O) relationships indicate that probably chemical weathering in the source area and recycling processes have been more important in shale and sandstone rocks. The relatively high CIA values (70–90%) indicates moderate to high weathering conditions of the samples and the paleoclimate of the source area was warm. K2O/Na2O versus SiO2 and Na2O-CaO-K2O tectonic setting discrimination plots, suggest a passive continental margin. In the study of trace elements, triangular Th-Sc-Zr/10 and La-Th-Sc plots both suggest a passive margin setting of the basin. Petrographic and geochemical results of the samples suggest uplifted basement source areas dominated by sedimentary rocks and/or granite-gneiss rocks. The source rocks might have been the recycled pre-Soutpansberg Karoo Supergroup rocks and the metasedimentary rocks of the Soutpansberg Group. Other source rocks may have been the pre-Beit-Bridge basement rocks (granites and gneisses).

Morphological evolution of gold nuggets in proximal sedimentary environments, southern New Zealand

A rare collection of centimetre-scale proximal gold nuggets has been examined in the context of the Late Pleistocene to Holocene eluvial and colluvial sediments in which they occur, in a range of environments from arid temperate to periglacial. Liberation of supergene nuggets from their basement source in fault zones occurred progressively by physical erosion as the host basement rocks were uplifted between middle Miocene and Holocene. The host sediments are made up of poorly sorted angular greenschist facies schist debris. Some nuggets were recycled through several generations of these colluvial deposits, and additional nuggets were liberated to younger sediments during this time. The nuggets did not migrate laterally more than a few tens of metres during this repeated recycling, and were passively elevated vertically with the rising basement on to the crest and slopes of a 1600m high mountain range. The most recently liberated nuggets retain almost all the morphological features of their supergene origin, including coarse (cm scale) crystal shapes, delicate crystalline internal structure, and imprints of oxidised pyrite crystals. Minor transport in colluvium has caused some abrasion and rounding of gold crystals. Repeated recycling progressively obscured the crystal shapes, although relict crystals are still recognisable on parts of most nuggets. Differential timing of liberation from basement has resulted in a wide range of rounding effects in groups of closely-coexisting nuggets. Variably crystalline gold overgrowths (micron scale) coat abraded surfaces of all or part of most nuggets, commonly intergrown with authigenic smectite clay minerals. These overgrowths developed from alkaline groundwater (pH7–9) that had undergone extensive chemical interaction with labile minerals in the host colluvium. Abrasion during recycling removed some of the overgrowths, but this was replaced in the new sedimentary hosts. Physical and chemical processes affecting nuggets in these sediments have similarly affected eluvial and colluvial nuggets in a wide range of settings around the world, including Yukon (Canada), California (USA) and arid parts of Australia.

Procedencia del material granítico de los molinos de vaivén prehistóricos: el caso del yacimiento Cogotas I de Arroyo Mucientes-San Lázaro (Castronuño, Valladolid) y propuesta de una marcha metodológica general

Un elemento habitual en yacimientos prehistóricos como los del grupo arqueológico de Cogotas I son los molinos manuales, cuya materia prima es a menudo de carácter alóctono, por lo que establecer su procedencia es básico para el estudio del aprovechamiento de recursos y para planteamientos de Economía Política. El avance en esas direcciones exige un estudio de conjunto, con molinos de granito seleccionados para cubrir las diversas subzonas del área de dispersión de Cogotas I, y en especial por su alejamiento de los afloramientos graníticos. Tal estudio puede hoy incluir diversos procedimientos analíticos que combinan sucesivos criterios geográficos, petrográficos, mineralógicos y geoquímicos. Para probar su viabilidad, se ha llevado a cabo un primer ensayo experimental estudiando dos molinos obtenidos en la excavación de un yacimiento cogotense de Castronuño (Valladolid), cuya área fuente más probable se localiza a unos 50 km, en la localidad zamorana de Peñausende. El resultado obtenido avala la propuesta de una marcha metodológica generalizable a otras situaciones análogas.

Geochemical Characterization of Trace MVT Mineralization in Paleozoic Sedimentary Rocks of Northeastern Wisconsin, USA

Disseminated Mississippi Valley-type (MVT) mineralization occurs throughout northeastern Wisconsin, USA, and is recognized as the source of regionally extensive natural groundwater contamination in the form of dissolved arsenic, nickel, and other related metals. Although considerable attention has been given to arsenic contamination of groundwater in the region, limited attention has been focused on characterizing the bedrock sources of these and other metals. A better understanding of the potential sources of groundwater contamination is needed, especially in areas where groundwater is the dominant source of drinking water. This article describes the regional, stratigraphic, and petrographic distribution of MVT mineralization in Paleozoic rocks of northeastern Wisconsin, with a focus on sulfide minerals. Whole-rock geochemical analysis performed on 310 samples of dolomite, sandstone, and shale show detectable levels of arsenic, nickel, cobalt, copper, lead, zinc, and other metals related to various sulfide mineral phases identified using scanning electron microscopy. MVT minerals include pyrite, marcasite, sphalerite, galena, chalcopyrite, fluorite, celestine, barite, and others. We describe the first nickel- and cobalt-bearing sulfide mineral phases known from Paleozoic strata in the region. Arsenic, nickel, and cobalt are sometimes present as isomorphous substitutions in pyrite and marcasite, but discrete mineral phases containing nickel and cobalt elements are also observed, including bravoite and vaesite. Locally abundant stratigraphic zones of sulfide minerals occur across the region, especially in the highly enriched Sulfide Cement Horizon at the top of the Ordovician St. Peter Sandstone. Abundant quantities of sulfides also appear near the contact between the Silurian Mayville Formation and the underlying Maquoketa and Neda formations in certain areas along and east of the Niagara escarpment. This article illustrates how a detailed geochemical and mineralogical investigation can yield a better understanding of groundwater quality problems.