Quartz-pebble Conglomerates Research Articles

Paleoproterozoic Huronian basin: product of a Wilson cycle punctuated by glaciations and a meteorite impact

The Huronian Supergroup (∼2.4–2.2 Ga) comprises up to 12 km of mainly sedimentary supracrustal rocks. The oldest Huronian unit, the Livingstone Creek Formation, is chemically and mineralogically immature. Significant chemical weathering and deposition of U-rich quartz pebble conglomerates followed deposition of the Livingstone Creek Formation and extrusion of the overlying, rift-related lavas of the Thessalon Formation. Much of the succeeding Huronian comprises climatically(?)-controlled tripartite cycles, each of which begins with glaciogenic diamictites, followed by mudstones and cross bedded arenaceous units. Huronian deposition is interpreted as the result of a partial Wilson cycle, involving rifting and development of a southward-facing passive margin. The ∼2.4 Ga Murray and Creighton granites have been considered by others to be co-eval with an early orogenic episode (Blezardian). Expected unconformities are, however, lacking and these granites, together with co-eval volcanic rocks and abundant soft sediment deformation structures are considered to be due to anorogenic processes during Huronian basin subsidence. The main deformation of the Huronian is probably related to the Penokean orogeny (∼1.89–1.8 Ga). Geochemical investigations have contributed to paleoclimatic and provenance studies and have helped to define basin-wide metasomatic events. A large impact at ∼1.85 Ga likely played an important role in formation of the Sudbury Igneous Complex (SIC) and deposition of fallback breccias of the Onaping Formation (basal Whitewater Group in the Sudbury basin). Overlying sedimentary rocks of the Whitewater Group are considered to be a fortuitously-preserved portion of a widespread flysch apron that spread across the southern margin of the Superior Province as a foreland basin fill, in response to the closure phase of the Wilson cycle during the Penokean orogeny. Geochronological data suggest that these events occurred during the Penokean orogeny.

Gold mineralization in the Serra de Jacobina region, Bahia Brazil: tectonic framework and metallogenesis

Gold deposits in the Serra de Jacobina region in Bahia, northeastern Brazil, occur in a belt of siliciclastic metasedimentary rocks intercalated with mafic and ultramafic rocks, and underlain by a tonalite–trondhjemite–granodiorite gneiss-dominated (TTG) basement. The siliciclastic sequence probably represents the remnants of a sedimentary basin, which formed in an Archean passive margin-type setting. The basin was subsequently subjected to a complex history of deformation, metamorphism, and hydrothermal activity, as a result of oblique collisional events in Late Archean and Paleoproterozoic. The majority of the auriferous occurrences in the area are hosted by quartz-pebble conglomerates, and have been noted to resemble placer-type deposits. However, structurally-controlled hydrothermal orebodies, and the formation of gold occurrences also in quartzites and mafic and ultramafic rocks, support an epigenetic model for the mineralizing event. Gold mineralization is interpreted to be an integral part of the late (~1.9 Ga) tectonothermal evolution of the Serra de Jacobina region. It was roughly coeval with the emplacement of large volumes of post-collisional type, peraluminous granitic magmas, during a regional strike-slip regime.

Basin-internal derivation of hydrocarbons in the Witwatersrand Basin, South Africa: evidence from bulk and molecular δ13C data

Gold in the quartz–pebble conglomerates of the late Archean Witwatersrand Basin, South Africa, is often intimately associated with carbonaceous matter of organic/biogenic origin which occurs in the form of stratiform carbon seams and paragenetically late bitumen nodules. Both carbon forms are believed to be formed by solidification of migrating hydrocarbons. This paper presents bulk and molecular chemical and stable carbon isotope data for the carbonaceous matter, all of which are used to provide a clue to the source of the hydrocarbons. These data are compared with those from intra-basinal shales and overlying dolostone of the Transvaal Supergroup. The δ 13C values of the extracts from the Witwatersrand carbonaceous material show small differences (up to 2.4‰) compared to the associated insoluble organic matter. This suggests that the auriferous rocks were stained by mobile hydrocarbons produced by thermal and oxidative alteration of indigenous bitumens. A contribution from hydrocarbons derived from intra-basinal Witwatersrand shales cannot be excluded. Individual aliphatic hydrocarbons of the various carbonaceous materials were subjected to compound specific isotope analysis using on-line gas chromatography/combustion/stable isotope ratio mass spectrometry (GC/C/IRMS). The limited variability of the molecular parameters and uniform δ 13C values of individual n-alkanes (−31.1±1.7‰) and isoprenoids (−30.7±1.1‰) in the Witwatersrand samples exclude the mixing of oils from different sources. Carbonaceous matter in the dolostones shows distinctly different bulk and molecular isotope characteristics and thus cannot have been the source of the hydrocarbons in the Witwatersrand deposits. All the various forms of Witwatersrand carbon appear indigenous to the Witwatersrand Basin, and the differences between them are explained by variable, in general probably short (centimeter- to meter-scale) hydrocarbon migration during diagenesis and subsequent hydrothermal infiltration.

Study of environmental radioactivity at the horizon of uranium bearing quartz pebble conglomerates near Chickmagalur, India

Study of environmental radioactivity at the horizon of uranium bearing quartz pebble conglomerates near Chickmagalur, India

Two-Fluids (Oxic and Reduced) Model for the Formation of Uraninite in Early Proterozoic Quartz-Pebble Conglomerates of the Elliot Lake District, Ontario, Canada

Uraninite in quartz-pebble conglomerate beds of ~2.8 to ~ 2.2 Ga has been considered to be detrital in origin by many previous researchers because of the presence of rounded grains of uraninite and pyrite. The survival of these minerals during fluvial transport and deposition has been used as an important line of evidence for an anoxic atmosphere prior to 2.2 Ga, because both minerals are unstable under an oxygen-rich environment. An alternative model, diagenetic/hydrothermal model, has been suggested for uraninite and pyrite in the q u a r t s p e b b l e c o n g l o m e r a t e beds in the Wi twa te r s rand dis t r ic t , South Af r ica (e.g. Barnicoat et al., 1997). If uraninite and pyrite are not of detrital in origin, the occurrence of this type of deposits may not be used as evidence for an anoxic atmosphere. On the other hand, if these minerals were formed by diagenetic or hydrothermal processes, a series of redox reactions must have occurred to form uranium-bearing minerals, such as the reactions 6+ between U -beanng, oxic groundwater and reductants (e.g. organics and sulphides) to precipitate UO2 (uraninite). Such formational processes would have been favoured under an oxic atmosphere. The objective of this research is, therefore, to better understand the origin(s) of uraninite in quartz conglomerate beds of >2.2 Ga, and to constraint the atmospheric pO2 level. This objective has been pursued mostly from a detailed study, using ore microscopy and electron microprobe analyser, of the textures and paragenesis of U-Th-bearing minerals in ores from the Stanleigh mine, Elliot Lake district, Ontario, Canada. The ores are hosted in quartzpebble conglomerate beds of the Matinenda Formation which is ~2.4 Ga in age. Paragenesis and textures of U-Th.bearing minerals

Quartz-Pebble Conglomerates of Chikmagalur Area, Karnataka, India: Implications on Uranium Concentration

The Quartz - Pebble Conglomerates (QPC) represent the angular unconformity prevailed between the older group of Sargur rocks and younger Dharwar volcano-sedimentary association in Chikmagalur area of Karnataka state. The older group of rocks consist of amphibolites, Peninsular gneisses, granites, quartzites and quartz-sericite schists whereas the younger group has amphibolites, quartzites, phyllites, chlorite schists, differentiated volcanic rocks and banded iron formations. The QPCs have rutile, ilmenite, pyrite, pyrrhotite, chalcopyrite and rarely uraninite as the major opaque phases. Gamma ray spectrometry, autoradiography and ore petrographic studies have shown that the concentration of uranium in the QPCs is due to solution redeposition of uranium by adsorption or absorption on rutile generated by the decomposition of ilmenite with hydrous iron as another product. The genesis of pyrite is described through a polyplylitic process . The oxidising environments prevailed in Pre-cambrian might have promoted the transportation of U +6, S +6 and Cr +6 and were concentrated in reducing pockets of Bababudan as U +4, S −2 and Cr +3. Cr +3 is represented by fuchsite mica abundant in pyrite rich zones. The scarcity of uraninite is due to the destruction of these grains by oxidation during transportation.

The Proterozoic Thule Supergroup, Greenland and Canada: history, lithostratigraphy and development

A new lithostratigraphic scheme for the Proterozoic (Neohelikian-Hadrynian) intracratonic Thule Basin of northern Baffin Bay is presented. This basin, preserved between 76° and 79° N in Greenland and Ellesmere Island, Canada, contains little deformed and unmetamorphosed strata at least 6 km thick that are referred to the Thule Supergroup. The succession is composed of continental to shallow marine sediments with prominent red bed units, and one main interval of basaltic volcanic rocks. Diabase sills representing both the Mackenzie (Neohelikian) and Franklin (Hadrynian) magmatic episodes occur at certain levels.
 The bulletin has three main parts. The forerunner of the new supergroup, the Thule Group, is infamous for the nomenclatorial chaos that has surrounded its use. Consequently the first part is a historical summary as background to the definitive nomenclatorial revision. The second and main part on lithostratigraphy splits the Thule Supergroup into 36 formal divisions: 5 groups, 15 formations and 16 members. Twenty geological maps show the regional distribution of the defined units that are represented by 78 stratigraphic logs and sections. The third part summarises basin development as a restricted or semi-restricted depocentre on the northern margin of the Canadian-Greenlandic shield. Evolution was controlled by block faulting and basin sagging in a divergent plate regime.
 The Smith Sound Group, up to 700 m thick, represents the northern platform and basin margin equivalent of the Nares Strait and Baffin Bay Groups of the central basin. Composed of sandstones and shales with subordinate stromatolitic carbonates, the Smith Sound Group represents an overall shelf environment with long-lasting conditions for shallow water to subaerial deposition. Supratidal to marginally marine and intermittently lacustrine sedimentation prevailed.
 The Nares Strait Group, up to 1200 m thick and representing the basal strata of the central basin, is composed of sandstones and basaltic volcanics including flows, sills and volcaniclastic deposits, as well as shale- and carbonate-dominated intervals. The Group represents deposition in alluvial plain, littoral and offshore environments, with accompanying terrestrial tholeiitic volcanicity.The Baffin Bay Group, overlying conformably the previous group, represents the most widespread strata of the Thule Basin reaching a maximum thickness of up to 1300 m. Sandstones and quartz-pebble conglomerates, with important intervals of shales and siltstones, represent mixed continental to marine shoreline environments with an interval of deeper water deposition, possibly in a prodelta or offshore basin.
 The Dundas Group, 2 to 3 km thick and following the previous group along a gradational contact, comprises sandstones, siltstones and shales with lesser amounts of carbonate and evaporit. Deposition was in an overall deltaic to offshore environment.
 The youngest strata, the Narssarssuk Group, 1.5 to 2.5 km thick, are preserved in a graben on the south-eastern margin of the basin. The cyclic carbonate - red bed siliciclastic sequence with evaporites represents deposition in a low-energy, hypersaline, peritidal environment in conditions perhaps analogous to modern coastal sabkhas.

Initiation of ∼2.45-2.1 Ga intracratonic basin sedimentation of the Hurwitz Group, Keewatin Hinterland, Northwest territories, Canada

Continental to shallow-marine rocks of the Hurwitz Group, exposed as outliers across the Hearne Province in northern Canada, were deposited between ∼2.45 and 2.11 Ga on a 3.3-2.6 Ga granite-greenstone basement. Initiation of the Hurwitz Basin was characterized by low relief and rates of subsidence that never progressed beyond the intensity of regional sagging. During stage 1, polymictic conglomerate (lower Noomut Formation) was deposited in palaeovalleys cut into Archaean basement, and subarkose to quartz arenite ± quartz pebble conglomerate (upper Noomut Formation) was deposited on a low-relief sand plain. Abundant first-cycle quartz-rich sandstones are attributed to intense humid climate weathering in low-relief source areas and at sites of temporary storage, as well as to repeated fluvial and eolian reworking. Interbedding of pyritic palaeoplacers and continental redbeds denotes fluctuating levels of atmospheric oxygen; disappearance of pyritic palaeoplacers upsection signifies ultimate changeover to an atmosphere with sustained free oxygen. Stage 2 (Padlei Formation) was marked by a climatic shift toward cold (glacial?) conditions as indicated by dropstones, greywacke (till?) pellets and possible ikaite pseudomorphs in lacustrine rhythmites. Stage 3 (Kinga Formation) represents a return to wet/warm conditions with fluvial (± eolian) deposition of subarkose and quartz arenite on a low-relief sand plain (Maguse Member) and deposition of supermature quartz arenite in a vast (100,000 km 2 minimum), shallow-water lake or series of lakes (Whiterock Member). Local chert beds at the top of the Kinga Formation (Hawk Hill Member) formed by spring discharge due to regional, gravity-driven, convective groundwater flow. An intracratonic setting is implied by: (1) a symmetric, basin-centred fill; (2) absence of features indicating a shelf-slope break; (3) absence of rift rocks; (4) a prolonged slow-subsidence history; (5) likely correlation with other continental to shallow-marine deposits scattered across the Keewatin Hinterland; and (6) positioning 1000–2000 km away from closest coeval passive margin sequences. We speculate that Hurwitz Basin formed due to lithospheric stretching related to a large-scale mantle upwelling during the protracted breakup of an earliest Palaeoproterozoic supercontinent (‘Kenorland’).

Relationship between the Montgomery Lake and Hurwitz groups, and stratigraphic revision of the lower Hurwitz Group, District of Keewatin

Recent detailed mapping has resulted in a possible solution to a long-standing stratigraphic controversy as to whether the contact between the Montgomery Lake Group (age uncertain) and the Hurwitz Group (Paleoproterozoic) is an unconformity. In its type area, the Montgomery Lake Group consists of mainly immature litharenite and lithic wacke, and is separated from underlying Archean greenstones and the overlying Hurwitz Group by angular unconformities. In the Padlei belt, a unit previously referred to as "Montgomery Lake sedimentary rocks," "Montgomery Lake Group," or "map unit X" consists of mainly submature to mature subarkose to quartz arenite, with interbeds of pyritic (auriferous) and nonpyritic (locally red) quartz pebble conglomerate. These rocks differ from the Montgomery Lake Group in its type area, but bear strong lithologic and stratigraphic resemblence to rocks in the lower Hurwitz Group. Furthermore, interfingering between map unit X and Hurwitz Group lithotypes suggests that map unit X is an integral part of the Herwitz Group and that its upper contact is transitional. We formally propose that map unit X be called the "Noomut Formation," a new unit at the base of the Hurwitz Group. In the absence of an unconformity in its lower part, the lower Hurwitz Group provides a continous record of the time during which free oxygen was sustained in Earth's atmosphere, and of paleoclimatic changes that may be related to the breakup of an earliest Paleoproterozoic supercontinent.

Oxygen isotope sedimentology of gold- and uranium-bearing Witwatersrand and Huronian Supergroup quartz-pebble conglomerates

The relation between Au and U content and provenance has been evaluated for a number of quartz-pebble conglomerates from the Archean Witwatersrand and Early Proterozoic Huronian Supergroups, through analysis of the oxygen isotope composition of siliceous detritus within these rocks. Oxygen isotope compositions of quartz, quartzite, and chert pebbles, as well as of individual sand-sized quartz grains in the matrix of the conglomerates, were measured using conventional and laser fluorination techniques. Significant heterogeneity in the delta(18)O values of quartz and quartzite pebbles and quartz sands within individual samples of these conglomerates, suggests that they have preserved delta(18)O values of their source rocks. Preservation of original delta(18)O values of individual sand grains in conglomerates whose matrix displays a typical metamorphic texture implies limited oxygen and silica mobility, and recrystallization by pressure solution in the presence of small amounts of intergranular fluids. Oxygen isotope analyses of quartz pebbles along the Main `'reef'' (conglomerate) of the West Rand Consolidated gold mine, Witwatersrand, indicate that quartz-pebble delta(18)O values reflect local variations in provenance. Yet, the overall delta(18)O distributions for quartz pebbles and quartz sands from the Au- and U-bearing Main reef and the directly overlying U-bearing Monarch `'reef'' (conglomerate) in the same mine are identical, implying an overall similarity in their source rocks. Comparison of the quartz-pebble delta(18)O distributions between these two reefs and other Witwatersrand conglomerates, including polymictic conglomerates from the Welkom area of the Witwatersrand basin, moreover, suggests that the provenance was similar throughout deposition of much of the Central Rand Group. Differences in the ore content and Au/U ratio between conglomerates of the Central Rand Group are compatible with differences in their sedimentology. In contrast to conglomerates from the Witwatersrand Supergroup, delta(18)O distributions of quartz pebbles from different conglomerates throughout the Huronian Supergroup indicate distinct differences in provenance. A erude positive correlation between Au content and proportion of quartz pebbles with delta(18)O values greater than or equal to 11.5 per mil exists for these conglomerates. The U-bearing Matinenda Formation conglomerates in the Elliot Lake area have a very low proportion of pebbles with delta(18)O values above 11.5 per mil. Matinenda Formation conglomerates sampled closer to present outcrops of Archean greenstone terranes, as well as conglomerates of the Lorrain and Livingstone Creek Formations, have a higher proportion of quartz pebbles with delta(18)O values greater than or equal to 11.5 per mil and also higher Au contents. Differences between the delta(18)O distributions for quartz sands and quartz pebbles help to identify sediment sources. Absence of quartz sands with delta(18)O greater than or equal to 11.5 per mil, but presence of quartz pebbles with such values in the same sediment, may indicate a source which yields little or no sand-sized material in fluvial settings. Comparison of the delta(18)O values of quartz pebbles and quartz sands in quartz-pebble conglomerates to delta(18)O values for quartz from likely source rocks suggests that compositionally homogeneous granites and related pegmatites are the dominant source of uranium, whereas hydrothermal lode gold veins are the dominant source of gold.

Phanerozoic analogues for carbonaceous matter in Witwatersrand ore deposits

Thoriferous bitumen nodules in quartz-pebble conglomerates and pebbly sandstones of Phanerozoic age in the British Isles are analogues for the uraniferous bitumen nodules (flyspeck) in the Archean Witwatersrand gold-uranium deposits. They are comparable in morphology, relationship to bedding planes, replacive behavior toward quartz, mineralogy, fragmentation, and displacement of inclusions. In each case the quartz-rich sedimentary host rocks represent regionally important transgressive deposits.The contrast in dominant radioelement (thorium or uranium) probably reflects differences in mobility of the elements as a function of variable Eh conditions. The Phanerozoic thoriferous nodules occur only in pale (low Eh) beds which lack uranium deposits, but in the Archean Witwatersrand deposits fractionation of uranium and thorium may not have been as marked.In the Phanerozoic examples there is good evidence for migration of hydrocarbons through the host rocks and the bitumen nodules can confidently be attributed a hydrocarbon origin. By analogy, this supports proponents of a hydrocarbon origin for the Witwatersrand carbon, rather than a microbial origin.

Geochemistry and origin of Archean banded iron-formation from the Bababudan schist belt, India

Oxide and mixed oxide-silicate-carbonate facies banded iron-formation of the Bababudan schist belt are metamorphosed to a lower amphibolite facies. A wide spectrum of minerals including quartz, magnetite, grunerite, biotite, ankerite, siderite, and calcite is responsible for variation in major element composition. High Al 2 O 3 , TiO 2 , K 2 O, and MgO contents characterize the shaly banded iron-formation, whereas the cherty banded iron-formation is depleted in these constituents. Zr, Hf, Rb, St, Ba, U, and Th distinguish the felsic clastic flux from mafic flux, which is characterized by an abundance of Cr, Ni, Co, Sc, V, and Y. Cherty banded iron-formation has depleted and less fractionated rare earth element (BEE) patterns coupled with prominent positive Eu anomalies. These features closely resemble BEE distribution in the present-day hydrothermal fluids added at midocean ridge vent sites. In contrast the elevated Epsilon REE, fractionated patterns, and negative Eu anomalies characterize shaly banded iron-formation, which is more influenced by the supply of terrigenous debris. Both light and heavy REE increase simultaneously and the ratio between the two is relatively constant. Nd and its relationship with Al 2 O 3 and Fe 2 O (sub 3(total)) indicates that both hydrothermal and continental flux have been significant sources to varying degrees for banded iron-formation at various stratigraphic levels. Intermittent availability of oxygen appears to have resulted in rhythmic precipitation of iron hydroxide, and thus the banding of iron- and silica-rich layers. The essential components iron and silica of the banded iron-formation were supplied through hydrothermal fluids added to ocean water in the deep-marine part of the basin and transported due to the thermo-chemocline for precipitation in the shallow shelf zone. Seasonal suspended and dissolved fluvial flux from a continental source played a significant role in generating the composition of the banded iron-formation suite. Occurrence of the banded iron-formation with underlying quartz-pebble conglomerate and quartzites and the overlying turbidites might be explained by assuming a faster rate of generation and consumption of the Archean oceanic lithosphere.

EVOLUÇÃO SEDIMENTAR DO SUPERGRUPO MINAS: 500 MA. DE REGISTRO GEOLÓGICO NO QUADRILÁTERO FERRÍFERO, MINAS GERAIS, BRASIL

It is now possible to bracket the age of the Minas Supergroup with reasonable precision between about2580 Ma. (base of the Moeda Formation) and 2050 Ma. (top of the Sabará Formation) on the basis of precisezircon U-Pb age determinations from both granitic rocks of the basement and detrital zircons from quartzitesand graywackes of the Proterozoic metasedimentary sequences of the Quadrilátero Ferrífero.Three major unconformities subdivide the stratigraphic column into three sequences. The first sequencecomprises the Moeda, Batatal, Cauê and Gandarela Formations from 2580 to 2400 Ma.. The second sequencecomprises the Cercadinho, Fecho do Funil, Taboões and Barreiro Formations (the Piracicaba Group) datedat 2400 to 2150 Ma.. The third sequence contains only the synorogenic sediments of the Sabará Formation,deposited during the Transamazonian Event between 2150 and 2050 Ma.. This event folded andmetamorphosed the Minas Supergroup for the first time.The Sabará Formation of the Piracicaba Group has very different sedimentological characteristics fromthe other formations of this group and is exceptionally thick (about 3500 m), we therefore propose to upgradeit to the rank of a Group.Zircon age histograms of the Moeda quartzite (sample QF-1) and Sabará graywacke are similar, indicatinga common Archean source, i.e. the Belo Horizonte Complex north of the Quadrilátero Ferrífero. This impliesthat sedimentation of the Minas Supergroup in the western Quadrilátero Ferrífero is autochthonous.Periods of carbonate and Iron Formation platform sedimentation (Cauê, Gandarela and partly Fecho doFunil Formations) coincide with minor frequency of age plots of detrital zircons, indicating also a stabletectonic environment.Comparison of the Minas Supergroup with the Transvaal Sequence of South Africa revealscorrespondence not only in age but also in thickness, position of the major unconformities, lithologicalcompositions and in the position of gold and pyrite bearing quartz-pebble conglomerates.

Composition of the Early Precambrian Atmosphere and Hydrosphere Thermodynamic Constraints from Mineral Deposits

A chemical equilibrium model for the early Precambrian atmosphere and hydrosphere is presented. Thermodynamic calculations based on observed stable minerals and mineral assemblages in banded iron formation (BIF), pyritic, uraniferous and auriferous quartz pebble conglomerates, go ld -bea r ing cong lomera t e s o f the Tarkwaian in Ghana, and red bed sediments, yield the following conditions: Before approx. 2350 Ma, the atmosphere was

Anomalous pebble size variation in an erosive, Late Archaean braided stream: the Middelvlei gold placer, Witwatersrand, South Africa

ABSTRACTA characteristic of fluvial conglomerates is a gradual decrease in pebble size in the direction of the palaeocurrents. However, for the ancient braided stream investigated in this study, a deviation from this norm was found.The Middelvlei gold placer is a quartz pebble conglomerate that is mined in a South African goldfield, known as the West Wits Line, situated west of Johannesburg. Stratigraphically the placer occurs near the base of the Central Rand Group of the Witwatersrand Supergroup.In previous studies, the placer and the underlying sequence have been shown to be fluvial in origin, constituting a coarsening upward genetic increment of strata. The complete conformable succession, however, is only preserved in the distal parts of the depositional environment. In most parts of the goldfield the placer is separated from the underlying sequence by a progressive unconformity, which disappears deeper into the basin. This unconformity is thought to have formed during a progradation.Palaeocurrents of the Middelvlei placer are towards the south‐southeast in the western parts of the study area and towards the south‐east in the east. Maximum pebble size, however, decreases towards the west‐southwest, that is in a direction roughly perpendicular to the palaeocurrents.A reconstruction of the palaeotopography of the base of the placer in the western parts of the study area revealed considerable variations in palaeoheight with a gradient of about 0·002 along depositional strike.The anomalous lateral pebble size variation of the placer is explained by postulating an eastward migration of the depositing braided stream, simultaneous with its progradation. During the eastward migration, erosion of the underlying sequence became progressively more severe. This created an eastward‐thinning wedge of underlying strata.The results of the study do not invalidate lateral pebble size variation as a regional palaeoslope indicator. However, the findings suggest that caution should be exercised in the application of this indicator.

Oxygen isotopic study of the nature and provenance of large quartz and chert clasts in gold-bearing conglomerates of South Africa and Stable isotope compositions of quartz pebbles and their fluid inclusions as tracers of sediment provenance: Implications for gold-and uranium-bearing quartz pebble conglomerates: Comment and Replies

Oxygen isotopic study of the nature and provenance of large quartz and chert clasts in gold-bearing conglomerates of South Africa and Stable isotope compositions of quartz pebbles and their fluid inclusions as tracers of sediment provenance: Implications for gold-and uranium-bearing quartz pebble conglomerates: Comment and Replies

Proterozoic mineral deposits near plate margins of the archean wyoming province, USA

Abstract Proterozoic mineral deposits located near the margin of the Archean Wyoming Province can be interpreted in a plate tectonic context. The southern margin of the Archean Wyoming Province is exposed in southeastern Wyoming where Early Proterozoic island areas have been sutured to an Early Proterozoic passive margin developed during rifting of the Archean Wyoming craton. Mineral deposits include uranium-thorium-gold bearing quartz pebble conglomerate deposited during rifting, stratiform sulphide in black shale of the foredeep basin, anomalous platinum group element values in layered complexes of the island arcs, and a variety of mineral deposits that formed during closure of the arcs and passive margin. The eastern margin of the Wyoming Province is probably located under Phanerozoic cover just east of the Black Hills of South Dakota. The Black Hills is a segment of the Early Proterozoic Trans-Hudson Orogen which is interpreted here as an area exhibiting evidence of opening and closing of an intracratonic basin. In the Black Hills, domes underlain by Archean rocks are overlain by an Early Proterozoic clastic successions that preserves evidence of rifting. These clastic successions are succeeded by marine quartzite which is, in turn, overlain by a eugeosynclinal succession containing iron formation, graywacke, and a variety of volcanic units. The basal Early Proterozoic clastic succession contains uranium and gold-bearing quartz pebble conglomerate formed during rifting. The eugeosynclinal succession contains the Homestake iron formation which is host to the Early Proterozoic Homestake type gold deposits. Geologic and geochemical evidence can be marshalled to support either an epigenetic or syngenetic origin for the Homestake-type gold deposits. If the gold deposits are syngenetic they may have formed in a foredeep basin developed during closure of the intracratonic basin.

Conflicting evidence on the timing of mesothermal and paleoplacer gold mineralisation in early Proterozoic rocks from Southwest Ghana, West Africa

Rocks of early Proterozoic age (ca. 2100 Ma) host the major gold deposits in Ghana. The deposits are either located in mesothermal quartz vein systems or hosted in a quartz pebble conglomerate that represents a paleoplacer. Both types of mineralisation are largely confined to the Ashanti Belt, one of four parallel northeast-trending volcanic belts. While the stratigraphy and structure of the belts are similar, the Ashanti belt is characterised by a more tectonised northwest margin where most of the epigenetic gold deposits are located. In these deposits, gold mineralisation is located in faults that parallel the regional trend of the belts and were active late in the deformation history of the terrane. The auriferous quartz pebble conglomerate is part of a clastic sequence that is largely derived from the adjacent volcanic and plutonic rocks with the gold widely regarded as having originated from eroded vein deposits. Structural data, however, show that both the volcanic rocks and clastic sequence were deformed jointly prior to epigenetic gold mineralisation. Thus, the quartz vein deposits could not have been the source of the paleoplacer mineralisation. The paleoplacer gold could have originated from one of several possible sources but none has been unequivocally identified.

Stable isotope compositions of quartz pebbles and their fluid inclusions as tracers of sediment provenance: Implications for gold- and uranium-bearing quartz pebble conglomerates

Oxygen isotope compositions of pebbles from late Archean to paleo-Proterozoic gold- and/or uranium-bearing oligomictic quartz pebble conglomerates of the Witwatersrand district, South Africa, and Huronian Supergroup, Canada, were determined in an attempt to define the nature of the source terrain. The [delta][sup 18]O values of quartz pebbles within any one sample typically vary by [approximately] 4[per thousand] or more, but occasionally by as much as 8[per thousand], even for adjacent pebbles within the same hand specimen. In addition, adjacent quartz pebbles of widely contrasting [delta][sup 18]O values also preserve distinct isotopic signatures of their fluid inclusions. This overall heterogeneity suggests that the pebbles did not undergo significant oxygen isotope exchange after incorporation in the conglomerates. Therefore, oxygen isotope analyses of such quartz pebbles, in combination with a detailed investigation of their mineral and fluid inclusions, can provide a useful method for characterizing pebble populations and hence dominant sediment source modes. Comparison of values found in this study with [delta][sup 18]O values of quartz from Archean granites, pegmatites, and mesothermal greenstone gold veins, i.e., [delta][sup 18]O values of sources commonly proposed for the conglomerate ores, suggests that uranium is derived from a granitic source, whereas gold has a mesothermal greenstone goldmore » source. Low [delta][sup 18]O values of chert pebbles (9[per thousand] to 11.5[per thousand]) relative to those expected for Archean and Proterozoic marine cherts (commonly [ge] 17[per thousand]) effectively exclude marine cherts, and therefore, auriferous iron formations and exhalatives, as likely sources of gold.« less

Placer formation during progradational fluvial degradation; the late Archean Middelvlei gold placer, Witwatersrand, South Africa

Previous workers have established experimentally that geomorphologic conditions of degradation are favorable for the transport and concentration of heavy minerals and should therefore be conducive to the formation of placers. This principle may apply generally to the late Archean gold-uranium placers of South Africa, most of which rest on erosion surfaces.The Middelvlei gold placer, a quartz-pebble conglomerate which occurs near the base of the Central Rand Group of the Witwatersrand Supergroup, is mined selectively in a gold field known as the West Wits Line situated west of Johannesburg. The placer is considered part of a depositional system which overlies a regional mudstone marker, the Green Bar Member, and comprises three members, the Footwall Member, the Middelvlei placer, and the Hanging-wall Member.The Footwall Member consists predominantly of trough crossbedded quartz wackes, with lag conglomerates and massive mudstone as subordinate lithofacies. Thickness of this member varies from about 35 to more than 100 m and is generally smaller in the eastern parts of the gold field, where it is overlain by a large-pebble-type Middelvlei placer. Paleocurrent distributions of the Footwall Member are unimodal in all parts of the gold field with mean directions toward the southeast in the eastern regions and toward the south-southeast in the western parts. Directional variances are typically smaller than 1,600.The Middelvlei placer overlies the Footwall Member unconformably in most parts of the gold field, but in some southern parts of the western region no sedimentary break between these two members is evident. The placer comprises interbeds of quartz-pebble conglomerate, mostly massive, cross- and horizontally bedded quartz wacke, and thin massive mudstone.Maximum pebble size is about -6.0 phi and sizes are generally greater in the eastern parts of the gold field. Paleocurrent distributions are also unimodal, and in any particular area the means are not significantly different from those of the underlying Footwall Member. Directional variances are typically smaller than 2,000.The Hanging-wall Member consists mainly of trough crossbedded quartz wackes and subordinate lag conglomerates. For the western regions of the gold field information on paleo-currents is sparse, but in the eastern region the distribution is unimodal, with a mean not significantly different from that of the underlying Middelvlei placer.The Middelvlei depositional system is thought to be fluvial in origin and was probably deposited in braided streams. The Footwall Member and the Middelvlei placer constitute a coarsening-upward megacycle and are thought to represent a prograding braid plain. In most areas the transport of coarse-grained gravel during progradation was accompanied by erosion of the underlying Footwall Member.Gold concentrations are generally higher in areas of severe degradation of the Footwall Member than in regions where little or no basal erosion by the placer has taken place and where a conformable relationship between the two members exists. Statistical analyses revealed that the gold-rich facies of the placer is a single, relatively thin bed of massive, clast-supported conglomerate with large pebbles.