Middle Proterozoic Research Articles

The Major Ore Clusters of Super-Large Iron Deposits in the World, Present Situation of Iron Resources in China, and Prospect

The metamorphosed sedimentary type of iron deposits (BIF) is the most important type of iron deposits in the world, and super-large iron ore clusters of this type include the Quadrilatero Ferrifero district and Carajas in Brazil, Hamersley in Australia, Kursk in Russia, Central Province of India and Anshan-Benxi in China. Subordinated types of iron deposits are magmatic, volcanic-hosted and sedimentary ones. This paper briefly introduces the geological characteristics of major super-large iron ore clusters in the world. The proven reserves of iron ores in China are relatively abundant, but they are mainly low-grade ores. Moreover, a considerate part of iron ores are difficult to utilize for their difficult ore dressing, deep burial or other reasons. Iron ore deposits are relatively concentrated in 11 metallogenic provinces (belts), such as the Anshan-Benxi, eastern Hebei, Xichang-Central Yunnan Province and middle-lower reaches of Yangtze River. The main minerogenetic epoches vary widely from the Archean to Quaternary, and are mainly the Late Archean to Middle Proterozoic, Variscan, and Yanshanian periods. The main 7 genetic types of iron deposits in China are metamorphosed sedimentary type (BIF), magmatic type, volcanic-hosted type, skarn type, hydrothermal type, sedimentary type and weathered leaching type. The iron-rich ores occur predominantly in the skarn and marine volcanic-hosted iron deposits, locally in the metamorphosed sedimentary type (BIF) as hydrothermal reformation products. The theory of minerogenetic series of mineral deposits and minerogenic models has applied in investigation and prospecting of iron ore deposits. A combination of deep analyses of aeromagnetic anomalies and geomagnetic anomalies, with gravity anomalies are an effective method to seeking large and deep-buried iron deposits. China has a relatively great ore-searching potential of iron ores, especially for metamorphosed sedimentary, skarn, and marine volcanic-hosted iron deposits. For the lower guarantee degree of iron and steel industry, China should give a trading and open the foreign mining markets.

The geology of the carbonate-hosted Blende Ag–Pb–Zn deposit, Wernecke Mountains, Yukon, Canada

The Ag–Zn–Pb Blende deposit is located in the Wernecke Mountains, Yukon and is hosted by the middle Proterozoic Gillespie Lake Group dolomitic siltstones. The sulphide mineralization is localized within the axial planar cleavage of a kilometre-scale anticline and is dominated by galena- and sphalerite-cemented mosaic, rubble and crackle breccias with minor pyrite, galena, sphalerite and dolomite veins. 206Pb/204Pb values from galena range from 16.355 to 16.600, 207Pb/204Pb from 15.430 to 15.461, and 208Pb/204Pb from 36.016 to 36.283, respectively, and yield model ages between 1,490 and 1,430 Ma. A hydrothermal alteration zone, which is younger than the mineralization, has a poorly constrained U–Pb monazite age of 1,307 ± 180 Ma, which suggests that the Blende deposit is Proterozoic in age. Dolomites associated with the main- and late-stage mineralization have δ13C values that range from −1.8 to 0.9 ‰ and δ18O values of 15.7 to 21.9 ‰. The total range of δ34S values from pyrite, galena and sphalerite is 9.4 to 58.1 ‰, indicating that the sulphur in the deposit was derived from reduction of seawater sulphate in a closed system. Strontium isotopes suggest there were three fluids involved in the Blende mineralizing system: Fluid 1 was derived from seawater and formed carbonate and quartz veins pre-mineralization; it has an 87Sr/86Sr ratio of 0.70948. Fluid 2 has a high 87Sr/86Sr ratio of 0.73866, and fluid 3 has a Sr isotopic ratio of 0.71602. Fluids 1 and 3 have similar isotopic compositions but different total Sr ion signals (a function of concentration). This suggests that fluids 1 and 3 may have ultimately been derived from Proterozoic seawater but have undergone different amounts of water–rock interaction. The isotopic and geochemical data suggest the mineralization formed when a H2S-rich fluid derived from seawater (fluid 3) mixed with a metal-bearing fluid (fluid 2) in the high permeability zones of the axial planar cleavage. The Blende deposit is an unusual carbonate-hosted massive sulphide deposit which has features of both Irish-type and clastic-dominated Zn–Pb deposits. The deposit formed in response to a change in tectonism from a compressional regime to extension and rifting.

Ultramafic and mafic magmatism in southwestern Tuva

This paper presents the results of the first comprehensive geological, petrographic, mineralogical, geochemical, and geochronological studies of ultramafic and mafic magmatism in the poorly explored southwestern Tuva (evidence from the Birdag and Khayalyg massifs). These massifs intrude ortho- and paraschists, which are presumably Middle Proterozoic. Amphibole gabbros in the Khayalyg massifs contain numerous relict host-rock xenoliths. The massifs are dominated by meso- and leucocratic amphibole gabbros with minor ultramafic (serpentinous plagioclase harzburgites and plagioclase lherzolites) and transitional (wehrlites, olivine clinopyroxenites, hornblendites, melanocratic olivine gabbronorites) rocks. The serpentinous plagioclase harzburgites and plagioclase lherzolites occur in the amphibole gabbros of the Birdag massif as small lenticular bodies. The latter are interpreted as xenoliths of ultramafic restites of earlier protrusion rather than mafic-melt differentiates. The wehrlites, olivine clinopyroxenites, hornblendites, and melanocratic olivine gabbronorites forming the outer zones of the xenogenic bodies of ultramafic restites are considered hybrid rocks. They resulted from the contact reactions of mafic melts and their fluids with the xenogenic bodies of ultramafic restites, which were feldspathized during this interaction. In the gabbros from both massifs, the chondrite-normalized content of MREE and HREE is lower and that of LREE is higher than those in N-MORB. The plagioclase peridotites, wehrlites, and olivine clinopyroxenites forming xenogenic bodies among the amphibole gabbros of the Birdag massif are richer in REE (especially LREE) than the ultramafic restites in ophiolite associations. This is because they were infiltrated by fluids enriched in these elements during their alteration under the influence of later mafic melts. The studies suggest that the Birdag and Khayalyg ultramafic–mafic massifs result from the spatial coexistence of (1) more ancient small protrusions of ultramafic restites, which occurred as allochthons among Middle Proterozoic(?) metamorphic rocks, (2) later gabbroic intrusions (from 494 ± 16 to 450–447.4 ± 5 Ma), and (3) hybrid transitional rocks (wehrlites, olivine clinopyroxenites, hornblendites, melanocratic olivine gabbros) making up contact-reaction zones along their boundaries.

Re–Os and Mo isotope systematics of black shales from the Middle Proterozoic Velkerri and Wollogorang Formations, McArthur Basin, northern Australia

Joint application of the Mo isotope paleoredox proxy and Re–Os deposition-age geochronometer to euxinic black shales has potential for tracing the evolution of ocean redox chemistry over geological time. Here, we report new Re–Os and Mo isotope data for the Mesoproterozoic Velkerri Formation (Roper Group) and Paleoproterozoic Wollogorang Formation (Tawallah Group), McArthur Basin, northern Australia. New Re–Os ages of 1361 ± 21 Ma (2 σ, n = 14, mean square of weighted deviates [MSWD] = 1.3, Model 1) and 1417 ± 29 Ma (2 σ, n = 12, MSWD = 1.3, Model 1) constrain the depositional age of the Velkerri Formation and its contained biomarkers, as well as acritarchs and microfossils from the Roper Group. Black shales from the upper Velkerri Formation have high Mo abundances (105–119 ppm) and degree of pyritization [DOP] values (0.90–0.92) implying quantitative conversion of molybdate (MoO 4 2−) to thiomolybdate (MoS 4 2−) in overlying bottom waters. The average δ 97/95Mo (0.72 ± 0.10‰, 2 σ, n = 6) of these shales is consistent with previous data, but represents a significantly more precise determination for global seawater δ 97/95Mo at 1.4 Ga. This value is lighter than present-day seawater by ∼0.85‰ and reflects expanded strongly euxinic deep ocean conditions ([H 2S] aq > 11 μM) relative to oxic, suboxic, and weakly/intermittently euxinic ([H 2S] aq < 11 μM) marine deposition in the 1.4 Ga oceans. Mass-balance modelling suggests Mo removal into strongly euxinic and oxic sediments may have comprised 30–70% and less than 15%, respectively, of the oceanic Mo sink at 1.4 Ga as opposed to 5% and 35% today, respectively. The Re–Os radioisotope system in organic-rich shales serves as a test for post-depositional alteration that could affect the usefulness of paleoredox tracers such as Mo stable isotopes. Re–Os isotope data for the Wollogorang Formation black shales are scattered and yield a highly imprecise date of 1359 ± 150 Ma (2 σ, n = 21, MSWD = 85, Model 3). This age is younger than U–Pb zircon ages from interbedded tuffs that constrain the age of deposition at ca. 1730 Ma. In conjunction with previous petrological, geochemical, and paleomagnetic data, the Re–Os isotope data suggest hydrothermal fluid flow through the Wollogorang Formation, possibly associated with formation of the ca. 1640 Ma McArthur River Pb–Zn–Ag sedimentary exhalative deposit, resulted in post-depositional mobilization of Re and Os. Based on the degree of deviation of the Re–Os data from a 1730 Ma reference line, open-system behavior of Re and Os was greatest near the base of the black shale unit. Wollogorang Formation black shales are enriched in Mo (41–58 ppm), but are characterized by variable δ 97/95Mo (0.3–0.8‰) and DOP (0.57–0.92). The lightest δ 97/95Mo values occur near the base of the black shale unit. Based on the Re–Os systematics, hydrothermal fluids have probably overprinted the authigenic δ 97/95Mo signature in those shales. However, the heaviest δ 97/95Mo values in the Wollogorang Formation come from stratigraphically higher shales, and are similar to those observed for the Velkerri Formation, and thus may reflect seawater δ 97/95Mo at 1.73 Ga.

Paleo-fluid composition determined from individual fluid inclusions by Raman and LIBS: Application to mid-proterozoic evaporitic Na–Ca brines (Alligator Rivers Uranium Field, northern territories Australia)

Detailed determination of the composition of individual fluid inclusions in the H 2O–NaCl–Ca(Mg)–Cl 2 system from microthermometry data may be difficult because (i) the melting of salt hydrates is often unclear, (ii) salts or salt hydrates are often metastable, and iii) eutectic temperatures are often difficult to relate precisely to a unique salt–water system. These problems can be circumvented by using a Raman microprobe to identify the salt hydrates and to measure the chlorinity, by using Laser Induced Breakdown Spectroscopy (LIBS) to determine cation ratios, and by cross-checking the data from these independent techniques at each step in the determination of the fluid inclusion composition. Data from individual inclusion analysis are then completed by crush–leach technique data on bulk samples. This methodology was applied to a case study, the mineralized faults and associated breccia located in the Alligator Rivers Uranium Field (middle Proterozoic Kombolgie Subgroup, Northern Territory, Australia). Faults are cemented by euhedral quartz containing a variety of fluid inclusions with a wide range of ice-melting temperatures, from near 0 °C down to about − 50 °C. The approach has permitted the identification of three types of fluids: an Na-rich brine with variable Mg concentrations, a Ca-rich brine with low Mg concentrations, and a lower salinity fluid. The Ca and Na-brines mixed with each other and a less saline end-member. On the basis of Cl/Br and cation ratios, the Na-brine is interpreted as a primary brine, resulting from the evaporation of seawater. It is inferred that the Na-brine evolved chemically during its interaction with the Ca-rich lithologies from Proterozoic basement, forming subsequently a Ca-brine through Na–Ca exchange reactions. Thus, the fault system, which affects both the base of the Kombolgie sandstone and its basement, represents active drainage zones where different fluid reservoirs were connected, and thus a place where fluid mixing was highly favored.

Mineralogy and geochemistry of Al-phosphate and Al-borosilicate-bearing metaquartzites of the northern Serra do Espinha�o (State of Bahia, Brazil)

The investigated Al-phosphate- and Al-borosilicate-bearing metaquartzite horizon belongs to the Middle Proterozoic Serra de Vereda member of the Sao Marcos Formation of the northern Espinhaco fold belt. Outcrops are located in the NNW-SSE trending hills of the Serra de Canas/Cana Brava (Northern Serra do Espinhaco, State of Bahia, NE Brazil) about 10 km west of the town of Boquira. Sky blue lazulite and deep blue to lilac dumortierite are enriched in layers which sometimes show relic cross bedding. The occurrence of hematite patches and layers is typical. In rocks of type A, phosphorus contents range from 1.89 wt.% to 10.73 wt.% P2O5 and boron contents are mostly below 10 ppm whereas in the rocks of type B phosphorus contents are below 1% and boron contents reach 2600 ppm with a mean value of 1139 ppm. REE distribution patterns of rocks are predominantly controlled by the amount of xenotime, monazite and zircon. Oxygen thermometry on quartz and hematite and the stability of kyanite, augelite, trolleite and berlinite allow to estimate the minimum metamorphic peak P-T conditions at 475 degreesC and 3.8 kbar i.e. within the stability field of scorzalite-lazulite and dumortierite. The association of Al-phosphates with hematite layers provides evidence for the deposition of the metaquartzite protolith in a seawater-fed sabkha-like sand flat fringing the Espinhaco rift. It is proposed that apatite, Al-phosphates and Al-borosilicates were chemically precipitated together with iron hydroxides from pore fluids. The antithetic behaviour of phosphorus and boron contents in the metaquartzites is the consequence of an influx of toxic boron-rich water that drastically affected algae populations and consequently reduced phosphorus precipitation.

Study of coal chemical group composition. Part (II). Optimization of coal dissolution system : Tian, Y. et al. Meitan Zhuanhua, 2002, 25, (1), 28–32. (In Chinese)

Organic-rich samples derived from a Middle Cambrian Formation in the Georgina Basin, and from the Middle Proterozoic of the McArthur Basin in northern and central Australia, yielded alginite ranging from immature oil shale material to overmature residue. A maturation scale has been developed based on the thermal evolution of alginite as determined from reflectance and fluorescence. The coalification path of alginite is marked by jumps in contrast to the linear path of wood-derived vitrinite. Six zones have been recognised, ranging from undermature (zone I), through the mature (zones II/III), followed by a stable stage of no change (zone IV) to the overmature (zones V and VI). The onset of oil generation in alginite as evident from the present study is at 0.3% Ro Alg. and is expressed in a change of fluorescence from yellow to brown, and a coalification jump from 0.3 to 0.6% Ro of Alg. In many boreholes zone III can be distinguished between 0.6 and 0.8% Ro of Alg. where subsequent oil generation occurs. Zones II and III represent the oil window.A zone of little or no change designated zone IV, at 0.91.0% Ro of alginite follows zones II/III. A marked coalification jump characterises zone V, where a pronounced change in reflectance occurs to >1.0% Ro Alg., signifying peak gas generation. The border of oil preservation lies at the transition of zone V and VI, at 1.6% Ro Alg. In zone VI gas generation only occurs.Comparison of reflectance results with experimental and geochemical pyrolysis data supports high activation energies for hydrocarbon generation from alginite, and therefore a later onset of oil generation than other liptinite macerals (i.e. cutinite, exinite, resinite) as well as a narrow oil window.Transmission electron microscopy (TEM) confirms that alginite does not go through a distinct intermediate stage but that the percentage of unreacted organic matter decreases as maturation proceeds. A clear distinction can be made in TEM between immature alginite, alginite after oil generation, and alginite residue following gas generation. Alginite beyond 1.6% Ro acquires very high densities and the appearance of inertinite in TEM.Bitumens/pyrobitumens make a pronounced contribution to the organic matter throughout the basins and have been shown to effect pyrolysis results by suppressing Tmax. The bitumens/pyrobitumens have been divided into four groups, based on their reflectance and morphology, which in turn appears to be an expression of their genetic history. Their significance is in aiding the understanding of the basins' thermal history, and the timing of oil and gas generation.

STRUCTURAL STATE OF K-FELDSPAR IN THE FELSIC VOLCANIC ROCKS AND RING PLUTON GRANITES OF THE BUTLER HILL CALDERA, ST. FRANCOIS MOUNTAINS, SOUTHEASTERN MISSOURI

The K-feldspar in all major rock units exposed in the Middle Proterozoic Butler Hill Caldera, St. Francois Mountains, southeastern Missouri, shows a high, but not maximum, degree of structural order. The structural state parameter Z for phenocryst material from the felsic volcanic rock units averages 0.89, with a standard deviation of 0.06 ( Z would be 1 in fully ordered microcline). K-feldspar extracted from the matrix of these same samples yields an average Z value of 0.86, with a standard deviation of 0.07. The Z parameter for the K-feldspar in the ring pluton granites averages 0.87, with a standard deviation of 0.06. The structural state of the K-feldspar in these ring pluton granites is virtually identical to that reported by our group for the Butler Hill – Breadtray Granite, the subvolcanic granite massif exposed in the center of the Butler Hill Caldera. There is no statistically significant variation of K-feldspar structural state with inferred structural elevation, present topographic elevation, or proximity to the Simms Mountain Fault. We interpret the homogeneity of the K-feldspar structural state throughout the Butler Hill Caldera complex, and especially the unexpectedly high degree of structural order in the volcanic K-feldspar, as evidence for a pervasive alteration event, which re-ordered the K-feldspar throughout the entire volcanic–plutonic complex. The most likely cause of the homogenization of the K-feldspar structural state is the late-stage alkaline magmatism inferred to have occurred throughout the St. Francois Mountains at approximately 1.36 Ga.

Genesis and organic geochemical characteristics of the carbonaceous rock stratabound gold deposits, South China

The organic matter of three different chronological major carbonaceous rock gold-bearing formations of South China (Middle Proterozoic Shangqiaoshan group of northeastern Jiangxi, Lower Cambrian Shuikou group of northern Guangxi and Devonian Shetianqiao group of eastern Hunan) and related carbonaceous stratabound gold deposits such as Jinshan, Longshui and Shixia deposits, respectively, has been characterized by organic geochemical techniques. These organic geochemical results show that the average total organic carbon (TOC) content of the three chronological carbonaceous rock gold-bearing formations of South China ranges from 0.15% to 1.56%. The thermal maturity of the organic matter of host rocks in the three gold-bearing formations is high. The micro-component of the organic matter of the host rocks consists primarily of solid bitumen and graphite. The organic carbon and gold of the host rocks appear to syndeposit in situ during the formation of the gold-bearing formations. The organic carbon played a certain role in controlling the geochemical environment of the gold-bearing formations. The metallogenetic mechanism of the carbonaceous rock stratabound gold deposits of South China is closely associated in genesis with the sedimentation, diagenesis and thermal evolution history of the organic matter of host rocks in the gold-bearing formations.

Petrology, geochemistry, and diabase-granophyre relations of a thick basaltic sill emplaced into wet sediments, western Montana

The Plains Sill is a thick diabase-granophyre body that intruded the wet sediments of the Middle Proterozoic Prichard Formation of the Belt-Purcell Supergroup. The diabase is a high-iron tholeiite geochemically compatible with large-volume mantle melting in an intracratonic rift environment. Evidence of emplacement into wet sediments includes thick zones of homogenized granosediments adjacent to the sill, soft-sediment deformation at sill contacts, and sedimentary ovoid structures possibly formed by local fluidization of sediments. Utilizing sediment pore water and driven by heat from the sill, the diabase was metamorphosed during crystallization and cooling, leaving hornblende as the dominant mafic phase. Continued retrograde alteration resulted in overgrowths of secondary hornblende and variable alteration of plagioclase to epidote. A miarolitic granophyre layer, up to 150 m thick, caps the diabase and appears igneous in origin. Locally the granophyre is anomalously thick, perhaps reflecting updip migration of granophyric fluid where the Plains Sill cuts upsection through the Prichard Formation stratigraphy.

Introduction to and classification of manganese deposits of China

Manganese (Mn) ore deposits are widely distributed in China and can be divided into six types based on origin and subsequent modifications: (1) sedimentary, (2) volcanic-sedimentary, (3) metamorphosed, (4) hydrothermally modified, (5) hydrothermal, and (6) supergene. Sedimentary and supergene Mn ore deposits are economically the most important. Mn ore deposits and ore-bearing sequences are distributed temporally from the Proterozoic through the Quaternary, but they principally are Middle Proterozoic, Sinian, Cambrian, Ordovician, Devonian, Carboniferous, Permian, Triassic and Quaternary ages. The largest Mn metallogenic periods in China are Sinian and Devonian, in contrast to the widespread Proterozoic, Cretaceous and Oligocene Mn deposits found elsewhere in the world. Mn-bearing stratigraphic successions in China can be divided into six types of lithologic associations: (1) black shale series, (2) chert–mudstone–limestone, (3) carbonate, (4) mudrock–carbonate, (5) volcanic-sedimentary, and (6) Epeirogenic clastic rock types. The most important Mn metallogenic regions in China are: (1) Yanliao, (2) border of Hunan–Sichuan–Guizhou–Hubei Provinces, (3) Southern Shaanxi and northeastern Sichuan, (4) Middle Guizhou and eastern Yunnan, (5) Southwestern Guangxi, (6) Southeast Yunnan, (7) Hunan–Guangxi–Guangdong–Fujian, and (8) Xinjiang. The most striking characteristics of Mn ore deposits in China are the wide distribution in geological ages and variety of compositional types of ores, such as Fe–Mn, B–Mn, P–Mn, Co–Mn, Ca–Mn, Ca–Mg–Mn, among others. The primary Mn ores of China are mainly Mn carbonates.

Provenance analysis of lower Paleozoic cratonic quartz arenites of the North American midcontinent region: U-Pb and Sm-Nd isotope geochemistry

Lower Paleozoic supermature quartz arenites in Wisconsin and Michigan were derived from several Proterozoic and Archean terranes. Single-grain, detrital zircon populations from the Galesville and St. Peter Sandstones in Wisconsin yield very similar, closely concordant age distributions (Cambrian Galesville Sandstone: 1.1 Ga [n = 4], 1.4 Ga [n = 2], 1.8 Ga [n = 1], and 2.7 Ga [n = 2]; Ordovician St. Peter Sandstone: 1.1 Ga [n = 2], 1.8 Ga [n = 1], and 2.7 Ga [n = 6]). In contrast, most of the nine detrital zircons that were analyzed from the St. Peter Sandstone in the Michigan basin have high U contents (450‐2500 ppm) and are strongly discordant (60%‐90%). Six zircons from the Michigan basin that have ca. 1.0 Ga 207 Pb*/ 206 Pb* ages define a regression line that has an upper intercept of about 1100 Ma and a lower intercept of 15 Ma; one zircon has a 2.7 Ga 207 Pb*/ 206 Pb* age. The zircon data indicate that although a number of different terranes contributed detrital material to the Paleozoic quartz arenites in Wisconsin, 1.1 and 2.7 Ga terranes were the dominant sources, and not the local basement, which primarily consists of the ♢2.7 Ga Marshfield terrane, the 1.8 Ga Penokean orogen, and the 1.4 Ga Wolf River batholith. A terrane that has a 1.1 Ga age is probably the main source for the St. Peter Sandstone in the Michigan basin. Quartz separates were also analyzed for PbPb and Sm-Nd isotope variations, and the data do not indicate significant source differences between the heavy mineral fraction (zircons) and the quartz framework grains. Pb-Pb isochrons and Sm-Nd isotope data for quartz separates reflect mixing of the age groups and approximate relative proportions that are identified from the single zircon results. All isotope data on quartz separates and UPb zircon data indicate that the detrital constituents (zircons and quartz framework grains) that compose the lower Paleozoic quartz arenites in Wisconsin were primarily derived from the 2.7 Ga granite-greenstone terrane of the southern Superior Province and a 1.1 Ga terrane. The latter terrane is either the silicic volcanic rocks associated with the Midcontinent rift system, or, more likely, the voluminous granitic rocks that are associated with the Grenville Province on the eastern margin of North America. The Middle Proterozoic Grenville Province was the most important ultimate source of quartz and zircons to the St. Peter Sandstone in the Michigan basin; lesser amounts of material were contributed from the Archean Superior Province.

Evidence for extensive tabular intrusions in the Precambrian shield of western Canada: A 160-km-long sequence of bright reflections

The Trans-Hudson orogen is the preserved Early Proterozoic collisional zone between the Archean Superior and Hearne-Rae cratons and the focus of Lithoprobe's Trans-Hudson orogen transect where seismic reflection line S2b was recorded. The most intriguing structure in the data is a continuous band of strong reflections in the upper crust, herein named the Wollaston Lake reflector, which extends for 160 km across the northwestern hinterland of the Trans-Hudson orogen. The reflections range from 2.0 to 4.5 s two-way traveltime (6.0–13.5 km depth) and show a heterogeneous internal structure with multicyclic reflected arrivals and numerous diffractions. Our studies indicate that a series of high-velocity-high-density reflectors, having lengths of 800 m to a few kilometres and thicknesses from 50 to 150 m, most likely form the reflecting elements. Geologic mapping and dating document diabase intrusions associated with the Middle Proterozoic Mackenzie igneous event. Sills drilled in the region have thicknesses coinciding with those of our seismic model. We conclude that the Wollaston Lake reflector represents a series of diabase sheetlike intrusions emplaced during Mackenzie (i.e., post-Hudsonian) magmatic activity. Regionally, it provides constraints for the Middle Proterozoic paleo–stress field and for the late-stage tectonic history of the northwestern hinterland of the Trans-Hudson orogen. More generally, the detection of the reflector emphasizes the importance of large-scale horizontal injection of tabular bodies into the crust, over long distances and across tectonic boundaries.

96/04641 Structural elucidation of Argonne premium coals: Molecular weights, heteroatom distribution and linkages between clusters

Organic-rich samples derived from a Middle Cambrian Formation in the Georgina Basin, and from the Middle Proterozoic of the McArthur Basin in northern and central Australia, yielded alginite ranging from immature oil shale material to overmature residue. A maturation scale has been developed based on the thermal evolution of alginite as determined from reflectance and fluorescence. The coalification path of alginite is marked by jumps in contrast to the linear path of wood-derived vitrinite. Six zones have been recognised, ranging from undermature (zone I), through the mature (zones II/III), followed by a stable stage of no change (zone IV) to the overmature (zones V and VI). The onset of oil generation in alginite as evident from the present study is at 0.3% Ro Alg. and is expressed in a change of fluorescence from yellow to brown, and a coalification jump from 0.3 to 0.6% Ro of Alg. In many boreholes zone III can be distinguished between 0.6 and 0.8% Ro of Alg. where subsequent oil generation occurs. Zones II and III represent the oil window.A zone of little or no change designated zone IV, at 0.91.0% Ro of alginite follows zones II/III. A marked coalification jump characterises zone V, where a pronounced change in reflectance occurs to >1.0% Ro Alg., signifying peak gas generation. The border of oil preservation lies at the transition of zone V and VI, at 1.6% Ro Alg. In zone VI gas generation only occurs.Comparison of reflectance results with experimental and geochemical pyrolysis data supports high activation energies for hydrocarbon generation from alginite, and therefore a later onset of oil generation than other liptinite macerals (i.e. cutinite, exinite, resinite) as well as a narrow oil window.Transmission electron microscopy (TEM) confirms that alginite does not go through a distinct intermediate stage but that the percentage of unreacted organic matter decreases as maturation proceeds. A clear distinction can be made in TEM between immature alginite, alginite after oil generation, and alginite residue following gas generation. Alginite beyond 1.6% Ro acquires very high densities and the appearance of inertinite in TEM.Bitumens/pyrobitumens make a pronounced contribution to the organic matter throughout the basins and have been shown to effect pyrolysis results by suppressing Tmax. The bitumens/pyrobitumens have been divided into four groups, based on their reflectance and morphology, which in turn appears to be an expression of their genetic history. Their significance is in aiding the understanding of the basins' thermal history, and the timing of oil and gas generation.

Isotopic constraints on the age and provenance of the Lesser and Greater Himalayan sequences, Nepalese Himalaya

The Main Central thrust of the central Himalaya juxtaposes the Lesser Himalayan sequence (footwall) with the Greater Himalayan sequence (hanging wall), both of which are regarded as components of the northern passive margin of India. Considerable uncertainty surrounds their respective ages and sedimentary derivation. U-Pb and Sm-Nd isotopic studies of samples from the Langtang area, central Nepal, demonstrate important distinctions between the two rock packages. The Greater Himalayan sequence had a sedimentary provenance that included a major source of 0.8–1.0 Ga zircons, implying a Late Proterozoic age. The source of the Precambrian part of the Lesser Himalayan sequence contained 1.87–2.60 Ga zircons, and its depositional age is probably Middle Proterozoic. Nd isotopic characteristics of the two sequences are different: at 21 Ma, the Greater Himalayan rocks had eNd values between −14.6 and −18.5 while their Lesser Himalayan counterparts had values between −21.4 and −25.9, giving rise to major differences in their model ages. The Lesser and Greater Himalayan sequences at Langtang are interpreted as correlative to the lower and upper parts of the Vindhyan Supergroup, currently exposed south of the Gangetic plain in northern India. On the basis of available age data, the exposed hanging wall of the Main Central thrust in central and eastern Nepal contains no Archean-Early Proterozoic basement. If the fault does root into the basement at depth, it must do so farther north than recently published cross sections indicate. This implies a total displacement on the fault that is substantially greater than the 140–210 km required by tectonic overlap in the eastern Himalayas.

An integrated analytical approach for determining the origin of solid bitumens in the McArthur Basin, northern Australia

An integrated analytical approach using petrological, geochemical and isotopic techniques has been used to determine the origin of solid bitumens in the Middle Proterozoic Roper Group, McArthur Basin, northern Australia. Sandstone and dolerite samples, which in hand specimen contain dark material believed to be solid bitumens, were examined from six wells. Under ultraviolet illumination, non-fluorescent solid bitumens can be distinguished from two generations of crude oil, one of lower maturity (orange fluorescence), the other of higher maturity (white fluorescence). Other petrographic observations indicate that the precursor hydrocarbons to the solid bitumens and the subsequent charges of crude oil postdate all diagenetic clay and quartz cementation. Geochemical analyses indicate that many of the sandstones contain both an aliphatic-rich oil and solid bitumen, mixed in varying proportions, thus corroborating the petrological results. On the basis of n-alkane distributions, the oil appears to have two 3ubtly-different source inputs, one of which is typified by a greater proportion of high molecular weight n-alkanes which have a slight even carbon-number predominance and an isotopically lighter aliphatic hydrocarbon fraction. One sandstone sample contains solid bitumen in a fracture which is characterized by a high abundance of asphaltenes, polar compounds and sulphur and may have formed by deasphalting of a precursor oil. The solid bitumens recovered from fractures in dolerite sills were formed by thermal processes which in one sample resulted in an unusual series of ketone isomers. However most solid bitumens, particularly where present in the coarser sandstones, are related to the generation of secondary porosity. The large proportion of unresolved complex mixture, the lack of n-alkanes and the isotopically heavier aliphatic hydrocarbon fractions of samples containing predominantly solid bitumens indicate that bitumen formation occurred mainly by biodegradation of a precursor oil.

Depositional History of the Middle Proterozoic Castner Marble and Basal Mundy Breccia, Franklin Mountains, West Texas

ABSTRACT The ca. 1260 Ma Castner Marble is composed of six metamorphosed lithologies: stromatolitic limestone, cryptalgalaminite, massive limestone, hornfels (mudstone and tuff), rhythmite, and flat-pebble conglomerate. On the basis of the associations of these lithologies, the Castner Marble can be divided into three distinct intervals: a lower section containing stromatolitic and cryptalgalaminite units, a middle section with thick hornfels and massive limestone beds, and an upper section of rhythmites (interlaminated to thinly interbedded carbonate and hornfels) with interbedded flat-pebble conglomerate. The Castner succession was deposited during a transgressive event on a low-energy carbonate ramp. The contact between the Castner Marble and the overlying basaltic Mundy Breccia marks a catastrophic event or series of events, in which rhythmite units were disrupted and/or displaced along arcuate detachment surfaces and redeposited as lenses of rhythmite megabreccia unconformably overlying intact rhythmite sequences. These megabreccia lenses consist of large, locally deformed rhythmite blocks in a matrix of muddy flat-pebble conglomerate. One megabreccia lens contains intervals bearing irregular-shaped basalt clasts possible hyaloclastite layers, and pod-like basalt intrusions. This polymictic lens may have been produced by submarine syn-eruptive mixing of basalt and sediment. Local basalt intrusions may be part of the feeder system for this material and also the overlying basaltic Mundy Breccia. The Mundy Breccia covered the irregular topography created by the megabreccia lenses, locally loading and deforming the megabreccia into flame-like structures. The polymictic (carbonate and basalt) nature of one lens and presence of load features suggests that little time elapsed between deposition of the Castner Marble and the Mundy Breccia, and therefore that the Mundy Breccia was most likely deposited in a submarine setting.

Lower palaeozoic and precambrian petroleum source rocks and the coalification path of alginite

Organic-rich samples derived from a Middle Cambrian Formation in the Georgina Basin, and from the Middle Proterozoic of the McArthur Basin in northern and central Australia, yielded alginite ranging from immature oil shale material to overmature residue. A maturation scale has been developed based on the thermal evolution of alginite as determined from reflectance and fluorescence. The coalification path of alginite is marked by jumps in contrast to the linear path of wood-derived vitrinite. Six zones have been recognised, ranging from undermature (zone I), through the mature (zones II/III), followed by a stable stage of no change (zone IV) to the overmature (zones V and VI). The onset of oil generation in alginite as evident from the present study is at 0.3% R o Alg. and is expressed in a change of fluorescence from yellow to brown, and a coalification jump from 0.3 to 0.6% R o of Alg. In many boreholes zone III can be distinguished between 0.6 and 0.8% R o of Alg. where subsequent oil generation occurs. Zones II and III represent the oil window. A zone of little or no change designated zone IV, at 0.9 1.0% R o of alginite follows zones II/III. A marked coalification jump characterises zone V, where a pronounced change in reflectance occurs to >1.0% R o Alg. , signifying peak gas generation. The border of oil preservation lies at the transition of zone V and VI, at 1.6% R o Alg. In zone VI gas generation only occurs. Comparison of reflectance results with experimental and geochemical pyrolysis data supports high activation energies for hydrocarbon generation from alginite, and therefore a later onset of oil generation than other liptinite macerals (i.e. cutinite, exinite, resinite) as well as a narrow oil window. Transmission electron microscopy (TEM) confirms that alginite does not go through a distinct intermediate stage but that the percentage of unreacted organic matter decreases as maturation proceeds. A clear distinction can be made in TEM between immature alginite, alginite after oil generation, and alginite residue following gas generation. Alginite beyond 1.6% R o acquires very high densities and the appearance of inertinite in TEM. Bitumens/pyrobitumens make a pronounced contribution to the organic matter throughout the basins and have been shown to effect pyrolysis results by suppressing T max. The bitumens/pyrobitumens have been divided into four groups, based on their reflectance and morphology, which in turn appears to be an expression of their genetic history. Their significance is in aiding the understanding of the basins' thermal history, and the timing of oil and gas generation.

Preserved initial [formula omitted] in apatite from altered felsic igneous rocks: A case study from the Middle Proterozoic of South Australia

Strontium isotope data from a suite of hydrothermally altered Middle Proterozoic granitoids and a related felsic volcanic rock show that the total-rock systems have been disturbed as a result of hydrothermal alteration. As such, unambiguous determination of initial 87Sr 86Sr is not possible using total-rock data, with consequent loss of valuable petrogenetic information. In contrast, initial 87Sr 86Sr values derived from apatite are uniform when compared with the discordant total-rock analyses. Evaluation of strontium diffusion kinetics in apatite, together with geological and isotopic evidence presented here suggest that initial 87Sr 86Sr derived from apatite preserves the igneous initial 87Sr 86Sr and that 87Sr 86Sr in apatite is unlikely to have been modified as a result of igneous cooling or hydrothermal alteration. This study shows that under some conditions, apatite is resistant to 87Sr 86Sr modification and can yield petrogenetic information not attainable from total-rock analyses. Data presented here, together with data obtained in other studies, indicate that the strontium isotope system in apatite is more resistant to modification than feldspar and biotite. This approach may be of widespread application in determining initial 87Sr 86Sr of similarly altered ancient igneous rocks.

Yardea Dacite—Large-volume, high-temperature felsic volcanism from the Middle Proterozoic of South Australia

The Yardea Dacite is a large-volume felsic volcanic unit from the Middle Proterozoic Gawler Range Volcanics of South Australia; it has been previously described as an ignimbrite. However, some samples contain no petrographic evidence for a pyroclastic origin, but have characteristics compatible with final crystallization from a nonfragmented magma. These samples may have erupted as lavas, but others are likely to be extremely densely welded ignimbrites, suggesting a compound nature for the unit. Geothermometry and phase equilibria indicate that the Yardea Dacite originated from a high-temperature (∼1000 °C) felsic magma with a low water content (≤2%). The Yardea Dacite is not associated with a known caldera of the Valles type, and shares many characteristics of recently described Cenozoic felsic volcanic rocks from the western United States, interpreted as rheoignimbrites or as unusually extensive lavas.