Main Seam Research Articles

Stratified and unstratified bordism of pseudomanifolds

We study bordism groups and bordism homology theories based on pseudomanifolds and stratified pseudomanifolds. The main seam of the paper demonstrates that when we uses classes of spaces determined by local link properties, the stratified and unstratified bordism theories are identical; this includes the known examples of pseudomanifold bordism theories, such as bordism of Witt spaces and IP spaces. Along the way, we relate the stratified and unstratified points of view for describing various classes of pseudomanifolds.

Spatial variation in the stratigraphic architecture of the Fort Cooper and equivalent coal measures, Bowen Basin, Queensland

The major stratigraphic units of the Fort Cooper Coal Measures (FCCM) have been reviewed, and an internal coal seam architecture using 500 wireline logs in the Bowen Basin, southeast Queensland has been developed. The subdivisions of the coal measures include the Burngrove Formation, Middle Main Seam and Fair Hill Formation in the northern Bowen Basin. To the south of the basin, the Middle Main Seam is interpreted as the stratigraphic equivalent of Black Alley Shale Formation. The Yarrabee Tuff forms the top boundary of the coal measures and is distributed basin-wide except over the southern Denison and southern Taroom troughs. Accessory tuffs beneath the Yarrabee Tuff aided seam level correlation. The geometries of coal seams have been analysed for each morphotectonic zone of the Bowen Basin using gravity data, which highlight high and low topographic sites. The Collinsville Shelf in northwest of the Bowen Basin comprises thick and merged Burngrove and Fair Hill formation coals that split towards northeast in the Nebo Synclinorium and southeast in Taroom Trough. Coal seams split into daughter seams from north to south and are named as first, second or third order, based on the number of splits that occurred. First- and second-order splitting occurs at the boundary of the Nebo Synclinorium and Collinsville Shelf, at the Narrows of Nebo Synclinorium and Comet Ridge. Further, third- and fourth-order splitting occurs at the boundary of the Comet Ridge and Taroom Trough. Splitting zones found at the junction of morphotectonic zones are related to a change in subsidence. In addition, the association of split seams with coarse grain-dominated interburden reflects increased accommodation in low-lying areas such as the Nebo Synclinorium and the troughs. Thick and coalesced coal seams are associated with shelves and ridges of the basin reflecting low accommodation sites. Hence, the variation in the coal seam architecture of FCCM responds to the subsidence associated with different morphotectonic zones of the basin.

Simulation of water resource loss in short-distance coal seams disturbed by repeated mining

Physical simulations and field measurements were performed to study the movement of overlying strata and water conductive fractures (WCF) under conditions of repeated mining in short-distance coal seams. In addition, the feasibility of water resource conservation mining (WRCM) was analyzed in areas disturbed by repeated mining. The results of the simulation showed that the aquiclude remained intact after the mining of Coal Seam 11 (the 1st main seam) in Shigetai coal mine in northwest China. According to the findings, WRCM can be conducted in this area. After the mining of Coal Seam 12 (the 2nd main seam), the WCF in overlying strata above the central section of the mined-out area will gradually compress and close. However, due to the impact of repeated mining in short-distance coal seams, WCF in overlying strata at the edge of the mined-out area are connected to the Quaternary loose aquifer and are not likely to close, resulting in the loss of the water resource. Thus, WRCM in this area would be difficult to conduct. Moreover, field observations showed that the aquifer can likely remain intact after Coal Seam 11 is mined, but the aquifer’s water level will probably not fully recover if Coal Seam 12 is mined. This is consistent with the simulation results. Therefore, appropriate technical mining measures must be taken to conduct WRCM at the 2nd main seam in the shallow-buried short-distance coal seams. These research results can be used as a reference for future WRCM endeavors in the arid and semi-arid regions of northwest China.

Platinum-group element distribution in base-metal sulfides of the UG2 chromitite, Bushveld Complex, South Africa—a reconnaissance study

Two drill cores of the UG2 chromitite from the eastern and western Bushveld Complex were studied by whole-rock analysis, ore microscopy, SEM/Mineral Liberation Analysis (MLA), and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analysis. The top and base of the UG2 main seam have the highest bulk-rock Pd and Pt concentrations. Sulfides mostly occur as aggregates of pentlandite, chalcopyrite, and rare pyrrhotite and pyrite or as individual grains associated mostly with chromite grains. In situ LA-ICP-MS analyses reveal that pentlandite carries distinctly elevated platinum-group element (PGE) contents. In contrast, pyrrhotite and chalcopyrite contain very low PGE concentrations. Pentlandite shows average maximum values of 350–1,000 ppm Pd, 200 ppm Rh, 130–175 ppm Ru, 20 ppm Os, and 150 ppm Ir, and is the principal host of Pd and Rh in the studied ores of the UG2. Mass balance calculations were conducted for samples representing the UG2 main seam of the drill core DT46, eastern Bushveld. Pentlandite consistently hosts elevated contents of the whole-rock Pd (up to 55 %) and Rh (up to 46 %), and erratic contents of Os (up to 50 %), Ir (2 to 17 %), and Ru (1–39 %). Platinum-group mineral (PGM) investigations support these mass balance results; most of the PGM are Pt-dominant such as braggite/cooperite and Pt-Fe alloys or laurite (carrying elevated concentrations of Os and Ir). Palladium and Rh-bearing PGM are rare. Both PGE concentrations and their distribution in base-metal sulfides (BMS) in the UG2 largely resemble that of the Merensky Reef, as most of the Pd and Rh are incorporated in pentlandite, whereas pyrrhotite, chalcopyrite, and pyrite are almost devoid of PGE.

Integration of vertical and in-seam horizontal well production analyses with stochastic geostatistical algorithms to estimate pre-mining methane drainage efficiency from coal seams: Blue Creek seam, Alabama

Coal seam degasification and its efficiency are directly related to the safety of coal mining. Degasification activities in the Black Warrior basin started in the early 1980s by using vertical boreholes. Although the Blue Creek seam, which is part of the Mary Lee coal group, has been the main seam of interest for coal mining, vertical wellbores have also been completed in the Pratt, Mary Lee, and Black Creek coal groups of the Upper Pottsville formation to degasify multiple seams. Currently, the Blue Creek seam is further degasified 2–3years in advance of mining using in-seam horizontal boreholes to ensure safe mining.The studied location in this work is located between Tuscaloosa and Jefferson counties in Alabama and was degasified using 81 vertical boreholes, some of which are still active. When the current longwall mine expanded its operation into this area in 2009, horizontal boreholes were also drilled in advance of mining for further degasification of only the Blue Creek seam to ensure a safe and a productive operation. This paper presents an integrated study and a methodology to combine history matching results from vertical boreholes with production modeling of horizontal boreholes using geostatistical simulation to evaluate spatial effectiveness of in-seam boreholes in reducing gas-in-place (GIP).Results in this study showed that in-seam wells' boreholes had an estimated effective drainage area of 2050acres with cumulative production of 604MMscf methane during ~2years of operation. With horizontal borehole production, GIP in the Blue Creek seam decreased from an average of 1.52MMscf to 1.23MMscf per acre. It was also shown that effective gas flow capacity, which was independently modeled using vertical borehole data, affected horizontal borehole production. GIP and effective gas flow capacity of coal seam gas were also used to predict remaining gas potential for the Blue Creek seam.

Effect of relative lake-level changes in mire–lake system on the petrographic and floristic compositions of a coal seam, in the Most Basin (Miocene), Czech Republic

Exposures in the Bílina opencast mine provide a direct evidence that the 3.5m thick upper bench of the Main Seam in the Most Basin passes over a distance of about a kilometre into a hundred metres thick equivalent of clastic deltaic sediments with Gilbert-type mouth bars and thus clearly corroborates the co-existence of lacustrine delta–mire system. Deposition of the delta and the upper bench of the Main Seam were affected by five short-term lacustrine incursions, possibly related to tectonic subsidence before final lacustrine transgression took place. Between the temporary lacustrine incursions, delta mouth bars prograded into an extensive mire being separated only by a shallow compaction lake along mouth bars foreland. However, most clastics, introduced from delta were caught by dense vegetation in a narrow proximal zone of mire and did not affect more distal parts of relatively dry woodland mire with water table fluctuating around the surface. It is therefore characterised by low ash yield and increased resinite, fusinite and inertodetrinite contents. Palynological data indicate that the peat-forming vegetation of the upper bench of the Main Seam was dominated by Taxodiaceae (Glyptostrobus, Athrotaxis) and Calamus palm accompanied by paludal species of the family Pinaceae and rich undergrowth of ferns and Vaccinium-type Ericaceae. Progradation of delta mouth bars to the proximity of the study section is documented by increasing proportion of allochthonous elements produced by plants colonizing the flood plain and delta plain areas (Nyssa–Taxodium, Parrotia–Ulmus pyramidalis and Comptonia–Pinus oviformis associations). In contrast, during periods of temporary lacustrine transgressions the mire was replaced by open water environment, which favoured deposition of clay-rich coals or partings (>30% of ash yield) of the flooding horizons in spite of fact that delta body retreated to the basin margin and the clastics were introduced from a larger distance. Rare palynomorphs indicate only sparse vegetation cover of aquatic plants during this time. Clay content seems to be the best indicator of the water level fluctuations. Facies indices (e.g. TPI and GI) show only poor correlation to hydrological and floristic changes detected in the mire–lake system.

Macerals of C1 Coal Seam in Parvade Coalfield (Tabas- Ira)

This paper deals with the study of the Late Triassic coal deposits from Parvade coalfield in south of Tabas town and located geologically this is part of Central Iran zone. Coal main seam in Parvade coalfield named C1.Coals are rich in vitrinite, with average contents of 90%.Collinite is main part of vitrinite. their structures are homogenous .Telinite is low amount of vitrinite, based on ratio of collinite to telinite , subsidence of swamp is gradually and conditions of environments is reduction and PH is acidic. Lithotype of coals is most ultraclarain with high amount of vitrinite .some of amounts presents of semi-vitrinite and fusinite, micronite, skelerutinite and semi-fusinite macerals. High content of the vitrinite group macerals in these coals shows that water level was relatively high .Amount of vitrinite played key roles in coking.

Coal facies in a Cenozoic paralic lignite bed, Krabi Basin, southern Thailand: Changing peat-forming conditions related to relative sea-level controlled watertable variations

Abstract The Cenozoic Krabi Basin in the southern part of peninsular Thailand contains about 112 million tons proven coal reserves. At present, coal is only produced from the Bang Mark mine located in the southern part of the basin, where the main lignite bed is 7–20 m thick. The lignite bed occurs in an overall paralic succession. The present paper investigates the depositional conditions of an approximately 8 m thick lignite bed (main seam) in the Bang Mark mine using organic petrography, including maceral ratios, and geochemistry. The results are further interpreted in a sequence stratigraphic context. The lignite is of low rank and is completely dominated by huminite indicating generally oxygen-deficient conditions in the precursor mire. Very low inertinite contents suggest rare occurrences of wildfires. The lower part of the lignite bed represents a topogenous fresh water peat mire with open water areas that in few cases may have experienced influx of saline water. The peat mire was subjected to periodic inundations and deposition of siliciclastics. Tissue preservation was relatively poor. The upper part of the lignite bed represents a slightly domed fresh water ombrogenous peat mire with a stable watertable and a balance between peat accumulation and accommodation space creation that favoured preservation of plant tissues. In general, the mire vegetation changed from less woody in the topogenous mire to more arborescent in the ombrogenous mire, where plants with suberinised wood cell walls also were more frequent. Decompacted, the lignite bed corresponds to a minimum ~ 11 m thick peat deposit that records from ~ 22,000 to 55,000 years of peat accumulation. Watertable rise in the peat mire was controlled overall by relative sea-level rise. In a sequence stratigraphic context, the lignite bed overlies a terrestrialisation surface (TeS; sensu Diessel, 2007) and the lowermost part records peat formation during a falling watertable and a decreasing accommodation/peat accumulation ratio (terrestrialisation). An accommodation reversal surface (ARS; sensu Diessel, 2007) indicates a change to paludification style of peat formation characterised by rising watertable and a high accommodation/peat accumulation ratio. Another ARS marks a gradual change to a situation with a balanced accommodation/peat accumulation ratio. The overall watertable rise throughout peat formation, but at a gradually slower rate from base to top, suggests that the lignite bed could be located in the late transgressive systems tract (TST).

COAL BED METHANE POTENCYOF TANJUNG FORMATION IN TANAH BUMBU SOUTH KALIMANTAN

Coal deposits distributes widely in Indonesia, mainly in Sumatera and Kalimantan. These deposits become an indication for a huge quantity of coal-bed methane (CBM) potency. As an environmentally friendly source of energy, CBM could be one of Indonesia's future energy sources. Centre for Geological Resources (CGR) started the exploration for this energy since 2005, while the study was conducted in 2008 at Tanjung Formation in Tanah Bumbu Area, South Kalimantan.CSAT-1 well was drilled to provide CBM resources and deep coal mine potency data for Tanjung Formation (Eocene) in Asem-asem Basin. This well went through 10 coal seams with three main seam, which is E seam at 212.34-213.30 m depth, I seam at 261.93-264.20 m, and J seam at 270.20-275.35 m depth. Calorific value from this three main seam categorized as high rank coal, vary from 6197-6745 cal/gram (adb), with total moisture between 4.51-7.11 %, adb.Total coal resources used for CBM resources quantification is 112,733,226 tons (between 300 to 1000 meters depth). Based on desorption test and gas chromatograph measurement from samples at various depth from three main seams, those coal gives methane resources estimation about 430 MSCF, with methane content vary between 1.2 - 6.6 cu-ft/ton of coal.

Zoning of platinum group mineral assemblages in the UG2 chromitite determined through in situ SEM-EDS-based image analysis

In situ scanning electron microscopy–energy dispersive X-ray spectrometry analysis of platinum group minerals (PGM) and base metal sulfides in the UG2 chromitite shows that this ore body is zoned along at least ∼6 km of strike. The uppermost part of the UG2 chromitite, referred to as the leader seam, is ∼16 cm thick and has a PGM assemblage that is dominated by PGE arsenides, sulpho-arsenides, and alloys (∼70 vol.% of all PGM), which are typical secondary PGM assemblages in other segments of UG2. This is the first time such laterally persistent secondary assemblages have been identified in the UG2 chromitite, as previously, they were only known to occur adjacent to transgressive fluid-bearing structures (e.g., pipes, faults). The underlying main seam is thicker (one to nine seams totaling ∼130 cm) and has a PGM assemblage that consists mostly of Pt sulfide, Pt–Pd sulfide, Pt–Rh–Cu sulfide, laurite, and Fe–Pt alloys (∼85 vol.% of all PGM), typically regarded as primary magmatic constituents of UG2 chromitite. There are, however, some subtle vertical changes in the PGM assemblages of the main seam that include the occasional presence of secondary assemblages in the top and bottom parts. The origin of these secondary PGM assemblages is related to alteration by hydrothermal fluids and/or fluid-rich melts that infiltrated during crystallization of the UG2 and may possibly have been derived from the UG2 chromitite itself.

Platinum-group elements in the UG1 and UG2 chromitites, and the Bastard reef, at Impala platinum mine, western Bushveld Complex, South Africa: Evidence for late magmatic cumulate instability and reef constitution

We have examined the UG1 and UG2 chromitites, the Bastard Reef, and ultramafic rocks in the footwall of the Merensky Reef that are possibly correlatives to the Pseudoreef, in drill core at Impala platinum mine. The UG2 consists of a 66 cm main seam and, ca 5 m above the UG2, four subsidiary seams with a combined thickness of 30 cm. The UG1 consists of a 112 cm main seam and, in its hanging- and foot wall, 50 subsidiary seams/stringers with a combined thickness of 91 cm. The UG2 main seam contains 2111 ppb Pt, 1214 ppb Pd, 362 ppb Ru, and 98 ppb Ir, with the subsidiary seams containing broadly similar PGE levels. The UG1 main seam contains 628 ppb Pt, 265 ppb Ru, and 45 ppb Ir over 112 cm, but only 13 ppb Pd. The subsidiary seams have variable compositions broadly overlapping with the main seam. Well defined positive correlations between Pt and Pd with the IPGE suggest that in the chromitites the PGE were originally concentrated by a sulfide liquid. In both the UG2 and the UG1 chromitites the highest PGE contents occur at the top and bottom of the seams, analogous to other localities in the Bushveld Complex. Such element distribution patterns resemble those in certain dykes interpreted to result from flow differentiation, suggesting that the seams formed via injection of chromite slurries into their semi-consolidated footwall cumulates, possibly during sagging of the centre of the complex in response to emptying of staging chambers at depth. Down-dip sliding of the semi-consolidated cumulates was also responsible for the phase layering of the chromitites, as represented by the sharp top and bottom contacts of the seams, and the extremely well defined correlation between PGE and chromitites. One of the last processes during cumulate solidification involved removal of some of the sulfur from the chromitites by late magmatic and/or hydrothermal fluids. Pd and Cu have also been mobilized locally (particularly in the UG1 chromitite), but the other PGE appear to have been largely immobile.

To the stratigraphic division of the Upper Eocene Main Seam Complex (Thuringian and Borna Main Seam) in the Leipzig Bay

To the stratigraphic division of the Upper Eocene Main Seam Complex (Thuringian and Borna Main Seam) in the Leipzig Bay

Provenance of Coals Recovered from the Wreck of HMAV Bounty

Coal samples from HMAV Bounty were analysed using standard techniques to shed light on their provenance. Petrographic analysis indicated they were Carboniferous, with high vitrinite and liptinite content and a mean random reflectance of vitrinite of 0.99%. Palynological analysis indicated the samples were derived from the Middle Coal Measures, Westphalian B. Combining coal rank (vitrinite reflectance), age, knowledge of seam distributions and coalfield history indicates the most like source to be the Durham Coalfield, possibly the Hutton or Low Main Seams. These coals were mined along the valley of the Weir in the latter part of the 18th century. © 2007 The Authors

Influence of the submerged arc welding in the mechanical behaviour of the P355NL1 steel—part II: analysis of the low/high cycle fatigue behaviours

A normalized fine grain carbon low alloy steel, P355NL1 (EN10028-3), intended for service in welded pressure vessels, where notch toughness is of high importance, has been investigated. Applications with this steel usually require the intensive use of welds. One of the most common welding processes that are used in the manufacturing of pressure vessels is the submerged arc welding. This welding process is often automated in order to perform the main seam welds of the body of the vessels. The influence of the automated submerged arc welding, in the mechanical performance, is investigated. In this paper (Part II) the low and high cycle fatigue and crack propagation behaviours are compared between the base and welded materials. Several series of small and smooth specimens as well as cracked specimens made of base, welded and heat affected materials, respectively, were fatigue tested. Strain, stress and energy based relations for fatigue life assessment, until crack initiation, are evaluated based on experimental results and compared between the base and welded materials. Finally, the fatigue crack propagation behaviours are compared between the base, welded and heat affected materials.

Slagging and fouling characteristics of seam 32/33, Panian coalfield, Semirara Island, Philippines

Twenty samples of seam 32/33, the main seam of Panian coalfield in Semirara Island, Antique Province, Philippines, were collected from a borehole drilled at the northeastern edge of the coalfield. The samples were analyzed to characterize the coal geochemistry of the seam and understand why the coals of Semirara Island exhibit a high tendency for slagging and fouling despite its low average ash content. Analysis of the slagging and fouling characteristics of this seam is important because it supplies five electric power utilities and several cement plants in the Philippines. Proximate analyses and vitrinite reflectance measurements designate the rank of the seam as sub-bituminous C, based on ASTM coal classification. H/C versus O/C ratios indicate that the seam is made up largely of huminite, denoting early stages of coalification. Chemical analysis of the ash reveals high contents of Na, Mg, Fe, Ca, Ba and Sr. The strongly negative correlation of these elements with the ash content indicates an organic affinity of the chemical elements of the seam. Owing to enrichment in alkali and alkali-earth elements, slagging and fouling indices indicate that the seam has medium to high propensity for slagging and a severe tendency for fouling. The detrimental characteristics of coal feedstock from Panian mine is mitigated by modifications to the boiler design and operational conditions and by blending with coals imported from Indonesia, China and Australia.

Estimate of the Effect of the Time Interval between Technical Inspections on the Strength Reliability of Certain First-Loop Components of a VVER-1000 Reactor

The results of a computational investigation of the possibility of and the safety conditions for switching from a 4- to an 8-yr time interval between technical inspections of the main circulation pipeline and the pressure compensator vessel in VVER-1000 reactors are presented. To this end, calculations of the critical and admissable sizes of defects in the main metal and weld seams in the operating regime, in accident situations, and during earthquakes have been performed. Calculations of the time for a through defect to reach a critical size for different operating periods have been performed. The influence of the hydrotesting pressure and the time interval between such tests on the operational safety and the effect of the time between technical inspections on the reliability of the indicated first-loop components of a VVER-1000 reactor with respect to the criterion of fracture resistance, taking account of the probabilistic nature of the initial data, is analyzed. The calculations were performed using normative and certified procedures geared toward the typical characteristics of steel and structures and the conditions of fabrication, assembly, and operation.

The Lower Karoo coal (k2–3) of the Mid-Zambezi basin, Zimbabwe: depositional analysis, coal genesis and palaeogeographic implications

Intensive lithological study and correlation of borehole records from the k2–3 coalfields or coal occurrences in the Mid-Zambezi basin led to the identification of two sedimentological types of coal: the Alluvial plain coal and the freshwater-lake shoreline coal. The Alluvial plain coal was found only at Gokwe and in the Nyamandlovu area. Its depth of more than 200 m below surface, the thinness and discontinuous nature of the seams, and the high ash content of the coal make the economic significance extremely small. In clear contrast, lithologically and economically, stand the lake shoreline coal fields at Wankie, Lubimbi, Lusulu, Lubu, Busi, and Sengwa. The pay-zone is the basal Main Seam, up to 17-m thick. The shoreline coal is either more or less massive (Wankie, Lusulu-Lubu) or is thin coal bands alternating with carbonaceous mudstone (Lubimbi, Sengwa). The clearest evidence for a lake shoreline environment comes from the lateral lithofacies change of the coal, e.g., at Wankie where it turns down-dip into sapropelic mudstone of the lake, and up-dip into terrestrial sediments of the coastal plain. The lake shoreline interpretation results finally in the delineation of a 20- to 40-km-wide coal-belt stretching from Wankie in the W to Sengwa in the E. The new model also opens up new perspectives for more coal within and between the coalfields. The study of quality and petrography of the shoreline coal supports the above depositional environment and reveals a standard maceral profile characterized by a basal vitrinite-rich coal passing upwards into inertinite-rich coal forming the major upper part of the sequence (typical Gondwana coal). The profile reflects an initial swamp phase generating a wet-forest swamp with Glossopteris trees, but this turned soon to a dry-forest swamp, with oxidation and decomposition of the vegetation, before it was finally overlain by fluviodeltaic sandstones of k4. The paludification is referred to an eustatic rise of the water-table caused by post-ice-age meltwater, but soon the water level dropped, due to the warmer climate. The local and regional controls of the peatswamp formation were considered, as well as the autochthonous and diachronous nature of the coal. The two coal types led to a new palaeogeographic setting for the Mid-Zambezi basin which is in agreement with the new rift concept. It was more of a trough having a SW–NE trend axis which was in the centre filled by a shallow freshwater lake. The above coal-belt was formed out of a peatswamp zone along its palaeo-shoreline. South of this stretched a ca. 100-km-wide shallow alluvial plain drained towards the NW by some meandering rivers, with adjacent flood plains temporarily occupied by local swamps. The alluvial plain was bounded on the SE by crystalline highlands representing the source of clastic sediments for the basin.

Petrographic and geophysical assessment of coal quality as related to briquetting: the Miocene lignite of the Lower Rhine Basin, Germany

In the Lower Rhine Basin, Germany, Tertiary lignites are mined primarily for electrical power generation and briquetting purposes. Until 2000, when the mine was closed, coals from the Bergheim open pit mine were used for briquetting. After 2000, briquettes were produced from the Hambach open pit mine but did not show the same quality and hardness as those from the Bergheim pit. In this context, the macropetrographic and microlithotype composition of the Hambach lignites was studied in detail. Samples were taken from the three profiles, S1, S2 and S3, and a well, R1. All of the samples were macropetrographically described, analyzed in their microlithotype composition, and coals from profile S3 were analyzed for their briquetting compression strength. Three horizons of high gelite content were detected in the Main Seam, two of them with negligible thickness and one with a thickness of up to 1 m. Briquettes produced from these horizons are characterized by significantly reduced hardness. By careful interpretation of subtle changes in geophysical log data, it was possible to map the occurrence of these gelite horizons. As a consequence of these studies, an excavation plan was set up by the RWE Rheinbraun to avoiding mining of critical strata for briquetting purposes.

Chromite Geochemistry and PGE Fractionation in the Campo Formoso Complex and Ipueira-Medrado Sill, Bahia State, Brazil

Two districts of stratiform deposits are situated near the northern margin of the Sao Francisco craton. The ultramafic host rocks are remnants of early Precambrian layered complexes that intruded Archean gneissic blocks prior to Transamazonian collisional events. The intrusions contain abundant chromite, exhibit orthopyroxene-rich sequences, and are similar geochemically, with highly magnesian bulk compositions (Mg/Mg + Fe = 0.91–0.67) and fractionation trends consistent with komatiitic parent magmas. The intrusion form, alteration history, and current metamorphic grade of each are distinct. At the Ipueira-Medrado mines, representative of the Jacurici Valley district, a folded segment of a 200- to 250-m-thick ultramafic-mafic sill contains a 5- to 8-m-thick highly massive chromitite layer. Green spinel in the orthopyroxene-rich peridotite and ubiquitous granoblastic textures indicate recrystallization during upper amphibolite or granulite facies metamorphism, now with incomplete retrograde alteration and serpentinization. In Campo Formoso a deeply eroded stratiform complex contains at least seven different economic seams, comprising varying proportions of disseminated, net-textured, layered, and massive ores. Here greenschist alteration of the ultramafic host rocks is complete, apart from orthopyroxene relicts in an upper unit at the Cascabulhos mine. Microprobe studies of chromite-rich specimens from representative stratigraphic sections of each intrusion suggest that unusually low TiO2 levels (<0.9 wt %) combined with high Cr2O3 contents (45–55 wt %) reflect the primitive parental magma composition in both intrusions. Core compositions of from Cascabulhos retain high XCr (0.57–0.72) but show a wide range of XMg (0.21–0.47), consistent with Fe-Mg exchange at grades at or below amphibolite facies. Some relict igneous fractionation is preserved, since TiO2 increases (0.2–0.4 wt %) while Cr/Al or XCr decreases (0.70–0.55) from the massive chromitites hosted by serpentinite, through the disseminated seams above, to the two high-level seams in pyroxenite. Chromite from Ipueira-Medrado is more magnesian (XMg = 0.57–0.64) but also more aluminous (XCr = 0.60–0.64) when massive types are compared. Here the submassive minor seams stratigraphically above and below the main seam both have relatively lower XMg and XCr (0.36–0.45 and 0.45–0.57). This suggests that combined Cr-Al and Mg-Fe exchange occurred between and silicates during postemplacement progressive metamorphism of the seams. Close-spaced sampling of representative drill core sections at <2- or <4-m intervals has allowed the precious metal potential of each intrusion to be assessed. In both cases the highest total platinum group element (PGE) concentrations occur exclusively in chromitite, illustrating the well-known phenomenon of chromite control. The Ipueira-Medrado chromitite contains lower concentrations (<350 ppb) and has unfractionated PGE patterns. Chalcophile element analyses provide little evidence of precious metal scavenging by immiscible sulfide segregation and so indicate little potential for mineralization elsewhere in the sill. At Campo Formoso the alternation of PGE-enriched chromitite and barren silicates is suggestive of repeated influxes of primitive magma during open-system fractionation. An increase in the Pt, Pd, and Rh tenor of the chromitite (to ca. 1 ppm) occurs from the lower massive group, through the upper disseminated group, to the high-level seams hosted by pyroxenite. Thus upward evolution of the host mineral compositions is matched by increasing (Pt + Pd + Ru)/(Os + Ir + Ru) ratio. The covariation of PGE ratios with mineral chemistry is analogous to that occurring over the sequence of chromitite seams in the Lower Critical zone of the Bushveld Complex. This provides evidence for a fundamental link in the fractionation processes controlling and PGE mineralization in stratiform intrusions, which has clear implications for exploration models.

Organic facies and depositional palaeoenvironment of lignites from Rio Maior Basin (Portugal)

The Rio Maior Basin (Portugal) is a tectonic depression, filled by a Pliocene sequence that comprises, from floor to roof: (i) kaoliniferous fine sands, (ii) diatomites and lignites, (iii) recent deposits of sandstone and clay. The diatomites and lignites form a small dissimetric syncline with alternating seams. Ten lignite seams were identified and named from floor to roof as F, E, D, C.2, C.1, C, B, A, a and a′. Seams A, D, E and F are considered to be the main seams. The organic fraction consists mainly of macerals of the huminite group, with small percentages of inertinite and liptinite groups. However, the petrographic composition of each seam is distinct, particularly with regards to macerals of the huminite and liptinite groups. Calculation of petrographic indices permitted to plot the coals in facies and palaeoenvironment diagrams. Five facies have been defined: (i) aquatic, (ii) herbaceous swamp, (iii) mixed swamp with forest and herbaceous vegetation, (vi) forest swamp (wetter) and (v) forest swamp (drier). These lignites are humic coals formed from organic matter of terrestrial origin. The peat biomass at the origin of these coals formed from a very diverse vegetation comprising gymnosperms and angiosperms. In seams F, and occasionally in seams E and D, Botryococcus algae have also contributed to the biomass. Peat deposition corresponded to a rheotrophic hydrological regime: the water level always remained above the topographic surface of the basin. Nevertheless, during the deposition of seam A in the northern part of the basin, the water level was slightly below the topographic surface. The organic matter was preserved in anaeorobic conditions.