Bitumens in Ore Deposits

It is widely documented that concentrations of metal may be associated with diverse organic materials, from living plants and animals through organic-rich sediments to crude oil, solid bitumen/pyrobitumen, and graphite. The signifi­cance of organic matter in mineralizing processes has been the subject of several special publications, including the proceedings of symposia on Oil and Ore (Garrard 1977), Organics in Ore Deposits (Dean 1986), the Role of Organisms and Organic Matter in Ore Deposition (MacQueen 1985), and Organic Matter in Hydrothermal Systems (Simoneit 1990). Recent research has made notable advances in the use of organic geochemical/pyrolysis data to assess the thermal maturity of ore deposits (e.g., MacQueen and Powell 1983), the transport of metals in fluids which contain organic compounds (e.g., Manning 1986), the role of microbiota in fixing metals (e.g., Morton and Changkakoti 1987) and the nature of sulphate reduction in sulphide ores associated with hydrocarbons (e.g., Leventhal 1990). The roles of fluid hydro­carbons in ore metal transport and solid hydrocarbons (bitumens) in ore metal deposition have been reviewed by Manning (1986) and Parnell (1988) respectively.

Migrabitumens often occur in paragenesis with ore minerals. Apart from some rare bitumens, the following are of importance: ozocerite, asphalt, asphaltites (gilsonite, glance pitch, and grahamite), wurtzilite, albertite, and impsonites (epi-, meso- and cata-impsonite). In a genetic sense there are two main groups with ozocerite alone in one group and all the other migrabitumens in the other. Ozocerite is formed mainly from terrestrial source material. The second, main group of migrabitumens developed from a predominantly marine source. The migrabitumens of the second group developed along two pathways. One path-way involves development by physical fractionation with the asphaltites as solid products. The other pathway involves development by chemical breakdown with wurtzilite and albertite as solid bitumen products. Metamorphism of asphaltites and wurtzilite-albertite results in impsonites. By weathering at out-crops etc., all migrabitumens become more or less oxidized. Such oxidized bitumens can have characteristics similar to diverse unaffected migrabitumens.

The following methods are used for microscopic analysis: microscopy, reflectance photometry, fluorescence photometry, micro-flowpoint, and microsolubility.

The analytical approach used to study organic matter in ore deposits is dictated by the questions asked about the organic-ore association. The two key parameters affecting organic composition are the biological source and thermal maturity. Microscopic techniques provide a rapid method of evaluating biological sources, thermal maturity, and paragenesis. In higher temperature ore deposits, the organic matter may develop a thermotropic mesophase. Elemental analysis and spectroscopic techniques supply a generalised overview of chemical composition and structure. Gas chromatography-mass spectrometry provides quantitative analysis and molecular structure. The use of several techniques in parallel is recommended.

Shungite is a solid bitumen which represents highly condensed carbonaceous matter from a Precambrian regionally metamorphosed area in Shunga (Karelia, USSR). Its chemical structure differs considerably in comparison with other types of solid bitumen. Structural inhomogeneity is a characteristic feature of this carbonaceous matter. The structure of shungite is a result of some process which oxidized primary organic matter of probably algal origin, and of the conditions of regional metamorphism.

In Mitov, Czechoslovakia, solid bitumens, quartz, calcite, and chlorite form the inter-pillow fillings of Upper Proterozoic pillow lavas. The bitumens, which are similar to poorly ordered graphite, show a marked mosaic and areal anisotropy typical of thermal mesophase that develops during industrial coking processes. The isotopic composition of the bitumen carbon (X δ¹³C –30.1‰) is within the range typical of organic carbon in the adjacent sedi-mentary rocks (δ¹³C = −24.2 to –35.3‰). Mineral inclusions in the bitumen contain an anomalous amount of vanadium. The origin of bitumen may be sought in the processes of pyrolysis and the thermal effects of submarine basalt extrusion on the organic matter of Proterozoic sediments and stromatolites.

The conversion of organic matter to petroleum products by hydrothermal activity is an easy process, occurring in nature in many types of environments. Geologically immature organic matter of marine sediments is being altered by this process to petroleum in the Guaymas Basin (Gulf of California), Escanaba Trough and Middle Valley (NE Pacific), Bransfield Strait (Antarctica), and Atlantis II Deep (Red Sea). Contemporary organic detritus and/or viable microorganisms are also converted in part to petroleum-like products when they become entrained by turbulent mixing into the discharging vent waters, resulting in instantaneous hydrous pyrolysis. This latter case is ubiquitous to all hydrothermal systems, but can be studied in hydrothermal vent fields without a sedimentary cover emanating directly from oceanic ridges, as for example on the East Pacific Rise at 13°N and 21°N and on the Mid-Atlantic Ridge at 26°N. The hydrocarbon products (methane to asphalt) generated in all these areas have been compared in terms of composition, organic matter sources, and analogy to reservoir petroleum. In addition, this type of organic matter rep¬resents a major input of carbon to the primary chemosynthetic bioproductivity of hydrothermal vent systems and may be important to interactions with the metals of hydrothermal mineral ores.

Massive sulphides and carbonate mineral deposits as well as hydrocarbons associated with sublacustrine hydrothermal activity were discovered at two sites, Pemba and Cape Banza, along the western side of the north Tanganyika trough (Zaire). This hydrothermal activity was investigated by scuba diving at a maximum depth of 20 m. Results presented here are from mineralogical and geochemical analyses of sulphide and carbonate deposits, hydrothermal fluids, and associated hydrocarbons. At Cape Kalamba, south of Cape Banza, a sublacustrine oil seepage has also been observed, sampled, and analyzed. This oil has a carbon-14 age of 25 000 a B.P. and is a biodegraded residue with biomarkers similar to typical crude oils. There are no hydrothermal poly-nuclear aromatic hydrocarbons detectable.

Present analytical results are interpreted in terms of fluid temperature at depth, nature of organic matter forming the hydrocarbons, and origin of heat.

Hydrothermal solutions carrying gold and organic matter formed the veinlets which comprise the Cherry Hill hot-spring deposit. Solid bitumen and/or primary fluid inclusions containing oil occur in Stages I–II and VII–XI of 12 paragenetic stages. Stages during which gold was deposited in some cases trapped bitumen and/or oil (IX) and in other cases did not (V and VI). Homogenization temperatures (T_h) of primary oil inclusions in general de-crease from Stage VIII to Stage XI, following the same trend as aqueous inclusions. As Th drops, the transmitted light color of the oils changes from orange/brown to light yellow to colorless and the UV-excited fluorescence emission changes from yellow/orange to blue, whereas the blue light-excited emission changes from weak orange to intense yellow/green. Oils were analyzed by crushing 1–7 mg samples in the inlet of a capillary column gas chromatograph. The column was programmed from –20 to 300°C and a flame ionization detector was used. All samples are dominated by an “unresolved hump” in the carbon number (C_n) range from about C₁₅ to C₂₈. Most samples have no n-alkanes and only traces of hydrocarbons in the range C₁–C₁₄. Four samples of Stage IX bitumen (trapped during a gold-precipitating stage) and one sample of Stage I bitumen (trapped during a nongold-bearing stage) were hand-picked, dissolved in a polar solvent mixture, and both soluble and insoluble fractions were analyzed for trace metals (including Au, As, Sb, S, Fe, and V) by neutron activation. Gold contents of the soluble fraction ranged from 4 to 120 ppb but were always below the solvent blank (280 ppb). The insoluble fractions contain up to 47 ppm gold and consist of insoluble organic matter, needles of valentinite (Sb₂O₃) and iron oxide.

Graphite, as a pure form of carbon, is found in Sri Lanka as disseminated flakes and in veins. In veins found in high grade metamorphic rocks, graphite often attains 99% purity. A variety of metals are found to accumulate in the graphite, mainly at the margin of these veins. Gold, in particular, becomes concentrated at vein margins, suggesting a hydrothermal transport mechanism and accumulation by carbon. It has been postulated that graphite becomes activated at the vein margins under high temperature conditions and it is this activation that provides sites for the accumulation of metals. Graphite has been reported from hydrothermal vents in the sea floor and these are associated with sulfides, particularly those of antimony. Even though the origin of graphite is still being debated, a deep-seated carbon source and metal-rich solutions appear to play a major role in the accumulation of metals in vein graphite.

The Kupferschiefer contains from 0.3 to 30wt.% organic matter. Only 1–3% of the organic matter is soluble; the remaining 97–99% occurs as a tightly intermixed, immobile, clay-organic matrix. A major component of the organic matter is vitrinite, with lesser alginite.

The clay-organic matrix of the shale contains small accumulations of Au, Pt, and Pd, probably as organometallic compounds.

Thucholite is present locally, and contains inclusions of uraninite, brannerite, and noble metal minerals located at the boundary between optically isotropic and anisotropic components. The minerals were exsolved during graphitization of the isotropic component of the thucholite. The optically homogeneous organic matrix of the thucholite contains discrete inclusions of UO₂, brannerite and organic compounds of U, Au, Pt, and Ni varying in size from 0.008 to 0.02μm, as shown by electron microscopy. The anisotropic component of the thucholite is composed of three types of ordered domains distinguished by three different values of graphite ring thickness (c₀, nm): 0.672, 0.740, and higher.

During replacement of thucholite by calcite at the boundary of the two minerals, native Au, Ag, Bi, PbBi, AuPb₂, Pb, Pd-arsenides, Bi-sulfides, PbSe, and TiO₂ developed.

Abundant disseminated organic matter and bitumen have been found in some Hg, Sb, Au, Pb, and Zn stratabound ore deposits in Cambrian, Devonian, and Permian formations in South China. According to their genesis, and time of formation, the bitumen can be divided into the following types:

The textural-structural characteristics of organic matter and bitumen in stratabound ore deposits help in understanding their genesis and are useful in exploration.

A review of the literature from around the world indicates that the association of bitumen and cinnabar in mercury deposits is widespread. Such deposits are commonly associated with sedimentary rocks; the compositions of igneous rocks in the vicinity of these deposits are highly variable, suggesting that anomolous heat rather than igneous rock type may be important to their formation. These deposits are commonly spatially coincident with anticlines or domes, which suggests that the focusing of buoyant fluids may be critical to their formation. The morphology of mercury ore bodies throughout the world is controlled by permeability, whether primary or secondary. On the deposit scale, mercury ore is spatially correlated with solid or liquid bitumens, or gas.

Within the USA, cinnabar-bitumen ore deposits are most prevalent in the California Coast Ranges. The variability of physical occurrence and chemical composition of bitumen is illustrated by samples of ore and gangue from nearly a dozen currently defunct mercury deposits. Common modes of occurrence for bitumen include: (a) clots and masses along the centers of silica and/or carbonate veins, (b) thin films along vein margins and/or crystal growth fronts, (c) masses within vugs of breccia veins, particularly along the hanging walls, (d) discrete microscopic fluid inclusions, (e) droplets within so-called “froth veins” of silica and/or carbonate, and (f) fine disseminations within siliceous sinter. At the hand-sample and microscopic scales, bitumen and cinnabar are spatially correlated. Bitumens are generally low in saturates and contain significant amounts of aromatics, NSOs, and asphaltenes. Some of the most abundant compounds have been identified as methylphenanthrenes, dimethylphenanthrenes, chrysene, methylchrysenes, benzofluoranthene(s), and benzopyrene(s). The range in carbon number of bitumens is typically small, possibly reflecting a process of natural fractionation arising from involvement in hydrothermal systems.

The chemical compositions of these bitumen samples strongly reflect the influence of thermal alteration. Results of recent shale retort experiments have shown that trace amounts of mercury in sedimentary rocks can be liberated to a gas phase in response to heating (Olsen et al. 1985). It is likely that the combination of high geothermal gradients, high fluid fluxes, and focusing of fluids all may have contributed to the generation, migration, and concentration of bitumen along with mercury in these deposits.

The association of uranium mineralization with solid bitumens is widely recorded. Uranium ore-related bitumens are generally the altered residues of crude oils. Even if petroleum cannot be considered either as a source or as a carrier fluid for uranium, spatial relationships between source-rocks or oil fields with uranium deposits are evident. Uraniferous bitumens occur in various reservoir facies (sandstones, faults, breccias) and have been recognized in Precambrian, Cambrian, Permian, Triassic, and Tertiary sedimentary deposits. They are generally insoluble in the usual organic solvents and display quite high aromaticity factors. Such characteristics are frequently induced by radiolytic degradation, which is known to drastically transform petroleum by dehydrogenation and polymerization. Microspectroscopic and isotopic investigations on mineralized and unmineralized bitumens from Precambrian deposits are presented in this chapter.

Oil-to-source rock correlations have been established down to the molecular level and, when combined with geological and thermal information, provide complementary data on the age of uranium mineralization.

Biodegradation has also been quoted as a major phenomenon for the formation of solid bitumen in uranium deposits. Two different alteration pathways have been recorded and respectively correspond to an oxidation and a CH₂ loss.

The importance of bitumens in the mineralizing process is not precisely known. Nevertheless, most of the studies dealing with this subject emphasize the role of a reducing substrate played by bitumens when interacting with uranium-carrying aqueous solutions.

In this chapter, several examples of uranium-bitumen associations are discussed with special emphasis on the geochemical modifications undergone by bitumen during alteration processes.

Pore-filling organic material concentrated uranium to form sandstone-hosted primary uranium deposits in the Jurassic Morrison Formation of the Grants uranium region, New Mexico. Because this organic material is the main ore control, determination of the nature, source, and time of emplacement of the organic material is critical to understanding the primary uranium ore deposits. Petrographic observations and radiometric ages for primary ore in the host sandstones (Westwater Canyon and Jackpile Sandstone Members) require that the organic material was emplaced early in their diagenetic histories; in fact, organic material was emplaced before significant compaction. Reconstruction of the burial history of the basin indicates that Late Jurassic-Early Cretaceous time, the interval of inferred emplacement of the organic material, was probably not a time of hydrocarbon generation and migration. Nuclear magnetic resonance analyses, elemental abundances, and isotopic studies provide further constraints on the nature of the organic material, and data from these studies are consistent with a humic acid rather than an oil origin. In addition, the overall tabular shape of primary uranium orebodies and the way these orebodies are suspended in the host sandstone mimics recent deposits of humate, the precipitated form of humic acids.

The critical clue that favors a humic acid origin for ore-related organic material is provided by the nature and patterns of postdepositional alterations that are spatially and temporally related to primary ore, specifically destruction of detrital magnetite and ilmenite (iron-titanium oxide) grains. Humic acids are implicated in the destruction of iron-titanium oxide grains because these organic acids are one of the most effective natural agents in leaching of iron from these grains. Thus, timing constraints, chemical evidence, orebody geometry, and the nature and patterns of ore-related alterations favor a humic acid origin and suggest that an oil origin is highly unlikely.

Because alteration of iron-titanium oxide grains tracks the pathways of humic-bearing solutions through the host sandstones, it is possible to trace these solutions back to their source. Alteration patterns in the Westwater Canyon and Jackpile Sandstone Members suggest that the humic-bearing solutions originated as pore fluids in the smectite diagenetic mineral zone of a large alkaline-saline lake complex in the intervening Brushy Basin Member and moved into adjacent sandstones during early dewatering of the smectitic mudstones. Within the sandstones, the humic acids precipitated by the process of cation loading, in tabular layers. Uranium was concentrated within these tabular layers by complexation with the humic acids to form primary uranium orebodies.

A second episode of uranium mineralization was associated with incursion of oxidizing groundwater during the Laramide orogeny. At this time, uranium was remobilized from some primary ores and reprecipitated at a regional reduction-oxidation interface, commonly along faults and fractures. These redistributed ores yield dates that are consistently less than 10 Ma.

The two episodes of ore genesis can be related to the hydrologic history of the San Juan Basin since deposition of the Morrison Formation. Formation of primary ore can be related to the movement of compaction-driven humic-acidbearing pore waters from the Brushy Basin Member into the host sandstones early in the burial history of the Morrison Formation. Redistribution of this primary ore occurred much later in the hydrologic history of the basin, when Tertiary uplift of the basin margins resulted in recharge of the host sandstones. At this time, oxidizing groundwaters removed uranium from some primary deposits and precipitated it along reduction-oxidation boundaries not too distant from the primary deposits.

The uranium ore deposits of the Francevillian Series (Gabon) represent the oldest (2.0 Ba) high grade uranium ore deposit located in a sedimentary environment. In these deposits, uranium is associated with migrated organic matter which occurs as infillings of secondary porosity in deltaic sandstones. Sedimentologic, tectonic, petrographic, and geochemical studies have permitted a reconstruction of the geologic conditions in which uranium mineralization took place. The combined results indicate that these uranium ore deposits are located in sandstone reservoirs associated with hydrocarbon traps and are capped by impermeable black shales which are also good oil source rocks.

Organic matter-mineral associations in the Witwatersrand, Elliot Lake, and Oklo ore deposits help to elucidate the various roles of kerogens and bitumens in uranium metallogeny. Sedimentological observations imply that at least in the Witwatersrand, ancient mats of cyanobacteria helped initially to trap and concentrate detrital uranium minerals from low-energy water transport close to 3 Ga ago. These microbial mats were converted to kerogen during diagenesis while retaining their acquired minerals. In the 2.15–2.37 Ga old Elliot Lake ores, long after deposition apparent hydrothermal events helped bitumen to evolve in, and then move away from, thin layers of syngenetic kerogen. However, bitumen mobilization did not allow uranium migration. At the 1.82–2.1-Ga-old uranium deposits in the vicinity of Oklo, kerogen abetted the post-depositional concentration of uranium in high grade ores and the natural fission reactors. Nuclear criticality was maintained for more than a hundred thousand years when the contained water, acting as moderator, was expelled. The Oklo natural reactors are time-tested analogs of radioactive waste containment models. Consideration of the elemental, molecular compositions, maturity, graphite cryptocrystallinity and the mineral associations of Witwatersrand, Elliot Lake, and Oklo kerogens and bitumens together, helps to clarify a number of common and distinct effects of organic matter on uranium mineralization in these three Precambrian ore deposits.

Globular blebs of solid pyrobitumen occur in veins that cut uraniferous conglomerates of the Lower Proterozoic Matinenda Formation in the Panel Mine in the Elliot Lake District, Ontario. The blebs are small (1–10mm) and vary in shape from round to discoid, kidney to saddle, twisted or elongate. Their surfaces are shiny and permeated with vesicles. The blebs are composed predominantly of carbon with a H/C ratio of 0.57, a reflectivity (%R_m) of 0.9%, and a δ ¹³C value of −33‰ (PDB). The paragenetic sequence of minerals and pyrobitumen in the veins is: quartz, pyrite 1, pyrobitumen, sepiolite, pyrite 2, pyrrhotite and galena, and finally calcite. The pyrobitumen blebs in the Panel mine are the result of natural migration and maturation of Precambrian petroleum. Tarry masses in the veins were polymerized to the catagenesis stage by outgasing, water-washing, and thermal cracking.

Thoriferous bitumen nodules in the Northwest Irish Basin are composed of an envelope of bitumen surrounding numerous inclusions of a range of thorium-calcium phospho-silicates. One nodule exhibits physical and chemical zonation, indicating episodic influx of mineralizing fluids with changing chemistry through time. Calculated chemical ages for the inclusions show a wide range of results indicating that Pb was present at the time of nodule formation and has undergone leaching subsequent to this.

Reduction spots in marine and continental red beds were investigated for the presence of organic matter. The samples are from a wide variety of localities and from host rocks ranging in age from Precambrian to Cretaceous. It was found that organic carbon is present in a minor number of samples only, where it is a major component. Organic carbon in reduction spot cores is always associated with uranium minerals. Rock-Eval pyrolysis shows that this organic matter is depleted in H and enriched in O relative to possible precursor materials. Pyrolysis-GC-MS indicates that this insoluble, high-molecular weight organic material primarily consists of aromatic structures, similar to other occurrences of organic matter associated with uranium mineralization. The stable isotopic composition of the organic matter is highly variable (−23.1 to −49.8% relative to PDB) but consistent with a derivation from oil, gas, or bacterial biomass. In reduction spots devoid of organic carbon, stable isotopes (C, O) of carbonates are not significantly different in cores and in host rocks and therefore do not provide evidence for the former presence of organic carbon. Several possibilities for the origin of organic carbon in reduction spots are discussed but no conclusive evidence can be presented yet. Mineralogically, reduction spots with organic matter are generally similar to those without. A significant difference is the abundance of roscoelite in organic-poor reduction spots, while this mineral is absent in organic-rich ones.

The restriction of a variety of minerals and alteration effects of Mississippi Valley-type ores to specific sites within huge volumes of potential host rock suggests that some peculiar characteristic of the sites caused them to become the focus of mineralisation. We propose that the ores were localised where a hot (80–220 °C) brine bearing thiosulphate (S₂O₃ ²⁻) and metals interacted with organic matter. By acting as a reductant, a source of organic acid, a source of CO₂, and a substrate for bacterial metabolism, organic matter played a critical role in each stage of the paragenesis of Mississippi Valley-type deposits.

The transport of sulphur in the form of thiosulphate inhibited the precipitation of extremely insoluble Pb and Zn sulphides and barium sulphate; thus metals and sulphur as thiosulphate could be carried in one mineralising solution. In contrast to sulphate, which is reduced so slowly at temperatures less than 230 °C that it would flush through the ore zone long before it could be reduced by organic matter, thiosulphate is readily reduced by organic matter to produce the −2 and −1 valent sulphur needed to precipitate sulphide (PbS, ZnS) and disulphide (FeS₂, NiS₂) minerals.

Reactions induced by the heating of organic matter at the sites of mineralisation may have produced the carbonate paragenesis typical of Mississippi Valley-type ores. Prior to the sulphide stage of mineralisation, carbonates first dissolved and then precipitated; during the sulphide stage, carbonates again dissolved. The initial dissolution may have been caused by organic acids that were produced by the heating of organic matter. With continued heating to about 120 °C, the organic acids began to break down to CO₂ and CH₄. The addition of CO₂ in the presence of organic acid pH buffers, provided by organic acids that had yet to degrade, precipitated carbonate minerals. At still higher temperatures (−140°C), organic acids quickly degraded, so the pH buffer was lost. Without a buffer, the continual addition of CO₂ lowered the pH, and the carbonates dissolved. The addition of CO₂ also may have triggered the precipitation of fluorite by forming MgHCO₃ ⁺ and F⁻ at the expense of MgF⁺.

Late-stage baryte and calcite appear to have formed at temperatures low enough for bacteria to survive. Bacteria can metabolise thiosulphate in the presence of organic matter. Products of this metabolism include SO₄ ²⁻; and CO₂; the former precipitated as baryte and the latter formed late-stage (postmain-sulphide-stage) calcite.

Bitumen occurs closely associated with sulphides in carbonate-hosted vein mineralization at Navarana Fjord, northeastern Freuchen Land, central North Greenland.

The bitumen in the vein is strongly degraded (carbonized) and occurs typically as small fragments associated with sphalerite (±galena and pyrite). The bitumen has a very low sulphur content and seems to have lost sulphur compared to any other bitumen from the source rocks in the region. The isotopic compositions of sulphur in bitumen and sulphides from the vein are similar with δ ³⁴S values of about +14‰.

The presence of bitumen in the vein was essential for the sulphide precipitation mechanism. It is suggested that in situ thermal cracking of sulphur-rich organic material added H₂S to a metal-bearing fluid, resulting in the precipitation of sulphides. Such a process would probably cause a rapid precipitation of sulphides, thus incorporating bitumen. Furthermore, such a process would not cause any isotopic fractionation of the H₂S produced although the bitumen itself would lose sulphur. The thermal cracking and carbonization of the organic material was caused by mixing of hydrocarbons with a hot hydrothermal (200°C) metal-bearing fluid, probably in the vein zone itself or just below in a sandstone succession.

The bitumen in the vein has not been correlated with any of the two known source rocks in the area but the S isotopic data suggest that the vein bitumen is similar to a long-distance migrated bitumen from an unknown source rock.

Stratabound ore deposits in China commonly occur in organic-rich formations (TOC > 1%, up to 19%), including black shales which are host rocks for stratabound U, Ni, V and Cu ore deposits, and source beds for Pb-Zn, Hg, Sb, Au and Sn ore deposits. Spatially, some stratabound ore deposits are adjacent to oilfields, or associated with oil traps.

The organic geochemical indicators show that: (1) the kerogen is mostly of type-I and type-III, 6¹³C generally −26.8 to −31.95‰; (2) the kerogens have reached the metagenesis and metamorphism stages (R_o = 2% and 4%); (3) the H/C atomic ratios for the kerogens are low (0-45-0.07) and the O/C ratios are very varied (0297–0.01); (4) the organic extracts are low (0.0n–0.00n%); (5) the maturity indicators of soluble organic matter including OEP, EOP, C32 homohopane C₋₂₂S/(R + S) and C29 sterane C₋₂₀S/(R + S) ratios and the thermal degradation yield of whole rock samples were controlled by (a) the integrated effects of temperature and time, (b) the type of host rock; and (c) the contents of amino acids are 90 to 850 ppm.

In gas inclusions from the Gongguan Hg-Sb deposits, the concentrations of CH₄, CO₂ and N2 are rather high with maximum values of 2290, 81700, and 16 300 ppm respectively. These gas compositions are similar to coal-generated gas. Fluid inclusion analyses yield Na/K atomic ratios of 15.64–66.82. This ratio range is quite similar to that of oilfield brines. The palaeogeothermal gradient, estimated by R₀ (%), was up to 43 °C/km.

Geochemical data for the organic matter (OM) in the Fore-Sudetic copper-bearing shales have been collected and evaluated. They were also compared with those for OM from the German Kupferschiefer. The organic matter (up to 20wt.%) has been one of the principal agents in the formation and transformations of the Kupferschiefer deposit. Bitumens represent about 3% of the whole OM. The organic matter is mainly of sapropelic origin (phytoplankton, algae, bacteria) and was deposited under highly reducing conditions. The specific composition of OM in the mineralized areas might be attributed to mineralization processes, e.g., microbial reduction of metal ions and sulphates. The OM of the “Rote Faule” areas has been secondarily oxidized by different processes, connected not only with ore mineralization. Some metals (V, Ni, noble metals) are still present in bitumens and probably also in kerogen in the form of organometallic compounds. The organic matter could take part in metal accumulation, reduction, precipitation and remobilization of sulphides, and also transformations of clay minerals. It is shown that some typical maturation parameters can be of dubious value in the case of organic matter from the ore deposits.

Xiangtan-type manganese carbonate deposits occur in early Sinian interglacial black shales, which are distributed widely and contain large manganese reserves in China. This type of manganese ore is black or greyish-black in colour and has massive, laminated, and banded structures. Black anthraxolite spheroids with white encircling haloes of quartz-Mn-calcite occur, mainly in the massive manganese ores. The geological setting, distribution, mineralogy, and chemical composition of the anthraxolite spheroids as well as the manganese carbonate ores are reported in detail. It is proposed that the anthraxolite spheroids were formed during the catagenetic stage, and underwent a rather low degree of thermal metamorphism (about 200°C). White quartz-Mn-calcite haloes and penetrating veinlets in the anthraxolite spheroids were formed later.

First-series transition element concentrations (Co, Cr, Cu, Fe, Mn, Ni, V, Zn) are examined in 27 solid bitumens from the United States, Canada, Mexico and Spain. Results are compared with organic geochemical parameters for whole bitumen and extractable organic matter, and the origin of metals in solid bitumens is discussed. In general, Co, Cr, and Cu are present in concentrations less than 100 ppm; Mn, Ni, and Zn from 10 to 1000 ppm; and Fe and V at levels greater than 1000 ppm.

¹³C NMR aromaticity values (f_a) in the sample set are controlled predominantly by maturity level (rather than source influence). Whereas the V/(V+Ni) ratio and the V concentration do not correlate with f_a in the sample set, Ni concentration increases with increasing f_a. These metal-f_a relationships are probably due to the occurrence of V and Ni in host complexes of different stabilities.

The chemical composition of sedimentary organic matter is controlled by original source influences and the effects of maturation, migration, and post-migration alteration. Data presented here provide support for source-control and maturity effects (particularly for Ni) on the composition of solid bitumens. The effect of metal gain and loss from migrating fluids (both water and oil) is not understood. Implications of future technological developments, including on-line liquid chromatography-inductively coupled plasma-mass spectrometry and liquid chromatography-mass spectrometry, are noted.

The Carboniferous rocks of Ireland contain widespread occurrences of solid bitumen. Samples of bitumen from hydrothermal mineral veins in limestones, accretionary bitumen in sandstones, and reservoir bitumens in dolomitized limestones were determined to be enriched in metals in the form of micron-scale inclusions of ore minerals. The inclusions are most abundant in bitumens associated with ore mineralization. The inclusions are more likely to contain base metal-bearing phases in the vicinity of sulphide ore deposits. These results suggest that metal anomalies in bitumens may have potential in the exploration for ore deposits.

Solid bitumen envelopes around radioactive clastic grains in sandstones from the Perth and Bonaparte Basins of Western Australia display textures that resemble those induced by the irradiation of oil in the laboratory. The envelopes surround monazite grains in the Perth Basin and Bonaparte Basin, and locally surround zircon and rare xenotime in the Bonaparte Basin.

Envelopes in the northern and central Perth Basin formed at different stages in the complex diagenetic histories of the sedimentary rocks, and examination of the envelopes has helped unravel the diagenetic sequences. In the Bonaparte Basin, some envelopes around monazite contain an inner, highly polymerised, non-fluorescent zone, and an outer, less polymerised, fluorescent zone. The inner zone is attributed to polymerisation of an early oil, and the outer zone to polymerisation of a later oil, an interpretation which accords with geochemical evidence indicating two episodes of oil migration. Where envelopes surround zircon, which is not as radioactive as monazite, they are not strongly polymerised, and fluoresce.

This investigation shows that the combined techniques of optical microscopy, blue-violet fluorescent microscopy and SEM examination are useful in establishing the history of oil in sedimentary rocks. The presence of solid bitumen envelopes may reveal more than one generation of oil, or may demonstrate the former presence of oil in dry rock. No petrological examination of sandstone is complete until the radioactive clastic grains have been examined for signs of solid bitumen envelopes.

Uraniferous bitumens from Permian rocks at Vrchlabi, Czechoslavakia, contain abundant inclusions of uraninite and also inclusions of gersdorffite. The uraninite consistently contains radiogenic lead which does not appear to have been mobilized and redistributed. Chemical age dating of large uraninite crystals yields Jurassic ages which are interpreted to represent the date of hydrocarbon migration into the rocks.