J.R. Ridley

Biomarkers, brines, and oil in the Mesoproterozoic, Roper Superbasin, Australia

Gas chromatography–mass spectrometry of oil inclusions from the ca. 1430 Ma marine Roper Group in the Roper Superbasin, Australia, provides a new source of information about the early biosphere and Proterozoic petroleum systems. Oil most likely derived from an overlying shale was trapped at ∼60 °C as abundant oil inclusions within transgranular microfractures in detrital quartz during Mesoproterozoic basin inversion. The oil is very mature and has a wide range of biomarkers, derived mainly from cyanobacteria, but lacks eukaryote biomarkers. Unlike associated solid bitumens, the inclusion oil is nonbiodegraded. Evidently, the inclusions remained closed systems, sheltered from postentrapment alteration and contamination. Because fluid inclusions have preserved biomarkers for >1000 m.y., they constrain the diversity of primordial ecosystems, whereas other forms of early Precambrian organic matter are usually absent or metamorphosed.

Interpretation of post-entrapment fluid-inclusion re-equilibration at the Three Mile Hill, Marvel Loch and Griffins Find high-temperature lode-gold deposits, Yilgarn Craton, Western Australia

Abstract Fluid inclusions that are interpreted to be related to vein filling of quartz veins at the Three Mile Hill and Marvel Loch gold deposits in amphibolite-facies rocks, and the Griffins Find deposits in granulite-facies rocks in the Yilgarn Craton of Western Australia are dominantly low-salinity H 2 O–CO 2 ±CH 4 mixtures or CO 2 –CH 4 fluids. At each deposit, inclusions have a wide range of compositions with respect to both the aqueous:carbonic ratio and the CO 2 :CH 4 ratio of the carbonic phase, and most fluids could not have been in equilibrium with the mineral assemblage in the vein and adjacent rock. Inclusion densities suggest a range of P–T conditions of entrapment, and are in general not consistent with the conditions of vein formation indicated by vein assemblages. Diffusional addition of H 2 into inclusions, diffusional loss of H 2 O, and reduction of inclusion volume are possible during cooling and uplift along the inferred P–T path. The inclusion populations could have been derived from an originally uniform population of low-salinity, aqueous dominated H 2 O–CO 2 inclusions by a combination of these processes. Inclusion modification as the cause of the complex inclusion populations is supported by relations of molar volume to composition, and, to an extent, by variations in fluid salinity. If inclusion re-equilibration is the cause of inclusion variability, it was of variable intensity within a single vein system, and within individual clusters of inclusions in some samples, and is suggested to have been a function of the local petrological and textural environment.

Oil-bearing CO2–CH4–H2O fluid inclusions: oil survival since the Palaeoproterozoic after high temperature entrapment

The 2.45-Ga fluvial quartz arenites and conglomerates of the Matinenda Formation at Elliot Lake, Canada, contain evidence for two episodes of oil migration with entrapment of oil-bearing fluid inclusions. The first episode was during diagenesis, the second during a subsequent sub-greenschist facies metamorphic event at ca. 2.2 Ga at temperatures exceeding 250 °C. Remnants of the migrating hydrocarbon fluids are preserved as radiogenic pyrobitumen nodules occupying former inter-granular pore spaces and as complex fluorescing oil-bearing fluid inclusions. The latter include two dominant types: (1) aqueous-oil inclusions with a minor oil phase, located within original detrital grains and clasts and within rare quartz overgrowths, and (2) aqueous–carbonic inclusions with a minor oil phase, light hydrocarbons and solids, located within microfractures in detrital quartz grains and clasts. The majority of Type 1 inclusions were trapped with a high salinity fluid (up to 25 wt.% NaCl equivalent) at relatively low temperatures between 80 and 200 °C and pressures of less than 2 kbar early in the burial history of the Matinenda Formation. Type 2 inclusions were trapped during metamorphism with a much lower salinity aqueous–carbonic fluid (1.2 to 17 wt.% NaCl equivalent) at temperatures between 280 and 350 °C and pressures between 1 and 1.5 kbar. Under these conditions oil may have been miscible in the mixed fluid. δ13C of −25.5‰ and −24.9‰ for the pyrobitumen nodules and fluid inclusion hydrocarbon gas indicate a biogenic origin for the petroleum. δ13C of −16.2‰ for the bulk inclusion gas suggests a value of −12‰ for the carbonic phase, hence a mix of inorganic CO2 and organic CO2 from either thermal alteration of organic matter or in situ chemical oxidation of hydrocarbons. This study has implications for the depth and temperature at which oil may be present, suggesting that deep, old reservoirs may not be completely barren of petroleum. The fact that hydrocarbons in the Matinenda Formation fluoresce indicates that aromatic compounds are able to survive temperatures of up to 350 °C in a suitable geological environment.

Listric normal faulting and the reconstruction of the synmetamorphic structural pile of the Cyclades

Summary There are late, post-metamorphic faults on the Aegean island of Syros that are demonstrably listric normal. Breccias associated with these faults are identical to those of late low-angle faults elsewhere on the island. It is therefore proposed that these low-angle faults are the toes of listric normal faults. This provides a basis for delineating a structural succession prior to faulting in which apparently ‘allochthonous’ rocks were originally at a higher level in the pile. This is consistent with the observed structural succession on Syros. The highest structural units are greenschists with similar kinematic axes to those of the blueschists below, but with a contrasting deformation style. These greenschists may have been derived from the ‘roof’ of the convergence zone in which the blueschist metamorphism took place. Similar fault patterns can be inferred for many of the neighbouring islands, which are horst blocks. Many show peripheral ‘upper units’ of weakly- or non-metamorphic rocks. The ‘upper units’ are inferred to have been originally at a high level in a pile built up during early Tertiary tectonism and are not remnants of a far-travelled nappe emplaced in the Miocene.

Vertical movement in orogenic belts and the timing of metamorphism relative to deformation

Summary Any phase of deformation in an orogenic belt that causes relative vertical movements of bodies of rock by a few kilometres or more should have a recognizable imprint in the metamorphic history. The effect seen in a P-T-t path will depend on whether the rock body examined moved relatively up or down in the orogenic pile. An upthrown block is predicted to cool soon after the onset of movement, but the resulting P-T path will be insignificantly different to that resulting from a cycle of thickening and erosion. A downthrown block will undergo renewed heating after movement, and evidence of the earlier metamorphic history will probably be overprinted. It is suggested that the recognition of phases of relative vertical movement in an orogenic belt is best made by combined studies of deformation-metamorphism textural relationships, and P-T-t paths. The Lukmanier Pass area of the Alps illustrates the textural patterns predicted: here, a phase of deformation immediately post-dates the metamorphic peak, suggesting uplift of the area relative to the Helvetic nappes to the north.

The significance of deformation associated with blueschist facies metamorphism on the Aegean island of Syros

Summary A thorough examination of the syn-metamorphic minor structures in the blueschists of the Aegean island of Syros allows a determination of the kinetics and nature of the deformation. The deformation is essentially related to ductile thrusting at up to 45 km depth. The change in fold geometry with orientation, and the presence of a locally developed mineral lineation at a high-angle to the fold axes give two independent determinations of the direction of thrusting. The direction, 130 ± 10°, suggests a link between the tectonism here and that in the Izmir-Ankara zone of mainland Turkey. The nature of the small-scale asymmetrical folding indicates shortening of the pile parallel to this thrust displacement. This, together with the suggestion from the metamorphic textures of increasing pressure and temperature during deformation suggest that the deformation is related to burial and is compressional, and hence is presumably related to a major collision event.

Oil migration in the Middle Proterozoic Roper Superbasin,Australia: evidence from oil inclusions and their geochemistries

Abstract The geochemistry and petrography of oil inclusions in the ca. 1.4 Ga dessie Creek Sandstone in the Roper Superbasin,Australia, provide insights into poorly understood Middle Proterozoic petroleum systems. Oil inclusions were trapped after the intrusion of ca. 1.3 Ga dolerite sills during basin reactivation and peak maturation. Biomarkers reflect the predominance of prokaryotes in the Mid-Proterozoic biosphere and the most likely source rock is the directly overlying Velkerri Formation.

Evidence for exsolution of Au-ore fluids from granites crystallized in the mid-crust, Archaean Louis Lake Batholith, Wyoming

Abstract The Louis Lake Batholith is contemporaneous with and sits adjacent to Archaean lode gold deposits and minor copper stockwork vein systems of the South Pass Greenstone Belt, Wyoming Province, and may thus have contributed fluid into these hydrothermal systems. The batholith is granodiorite to granite, lacks extreme fractionation, and contains evidence for early formation and later replacement of an immiscible early sulphide phase. Mafic enclaves contain anomalous concentrations of gold and copper, suggesting that metals were introduced from in-mixed mafic magma. Apatite SO3 and Cl values imply relatively early exsolution of sulphur- and chlorine-bearing fluids during fractionation. Exsolution of possibly immiscible high- and low-salinity fluids is implied from fluid inclusion petrography, even though crystallization pressures were greater than 3 kbar. Low salinity inclusions share affinities with those of orogenic gold systems, but contain notably higher Pb and Zn concentrations. High salinity inclusions have similar elemental compositions to brines of intrusion-centred ore systems. It is proposed that sulphide melt separated early during magma fractionation, scavenging Au and Cu, but became later destabilized, possibly because of increased fO2, releasing scavenged metals. Metals were transported from the pluton in sulphur- and chlorine-bearing hydrothermal fluids of low and high salinity, with Au being preferentially transported in the low-salinity fluid. Supplementary material: Whole Rock Geochemical data are available at http://www.geolsoc.org.uk/SUP18751.

Hydrocarbon Pseudo-Inclusions in Barite: How to Recognize and Avoid Artifacts

Petrographic recognition of oil inclusions under ultraviolet excitation is a relatively simple technique that plays an important role in investigations of hydrocarbon migration. We observed brightly fluorescing, mostly two-phase, oil-bearing fluid inclusions in and near microfractures in a sample of a 3.5-billion-year-old hydrothermal barite. The oil inclusions are strikingly similar to those found in conventional Phanerozoic reservoirs. However, careful petrographic observations combined with nondestructive analytical methods and modified sample preparation techniques using colored dyes revealed that these inclusions are artifacts. Most likely the inclusions contain oil-based polishing fluids introduced during sample preparation. Evidently, the fluids infiltrated opened or underpressured inclusion cavities via micropores and microcracks initiated during sample polishing and grinding. They mixed with residual fluids and air, and as the capillary pressure and surface tension reached equilibrium the inclusions came to resemble natural oil-bearing inclusions. We suggest that artifacts can be reliably recognized through careful petrographic observation and sample preparation using dyes. Oil-inclusion artifacts are particularly likely in soft, easily cleavable and oil-wetting minerals, including evaporites and carbonates, which are common components of petroleum reservoirs around the world. Also pseudo-inclusions could form in hard minerals and have aqueous compositions. In addition to pointing out a potential interpretive hazard, the study has implications for oil migration in cold, young reservoirs, because it illustrates that oil inclusions can form by refilling of cavities without the need for cementation.

Gold-Transporting Hydrothermal Fluids in the Earth’s Crust

Hydrothermal ore deposits that are exploited for gold include both gold-only deposits, such as orogenic deposits, and gold-bearing examples of the common hydrothermal deposits types that are formed around upper-crustal magmatic centres, in particular porphyry and epithermal deposits. Fluid-inclusion data have shown that ore fluids of gold-only deposits are compositionally distinct compared to fluids of other deposit types. This Special Publication includes an up-to-date summary of thermodynamic parameters of aqueous Au species at high temperatures and pressures; a dataset of fluid inclusion properties and compositions from orogenic deposits of different parts of the world; several comprehensive case studies of different types of gold deposits and their fluids from USA, Brazil, Egypt, Slovakia and Bulgaria; and numerical modelling aimed to define key parameters that affect fluid flow and gold deposition at a range of scales.