Superposition of gold mineralisation on pre‐existing carbonate alteration: Structural evidence from the Mulgarrie gold deposit,Yilgarn Craton

Carbonate alteration at the Mulgarrie gold mine in the Eastern Goldfields of Western Australia, is represented by porphyroblasts, veins and pervasive, texturally destructive, carbonatisation. Two foliations, S_1M and S_2M, were produced by two separate deformation events at the mine‐scale, D_1M and D_2M. D_1M and D_2M both occurred in response to regional D₂ tectonism. Carbonate alteration was the product of two separate episodes of fluid ingress: the earlier produced magnesite and the latter Fe‐dolomite. Both periods of carbonate alteration occurred pre‐ to early syn‐D_2M, when mafic to ultramafic komatiitic rocks reacted with fluids that moved along regional faults and pre‐date the alteration associated with regional peak metamorphism. Gold at Mulgarrie overprints pre‐ and late syn‐D₂ quartz veins in zones of massive carbonate alteration, suggesting it has a late‐ to post‐D₂ timing. This late timing agrees with the generally accepted syn‐D₃ (and younger) age for gold mineralisation in the Eastern Goldfields. We suggest that carbonate alteration at Mulgarrie is not a product of the hydrothermal event responsible for the gold mineralisation. Rather, the different relative timing of magnesite, Fe‐dolomite and gold indicates there were two carbonate‐producing fluid systems and a fluid transporting the gold overprinted these. Similarly, early carbonate alteration may play a role in localising auriferous vein deposits throughout the Yilgarn and other Archaean cratons.     

Upper Oligocene-Lower Miocene shallowing-upwardlacustrine sequences controlled by periodic and non-periodic processes(Ebro Basin, northeastern Spain)

The lacustrine facies from two sections (Candasnos and Fraga) ofthe Oligocene-Miocene Torrente de Cinca lithostratigraphic Unit in thecentral part of the Ebro Basin (Spain) have been analysed to determine theinfluence of orbital parameters in lacustrine sedimentation. The unit ispredominantly composed of limestones and marls, and represents a shallowlacustrine freshwater system. The sedimentological features of the faciesstudied demonstrate that the lower part of the Candasnos section representsoffshore lacustrine subenvironments whereas the upper part, and the whole ofthe Fraga section, characterise marginal lacustrine areas. Series of stratalthickness variations of limestone, marl, and limestone/marl couplets fromboth sections have been analysed using spectral analysis. This shows thatinformation corresponding to periodic cycles only appears in the offshorefacies, that is to say, in the lower part of Candasnos section, and disappearsin the marginal facies where non-periodic cycles exist. Furthermore, thespectral analysis of the offshore facies highlights the existence of a peak inthe power spectrum with a period of around 7 (6.8 to 7.8) that can berecognised in the field as shallowing-upward lacustrine sequences.Magnetostratigraphic data from the Candasnos section allow us to establish atime span of 2,808 years for the limestone/marl couplet from the lower partof this section, and between 19,000 and 22,000 years for the periodic cycleidentified, thus representing the climatic precession cycle. Shallowingsequences from marginal areas do not correspond with any periodiccycle.     

环境同位素记录中共生碳酸盐氧同位素分馏差值的影响

湖泊化学沉积碳酸盐δ18O是研究区域气候演变的重要环境指标之一。青海湖等闭流湖泊的研究结果证实,在对δ18O环境记录进行共生碳酸盐氧同位素分馏效应校正时,应依据具体情况,采用0-1‰的分馏差值。由高温实验结果推断出的常温分馏差值(4‰-7‰)不能被应用到自然条件下湖泊共生碳酸盐氧同位素分馏效应的校正。     

Cyclic palaeokarst surfaces in Aptian peritidal carbonate successions (Taurides, southwest Turkey): internal structure and response to mid-Aptian sea-level fall

The sedimentology and cyclic stratigraphy of palaeokarst structures in Aptian peritidal carbonate successions are interpreted using field and laboratory microfacies analyses of closely spaced samples from measured outcrop stratigraphic sections in southwest Turkey. Cycles displaying shallowing-upward metre-scale cyclicity are generally composed of lime mudstones/wackestones/packestones at the bottom and stromatolites or lime mudstones with charophytes and ostracods at the top. Subaerial exposure structures such as in situ karst breccias, dissolution vugs/pipes, mud cracks and sheet cracks are encountered at the top of the cycles. The presence of limestone layers between the successive karst breccia levels indicates that they are in situ palaeokarst structures, not recent karstifications or deep penetration from the upper palaeokarst surface down to the older strata. Palaeokarst breccia deposits are interpreted as mantling breccia formed as a result of epikarstification. Three main palaeokarst levels are recorded in nearly all sections. The sedimentology of the palaeokarst breccias, their position in cyclic peritidal carbonates and the biostratigraphic framework are used to trace the record of the global mid-Aptian sea-level fall in the southwest Taurides. The successive occurrences of three karst breccia levels close to the mid-Aptian sea-level fall correspond to falling periods of high-amplitude sea-level fluctuation within a late high-stand or early fall condition of a third-order sea level.     

Authigenic carbonates from the eastern Black Sea as an archive for shallow gas hydrate dynamics – Results from the combination of CT imaging with mineralogical and stable isotope analyses

Authigenic carbonates associated with cold seeps provide valuable archives of changes in the long-term seepage activity. To investigate the role of shallow-buried hydrates on the seepage strength and fluid composition we analysed methane-derived carbonate precipitates from a high-flux hydrocarbon seepage area (“Batumi seep area”) located on the south-eastern Black Sea slope in ca. 850 m. In a novel approach, we combined computerized X-ray tomography (CT) with mineralogical and isotope geochemical methods to get additional insights into the three-dimensional internal structure of the carbonate build-ups.     

Vast amount of accommodation space controlled evolution of a continuous Pliocene–Pleistocene mixed cool-water carbonate-siliciclastic prograding wedge in the Bay of Oran (Western Mediterranean)

The Bay of Oran is part of the northern Algerian continental margin, located in the Western Mediterranean Sea between Europe and northern Africa. A regional terrace in ca. 320 m water depth described in earlier studies and a second deeper located one (∼1200 m water depth) provide an unusually vast amount of accommodation space for an observed prograding wedge. Seismo-stratigraphic interpretation of high-resolution reflection seismic data show different phases of mixed cool-water carbonate-siliciclastic deposition: (Ia) Initial aggradation with low dipping foreset deposition during early-Pliocene relative sea-level highstand. (Ib) Deposition transitions to progradation when aggradation reaches the base level. (IIa) Once progradation reaches the shelf break, terrace deposition is reduced to coarse fraction foreset deposits until it ceases entirely. (IIb) Finer sediments are bypassed and start to aggrade on the lower slope terrace until deposits reach the shelf terrace depth. (III) Due to accommodation space prolongation progradation recommences. Phase IIa and phase III deposits are separated by a hiatus. A drop in mean sea-level during the mid-Pleistocene will have caused the base level to fall below the upper strata, hence causing some reworking and redeposition. However, sea-level variations are not considered to be a main controlling factor of the depositional sequences. The evolution of this continuous Pliocene–Pleistocene mixed cool-water carbonate-siliciclastic prograding wedge is instead attributed to the controlling factor of this unusually vast amount of accommodation space. In closest proximity to the sea-floor, sparse recent sedimentation in form of 5–10 m thick sediment lobes can be observed in subbottom profiler data only. From a tectonic point of view, a prolongation of the Yusuf Fault into the survey area though expected by other authors could not be supported with the available dataset.     

Numerical modeling study of mineralization induced by methane cold seep at the sea bottom

In order to investigate the response of authigenic minerals to gas hydrate geo-systems, the biogeochemical processes and its induced mineralization were predicted by employing the comprehensive reactive transport modeling approach. Based on the available data extracted from the northern continental slope area of the South China Sea, a 1-D vertical column model was developed. Three cases with different upward methane flux rates and three cases with different mineral compositions, i.e., a total of six cases were designed to investigate the effects of variations in the depth of sulfate methane transition zone (SMTZ) and in the mineral composition on the formation of authigenic minerals. The simulation results indicate that the SMTZ depth influenced by both the upward methane flux rate and the initial composition played an important role in the formation of authigenic minerals. The AOM reaction is intensive at the interface, and the precipitation amount of calcite is large, which is mainly controlled by AOM. When the methane leakage rate is 20 times higher than the base case, aragonite starts to precipitate. During the simulation, oligoclase, k-feldspar, smectite-Na, smectite-Ca, chlorite dissolved. Our study specific to this area as a starting point may provide a quantitative approach for investigating carbonate and pyrite formation in hydrate-bearing sediments accounting for methane oxidation and sulfate reduction. The method presented here and the model built in this study can be used for other sites with similar conditions. In addition, this study may serve as an indication for the potential natural gas hydrate reservoir in depth, and is also significant for marine carbon and sulfur cycle.     

Seismic and stratigraphic evidence for reef expansion and onset of aridity on the Northwest Shelf of Australia during the Pleistocene

Modern reef (the Great Barrier Reef and Ryukyu Reef) distribution in the Indo-Pacific region is strongly controlled by warm currents (East Australian and Kuroshio Currents) that radiate from the Indo-Pacific Warm Pool. The modern distribution of reefs (south of 15°S) on the Western Australian shelf is related to the presence of the warm Leeuwin Current. However, the age of the reefs south of 15°S, and hence their temporal relationship to the Leeuwin Current, has been largely unknown. Seismic and subsurface stratigraphic data show that reef growth and expansion on the Northwest Shelf of Australia began in the Middle Pleistocene (∼0.5 Ma). The oldest ooids in the region are approximately synchronous with reef growth. We suggest a two stage process for the spread of reefs to higher latitudes on the Western Australian coast; first an increase in Leeuwin Current activity at approximately 1 Ma brought warm waters and a tropical biota to the region; and second, increased aridity after ∼0.6 Ma led to a decline in clastic input and increased alkalinity, triggering ooid formation and reef expansion to higher latitudes associated with the switch to higher amplitude glacio-eustatic cycles at the end of the Middle Pleistocene Transition. The timing and mechanisms for reef expansion south along the Western Australian coast has implications for the origin of the Eastern Australian Middle Pleistocene Great Barrier Reef, the New Caledonia Barrier Reef and Japanese Ryukyu Reef systems.     

Fluid sources and stable isotope signatures in authigenic carbonates from the Northern Apennines,Italy

Authigenic carbonates are frequently associated with methane cold-seep systems, which extensively occur in various geologic settings worldwide. Of interest is the relation between the fluids involved in their formation and the isotopic signals recorded in the carbonate cements. Along the Northern Apennines foothills (Italy), hydrocarbons and connate waters still seeping nowadays are believed to be the primary sources for the formation of fossil authigenic carbonate found in Plio-Pleistocene marine sediments. Four selected outcrops of dolomitic authigenic carbonates were analysed to compare signature of seeping fluids with fractionation of stable carbon and oxygen isotopes recorded in the carbonate.Along the foothills, deep methane-rich fluids spontaneously rise to the surface through mud volcanoes or are exploited in wells drilled nearby to the fossil Plio-Pleistocene authigenic carbonates. The plumbing system providing fluids to present-day cold seeps was structurally achieved in Late Miocene and Plio-Pleistocene. δ¹³C values of methane, which vary from −51.9 to −43.0‰ VPDB, indicate that gas composition from the deep hydrocarbon reservoirs is relatively uniform along the foothills. On the contrary, δ¹³C in fossil authigenic carbonates strongly varies among different areas and also within the same outcrop.The different carbon sources that fed the investigated carbonates were identified and include: thermogenic methane from the deep Miocene reservoirs, ¹³C-enriched CO₂ derived from secondary methanogenesis and microbial methane from Pliocene successions buried in the Po Plain. The δ¹³C variability documented among samples from a single outcrop testifies that the authigenic carbonates might represent a record of varying biogeochemical processes in the hydrocarbon reservoirs. The sources of stable oxygen isotopes in authigenic carbonates are often ascribed to marine water. Oxygen isotopic fractionation in the dolomite cements indicates that marine pore water couldn't be the sole source of oxygen. δ¹⁸O values provide a preliminary evidence that connate waters had a role in the carbonates precipitation. The concomitant occurrence of active cold seepages and fossil record of former plumbing systems suggests that generation and migration of hydrocarbons are long-lasting and very effective processes along the Northern Apennines foothills.     

Seep mounds on the Southern Vøring Plateau (offshore Norway)

Multidisciplinary study of seep-related structures on Southern Vøring Plateau has been performed during several UNESCO/IOC TTR cruises on R/V Professor Logachev. High-resolution sidescan sonar and subbottom profiler data suggest that most of the studied fluid discharge structures have a positive relief at their central part surrounded by depression. Our data shows that the present day fluid activity is concentrated on the top of these “seep mounds”. Number of high hydrocarbon (HC) gas saturated sediment cores and 5 cores with gas hydrate presence have been recovered from these structures. δ¹³C of methane (between −68 and −94.6‰ VPDB) and dry composition of the gas points to its biogenic origin. The sulfate depletion generally occurs within the upper 30–200 cm bsf and usually coincides with an increase of methane concentration. Pore water δ¹⁸O ranges from 0.29 to 1.14‰ showing an overall gradual increase from bottom water values (δ¹⁸O ∼ 0.35‰). Although no obvious evidence of fluid seepage was observed during the TV surveys, coring data revealed a broad distribution of living Pogonophora and bacterial colonies on sea bottom inside seep structures. These evidences point to ongoing fluid activity (continuous seepage of methane) through these structures. From other side, considerable number and variety of chemosynthetic macro fauna with complete absence of living species suggest that present day level of fluid activity is significantly lower than it was in past. Dead and subfossil fauna recovered from various seep sites consist of solemyid (Acharax sp.), thyasirid and vesicomyid (cf. Calyptogena sp.) bivalves belonging to chemosymbiotic families. Significant variations in δ¹³C (−31.6‰ to −59.2‰) and δ¹⁸O (0.42‰ and 6.4‰) of methane-derived carbonates collected from these structures most probably related to changes in gas composition and bottom water temperature between periods of their precipitation. This led us to ideas that: (1) seep activity on the Southern Vøring Plateau was started with large input of the deep thermogenic gas and gradually decries in time with increasing of biogenic constituent; (2) authigenic carbonate precipitation started at the near normal deep sea environments with bottom water temperature around +5 °C and continues with gradual cooling up to negative temperatures recording at present time.