Fluid Flow Events Research Articles

Anomalously silver-rich vein-hosted mineralisation in disseminated-style gold deposits, Jiaodong gold district, China

Structural measurements and geochemical analyses, including bulk and in situ pyrite geochemistry, sulfur isotopes, and whole-rock geochemistry, are presented for the No. 3 orebody of the Jiaojia gold deposit (JJ3), located in the Jiaodong district of northeast China. The JJ3 orebody is distinct from the main orebody of the Jiaojia deposit (JJ1) because it is characterised by steeply dipping sub-metric quartz-pyrite veins with up to 300ppm of gold, whereas the JJ1 orebody represents an archetypal example of the disseminated and veinlet style mineralisation characteristic of regional faults in the Jiaodong district. Measurements on JJ3 veins and the host Jiaojia-Xincheng regional fault are consistent with development of mineralised, steeply dipping extension fractures during normal faulting, which produced the fault-hosted disseminated-style JJ1 orebody. Trace element geochemistry of pyrite in these veins shows that JJ3 pyrite is geochemically distinct from those of the main Jiaojia and Xincheng orebodies, being relatively enriched in Ag and Pb, as well as Ba, Bi, Te and Au, and relatively depleted in Cu and As. Enrichment in Ag and Pb is possibly related to infiltration of a saline hydrothermal fluid, as both are effectively transported as chloride complexes; however, depletion of Cu, which is also mobile as chloride complexes, requires a low temperature saline fluid where Cu is no longer soluble. The textural setting of the ore minerals suggests that these cooler fluids likely infiltrated during the waning stages of the hydrothermal system. The relative abundance of barite in the JJ3 orebody, which formed from late-stage oxidised magmatic–hydrothermal fluids, also supports the interpretation that the JJ3 orebody represents a late mineralisation event. The pervasive alteration surrounding the JJ3 orebody is K-feldspathic with a minor sericitic overprint, indicating an earlier higher temperature pervasive fluid flow event that was followed by low-temperature mineralising fluids. This interpretation implies that fracture dilation post-dated the earliest alteration, and that mineralisation and pervasive alteration in the JJ3 orebody are geochemically disconnected. Thus structural analysis is expected to be the most effective targeting method in future exploration for similar ore bodies.

Authigenic monazite and detrital zircon dating from the Proterozoic Rocky Cape Group, Tasmania: Links to the Belt-Purcell Supergroup, North America

The oldest known rocks in Tasmania occur in the Proterozoic Rocky Cape Group, a ∼10km thick quartzarenite-siltstone-pelite-dominated succession, previously constrained to have been deposited between 1450Ma and 750Ma. The Rocky Cape Group contains the enigmatic fossil Horodyskia (‘string of beads’) and has the potential to place Tasmania within supercontinent reconstructions. Detrital zircon and authigenic monazite grains dated via U–Pb laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS) analysis yield a deposition window between c. 1450Ma (youngest zircon populations) and c. 1330Ma (oldest authigenic monazite population) for the ∼9km thick lower-middle units (Pedder River Siltstone, Lagoon River Quartzite, Balfour Subgroup which hosts Horodyskia, Detention Subgroup). The upper units (∼1km) include the Irby Siltstone, which is younger than c. 1310Ma; this unit is likely separated from both the lower-middle units and the overlying c. <1010Ma Jacob Quartzite by disconformities. The lower-middle Rocky Cape Group is dominated by detrital zircon populations between 1600 and 1900Ma, with subordinate populations at c. 1450Ma and a spread of older grains mostly between 2300 and 2900Ma. The overlying Irby Siltstone has a bimodal detrital zircon distribution with a dominant peak at c. 1340Ma and a secondary peak at c. 1720Ma; no >1900Ma grains were identified.Authigenic monazite age distributions are complex, with multiple age domains within most samples. The common Pb corrected 206U/238Pb ages, defined by oldest grains in each sample, identify three statistically significant groups: (1) 1358–1292Ma (inclusive of two sigma errors) (Lagoon River Quartzite and Pedder River Siltstone), (2) 1283–1239Ma (Cowrie Siltstone and Balfour Subgroup), and (3) 1085±9Ma (Detention Subgroup). We suggest monazite was precipitated during episodic fluid flow events at these three stages in various parts of the basin. The original source for REE-bearing fluids could be detrital monazite, which is rarely preserved, and/or organic matter from the interbedded carbonaceous shales.The Rocky Cape Group has a shared provenance with the higher-grade metasediments (Surprise Bay and Fraser formations) of nearby King Island; the newly derived depositional ages also overlap and support the correlation of these rock associations. On the basis of current datasets, there are no obvious correlations that can be made with Mesoproterozoic basins preserved in mainland Australia. Instead, an overlap in the timing of deposition, similarities in detrital zircon signatures and analogous depositional environment suggests the c. 1.45–1.37Ga upper Belt-Purcell Supergroup (Missoula and Lemhi groups) of western North America constitutes a plausible correlation with the Tasmanian Mesoproterozoic succession. If the (unexposed) Palaeoproterozoic basement of Tasmania correlates with the Transantarctic Mountains region of East Antarctica as previously proposed, we suggest that the overlying Mesoproterozoic sequences were deposited during rifting of the supercontinent Nuna, between proto-Australia (including the Mawson craton of Antarctica) and Laurentia as predicted by the most recent palaeogeographic reconstructions. Both the Tasmanian and western Laurentian packages were affected by episodic post-depositional fluid flow events between c. 1.35 and 1.05Ga, possible thermotectonic imprints of the subsequent assembly of Rodinia.

Chemical and stable isotopic characteristics of syn-tectonic tourmaline from the Western fold belt, Mount Isa inlier, Queensland, Australia

Understanding hydrothermal fluid flow events related to orogenic events is important because they provide large-scale redistribution of metals which may become concentrated in ore deposits. In this study we determine the origin of tourmaline-bearing quartz veins and pegmatites in the Western fold belt of the Proterozoic Mount Isa inlier, a region which host numerous syn-orogenic Cu deposits. This paper uses tourmaline mineral chemistry and isotopic analyses (in-situ B and bulk O and H isotopes) to fingerprint the fluid reservoirs that contributed to the fluid budget during the Isan orogeny. All the tourmaline samples have schorl–dravite solid solution compositions. Quartz–tourmaline δ18O (11.0–12.6 and 7.7–9.4‰ V-SMOW, respectively) pairs yield equilibrium formation temperatures of 305 to 575°C with the higher temperatures related to pegmatitic tourmaline. Tourmaline δD values have a relatively narrow range from −68 to −56‰ V-SMOW. In most cases calculated δ18OFluid and δDFluid values are compatible with primary magmatic water with a small component of metamorphic/18O-enriched formation waters. However, there are large variations in tourmaline δ11B values which range from +10.7 to −17.0‰, with the majority of values between −10 and −15‰. Variations in δ11BTur in the pegmatites suggest that boron degassing generated relative 11B depletion during crystallization. Similar δ11BTur values in quartz–tourmaline veins spatially associated with the pegmatites, in cordierite- and sillimanite-bearing metamorphic rocks west of the Mount Isa fault, demonstrate a similar origin for B in this tourmaline. In contrast, quartz–tourmaline veins of similar timing hosted by greenschist grade metamorphic rocks east of the Mount Isa fault have much higher δ11BTur values (+6.2 to +10.7‰) which reflect the involvement of a distinct B isotopic reservoir; mostly likely a near-surface derived evaporitic brine. Thus, the B isotopic data reveal important differences in fluid origin that are not reflected in the δ18O values. Therefore, at least two isotopically distinct fluids, one sourced from fractionating anatectic pegmatites and the other from external basinal brines, circulated contemporaneously in separate areas during the Isan orogeny. There is no evidence, based on the tourmaline isotopic data, to suggest that these fluids mixed, which demonstrates that the hydrothermal plumbing system in the Western fold belt during the Isan orogeny was disconnected, probably due to differences in thermal structure/metamorphic grade across the Western fold belt during orogenesis.

Polygonal faults in chalk: Insights from extensive exposures of the Khoman Formation, Western Desert, Egypt

Research Article| June 01, 2014 Polygonal faults in chalk: Insights from extensive exposures of the Khoman Formation, Western Desert, Egypt Barbara J. Tewksbury; Barbara J. Tewksbury 1Department of Geosciences, Hamilton College, Clinton, New York 13323, USA Search for other works by this author on: GSW Google Scholar John P. Hogan; John P. Hogan 2Geology and Geophysics Program, Missouri University of Science and Technology, Rolla, Missouri 65409, USA Search for other works by this author on: GSW Google Scholar Simon A. Kattenhorn; Simon A. Kattenhorn 3Department of Geological Sciences, University of Idaho, Moscow, Idaho 83844, USA Search for other works by this author on: GSW Google Scholar Charlotte J. Mehrtens; Charlotte J. Mehrtens 4Department of Geology, University of Vermont, Burlington, Vermont 05405, USA Search for other works by this author on: GSW Google Scholar Elhamy A. Tarabees Elhamy A. Tarabees 5Department of Geology, Damanhour University, Damanhour 22516, Egypt Search for other works by this author on: GSW Google Scholar Author and Article Information Barbara J. Tewksbury 1Department of Geosciences, Hamilton College, Clinton, New York 13323, USA John P. Hogan 2Geology and Geophysics Program, Missouri University of Science and Technology, Rolla, Missouri 65409, USA Simon A. Kattenhorn 3Department of Geological Sciences, University of Idaho, Moscow, Idaho 83844, USA Charlotte J. Mehrtens 4Department of Geology, University of Vermont, Burlington, Vermont 05405, USA Elhamy A. Tarabees 5Department of Geology, Damanhour University, Damanhour 22516, Egypt Publisher: Geological Society of America Received: 08 Dec 2013 Revision Received: 02 Mar 2014 Accepted: 04 Mar 2014 First Online: 09 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 © 2014 Geological Society of America Geology (2014) 42 (6): 479–482. https://doi.org/10.1130/G35362.1 Article history Received: 08 Dec 2013 Revision Received: 02 Mar 2014 Accepted: 04 Mar 2014 First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Barbara J. Tewksbury, John P. Hogan, Simon A. Kattenhorn, Charlotte J. Mehrtens, Elhamy A. Tarabees; Polygonal faults in chalk: Insights from extensive exposures of the Khoman Formation, Western Desert, Egypt. Geology 2014;; 42 (6): 479–482. doi: https://doi.org/10.1130/G35362.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Although polygonal fault systems and related features are common in fine-grained sediments in modern submarine basins and have been studied in basins worldwide using three-dimensional (3-D) seismic data, extensive on-land exposures have remained elusive. We report here on the discovery of a polygonal fault system occurring in nearly continuous surface exposure over ∼900 km2 in chalk of the Cretaceous Khoman Formation near Farafra Oasis, Egypt. Field exposures reveal polygon boundaries defined by clusters of dozens of normal faults with strongly grooved fault surfaces and coarse calcite veins along faults with evidence for multiple fluid flow events. Geometric patterns and fault intersections reveal that mechanically interacting normal faults with multiple orientations were active contemporaneously in a horizontal strain field that was essentially isotropic and extensional. We interpret the very steep dips (∼80°) to reflect fault initiation in response to elevated pore fluid pressures. In the uppermost part of the Khoman Formation, a terrain of isolated circular structures displaying shallow inward dips overlies the polygonal fault network. The spatial relationship to the underlying faults is consistent with these small circular basins having formed as fluid escape structures as the polygonal fault system evolved. Outcrops in the Khoman Formation provide an unprecedented look into the 3-D geometry of a polygonal fault system, providing context for the analysis of analogous systems in marine basins and other on-land exposures. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

A review of the genesis of the world class Bayan Obo Fe–REE–Nb deposits, Inner Mongolia, China: Multistage processes and outstanding questions

The Bayan Obo Fe–REE–Nb deposit is the world's largest rare earth element (REE) resource and with the increasing focus on critical metal resources has become a focus of global interest. The deposit is hosted in the Palaeoproterozoic Bayan Obo Group, mainly concentrated in the H8 dolomite marble. The ores consist of light REE enriched monazite and bastnäsite, with a wide array of other REE minerals. Niobium mineralisation is hosted primarily in aeschynite and pyrochlore, although there are a wide range of other Nb-minerals. The origin of the host dolomite and ore bodies has been a subject of intense debate. The host dolomite has been proposed to be both of sedimentary origin and an igneous carbonatite. Carbonatite dykes do occur widely in the area, and consideration of the textural, geochemical and isotopic composition of the dolomite suggests an origin via intrusion of magmatic carbonatite into meta-sedimentary marble, accompanied by metasomatism. The origin of the ore bodies is complex, indicated most strongly by an ~1Ga range in radiometric age determinations. Compilation of available data suggests that the ores were originally formed around 1.3Ga (Sm–Nd isochron ages; Th–Pb ages of zircon), close in time to the intrusion of the carbonatite dykes. The ores were subsequently subjected to several stages of deformation and hydrothermal overprint, culminating in deformation, metamorphism and fluid flow related to the Caledonian subduction of the Mongolian Plate under the North China Craton from ~450 to 420Ma (Th–Pb ages of monazite). This stage resulted in the formation of the strong foliation (‘banding’) of the ore. The presence of undeformed veins with alkali mineral fills, and the overprinting of the foliation by Nb minerals suggest that secondary fluid flow events may also have contributed to the metal endowment of the deposits, as well as remobilising the original Fe and REE mineralisation. The alteration mineralogy and geochemistry of the ores are comparable to those of many REE mineralised carbonatites. Initial Nd isotope ratios at 450Ma, however, suggest crustal sources for the metals. These conflicting lines of evidence can be reconciled if a (at least) two stage isotopic evolution is accepted for the deposits, with an original mantle-sourced, carbonatite-related metal accumulation forming around 1.3Ga with εNd close to 0. The ore was remobilised, with associated re-equilibration of Th–Pb isotope systematics during deformation at ~450Ma. A further stage of alkaline hydrothermal fluid was responsible for Nb mineralisation at this stage. The complex geological history, with multiple stages of alkaline, high field strength element-rich, metasomatic fluid flow, is probably the main reason for the exceptional metal endowment of the Bayan Obo area.

Integrating 40Ar–39Ar, 87Rb–87Sr and 147Sm–143Nd geochronology of authigenic illite to evaluate tectonic reactivation in an intraplate setting, central Australia

The Warburton–Cooper basins, central Australia, include a multitude of reactivated fracture–fault networks related to a complex, and poorly understood, tectonic evolution. We investigated authigenic illites from a granitic intrusion and sedimentary rocks associated with prominent structural features (Gidgealpa–Merrimelia–Innamincka Ridge and the Nappamerri Trough). These were analysed by 40Ar–39Ar, 87Rb–87Sr and 147Sm–143Nd geochronology to explore the thermal and tectonic histories of central Australian basins. The combined age data provide evidence for three major periods of fault reactivation throughout the Phanerozoic. While Carboniferous (323.3±9.4Ma) and Late Triassic ages (201.7±9.3Ma) derive from basin-wide hydrothermal circulation, Cretaceous ages (∼128 to ∼86Ma) reflect episodic fluid flow events restricted to the synclinal Nappamerri Trough. Such events result from regional extensional tectonism derived from the transferral of far-field stresses to mechanically and thermally weakened regions of the Australian continent. Specifically, Cretaceous ages reflect continent-wide transmission of tensional stress from a >2500km long rifting event on the eastern (and southern) Australian margin associated with break-up of Gondwana and opening of the Tasman Sea. By integrating 40Ar–39Ar, 87Rb–87Sr and 147Sm–143Nd dating, this study highlights the use of authigenic illite in temporally constraining the tectonic evolution of intracontinental basins that would otherwise remain unknown. Furthermore, combining Sr- and Ar-isotopic systems enables more accurate dating of authigenesis whilst significantly reducing geochemical pitfalls commonly associated with these radioisotopic dating methods.

Structural Evolution of Auriferous Veins at the Endeavour 42 Gold Deposit, Cowal Mining District, NSW, Australia

Polyphase veins at the low-grade, structurally controlled epithermal gold deposit Endeavour 42 in the Junee-Narromine volcanic belt of the Ordovician to Silurian Macquarie Arc, central New South Wales (Australia), record multiple tectonically driven fluid-flow events during the mid-Paleozoic. Ordovician host rocks consist of a volcano-sedimentary succession intruded by trachyandesite and andesitic lavas and intrusions, as well as a volumetrically significant Ordovician diorite sill. The entire succession was tilted and faulted prior to the emplacement of multiple mafic to intermediate dike phases, commonly localized along faults. Gold-bearing veins cut all lithologic units at the Endeavour 42 deposit, with the exception of rare late-stage intrusions. Economic gold is restricted to faults, fault-hosted breccias, and veins with two distinct populations: (1) an older, steeply dipping conjugate vein set, cut by (2) two younger, moderately dipping vein sets. Both steep and inclined, moderately dipping vein sets contain a similar mineral assemblage, implying formation during a single epithermal system. Veins are generally 1 to 10 mm thick, commonly straight walled, but locally form overlapping veins with hard linkages. The steep veins reactivated two sets of older faults, fault-parallel fractures, and dike contacts, and accommodated subhorizontal extension. High gold grade, hydrothermally cemented breccias are associated with early gold-bearing veins and localized along deposit-scale faults. Crosscutting the early steep gold-bearing vein sets are inclined gold-bearing veins with moderate dips to the southwest and west. These opened as a new vein set and as veins localized along and at a slight angle to westward-dipping bedding planes, respectively. The geometry of the inclined, moderately dipping veins either represents a sudden change in the far-field stress state to near-vertical minimum compressive stress, thereby suggesting a change from an extensional to a contractional environment, or it may reflect local changes in the differential stress state with subhorizontal northeast-southwest tension. Late veins and thrust faults in the deposit indicate a postmineral contractional environment. The Endeavour 42 deposit underwent tectonic burial until it was exhumed in the early Carboniferous, as indicated by 40Ar/39Ar plateau ages of 362.7 ± 2.1, 356.3 ± 1.8, and 344.5 ± 2.1 Ma, toward the end of the Kanimblan Cycle.

Effects of episodic fluid flow on hydrocarbon migration in the Newport‐Inglewood Fault Zone, Southern California

AbstractFault permeability may vary through time due to tectonic deformations, transients in pore pressure and effective stress, and mineralization associated with water‐rock reactions. Time‐varying permeability will affect subsurface fluid migration rates and patterns of petroleum accumulation in densely faulted sedimentary basins such as those associated with the borderland basins of Southern California. This study explores the petroleum fluid dynamics of this migration. As a multiphase flow and petroleum migration case study on the role of faults, computational models for both episodic and continuous hydrocarbon migration are constructed to investigate large‐scale fluid flow and petroleum accumulation along a northern section of the Newport‐Inglewood fault zone in the Los Angeles basin, Southern California. The numerical code solves the governing equations for oil, water, and heat transport in heterogeneous and anisotropic geologic cross sections but neglects flow in the third dimension for practical applications. Our numerical results suggest that fault permeability and fluid pressure fluctuations are crucial factors for distributing hydrocarbon accumulations associated with fault zones, and they also play important roles in controlling the geologic timing for reservoir filling. Episodic flow appears to enhance hydrocarbon accumulation more strongly by enabling stepwise build‐up in oil saturation in adjacent sedimentary formations due to temporally high pore pressure and high permeability caused by periodic fault rupture. Under assumptions that fault permeability fluctuate within the range of 1–1000 millidarcys (10−15–10−12 m2) and fault pressures fluctuate within 10–80% of overpressure ratio, the estimated oil volume in the Inglewood oil field (approximately 450 million barrels oil equivalent) can be accumulated in about 24 000 years, assuming a seismically induced fluid flow event occurs every 2000 years. This episodic petroleum migration model could be more geologically important than a continuous‐flow model, when considering the observed patterns of hydrocarbons and seismically active tectonic setting of the Los Angeles basin.

Chemical changes of fluid inclusion oil trapped during the evolution of an oil reservoir: Jabiru-1A case study (Timor Sea, Australia)

This study of the Jabiru oil field, Timor Sea (Australia), shows how the variability of visual properties of fluid inclusions in palaeo-oil zones can be mapped to identify internal structures that result from fluid interactions in the reservoir. Variability in the attributes of oil inclusion assemblages (OIAs) is greater within an inferred palaeo-gas cap (Zone 1) and current residual zone (Zone 3) and less within the current oil zone (Zone 2). In Zone 3, where water imbibed, a higher proportion of OIAs with visual white fluorescing oil inclusions correlates with less mono-aromatic compounds and a higher abundance of polar compounds determined on a bulk extract of inclusion oil using High Performance Liquid Chromatography (HPLC). The CH2/CH3 ratios and water contents, determined by Fourier Transform-Infrared Spectroscopy (FT-IR), are higher. In Zone 1, where gas drained, there is a higher proportion of OIAs with visual yellow fluorescing oil inclusions, and these correlate with low HPLC mono-aromatic and moderate polar contents, and low FT-IR CH2/CH3 ratios and water contents. Interpretation of the grains with oil inclusions (GOI™) and pressure–volume–temperature-composition (PVTx) fluid inclusion data suggests that a gas-saturated blue fluorescing palaeo-oil was present in the three zones at the same Pressure–Temperature fluid inclusion forming event PT1. Lighter gas-saturated blue fluorescing oil was present in Zone 2, but was rarer in Zones 1 and 3 at the later fluid inclusion forming event PT2. The yellow and white inclusion oils, trapped in Zones 1 and 3 respectively, are gas-depleted and interpreted to result from the interaction between non-movable residual oil around grains with the surrounding gas and water, respectively. Comparison between the PVTx data and the published burial curve suggests that most of the fluid inclusions were trapped during two different hot fluid flow events. The full oil column is interpreted to have been present during Early–Mid Tertiary. The palaeo-oil column was reduced during the Late Miocene by imbibition of water in the present residual zone and drainage of gas at the top of the reservoir.

Volcanic arcs fed by rapid pulsed fluid flow through subducting slabs

During subduction, the seawater-altered lithosphere becomes dehydrated and expels fluids. Isotopic analysis of an exhumed oceanic slab in the Tianshan Mountain Range shows that although subduction can continue for many millions of years, fluids are expelled in short-lived channels over periods of just a few hundred years. At subduction zones, oceanic lithosphere that has interacted with sea water is returned to the mantle, heats up during descent and releases fluids by devolatilization of hydrous minerals. Models for the formation of magmas feeding volcanoes above subduction zones require largescale transport of these fluids into overlying mantle wedges1,2,3. Fluid flow also seems to be linked to seismicity in subducting slabs. However, the spatial and temporal scales of this fluid flow remain largely unknown, with suggested timescales ranging from tens to tens of thousands of years3,4,5. Here we use the Li–Ca–Sr isotope systems to consider fluid sources and quantitatively constrain the duration of subduction-zone fluid release at ∼ 70 km depth within subducting oceanic lithosphere, now exhumed in the Chinese Tianshan Mountains. Using lithium-diffusion modelling, we find that the wall-rock porosity adjacent to the flowpath of the fluids increased ten times above the background level. We show that fluids released by devolatilization travelled through the slab along major conduits in pulses with durations of about ∼ 200 years. Thus, although the overall slab dehydration process is continuous over millions of years and over a wide range of pressures and temperatures, we conclude that the fluids produced by dehydration in subducting slabs are mobilized in short-lived, channelized fluid-flow events.

Deep crustal xenoliths from central Montana, USA: Implications for the timing and mechanisms of high-velocity lower crust formation

Integration of petrologic, chronologic and petrophysical xenolith data with geophysical observations can offer fundamental insights into understanding the evolution of continental crust. We present the results of a deep crustal xenolith study from the northern Rocky Mountain region of the western U.S., where seismic experiments reveal an anomalously thick (10–30 km), high seismic velocity (compressional body wave, Vp > 7.0 km/s) lower crustal layer, herein referred to as the 7.x layer. Xenoliths exhumed by Eocene minettes from the Bearpaw Mountains of central Montana, within the Great Falls tectonic zone, include mafic and intermediate garnet granulites, mafic hornblende eclogite, and felsic granulites. Calculated pressures of 0.6–1.5 GPa are consistent with derivation from 23–54 km depths. Samples record diverse and commonly polymetamorphic pressure-temperature histories including prograde burial and episodes of decompression. Samples with barometrically determined depths consistent with residence within the seismically defined 7.x layer have calculated bulk P-wave velocities of 6.9–7.8 km/s, indicating heterogeneity in the layer. Shallower samples have markedly slower velocities consistent with seismic models. New monazite total U-Th-Pb data and a variety of additional published geochronology indicate a prolonged and episodic metamorphic history, beginning with protolith ages as old as Archean and followed by metamorphic and deep crustal fluid-flow events ca. 2.1 Ga, 1.8–1.7 Ga, and 1.5–1.3 Ga. We suggest that the 7.x layer in this region owes its character to a variety of processes, including magmatic underplating and intraplating, associated with multiple tectonic events from the Neoarchean to the Mesoproterozoic.

Dynamics of fault-fluid-hydrate system around a shale-cored anticline in deepwater Nigeria

[1] Gas hydrates were recovered by coring at the eastern border of a shale-cored anticline in the eastern Niger Delta. To characterize the link between faults and fluid release and to identify the role of fluid flow in the gas hydrate dynamics, three piezometers were deployed for periods ranging from 387 to 435 days. Two of them were deployed along a major fault linked to a shallow hydrocarbon reservoir while the third monitored the fluid pressure in a pockmark aligned above the same major fault. In addition, 10 ocean-bottom seismometers (OBS) were deployed for around 60 days. The piezometers simultaneously registered a prolonged fluid flow event lasting 90 days. During this time, OBS measurements record several episodic fluid release events. By combining and analyzing existing and newly acquired data, we show that the fluid-fault system operates according to the following three stages: (1) upward pore fluid migration through existing conduits and free gas circulation within several shallow sandy layers intersecting the major fault, (2) gas accumulation and pore pressure increases within sandy-silty layers, and (3) hydrofracturing and fluid pressure dissipation through sporadic degassing events, causing pore fluid circulation through shallow sandy layers and drawing overlying seawater into the sediment. This paper clearly demonstrates how an integrated approach based on seafloor observations, in situ measurements, and monitoring is essential for understanding fault-fluid-hydrate systems.

From outcrop to 3D modelling: a case study of a dolomitized carbonate reservoir, Zagros Mountains, Iran

ABSTRACT Outcrop data derived from fieldwork, remote sensing satellite and LiDAR-derived 3D models were used to build an integrated dual porosity-permeability static reservoir model which captured stratigraphic, diagenetic and structural heterogeneities. The study focuses upon the Mishrif-Mauddud/Sarvak interval, one of the most prolific reservoir units in the Middle East. The study area is exceptionally well exposed in deep gorges which cut transversally across anticlines of the Simply Folded Belt of the Zagros Mountains. The outcrops reveal volumetrically significant dolomitization of the latest Albian to Turonian carbonates of the Lower and Upper Sarvak formations. Three different dolomite bodies, which are spatially connected and genetically linked to the same fluid flow event, were recognized and mapped: (1) a thick dolomite body replacing the Lower Sarvak and forming a massive dolomite core; (2) horizontally extensive stratabound dolomite bodies (sheets), emanating laterally from the massive dolomite; and (3) vertically elongated dolomite pipes, rooted in the massive dolomite and typically replacing slope facies of the Upper Sarvak Formation. The widespread development of tight, non-planar dolomite textures (a typical feature of high temperature dolomitization) drastically reduces the reservoir potential of the dolomitized geobody in the study area. In particular, this is present in the massive dolomitized body. Vuggy porosity seems to increase porosity only locally and to a limited extent, developing a non-connected pore network. The dominant porous dolomite textures are more abundant in the peripheral part of the geobody (dolomite sheets), where they are strongly controlled by precursor facies and diagenesis. Three main dolomite pore types were identified (intercrystalline, interparticle and mouldic), linked to the depositional environment of the precursor limestone. These pore types were used for petrophysical modelling. The approach adopted in this study allowed the distribution of rock properties in the dolomitized geobody to mimic the main depositional facies architecture. The study area is characterized by a simple fracture network. Two main fracture sets and two major sets of conjugate normal faults were recognized in the field and mapped on 3D virtual outcrop data. Non-stratabound fracture density varied according to stratigraphic unit and/or dolomite body type (pipes/massive/sheets), showing a general increase from precursor limestone to dolomite. Fracture density also varied according to distance from faults (fault damage zone). This was particularly true in the limestone. The data also showed a prominent increase in fracture height from limestone to dolomite bodies, indicating that the dolomitized geobodies are likely sites for high production and early water breakthrough.

CO 2 degassing and trapping during hydrothermal cycles related to Gondwana rifting in eastern Australia

Intensive carbonate and clay mineral authigenesis took place throughout the Late Permian Bowen–Gunnedah–Sydney basin system in eastern Australia. We conducted isotopic and trace element analyses of carbonate and clay minerals from clastic sedimentary rocks of the Gunnedah Basin and the Denison Trough in the Bowen Basin. Rb–Sr isochron age data of the illitic clays are consistent with episodic hydrothermal fluid flow events that occurred in association with Gondwana rifting accompanied by alkaline magmatism at ∼85 Ma and ∼95 Ma. Stable isotope data of carbonate and clay minerals from the Gunnedah Basin are indicative of meteoric waters from a high-latitude environment as the main fluid source, whereas trace element, Sr and Nd isotope data highlight mixing of meteoric fluids with magmatic and/or crustal components, with a possible input from marine carbonates for some samples. Trace metals, oxygen and strontium isotopes of dawsonites from the Denison Trough are interpreted to have been mobilised by fluids that interacted with evolved clastic sedimentary and marine carbonate end members. According to the carbon isotope data, CO 2 for calcite and ankerite precipitation was sourced mainly from thermal degradation of organic matter and magmatism, whereas the CO 2 used for dawsonite formation is inferred to have been derived from magmatic and marine sources. In the low permeability environments (particularly in coal seams), the increasing accumulation and oversaturation of CO 2 particularly promote the precipitation of dawsonite.

Fluid Flow Events in the Southern Dabashan Fold-thrust Belt, Central China: Insight From Fluid Inclusion and Isotope Data in Calcite Veins

The authors systematically analyzed fluid inclusions in calcite veins from 33 samples collected from the fractures zones in 3 different units of southern Dabashan (SDBS) fold-thrust belt to study the characteristics of fluids in the different units and determine possible migration pathways of the fluids with particular reference to the petroleum exploration potential in the region. Homogenization temperatures and salinities of fluid inclusions and their stable isotope (δD, δ18O) showed a distinct zonation and variations of fluid characteristics in the different tectonic units of the SDBS fold-thrust belt. Homogenization temperature measurements and fluorescence observation results indicated that faults are the main conduits for fluids during their active periods. Oxygen and hydrogen isotopic compositions from the imbricate thrust belt to the fold-thrust belt and to the front detachment-fold belt shift from a mixing of surface meteoric water and sedimentary formation water box toward the magmatic water zone and metamorphic zone. The front detachment-fold belt has the best condition for petroleum preservation and is the potentially favorable area for future exploration.

REE redistribution during hydrothermal alteration of ores of the Kalahari Manganese Deposit

The Kalahari Manganese Deposit (KMD) is the largest land-based manganese deposit, hosting approximately 80% of the world's known, mineable manganese resources. The deposit, located near Kuruman in the Northern Cape Province of South Africa, is one of five erosional relics of the Paleoproterozoic (ca. 2.2Ga) Hotazel Formation, with sedimentary manganese ores occurring as up to 50m thick beds interbedded with banded iron-formation (BIF) and hematite lutite.The study focuses on the manganese ores of the Nchwaning–Gloria mining area of the northern KMD. In this area, pronounced mineralogical and major element alteration was imparted on the sedimentary manganese ores by a structurally-controlled hydrothermal fluid flow event. Most notable effects of hydrothermal alteration are the decomposition and leaching of Ca- and Mg-carbonate, and marked residual enrichment of manganese. On the basis of mineral assemblage, grade, texture and geochemical characteristics, three ore types were distinguished in the studied sample set, classified into least altered (LA), partially altered (PA) and advanced altered (AA) types. Advanced altered ores may be further classified into five different types, based on mineral assemblages that contain hausmannite and/or braunite as significant minerals. The rare earth element (REE) geochemistry of these fundamental ore types was studied in detail, to document REE mobility during hydrothermal alteration.Total REE concentrations in LA ores were found to be very low (14–22ppm) and remarkably uniform, within the range typically observed for BIF. Hydrothermal alteration results in residual enrichment and a much larger scatter in REE contents. A small Ce anomaly observed in the protolith remains similar in magnitude when observed in PAAS-normalised REE plots. The data define, however, a power trend in the (Ce/Ce*) vs (Pr/Pr*) diagram. Such behaviour is interpreted in terms of a conservative system that was predominantly protolith-buffered. Local remobilisation of REE during hydrothermal alteration is attributed to the dissolution of diagenetic apatite and redistribution of hydrothermal trace minerals, including neoformed apatite, monazite and cerianite.

Pyrite and Pyrrhotite Textures and Composition in Sediments, Laminated Quartz Veins, and Reefs at Bendigo Gold Mine, Australia: Insights for Ore Genesis

The various types, textures, and compositional zoning of pyrite in the gold-bearing saddle reefs, quartz veins, and surrounding sedimentary rocks provide new information on the potential source and timing of gold and arsenic and related fluid processes responsible for mineralization at Bendigo. Nodular diagenetic pyrite in the black shale tops to sandstone turbidites is enriched in invisible gold and arsenic with mean values of 0.61 ppm Au, 1,300 ppm As, and Au/Ag <1, based on LA-ICPMS analyses. Other elements enriched in the diagenetic pyrite within the organic-rich shales are Mn, Zn, Mo, Cu, V, Ba, Ag, Cd, Tl, Co, Ni, Bi, Pb, and Te. In contrast, euhedral- and growth-zoned hydrothermal pyrite in the turbidites and bedding-parallel laminated quartz veins contains lower contents of most trace elements but has higher contents of invisible Au and As, especially on the outermost rim of the pyrite. The gold-rich pyrite rims generally become thicker (a few to hundreds of microns) in proximity to the gold-bearing saddle reefs. Pyrite in the reef commonly has the highest levels of invisible Au and As and the lowest levels of other trace elements. It is characterized by Au/Ag >1 and Au/Pb >0.01. In the deepest stratigraphic levels, below known productive gold reefs, diagenetic pyrite in the most carbonaceous shales has been replaced by pyrrhotite during metamorphism. LA-ICPMS analyses reveal that the disseminated pyrrhotite contains similar levels of Ni and Co to the diagenetic pyrite but is strongly depleted in As and Au. The spatial relationships between organic-rich shales, folded bedding-parallel laminated quartz veins, and gold-arsenic−bearing saddle reefs, combined with the consistent trends in the trace element composition of pyrite hosted by these three geological elements, is interpreted to indicate that the black shales were an initial source of Au and As, and the laminated quartz veins acted as the initial pathways for hydrothermal fluid flow carrying Au and As from the source shales to the saddle reefs. Maximum gold and arsenic input into the reefs, principally as free gold plus arsenopyrite, occurred late during deformation toward the end of the hydrothermal cycle and is expressed by the Au-As−rich rims to hydrothermal pyrite in the sedimentary host rocks, laminated quartz veins, and reefs. This corresponds with final fold lockup and the development of through-going fault arrays linking adjacent anticlines. The source of Au and As for this final, and most economically important, fluid-flow event is considered to be from carbonaceous shales deeper in the basin, where original gold-bearing diagenetic arsenian pyrite reacts with organic matter and is converted to pyrrhotite, with release of Au, As, and S to the metamorphic fluid.

Combining digital terrain and surface textures with large-scale particle-based computational models to predict dam collapse and landslide events

Computational modelling of extreme rock and fluid flow events, such as landslides and dam collapses, can provide increased understanding of their post-initiation course. This can provide valuable insight into opportunities for designing mitigation strategies and to enable more informed management of such disaster scenarios. For the prediction of fluid-based geo-hazards, such as dam breaks, the smoothed particle hydrodynamics method is ideally suited to predicting the resulting complex highly three-dimensional free surfaces flows, which involve splashing, fragmentation and interaction with complex topography and engineering structures. For landsides, which are collision dominated flows of rocks, the discrete element method provides an ideal method for prediction. In all cases, the quality of prediction is controlled by the availability of high-quality digital terrain data produced typically by photogrammetry. Interpretation of simulation results is enhanced when photogrammetric-based surface textures are used in the surface renderings for flow visualisation. Advanced geo-hazard modelling therefore requires a close combination of both photogrammetry and particle-based modelling. Two case studies of dam breaks and one landslide event are used to demonstrate the level of prediction that is now possible using these combined methods.

Composition, nucleation, and growth of iron oxide concretions

Iron oxide concretions are formed from post depositional, paleogroundwater chemical interaction with iron minerals in porous sedimentary rocks. The concretions record a history of iron mobilization and precipitation caused by changes in pH, oxidation conditions, and activity of bacteria. Transport limited growth rates may be used to estimate the duration of fluid flow events. The Jurassic Navajo Sandstone, an important hydrocarbon reservoir and aquifer on the Colorado Plateau, USA, is an ideal stratum to study concretions because it is widely distributed, well exposed and is the host for a variety of iron oxide concretions. Many of the concretions are nearly spherical and some consist of a rind of goethite that nearly completely fills the sandstone porosity and surrounds a central sandstone core. The interior and exterior host-rock sandstones are similar in detrital minerals, but kaolinite and interstratified illite–smectite are less abundant in the interior. Lepidocrocite is present as sand-grain rims in the exterior sandstone, but not present in the interior of the concretions. Widespread sandstone bleaching resulted from dissolution of early diagenetic hematite grain coatings by chemically reducing water that gained access to the sandstone through fault conduits. The iron was transported in solution and precipitated as iron oxide concretions by oxidation and increasing pH. Iron diffusion and advection growth time models place limits on minimum duration of the diagenetic, fluid flow events that formed the concretions. Concretion rinds 2 mm thick and 25 mm in radius would take place in 2000 years from transport by diffusion and advection and in 3600 years if transport was by diffusion only. Solid concretions 10 mm in radius would grow in 3800 years by diffusion or 2800 years with diffusion and advection. Goethite (α-FeO (OH)) and lepidocrocite (γ-FeO (OH)) nucleated on K-feldspar grains, on illite coatings on sand grains, and on pore-filling illite, but not on clean quartz grains. Model results show that regions of detrital K-feldspar in the sandstone that consume H + more rapidly than diffusion to the reaction site determine concretion size, and spacing is related to diffusion and advection rates of supply of reactants Fe 2+, O 2, and H +.

The importance of early crustal architecture for subsequent basin-forming, magmatic and fluid flow events. The Granites-Tanami Orogen example

The Paleoproterozoic Granites-Tanami Orogen (GTO) consists of tightly folded, predominantly greenschist facies rocks. The orogen, a component of the North Australian Craton includes a ca. 1864 Ma succession of turbiditic and pelagic sedimentary rocks interbedded with volcanic rocks and abundant dolerite sills in a back-arc basin setting, which is intruded by collision-related ca. 1800 Ma granitic rocks. Integrated 4D interpretation of deep seismic profiles, gravity and magnetic field observations associated with available surface geology and igneous isotope geochemistry suggest that the GTO forms an imbricated crust developed on a partially inverted S-dipping, rifted Archean basement. The rifting took place in the Paleoproterozoic around 1864 Ma, and was followed by E–W orientated compressional deformation ca. 1850 Ma with the development of northerly trending isoclinal folds and associated faults. Orthogonal collisional tectonics followed at ca. 1800 Ma, which is associated with S-verging kilometre-scale, folds, faults, and orogenic lode-gold deposits in the region. Recognition of the preserved early rift architecture in the GTO has major implications for the subsequent evolution of the orogen. The early architecture has influenced all subsequent deposition, deformation, magmatic and fluid events, including spatial control of the ca. 1864 Ma rock succession to the location of intrusive bodies and fluid transfer during formation of ca. 1800 Ma orogenic gold mineralization.