Crustal Regime Research Articles

Punctuated geochronology within a sustained high-temperature thermal regime in the southeastern Gawler Craton

The Yorke Peninsula region in the southeastern Gawler Craton forms the southern portion of the Olympic Cu-Au Province that hosts the giant Olympic Dam, Carrapateena and Prominent Hill deposits. The Yorke Peninsula hosts substantial Cu-Au mineralisation, with deposits at Hillside and in the Moonta–Wallaroo district that are broadly considered to have formed during the same event as Olympic Dam, the c. 1595–1575 Ma Hiltaba thermal event. Despite the region's economic significance, there is very little information on the nature and history of metamorphism in the Yorke Peninsula region, leaving a considerable gap in the knowledge of the tectonic controls on mineralisation in this area. New U–Pb LA-ICP-MS geochronology on metamorphic titanite, monazite and apatite from Wallaroo Group metasediments indicate the Yorke Peninsula region has a protracted thermal history, with three groupings of metamorphic ages at c. 1585 Ma, 1560–1550 Ma and 1530–1500 Ma, and apatite cooling ages that cluster around c. 1450 Ma. Phase equilibria modelling of andalusite-sillimanite bearing schists and cordierite-sillimanite bearing migmatitic gneisses indicate that metamorphism occurred between 1560 and 1500 Ma occurred at P–T conditions between 3.6 and 4.5 kbar and 660–750 °C. These high thermal gradient conditions (45–50 °C km−1) are recorded at least 20 million years after Hiltaba Suite magmatism; however, they occur within the thermo-chemically anomalous crust of the South Australian Heat Flow Anomaly, which predominantly consists of rock packages with heat production rates 60–100% greater than the global median for rocks of their age. One-dimensional thermal models constrained by the crustal architecture and thermal regime of the SE Gawler Craton suggest the metamorphic conditions recorded between 1560 and 1500 Ma can be achieved by burial of this heat producing element -enriched crust, without the influence of an advective heat source. Within this thermally-anomalous crustal regime in which high thermal gradients can be sustained, small changes in burial depth are considered sufficient to stimulate metal remobilisation in the metal-rich crust of the Yorke Peninsula region. This potential for burial-triggered, thermally-induced fluid liberation and subsequent metal remobilisation may be applicable to the entire eastern Gawler Craton, including the Olympic Cu-Au Province.

Partial Melting of Lithospheric Mantle and Formation of the Early Cretaceous Alkaline Rocks in the Guandimiao REE Deposit, Luxi Terrane, Eastern China

The Luxi Terrane (eastern China) exposes widespread Early Cretaceous alkaline rocks, whereas their petrogenesis remains controversial, including fractional crystallization, partial melting and crustal contamination regime. Here, we present petrology, geochemistry, sphene U-Pb geochronology and trace element data from the syenogranite, quartz syenite and quartz monzonite of the Guandimiao alkaline complex rocks to investigate their petrogenesis. Geochemical data suggest that these alkaline rocks show alkalic and peralkaline characters, and high Ga/Al ratios, SiO2, light rare-earth element (LREE), Zr and Nb, and low MgO, CaO, Eu contents, corresponding to A-type granites. Sphene trace elements in syenogranite and quartz monzonite show obvious fractionation between LREE and heavy rare-earth element (HREE) and high Th/U ratios, indicating a magmatic origin. They yield U-Pb lower intercept ages of 128 ± 2.3 Ma and 127 ± 1.3 Ma, representing the crystallization ages of these alkaline rocks. The negative correlations between CaO, Fe2O3 (Total), MgO, P2O5, TiO2, MnO and the pronounced depletion in Nb, Ta and Ti suggest that the alkaline rocks were formed by fractional crystallization. Additionally, the positive correlation between La/Hf and La, Th and Th/V, Ce/Yb and K2O, and Tb/Yb and Yb suggest that the alkaline melts are generated by partial melting. Such high Rb/Nb, (Th/Nb)N and Nb/Th ratios indicate crustal contamination during the magma emplacement. We, therefore, propose the magma source of the alkaline rocks in the Guandimiao complex originated by partial melting of lithospheric mantle, which experienced fractional crystallization and crustal contamination processes during its emplacement. Such complex alkaline rocks were probably formed in an extensional back-arc setting induced by the retreat of the subducting Izanagi plate.

Geology and evolutionary stages of the Late Precambrian Hammamat sediments at Gebel Um Tawat, North Eastern Desert, Egypt

The Um Tawat Late Precambrian immature molasse-type Hammamat sedimentary rocks, located in the Northern part of the Eastern Desert of Egypt, were deposited at the late stage evolution of the Pan-African Orogeny. The Um Tawat molasse sediments are mainly derived from Dokhan volcanics and rest unconformably on these volcanics. They are intruded by the younger granites of Sala’at El-Belih granites and Gebel Gattar batholith. Hammamat sediments consist of repeated cycles of conglomerates, sandstones, siltstones, and minor mudstones deposited in an inter-mountainous basin by braided stream and alluvial fan regime constituting a stratigraphic section of about 500 m thick. Petrographically, the conglomerates are of polymictic type. Sandstone could be termed arkosic greywacke and shows ripple marks and in some places cross bedding. Siltstone is characterized by clear lamination and graded bedding. Mudstone is characterized by very fine grains of silt and clay size particles. The evolutionary stages of the study area is divided into five stages: (1) compressional regime resulted in the formation of the metavolcanics and metagabbro-diorite complex; (2) tension and relaxation crustal regime and uplift of the old basement forming G. Dokhan; (3) relaxation period, weathering, and erosion processes of the Dokhan volcanics formed the Hammamat molasse-type sediments; (4) tensional regime or weak rift-like processes led to the eruption of the younger granites and (5) regional compressional deformation subjected to the Hammamat sediments. This phase form G. Um Tawat and forming one regional open anticline fold whose fold axis is trending N47°E–S47°W and plunges 5° toward SW.

Evidence of Tectonic Control on the Geochemical Features of the Volatiles Vented along the Nebrodi-Peloritani Mts (Southern Apennine Chain, Italy)

Investigations carried out over the southernmost portion of the Apennine chain (Nebrodi-Peloritani Mountains, Sicily, Italy) reveal a close connection between the tectonic setting and the regional degassing of CO2-dominated volatiles. The geochemical features of the collected gases show that the pristine composition has been modified by gas-water interaction (GWI) and degassing processes. The 3He/4He isotopic ratio in the range of 0.7-2.8 Ra highlights variable contributions of mantle-derived helium, representing an unusual feature for the crustal regime of the study areas characterized by the widespread presence of 4He-producer metamorphic rocks. The degassing of mantle helium is coherent with the tectonics and related to the NW-SE extensional regime of the Calabro-Peloritan Arc (CPA). We propose that the degassing regime as well as the geochemical features of both the dissolved and bubbling gases is closely connected to the strain accumulation rate, inducing almost no temporal changes and insignificant deep-originated fluid contributions to the locked fault volumes. Investigations including discrete and continuous monitoring and degassing-rate estimations are useful tools to gain a better insight into the evolution of seismogenesis, considering the fault rupture as the final stage of a seismic cycle.

Source of ore-forming fluids of the Yangshan gold field, western Qinling orogen, China: Evidence from microthermometry, noble gas isotopes and in situ sulfur isotopes of Au-carrying pyrite

The Yangshan gold ore belt contains a series of sediment-hosted disseminated gold deposits in the southern subzone of the western Qinling orogen. The ore deposits formed in the Middle Devonian carbonaceous clastic-carbonate sequences that underwent strong deformation and low greenschist facies regional metamorphism. The shapes and distribution of the ore bodies are strictly confined by the E-W-trending Anchanghe-Guanyinba thrust fault zone. Pyrite and arsenopyrite are the major Au-carrying minerals. Microthermometric results show that the ore-forming fluids are CO2-rich aqueous solutions. The homogenization temperatures range from 221 to 303.5 °C, and salinities range from 2.0 to 7.2 wt% NaCl equiv. These characteristics are consistent with orogenic gold deposit. Noble gas isotopic measurement shows that 3He/4He ratios of pyrite and quartz grains inherited from the mineralized carbonaceous phyllite, ranging from 0.033 to 0.081 Ra, implying that the origins of ore fluids are restricted within the crustal regime. The 40Ar/36Ar ratios range from 434 to 863, indicating 34 to 68% crust-derived radiogenic 40Ar* in the ore fluids. Based on the backscattered electron (BSE) maps of pyrite, three generations (i.e. core, mantle, rim) are distinguished: pre-ore pyrite including framboidal pyrites and the cores of zoned pyrites both in the phyllite and plagiogranite dikes; ore stage pyrites including the middle zones of the three-zoned pyrites in the granitic dikes and the outer zones of the zoned pyrites in the phyllite; post-ore pyrites, i.e. the outermost rims of the zoned pyrites in the dikes. The arsenopyrite in the phyllite has similar texture to the pyrite, and is regarded as the peak of the mineralization due to its highest Au concentrations. High spatial resolution NanoSIMS multi-elemental maps of the pyrites reveal that (i) gold is distributed heterogeneously as lattice-bounded Au in the zoned pyrites with higher Au (As) in the middle zones and lower Au (As) in the inner and outermost zones. No nano-particles of native gold was identified in pyrite; (ii) gold mineralization occurred in a single event instead of multistage ore formation; (iii) gold precipitation event is followed by an overgrown zone with low Au and As concentrations. In situ sulfur isotopes of Au-bearing pyrite identified two types of sulfur sources: one is framboidal pyrite and the barren core of pyrite in the carbonaceous phyllite, which have very negative δ34S values (<−20%); the other is the Au-bearing overgrowth zone of pyrite both from the granitic dike and phyllite, which have higher δ34S values ranging from −7.3 to 1.3%. This result indicates that the barren pyrites have the sulfur isotopic feature of reduction of seawater sulfate, whereas the As-, Au-bearing pyrites are likely derived from a magmatic sulfur contaminated by a sedimentary sulfur.Combining Au concentrations with the corresponding δ34S values in the pyrites reveals that the zones with high Au have lower δ34S values (and vice versa). This coupling relation between Au and S isotope has been also identified in other orogenic and Carlin-type gold deposits. The oxidation of ore fluid is taken as the key for gold precipitation during formation of orogenic gold deposit. However, the oxidation of ore fluids accompanied by the escape of reduced gas (e.g. H2S) should result in high Au coupled with high δ34S. Therefore, we propose that the Yangshan gold deposit forms through partitioning aqueous vapor with high Au, As, and lower δ34S from a deep magma chamber under the ore field, which leads to overgrowth of the pre-existing pyrites.Based on the geological features, texture of pyrite, and sulfur isotopic characteristics, we suggested that the Yangshan gold deposits have the characteristics between Carlin-type and orogenic gold deposits. They are the product of the western Qinling orogeny during the Late Triassic.

Tectonic evolution of the North Patagonian Andes (41°–44° S) through recognition of syntectonic strata

The North Patagonian fold-thrust belt (41°–44° S) is characterized by a low topography, reduced crustal thickness and a broad lateral development determined by a broken foreland system in the retroarc zone. This particular structural system has not been fully addressed in terms of the age and mechanisms that built this orogenic segment. Here, new field and seismic evidence of syntectonic strata constrain the timing of the main deformational stages, evaluating the prevailing crustal regime for the different mountain domains through time. Growth strata and progressive unconformities, controlled by extensional or compressive structures, were recognized in volcanic and sedimentary rocks from the cordilleran to the extra-Andean domain. These data were used to construct a balanced cross section, whose deep structure was investigated through a thermomechanical model that characterizes the upper plate rheology. Our results indicate two main compressive stages, interrupted by an extensional relaxation period. The first contractional stage in the mid-Cretaceous inverted Jurassic–Lower Cretaceous half graben systems, reactivating the western Cañadón Asfalto rift border ~500km away from the trench, at a time of arc foreland expansion. For this stage, available thermochronological data reveal forearc cooling episodes, and global tectonic reconstructions indicate mid-ocean ridge collisions against the western edge of an upper plate with rapid trenchward displacement. Widespread synextensional volcanism is recognized throughout the Paleogene during plate reorganization; retroarc Paleocene-–Eocene flare up activity is interpreted as product of a slab rollback, and fore-to-retroarc Oligocene slab/asthenospheric derived products as an expression of enhanced extension. The second stage of mountain growth occurred in Miocene time associated with Nazca Plate subduction, reaching nearly the same amplitude than the first compressive stage. Extensional weakening of the upper plate predating the described contractional stages appears as a necessary condition for abnormal lateral propagation of deformation.

인제단층의 중생대 이 후 재활동 연대

Recently developed illite-age-analysis(IAA) approach was applied to determine the fault-reactivated events for the Inje fault that cut through Precambrian biotite granitic gneiss with NNE-SSW trend in the middle of Korean peninsula. Three distinct fault-reactivated events of shallow crustal regime were recognized using the combined approach of optimized illite-polytype quantification and K-Ar age-dating of clay fractions separated from 4 fault clay samples: 87.0±0.12 Ma, 65.5±0.05 and 66.6±1.38 Ma, 45.6±0.15 Ma, respectively. As well, 2M1 illite ages of 193~196 Ma and 254.3±6.96 were discernible, which may be related to the fault-activated time in the relatively deep crust. The study results suggest that the Inje fault would be firstly formed at 254.3°ae6.96 Ma and sporadically reactivated in shallow regime since about 87 Ma. These reactivation events in shallow regime might be due to the Bulguksa orogeny that would be strongly influenced in Korean peninsula at that time.

STRUCTURE AND DYNAMICS OF COLD WATER SUPER-EARTHS: THE CASE OF OCCLUDED CH4AND ITS OUTGASSING

We study the transport of methane in the external water envelopes surrounding water-rich super-Earths and estimate its outgassing into the atmosphere. We investigate the influence of methane on the thermodynamics and mechanics of the water mantle. We find that including methane in the water matrix introduces a new phase (filled ice) resulting in hotter planetary interiors. This effect renders the super-ionic and reticulating phases accessible to relatively low mass planets lacking a H/He atmosphere. We model the thermal and structural profile of the planetary crust and discuss five possible crustal regimes. The formation of methane clathrate in the subsurface is shown to inhibit the formation of a subterranean ocean. This effect results in increased stresses on the lithosphere making modes of ice plate tectonics possible. The dynamics of the tectonic plates are analysed. We derive overturn and resurfacing time scales as well as the melt fraction underneath spreading centers. Ice mantle dynamics is found to be important for assessing the composition of the atmosphere. We formulate the relation between the outgassing flux of methane and the tectonic mode dynamics. We give numerical estimates for the global outgassing rate of methane into the atmosphere.

Petrologic, tectonic, and metallogenic evolution of the southern segment of the ancestral Cascades magmatic arc, California and Nevada

Ongoing arc magmatism along western North America was preceded by ancestral arc magmatism that began ca. 45 Ma and evolved into modern arc volcanism. The southern ancestral arc segment, active from ca. 30 to 3 Ma, adjoins the northern segment in northern California across a proposed subducted slab tear. The east edge of the Walker Lane approximates the east edge of the southern arc whose products, mostly erupted from stratovolcanoes and lava dome complexes arrayed along the crest of the ancestral arc, extend down the west flank of the Sierra Nevada. Southern arc segment rocks include potassic, calc-alkaline intermediate- to silicic-composition lava flows, lava dome complexes, and associated volcaniclastic deposits. Northern and southern segment rocks are similar to other convergent-margin magmatic arc rocks but are compositionally distinct from each other. Southern segment rocks have lower TiO 2 , FeO*, CaO, and Na 2 O contents and higher K 2 O contents, and exhibit less compositional-temporal variation. Compositional distinctions between the northern and southern segment rocks reflect the composition and thickness of the crust beneath which the associated magma systems were sourced. Northern segment rock compositions are consistent with generation beneath thin, primitive crust, whereas southern segment rocks represent magmas generated and fractionated beneath thicker, more evolved crust. Although rocks in the two arc segments have similar metal abundances, they are metallogenically distinct. Small porphyry copper deposits are characteristic of the northern segment whereas significant epithermal precious metal deposits are most commonly associated with the southern segment. These metallogenic differences are also fundamentally linked to the tectonic settings and crustal regimes within which these two arc segments evolved.

Thermo-mechanical modeling of lower crust exhumation—Constraints from the metamorphic record of the Palaeoproterozoic Eburnean orogeny, West African Craton

The aim of this paper is to explore the link between the metamorphic record of a Palaeoproterozoic orogeny and its thermo-mechanical evolution, in order to discuss the burial and exhumation processes of moderate- to high-pressure volcano-sedimentary rocks (6–8 to >10kbar) in the Birimian Province (2.2–2.0Ga) of the West African Craton. Metamorphic data collected in Burkina Faso, southwest Ghana and eastern Senegal suggests a Palaeoproterozoic Birimian crust dominated by moderate apparent geothermal gradients of 20–30°C/km (M2a), which fit a purely conductive thermal crustal regime. This produced greenschist- to amphibolite-facies metamorphic assemblages that developed during the Eburnean orogeny (∼2.1Ga), associated with regional shortening and granitoid intrusions. The M2a gradient is superimposed on an early thermal regime (M1: <10–15°C/km) that produced high-P greenschist- to blueschist-facies metamorphic assemblages. M1 most likely recorded an earlier thermal history in the volcano-sedimentary basins where the protolith of the metamorphic terrains formed. Whether M1 is related to distributed burial associated with sediment accumulation or early subduction-related tectonics remains an open question, not directly addressed here.Thermo-mechanical models were built based on the assumption of an initial crust made up of a succession of volcanic island arcs emplaced on top of a basement dominated by CaO-rich Tonalite–Trondhjemite–Granodiorite suites (TTGs), and tectonically paired with sedimentary basins. From the results of these thermo-mechanical models, we propose a mechanism of burial, heating and exhumation of meta-sediments through the Birimian crust controlled by simultaneous folding/shortening and gravitational instabilities in the juvenile crust. We postulated here the pre-existence of thick sedimentary basins (depth 15km) in the Birimian orogenic crust to model the late-stage exhumation of meta-sediments, collected in the thermal aureole of CaO-poor granitoids, and which record elevated metamorphic pressures (P>6–8kbar). At around 25km depth, TTGs reached appropriate PT conditions to start melting. The buoyant and low-viscosity partially molten material, comprising a fraction of CaO-poor melt and residual material, then ascended through the overlying sediments. It entrained upwards lower crustal material as well as the surrounding sediments. This scenario explains the important breaks in metamorphic conditions observed in the Birimian province between the thin slivers (<2km thick) of high-pressure rocks (P>8kbar) preserved in the thermal aureoles of granites and the adjacent synforms of greenstones, which consist of low-grade metasediments (P<6kbar; T<450°C). This scenario is in agreement with a significant recycling of the TTGs in the genesis of CaO-poor granitoid melts. We discuss the implications of our results for the tectono-metamorphic and mechanical evolution of Palaeoproterozoic orogenies.

New U-Pb data on the age of rhyolite from Middle Zauralye: Evidence for change of crustal geodynamic regimes

775 The Turinskaya Series represented by Cenotype rocks of rhyolite-basalt composition is widely abun� dant along the eastern slope of the Urals and in West Siberia filling large grabens in the preJurassic base� ment. Formations of the Turinskaya volcanogenic series are usually distinguished as the taphrogenetic structural level together with the Chelyabinsk carbon� iferous series. In the opinion of A.P. Karpinskii (5), the dislocated character of Triassic-Jurassic deposits in this long zone provides evidence for posthumous orog� eny in the Urals during the Early Mesozoic period. V.E. Khain (10) distinguished the taphrogenetic struc� tural level as the final stage in the evolution of the geo� syncline orogenic system, which is supported by many researchers, in particular, V.N. Puchkov (8). However he attributed rhyolite of the Turinskaya Series to the epoch of 250 Ma and suggested to relate its formation to the Ural-Siberian superplume, which formed Sibe� rian trap basalts, in the opinion of N.L. Dobretsov (3). According to the investigations of A.L. Yanshin (11, 12), Ural orogeny and the formation of thrusts were finished by the beginning of the Middle Triassic, since the deposits of the Surakaiskaya Suite of the Ladin� ian-Carnian age occur horizontally and discordantly overlie dislocated Permian deposits in the Uralian front trough. Thus, solution of the problem of consolidation of the Uralides required quite precise dating of rhyolite of the Turinskaya Series. For this purpose we collected the samples of porphyric rhyolite on the Sinara River in the Middle Zauralye upstream the Borisovo quarry (Fig. 1) and occurring 120 m upwards the base of the Turinskaya Series. Note that dating of the upper part of the Turinskaya Series by different methods provided significant variations in the ages of volcanic rocks, from 250 to 221 Ma (2, 4). Boundary deposits of the Paleozoic and Mesozoic are located in the mouth area of the Chernushka River, left tributary of the Sinara River. They are represented by boulders of pebble conglomerate, mostly lime� stone, with a fragment size up to 20 cm. Pebbles con� sist of various darkgray, almost pelitoform carbonates and yellowishgray clayey limestone with crinoids, sometimes containing syringopora colonies. There are small honeyyellow rounded flints and large (up to 20 cm) angular-rounded gray flints. Fragments are cemented (porous type of cement) by volcanic mate� rial of cherry color. The thickness of conglomerate layer varies from 2 to 10 m.

Alpine tectonic evolution and thermal water circulations of the Argentera Massif (South-Western Alps)

Three groups of thermal springs with temperatures close to 70 °C discharge both in the core (at Bagni di Vinadio and Terme di Valdieri) and on the external margin (at Berthemont-Les-Bains) of the Argentera Massif. Detailed structural field analysis carried out on the hydrothermal sites allows us to delineate both a model of Alpine tectonic evolution of the Argentera Massif and the patterns of hydrothermal circulation that were active during its final exhumation. The observed fault rock assemblages provide information relative to deformation that occurred in viscous, frictional-to-viscous and frictional crustal regimes. During the Early Miocene, the Bersezio Fault Zone and the Fremamorta Shear Zone, two main mylonitic shear zones, mainly accommodated regional transpression and provided pathways for fluid flow promoting mineral reactions in greenschist facies. During the Late Miocene–Early Pliocene, frictional-to-viscous deformation affected the massif, which underwent predominant transpression in the internal sectors and extension on the external margin. During the Plio-Pleistocene, deformation in frictional condition accompanied the final exhumation of the massif in a transpressive regime and resulted in the development of the NW–SE striking cataclastic zones. The hydraulic properties of these structures mainly influence the patterns of the active thermal circulations and the localization of the recharge and discharge zones. At Berthemont these faults represent conduits, whereas at Vinadio and Valdieri they form complex systems of conduits and barriers. In these two latter sites, the cataclastic faults compose flower structures that constrain laterally the thermal fluid flows while intensely fractured granites sited at depth constitute a highly-transmissive geothermal reservoir. Less permeable migmatitic gneisses overlaying the granites prevent a massive infiltration of the cold fluids at depth. This context favours within the high-permeability fractures granites the development of buoyancy-driven flows which combined with topographically-driven flows, provided the conditions for the upflow of the high-temperature waters.

Crustal structure above a retreating trench: Receiver function study of the northern Apennines orogen

Teleseismic receiver functions recorded along a densely-spaced transect of 20 broad-band seismic stations highlight systematic spatial variation of crustal thickness across the Northern Apennines (Italy), revealing the two sides of an active continental orogenic zone. The crustal structure has been investigated using a 1D grid search technique, based on the coherent arrivals of forward and reverberated P-to-S converted phases from the Moho. At the SW end of the transect, the Tyrrhenian Sea and Tuscany are characterized by a shallow Moho (20–25 km) and a sharp impedance contrast. At the opposite edge, the definition of the crust–mantle boundary is problematic, suggesting a gradual and/or complicated transition from the crust to the mantle in the Adriatic domain. However, where we can resolve it, the crustal thickness is larger (∼ 35 km). The boundary between these two crustal regimes coincides with the high crest of the Apennines. Shear-wave splitting measurements (an upper-mantle texture indicator) in the area reveal a correspondence between crustal thickness and mantle fabric transitions. We suggest that these spatially coincident changes in the crustal structure and the upper-mantle texture beneath high Apennines mark the edge of the Eurasian plate in Italy.

Permeability evolution in quartz fault gouges under hydrothermal conditions

The permeability (k) of fine‐grained quartz aggregates were measured in situ during hot pressing (HPing) experiments to explore the evolution of fluid transport properties of fault zones during the interseismic period. Experiments were conducted at temperatures of 150°C and between 700 and 850°C, with confining and pore water pressures of 250 and 150 MPa, respectively. Significant permeability reduction was observed between 700 and 850°C, with permeability reduction rates (r = (1/t) ln (kto/kt)), ranging from approximately 6 × 10−5 s−1 at 700°C to a maximum of approximately 7.4 × 10−4 s−1 at 850°C. Permeability decreased exponentially with time, and the permeability reduction rate increased with increasing temperature, increasing differential stress, and decreasing grain size. Analysis of the permeability‐porosity relationships indicates that permeability in the simulated gouge at high temperature shuts off at a critical porosity of 0.045 ± 0.004. The presence of microstructures, such as grain interpenetration, grain shape truncation, arrays of fluid inclusions, and development of quartz overgrowths on grains, indicate that k reduction was controlled by dissolution‐precipitation creep processes. Extrapolation of the permeability reduction rates, measured in this study, to temperatures typical of the continental seismogenic regime highlights the strongly time‐dependent nature of permeability in natural fault wear products at depths of nucleation of major earthquakes. Within the recurrence time of large earthquakes, quartz‐rich fault zones in the fluid‐active midcrustal to lower continental crustal regimes can evolve from high‐permeability conduits to low‐permeability seals. Episodic changes in the fluid transport properties of faults during the interseismic period are likely to impact on the pore pressure evolution of fault wear products.

Uranium mineralization in the muscovite-rich granites of the Shalatin region, Southeastern Desert, Egypt

The younger granitoids of the Shalatin district in the Southeastern Desert of Egypt, are of biotite and two-mica granite compositions. The geochemistry of rare-earth elements (REE), yttrium, thorium and uranium forms the basis for many important methods to reconstruct igneous petrogenesis. Since the recognition that REE, Y, Th, U-rich accessories may play an important role in controlling the geochemistry of crustal melts, a considerable amount of work has been done in an attempt to understand their effects. However, this effort has been almost exclusively focused on three minerals: zircon, monazite and apatite. Nevertheless, the variety of REE-Th-U-rich accessories in granite rocks are neither limited to these three minerals nor are they always the main REE, Y, Th carriers. The geochemistry of REE, Y, Th and U reflects the behavior of accessories and some key major minerals such as garnet and feldspars, and may therefore give valuable information about the conditions of partial melting, melt segregation and crystallization of granite magmas in different crustal regimes. The geochemistry of U and Th during magmatic differentiation has been studied in many granites from different areas and it has been known that the U and Th contents of granitic rocks generally increase during differentiation, although in some cases they decrease. The Th/U ratio can either increase or decrease, depending on redox conditions, the volatile content or alteration by endogene or supergene solutions. The accessory assemblage of muscovite-rich granites and high-grade rocks is composed of monazite, xenotime, apatite, Th-orthosilicate, secondary U-mineralization and betafite-pyrochlore. REE, Y, Th and U are not suitable for geochemical modeling of granitoids by means of equilibrium-based trace element fractionation equations, but are still useful petrogenetic tools.

Curie Point Depths Beneath Precordillera Cuyana and Sierras Pampeanas Obtained from Spectral Analysis of Magnetic Anomalies

A gravity and magnetic survey has been carried out with the purpose of investigating geophysical features of the crusts beneath three geological provinces in western Argentina: Cuyo Precordillera, the Sierras Pampeanas of San Juan and La Rioja, and Famatina System, the results of which are displayed in three maps: Bouguer anomaly, total field magnetic anomaly and total field reduced to the pole. The top and bottom boundaries of the magnetized crust were calculated from power-density spectra of the total-field anomalies from our terrestrial database in 90 2D windows. The depths obtained for the bottom of magnetized crust are assumed to correspond to Curie point depths. The values thus obtained for the Precordillera range between 29 and 40 km, whereas for the Sierras Pampeanas, in the Sierra de Pie de Palo, and other mountain chains along the Bermejo-Desaguadero lineament or Valle Fértil lineament, such depth ranges between 20 and 35 km. These results are consistent with Curie point depths determined on different continental regions in the world. A map of regional heat flow has been prepared in the present work based on the depth of the Curie point isotherm, which shows that heat flow patterns in Precordillera are different from those found in Sierras Pampeanas. A significant heat flow extending in Northeast-Southwest direction seems to be directly related to Juan Fernández Ridge trace. The results of the present investigations also point out a possible relationship between the base of the magnetized crust and the boundary separating the brittle from the ductile crustal regime.

Geology of the Skorpion Supergene Zinc Deposit, Southern Namibia

The Skorpion zinc deposit in southwestern Namibia comprises a significant nonsulfide orebody (24.6 Mt at 10.6% Zn) and subordinate amounts of primary base metal sulfide mineralization, which structurally underlies the nonsulfide ores at depth. The host rocks of the blind, nonsulfide orebody and relict sulfide mineralization consist of a mixed volcano-sedimentary sequence of Neoproterozoic age that has formed in an initial continental rift system between the Kalahari cratonic province and the Rio de la Plata cratonic province. The metavolcanic and subvolcanic rocks display a distinct bimodal geochemical composition of within-plate affinity and have been emplaced under an extensional crustal regime. Zircons from a rhyolitic vesicular flow, extruded approximately contemporaneously with the volcanic-hosted sulfide mineralization, have been dated by SHRIMP U-Pb analysis and yield an age of 751.9 ± 5.5 Ma. The basin that hosts the Skorpion deposit has been shaped by horst and (half-)graben tectonics, which also controlled the sedimentary deposition of siliciclastic and carbonate sediments within both shallow and deeper water environments. The rock sequence was affected by a major tectono-metamorphic event the Pan African-Brasiliano orogeny at approximately 550 to 545 Ma. Metamorphism reached uppermost greenschist to lowermost amphibolite facies, and the complex deformational style of the region is the result of intense faulting, folding, and thrusting. The Skorpion orebody is hosted by arkosic metaarenites and subordinately by volcaniclastic metasedimentary rocks. It is irregularly shaped, transgressive to sedimentary layering, and has a relatively flat top, which is covered by a blanket of unmineralized overburden consisting of calcrete, boulder beds and recent sand dunes. At depth, the orebody has a relatively sharp contact with a barren marble unit on the east and south and a more gradational boundary within the siliciclastic host rocks on the west and north. The nonsulfide ore minerals comprise predominantly sauconite (Zn smectite), substantial amounts of hemimorphite and smithsonite, and subordinate amounts of scholzite, tarbuttite, atacamite, hydrozincite, and hetearolite. The ore minerals are mainly euhedral and subhedral crystals and occur as open space fillings in intergranular voids, fractures, and breccias and as replacements of feldspar and mica. The metasedimentary host rocks of the nonsulfide ore contain abundant iron hydroxide pseudomorphs after disseminated sulfides and stringers and after rare semimassive stratiform layers of sulfides. Gossanous, manganese and iron hydroxide-rich, siliciclastic, metasedimentary rocks are common in the western and northwestern vicinity of the nonsulfide orebody. Alteration of the supergene mineralized host rocks includes removal of calcite cement and the breakdown of detrital albite-rich plagioclase and of minor orthoclase. Local alteration phenomena also include silicification and rare secondary barite, both a result of the alteration and breakdown of feldspar. Minor base metal sulfide mineral deposits occur in metarhyolitic flows and felsic hyaloclastic rocks, structurally below the nonsulfide orebody. The primary sulfide minerals comprise pyrite, pyrrhotite, sphalerite, minor chalcopyrite, and galena as well as secondary chalcocite and colloidal sphalerite (brunckite). The volcanic-hosted sulfide ore minerals are deformed and partly recrystallized and thus of premetamorphic and predeformation origin. In contrast, the Skorpion nonsulfide zinc deposit is of postmetamorphic origin, as shown by two hydrous, low-P, low-T ore assemblages. The secondary assemblages occur, at least partly, as delicate euhedral and subhedral crystals in secondary pore space, open fractures, veins, and breccias. The nonsulfide ore contains pseudomorphs from in situ oxidation and replacement of preexisting sulfides and ore and also from the remobilization or precipitation of base metals. The Skorpion nonsulfide zinc deposit is of supergene origin and formed from the oxidation of sulfide protore by circulating meteoric water, as suggested by textural evidence and by initial stable isotope data. The timing of ore formation is still undefined but lies within the large age bracket between 545 Ma (peak metamorphism) and subsequent partial exhumation and the deposition of the nonmineralized overburden of Namib Desert gravels (<21 Ma) and dune sands.

Brittle Origins for Disseminated Gold Mineralization inMylonite: Gaocun Gold Deposit, Hetai Goldfield, Guangdong Proince, SouthChina

Gold mineralization in the Hetai goldfield in South China is almost entirely confined to a series of steeply dipping ductile shear zones generated during Hercynian-Indosinian orogenesis of Sinian host rocks. Approximately half of the mineralization is in the form of disseminations in the mylonite and the remainder was clearly emplaced as a result of brittle deformation in veinlets and as sulfide cement to brecciated mesoscopic quartz veins. Heaily influenced by the presence of disseminated mineralization, all publications on Hetai determine a major role for ductile deformation in the introduction of gold into the deposit. The brittle deformation is typically regarded as redistributing, within the host structures, the gold initially introduced as a result of ductile deformation mechanisms. However, textures of the sulfides associated with the gold indicate an entirely postductile deformation timing as they display none of the features attributed to ductile deformation. During the evident brittle deformation phase, competence contrasts at the microscopic scale allowed fracturing to focus on the quartz-sericite domain boundaries of the mylonite foliation, thus permitting the introduction of auriferous fluid into the mylonite to create disseminated mineralization. The control exerted by preexisting structures is further reflected in the restriction of disseminated mineralization to the quartz domains, presumably because the sericite domains accommodated the deformation in a more distributed fashion. Seeral other lines of evidence support an origin of the Hetai deposit in an upper crustal regime. A similar style of disseminated mineralization, to that seen in the mylonite, surrounds microveinlet terminations in the brecciated quartz veins, as well as overprinting pseudotachylite, which is associated with cataclasite. There are also large volumes of unmineralized mylonite indicating no connection between the intensity of strain and gold grade. We conclude that, in structurally controlled deposits, the textural category of disseminated gold mineralization can be generated either under entirely ductile or entirely brittle conditions, and that microtextural evidence has to be carefully analyzed to determine the mode of formation of a deposit.

Geochemistry, Petrogenesis and Tectonic Setting of Metavolcanics and their Implications for Gold Mineralisation in Gadag Gold Field, Southern India

Gold bearing metavolcanics of Gadag Gold Field (GGF) are represented by mafic (metabasalt, metabasaltic andesite), intermediate (metaandesite) and felsic (metadacite, metarhyolite) rocks. Mafic metavolcanic rocks are low-K Fe-rich tholeiites and were derived by partial melting of the upper mantle sources with high Fe/Mg ratios and low M values. Intermediate and felsic metavolcanics were formed by remelting of these tholeiites mainly in crustal regimes. Although a complete sequence of metavolcanic rocks from mafic to intermediate to felsic fractions occurs, these products were not the result of differentiation from a single magma, crustal contamination was involved in the formation of intermediate and felsic rocks. A clear gap in the chemical composition as well as index of differentiation among the mafic, intermediate and felsic fractions indicate that these metavolcanics constitute a typical bimodal character. It is suggested that these metavolcanics were emplaced in an active continental margin or a continental island arc setting. The petrogenetic processes of formation of Fe-rich tholeiites that evolved in an active continental margin or a continental island arc setting could have provided a favourable geochemical environment for gold mineralisation under the conditions of deformation and metamorphism.

Mineral occurrences in central East and North-East Greenland—new possibilities?

The geological provinces of central East and northeast Greenland host numerous mineral occurrences. Pre-Caledonian mineralization is scarce and its structural affiliation is not always clear. Mineral occurrences related to Caledonian lineaments comprise ores of tungsten, arsenic and antimony. Mineral occurrences related to post-Caledonian lineaments take the form of vein-type mineralization, which is widespread along the eastern fault zone, and occurrences of lead, zinc, copper and silver that are known along the lineament from Jameson Land, Traill Ø, Gauss Halvø and Hudson Land. Three major types of mineralization are related to the post-Caledonian evolution of East Greenland: (1) epigenetic, mainly base-metal mineralization occurring over a north-south extension of several hundred kilometres; (2) strata-bound-stratiform base-metal mineralization associated with red beds and carbonates; and (3) molybdenum and base-metal mineralization related to Tertiary felsic, subvolcanic intrusions. Skarn formation is found in connection with Tertiary mafic sill intrusions.When the huge area of central and North-East Greenland is taken into consideration exploration activity has been sparingly funded and sporadic by comparison with other explored areas. The post-Caledonian area of East and North-East Greenland is a crustal extension regime with good potential for mineral deposit formation. Further exploration should be focused on the investigation of extension faults, wrench faults and zones of dilatation. These fluid pathways create alteration, which must be carefully studied to improve understanding of the mineralizing system.