Miocene formations and NE-trending right-lateral strike–slip tectonism in Thrace,northwest Turkey: geodynamic implications

In the Thrace Peninsula, Neogene units were deposited in two areas, the Enez Basin in the south and the Thrace Basin in the north. In the southwesternmost part of the peninsula, upper lower–lower upper Miocene continental to shallow marine clastics of the Enez Formation formed under the influence of the Aegean extensional regime. During the last stage of the transpressional activity of the NW-trending right-lateral strike–slip Balkan–Thrace Fault, which had controlled the initial early middle Eocene deposition in the Thrace Basin, a mountainous region extending from Bulgaria eastwards to the northern Thrace Peninsula of Turkey developed. A river system carried erosional clasts of the metamorphic basement southwards into the limnic depositional areas of the Thrace Basin during middle Miocene time. Deposition of fluvial, lacustrine, and terrestrial strata of the Ergene Formation, which conformably and transitionally overlie the Enez Formation, began in the late middle Miocene in the southwest part and in the late Miocene in the north‐northeast part of the basin. Activity along the NE-trending right-lateral strike–slip faults (the Xanthi–Thrace Fault Zone) extending from northeast Greece northeastwards through the Thrace Peninsula of Turkey to the southern shelf of the western Black Sea Basin began during the middle Miocene in the northern Aegean, at the beginning of the late Miocene in the southwest part, and at the end of the late Miocene in the northeast part of the Thrace region. Although the Neogene deposits in the Thrace Basin were evaluated as the products of a northerly fault, our data indicate that the NW-trending northerly fault zone became effective only during the initial stage of the basin development. The later stage deposition in the basin was controlled by the NE-trending Xanthi–Thrace Fault Zone, and the deposits of this basin progressively evolved north/northeastwards during the late Miocene. During the late early Miocene–late Miocene interval, extension within the Thrace region was part of the more regional Aegean extensional realm, but from latest Miocene time, it has been largely decoupled from the Aegean extensional realm to the south.     

Temporal–spatial distribution and tectonic setting of porphyry copper deposits in Iran: Constraints from zircon U–Pb and molybdenite Re–Os geochronology

Porphyry copper deposits (PCDs) in Iran are dominantly distributed in Arasbaran (NW Iran), the middle segment of the Urumieh–Dokhtar Magmatic Arc (UDMA), and Kerman (central SE Iran), with minor occurrences in eastern Iran and the Makran arc. This paper provides a temporal–spatial and geodynamic framework of the Iranian porphyry Cu (Mo–Au) systems, based on geochronologic data obtained from zircon U–Pb and molybdenite Re–Os dating of host porphyritic rocks and molybdenites in 15 major PCDs. The dating results define a long metallogenic duration (39–6 Ma), and suggest a long history of tectonic evolution from the accretionary orogeny related to early Cenozoic closure of the Neo-Tethys Ocean to subsequent collisional orogeny for the Iranian porphyry copper systems.The oldest porphyry mineralization occurred in the eastern part of Iran after the closure of a branch of the Neo-Tethyan (Sistan) Ocean between the Lut and Afghan blocks in the late Eocene (39–37 Ma). This was followed by mineralization in the Kerman porphyry copper belt over a time interval of about 20 m.y., where two metallogenic epochs have been recognized, including late Oligocene (29–27 Ma) and Miocene (18–6 Ma). The Bondar-e-Hanza deposit formed in the late Oligocene, while and the remaining dated deposits belong to Miocene epoch. According to the deposits' characteristics and their ages, the Miocene epoch can be divided into early, middle, and late stages. The Darreh Zar, Bakh Khoshk, Chah Firouzeh and Sar Kuh deposits formed during the early–middle Miocene. The largest porphyry deposits occur in the middle stage during the middle Miocene (14–11 Ma) and include the Sar Cheshmeh, Meiduk, Dar Alu and Now Chun deposits. These deposits were formed during crustal thickening, uplift, and rapid exhumation of the belt. The final stage of porphyry mineralization occurred during the late Miocene (9–6 Ma), and formed the Iju, Kerver, Kuh Panj and Abdar deposits.There were two porphyry mineralization stages in the Arasbaran porphyry copper belt in NW Iran, including an older late Oligocene (29–27 Ma) and a younger early Miocene (22–20 Ma) events. The Haft Cheshmeh deposit belongs to the older stage, and the world-class Sungun and Masjed Daghi deposits formed during the early Miocene.In the middle segment of the UDMA (Saveh–Yazd porphyry copper belt), PCDs formed during middle Miocene time (17–15 Ma). The geochronological results reveal that the porphyry mineralization moved from the northwest to southeast of UDMA over the time.Our dating results, combined with the possible late Eocene–Oligocene timing for collision between the Arabian and Iranian plates, support a model for Iranian PCD formation by partial melting of previously subduction-modified lithosphere in a post-subduction and post-collisional tectonic setting.     

Basin shape and sediment architecture in the Gun Supersequence: A strike‐slip model for Pb–Zn–Ag ore genesis at Mt Isa

Sequence‐stratigraphic correlations provide a better understanding of sediment architecture in the Mt Isa and lower McNamara Groups of northern Australia. Sediments record deposition in a marine environment on a broad southeast‐facing ramp that extended from the Murphy Inlier in the northwest to the Gorge Creek, Saint Paul and Rufous Fault Zones in the southeast. Depositional systems prograded in a southeasterly direction. Shoreline siliciclastic facies belts initially formed on the western and northern parts of the ramp, deeper water basinal facies occurred to the east and south. The general absence of shoreline facies throughout the Mt Isa Group suggests that depositional systems originally extended further to the east and probably crossed the Kalkadoon‐Leichhardt Block. Fourteen, regionally correlatable fourth‐order sequences, each with a duration of approximately one million years, are identified in the 1670–1655 Ma Gun Supersequence. Stratal correlations of fourth‐order sequences and attendant facies belts resolve a stratigraphic architecture dominated by times of paired subsidence and uplift. This architecture is most consistent with sinistral strike‐slip tectonism along north‐northeast‐oriented structures with dilational jogs along northwest structures as the primary driver for accommodation. Although reactivated during deformation, the ancestral northwest‐trending May Downs, Twenty Nine Mile, Painted Rocks, Transmitter, Redie Creek and Termite Range Fault Zones are interpreted as the principal synsedimentary growth structures. Sinistral strike‐slip resulted in a zone of long‐lived dilation to the north of the May Downs/Twenty Nine Mile and Gorge Creek Fault Zones and a major basin depocentre in the broad southeast‐facing ramp. Subordinate depocentres also developed on the northern side of the ancestral Redie Creek and Termite Range fault zones. Transfer of strike‐slip movement to the east produced restraining or compressive regions, localising areas of uplift and the generation of local unconformities. Northwest‐ and north‐northeast‐oriented magnetic anomalies to the south and west of Mt Isa, identify basement heterogeneities. Basement to the south and west of these anomalies is interpreted to mark intrabasin siliciclastic provenance areas in the Gun depositional system. Pb–Zn–Ag deposits of the Mt Isa valley are interpreted as occurring in a major basin depocentre in response to a renewed phase of paired uplift and subsidence in late Gun time (approximately 1656 Ma). This event is interpreted to have synchronously created accommodation for sediments that host the Mt Isa deposit, while focusing topographically and thermobarically driven basinal fluids into the zone of dilation.     

A review of sulfur to selenium ratios in magmatic nickel–copper and platinum-group element deposits

Deviations in the sulfur to selenium ratios (S/Se) from mantle values in magmatic Ni–Cu–Platinum Group Elements (PGE) sulfide deposits have been widely used to constrain the ore forming processes. Basically, S/Se ratios greater than mantle values are interpreted to be the result of contamination of the mantle derived magma by S-rich sedimentary rocks, whereas S/Se ratios lower than mantle values are thought to be the result of S loss during post-crystallization. However, there are many other processes involved in producing a deposit and it is possible that these may be also important in controlling S/Se ratios. In order to investigate the relative importance of these processes, we have compiled a data base of S, Se, δ³⁴S and metal values from Ni–Cu–PGE sulfide deposits. This compilation shows that processes affecting S/Se ratios can be divided into two main classes: the magmatic processes and the late- to post-magmatic processes.

• 1)Magmatic processes include the well-known addition of S from sedimentary rocks, variations in the sulfide to silicate liquid ratio (R-factor), depletion of the silicate magma in Se by early segregation of the sulfide liquid, and the moderate incompatibility of Se into the first sulfide minerals to crystallize from a sulfide liquid, the monosulfide-solid-solution (MSS). This incompatibility results in a change in S/Se ratio between the Fe-rich and Cu-rich zones of magmatic sulfide ores. The fractionation of Se during crystallization of sulfide liquids has not previously been appreciated.
• 2)Late- to post-magmatic processes include: hydrothermal alteration, high-grade metamorphism, serpentinization and supergene weathering. Some metamorphosed Cu-deposits have low S/Se ratios suggesting S-loss by breakdown of sulfide minerals during a high-grade metamorphic event. However, the effectiveness of this process remains unclear and alternative models exist. The preferential remobilization of S relative to Se during hydrothermal alteration, serpentinization and supergene weathering leads to a moderate decrease of S/Se ratios values and can mask the initial S/Se ratio.

     

Zircon U–Pb and molybdenite Re–Os geochronology of copper–molybdenum deposits in southeast Liaoning Province,China

ABSTRACT

Liaoning Province in China is an area known for the occurrence of numerous copper and/or molybdenum deposits of variable size. However, the age of mineralization and tectonic setting in this region are still a subject of debate. In this study we describe the geology of these deposits and apply zircon U–Pb and molybdenite Re–Os isotopic dating to constrain their ages and define the metallogenic epochs of this province. The Huatong Cu–Mo deposit yields molybdenite Re–Os model ages of 127.6–126.3 Ma and an isochron age of 127.4 ± 0.7 Ma. The Dongbeigou Mo deposit yields molybdenite Re–Os model ages of 132.6–127.1 Ma, an isochron age of 128.1 ± 5.1 Ma, and a zircon U–Pb age of 129.4 ± 0.3 Ma for the associated monzogranite. The granodiorite associated with the Wanbaoyuan Cu–Mo deposit yields a zircon U–Pb age of 128.4 ± 1.1 Ma; the plagiogranite associated with the Yaojiagou Mo deposit yields an age of 167.5 ± 0.9 Ma; and the biotite–plagioclase gneiss from the Shujigou Cu deposit yields an age of 2549.4 ± 5.6 Ma. These results, together with previous geochronology data, show that intense Cu–Mo porphyry and skarn mineralization were coeval with Early–Middle Jurassic and Early Cretaceous granitic magmatism. The former was associated with the orogeny that followed the collision of the Siberian and North China plates and the resulting closure of the palaeo-Asian Ocean, and the latter with rifting that followed the subduction of the palaeo-Pacific Plate and associated lithospheric thinning. Volcanogenic massive sulfide Cu deposit. mineralization took place much earlier, in the late Archaean, and was related to continent–continent collision, palaeo-ocean closure, the formation of a united continental landmass, bimodal volcanism, magma emplacement, and subsequent metamorphism and deformation of syn-collisional granites.     

Young,active conjugate strike–slip deformation in West Sichuan: evidence for the stress–strain pattern of the southeastern Tibetan Plateau

The active kinematics of the eastern Tibetan Plateau are characterized by the southeastward movement of a major tectonic unit, the Chuan-Dian crustal fragment, bounded by the left-lateral Xianshuihe–Xiaojiang fault in the northeast and the right-lateral Red River–Ailao Shan shear zone in the southwest. Our field structural and geomorphic observations define two sets of young, active strike–slip faults within the northern part of the fragment that lie within the SE Tibetan Plateau. One set trends NE–SW with right-lateral displacement and includes the Jiulong, Batang, and Derong faults. The second set trends NW–SE with left-lateral displacement and includes the Xianshuihe, Litang, Xiangcheng, Zhongdian, and Xuebo faults. Strike–slip displacements along these faults were established by the deflection and offset of streams and various lithologic units; these offsets yield an average magnitude of right- and left-lateral displacements of ～15–35 km. Using 5.7–3.5 Ma as the time of onset of the late-stage evolution of the Xianshuihe fault and the regional stream incision within this part of the plateau as a proxy for the initiation age of conjugate strike–slip faulting, we have determined an average slip rate of ～2.6–9.4 mm/year. These two sets of strike–slip faults intersect at an obtuse angle that ranges from 100° to 140° facing east and west; the fault sets define a conjugate strike–slip pattern that expresses internal E–W shortening in the northern part of the Chuan-Dian crustal fragment. These conjugate faults are interpreted to have experienced clockwise and counterclockwise rotations of up to 20°. The presence of this conjugate fault system demonstrates that this part of the Tibetan Plateau is undergoing not only southward movement, but also E–W shortening and N–S lengthening due to convergence between the Sichuan Basin and the eastern Himalayan syntaxis.     

The Qian’echong and Tangjiaping Porphyry Mo deposits, Dabie, East China:Origin of the Ore-related Granites and Tectonic Settings

Please refer to the attachment(s) for more details     

Pb-Zn “SEDEX” deposits and their copper stockwork roots, western Cuba

Base metal deposits in western Cuba belong to the SEDEX type, as defined by Carne and Cathro (1982). The age of the mineralisation is Jurassic. The host rocks of the deposits are gritstones, sandstones and black shales, with no igneous rocks involved. Sulphur isotopic data from pyrite and chalcopyrite indicate a biogenic and magmatic origin, at least for part of the Cuban sulphides. The inferred tectonic setting of these deposits is that of a passive continental margin. The Pb-Zn-Cu mineralisation in Cuba is hosted in the siliciclastic sedimentary rocks of the San Cayetano Formation (Lower-Upper Jurassic in age), which indicates derivation of quartzose grains from recycled orogen source areas, related to either the Grenville or Pan African granitic-metamorphic belts. Received: 7 January 1997 / Accepted: 25 September 1997     

Arsenite removal from water by electro-coagulation on zinc–zinc and copper–copper electrodes

Removal of arsenite from aqueous solution was carried out using electro-coagulation method. The experiments were conducted using copper–copper and zinc–zinc electrodes. The optimized experimental parameters were 2.0 mg/L initial concentration, 16.0-min processing time, 6.0 pH, 3.0-V applied voltage and 30 °C temperature for zinc–zinc electrodes while these values for copper–copper electrodes were 2.0 mg/L initial concentration, 20.0-min processing time, 7.0 pH, 5.0-V applied voltage and 30 °C temperature. The results demonstrated that zinc–zinc and copper–copper electrodes removed arsenite up to 99.89 and 99.56 %, respectively. The treated water was clear, colorless and odorless without any secondary contamination. There was no change in water quality after the removal of arsenite. The reported method is capable to remove arsenite from water at 6–7 pH range, which is a pH range of natural water. Therefore, this method may be the choice of arsenite removal from natural ground water.     

Fe–Cu deposits in the Kangdian region,SW China: a Proterozoic IOCG (iron-oxide–copper–gold) metallogenic province

Numerous Fe–Cu deposits are hosted in the late Paleoproterozoic Dongchuan and Dahongshan Groups in the Kangdian region, SW China. The Dongchuan Group is composed of siltstone, slate, and dolostone with minor volcanic rocks, whereas the Dahongshan Group has undergone lower amphibolite facies metamorphism and consists of quartz mica-schist, albitite, quartzite, marble and amphibolite with local migmatite. Deposits in the Dongchuan Group are commonly localized in the cores of anticlines, in fault bends and intersections, and at lithological contacts. Orebodies are closely associated with breccias, which are locally derived from the host rocks. Fe-oxides (magnetite and/or hematite) and Cu-sulfides (chalcopyrite, bornite) form disseminated, vein-like and massive ores, and typically fill open spaces in the host rocks. The deposits have extensive albite alteration and local K-feldspar alteration overprinted by quartz, carbonate, sericite and chlorite. Deposits in the Dahongshan Group have orebodies sub-parellel to stratification and show crude stratal partitioning of metals. Fe-oxide ores occur as massive and/or banded replacements within the breccia pipes, whereas Cu-sulfide ores occur predominantly as disseminations and veinlets within mica schists and massive magnetite ores. Ore textures suggest that Cu-sulfides formed somewhat later than Fe-oxides, but are possibly within the same mineralization event. Both ore minerals predated regional Neoproterozoic metamorphism. Both orebodies and host rocks have undergone extensive alteration of albite, scapolite, amphibole, biotite, sericite and chlorite. Silica and carbonate alterations are also widespread. Ore-hosting strata have a LA-ICP-MS zircon U–Pb age of 1681 ± 13 Ma, and a dolerite dyke cutting the Fe-oxide orebodies has an age of 1659 ± 16 Ma. Thus, the mineralization age of the Dahongshan deposit is constrained at between the two. All ores from the two groups have high Fe and low Ti, with variable Cu contents. Locally they are rich in Mo, Co, V, and REE, but all are poor in Pb and Zn. Sulfides from the Fe–Cu deposits have δ³⁴S values mostly in the range of +2 to +6 per mil, suggesting a mix of several sources due to large-scale leaching of the strata with the involvement of evaporites. Isotopic dating and field relationships suggest that these deposits formed in the late Paleoproterozoic. Ore textures, mineralogy and alteration characteristics are typical of IOCG-type deposits and thus define a major IOCG metallogenic province with significant implications for future exploration.     

Porphyry Cu (–Mo–Au) deposits related to melting of thickened mafic lower crust: Examples from the eastern Tethyan metallogenic domain

Most porphyry Cu deposits in the world occur in magmatic arc settings and are formed in association with calc-alkaline arc magmas related to subduction of oceanic lithosphere. This contribution reviews a number of significant porphyry Cu deposits in the eastern Tethyan metallogenic domain. They widely occur in a variety of non-arc settings, varying from post (late)-collisional transpressional and extensional environments to intracontinental extensional environments related to orogenic and anorogenic processes. Their spatial–temporal localization is controlled by strike–slip faults, orogen-transverse normal faults, lineaments and their intersections in these non-arc settings. These deposits are dominated by porphyry Cu–Mo deposits with minor porphyry Cu–Au and epithermal Au deposits, and exhibit a broad similarity with those in magmatic arcs. The associated magmas are generally hydrous, relatively high fO₂, high-K calc-alkaline and shoshonitic, and show geochemical affinity with adakites. They are distinguished from arc magmas and/or oceanic-slab derived adakites, by their occurrence as isolated complexes, high K₂O contents (1.2–8.5%), and much wider range of ε_Nd(t) values(? 10 to + 3) and positive ε_Hf(t) values (+ 4.6 to + 6.9). These potassic magmas are most likely formed by partial melting of thickened juvenile mafic lower-crust or delaminated lower crust, but also involving various amounts of asthenospheric mantle components. Key factors that generate hydrous fertile magmas are most likely crust/mantle interaction processes at the base of thickened lower-crust in non-arc settings, rather than oceanic-slab dehydration (as in arc settings). Breakdown of amphibole in thickened lower crust (e.g., amphibole eclogite and garnet amphibolite) during melting is considered to release fluids into the fertile magmas, leading to an elevated oxidation state and higher H₂O content necessary for development of porphyry Cu–Mo–Au systems. Copper and Au in hydrous magmas are likely derived from mantle-derived components and/or melts, which either previously underplated and infiltrated at the base of the thickened lower crust, or were input into the primitive magmas by melt/mantle interaction. In contrast, Mo and (part of the) S in the fertile magmas are probably supplied by old crust during melting and subsequent ascent.     

E–W strike slip shearing of Kinwat granitoid at South East Deccan Volcanic Province,Kinwat, Maharashtra,India

We study the margin of South East Deccan Volcanic Province around Kinwat lineament, Maharashtra, India, which is NW extension of the Kaddam Fault. Structural field studies document \(\sim \)E–W strike-slip mostly brittle faults from the basement granite. We designate this as ‘Western boundary East Dharwar Craton Strike-slip Zone’ (WBEDCSZ). At local level, the deformation regime from Kinwat, Kaddam Fault, micro-seismically active Nanded and seismically active Killari corroborate with the nearby lineaments. Morphometric analyses suggest that the region is moderately tectonically active. The region of intense strike-slip deformation lies between seismically active fault along Tapi in NW and Bhadrachalam in the SE part of the Kaddam Fault/lineament. The WBEDCSZ with the surface evidences of faulting, presence of a major lineaments and intersection of faults could be a zone of intraplate earthquake.     

Active normal faulting during the 1997 seismic sequence in Colfiorito,Umbria: Did slip propagate to the surface?

In order to determine whether slip during an earthquake on the 26th September 1997 propagated to the surface, structural data have been collected along a bedrock fault scarp in Umbria, Italy. These collected data are used to investigate the relationship between the throw associated with a debated surface rupture (observed as a pale unweathered stripe at the base of the bedrock fault scarp) and the strike, dip and slip-vector. Previous studies have suggested that the surface rupture was produced either by primary surface slip or secondary compaction of hangingwall sediments. Some authors favour the latter because sparse surface fault dip measurements do not match nodal plane dips at depth. It is demonstrated herein that the strike, dip and height of the surface rupture, represented by a pale unweathered stripe at the base of the bedrock scarp, shows a systematic relationship with respect to the geometry and kinematics of faulting in the bedrock. The strike and dip co-vary and the throw is greatest where the strike is oblique to the slip-vector azimuth where the highest dip values are recorded. This implies that the throw values vary to accommodate spatial variation in the strike and dip of the fault across fault plane corrugations, a feature that is predicted by theory describing conservation of strain along faults, but not by compaction. Furthermore, published earthquake locations and reported fault dips are consistent with the analysed surface scarps when natural variation for surface dips and uncertainty for nodal plane dips at depth are taken into account. This implies that the fresh stripe is indeed a primary coseismic surface rupture whose slip is connected to the seismogenic fault at depth. We discuss how this knowledge of the locations and geometry of the active faults can be used as an input for seismic hazard assessment.     

Physical and chemical characteristics of the host rocks in controlling the mineralization of the Chinkuashih high-sulfidation gold–copper deposits,northeastern Taiwan

Epithermal high-sulfidation gold–copper deposits at the Chinkuashih area in northeastern Taiwan occur both within Pleistocene andesite and Miocene sedimentary rocks. Spatially associated Penshan and Shumei deposits of a major gold–copper vein, the “Main Vein”, were both mineralized along an extended normal fault zone. These deposits appear to have formed from the same original hydrothermal fluids, but in different host rock types. However, the results of trace element analyses indicate that the andesite-hosted Penshan deposit has distinctly higher ore-metal and lower LREE contents than the sediment-hosted Shumei deposit. The development of higher grade ore at Penshan deposit resulted from the presence of ferrous Fe-rich minerals in andesite that caused the deposition of a larger amount of pyrite and gold during the sulfidation–reduction reactions of acidic fluid with host rocks. Moreover, the porous–permeable silicic alteration facies of the Penshan deposit provided conduits for the circulation of ore-metal bearing fluids and the trapping of metal-bearing magmatic volatile to precipitate ore minerals. On the other hand, the higher LREE contents of the Shumei open pit reflect the low pH and abundance of mainly SO₄^2? ion in the hydrothermal fluid perhaps because sedimentary host rocks were not able to neutralize and to reduce the acidic fluid effectively through the reactions of fluid and host rocks. Moreover, the Fe-poor host rocks have lower capacity to consume H₂S and precipitate pyrite and gold. In addition, the circulation of ore-metal bearing fluids and trapping of metal-bearing magmatic volatile to precipitate ore minerals could be handicapped by the low permeability and porosity of the silicified sedimentary rocks. It is apparent from these observations that physical and chemical characteristics of host rocks are important factors in controlling the ore grade of the Chinkuashih high-sulfidation gold–copper deposits.     

Application of ASTER data for exploration of porphyry copper deposits: A case study of Daraloo–Sarmeshk area,southern part of the Kerman copper belt,Iran

The Cenozoic Urumieh–Dokhtar Magmatic Belt (UDMB) of Iran is a major host to porphyry Cu ± Mo ± Au deposits (PCDs). Most known PCDs in the UDMB occur in the southern section of the belt, also known as the Kerman Copper Belt (KCB). Three major clusters of PCDs are distinguished in the KCB and include the Miduk, Sarcheshmeh and Daraloo clusters. The Daraloo and Sarmeshk deposits occur in a northwest–southeast-trending fault zone that is characterized by the presence of a narrow zone of alteration–mineralization that contains a series of Oligocene granitoids and Miocene porphyritic tonalite–granodiorite plutons that cut Eocene andesitic lava flows and pyroclastic rocks. Here we use various techniques, including different ratio images, minimum noise fraction, pixel purity index, and matched filter processing to process ASTER data (14 bands) and generate maps that portray the distribution of hydrothermal minerals (e.g., sericite, kaolinite, chlorite, epidote and carbonate) related to PCD alteration zones. In order to validate the ASTER data, follow-up ground proofing and related mineralogical work was done which, in all cases, proved to be positive. The results of this work have identified the regional distribution of hypogene alteration zones (i.e., phyllic, argillic, propylitic and silicic), in addition to areas of secondary Fe-oxide formation, which are coincident with known sites of PCDs. The regional distribution and extent of the alteration zones identified also highlighted the role of regional structures in focusing the mineralizing/altering fluids. These results demonstrate very convincingly that ASTER imagery that uses the appropriate techniques is reliable and robust in mapping out the extent of hydrothermal alteration and lithological units, and can be used for targeting hydrothermal ore deposits, particularly porphyry copper deposits where the alteration footprint is sizeable.     

`Crack–seal', slip: a new fault valve mechanism?

This paper presents a new model of fault development in carbonate rocks involving a crack–seal–slip sequence. The structures of sheared calcite veins from the Les Matelles outcrop, Languedoc (S. France), and the observations used to construct this new model which integrates aspects of `crack–seal' evolution of calcite-filled veins with concepts of fault valve behaviour are described. In our model, hydraulic mode I reopening of an oblique pre-existing vein in an overall strike-slip stress regime is accompanied by precipitation of calcite, but significant fault slip cannot occur initially despite this obliquity because the ends of the pre-existing structure limit further reopening propagation beyond the tips. The rate of aligned calcite precipitation keeps pace with the rate of dilation of the structure, so that calcite cement essentially seals the system. Stress concentrations at the tips are allowed to rise with reopening until failure of the tip zone results in branch crack formation, triggering both slip along the vein and hydraulic pressure drop. This is followed by sealing within the branch cracks. Such a crack–seal–slip cycle may be repeated several times, as evidenced by fault-perpendicular calcite vein growth interlayered with calc-mylonite lamellae within these structures. Later cycles will become less pronounced because strength recovery of the sealed branch cracks does not regain the initial strength of the intact rock. This model could apply at various scales, and could be a mechanism for triggering earthquakes.     

Special Xenoliths in an Aegirine–Augite Syenite Porphyry in Liuhe, Yunnan, China: Discovery and Implications

Three special types of xenoliths have recently been found in an aegirine–augite syenite porphyry in Liuhe, Yunnan, China. Petrographical, petrochemical, electron microprobe, and scanning electron microscopy studies indicate that pure calcite xenocrysts and quartz-bearing topaz pegmatite xenoliths result from the degassing of mantle fluids during their migration, and that black microcrystalline iron-rich silicate-melt xenoliths are the product of the extraction of mantle fluids accompanying degassing and are composed dominantly of quartz, chlorite, and iron-rich columnar and sheet silicate minerals with characteristic minerals, such as native iron, apatite, and zircon. According to the bulk-rock chemical and mineral compositions and crystallization states, the microcrystalline melt xenoliths are not the product of conventional magmatism, and especially the existence of native iron further proves that the xenoliths were mantle fluid materials under reduction or anoxic conditions. The study of the special xenoliths furnishes an important deep-process geochemical background of polymetallic mineralization in different rocks and strata in the study area.