Breccia

Distribution by Scientific Domains
Distribution within Earth and Environmental Science


Selected Abstracts


Tectonic and stratigraphic significance of the Middle Ordovician carbonate breccias in the Ogcheon Belt, South Korea

ISLAND ARC, Issue 3 2002
In-Chang Ryu
Abstract Carbonate breccias occur sporadically in the Lower,Middle Ordovician Maggol Limestone exposed in the Taebacksan Basin in the northeastern part of the northeast,southwest-trending Ogcheon Belt, South Korea. These carbonate breccias have been previously interpreted as intraformational or fault-related breccias. Thus, little attention has been focused on tectonic and stratigraphic significance of these carbonate breccias. The present study, however, indicates that the majority of these carbonate breccias are solution,collapse breccias, which are causally linked to paleokarstification. Carbonate facies analysis in conjunction with conodont biostratigraphy suggests that an overall regression toward the top of the Maggol Limestone probably culminated in subaerial exposure of platform carbonates during the early Middle Ordovician (earliest Darriwilian). Extensive subaerial exposure of platform carbonates resulted in paleokarst-related solution,collapse breccias in the upper Maggol Limestone. This subaerial exposure event is manifested as a major paleokarst unconformity at the Sauk,Tippecanoe sequence boundary elsewhere beneath the Middle Ordovician succession and its equivalents, most notably North America and North China. Due to its global extent, this paleokarst unconformity has been viewed as a product of second- or third-order eustatic sealevel drop during the early Middle Ordovician. Although a paleokarst breccia zone is recognized beneath the Middle Ordovician succession in South Korea, the Sauk,Tippecanoe sequence boundary appears to be a conformable transgressive surface on the top of the paleokarst breccia zone in the upper Maggol Limestone. The paleokarst breccia zone beneath the conformable transgressive surface is represented by a thinning-upward stack of exposure-capped tidal flat-dominated cycles that are closely associated with multiple occurrences of paleokarst-related solution,collapse breccias. This paleokarst breccia zone was a likely consequence of repeated fourth- and fifth-order sealevel fluctuations. It suggests that second- and third-order eustatic sealevel drop may have been significantly tempered by substantial tectonic subsidence near the end of the Maggol deposition. The tectonic subsidence in the basin is also evidenced by the occurrence of coeval off-platform lowstand siliciclastic quartzite lenses as well as debris flow carbonate breccias (i.e. the Yemi Breccia). With the continued tectonic subsidence, subsequent rise in the eustatic cycle caused drowning and deep flooding of carbonate platform, forming a transgressive surface on the top of the paleokarst breccia zone. This tectonic implication contrasts notably with the slowly subsiding carbonate platform model for the basin as has been previously interpreted. Thus, it is proposed that the Taebacksan Basin in the northeastern part of the Ogcheon Belt evolved from a slowly subsiding carbonate platform to a rapidly subsiding intracontinental rift basin during the early Middle Ordovician. The proposed tectonic model in the basin gives much better insight to unravel the stratigraphic response to tectonic evolution of the Ogcheon Belt, which remains an enigmatic feature in formulating a tectonic framework of the Korean peninsula. The present study also provides a good example that the falling part of the eustatic sealevel cycle may not produce a significant event in a rapidly subsiding basin where the rate of eustatic fall always remained lower than the rate of subsidence. [source]


Patterns of damage in igneous and sedimentary rocks under conditions simulating sea-salt weathering

EARTH SURFACE PROCESSES AND LANDFORMS, Issue 1 2003
C. Cardell
Abstract A saline-spray artificial ageing test was used to simulate the effects produced in granites and sedimentary rocks (calcarenites, micrites and breccia) under conditions in coastal environments. Three main points were addressed in this study: the durability of the different kinds of rock to salt decay, the resulting weathering forms and the rock properties involved in the weathering processes. For this, mineralogical and textural characterization of each of the different rocks was carried out before and after the test. The soluble salt content at different depths from the exposed surfaces was also determined. Two different weathering mechanisms were observed in the granite and calcareous rocks. Physical processes were involved in the weathering of granite samples, whereas dissolution of calcite was also involved in the deterioration of the calcareous rocks. We also showed that microstructural characteristics (e.g. pore size distribution), play a key role in salt damage, because of their influence on saline solution transport and on the pressures developed within rocks during crystallization. Copyright © 2003 John Wiley & Sons, Ltd. [source]


Rockshelter sedimentation in a dynamic tropical landscape: Late Pleistocene,Early Holocene archaeological deposits in Kitulgala Beli-lena, southwestern Sri Lanka

GEOARCHAEOLOGY: AN INTERNATIONAL JOURNAL, Issue 6 2009
Nikos Kourampas
Kitulgala Beli-lena, a rockshelter in gneiss in humid tropical southwestern Sri Lanka, was inhabited by Late Pleistocene,Early Holocene (>31,000,7880 B.P.) hunter-gatherers who made geometric microliths and exploited rainforest resources. Micromorphological analysis of a ca. 3-m-thick succession of diamictic loams, clays, and breccia with cultural content suggests that relative contribution of episodic colluviation and roof fall, water seepage through joints and diverse human activity varied through time. Facies changes across the profile reflect monsoon weakening ca. 20,000,16,000 cal B.P. and abrupt intensification ca. 15,700 cal B.P., on the wane of the global Last Glacial Maximum (LGM). Post-depositional modification included clay, sesquioxide, and minor phosphate translocation; termite and other arthropod bioturbation; and clast weathering on the rockshelter floor. Human input (tools and tool-making refuse, reworked charcoal and associated combustion by-products) is markedly higher in sediments younger than ca. 15,700 cal B.P., suggesting intensification of site use immediately after the LGM. © 2009 Wiley Periodicals, Inc. [source]


The formation and sedimentary infilling of the Cave of Hearths and Historic Cave complex, Makapansgat, South Africa

GEOARCHAEOLOGY: AN INTERNATIONAL JOURNAL, Issue 4 2004
A.G. Latham
The archaeology of caves is best served by including a study of natural effects prior to and during anthropogenic input. This is especially true for the Cave of Hearths because not only has erosion determined the area of occupation, but also subsequent undermining has caused collapse of some of the rearward parts of the site during Early Stone Age (Acheulian) and later times; and this had a major impact on excavation. The key to understanding the nature of the collapsed layers was the rediscovery of a lower part of the cavern below the whole site. This lower cavern is no longer accessible, but the evidence for it was revealed in a swallow hole by R.J. Mason, and in archived material at the Department of Archaeology, University of Witwatersrand. The creation and dissolution of dolomite fragments in the upper layers has resulted in the formation of thick, carbonate-cemented breccia that has preserved underlying layers and prevented further collapse. We agree with Mason that further archaeological and hominid finds await excavation under the proximate Historical Cave west entrance. This area has the potential for archaeological and palaeoanthropological material that predates the layers in the Cave of Hearths. © 2004 Wiley Periodicals, Inc. [source]


A simulation of the hydrothermal response to the Chesapeake Bay bolide impact

GEOFLUIDS (ELECTRONIC), Issue 3 2005
W. E. SANFORD
Abstract Groundwater more saline than seawater has been discovered in the tsunami breccia of the Chesapeake Bay Impact Crater. One hypothesis for the origin of this brine is that it may be a liquid residual following steam separation in a hydrothermal system that evolved following the impact. Initial scoping calculations have demonstrated that it is feasible such a residual brine could have remained in the crater for the 35 million years since impact. Numerical simulations have been conducted using the code HYDROTHERM to test whether or not conditions were suitable in the millennia following the impact for the development of a steam phase in the hydrothermal system. Hydraulic and thermal parameters were estimated for the bedrock underlying the crater and the tsunami breccia that fills the crater. Simulations at three different breccia permeabilities suggest that the type of hydrothermal system that might have developed would have been very sensitive to the permeability. A relatively low breccia permeability (1 × 10,16 m2) results in a system partitioned into a shallow water phase and a deeper superheated steam phase. A moderate breccia permeability (1 × 10,15 m2) results in a system with regionally extensive multiphase conditions. A relatively high breccia permeability (1 × 10,14 m2) results in a system dominated by warm-water convection cells. The permeability of the crater breccia could have had any of these values at given depths and times during the hydrothermal system evolution as the sediments compacted. The simulations were not able to take into account transient permeability conditions, or equations of state that account for the salt content of seawater. Results suggest, however, that it is likely that steam conditions existed at some time in the system following impact, providing additional evidence that is consistent with a hydrothermal origin for the crater brine. [source]


Canopy recovery after drought dieback in holm-oak Mediterranean forests of Catalonia (NE Spain)

GLOBAL CHANGE BIOLOGY, Issue 12 2004
Francisco Lloret
Abstract Climate change is likely to produce more frequent and longer droughts in the Mediterranean region, like that of 1994, which produced important changes in the Quercus ilex forests, with up to 76% of the trees showing complete canopy dieback. At the landscape level, a mosaic of responses to the drought was observed, linked to the distribution of lithological substrates. Damage to the dominant tree species (Q. ilex) and the most common understorey shrub (Erica arborea) was more noticeable on the compact substrates (breccia) than on the fissured ones (schist). This result was consistent with observations documenting deeper root penetration in schist than in breccia materials, allowing the plants growing on fissured substrates to use water from deeper soil levels. Smaller plants were more vulnerable to drought than larger plants in the trees, but not in the shrubs. Overall, Q. ilex was more affected than E. arborea. The resilience of the system was evaluated from the canopy recovery 1 year after the episode. Stump and crown resprouting was fairly extensive, but the damage pattern in relation to substrate, plant size, and species remained similar. The effect of recurrent drought episodes was studied on vegetation patches of Q. ilex located on mountain slopes and surrounded by bare rock. We observed that plants that resprouted weakly after a previous drought in 1985 were more likely to die or to produce poor regeneration in 1995 than plants that had resprouted vigorously. Vegetation patches located on the lower part of the slope were also less damaged than patches situated uphill. The study provides evidence of relevant changes in forest canopy as a consequence of extreme climate events. The distribution of this effect across the landscape is mediated by lithological substrate, causing patchy patterns. The results also support the hypothesis that recurrent droughts can produce a progressive loss of resilience, by depleting the ability of surviving plants to regenerate. [source]


In situ hydraulic tests in the active fault survey tunnel, Kamioka Mine, excavated through the active Mozumi-Sukenobu Fault zone and their hydrogeological significance

ISLAND ARC, Issue 4 2006
Tsuyoshi Nohara
Abstract The spatial hydrogeological and structural character of the active Mozumi-Sukenobu Fault (MSF) was investigated along a survey tunnel excavated through the MSF in the Kamioka Mine, central Japan. Major groundwater conduits on both sides of the MSF are recognized. One is considered to be a subvertical conduit between the tunnel and the surface, and the other is estimated to be a major reservoir of old meteoric water alongside the MSF. Our studies indicate that part of the MSF is a sub-vertical continuous barrier that obstructs younger meteoric water observed in the south-eastern part of the Active Fault Survey Tunnel (AFST) and water recharge to the rock mass intersected by the north-western part of the AFST. The MSF might be a continuous barrier resulting in the storage of a large quantity of older groundwater to the northwest. The observations and results of in situ hydraulic tests indicate that the major reservoir is not the fault breccia associated with the northeast,southwest trending faults of the MSF, but the zone in which blocks of fractured rocks occur beside high angle faults corresponding to X shears whose tectonic stress field coincides with the present regional stress field and antithetic Riedel shears of the MSF. The results from borehole investigations in the AFST indicate that secondary porosity is developed in the major reservoir due to the destruction of filling minerals and fracture development beside these shears. The increase in hydraulic conductivity is not directly related to increased density of fractures around the MSF. Development of secondary porosity could cause the increase in hydraulic conductivity around the MSF. Our results suggest that minor conduits of the fracture network are sporadically distributed in the sedimentary rocks around the MSF in the AFST. [source]


Fault rock analysis of the northern part of the Chelungpu Fault and its relation to earthquake faulting of the 1999 Chi-Chi earthquake, Taiwan

ISLAND ARC, Issue 1 2005
Kohtaro UjiieArticle first published online: 3 MAR 200
Abstract The 1999 Chi-Chi earthquake in Taiwan (Mw = 7.6) produced a surface rupture along the north,south-striking Chelungpu thrust fault with pure dip-slip (east side up) and left lateral strike-slip displacements. Near-field strong-motion data for the northern part of the fault illustrate a distinct lack of the high-frequency seismic radiation associated with a large slip (10,15 m) and a rapid slip velocity (2,4 m/s), suggesting a smooth seismic slip associated with low dynamic frictional resistance on the fault. A drillhole was constructed at shallow depths in the possible fault zones of the northern part of the Chelungpu Fault, which may have slipped during the 1999 earthquake. One of the zones consists of a 20-cm-thick, unconsolidated fault breccia with a chaotic texture lacking both discrete slip surfaces (e.g. Riedel shears) and grain crushing. Other possible fault zones are marked by the narrow (less than a few centimeters) gouge zone in which clayey material intrudes into the damaged zone outside of the gouge zone. These characteristic fault rock textures suggest that the slip mechanisms at shallow levels during the earthquake involved either granular flow of initially unconsolidated material or slip localization under elevated pore pressure along the narrow clayey gouge zone. Because both mechanisms lead to low dynamic frictional resistance on the fault, the rapid seismic slip in the deep portions of the fault (i.e. the source region of strong-motion radiation) could have been accommodated by frictionless slip on the shallow portions of the fault. The combination of strong-motion data and fault rock analysis suggests that smooth slip associated with low dynamic friction occurred on both the deep and shallow portions of the fault, resulting in a large slip between the source region and the surface in the northern region. [source]


Hardened foliated fault gouge from the Nojima Fault zone at Hirabayashi: Evidence for earthquake lightning accompanying the 1995 Kobe earthquake?

ISLAND ARC, Issue 3-4 2001
Yuji Enomoto
Abstract Two anomalous features were found in the Nojima Fault zone at Hirabayashi in Awaji Island, south-west Japan: (i) hard foliated gouge between weathered granitic fault breccia and weakly consolidated mudstone of the Osaka Group; and (ii) mudstone near the gouge showing anomalous magnetization behavior. Roots of herbaceous vegetation near the foliated gouge were extraordinarily charred. In order to understand the nature of the gouge, shallow drillings were made to a depth of 3,14 m across the fault zone. Various physicochemical measurements of the gouge at depths and charred roots of herbaceous vegetation were conducted. The main results were: (i) Using electron spin resonance (ESR) analysis, the carbon radical peak (g = 2.006) of the charred roots was found to be 25 times larger than that of the non-charred roots of the same vegetation taken near the fault, indicating that the charred roots were subjected to baking; (ii) the hard foliated gouge clearly showed a lamellar structure consisting alternately of gray and black layers; (iii) the black layers in most of the foliated gouge showed flow structures almost parallel to the fault, but the gray layers rarely showed flow patterns; (iv) natural remanent magnetization (NRM) of the foliated gouge was 430 times greater than that of the granitic fault breccia and approximately 70 times greater than that of the mudstone; (v) the NRM intensity of the mudstone near the fault was highest near the ground level and decreased as the depth increased, although the magnetic susceptibility of the mudstone was almost constant and independent of depth; (vi) the high-coe civity magnetization component vectors of both the mudstone and the foliated gouge in a Schmidt equal-area projection was quite different from that of the present direction of the Earth's field; and (vii) using a magnetic force microscope, intense magnetic force lines were found in the black parts of the foliated gouge. It is suggested that these anomalies were possibly caused by earthquake lightning that accompanied the 1995 Kobe earthquake. In a spark plasma sintering test, which was conducted to simulate the possibility of earthquake lightning-induced sintering of the gouge, weakly altered gouge was successfully sintered within 10 s. The hardness of sintered sample was comparable to that of the hard foliated gouge. [source]


Zeolites in fissures of granites and gneisses of the Central Alps

JOURNAL OF METAMORPHIC GEOLOGY, Issue 8 2010
T. WEISENBERGER
Abstract Six different Ca-zeolite minerals are widespread in various assemblages in late fissures and fractures in granites and gneisses of the Swiss Alps. The zeolites formed as a result of water,rock interaction at relatively low temperatures (<250 °C) in the continental upper crust. The zeolites typically overgrow earlier minerals of the fissure assemblages, but zeolites also occur as monomineralic fissure fillings. They represent the youngest fissure minerals formed during uplift and exhumation of the Alpine orogen. A systematic study of zeolite samples showed that the majority of finds originate from three regions particularity rich in zeolite-bearing fissures: (i) in the central and eastern part of the Aar- and Gotthard Massifs; (2) Gibelsbach/Fiesch, in a fissure breccia located at the boundary of Aar Massif and Permian sedimentary rocks; and (3) in Penninic gneisses of the Simano nappe at Arvigo (Val Calanca). Rail and road tunnel construction across the Aar- and Gotthard Massif provided excellent data on zeolite frequency in Alpine fissures. It was found that 32% (Gotthard NEAT rail base tunnel, Amsteg section) and 18% (Gotthard road tunnel) of all studied fissures are filled with zeolites. The number of different zeolites is limited to six species: laumontite, stilbite and scolecite are abundant and common, whereas heulandite, chabazite and epistilbite occur occasionally. Calcium is the dominant extra-framework cation, with minor K and Na. Heulandite and chabazite contain Sr up to 29 and 10 mol.% extra-framework cations respectively. Na and K contents in zeolites tend to increase during growth as a result of changes in fluid composition and/or temperature. The K enrichment of stilbite found in surface outcrops compared to subsurface samples may indicate late stage cation exchange with surface water. Texture data, relative age sequences derived from fissure assemblages and equilibrium calculations show that the Ca-dominated zeolites precipitated from fluid with decreasing temperature in the order (old to young = hot to cold): scolecite, laumontite, heulandite, chabazite and stilbite. The necessary components for zeolite formation are derived from dissolving primary granite and gneiss minerals. The nature of these minerals depends, among other factors, on the metamorphic history of the host rock. Zeolites in the Aar Massif derived from the dissolution of epidote, secondary calcite and albite that were originally formed during Alpine greenschist metamorphism from primary granite and gneiss assemblages. Zeolite fissures occur in areas of H2O-dominated fluids. This is consistent with equilibrium calculations that predict a low CO2 tolerance of zeolite assemblages, particularly at low temperature. [source]


CYLINDRICAL AND CONICAL FOLD GEOMETRIES IN THE CANTARELL STRUCTURE, SOUTHERN GULF OF MEXICO: IMPLICATIONS FOR HYDROCARBON EXPLORATION

JOURNAL OF PETROLEUM GEOLOGY, Issue 3 2006
J. J. Mandujano V.
The NW-SE trending Cantarell structure in the Gulf of Campeche hosts the largest oilfield in Mexico. The oil occurs predominantly in latest Cretaceous , earliest Tertiary breccias with subsidiary reserves in Upper Jurassic (Oxfordian and Kimmeridgian) and Lower Cretaceous oolitic and partially dolomitized limestones, dolomites and shaly limestones. Cantarell has been interpreted both as a fold-and-thrust zone and as a dextral transpressional structure. Analysis of structure contours at 100m intervals, on the tops of the Tertiary breccia and the Kimmeridgian (Upper Jurassic) dolomite, indicates that the structure is an upright cylindrical fold with gently plunging conical terminations; there is also a conical portion in the central part of the structure. The axes of the central, NW and SE cones are subvertical. This geometry indicates that the two fold terminations and the central cone are aprons rather than points, with the NW and central cone axes intersecting the cylindrical fold axis at the point where the geometry switches from conical to cylindrical. The apical angle (i.e. the angle between the fold and cone axes) varies as follows: (i) in the NW cone, it is ,70° in the breccia and ,76° in the Kimmeridgian dolomite; (ii) in the central cone, it is ,77° in the breccia and ,73° in the Kimmeridgian dolomite; and (iii) in the SE cone, it is ,64° in the breccia and ,57° in the Kimmeridgian dolomite. This indicates that whereas the fold opens with depth in the NW cone, it tightens with depth in the central and SE cones. Assuming a parallel fold geometry, these apical angles indicate an increase in volume in the NW cone (i.e. larger hydrocarbon reservoirs), compared to the central and SE cones. Theoretical considerations indicate that the curvature increases dramatically towards the point of the cone. In the case of the Cantarell structure, the apices of the cones are located at the conical-cylindrical fold junctions, where the highest curvature may have resulted in a higher degree of fracturing. The coincidence of maximum curvature and the intersection of the conical and cylindrical fold axes in the fold culminations with porous and permeable reservoir rocks may have made these locations favourable for the accumulation of hydrocarbons. [source]


Homogeneity of fossil assemblages extracted from mine dumps: an analysis of Plio-Pleistocene fauna from South African caves

LETHAIA, Issue 4 2005
FRANK SÉNÉGAS
Mine dumps associated with limestone cave deposits are common in dolomitic areas of southern Africa. The dumps often contain blocks of breccia, which are rich in micro-mammalian fossils (especially rodents, shrews and bats). Unfortunately, these fossiliferous breccia blocks are out of geological and stratigraphic context. Nevertheless, they provide a large amount of palaeontological material of great interest. In order to use this kind of material, a first approach is to test for homogeneity of the fossil assemblages extracted from the breccia blocks. Fisher's exact test can be used. Two analyses were undertaken. The first was performed on block samples taken in situ from breccia at the Drimolen hominid site. The results indicated that the samples were homogenous, as expected. The second analysis was carried out on different samples extracted from blocks of breccia collected from a dump at the Gondolin site. The results show that it is possible to group several samples in a single representative assemblage. Some blocks could be grouped together and then used to address taphonomic issues. Once these problems are solved, the data set can be used with greater confidence to address matters concerning palaeoenvironmental reconstructions associated with Plio-Pleistocene hominids. [source]


Petrography, mineralogy, and trace element geochemistry of lunar meteorite Dhofar 1180

METEORITICS & PLANETARY SCIENCE, Issue 9 2009
Aicheng Zhang
Dhofar 1180 is predominantly composed of fine-grained matrix with abundant mineral fragments and a few lithic and glassy clasts. Lithic clasts show a variety of textures including cataclastic, gabbroic, granulitic, ophitic/subophitic, and microporphyritic. Both feldspathic and mafic lithic clasts are present. Most feldspathic lithic clasts have a strong affinity to ferroan anorthositic suite rocks and one to magnesian suite rocks. Mafic lithic clasts are moderately to extremely Fe-rich. The Ti/[Ti+Cr]-Fe/[Fe+Mg] compositional trend of pyroxenes in mafic lithic clasts is consistent with that of low-Ti mare basalts. Glasses display a wide chemical variation from mafic to feldspathic. Some glasses are very similar to those from Apollo 16 soils. KREEP components are essentially absent in Dhofar 1180. One glassy clast is rich in K, REE and P, but its Mg/[Mg+Fe] is very low (0.25). It is probably a last-stage differentiation product of mare basalt. Molar Fe/Mn ratios of both olivine and pyroxene are essentially consistent with a lunar origin. Dhofar 1180 has a LREE-enriched (La 18 × CI, Sm 14 × CI) pattern with a small positive Eu anomaly (Eu 15 × CI). Th concentration is 0.7 ppm in Dhofar 1180. Petrography, mineralogy, and trace element geochemistry of Dhofar 1180 are different from those of other lunar meteorites, indicating that Dhofar 1180 represents a unique mingled lunar breccia derived from an area on the lunar nearside but far away from the center of the Imbrium Basin. [source]


Shocked quartz grains in the polymict breccia of the Granby structure, Sweden,-Verification of an impact

METEORITICS & PLANETARY SCIENCE, Issue 8 2009
Carl Alwmark
In this study, drill core samples of a sedimentary breccia from the Granby structure have been investigated for microscopic shock metamorphic evidence in an attempt to verify the impact genesis of the structure. The finding of multiple sets of decorated planar deformation features (PDFs) in quartz grains in these samples provides unambiguous evidence that the structure is impact derived. Furthermore, the orientation of the PDFs, e.g., , {101}, , {101} and r, z {101}, is characteristic for impact deformation. The fact that a majority of the PDFs are decorated implies a water-bearing target. The shocked quartz grains can be divided into two groups; rounded grains found in the breccia matrix likely originated from mature sandstone, and angular grains in fragments from crystalline target rocks. The absence of melt particles provides an estimated maximum shock pressure for the sedimentary derived quartz of 15,20 GPa and the frequency distribution of PDF orientations in the bedrock quartz implies pressures of the order of 10 GPa. [source]


Archaeabacterial lipids in drill core samples from the Bosumtwi impact structure, Ghana

METEORITICS & PLANETARY SCIENCE, Issue 11 2008
Marina ESCALA
The Bosumtwi crater in Ghana (West Africa) is a relatively young (1.07 Myr) structure with a rim-to-rim diameter of about 10.5 km. In a preliminary study targeting the subsurface microbial life in the impact structure, seven samples of the impact breccia from the central uplift of the Bosumtwi crater were analyzed for the presence of typical archaeal membrane-lipids (GDGTs). These have been detected in four of the samples, at a maximum depth of 382 m below the lake surface, which is equivalent to 309 m below the surface sediment. The concentration of the GDGTs does not show a trend with depth, and their distribution is dominated by GDGT-0. Possible origins of these lipids could be related to the soils or rocks predating the impact event, the hydrothermal system generated after the impact, or due to more recent underground water [source]


An international and multidisciplinary drilling project into a young complex impact structure: The 2004 ICDP Bosumtwi Crater Drilling Project,An overview

METEORITICS & PLANETARY SCIENCE, Issue 4-5 2007
Christian KOEBERL
It is the source crater of the Ivory Coast tektites. The structure was excavated in 2.1,2.2 Gyr old metasediments and metavolcanics of the Birimian Supergroup. A drilling project was conceived that would combine two major scientific interests in this crater: 1) to obtain a complete paleoenvironmental record from the time of crater formation about one million years ago, at a near-equatorial location in Africa for which very few data are available so far, and 2) to obtain a complete record of impactites at the central uplift and in the crater moat, for ground truthing and comparison with other structures. Within the framework of an international and multidisciplinary drilling project led by the International Continental Scientific Drilling Program (ICDP), 16 drill cores were obtained from June to October 2004 at six locations within Lake Bosumtwi, which is 8.5 km in diameter. The 14 sediment cores are currently being investigated for paleoenvironmental indicators. The two impactite cores LB-07A and LB-08A were drilled into the deepest section of the annular moat (540 m) and the flank of the central uplift (450 m), respectively. They are the main subject of this special issue of Meteoritics & Planetary Science, which represents the first detailed presentations of results from the deep drilling into the Bosumtwi impactite sequence. Drilling progressed in both cases through the impact breccia layer into fractured bedrock. LB-07A comprises lithic (in the uppermost part) and suevitic impact breccias with appreciable amounts of impact melt fragments. The lithic clast content is dominated by graywacke, besides various metapelites, quartzite, and a carbonate target component. Shock deformation in the form of quartz grains with planar microdeformations is abundant. First chemical results indicate a number of suevite samples that are strongly enriched in siderophile elements and Mg, but the presence of a definite meteoritic component in these samples cannot be confirmed due to high indigenous values. Core LB-08A comprises suevitic breccia in the uppermost part, followed with depth by a thick sequence of graywacke-dominated metasediment with suevite and a few granitoid dike intercalations. It is assumed that the metasediment package represents bedrock intersected in the flank of the central uplift. Both 7A and 8A suevite intersections differ from suevites outside of the northern crater rim. Deep drilling results confirmed the gross structure of the crater as imaged by the pre-drilling seismic surveys. Borehole geophysical studies conducted in the two boreholes confirmed the low seismic velocities for the post-impact sediments (less than 1800 m/s) and the impactites (2600,3300 m/s). The impactites exhibit very high porosities (up to 30 vol%), which has important implications for mechanical rock stability. The statistical analysis of the velocities and densities reveals a seismically transparent impactite sequence (free of prominent internal reflections). Petrophysical core analyses provide no support for the presence of a homogeneous magnetic unit (= melt breccia) within the center of the structure. Borehole vector magnetic data point to a patchy distribution of highly magnetic rocks within the impactite sequence. The lack of a coherent melt sheet, or indeed of any significant amounts of melt rock in the crater fill, is in contrast to expectations from modeling and pre-drilling geophysics, and presents an interesting problem for comparative studies and requires re-evaluation of existing data from other terrestrial impact craters, as well as modeling parameters. [source]


Drill core LB-08A, Bosumtwi impact structure, Ghana: Geochemistry of fallback breccia and basement samples from the central uplift

METEORITICS & PLANETARY SCIENCE, Issue 4-5 2007
Ludovic FERRIČRE
Two boreholes were drilled to acquire hard-rock samples of the deep crater moat and from the flank of the central uplift (LB-07A and LB-08A, respectively) during a recent ICDP-sponsored drilling project. Here we present results of major and trace element analysis of 112 samples from drill core LB-08A. This core, which was recovered between 235.6 and 451.33 m depth below lake level, contains polymict lithic breccia intercalated with suevite, which overlies fractured/brecciated metasediment. The basement is dominated by meta-graywacke (from fine-grained to gritty), but also includes some phyllite and slate, as well as suevite dikelets and a few units of a distinct light greenish gray, medium-grained meta-graywacke. Most of the variations of the major and trace element abundances in the different lithologies result from the initial compositional variations of the various target rock types, as well as from aqueous alteration processes, which have undeniably affected the different rocks. Suevite from core LB-08A (fallback suevite) and fallout suevite samples (from outside the northern crater rim) display some differences in major (mainly in MgO, CaO, and Na2O contents) and minor (mainly Cr and Ni) element abundances that could be related to the higher degree of alteration of fallback suevites, but also result from differences in the clast populations of the two suevite populations. For example, granite clasts are present in fallout suevite but not in fallback breccia, and calcite clasts are present in fallback breccia and not in fallout suevite. Chondrite-normalized rare earth element abundance patterns for polymict impact breccia and basement samples are very similar to each other. Siderophile element contents in the impact breccias are not significantly different from those of the metasediments, or compared to target rocks from outside the crater rim. So far, no evidence for a meteoritic component has been detected in polymict impact breccias during this study, in agreement with previous work. [source]


Integrated 3-D model from gravity and petrophysical data at the Bosumtwi impact structure, Ghana

METEORITICS & PLANETARY SCIENCE, Issue 4-5 2007
Hernan UGALDE
A vast amount of geoscience data is available from the pre-site surveys and the actual drilling phase. A 3-D gravity model was constructed and calibrated with the available data from the two ICDP boreholes, LB-07A and LB-08A. The 3-D gravity model results agree well with both the sediment thickness and size of the central uplift revealed by previously collected seismic data, and with the petrophysical data from the LB-08A and LB-07A core materials and the two borehole logs. Furthermore, the model exhibits lateral density variations across the structure and refines the results from previous 2.5-D modeling. An important new element of the 3-D model is that the thickness of the intervals comprising polymict lithic impact breccia and suevite, monomict lithic breccia and fractured basement is much smaller than that predicted by numerical modeling. [source]


Chesapeake Bay impact structure: Morphology, crater fill, and relevance for impact structures on Mars

METEORITICS & PLANETARY SCIENCE, Issue 10 2006
J. Wright Horton Jr.
It provides an accessible analog for studying impact processes in layered and wet targets on volatile-rich planets. The CBIS formed in a layered target of water, weak clastic sediments, and hard crystalline rock. The buried structure consists of a deep, filled central crater, 38 km in width, surrounded by a shallower brim known as the annular trough. The annular trough formed partly by collapse of weak sediments, which expanded the structure to ,85 km in diameter. Such extensive collapse, in addition to excavation processes, can explain the "inverted sombrero" morphology observed at some craters in layered targets. The distribution of crater-fill materials in the CBIS is related to the morphology. Suevitic breccia, including pre-resurge fallback deposits, is found in the central crater. Impact-modified sediments, formed by fluidization and collapse of water-saturated sand and silt-clay, occur in the annular trough. Allogenic sediment-clast breccia, interpreted as ocean-resurge deposits, overlies the other impactites and covers the entire crater beneath a blanket of postimpact sediments. The formation of chaotic terrains on Mars is attributed to collapse due to the release of volatiles from thick layered deposits. Some flat-floored rimless depressions with chaotic infill in these terrains are impact craters that expanded by collapse farther than expected for similar-sized complex craters in solid targets. Studies of crater materials in the CBIS provide insights into processes of crater expansion on Mars and their links to volatiles. [source]


Halite and stable chlorine isotopes in the Zag H3,6 breccia

METEORITICS & PLANETARY SCIENCE, Issue 5 2004
J. C. Bridges
The purity of the associated NaCl-H2O brine is implied by freezing characteristics of fluid inclusions in the halite and EPMA analyses together with a lack of other evaporite-like phases in the Zag H3,6 component. This is inconsistent with multi-stage evolution of the fluid involving scavenging of cations in the Zag region of the parent body. We suggest that the halite grains are clastic and did not crystallize in situ. Halite and water-soluble extracts from Zag have light Cl isotopic compositions, ,37Cl = ,1.4 to ,2.8%. Previously reported bulk carbonaceous chondrite values are approximately ,37Cl = +3 to +4%. This difference is too great to be the result of fractionation during evaporation, and instead, we suggest that Cl isotopes in chondrites are fractionated between a light reservoir associated with fluids and a heavier reservoir associated with higher temperature phases such as phosphates and silicates. Extraterrestrial carbon released at 600 °C from the H3,4 matrix has ,13C = ,20%, consistent with poorly graphitized material being introduced into the matrix rather than indigenous carbonate derived from a brine. We have also examined 28 other H chondrite falls to ascertain how widespread halite or evaporite-like mineral assemblages are in ordinary chondrites. We did not find any more to add to Zag (H3-6) and Monahans (H5), which suggests that such highly soluble phases were not usually preserved on the parent bodies. [source]


The importance of being cratered: The new role of meteorite impact as a normal geological process

METEORITICS & PLANETARY SCIENCE, Issue 2 2004
Bevan M. French
It also identifies some exciting scientific challenges for future investigators: to determine the full range of impact effects preserved on the Earth, to apply the knowledge obtained from impact phenomena to more general geological problems, and to continue the merger of the once exotic field of impact geology with mainstream geosciences. Since the recognition of an impact event at the Cretaceous-Tertiary (K-T) boundary, much current activity in impact geology has been promoted by traditionally trained geoscientists who have unexpectedly encountered impact effects in the course of their work. Their studies have involved: 1) the recognition of additional major impact effects in the geological record (the Chesapeake Bay crater, the Alamo breccia, and multiple layers of impact spherules in Precambrian rocks); and 2) the use of impact structures as laboratories to study general geological processes (e.g., igneous petrogenesis at Sudbury, Canada and Archean crustal evolution at Vredefort, South Africa). Other research areas, in which impact studies could contribute to major geoscience problems in the future, include: 1) comparative studies between low-level (,7 GPa) shock deformation of quartz, and the production of quartz cleavage, in both impact and tectonic environments; and 2) the nature, origin, and significance of bulk organic carbon ("kerogen") and other carbon species in some impact structures (Gardnos, Norway, and Sudbury, Canada). [source]


Geology and geochemistry of shallow drill cores from the Bosumtwi impact structure, Ghana

METEORITICS & PLANETARY SCIENCE, Issue 8 2003
Daniel Boamah
The interior of the structure is largely filled by the 8 km diameter Lake Bosumtwi, and the crater rim and region in the environs of the crater is covered by tropical rainforest, making geological studies rather difficult and restricted to road cuts and streams. In early 1999, we undertook a shallow drilling program to the north of the crater rim to determine the extent of the ejecta blanket around the crater and to obtain subsurface core samples for mineralogical, petrological, and geochemical studies of ejecta of the Bosumtwi impact structure. A variety of impactite lithologies are present, consisting of impact glassrich suevite and several types of breccia: lithic breccia of single rock type, often grading into unbrecciated rock, with the rocks being shattered more or less in situ without much relative displacement (autochthonous?), and lithic polymict breccia that apparently do not contain any glassy material (allochtonous?). The suevite cores show that melt inclusions are present throughout the whole length of the cores in the form of vesicular glasses with no significant change of abundance with depth. Twenty samples from the 7 drill cores and 4 samples from recent road cuts in the structure were studied for their geochemical characteristics to accumulate a database for impact lithologies and their erosion products present at the Bosumtwi crater. Major and trace element analyses yielded compositions similar to those of the target rocks in the area (graywacke-phyllite, shale, and granite). Graywacke-phyllite and granite dikes seem to be important contributors to the compositions of the suevite and the road cut samples (fragmentary matrix), with a minor contribution of Pepiakese granite. The results also provide information about the thickness of the fallout suevite in the northern part of the Bosumtwi structure, which was determined to be ,15 m and to occupy an area of ,1.5 km2. Present suevite distribution is likely to be caused by differential erosion and does not reflect the initial areal extent of the continuous Bosumtwi ejecta deposits. Our studies allow a comparison with the extent of the suevite at the Ries, another well-preserved impact structure. [source]


KREEPy lunar meteorite Dhofar 287A: A new lunar mare basalt

METEORITICS & PLANETARY SCIENCE, Issue 4 2003
Mahesh Anand
The main portion of this meteorite (Dho 287A) consists of a mare basalt, while a smaller portion of breccia (Dho 287B) is attached on the side. Dho 287A is only the fourth crystalline mare basalt meteorite found on Earth to date and is the subject of the present study. The basalt consists mainly of phenocrysts of olivine and pyroxene set in a finer-grained matrix, which is composed of elongated pyroxene and plagioclase crystals radiating from a common nucleii. The majority of olivine and pyroxene grains are zoned, from core to rim, in terms of Fe and Mg. Accessory minerals include ilmenite, chromite, ulvöspinel, troilite, and FeNi metal. Chromite is invariably mantled by ulvöspinel. This rock is unusually rich in late-stage mesostasis, composed largely of fayalite, Si-K-Ba-rich glass, fluorapatite, and whitlockite. In texture and mineralogy, Dho 287A is a low-Ti mare basalt, with similarities to Apollo 12 (A-12) and Apollo 15 (A-15) basalts. However, all plagioclase is now present as maskelynite, and its composition is atypical for known low-Ti mare basalts. The Fe to Mn ratios of olivine and pyroxene, the presence of FeNi metal, and the bulk-rock oxygen isotopic ratios, along with several other petrological features, are evidence for the lunar origin for this meteorite. Whole-rock composition further confirms the similarity of Dho 287A with A-12 and A-15 samples but requires possible KREEP assimilation to account for its rare-earth-element (REE) contents. Cooling-rate estimates, based on Fo zonation in olivine, yield values of 0.2,0.8°C/hr for the lava, typical for the center of a 10,20 m thick flow. The recalculated major-element concentrations, after removing 10,15% modal olivine, are comparable to typical A-15 mare basalts. Crystallization modeling of the recalculated Dho 287A bulk-composition yields a reasonable fit between predicted and observed mineral abundances and compositions. [source]


Lunar regolith breccia Dhofar 287B: A record of lunar volcanism

METEORITICS & PLANETARY SCIENCE, Issue 4 2003
S. I. Demidova
The present study is directed mainly at the breccia portion of this meteorite. This breccia consists of a variety of lithic clasts and mineral fragments set in a fine-grained matrix and minor impact melt. The majority of clasts and minerals appear to have been mainly derived from the low-Ti basalt suite, similar to that of Dho 287A. Very low-Ti (VLT) basalts are a minor lithology of the breccia. These are significantly lower in Mg# and slightly higher in Ti compared to Luna 24 and Apollo 17 VLT basalts. Picritic glasses constitute another minor component of the breccia and are compositionally similar to Apollo 15 green glasses. Dho 287B also contains abundant fragments of Mg-rich pyroxene and anorthite-rich plagioclase grains that are absent in the lithic clasts. Such fragments appear to have been derived from a coarse-grained, Mg#-rich, Na-poor lithology. A KREEP component is apparent in chemistry, but no highlands lithologies were identified. The Dho 287 basaltic lithologies cannot be explained by near-surface fractionation of a single parental magma. Instead, magma compositions are represented by a picritic glass; a low-Ti, Na-poor glass; and a low-Ti, Na-enriched source (similar to the Dho 287A parental melt). Compositional differences among parent melts could reflect inhomogeneity of the lunar mantle. Alternatively, the low-Ti, Na-poor, and Dho 287A parent melts could be of hybrid compositions, resulting from assimilation of KREEP by picritic magma. Thus, the Dho 287B breccia contains lithologies from multiple magmatic eruptions, which differed in composition, formational conditions, and cooling histories. Based on this study, the Dho 287 is inferred to have been ejected from a region located distal to highlands terrains, possibly from the western limb of the lunar nearside, dominated by mare basalts and KREEP-rich lithologies. [source]


The irradiation history of the Ghubara (L5) regolith breccia

METEORITICS & PLANETARY SCIENCE, Issue 3 2002
T. E. Ferko
The xenoliths, like the host, have high concentrations of trapped solar gases and are heavily shocked. While contents of noble gases and degree of shock-loading in this individual and three others differ somewhat, the data indicate that Ghubara is a two-generation regolith breccia. Contents of cosmogenic 26Al and 10Be and low track densities indicate that the Ghubara individuals were located more than 15 cm below the surface of an 85 cm meteoroid. Because of its large size, Ghubara's cosmic-ray exposure age is poorly defined to be 15,20 Ma from cosmogenic nuclides. Ghubara's terrestrial age, based on 14C data, is 2,3 ka. Not only is Ghubara the first known case of a two-generation regolith breccia on the macroscale, it also has a complicated thermal and irradiation history. [source]


Origin of a late Eocene to pre-Miocene buried crater and breccia lens at Fohn-1, North Bonaparte Basin, Timor Sea: A probable extraterrestrial connection

METEORITICS & PLANETARY SCIENCE, Issue 2 2000
John d. Gorter
The crater displays the classic elements of impact structures, including a central uplift, ring syncline, and upraised rims. The presence in the breccia of redeposited Campanian and Maastrichtian microfossils suggests rebound of strata from levels deeper than 1250 m below the pre-Miocene unconformity. Morphometric modelling suggests an original crater at least 1400 m deep, which is consistent with the excavation of Cretaceous strata. Stratigraphic and palaeontological evidence suggests that the impact occurred between 36 and 24.6 Ma. The breccia contains a pseudotachylite component enriched in the inert Pt group elements (PGE) (Ir, Ru) by factors of 5,12 above the values of common sediments. The more mobile PGE (Os, Pt, Pd) show a wide scatter and terrestrial-type values. Opposite geochemical/stratigraphic trends pertain to different PGE species,the relatively inert Ir-Ru group shows an overall concentration at the base of the section, whereas the more mobile Os shows peaks at median levels of the section,suggesting upward diagenetic leaching. The near-chondritic PGE patterns at the base of the breccia pile are accompanied by near-chondritic Ni/Cr, Co/Cr, Ni/Ir, Ni/Pt, and Cu/Pd ratios. Departure from these values related to alteration at higher levels in the breccia pile is accompanied with high S levels (,1%). [source]


A cranial base of Australopithecus robustus from the hanging remnant of Swartkrans, South Africa

AMERICAN JOURNAL OF PHYSICAL ANTHROPOLOGY, Issue 4 2006
Darryl J. de Ruiter
Abstract SKW 18, a partial hominin cranium recovered from the site of Swartkrans, South Africa, in 1968 is described. It is derived from ex situ breccia of the Hanging Remnant of Member 1, dated to approximately 1.5,1.8 Mya. Although partially encased in breccia, it was refit to the facial fragment SK 52 (Clarke 1977 The Cranium of the Swartkrans Hominid SK 847 and Its Relevance to Human Origins, Ph.D. dissertation, University of the Witwatersrand, Johannesburg), producing the composite cranium SKW 18/SK 52. Subsequent preparation revealed the most complete cranial base attributable to the species Australopithecus robustus. SKW 18 suffered weathering and slight postdepositional distortion, but retains considerable anatomical detail. The composite cranium most likely represents a large, subadult male, based on the incomplete fusion of the spheno-occipital synchondrosis; unerupted third molar; pronounced development of muscular insertions; and large teeth. Cranial base measures of SKW 18 expand the range of values previously recorded for A. robustus. SKW 18 provides information on anatomical features not previously visible in this taxon, and expands our knowledge of morphological variability recognizable in the cranial base. Morphological heterogeneity in the development of the prevertebral and nuchal muscular insertions is likely the result of sexual dimorphism in A. robustus, while differences in cranial base angles and the development of the occipital/marginal sinus drainage system cannot be attributed to size dimorphism. Am J Phys Anthropol, 2006. © 2006 Wiley-Liss, Inc. [source]


Magmatic evolution of the Mantos Blancos copper deposit, Coastal Range of northern Chile: insight from Sr,Nd isotope, geochemical data and silicate melt inclusions

RESOURCE GEOLOGY, Issue 2 2008
Luis E. Ramírez
Abstract The Mantos Blancos copper deposit (500 Mt at 1.0% Cu) was affected by two superimposed hydrothermal events: (i) phyllic alteration related to a rhyolitic dome emplacement and brecciation at ca 155 Ma; and (ii) potassic, sodic and propylitic alteration at ca 142 Ma, coeval with stocks and sills emplacement of dioritic and granodioritic porphyries, that locally grade upwards into polymictic magmatic hydrothermal breccias. Major hypogene copper sulfide mineralization is related to the second event. A late-ore mafic dike swarm cross-cuts all rocks in the deposit. Two types of granodioritic porphyries can be distinguished from petrographic observations and geochemical data: granodiorite porphyry I (GP I) and granodiorite porphyry II (GP II), which resulted from two different trends of magmatic evolution. The concave shape of the rare earth element (REE) distribution pattern together with the weak or absence of negative Eu anomalies in mafic dikes, dioritic and GP I porphyries, suggest hornblende-dominated fractionation for this magmatic suite. In contrast, distinct negative Eu anomalies and the flat REE patterns suggest plagioclase-dominated fractionation, at low oxygen fugacity, for the GP II porphyry suite. But shallow mixing and mingling between silicic and dioritic melts are also likely for the formation of the GP II and polymictic breccias, respectively. Sr-Nd isotopic compositions suggest that the rhyolitic dome rocks were generated from a dominantly crustal source, while the GP I has mantle affinity. The composition of melt inclusions (MI) in quartz crystals from the rhyolitic dome is similar to the bulk composition of their host rock. The MI analyzed in quartz from GP II and in the polymictic magmatic hydrothermal breccia of the deposit are compositionally more evolved than their host rocks. Field, geochemical and petrographic data provided here point to dioritic and siliceous melt interaction as an inducing mechanism for the release of hydrothermal fluids to form the Cu mineralization. [source]


Variations in Chemical Composition of Clay Minerals and Magnetic Susceptibility of Hydrothermally Altered Rocks in the Hishikari Epithermal Gold Deposit, SW Kyushu, Japan

RESOURCE GEOLOGY, Issue 1 2008
Hiroyasu Murakami
Abstract Hydrothermal alteration, involving chiefly chlorite and illite, is extensively distributed within host rocks of the Pleistocene Hishikari Lower Andesites (HLA) and the Cretaceous Shimanto Supergroup (SSG) in the underground mining area of the Hishikari epithermal gold deposit, Kagoshima, Japan. Approximately 60% of the mineable auriferous quartz-adularia veins in the Honko vein system occur in sedimentary rocks of the SSG, whereas all the veins of the Yamada vein system occur in volcanic rocks of the HLA. Variations in the abundance and chemical composition of hydrothermal minerals and magnetic susceptibility of the hydrothermally altered rocks of the HLA and SSG were analyzed. In volcanic rocks of the HLA, hydrothermal minerals such as quartz, chlorite, adularia, illite, and pyrite replaced primary minerals. The amount of hydrothermal minerals in the volcanic rocks including chlorite, adularia, illite, and pyrite as well as the altered and/or replaced pyroxenes and plagioclase phenocrysts increases toward the veins in the Honko vein system. The vein-centered variation in mineral assemblage is pronounced within up to 25 m from the veins in the peripheral area of the Honko vein system, whereas it is not as apparent in the Yamada vein system. The hydrothermal minerals in sandstone of the SSG occur mainly as seams less than a few millimeters thick and are sporadically observed in halos along the veins and/or the seams. The alteration halos in sandstone of the SSG are restricted to within 1 m of the veins. In the peripheral area of the Honko vein system, chlorite in volcanic rocks is characterized by increasing in Al in its tetrahedral layer and the Fe/Fe + Mg ratio toward the veins, while illite in volcanic rocks has relatively low K and a restricted range of Fe/Fe + Mg ratios. Temperature estimates derived from chlorite geothermometry rise toward the veins within the volcanic rocks. The magnetic susceptibility of tuff breccia of the HLA varies from 21 to less than 0.01 × 10,3 SI within a span of 40 m from the veins and has significant variation relative to that of andesite (27,0.06 × 10,3 SI). The variation peripheral to the Honko vein system correlates with an increase in the abundance of hematite pseudomorphs after magnetite, the percentage of adularia and chlorite with high Fe/Fe + Mg ratios, and the degree of plagioclase alteration with decreasing distance to the veins. In contrast, sedimentary rocks of the SSG maintain a consistent magnetic susceptibility across the alteration zone, within a narrow range from 0.3 to 0.2 × 10,3 SI. Magnetic susceptibility of volcanic rocks of the HLA, especially tuff breccia, could serve as an effective exploration tool for identifying altered volcanic rocks. [source]


Epithermal Gold-Silver Mineralization of the Asachinskoe Deposit in South Kamchatka, Russia

RESOURCE GEOLOGY, Issue 4 2007
Ryohei Takahashi
Abstract The Asachinskoe epithermal Au-Ag deposit is a representative low-sulfidation type of deposit in Kamchatka, Russia. In the Asachinskoe deposit there are approximately 40 mineralized veins mainly hosted by dacite,andesite stock intrusions of Miocene,Pliocene age. The veins are emplaced in tensional cracks with a north orientation. Wall-rock alteration at the bonanza level (170,200 m a.s.l.) consists of the mineral assemblage of quartz, pyrite, albite, illite and trace amounts of smectite. Mineralized veins are well banded with quartz, adularia and minor illite. Mineralization stages in the main zone are divided into stages I,IV. Stage I is relatively barren quartz,adularia association formed at 4.7 ± 0.2 Ma (K-Ar age). Stage II consists of abundant illite, Cu-bearing cryptomelane and other manganese oxides and hydroxides, electrum, argentite, quartz, adularia and minor rhodochrosite and calcite. Stage III, the main stage of gold mineralization (4.5,4.4 ± 0.1,3.1 ± 0.1 Ma, K-Ar age), consists of a large amount of electrum, naumannite and Se-bearing polybasite with quartz,adularia association. Stage IV is characterized by hydrothermal breccia, where electrum, tetrahedrite and secondary covellite occur with quartz, adularia and illite. The concentration of Au+Ag in ores has a positive correlation with the content of K2O + Al2O3, which is controlled by the presence of adularia and minor illite, and both Hg and Au also have positive correlations with the light rare-earth elements. Fluid inclusion studies indicate a salinity of 1.0,2.6 wt% NaCl equivalent for the whole deposit, and ore-forming temperatures are estimated as approximately 160,190°C in stage III of the present 218 m a.s.l. and 170,180°C in stage IV of 200 m a.s.l. The depth of ore formation is estimated to be 90,400 m from the paleo-water table for stage IV of 200 m a.s.l., if a hydrostatic condition is assumed. An increase of salinity (>CNaCl, 0.2 wt%) and decrease of temperature (>T , 30°C) within a 115-m vertical interval for the ascending hydrothermal solution is calculated, which is interpreted as due to steam loss during fluid boiling. Ranges of selenium and sulfur fugacities are estimated to be logfSe2 = ,17 to ,14.5 and logfS2 = ,15 to ,12 for the ore-forming solution that was responsible for Au-Ag-Se precipitation in stage III of 200 m a.s.l. Separation of Se from S-Se complex in the solution and its partition into selenides could be due to a relatively oxidizing condition. The precipitation of Au-Ag-Se was caused by boiling in stage III, and the precipitation of Au-Ag-Cu was caused by sudden decompression and boiling in stage IV. [source]