Massive Serpentinite Research Articles

Magnetic anisotropy of serpentinized peridotites from the MARK area: Implications for the orientation of mesoscopic structures and major fault zones

[1] Mantle-derived serpentinized peridotites are exposed both along fracture zones and in areas of extreme tectonic extension at slow to intermediate spreading ridges and may constitute a significant volume of the shallow crust in these environments. Here we examine the potential of magnetic remanence data and structural features in serpentinized peridotites from Ocean Drilling Program (ODP) Site 920 (Mid-Atlantic Ridge south of Kane, MARK) to provide insights into the tectonic processes responsible for the exposure of these deep-seated rocks at the seafloor. Paleomagnetic data from 214 samples from Site 920 document a remarkably consistent inclination (36.1° +0.8°/-1.4°) that is shallower than either the expected geocentric axial dipole inclination (40.7°) or present-day inclination (41.9°) at the site. We show that the nearly univectorial remanence in these samples is likely to be a partial thermoremanence, possibly augmented by viscous processes at moderate temperatures. These properties were acquired during cooling from the relatively high temperatures (>350°C) at which serpentinization occurred. The remanence directions therefore provide some information on the latest stages of uplift of the serpentinite massif. However, interpretation of this tectonic history is complicated by the presence of a pronounced magnetic fabric, which presumably resulted in a deflection of the remanence. We estimate the magnitude and direction of this deflection using a relationship between the anisotropy of magnetic susceptibility and remanence anisotropy. The corrected remanent inclinations (mean 39.5°) more closely approximates the time-averaged inclination at the site, indicating that the massif experienced little or no resolvable tilt after serpentinization and cooling to 350°C. Accounting for the anisotropy-related deflection of the remanence also allows us to more accurately restore various structural features within the core to their geographic orientation. After this reorientation the dominant mesoscopic foliation in these rocks, defined by the preferred orientation of orthopyroxene and subparallel serpentine veins, has an average orientation that closely parallels the regional-scale fault zones on the western median valley wall.

The Tugela Terrane, Natal belt: Kibaran magmatism and tectonism along the southeast margin of the Kaapvaal Craton

This paper is a preliminary report on the geology of the Mandleni tectonostratigraphic package (MTP) part of the Tugela terrane, the northernmost element of the Natal belt in southeastern Africa. The Dondwana gneiss unit, a metamorphosed bimodal magmatic sequence comprising feldspathic gneiss, with lesser amounts of amphibolite, underlies much of the MTP. All primary structures in these rocks have been obliterated during post-depositional metamorphism and deformation. Fabrics attributable to the oldest recognised tectonic event (D1) have largely been overprinted during subsequent deformation. D2 was characterised by syn-kinematic high-pressure (P>9 kbar) granulite facies metamorphism. The dominant S2 planar fabric is a penetrative gneissic foliation that parallels lithological contacts. It is defined by colour and compositional banding, and by parallelism of coarsely crystalline mineral grains. D3 tectonism produced asymmetric north-verging folds, faults and shear zones, which accommodated overthrusting of the MTP onto the Kaapvaal craton. Plagioclase ± biotite mantles on garnet developed at this time and are attributable to uplift of the MTP from the mid- to deep-crustal levels established during D2. Ultramafic rocks (the Evuleka ultramafics) tectonically interleaved with the Dondwana gneiss unit during D3, consist of talc schist, and massive serpentinite with disseminated chromite and magnetite. These ultramafic rocks are interpreted as altered and sheared mantle or cumulate from the lower portion of an ophiolite sequence. Granitoid magmatism associated with D3 is related to subduction beneath the MTP. This subduction led to basin-closure and emplacement of the MTP onto the Kaapvaal craton. The final tectonic event (D4) produced broad open south-plunging folds and sinistral shear zones.

空知‐エゾ帯の“未分離日高累層群”から高圧変成鉱物の発見

High P/T metamorphic minerals are found from a Late Cretaceous accretionary complex (Ganpi-yama Complex) separately occurring within a serpentinite mass in the Kamuikotan Zone of the Sorachi-Yezo Belt, Hokkaido. The complex was previously referred to as the "undivided Hidaka Supergroup". The Yon-no-sawa Unit, a major member of the complex, is composed of deformed alternation of siliciclastic sandstone and mudstone, greenstones and chert. Sodic pyroxenes coexisting with quartz are found from greenstone, and metamorphic aragonite from sandstone and greenstone. The finding of these high P/T metamorphic minerals suggests that the Ganpi-yama Complex has, at least, a metamorphic affinity to the Kamuikotan Zone. An exact tectonostratigraphic correlation of the complex, however, is still open to the argument.

Shear strength of landslide clay soil containing weathered serpentinite

Serpentinite is a rock consisting magnetite, brucite, and serpentine minerals. It is supposed to have formed on the deep sea floor by a sudden eruption of mantle matters or by cracking of the crust and coming in contact with water. It is generally associated with basic and sedimentary rocks. Among three constituent minerals, brucite easily dissolves in water, whereas magnetite does not. Hence, at a border between serpentinite and other rocks, formation of pores in the weathered serpentinite mass takes place due to leaching of brucite. These pores act as groundwater conduits all through the weathered rock mass, causing weathering of other rocks too. In the weathering process, chlorite changes partially to smectite and partially to expansive chlorite, which then form a clay layer with very weak strength. When this clay layer further loses strength, the soil mass resting on it starts moving, creating a landslide.
 Many landslide sites in Japan lie at the boundary of serpentinite and metamorphic basic rocks like greenschist and black (sedimentary) schist. The slip layer clays of these sites contain various minerals such as serpentine, talc, chlorite, and montmorillonite. The angle of internal friction for serpentinite is about 30°, whereas those of minerals like talc, chlorite, and montmorillonite is between 20° and 10°. In them, the decrease from peak to residual strength is high. Slopes of serpentinite soil containing a high amount of these weak clay minerals are prone to sliding.

Geochemistry and genesis of the black shale-hosted Ni-Cu-Zn deposit at Talvivaara, Finland

The Talvivaara deposit is hosted by metamorphosed black shales (black schists) and contains 300 million metric tons (Mt) of low-grade ore averaging 0.26 percent Ni, 0.14 percent Cu, and 0.53 percent Zn. As a result of tectonic deformation, the horizon containing > or = 0.8 percent Ni-Cu-Zn is up to 330 m thick. Talvivaara is situated in the Early Proterozoic Kainuu schist belt, which consists mainly of metasediments, with discontinuous serpentinite remnants 1.97 to 1.96 Ga in age. Serpentinite-associated Cu-Co-Zn-Au-Ni deposits and prospects of the Outokumpu type are present in the Kainuu-Outokumpu region; talc-carbonate rocks (altered serpentinites) are also found in the Talvivaara area.Pyrite and pyrrhotite are the dominant sulfide minerals in the Talvivaara deposit. Pyrite, pyrrhotite, chalcopyrite, sphalerite, alabandite, and pentlandite occur both as fine-grained disseminations ( 1% and S > 1%) were divided into three groups according to their Ni and Mn values: low Ni-Mn (Ni or = 0.1% and Mn or = 0.8%). Black calc-silicate rocks (Ca > or = 3.5%) are encountered as intercalations 10 cm to 3 m thick within the black schists. Even the so-called low Ni-Mn black schists at Talvivaara have anomalously high median C graphite , S, Ni, Cu, and Zn values compared with black schist intercalations within mica schists elsewhere in eastern Finland. The median C graphite values in the Talvivaara black schists are 7.1 to 7.7 percent for all three black schist types, and C graphite does not correlate significantly with any other element studied. Median S/C graphite ratios are 1.0 to 1.7 for the low Ni-Mn and Ni-rich black schists and 2.3 for the Mn-rich black schists. Black schists are also characterized by higher Al, Au, B, Ba, Fe, Hg, K, Li, Mo, Na, Pd, V, Zr, and rare earth element (REE) concentrations and by lower Ca, Mg, Ag, and F values compared with the intercalated black calc-silicate horizons.The degree of pyritization of black schists containing fine-grained sulfide disseminations is 0.8 to 1.0, suggesting an anoxic depositional environment. The precursors to the Talvivaara black schists were metalliferous organic-rich muds deposited in anoxic conditions. Geochemical evidence for hydrothermal influx includes elevated Ni, Cu, Zn, and Mn values relative to those of average Finnish and North American black shales. In addition, elevated S isotope delta 34 S values (median values of -5.2ppm for pyrrhotite, -4.3ppm for pyrite from the low Ni-Mn black schists, and -3ppm for both pyrrhotite and pyrite from the Ni-rich black schists) and positive Eu anomalies are regarded as further indications of hydrothermal activity. The Ni-rich black schists record Na depletion, which suggests hydrothermal alteration. Black calc-silicate rocks exhibit heavy S isotope values (-3.8-+20.8ppm for pyrrhotite and -7.1-+20.8ppm for pyrite) and are depleted in light REE, thus resembling Mn-rich seawater from recent hydrothermal sites. The Ni-rich and Mn-rich horizons with black calc-silicate rock intercalations probably result in part from precipitation of upwelling hydrothermal solutions through the Talvivaara sediments, analogously with the processes encountered, for example, in the recent Galapagos mounds hydrothermal field.The Talvivaara Ni-Cu-Zn deposit and the Outokumpu serpentinite-associated Cu-Co-Zn-Au-Ni deposits have features in common, such as the C concentration and texture of associated black schists and S isotope compositions. Cobalt and nickel concentrations of the Talvivaara Ni-rich black schists are similar to those of the Ni occurrences hosted by quartz rocks at Outokumpu. There are, however, distinct differences between the two deposits. In contrast to the black schists at Talvivaara, which have horizons with Mn values of 0.8 or more percent, those of the Outokumpu region do not contain elevated Mn concentrations. At Talvivaara, Ni predominates over Cu, and the deposit is hosted by graphite-rich rocks. At Outokumpu, Cu predominates over Ni and the deposit is mainly hosted by fine-grained quartz rocks. In the case of Talvivaara, the nearest serpentinite massif with Outokumpu-type fine-grained quartz rocks anomalous with respect to Cr and Ni is encountered 20 km to the south.The high C graphite concentrations, which are very characteristic of the rift-related black schists in the Kainuu-Outokumpu areas, point to vigorous organic productivity and extensive preservation of organic material in the sediment. Thick (>120 m) formations 2.1 to 1.9 Ga in age with a high median C graphite value are also encountered in several places elsewhere in the Fennoscandian Shield.

Siliceous volcanic centers as guides to mineral exploration; review and summary

The Talvivaara deposit is hosted by metamorphosed black shales (black schists) and contains 300 million metric tons (Mt) of low-grade ore averaging 0.26 percent Ni, 0.14 percent Cu, and 0.53 percent Zn. As a result of tectonic deformation, the horizon containing > or = 0.8 percent Ni-Cu-Zn is up to 330 m thick. Talvivaara is situated in the Early Proterozoic Kainuu schist belt, which consists mainly of metasediments, with discontinuous serpentinite remnants 1.97 to 1.96 Ga in age. Serpentinite-associated Cu-Co-Zn-Au-Ni deposits and prospects of the Outokumpu type are present in the Kainuu-Outokumpu region; talc-carbonate rocks (altered serpentinites) are also found in the Talvivaara area.Pyrite and pyrrhotite are the dominant sulfide minerals in the Talvivaara deposit. Pyrite, pyrrhotite, chalcopyrite, sphalerite, alabandite, and pentlandite occur both as fine-grained disseminations ( graphite >1% and S > 1%) were divided into three groups according to their Ni and Mn values: low Ni-Mn (Ni or = 0.1% and Mn or = 0.8%). Black calc-silicate rocks (Ca > or = 3.5%) are encountered as intercalations 10 cm to 3 m thick within the black schists. Even the so-called low Ni-Mn black schists at Talvivaara have anomalously high median C graphite , S, Ni, Cu, and Zn values compared with black schist intercalations within mica schists elsewhere in eastern Finland. The median C graphite values in the Talvivaara black schists are 7.1 to 7.7 percent for all three black schist types, and C graphite does not correlate significantly with any other element studied. Median S/C graphite ratios are 1.0 to 1.7 for the low Ni-Mn and Ni-rich black schists and 2.3 for the Mn-rich black schists. Black schists are also characterized by higher Al, Au, B, Ba, Fe, Hg, K, Li, Mo, Na, Pd, V, Zr, and rare earth element (REE) concentrations and by lower Ca, Mg, Ag, and F values compared with the intercalated black calc-silicate horizons.The degree of pyritization of black schists containing fine-grained sulfide disseminations is 0.8 to 1.0, suggesting an anoxic depositional environment. The precursors to the Talvivaara black schists were metalliferous organic-rich muds deposited in anoxic conditions. Geochemical evidence for hydrothermal influx includes elevated Ni, Cu, Zn, and Mn values relative to those of average Finnish and North American black shales. In addition, elevated S isotope delta 34 S values (median values of -5.2ppm for pyrrhotite, -4.3ppm for pyrite from the low Ni-Mn black schists, and -3ppm for both pyrrhotite and pyrite from the Ni-rich black schists) and positive Eu anomalies are regarded as further indications of hydrothermal activity. The Ni-rich black schists record Na depletion, which suggests hydrothermal alteration. Black calc-silicate rocks exhibit heavy S isotope values (-3.8-+20.8ppm for pyrrhotite and -7.1-+20.8ppm for pyrite) and are depleted in light REE, thus resembling Mn-rich seawater from recent hydrothermal sites. The Ni-rich and Mn-rich horizons with black calc-silicate rock intercalations probably result in part from precipitation of upwelling hydrothermal solutions through the Talvivaara sediments, analogously with the processes encountered, for example, in the recent Galapagos mounds hydrothermal field.The Talvivaara Ni-Cu-Zn deposit and the Outokumpu serpentinite-associated Cu-Co-Zn-Au-Ni deposits have features in common, such as the C concentration and texture of associated black schists and S isotope compositions. Cobalt and nickel concentrations of the Talvivaara Ni-rich black schists are similar to those of the Ni occurrences hosted by quartz rocks at Outokumpu. There are, however, distinct differences between the two deposits. In contrast to the black schists at Talvivaara, which have horizons with Mn values of 0.8 or more percent, those of the Outokumpu region do not contain elevated Mn concentrations. At Talvivaara, Ni predominates over Cu, and the deposit is hosted by graphite-rich rocks. At Outokumpu, Cu predominates over Ni and the deposit is mainly hosted by fine-grained quartz rocks. In the case of Talvivaara, the nearest serpentinite massif with Outokumpu-type fine-grained quartz rocks anomalous with respect to Cr and Ni is encountered 20 km to the south.The high C graphite concentrations, which are very characteristic of the rift-related black schists in the Kainuu-Outokumpu areas, point to vigorous organic productivity and extensive preservation of organic material in the sediment. Thick (>120 m) formations 2.1 to 1.9 Ga in age with a high median C graphite value are also encountered in several places elsewhere in the Fennoscandian Shield.

Metapyroxenites From New Idria, California, USA

Rocks consisting of chlorite, diopside, and titaniferous garnet from from New Idria, California, USA, are interpreted as metamorphosed pyroxenites that are part of a serpentinite massif. These rocks represent fragments of oceanic crust or underlying mantle, emplaced within the Franciscan complex during subduction of the now-defunct Farallon plate during the Jurassic. The metapyroxenites, along with other tectonic inclusions in the serpentinite, carry important geochemical clues to the nature of subduction zone processes. The metapyroxenites may themselves be tectonic inclusions. The New Idria occurrences have strong affinities to metapyroxenites from the Malenco serpentinite in the Alps.

Stable isotope and trace element studies of vein ophicarbonates at Gogolow-Jordanow serpentinite massif (Poland): A contribution to the origin of ophiaragonite and ophimagnesite.

Crust-form aragonite and needle-shape aragonite crystallized in cavities in magnesites as well as vein magnesite from ultramafic rocks related to the Sleza ophiolite (NE part of Bohemian Massif) have been examined for carbon and oxygen isotope ratios and for some trace elements. δ18OSMOW values of magnesite, corrected for acid-decomposition fractionation, comprise a narrow range of 23.7 to 27.9‰, which suggests near-surface temperature, high water-to-rock ratio, and meteoric origin of the ore-forming solution. This inference is further supported by δ13C values (–18.7 to –6.4‰) which indicate a biogenic origin of carbon. Moreover, chemical analyses of the magnesites showing lack of elements characteristic for intrusionrelated hydrothermal deposits support the meteoric-biogenic origin of the magnesite deposits of the Gogolow-Jordanow massif. The two types of aragonites are uniform in δ13C (–15.0 to –16.3‰) and δ18OSMOW (23.9 to 25.4‰). The Δ13Cmg-ar = –3.1 and –3.5‰ of two samples of magnesite-aragonite aggregates show the absence of carbon isotope equilibrium in this system. However, the Δ18Omg-ar = 4.0 and 1.8‰ for the respective samples suggest that the aragonites formed in the magnesites have possibly crystallized in oxygen isotope equilibrium with the host magnesite.

The nature and origin of eclogite blocks in serpentinite from the Tamworth Belt, New England Fold Belt, eastern Australia

Small eclogite blocks occur in a serpentinite mass in the eastern part of the Tamworth Belt of the New England Fold Belt. Mineral compositions are similar to those of group C eclogite from Oregon and California, suggesting metamorphic pressures of 1–1.4 GPa. Core compositions of garnet and clinopyroxene suggest temperatures of ∼ 412°‐422°C whereas rims indicate temperatures of ∼ 521°‐532°C. The blocks, which are of a composition similar to ocean floor basalt, are either accidental blocks incorporated into the ultramafic body during emplacement from a forearc substrate or were produced by high pressure metamorphism of a basaltic intrusion carried to depth in the ultramafic rocks of a downgoing slab. In both interpretations, rise of the ultramafic rocks probably occurred during crustal extension at the start of the Permian.

紀伊半島東部，御荷鉾緑色岩類に伴う蛇紋岩れき岩中の青色片岩

Blueschists associated with serpentinites are found in the Mikabu Greenstones of eastern Kii Peninsula. The serpentinites are classified into two types: massive serpentinites and serpentinite conglomerates. Blueschists occur as thin layers (6-250cm wide), and are intercalated with the serpentinite conglomerates. Talc-carbonate rocks are developed along the boundary between blueschists and serpentinite conglomerates. The blueschists, talc-carbonate rocks and surrounding Sanbagawa schists are folded with E-W trending fold axis. Both the blueschists and Sanbag-awa schists have similar mineral lineations plunging gently to the east or to the west. Major metamorphic minerals of the blueschists are magnesioriebeckite, aegirine-aegirine-augite, muscovite, calcite, quartz and hematite. The blueschists, talc-carbonate rocks and serpentinite conglomerates characteristically contain detrital minerals: brown amphibole, colorless amphibole and chromian spinel. These petrographic data suggest that (1) the serpentinite conglomerates and protoliths of blueschists and talc-carbonate rocks were sedimentary mixture of siliceous, calcareous and mafic materials, and (2) they were recrystallized during the Sanbagawa metamorphism. The blueschists and serpentinite conglomerates represent siliceous and mafic parts of the original sedimentary sequence, respectively.

関東山地，山中帯白亜系南縁部の緑色岩

Petrological characteristics of greenstones from the Sanchu belt were determined. The rocks are closely associated with serpentinite masses along the south marginal fault of the belt. A preliminary K-Ar age for the least metamorphosed rock is 120 Ma. They commonly have phenocrysts of plagioclase and olivine, which is totally altered into chlorite and has small euhedral inclusions of chromian spinel. Major-element bulk chemistry indicates they are tholeiitic basalts of MORB affinity. They especially resemble E-type MORB on the basis of chondrite-normalized rare-earth element distribution patterns as well as of Hf-Ta-Th ratios. Relic chromian spinels are enriched with Ti and Fes3+, showing a derivation from within-plate basalt or E-type MORB.Geochemical and mineralogical data indicate that the Sanchu greenstones are most likely E-type MORB.

Petrology of serpentine sandstone as a key to tectonic development of serpentine belts

The petrology of serpentine sandstone, which is ubiquitous in the serpentine belt, could provide some constraints on petrological characteristics for lost or eroded serpentinite masses. The chemistry of detrital chrome spinel and chrome diopside in particular is a useful indicator of the equilibrium temperature and lithology of the source peridotite. Combined with the petrology of the associated serpentinite masses this could provide the difference in the petrological characteristics, if any, between the lost serpentine and that now exposed in a serpentine belt, and consequently some constraints could be placed on the tectonic history of the serpentine belt itself. In the Mineoka belt, a narrow serpentine belt in central Japan, the serpentinite mass exposed during the Miocene has experienced a lower pyroxene temperature and wider lithology than the serpentinite masses now exposed in the same area. This fact possibly indicates that a vertical slice of the upper mantle peridotite has been uplifted and intermittently protruded since the Miocene. The Mineoka belt is probably placed on part of a fossil transcurrent fault, which had worked in the opening of the Shikoku Basin, of the northern end of the Philippine Sea plate. Along the transcurrent fault the mantle peridotite slices with various petrologic characteristics could have been continuously emplaced in the same way as in the present-day oceanic fracture zone. On the other hand, in the Kamuikotan belt of Hokkaido, northern Japan, the source of a Miocene serpentine sandstone at Furenai has the same petrologic characteristics as the now associated Sarugawa peridotite mass. The Sarugawa mass is a dismembered part of an ophiolite suite uplifted during the collision between the Eurasia plate and the Okhotsk (North America) plate in Miocene times. The Furenai serpentine sandstone was produced in a small sedimentary basin within the Sarugawa peridotite mass during the middle Miocene. Contrary to the Mineoka belt, large vertical uplift movements of the peridotite mass did not occur in the Kamuikotan zone after the Miocene.

The O'Toole nickel deposit, Morro do Ferro greenstone belt, Brazil

The O'Toole orebody is the first significant nickel sulfide deposit of the volcanic peridotitc association to be found in Brazil. It is in the Archean Morro do Ferro greenstone belt in the southwest part of Minas Gerais, Brazil. The deposit is hosted by the Morro do Niquel unit characterized by a komatiitic suite of rocks, with low CaO/Al 2 O 3 ratios ( approximately 1) and a high Al 2 O/TiO 2 ( approximately 20), including olivine peridotite, peridotite, pyroxenite, and basalt flows. Komatiitic lava flows are massive or layered with spinifex tops and basal olivine cumulates. Pillowed lava flows and breccias occur subordinately intercalated with chemical precipitates and tuffs that form a minor part of the unit. The Morro do Niquel assemblage has undergone at least three major deformational events accompanied by fold development resulting in a dominant north-west subvertical structural trend. The rocks have undergone upper greenschist to lower amphibolite metamorphism with postulated peak temperatures of 550 degrees C and pressures of 3 kbars or more.Four cycles of southwest-facing serpentinite-clinopyroxenite-amphibolite separated by banded iron-formation have been defined to date in the O'Toole sequence. The deposit is associated with the upper cycle that is 60 m thick and has been divided into five main units: ore horizon (4 m), serpentinite (13 m), clinopyroxenite (15 m), amphibolite (20 m), and banded iron-formation (16 m). The orebody is tabular, subvertical, and contains five interconnected shoots over a strike distance of 1,600 m and downdip extension of 500 m. The ore horizon occurs at the base of massive serpentinite with an iron-formation footwall. Five ore types are defined: breccia, matrix, disseminated, stringer, and banded ores. The ore mineral assemblage is pyrrhotite-pentlandite-chalcopyrite with significant cobaltite and platinum-group minerals (PGM). Secondary minerals occur in the oxidized cap.The ore appears to have formed in an embayment characterized by the thinning of the footwall sediments beneath the O'Toole deposit. This embayment is postulated to be primary and may have provided the trap for the accumulation of the immiscible sulfide droplets carried by the ultramaric flow host. The matrix and disseminated sulfides represent a primary distribution in the embayment whereas the breccia and stringer ores are dearly remobilized. The deposit is interpreted as a volcanic peridotite association type showing similarities to other Archean komatiite-associated deposits elsewhere in the world. An unusual feature of the deposit is its occurrence in the upper ultramafic unit of a cyclic sequence in contrast with the majority of Archean komatiite-hosted nickel deposits where mineralization occurs at the base of the first ultramafic unit.

Gold and platinum group elements in cobaltarsenide ores: Hydrothermal concentration from a serpentinite source-rock (Bou Azzer, Morocco)

The cobalt-arsenide ores of Bou Azzer are located along the borders of serpentinite massifs (Upper Proterozoic ophiolite complex) in carbonate-quartz lenses resulting from hydrothermal carbonate alteration of serpentinite. The cobalt ores contain an average gold content of 5–20 ppm; gold is mainly located in skutterudite (120 ppm av.), whereas the Fe-arsenide (loellingite) contains < 1 ppm Au. Similarly the highest PGE contents are found in skutterudite (up to 2 ppm total PGE). All the arsenide ores of Bou Azzer exhibit the same chondrite normalized PGE pattern displaying positive Rh and negative Pt anomalies, and a slight positive slope (Pd/Ir = 1 to 2). This uncommon PGE pattern closely resembles to that of sulphides of komatiites. In serpentinite, the PGE patterns are typical of slightly depleted mantle rocks, and the associated podiform chromitites are within the range of ophiolitic chromitites, except for Pd and Au enrichment. Horizons of sulphide-bearing serpentinites show relatively high contents of noble metals and display PGE patterns which closely resemble those of the Co-arsenide ores, although an order of magnitude lower. These sulphides probably correspond to the remobilization during serpentinization of primary magmatic sulphides. The sulphiderich horizons are a possible source-rock for the noble metals of the Bou Azzer cobaltarsenide ores.

A suspect serpentinite mass in the Tokyo Bay area:petrology and provenance of serpentinite pebbles in the upper Cenozoic system in the Boso Peninsula, central Japan

A suspect serpentinite mass in the Tokyo Bay area:petrology and provenance of serpentinite pebbles in the upper Cenozoic system in the Boso Peninsula, central Japan

Composition and origin of lateritic nickel ore at Tagaung Taung, Burma

The recently discovered lateritic nickel ore deposit on the summit area of the Tagaung Taung contains about 40 million DMT nickel-saprolite ore with approximately 2% Ni. The serpentinite massif is covered with a thick weathering mantle consisting of saprolite, limonite, and an allochthonous lateritic surface layer. The thickness, nickel content, and physical properties of the ore are very variable. The main minerals in the nickel saprolite ore are nickeliferous serpentine and smectite. Microprobe analyses show a strong depletion of Mg with increasing nickel content in the serpentine minerals; smectite generally contains less nickel than serpentine. Chemical analyses of samples from more than 100 drill holes are used to calculate the mean chemical compositions, mean accumulation factors, and gain and loss of major and trace elements for the various layers. This permits a quantitative approach to be made to the genetic history of the weathering mantle. Gain and loss determinations on the basis of constant chromium content and of constant volume give nearly identical results for the saprolite layer. Calculations on the basis of Ni/Cr ratios indicate that three-quarters of a former 20-m-thick limonite layer has been eroded away. Only the lower part of the original limonite layer with a relatively high SiO2 content is preserved. The kaolinite- and illite-rich surface layer is a mixture of limonite and weathered clayey material, which was probably derived from adjacent rocks. Part of the autochthonous weathering section is contaminated with infiltrated allochthonous surface material. It is not possible that colluvial-alluvial deposition of clayey material from nonultramafic rocks could have taken place under the present geomorphic regime. Thus, lateritization of the serpentinite and deposition of siallitic material must have occurred before exposure of the Tagaung Taung in its present form.

The origin of the chrysolite asbestos veins in southeastern Quebec

The chrysotile asbestos deposits of southeastern Quebec occur in serpentinized peridotites of Ordovician ophiolites. The asbestos is localized in veins within blocks of partly serpentinized peridotite bounded by zones containing schistose serpentinite and granitic masses. It is thought that these zones localized shear and provided the channelway s by which water entered the peridotite. Different generations of chrysotile veins can be identified locally by cross-cutting relationships, but these observations cannot be generalized regionally. The asbestos was produced late during the deformation of the peridotite, filling extensional fractures in the peridotite induced by stresses resulting from motion along the shear zones.The factors that control the formation of the chrysotile veins are the degree of serpentinization of the peridotite and the size of the peridotite mass. Other factors being equal, the smaller the peridotite mass is, the shorter the time necessary to serpentinize it. Smaller masses of peridotite, once serpentinized, weaken, develop a foliation, and will not develop cross-fiber veins. Larger peridotite masses will be less serpentinized and stronger and will develop cross-fiber veins.

Co–Ni arsenide deposits, with accessory gold, in ultramafic rocks from Morocco

A type of Co–Ni arsenide deposit with accessory gold and no silver can be recognized in ultramafic massifs of upper mantle origin. The Bou Azzer deposits (50 000 t Co) are distributed along the borders of serpentinite massifs in an Upper Proterozoic ophiolite (Anti-Atlas, Morocco). They consist of quartz–carbonate lenses with Co (Ni–Fe) arsenides. Gold is related to the skutterudite (8–15 ppm average content). In the Alpine lherzolite massif of Beni Bousera (Rif, Morocco) are found small chromite–Ni arsenides veins (300 t Ni) with accessory gold. They are associated with late orthopyroxene–vermiculite dikes. Several occurrences of As, arsenides, and gold-bearing carbonate rocks are found in serpentinites from various ophiolites. It is proposed that gold was leached by As- and Co2-rich hydrothermal solutions related to the serpentinization of mantle peridotites.

Application géophysique à l'interprétation structurale de la région du lac Bécancour, Thetford Mines, Québec

The present study, based on detailed gravity anomaly (Bouguer and residual) and magnetic maps of the Thetford Mines area, seeks to explain the mechanism by which the Lac Bécancour structural unit was emplaced. The Bouguer anomaly map shows: (1) generally more positive anomalies toward the northwest; (2) local positive anomalies in the southwest and north-northwest; and (3) a small local positive anomaly in the east. Residual anomalies are characterized by a weak high of the order of 2–3 × 10−5 m/s2 (2–3 mgal) over the center of the structure. The magnetic map shows: (1) negative to neutral intensities over the center of the structure; (2) two small positive anomalies on the inner border of the structure; (3) a large magnetic mass (the ophiolite complex) in the west-northwest; and (4) small and isolated magnetic masses to the southwest and east.The hypothesis of diapir-like rise caused by an underlying serpentinite mass, as proposed by some geologists, is refuted by the absence of a positive magnetic anomaly over the structure. A second hypothesis, which calls on post-nappe granitization of the core of the structure at depth, is unlikely as gneiss is now here exposed in the area and the structure is bounded by faults. Instead, the geophysical data suggest that the Lac Bécancour structural unit is an old (Early Cambrian or Eocambrian) block that has undergone polyphase deformation during the Taconic Oxogeny rather than being a horst-like body emplaced by vertical rise. Identical overthrust slices of Hadrynian basement in the centers of imbricated blocks and nappes of lower Paleozoic rocks are commonly observed in the Caledonides of Norway and Sweden where they generally have the appearance of antiforms or synforms. [Journal Translation]

Proterozoic tectonic evolution and late Svecokarelian plate deformation of the Central Baltic Shield

The continental crust of the Central Baltic Shield evolved by accretion towards the west during the Svecokarelian orogeny 1700–2200 Ma ago. The following features are consistent with a plate tectonic mechanism involving subduction of oceanic crust below an Archean craton in the east: flysch-sediments with serpentinite masses and pillow lavas, linear high-grade metamorphic zones, island-arc type volcanic belts and late tectonic batholiths with porphyry type Cu-Mo deposits. Semi-consolidated new crust was affected by late Svecokarelian deformation (Dn) after 1850 Ma; NNE-trending folds with crenulation cleavage were overprinted on older structures together with associated NW trending ductile transcurrent shear zones that curve the Fn folds into gentle S and Z shapes. The late tectonic batholiths intruded partly at the same time as and partly after the Dn deformation.