Volcaniclastic Units Research Articles

Petrographic and geochemical interpretation of the Late Cretaceous volcaniclastic deposits from the Hateg Basin

The present paper provides new petrographic and geochemical insights into the tectonic and depositional environment of the Late Cretaceous volcaniclastic deposits from the Hateg Basin, in the South Carpathians. These deposits are widespread in the Rachitova–Stei unit, which is tectonically delimitated from the South Carpathians' geological background by strike-slip faults. The conducted petrographic studies have revealed that the volcaniclastic rocks from the Rachitova–Stei unit are mainly pyroclastic (i.e. coarse tuffs, tuff-breccia and tuffites), with a wide variety of volcanic pyroclastic fragments, most commonly including andesites (amphibole andesites, biotite-bearing-amphibole andesites, pyroxene-bearing-amphibole andesites, pyroxene-andesites), and less frequently latite-andesites, latites, rhyolites and dacites. The rhyolites are found only in the lower sequence of the Rachitova–Stei unit. The volcaniclastic deposits were altered by diagenetic processes. The celadonite is the most common diagenetic mineral formed in these rocks, whose presence indicates that the pyroclastic fragments have interacted with sea water. The high percentage of pyroclastic fragments with mainly angular forms and the rich-hornblende content evidence a highly explosive volcanism. The poorly-sorted pyroclastic assemblages with larger blocks and with little reworked pyroclastic material indicate a very short distance of transportation and a deposition relatively close to the explosion centre. The huge quantity of pyroclastic material from the Rachitova–Stei unit suggests that the volcanic explosion had a centre of considerable dimensions, but there is no evidence of such a volcano neither in the Hateg Basin nor anywhere else in the South Carpathians. The whole-rock geochemical composition and the immobile trace element signature from the pyroclastic fragments indicate an island arc depositional environment that was placed close to an active continental margin. In addition, the andesitic nature of the pyroclasts indicates that the volcaniclastic rocks were most probably derived from a magma generated by the subduction of an oceanic plate under a thin continental plate. The presence of the non-volcanic pyroclasts (i.e. granites, quartz-muscovite schists, muscovite quartzites, graphitic phyllites etc.), detached from the continental crust by explosion, confirms that the overriding plate is of continental material. The presence of the rhyolitic magma in the first stage of eruption also supports the continental nature of the overriding plate, the magma having resulted by the partial melting of the rocks from the continental crust. The petrographic and geochemical data have evidenced that the volcanism took place on a thin continental crust. However, this crust is not characteristic to the Getic Unit from the South Carpathians, which is commonly accepted to represent the basement of the Hateg Basin. Most probably, the volcanism was generated in another geotectonic context, respectively, in a continental island arc with a thin overriding crust, most likely situated at a lower latitude. The volcaniclastic units of the Hateg Basin were displaced and moved from this tectonic context to the present location by strike-slip movements.

Evidence of slope instabilities and tsunami associated with the 3.5 ka Avellino eruption of Somma-Vesuvius volcano, Italy

Abstract The interpretation of high-resolution seismic profiles acquired close the eastern coast of the Bay of Naples offshore Somma–Vesuvius volcano, and calibrated by borehole data, allowed us to recognize a reflection-free seismic facies consisting of a volcaniclastic debris avalanche overlain by pyroclastic density current deposits. Both are associated with the 3.5 ka ‘Avellino eruption’ and are in turn overlain by a marine succession. The top of the volcaniclastic unit corresponds to a deep erosional surface covered by a coarse-grained bioclastic layer with rounded pumice. We argue that these features are related to a tsunami that was triggered by the sudden entrance into the sea of the gigantic (>1 km 3 ) volcaniclastic unit. The onshore–offshore correlation and mapping of this volcaniclastic unit have allowed us to evaluate its distribution west of Somma–Vesuvius. The pyroclastic density current deposits terminate seaward at 40 m water depth; at the same depth the debris avalanche is affected by a gravitational instability. Over the last 3.5 ka the slump has been in constant movement, as documented by the angular unconformities at the top of the chaotic seismic unit and within the overlying marine deposits.

A revised lithostratigraphic nomenclature for the Palaeogene Faroe Islands Basalt Group, NE Atlantic Ocean

ABSTRACTThe Palaeogene volcanic succession of the Faroe Islands in the NE Atlantic Ocean is formalised using a purely lithostratigraphic approach and following international guidelines. The Faroe Islands Basalt Group (FIBG) has a gross stratigraphic thickness of ∼6·6 km, dominated by subaerial basalt lava flows, and is subdivided into seven formations. The Lopra Formation forms the basal ∼1·1 km of the Lopra-1/1A borehole, dominated by hyaloclastites, volcaniclastic sandstones and invasive basaltic lavas/sills. It is overlain by the ∼3·25 km-thick Beinisvørð Formation, dominated by laterally extensive basalt sheet lobes separated by minor volcaniclastic lithologies. The Beinisvørð Formation is overlain by the <15 m-thick, inter-eruption, coal-bearing facies of the Prestfjall Formation and the <50 m-thick, syn-eruption, pyroclastic and sedimentary facies of the Hvannhagi Formation. Lava flow volcanic activity resumed with the <1·4 km-thick Malinstindur Formation, dominated by thinly bedded compound basalt lava flows. The top of this formation is marked by a regional disconformity surface, overlain by sandstone and conglomerate deposits of the maximum 30 m-thick Sneis Formation, a newly recognised stratigraphic unit. The final phase of volcanism recorded on the Faroe Islands consists of the >900 m-thick Enni Formation composed of a mixture of basalt sheet lobes and compound flows with abundant volcaniclastic units, e.g. the Argir Beds, that may require a further subdivision at this stratigraphic level. The new lithostratigraphy allows for more refined biostratigraphical and sequence stratigraphic correlations and prepares for a revised geological map of the Faroe Islands.

An Archean megacaldera complex: The Blake River Group, Abitibi greenstone belt

The Archean Blake River Group of the southern Abitibi greenstone belt is defined as a subaqueous megacaldera. Compelling evidence include: (1) a radial and concentric organization of synvolcanic mafic to intermediate dykes, (2) an overall dome geometry defined by the volcanic strata, (3) a peripheral distribution of subaqueous volcaniclastic units, (4) a zonal distribution of carbonate alteration, and (5) a distinct annular synvolcanic inner and outer ring fault pattern. Three caldera-forming events have been identified: (1) the early Misema caldera, (2) the New Senator caldera, and (3) the well-known Noranda caldera. The multi-vent BRG mafic volcanic complex, developed on a monotonous sequence of tholeiitic basalts, forming a submarine plain, and experienced a first major collapse that created the giant Misema Caldera (80 km in diameter). An endogenic dyke swarm intruded the synvolcanic fractures and an underlying magma chamber developed. Major volcaniclastic units were generated by local volcanic centers and summit calderas formed along the outer and inner ring faults. This fault system was used as a conduit for CO 2-rich hydrothermal activity. Renewed volcanic activity was associated with a resurgent central dome inside the Misema Caldera. This second collapse event created the 35 km by 14 km NW-trending New Senator Caldera. This caldera was produced by multi-step sagging after the underlying magmatic chamber migrated to the SE and formed the Flavrian-Powel Plutons. The final collapse resulted in the formation of the Noranda Caldera that generated a well-developed 070°-trending fracture pattern associated with several VMS deposits. The multiple-caldera setting provides an effective model to explain the presence of VMS mineralization along the synvolcanic fractures associated with the three collapse episodes. However, the importance of the New Senator Caldera fracture pattern is emphasized because of its role in the positioning of the giant Horne Mine deposit.

Lithofacies and architecture of a basinwide tuff unit in the Miocene Eoil Basin, SE Korea: Modes of pyroclastic sedimentation, changes in eruption style, and implications for basin configuration

The Miocene Eoil Basin, SE Korea, is a small half-graben basin that was rifted by both extensional and dextral strike-slip deformations during backarc opening of the East Sea (Sea of Japan). The basin was filled by fluviolacustrine sediments and abundant basaltic and dacitic volcanic deposits. The Paljori Tuff is a 2–20-m-thick, basinwide dacitic volcaniclastic unit that is intercalated in the topmost part of a fluviolacustrine formation. The tuff consists of two pyroclastic units (units II and III), which are underlain and overlain by two resedimented volcaniclastic units (units I and IV). Unit I at the base is composed mainly of fine-grained resedimented tephra; unit II is a massive and stratified tuff produced by a dense pyroclastic density current that was ponded in the southwestern part of the basin; unit III is a massive lapilli tuff emplaced by a voluminous and turbulent pyroclastic density current that swept across the basin toward the northeast without ponding or blocking by intrabasinal highs and basin-margin relief; and unit IV at the top is composed of coarse-grained resedimented tephra. The general coarsening-up of these units, the lack of nonvolcaniclastic sedimentary interbeds, and the offset stacking of the units toward the northeast suggest an episodic, short-duration eruption from the southwest that waxed in mass-eruption rate and changed eruption style from phreatomagmatic to magmatic. These units show basin-scale variations in geometry, including: (1) thick accumulations of unit I in the northeastern and southwestern extremities of the basin; (2) ponding of unit II in the southwestern part of the basin; (3) truncation of unit III in the same area; and (4) thickening of unit IV toward the northeastern basin margin. These provide important clues to the distribution of and temporal changes in accommodation space and, hence, the configuration and structural evolution of the Eoil Basin. It is inferred that the greatest subsidence occurred at the southwestern and northeastern corners of the basin; the southwest-northeast–trending axes of the basin plunge gently toward the northeast, and the major intrabasinal faults of the basin were produced before the eruption of the Paljori Tuff. The basinwide preservation of the tuff and its fairly uniform thickness suggest that the basin was undergoing rapid extension and subsidence because of the onset of the rift climax. The Paljori Tuff shows that important clues to, and high-resolution records of, the large-scale configuration and structural evolution of a sedimentary basin can be obtained from the study of lithofacies variations and the architecture of a single basinwide volcaniclastic unit.

Rudist bivalve assemblages from the Back Rio Grande Formation (Cretaceous: Campanian) of Jamaica and their stratigraphic significance

The lithostratigraphy of the Back Rio Grande Formation (Campanian) in the Blue Mountains of eastern Jamaica is described. Six informal members are recognised; three limestone and three volcaniclastic units. The rudist assemblages from the three limestone members are described, and the occurrence of larger foraminifers is discussed. The lower limestone member yields Barrettia monilifera Woodward, Durania krijneni sp. nov., Chiapasella cubensis Rutten, Bournonia thiadensi Vermunt, Branislavia stellaplicatus sp. nov. and Antillosarcolites? cf. macgillavryi Chubb together with several forms left in open nomenclature as well as the larger foraminifer Pseudorbitoides trechmanni Douvillé. The middle limestone member yields D. krijneni sp. nov. and C. cubensis. In the upper limestone member occur Barrettia sp. A, D. krijneni sp. nov., C. cubensis, Titanosarcolites sp. A and A.? cf. macgillavryi Chubb together with larger foraminifers including Pseudorbitoides and Vaughanina cubensis Palmer. The lower limestone member is placed in the middle Campanian, and the upper limestone member together with the overlying Rio Grande Formation are assigned a late Campanian age on the basis of their rudist and larger foraminifer assemblages. Chiapasella cubensis is recognised as a good marker for both the middle and upper Campanian.

Geologic, Geochemical, and Geochronological Constraints on the Genesis of Gold Mineralization at Poplar Mountain, Western New Brunswick, Canada

The Poplar Mountain gold occurrence in western New Brunswick is hosted in the Poplar Mountain volcanic complex (PMVC), which is located along the southern segment of the regional Woodstock fault zone. The PMVC consists of three principal units including, in ascending order, a porphyritic felsic volcanic unit, a volcaniclastic unit, and a mafic volcanic unit. U-Pb dating of zircon indicates that the age of the volcanic rocks is younger than 459 ± 3 Ma, and 40Ar/39Ar dating of mineralization-associated sericite indicates that the age of mineralization is 411 ± 3.7 Ma. Gold-mineralized zones occur in all the lithologic units but mainly in the porphyritic felsic volcanic unit. Gold zones are not controlled by individual faults but are characterized by high-density brittle fracturing, thin carbonate-quartz veining, and intensive ankerite-sericite alteration, superimposed on an earlier chlorite-cal- cite-quartz alteration. Gold is associated with arsenopyrite, which mostly occurs as disseminations in the host rocks, and to a lesser extent with quartz-carbonate-sericite veins and quartz-cemented breccias. Geochemical data indicate that mineralization-associated alteration is characterized by enrichment of K, Rb, Cs, Ca, Sr, Ba, As, Sb, W, C, and S. Fluid inclusion data indicate that the mineralizing fluids are composed of H 2O, salts, and CO2 with variable amounts of N2 and CH4; salinities range from 2.3 to 10.6 eq. wt.% NaCl, but mainly from 2.3 to 5.6 eq. wt.% NaCl, and homogenization temperatures lie mainly between 220° and 270°C. Fluid pressures at the site of mineralization are estimated to have been 770 to 1200 bars, corresponding to a depth of 2.9 to 4.6 km at lithostat- ic pressure. The δ18OSMOW and δ13CPDB values of mineralization-related ankerite range from +14.5‰ to +16.5‰, and -6.8‰ to -8.3‰, respectively. The δ18O values of the ore-forming fluids are estimated to be 6.4‰ to 8.3‰ at a temperature of 250°C. These carbon and oxygen isotope data fall in the field of magmatic fluids and in part the field of metamorphic fluids. Considering the geological setting of the region, abundant granitic intrusions, the similarity between the age of mineralization (411 ± 3.7 Ma) and the nearby Pokiok batholith (402-415 Ma), and the enrichment of granophile elements in mineralization-related alteration, the ore-forming fluids were probably derived from a granitic intrusion underneath the PMVC. These fluids were focused along structures related to the Woodstock fault. The Poplar Mountain gold occurrence might be compared to some granitic intrusion-related gold systems based on the geochemical data presented herein. © 2008 Canadian Institute of Mining, Metallurgy and Petroleum. All rights reserved.

The ~ 2500 yr B.P. Chicoral non-cohesive debris flow from Cerro Machín Volcano, Colombia

Cerro Machín Volcano (CMV) is located in the central part of the Colombian Andes (2750 m asl), 150 km southwest of Bogotá. It is considered the most dangerous active volcano of Colombia. CMV has experienced at least six major explosive eruptions during the last 5000 years. These eruptions have emplaced many types of pyroclastic deposits with associated lahars that have traveled more than 100 km. One of these lahars is called Chicoral Debris Flow Deposit (DFD2). This deposit is exposed as discontinuous terraces (3–20 m thick) along the Coello and Magdalena rivers up to 109 km from the source. The DFD2 covers a minimum area of 62 km 2 and has a minimum volume of 0.57 km 3. It comprises two dacite-rich volcaniclastic units. Grain-size analysis reveals that the matrix content and sorting increase with distance while the average grain size decreases. The clay content of the DFD2 matrix is approximately 1%, thus categorizing it as a non-cohesive debris flow. Radiocarbon dates obtained from underlying and overlying paleosols yielded ages of 2505 + 65 and 1640 + 45 yr B.P., respectively. These dates suggest that DFD2 is related to the ~ 2600 yr B.P. El Guaico eruption of CMV. This eruption produced a block-and-ash flow that filled and blocked the Toche River up to 5 km from the volcano. Subsequent remobilization of this loose material by runoff water generated a massive debris flow that traveled 91 km along the Toche and Coello rivers and continued across the Espinal Alluvial Fan debouching into the Magdalena River where it continued another 18 km prior to its transformation into a sediment-laden flow. Because the last eruption of the volcano occurred ca. 900 years ago, no historic activity of CMV is known among inhabitants of the region. Hence the region has developed without awareness of volcanic hazards. Therefore an assessment of volcanic hazards is essential for understanding and evaluating the vulnerability and risk to which people are exposed in case of a future eruption. Such assessment is critical for urban planning, development, contingency, emergency and education planning.

Upper Jurassic–Lower Cretaceous stratigraphy in south-eastern Tibet: a comparison with the western Himalayas

Lithostratigraphic studies of the Upper Jurassic–Lower Cretaceous sedimentary successions exposed in the Tingri–Gyangze area, south-eastern Tibet resulted in the establishment of a revised stratigraphic framework. A major crustal fault separates the southern Tibetan sedimentary successions into a Southern Zone and a Northern Zone. The Upper Jurassic–Lower Cretaceous strata of the Southern Zone are subdivided into the Menkadun Formation (Oxfordian, Kimmeridgian, up to lower Upper Tithonian), the Gucuo Formation (Upper Tithonian to Lower Albian) and the overlying Dongshan Formation (Upper Albian). The Gucuo Formation is further subdivided into a quartz arenite unit, which is overlain by a shale unit, in turn overlain by a volcaniclastic sandstone unit. The youngest cluster of detrital zircon absolute age data (127.7 ± 1.8 Ma) from the lower part of the volcaniclastic unit of the Gucuo Formation place the volcanic event before the Late Barremian. In the Northern Zone, the Upper Jurassic–Lower Cretaceous strata have been subdivided into four formations: the Zhera Formation (Upper Jurassic), the Weimei Formation (Tithonian), the Rilang Formation (?Berriasian) and the Gyabula Formation (? post-Valanginian). The appearance of silica cemented quartz arenites in both the quartz arenite unit of the Gucuo Formation and the Weimei Formation in the uppermost Jurassic is considered to provide strong evidence for an eustatic sea level drop affecting the northern margin of Greater India. In contrast the appearance of volcaniclastic sandstones in the Gucuo and Rilang formations are interpreted to represent a rift related volcanic event. The Menkadun–Zhera formations, the quartz arenite unit of the Gucuo Formation–Weimei Formation, the volcaniclastic sandstone unit–Rilang Formation in south-eastern Tibet are lithologically similar and broadly correlative with the Spiti Shale, Takh and Pingdon La formations in Zanskar (India), and Spiti Shale, Dangardzong, Kagbeni and Dzong formations in the Thakkhola area (Nepal). The onset of quartzose and subsequently volcaniclastic deposition becomes progressively younger westward, from the Gucuo region toward Zanskar.

The nature and timing of Palaeoproterozoic sedimentation at the southeastern margin of the Congo Craton; zircon U–Pb geochronology of plutonic, volcanic and clastic units in northern Zambia

This study is focused on the determination of the age and provenance of Proterozoic (meta-)sedimentary units on the southern margin of the Congo Craton, collectively known as the Muva Supergroup. Zircon U–Pb SHRIMP dating of crystalline basement units (granite and volcanic tuffs) that form the basement to the Muva Supergroup have yielded crystallisation ages of between 1868 ± 7 and 1860 ± 13 Ma. Discrete volcaniclastic units within the deformed portions of the Muva Supergroup in the Irumide Belt have yielded crystallisation ages of between 1879 ± 13 and 1856 ± 4 Ma. Detrital zircon studies in three samples of quartzite and one conglomerate from the various components of the Muva Supergroup, the Mporokoso Group on the Bangweulu Block and the Kanona and Manshya River groups in the Irumide Orogen on the southeastern margin of the Bangweulu Block, have yielded populations of Archaean and Palaeoproterozoic zircons in which the youngest detrital grains have an age of ca. 1800 Ma. These data indicate that the Muva Supergroup is a Palaeoproterozoic succession deposited at ca. 1800 Ma and that the Mporokoso, Kanona and Manshya River groups are all temporally equivalent and are likely parts of a single widespread succession. In contrast, the youngest detrital zircon in the Kasama Formation, a discrete succession of mature fluvial quartzite on the Bangweulu Block, has yielded an age of 1434 ± 14 Ma, indicating that it is a younger Mesoproterozoic succession. A striking similarity between the detrital zircon populations in the Kasama Formation and those in the Mporokoso Group support previous sedimentological interpretations that it is a second-cycle sediment derived from the Mporokoso Group.

The LaRonde Penna Au-Rich Volcanogenic Massive Sulfide Deposit, Abitibi Greenstone Belt, Quebec: Part I. Geology and Geochronology

The LaRonde Penna Au-rich volcanogenic massive sulfide (VMS) deposit is the largest Au deposit currently mined in Canada (58.8 Mt at 4.31 g/t, containing 8.1 Moz of Au). It is part of the Doyon-Bousquet-LaRonde mining camp located in the eastern part of the Blake River Group of the Abitibi greenstone belt which is host to several of the world’s most important, present and past, Au-rich VMS deposits (e.g., Horne, Quemont, Bousquet, Bousquet 2-Dumagami). The LaRonde Penna deposit consists of massive to semimassive sulfide lenses (Au-Zn-Ag-Cu-Pb), stacked in the upper part of a steeply dipping, south-facing homoclinal volcanic sequence composed of extensive tholeiitic basaltic flows (Hebecourt Formation) overlain by tholeiitic to transitional, mafic to intermediate, effusive and volcaniclastic units at the base (lower member of the Bousquet Formation) and transitional to calc-alkaline, intermediate to felsic, effusive and intrusive rocks on top (upper member of the Bousquet Formation). The mafic to felsic volcanism of the Hebecourt Formation and of the lower member of the Bousquet Formation formed an extensive submarine basement or platform on which the intermediate to felsic rocks of the upper member of the Bousquet Formation were emplaced at restricted submarine eruptive centers or as shallow composite intrusive complexes. The submarine felsic volcanic rocks of the upper member of the Bousquet Formation are characterized by dacitic to rhyodacitic autoclastic (flow breccia) deposits that are cut and overlain by rhyodacitic and rhyolitic domes and/or partly extrusive cryptodomes and by intermediate to mafic sills and dikes. This volcanic architecture is thought to have been responsible for internal variations in ore and alteration styles, not only from one lens to another, but also along a single mineralized horizon or lens. In the upper part of the mine, the 20 North lens comprises a transposed pyrite-chalcopyrite (Au-Cu) stockwork (20N Au zone) overlain by a pyrite-sphalerite-galena-chalcopyrite-pyrrhotite (Zn-Ag-Pb) massive sulfide lens (20N Zn zone). The latter was formed, at least in part, by replacement of footwall rhyodacitic autoclastic deposits emplaced within a subbasin located between two rhyolite domes or cryptodomes. The 20N Zn zone tapers with depth in the mine and gives way to the 20N Au zone. At depth in the mine, the 20N Au zone consists of semimassive sulfides (Au-rich pyrite and chalcopyrite) enclosed by a large aluminous alteration halo on the margin of a large rhyolitic dome or cryptodome. U-Pb zircon geochronology gives ages of 2698.3 ± 0.8 and 2697.8 ± 1 Ma for the footwall and hanging-wall units of the 20 North lens, respectively. Thus, the formation of the 20 North lens was coeval with other VMS deposits in the Bousquet Formation and in the uppermost units of the Blake River Group. Although deformation and metamorphism have affected the primary mineral assemblages and the original geometry of the deposit, these events were not responsible for the different auriferous ore zones and alteration at LaRonde Penna. Studies of the LaRonde Penna deposit show that the hydrothermal system evolved in time and space from near-neutral seawater-dominated hydrothermal fluids, responsible for Au-Cu-Zn-Ag-Pb mineralization, to highly acidic fluids with possible direct magmatic contributions, responsible for Au ± Cu-rich ore and aluminous alteration. The different ore types and alteration reflect the evolving local volcanic setting described in this study.

Structural Modification of the Komatiite-Associated Harmony Nickel Sulfide Deposit, Leinster, Western Australia

The Harmony nickel sulfide deposit (3.2 Mt of ore) is situated along the eastern margin of the Agnew-Wiluna greenstone belt, Eastern Goldfields, Western Australia. The deposit occurs 2 km north of the larger Rocky’s Reward (15.9 Mt) and Perseverance (50.7 Mt) deposits. The mine stratigraphy trends north-northwest–south-southeast, dips moderately (30°–80°) to the west, is overturned, and metamorphosed to middle-amphibolite facies grades (about 550°C, 3 kbars). Stratigraphic footwall units to the west of the nickel-bearing komatiite include ca. 2720 to 2725 Ma (volcano-) sedimentary rocks and rhyodacite volcanic and volcaniclastic units. Hanging-wall units comprise mainly reworked sedimentary or volcaniclastic rocks interlayered with at least two barren spinifex-textured komatiite units exposed in the mine area. Exhalative pyrite-rich pelitic units and thin amphibolite units are locally present in the footwall and hanging wall. The nickeliferous komatiite is thickest in the center of the deposit and thins to the north, south, and at depth. Although massive sulfides and minor disseminated sulfides are mainly located along the sheared western footwall contact, minor massive sulfides occur throughout the komatiite and country rock in fold hinges, boudin neck areas, brittle-ductile shear zones, piercement cusps, piercement veins, and brittle faults. Footwall and hanging-wall units have a strong composite fabric defined by compositional layering and several subparallel axial planar foliations. East-west shortening resulted in the folding of the composite fabric to form abundant north-northwest–trending, tight, asymmetric, moderately (20°–40°) N- and S-plunging folds. The asymmetric folds thicken the komatiite and orebodies in the center of the pit and control deposit-scale orebody plunge directions. The attenuated north-northwest–trending komatiite fold limbs are boudinaged and concentrate pods of massive sulfides in boudin neck positions. Relaxation in horizontal shortening coincided with open recumbent folding of the steeply dipping limbs of earlier upright asymmetric folds. A resumption of east-west shortening in a brittle stress regime resulted in the remobilization of massive sulfides into east-west–trending brittle faults. The concentration of massive sulfides in brittle-ductile to brittle structures indicates that massive sulfide remobilization spanned a diverse range of deformation and metamorphic conditions. The present geometry and distribution of stratigraphy and orebodies at Harmony reflect a complex interplay of primary magmatic and secondary overprinting processes. Remobilized massive sulfide orebodies are an important exploration target in other deformed and metamorphosed terranes that host primary Ni-Cu-(PGE) deposits. Although footwall units are the preferential host to remobilized massive sulfide shoots, massive sulfides are commonly remobilized within shear zones and faults zones and hosted by hanging-wall units up to 50 m from their primary footwall position. In cases of folding, orebodies tend to be elongate parallel to the plunge direction of fold axes or mineral stretching lineation, and potential exploration targets occur along the trend of known folded ore shoots and more specifically along the plunge line of such shoots.

Facies architecture of the Early Devonian Ural Volcanics, New South Wales

The Ural Volcanics are a early Devonian, submarine, felsic lava-sill complex, exposed in the western central Lachlan Orogen, New South Wales. The Ural Volcanics and underlying Upper Silurian, deepwater, basin-fill sedimentary rocks make up the Rast Group. The Ural Range study area, centrally located in the Cargelligo 1:100 000 map sheet area, was mapped at 1:10 000 scale. Seventeen principal volcanic facies were identified in the study area, dominated by felsic coherent facies (rhyolite and dacite) and associated monomictic breccia and siltstone-matrix monomictic breccia facies. Subordinate volcaniclastic facies include the pumice-rich breccia facies association, rhyolite – dacite – siltstone breccia facies and fiamme – siltstone breccia facies. The sedimentary facies association includes mixed-provenance and non-volcanic sandstone to conglomerate, black mudstone, micaceous quartz sandstone and foliated mudstone. The succession was derived from at least two intrabasinal volcanic centres. One, in the north, was largely effusive and intrusive, building a lava – sill complex. Another, in the south, was effusive, intrusive and explosive, generating lavas and moderate-volume (∼3 km3) pyroclastic facies. The presence of turbidites, marine fossils, very thick massive to graded volcaniclastic units and black mudstone, and the lack of large-scale cross-beds and erosional scours, provide evidence for deposition in a submarine environment below storm wave-base. The Ural Volcanics have potential for seafloor or sub-seafloor replacement massive sulfide deposits, although no massive sulfide prospects or related altered zones have yet been defined. Sparse, disseminated sulfides occur in sericite-altered, steeply dipping shear zones.

Origin of magnetic anomalies in volcaniclastic units of the Messel maar-diatreme (Germany)

Origin of magnetic anomalies in volcaniclastic units of the Messel maar-diatreme (Germany)

Geology and Volcanic Facies Architecture of the Lower Ordovician Waterloo Massive Sufide Deposit, Australia

The Waterloo massive sulfide deposit occurs in the Cambro-Ordovician Seventy Mile Range Group in the Charters Towers district in northern Queensland, one of the principal submarine successions in eastern Australia that contains abundant volcanic-hosted base metal deposits. The deposit consists of several small dismembered, stratiform, blanketlike, pyrite-chalcopyrite-sphalerite-galena massive sulfide lenses hosted by intensely altered volcanic rocks. The deposit and its host rocks have been deformed during regional deformation of the Middle to Late Ordovcian and were affected by two subsequent events of brittle deformation. The Waterloo deposit formed on top of a lava-dominated andesite succession comprising coherent volcanic units and related juvenile volcaniclastic rocks. The hydrothermal activity leading to the deposition of massive sulfides commenced during the waning stages or after the cessation of the andesite volcanism. The blanketlike mineralization is interpreted to have formed largely at or very close to the sea floor in a moderately deep to deep marine environment. Formation of the massive sulfides was accompanied by the mass-flow deposition of coarse quartz-feldspar crystal-rich volcanic debris that records broadly contemporaneous, probably explosive, rhyolitic volcanism occurring outside the immediate study area. This resedimented volcaniclastic unit retains a record of hanging-wall alteration, indicating that the Waterloo hydrothermal system was operative during and after burial by the coarse volcaniclastic material. The deposition of the coarse volcaniclastic rocks was followed by the intrusion of a feldspar-porphyritic dacite cryptodome that was also subject to hydrothermal alteration associated with the massive sulfide formation. Facies relationships suggest that the cryptodome locally breached the wet and unconsolidated coarse volcaniclastic material to emerge on the sea floor. The emplacement of the partially extrusive dacite cryptodome coincided with the waning of the Waterloo hydrothermal system. The subsequent period of relatively quiet sedimentation was occasionally interrupted by the emplacement of synsedimentary basalt and andesite sills as volcanism at Waterloo shifted back to more mafic compositions. This period was followed by the mass-flow deposition of coarse feldspar-quartz-vitric volcanic debris and the onset of another period of intense basalt- to andesite-dominated volcanism. Geochemical analyses of volcanic rocks from the host-rock succession of the Waterloo deposit and correlation with regional data suggest that the massive sulfide deposits in the Charters Towers district formed during a specific period of Cambro-Ordovician back-arc basin development dominated by mantle-derived volcanism. Comparison with the volcanic architecture of other known massive sulfide deposits in the district indicates that massive sulfide mineralization at Charters Towers is mainly associated with silicic shallow, intrusion-dominated volcanic centers and occurrences of this volcanic facies should be considered prospective for other deposits in the belt.

Early Silurian magmatism and the Scandian evolution of the Kalak Nappe Complex, Finnmark, Arctic Norway

Three orogenic events, Porsanger (pre-800 Ma), Finnmarkian (490–540 Ma) and Scandian (400–425 Ma) have been proposed to explain the evolution of the Kalak Nappe Complex, a major component of the Caledonian orogen in Norway. The Kalak Nappe Complex has been considered as Baltoscandian passive margin metasediments telescoped eastwards onto the Baltic Shield. Two granitoid bodies with a weighted mean U–Pb zircon age of 438 ± 2 Ma cut the Hellefjord Schist, regarded as the highest stratigraphic unit in the Kalak Nappe Complex. A volcaniclastic unit within the Hellefjord Schist yields an identical U–Pb zircon age of 438 ± 4 Ma. Magmatism and sedimentation are synchronous within analytical error. The Hellefjord Schist cannot have been affected by Finnmarkian or Porsanger orogenesis and cannot be part of a continuous stratigraphic package. A Laurentian affinity for these sediments is suggested and radical revision of the tectonostratigraphy is required. The Hellefjord Schist is reassigned to the Magerøy Nappe of the Upper–Uppermost Allochthon. As the entire stratigraphy is infolded with the Hellefjord Schist a major deformation phase of Scandian age affected the Kalak Nappe Complex. The Hellefjord Schist represents a Laurentian flysch sequence that was juxtaposed with Baltica during the Scandian Orogeny.

Petrology and Geochemistry of West Philippine Basin Basalts and Early Palau–Kyushu Arc Volcanic Clasts from ODP Leg 195, Site 1201D: Implications for the Early History of the Izu–Bonin–Mariana Arc

Site 1201D of Ocean Drilling Program Leg 195 recovered basaltic and volcaniclastic units from the West Philippine Basin that document the earliest history of the Izu–Bonin–Mariana convergent margin. The stratigraphic section recovered at Site 1201D includes 90 m of pillow basalts, representing the West Philippine Basin basement, overlain by 459 m of volcaniclastic turbidites that formed from detritus shed from the Eocene–Oligocene proto-Izu–Bonin– Mariana island arc. Basement basalts are normal mid-ocean ridge basalt (N-MORB), based on their abundances of immobile trace elements, although fluid-mobile elements are enriched, similar to back-arc basin basalts (BABB). Sr, Nd, Pb and Hf isotopic compositions of the basement basalts are similar to those of basalts from other West Philippine Basin locations, and show an overall Indian Ocean MORB signature, marked by high Pb/Pb for a given 206Pb/204Pb and high 176Hf/177Hf for a given 143Nd/ Nd. Trace element and isotopic differences between the basement and overlying arc-derived volcaniclastics are best explained by the addition of subducted sediment or sediment melt, together with hydrous fluids from subducted oceanic crust, into the mantle source of the arc lavas. In contrast to tectonic models suggesting that a mantle hotspot was a source of heat for the early Izu–Bonin– Mariana arc magmatism, the geochemical data do not support an enriched, ocean island basalt (OIB)-like source for either the basement basalts or the arc volcanic section.

Erste Ergebnisse von Suszeptibilitätsmessungen an Vulkaniklastika der Forschungsbohrung Messel 2001

Erste Ergebnisse von Suszeptibilitätsmessungen an Vulkaniklastika der Forschungsbohrung Messel 2001

Impact of a Quaternary volcano on Holocene sedimentation in Lillooet River valley, British Columbia

Lillooet River drains 3850 km 2 of the rugged Coast Mountains in southwestern British Columbia, including the slopes of a dormant Quaternary volcano at Mount Meager. A drilling program was conducted 32–65 km downstream from the volcano to search for evidence of anomalous sedimentation caused by volcanism or large landslides at Mount Meager. Drilling revealed an alluvial sequence consisting of river channel, bar, and overbank sediments interlayered with volcaniclastic units deposited by debris flows and hyperconcentrated flows. The sediments constitute the upper part of a prograded delta that filled a late Pleistocene lake. Calibrated radiocarbon ages obtained from drill core at 13 sites show that the average long-term floodplain aggradation rate is 4.4 mm a −1 and the average delta progradation rate is 6.0 m a −1. Aggradation and progradation rates, however, varied markedly over time. Large volumes of sediment were deposited in the valley following edifice collapse events and the eruption of Mount Meager volcano about 2360 years ago, causing pulses in delta progradation, with estimated rates to 150 m a −1 over 50-yr intervals. Two of the volcaniclastic units identified in drill core correlate with previously documented strong acoustic reflectors in Lillooet Lake at the downstream end of the basin. The Mount Meager massif constitutes only 2% of the Lillooet River drainage, but lithology counts of Lillooet River channel gravels indicate that a disproportionate percentage of the sediment is derived from the volcano. The data indicate that deposits of large debris flows are important elements of the sedimentary sequence and that Mount Meager dominates the sediment supply to Lillooet River.

Stratigraphy, geochronology, and geochemistry of the Georgie River area, northwest British Columbia, and implications for mineral exploration

Georgie River area is underlain by greenschist facies volcanic, clastic, and plutonic rock. High precision U–Pb isotopic ages on zircon enable regional stratigraphic correlation by constraining the Lower to lower Middle Jurassic depositional history. A sample from pyroxene-phyric tholeiitic basalt is Upper Triassic and (or) Lower Jurassic in age; its upper age limit is constrained by a crosscutting intrusion dated at 189.8 ± 0.3 Ma. The Bulldog Creek pluton, at 193.0 ± 0.3 Ma, is significantly older than the previously reported K–Ar age. Hazelton Group strata include a coarse-grained volcaniclastic and volcanic unit overlain by a fine-grained clastic and bimodal volcanic flow unit. The coarse volaniclastic unit is coeval and probably cogenetic with the Outram Lake porphyry intrusion (189.7 ± 0.3 Ma) and near the top includes andesitic tuff (189.8 ± 0.6 Ma). These data constrain the age of deposition of the unit and support correlation with the Lower Jurassic Hazelton Group and members of the Texas Creek plutonic suite. Age of the fine clastic and bimodal volcanic unit is no older than the youngest detrital zircons (ca. 185 Ma) near its base. That age and the age of a rhyolite unit (176.6 ± 1.1 Ma) at the top support correlation with the Middle Jurassic Salmon River Formation of the Hazelton Group. Resolution of the litho- and chronostratigraphy of the area reveal close similarities in age, rock types, succession, and composition with strata that host the base- and precious metal-rich stratiform Eskay Creek deposit, revealing the potential for Eskay-type volcanogenic massive sulphide (VMS) deposits in the Georgie River area.