Tuffaceous Sedimentary Rocks Research Articles

Microanalytical approaches on the silicification process of wood fossil from Jasinga, West Java, Indonesia

Worldwide, silicified woods are found in many geological formations. Significantly, the organic materials of wood are no longer dominant; almost all wood fossils have been mineralized into inorganic silica materials. These unique geological processes must be understood to develop better understanding on organic material fossilization, particularly in the micron scale. Therefore, our aim was to characterize the composition of silicified wood using comprehensive microanalysis. The methods utilized were XRF, ICP-MS, XRD, FTIR, and FE-EPMA. Specimens are from Jasinga, West Java, Indonesia. The results showed that wood silicification was controlled by the infiltration of silica from the host rock into the spaces of the wood structure. In Jasinga, they are controlled by Pliocene tuffaceous sedimentary rocks. The ratio of silica phases revealed a trend in the degree of silicification. Besides silica, the distribution of trace elements also demonstrates the geochemical interaction between the wood fossil and host rock. Wood fossils are affected by the gradual replacement of organic carbon-based materials with silica through silicification. Silica enrichment occurs in the internal of wood, facilitates permineralization and recrystallization. Silica replaces organic material and preserves the wood structures. The microanalytical approach provides comprehensive perspectives on wood petrification, leads to better insights for paleontological studies.

Triassic Diamondiferous Tuffaceous–Sedimentary Rocks in the Arctic Zone of Siberia

Abstract In the Arctic zone of Siberia, large diamond occurrences have been discovered in volcaniclastic, sedimentary–volcaniclastic, volcanosedimentary, and sedimentary rocks of the upper Ladinian strata and the base of the Carnian (Triassic) strata. They are confined to the Primorye mineragenic zone, which is traced along the Laptev Sea water area from the western Verkhoyansk area to eastern Taimyr. We have first identified a specific range of diamonds in these deposits. Among the rounded crystals of varieties I, II, V, and VII, there are grains with a light carbon isotope composition and high nitrogen contents. They have no analogues in typomorphic features in the known primary deposits of Yakutia but are completely similar to diamonds in the Rhaetian, Early Jurassic, Late Jurassic, Early Cretaceous, Neogene, and Quaternary commercial placers and placer occurrences, which suggests their formation as a result of the erosion of Triassic sources.

Late Palaeozoic extensional volcanism along the northern margin of Gondwana in southern Turkey: implications for Palaeotethyan development

The Late Palaeozoic–Early Mesozoic Tethyan development of the Eastern Mediterranean region remains debatable, especially in Turkey, where alternative northward and southward subduction hypotheses are proposed. Relevant to this debate, new whole-rock geochemical data are provided here for early Carboniferous (Late Tournaisian-Late Visean; c. 340–350 Ma) tuffaceous sedimentary rocks within the Çataloturan thrust sheet (Aladağ nappe), eastern Taurides. The tuffs accumulated from evolved alkaline volcanism, variably mixed with terrigenous and radiolarian-rich sediments. In addition, Late Palaeozoic meta-volcanic rocks, c. 150 km farther NE, within the Binboğa (= Malatya) metamorphics (a low-grade high-pressure unit), are indicative of a within-plate setting. An impersistent geochemical subduction signature in these volcanics may represent an inherited, rather than contemporaneous, subduction influence, mainly because of the absence of a continental margin arc or of arc-derived tuff. Both the Binboğa metamorphics and the Çataloturan thrust sheet (Aladağ nappe) restore generally to the north of the relatively autochthonous Tauride carbonate platform (Geyik Dağ), within the carbonate platform bordering north-Gondwana. The Çataloturan thrust sheet is interpreted, specifically, as a c. E–W, deep-water, volcanically active rift that progressively infilled. Regional geological evidence suggests that melange units (Konya Complex, Afyon zone), Teke Dere unit, Lycian nappes), and Chios–Karaburun melange, E Aegean) accreted to the north-Gondwana continental margin during the late Carboniferous; this was coupled with localised calc-alkaline granitic magmatism (Afyon zone of Anatolide crustal block). We propose an interpretation in which Late Devonian–Carboniferous alkaline intra-plate volcanism relates to extension/rifting along the north-Gondwana margin. In contrast, the melange accretion and granitic magmatism could relate to short-lived late Carboniferous southward subduction that accompanied the diachronous closure of Palaeotethys.

Bosoite, a new silica clathrate mineral from Chiba Prefecture, Japan

AbstractBosoite (IMA2014-023) is a new silica clathrate mineral containing hydrocarbon molecules in its crystal structure. Bosoite can be considered structurally as a silica analogue of the structure-H gas hydrate, where guest molecules are trapped in cage-like voids constructed of the host framework. The mineral occurs in the Miocene tuffaceous sedimentary rocks at Arakawa, Minami-boso City, Chiba Prefecture, Japan. Bosoite is hexagonal, and it crystallises as an epitaxial intergrowth on chibaite crystals, with the {0001} of bosoite parallel to octahedral {111} form of chibaite. Crystals are colourless and transparent with vitreous lustre. The calculated density is 2.04 g/cm3. The empirical formula (based on 2 O apfu and guest molecules assumed as CH4) is Na0.01(Si0.98Al0.02)Σ1.00O2⋅0.50CH4; the end-member formula is SiO2⋅nCxH2x+2. Bosoite has the space groupP6/mmm, with the unit-cell parametersa= 13.9020(3) Å,c= 11.2802(2) Å,V= 1887.99(6) Å3andZ= 34. The crystal structure of bosoite was refined by single-crystal X-ray diffraction and converged toR1= 4.26% for the average model andR1= 2.96% for the model where all oxygen sites are split.

Mineralogical characterization and diagenetic history of Permian marine tuffaceous deposits in Guangyuan area, northern Sichuan basin, China

Based on thin section identification, scanning electron microscope (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD) analysis of samples from cuttings and cores in drillings, tuffaceous deposits were identified to intercalate in marine carbonate rocks and mudstones in the upper part of Maokou Formation and the middle to lower part of the Wujiaping Formation in Guangyuan area, northern Sichuan basin, China. These tuffaceous deposits are of significance to understand the influence range of magmatic activity of the Emeishan Large Igneous Provinces (ELIP), and explore and exploit oil and gas reservoirs in Permian strata in the Sichuan basin. The research results indicated that tuffs formed 0.1–10 mm-thick laminae, and they formed lamellar interbedding with organic mudstones and carbonaceous mudstones, which were sandwiched among bioclast limestones and marls. Tuffaceous deposits in the study area include tuff, sedimentary tuff and tuffaceous sedimentary rock. Under the influence of sulfate reduction, acidic hydrotherm and alkaline fluid, tuffaceous deposits were altered by diagenesis such as dolomitization, argillization, pyritization and dissolution, and formed a variety of secondary rocks and pores which characterized by the micron-scale solution pores in tuff components and the nanometer-scale intercrystal pores among authigenic clay minerals. In addition, the primary directional sedimentary structure of tuffs and sedimentary tuffs and developed laminae in the rocks are conducive to the formation of fissures parallel to laminae, and effectively improve the reservoir property of rocks. The tuffaceous deposits in the study area are mainly the products of subaqueous pyroclastic flow that were dominated by acidic magmatism and indicated that they may not be directly related to the ELIP, which is located in the southwest Sichuan-Yunnan provinces and dominated by continental eruption of basalt.

On the Age of the Buor-Kemus Flora of Northeast Asia Based on the Data on the Nonmarine Cretaceous in Eastern Chukotka

On the basis of the analysis of literature and cartographic materials, our field observations, results of U–Pb isotope dating of zircons, and examination of paleobotanical collections, it was established that the “Mesophytic” Buor-Kemus fern–gymnosperm flora existed in Eastern Chukotka in the Late Cretaceous longer than in other regions. The volcanics of the Nyrvakinot sequence are partly facially replaced by the sedimentary deposits of the Olkhovaya Formation. Both units contain plant remains of the Buor-Kemus flora. The Amgen volcanogenic sequence lies conformably and without any hiatus on the Nyrvakinot sequence and contains “Cenophytic” plant assemblages with a significant amount of angiosperms, ascribed to the Turonian–Coniacian. These three units were previously considered as the three major constituents of the Etelkuyum Formation. The U–Pb isotope ages of zircons from volcanogenic and tuffaceous sedimentary rocks of the Nyrvakinot sequence show that they accumulated in the Late Сretaceous in the age range of 93–87 Ma (Turonian–Coniacian). Taking into account all data available, it is evident that the Buor-Kemus flora (at least within the Eastern Chukotka segment of the Okhotsk–Chukotka Volcanogenic Belt) existed in the Albian, Cenomanian, Turonian, and, probably, in the early Coniacian.

Discovery and identification of clay-sized tuffaceous rocks in Chang 7 Member of Upper Triassic Yanchang Formation in Ordos Basin, China

This study for the first time demonstrates that some of the so-called clay-sized mudstones observed by the naked eye, such as clay-sized black mudstones and clay-sized oil shales, which are rich in black organic matter (including oil and asphaltene), in the Chang 73 Submember of the Upper Triassic Yanchang Formation in the Ordos Basin of China are actually clay-sized tuffaceous rocks (including tuff, sedimentary tuff and tuffaceous sedimentary rock) with high hydrocarbon generation capacities. Thus, these rocks can be defined as clay-sized tuffaceous source rocks. Identification of this lithology has important theoretical and practical significance for the exploration and development of shale oil in the Chang 7 Member. Through the macroscopic observation of drill cores and outcrop profiles, microscopic observation of electron probe thin sections and whole-rock inorganic geochemical analysis (including major, trace and rare earth elements), this work demonstrates that the organic matter-rich clay-sized tuffaceous rocks, especially clay-sized tuffs, have the following characteristics. First, the clay-sized tuffaceous rocks with little black organic matter are mainly greyish white, yellowish brown and purplish grey, and mixed colors occur in areas with strong bentonite lithification. Second, the clay-sized tuffaceous rocks have experienced strong devitrification and recrystallization, forming abundant flaky aluminosilicate minerals with directional arrangement. In thin sections under a polarizing microscope, the interference colors generally show regular alternation between the lowest interference color of first-order yellow and the highest interference color of second-order blue-green. Third, the rock samples plot in the igneous rock field in the TiO2-SiO2 cross-plot and exhibit similar trace element and rare earth element patterns on spider diagrams, indicating that the samples are derived from the same source. The results prove that clay-sized tuffaceous rocks may be widespread in the Chang 73 Submember of the Upper Triassic Yanchang Formation in the Ordos Basin, China.

Platinum and platinoids in the Bolotisty gold-placer deposit (Khabarovsky Krai, Russia)

The article shows the results of studies the platinum mineralization (within the framework of studying the complexity of deposits) in the waste of heavy concentrate in the southeastern flank of the Bolotisty gold-placer deposit, which the located in the Khabarovsky Krai (Russia). The deposit is localized in the “erosion window” among the covers of Miocene basalts and Eocene andesites, where Cretaceous sedimentary, tuffaceous-sedimentary rocks of the basement of the volcano structure, broken through by an Eocene intrusive body of intermediate and basic composition, come to the surface. An abbreviated mineralogical analysis for monofraction of precious metals was carried out for a concentrate fraction weighing 1.813 kg with a grain size of -3.0 + 0.1 mm. Gold and platinum were found - 1.38 mg and 1.08 mg. Platinum has an iron impurity up to 3-5 % Fe and Cu, Ir, Rh impurities are variable. There are plate-like inclusions of native osmium with an admixture of ruthenium in 43 % of the studied platinum grains. There are single inclusions of ehrlichmanite. A large amount of high-temperature dipyramidal quartz, along with magnetite, chromite, tourmaline and other minerals is a characteristic feature of the concentrate and platinum grains of this area.

Deformed continental arc sequences in the South Tianshan: New constraints on the Early Paleozoic accretionary tectonics of the Central Asian Orogenic Belt

The tectonic affinity and structural evolution of the South Tianshan are key issues to understand the Paleozoic accretionary or collisional orogenesis in the southwestern Central Asian Orogenic Belt, but remain controversial. Geological and structural investigations on the Early Paleozoic low-grade metamorphic volcanic-sedimentary sequences around the Bayinbuluk basin demonstrate that they were prevalently reworked by north-vergent ductile deformation and overlain unconformably by Devonian to Carboniferous carbonates. Zircon U–Pb dating indicates that the volcanic rocks erupted during the Middle Ordovician to Early Devonian time (460–410 Ma), and contain Neoarchean to Neoproterozoic zircon xenocrysts (~2.75 Ga, ~2.46 Ga, ~1.60 Ga, 997–963 Ma and 827 Ma). Their whole-rock elemental, Sr–Nd isotopic (εNd(t) = −5.0 to +3.6) and zircon Lu–Hf isotopic (εHf(t) = −13.5 to +11.6) features resemble those of Andean-type continental arc volcanic rocks. The coexisting tuffaceous sedimentary rocks indicate a depositional environment amidst a continental volcanic arc at 421 to 404 Ma. The newly obtained and available geochronological, geochemical and structural data suggest that: (1) a wide active continental margin covering the South and Central Tianshan and northern Tarim was formed due to Early-Mid Paleozoic southward subduction of the Paleo-Tianshan Ocean beneath the Tarim Craton; and (2) a Mid-Paleozoic transition from advancing to retreating accretionary orogeny was followed by opening of the South Tianshan back-arc basins and break-up of the northern Tarim continental margin. The South Tianshan was subsequently subjected to regional deformation during the successive diachronous closure of the major and back-arc oceanic basins, and locally reworked by the ductile strike-slip faulting (307–255 Ma).

Characterization of lacustrine mixed fine-grained sedimentary rocks using coupled chemostratigraphic-petrographic analysis: A case study from a tight oil reservoir in the Jimusar Sag, Junggar Basin

Abstract Mixed deposits are sediments consisting of external clastic (epiclastic or terrigenous), intrabasinal components and pyroclastic components. The mixture, comprising variable amounts of the three components, is defined as “mixed sedimentary rocks”. The Permian Lucaogou Formation (P2l) in the Jimusar Sag of the Junggar Basin is a promising tight oil target in western China, the fine-grained mixed sedimentary rocks of which are rich in organic matter (OM) and two sweet spot intervals with relatively high porosity. However, the sediment composition, provenance and deposition environmental settings have not been studied in detail. In this study coupled chemostratigraphic-petrographic analysis were used to reconstruct their depositional environments. The results show that the fine-grained sedimentary rocks have three major sediment sources, external clastic input (terrigenous clastics), intrabasinal autochthonous to parautochthonous components (carbonates, siliceous skeletal debris and OM) and pyroclastic input. Main lithofacies include siltstone/fine sandstone, mudstone, dolomite and tuffite. The silt/sandstones were mainly sourced from rocks with calc-alkaline composition, while the tuffaceous sedimentary rocks were sourced from high-K calc-alkaline rocks. Elemental proxies suggest that the carbonate rocks were generally deposited under warm and arid conditions, whereas the fine-grained clastic sediments were deposited under relatively humid conditions. The muddy or silty tuffaceous mixed rocks were deposited under relatively reducing conditions compared with carbonates and sandstones. Variations of lithofacies and OM accumulation of different intervals reflect changing deposition environmental settings, and the frequently altered high TOC content rocks and good reservoirs are benefit for tight oil formation. The work may provide some useful insights and serve as a reference for studying other mixed fine-grained sedimentary rocks and tight oil plays in similar lacustrine basins elsewhere.

Time series analyses of hydrological parameter variations and their correlations at a coastal area in Busan, South Korea

Monitoring and time-series analysis of the hydrological parameters electrical conductivity (EC), water pressure, precipitation and tide were carried out, to understand the characteristics of the parameter variations and their correlations at a coastal area in Busan, South Korea. The monitoring data were collected at a sharp interface between freshwater and saline water at the depth of 25 m below ground. Two well-logging profiles showed that seawater intrusion has largely expanded (progressed inland), and has greatly affected the groundwater quality in a coastal aquifer of tuffaceous sedimentary rock over a 9-year period. According to the time series analyses, the periodograms of the hydrological parameters present very similar trends to the power spectral densities (PSD) of the hydrological parameters. Autocorrelation functions (ACF) and partial autocorrelation functions (PACF) of the hydrological parameters were produced to evaluate their self-correlations. The ACFs of all hydrologic parameters showed very good correlation over the entire time lag, but the PACF revealed that the correlations were good only at time lag 1. Crosscorrelation functions (CCF) were used to evaluate the correlations between the hydrological parameters and the characteristics of seawater intrusion in the coastal aquifer system. The CCFs showed that EC had a close relationship with water pressure and precipitation rather than tide. The CCFs of water pressure with tide and precipitation were in inverse proportion, and the CCF of water pressure with precipitation was larger than that with tide.

A Mesozoic orogenic cycle from post-collision to subduction in the southwestern Korean Peninsula: New structural, geochemical, and chronological evidence

The Early to Middle Mesozoic basins, distributed sporadically over the Korean Peninsula, preserve important records of the tectonic history of some of the major orogenic belts in East Asia. Here we present a comprehensive study of the structural, geochemical, geochronological, and paleontological features of a volcano-sedimentary package, belonging to the Oseosan Volcanic Complex of the Early to Middle Mesozoic Chungnam Basin, within the Mesozoic subduction-collision orogen in the southwestern Korean Peninsula. The zircon U-Pb data from rhyolitic volcanic rocks of the complex suggest Early to Middle Jurassic emplacement age of ca. 178–172Ma, harmonious with plant fossil taxa found from the overlying tuffaceous sedimentary rock. The geochemical data for the rhyolitic volcanic rocks are indicative of volcanic arc setting, implying that the Chungnam Basin has experienced an intra-arc subsidence during the basin-expanding stage by subduction of the Paleo-Pacific (Izanagi) Plate. The Jurassic arc-related Oseosan Volcanic Complex was structurally stacked by the older Late Triassic to Early Jurassic post-collisional basin-fill of the Nampo Group by the Jangsan fault during basin inversion. The Late Jurassic to Early Cretaceous K-feldspar and illite K-Ar ages marked the timing of inversion tectonics, contemporaneous with the magmatic quiescence in the southern Korean Peninsula, likely due to flat-lying or low-angle subduction. The basin evolution history preserved in the Mesozoic Chungnam Basin reflects a Mesozoic orogenic cycle from post-collision to subduction in the southwestern Korean Peninsula. This, in turn, provides a better understanding of the spatial and temporal changes in Mesozoic tectonic environments along the East Asian continental margin.

A multi‐disciplinary characterisation of Heath Steele belt deposits, Bathurst Mining Camp: a basis for identifying new targets for the exploration

ABSTRACTAlthough the Bathurst Mining Camp has been explored for more than 40 years, high‐resolution airborne geophysical survey has led to the detection of several new volcanogenic massive sulphide occurrences in the region. The discovery of these concealed exploration targets depended directly on the knowledge of the multi‐parameter signature of the major deposits and the search for the repeatability of these signatures in other regions. The Heath Steele B deposit, within the southern part of the Bathurst Mining Camp, together with an associated magnetite‐rich iron formation and adjacent tuffaceous sedimentary rocks, produced strong, co‐extensive magnetic and conductivity anomalies. However, the strong anomalies observed in the geophysical data can be related to non‐mineralised lithology such as mafic dykes or graphite. So a direct identification of exploration targets using geophysical data only can be difficult. For this reason, the joint interpretation of geophysical and geochemical signature can considerably improve the identification of new deposits. Using these techniques, we identify three new possible targets, characterised by local magnetic and apparent conductive highs. Two of these areas are located in the Heath Steele belt, and their geochemistry has been studied in detail. The proximity of these targets (less than 1.5 km) from known occurrences of hydrothermal sediments (magnetite, siderite, and stilpnomelane) confirms their exploration potential. The fourth identified area is most likely associated with a regional anomaly related to an unmapped ferromagnetic structure and was earlier disregarded as a possible target. The joint interpretation of geophysical and geochemical data allowed us to identify two new targets for exploration and to exclude one area interpreted as associated with a regional anomaly. Therefore, the search parameters (physical proprieties and hydrothermal sediment geochemical pattern distribution) are established with a success index of 75%. The index was limited by the absence of geochemistry data over the third target. The procedure presented in this paper can be used to search for new targets elsewhere in the Bathurst area and even in other base metal regions.

Ore potentiality of the Vanchin Graben, Primorye, Russia

New data on Cenozoic mineralization in the Vanchin coal basin are presented. In addition to the Au-Ag quartz-adularia veins known since the 1970s (Soyuzny deposit), stratified Au-Ag mineralization in tuffaceous-sedimentary rocks, porphyry tin mineralization in the extrusion of tourmalinized rhyolite, metalliferous coal seams enriched in Au, Ge, and REE are hosted in this graben. The localization and chemical and mineral compositions of these types of mineralization are considered. It is shown that Au-Ag mineralization is more widespread in the graben that was previously supposed. The anomalous and diverse mineralization of the Vanchin Graben is attributed, first of all, to long-term evolution of volcanism and hydrothermal activity developing synchronously with lacustrine and bog sedimentation and continuing after its termination. The main stages of ore formation are related to volcanic centers differing in age and conjugate hydrothermal systems functioning above a long-living magma chamber.

Geology and metallogeny of the Ar Rayn terrane, eastern Arabian shield: Evolution of a Neoproterozoic continental-margin arc during assembly of Gondwana within the East African orogen

The Neoproterozoic Ar Rayn terrane is exposed along the eastern margin of the Arabian shield. The terrane is bounded on the west by the Ad Dawadimi terrane across the Al Amar fault zone (AAF), and is nonconformably overlain on the east by Phanerozoic sedimentary rocks. The terrane is composed of a magmatic arc complex and syn- to post-orogenic intrusions. The layered rocks of the arc, the Al Amar group (>689 Ma to ∼625 Ma), consist of tholeiitic to calc-alkaline basaltic to rhyolitic volcanic and volcaniclastic rocks with subordinate tuffaceous sedimentary rocks and carbonates, and are divided into an eastern and western sequence. Plutonic rocks of the terrane form three distinct lithogeochemical groups: (1) low-Al trondhjemite-tonalite-granodiorite (TTG) of arc affinity (632–616 Ma) in the western part of the terrane, (2) high-Al TTG/adakite of arc affinity (689–617 Ma) in the central and eastern part of the terrane, and (3) syn- to post-orogenic alkali granite (607–583 Ma). West-dipping subduction along a trench east of the terrane is inferred from high-Al TTG/adakite emplaced east of low-Al TTG. The Ar Rayn terrane contains significant resources in epithermal Au–Ag–Zn–Cu-barite, enigmatic stratiform volcanic-hosted Khnaiguiyah-type Zn–Cu–Fe–Mn, and orogenic Au vein deposits, and the potential for significant resources in Fe-oxide Cu–Au (IOCG), and porphyry Cu deposits. Khnaiguiyah-type deposits formed before or during early deformation of the Al Amar group eastern sequence. Epithermal and porphyry deposits formed proximal to volcanic centers in Al Amar group western sequence. IOCG deposits are largely structurally controlled and hosted by group-1 intrusions and Al Amar group volcanic rocks in the western part of the terrane. Orogenic gold veins are largely associated with north-striking faults, particularly in and near the AAF, and are presumably related to amalgamation of the Ar Rayn and Ad Dawadimi terranes. Geologic, structural, and metallogenic characteristics of the Ar Rayn terrane are analogous to the Andean continental margin of Chile, with opposite subduction polarity. The Ar Rayn terrane represents a continental margin arc that lay above a west-dipping subduction zone along a continental block represented by the Afif composite terrane. The concentration of epithermal, porphyry Cu and IOCG mineral systems, of central arc affiliation, along the AAF suggests that the AAF is not an ophiolitic suture zone, but originated as a major intra-arc fault that localized magmatism and mineralization. West-directed oblique subduction and ultimate collision with a land mass from the east (East Gondwana?) resulted in major transcurrent displacement along the AAF, bringing the eastern part of the arc terrane to its present exposed position, juxtaposed across the AAF against a back-arc basin assemblage represented by the Abt schist of the Ad Dawadimi terrane. Our findings indicate that arc formation and accretionary processes in the Arabian shield were still ongoing into the latest Neoproterozoic (Ediacaran), to about 620–600 Ma, and lead us to conclude that evolution of the Ar Rayn terrane (arc formation, accretion, syn- to postorogenic plutonism) defines a final stage of assembly of the Gondwana supercontinent along the northeastern margin of the East African orogen.

Matter composition and formation conditions of the Pliocene-Pleistocene sediments in the Kuril basin (sea of Okhotsk)

An integrated (petrographical and micropaleontological) study of the sedimentary cover samples dredged from the lower slopes of the Kuril deep-sea basin was carried out. The Pliocene-Pleistocene sediments are mainly represented by tuffaceous sedimentary rocks (tuffites, tuffaceous muds, tuffaceous diatomites, tuffaceous silts, tuffaceous sandstones, etc.). Significant admixtures of pyroclastic matter, especially of volcanic glasses, indicates that the sedimentation process was accompanied by explosive volcanism. The data obtained give evidence about the intensification of the tectonomagmatic regime within the region under study during the Pliocene-Pleistocene time. By the beginning of the Pliocene, a deep-sea basin with a well-manifested continental and/or island slope and a narrow shelf already existed. The Pliocene-Pleistocene deposits accumulated in a cold well-aerated deep-sea basin under oxic conditions and downslope sediment transport.

The Camelback Zn-Pb-Cu Deposit: A Recent Discovery in the Bathurst Mining Camp, New Brunswick, Canada

Camelback is a small (≤200 000 tonnes), moderate grade (5%–7% Zn+Pb), volcanogenic massive sulfide deposit, which occurs within the Nepisiguit Falls Formation of the Ordovician Tetagouche Group. The host rocks are tuffaceous sedimentary rocks (Little Falls member) that overlie quartz-feldspar porphyritic tufflavas (Grand Falls member). The hanging-wall sequence comprises rhyolite of the Flat Landing Brook Formation, overlain by the Forty Mile Brook tholeiitic basalt. A dike of unaltered andesite was intersected beneath the massive sulfides, but was not found in the hanging-wall sequence in the vicinity of the deposit. The stratiform part of the deposit is made up of two, steeply south dipping, subparallel massive lenses that average approximately 4 m in thickness. Each lens is zoned, with an upper part consisting mainly of a pyrite and a basal part containing pyrite, sphalerite, and galena, with Ag values. The Au content in the massive sulfides is low (average = 52 ppb for 271 samples), but tends to be enriched in the massive pyrite near the top of each lens. Oxide facies iron formation is spatially associated with the massive sulfides, and has been traced up to 1800 m along strike. Footwall hydrothermal alteration is typical of VMS systems, and is characterized by depletion in K 2 O and Na 2 O, and enrichment in MgO and Fe 2 O 3 T with proximity to massive sulfides. The massive lenses are underlain by intensely chloritic, fine-grained, tuffaceous sedimentary rocks containing locally significant sulfide (chalcopyrite > pyrite > pyrrhotite) veins, which are interpreted to represent feeder zone mineralization. Unlike many VMS deposits, there is no evidence of silicification beneath the massive sulfides at the top of the feeder zone. The oxygen isotopic compositions (δ 18 O = −1.5‰ to −0.3‰) of hydrothermal chlorite coupled with sulfur isotopic compositions (δ 34 S ≈ 12‰) of the massive sulfides suggest that seawater was the dominant fluid in the hydrothermal system. Normalized rare earth element diagrams show flat Ce profiles and positive Eu anomalies, indicating that modified seawater was involved in ore formation; i.e., high-temperature (≥350°C), acidic, and reduced prior to entrainment into the hydrothermal cell. Although, seawater was the dominant fluid in the sulfide-forming system, δ 34 S values of stringer zone sulfides (8.5‰) coupled with elevated Sn (400 ppm) in one massive sulfide sample indicate that there was at least some magmatic component in the hydrothermal cell.

Timing of Cenozoic volcanism and Basin and Range extension in northwestern Nevada: New constraints from the northern Pine Forest Range

Eocene–middle Miocene volcanic rocks in the northern Pine Forest Range, Nevada, are ideally situated for reconstructing the timing and style of volcanism and extensional faulting in the northwesternmost part of the Basin and Range province. A conformable sequence of Cenozoic volcanic and sedimentary strata in the northern Pine Forest Range dips ~30°W, and 11 new 40 Ar/ 39 Ar ages from this sequence defi ne 3 major episodes of volcanic activity. Pre-Tertiary basement and older (ca. 38 Ma) Tertiary intrusive rocks are overlain unconformably by Oligocene (ca. 30–23 Ma) basalt fl ows and dacitic to rhyolitic ash-fl ow tuffs interbedded with fi ne-grained tuffaceous sedimentary rocks. Oligocene rocks are overlain by ~550 m of ca. 17–16 Ma basalt fl ows equivalent to the Steens Basalt in southern Oregon, and basalt fl ows are capped by a thin 16.3 Ma ignimbrite that likely is correlative with either the Idaho Canyon Tuff or the Tuff of Oregon Canyon. The northern Pine Forest Range is bounded to the east by a major down-to-the-east normal fault that dips ~40°E with well-developed fault striations indicative of dip-slip motion. This fault initiated at an angle of ~70° and was rotated ~30° during uplift of the range. A suite of 17 apatite fi ssion-track ages from the Pine Forest footwall block demonstrates that exhumation, uplift, and slip on the range-bounding fault began ca. 12–11 Ma and continued until at least 7 Ma, with moderate slip since then. The Pine Forest Range did not undergo signifi cant extension before or during peak Oligocene and Miocene volcanism, and similar geologic relationships in nearby ranges suggest that a larger region of northwestern Nevada was also little extended during this interval. Basin and Range faulting in northwestern Nevada appears to have begun no earlier than 12 Ma, making it distinctly younger than deformation in much of central and southern Nevada, where peak extension occurred in the middle Miocene or earlier.

The Small-Drillhole Minipermeameter Probe for In-Situ Permeability Measurement

Summary Laboratory measurement of permeability using a Hassler cell is the industry standard; however, consistently removing undisturbed rock samples from friable outcrops is difficult. Although various conventional surface-sealing mini-permeameters are developed as an alternative for permeability measurement, these devices generally suffer from difficulties in maintaining optimal forces on the tip seal when dealing with outcrop irregularities in the field; outcrop weathering is also problematic. Because a reliable field method is needed for studies of friable geological units, this paper presents an innovative technique for measuring permeability in situ. The design of the small-drillhole minipermeameter probe is discussed, as well as the accompanying analytical technique and the size and shape of the instrument's averaging volume. Small-diameter holes [i.e., 1.8 cm (0.7 in.)] are drilled into an outcrop with a masonry drill, followed by drillhole vacuuming, probe insertion, seal expansion, gas injection, and calculation of the intrinsic permeability through measurement of the injection pressure, gas-flow rate, and knowledge of the system geometry. Advantages of this approach include access to a nonweathered surface, an operator-independent sealing mechanism around the air-injection zone, and the potential for permeability measurement at multiple depths below an outcrop surface. To date, data have been collected from four diverse porous media: upper and lower shoreface sandstone (Escalante, Utah), saprolitic soils (Clemson, South Carolina), nonwelded and sintered ignimbrite (Bishop, California), and fluvially reworked tuffaceous sedimentary rock (Bishop, California). The probe has proved durable and robust, with a single probe sufficient for making thousands of measurements in a variety of environments. Data quality supports the conclusion that the drillhole probe is a practical field instrument.

Correlation of Olary and Broken Hill Domains, Curnamona Province:Possible Relationship to Mount Isa andOther North Australian Pb-Zn-Ag-Bearing Successions

The 300-Mt Broken Hill Pb-Zn-Ag orebody is in the Broken Hill Domain, which, together with the contiguous Olary Domain, constitutes the southern part of the Curnamona province of eastern Australia. The Curnamona province is dominated by medium- to high-grade metamorphic rocks of the Paleoproterozoic Willyama Supergroup and is the focus of exploration for base metals, gold, and uranium. A robust chronology of the ~1720 to 1640 Ma Willyama Supergroup provides the basis for temporal correlations on a regional scale between the Olary and Broken Hill Domains, and possibly on an interbasin scale between the southern Curnamona province and the Carpentaria zinc belt in northern Australia (Mount Isa, McArthur River). SHRIMP U-Pb zircon analyses from tuffaceous metasedimentary rocks provide ages for two new chronostratigraphic marker horizons in the Olary Domain. Firstly, the 1693 ± 3 Ma Plumbago Formation, and an equivalent metasiltstone in the Broken Hill Domain, confirm earlier suggestions that the base metal-enriched Bimba Formation is stratigraphically equivalent to the Ettlewood Calc-Silicate Member in the lower part of the ~1685 to 1695 Ma Broken Hill Group. Secondly, tuffaceous sedimentary rocks in the Mount Howden Subgroup of the Olary Domain have a maximum age of 1651 ± 7 Ma, thus suggesting correlation with a similar lithostratigraphic succession in the middle Paragon Group at Broken Hill. The lithostratigraphy, depositional ages, and inferred intervals of missing rock in the Curnamona province are broadly similar to the basin evolution of the Carpentaria zinc belt of northern Australia, suggesting possible chronostratigraphic correlations between Broken Hill and the Mount Isa-McArthur regions. The Broken Hill Group (~1695–1685 Ma) is considered to be time-equivalent to the Prize Supersequence in northern Australia, with possible implications for their respective depositional and metallogenic histories. Depositional ages from the middle Paragon Group (1655 ± 4 Ma, 1657 ± 4 Ma) at Broken Hill are closely comparable to the ca. 1655 Ma ages of tuff units associated with the Mount Isa and Hilton Pb-Zn-Ag orebodies (Gun Supersequence of northern Australia). The upper Paragon Group is considered to be the same age (ca. 1640 Ma) as that part of the Isa superbasin that hosts the McArthur River (HYC) deposit of the McArthur basin, the Walford deposit, and other Pb-Zn-Ag–mineralized sequences of the Lawn Hill platform.