Preliminary Geochemical Data Research Articles

Assessing the potential of Assarel porphyry copper deposit for critical raw materials: mineral-geochemical data for combination with agile exploration methods and better geo-modeling

The supply of European critical raw materials (CRMs) does not currently meet the European demand. This gap is predicted to increase, making Europe even more dependent on outside suppliers. AGEMERA is a project to help tackle this strategic problem by studying several CRM sites in Finland, Poland, Spain, the Balkans, and Zambia in different deposit types. To check the potential of the porphyry systems in Europe as a source of CRMs, the Assarel porphyry-Cu deposit was chosen as a test site. Here we present preliminary geochemical and mineral chemistry data for hydrothermal ore and alteration minerals in Assarel that reveal the potential of the deposit to source some CRMs as by-products. Further studies will combine the geochemical data with innovative geophysical methods for an improved deposit model and provide enhanced knowledge about effective exploration techniques for CRMs.

NEW TYPES OF BASIC ROCKS IN THE KORSUN-NOVOMYRHOROD ANORTHOSITE-RAPAKIVI GRANITE PLUTON AS AN INDICATOR OF ITS PETROGENESIS

Two species types of basic rocks contrasting in chemical and mineral composition were investigated within the Korsun-Novomyrhorod anorthosite-rapakivigranite pluton (KNP) of the Proterozoic age. In comparison to more typical basic rocks of KNP, the investigated rocks are presented by the extremely Fe-rich fayalite gabbroids and the most MgO enriched biotite gabbronorite. The first of them occupy a deeper level of the Horodishche massif in the central part of KNP. According to a high-Fe association of minerals and reduced or low An content in plagioclase they may represent the crystallization of a highly evolved basic melt (after anorthosite and gabbronorites separation), which were crystallized under abyssal conditions and low oxygen fugacity. Preliminary geochemical data indicate that trace elements concentration and negligible negative Eu-anomalies (0.72-0.95) are similar to most distributed basic rocks but unlike the last it is slightly differed by decreasing La/Yb and enriched in Sc (up to 118 ppm). Thus, we suppose those rocks might be crystallized as a result of mixing highly differentiated (iron and alkali enriched) melt with the early generation of anorthitic plagioclase, with subsequent dissolution of the last. Enrichment in iron of the mafic minerals and increasing of alkalinity of plagioclase in the basic rocks is consistent with the appearance of ferrodioritic melts as a product of prolonged crystalline differentiation of the initial melt. In contrast to fayalitic gabbroids, the pyroxene-biotite gabbronorites from the border zone in according to increased Mg# of the mafic minerals and rocks are obviously the least differentiated varieties of the anorthosite-gabbronorite series. The regularities in chemical composition in such type of rocks are consistent with the liquid line of dissent for basic rocks in KNP, which implies their crystallization at an earlier stage of magma ascending. By composition, such melt can be formed at an intermediate stage from slightly differentiated melt. This is indicated by enrichment in Sr (453-881 ppm) and Ba (910-930 ppm), Eu/Eu* (0.85-1.10), increased content MgO (up to 8 wt. %), Cr and V (59-193 and 169-350 ppm respectively). At the same time these rocks are enriched in Zr and Hf (378-478 and 10.3-12.02 ppm respectively), highly enriched in Rb (169-192 ppm), with moderate Nb and Ta content (14.6-18.1 and 0.91-2.84 ppm respectively) that point out to interaction and partial assimilation by crust material. Summarizing geological data of the deep drill-holes, it is possible to reveal a general direction of the mafic minerals evolution in the basic rocks and the evidences of cryptic layering. The last are quite clearly manifested both in the large gabbro-anorthosite massifs and individual intrusive bodies. We suppose that the evolution trend of mafic mineral composition are consistent with the tholeiitic trend differentiation of the primary melt with gradual increasing of iron content (under low oxygen fugacity) during differentiation and ascending in the upper crust. According to this interpretation of the inner structure of gabbro-anorthositic massifs, on the modern erosional level of KNP are often exposed the fragments of the upper (or lateral) layered series (Fe-enriched), less often the drill-holes reach up to the heads of the lower layered series.

Discovery of a large accumulation of natural hydrogen in Bourakebougou (Mali)

Recent exploratory wells in Mali (Bourabougou field) targeting natural hydrogen were a success and provide a new understanding of a first test well exploited by PETROMA (Bougou-1), and a comprehension of continental hydrogen functioning systems. Based on extensive gas data of a pioneer well and preliminary geochemical data obtained from a dozen exploratory wells in the vicinity, it is possible to confirm the presence of an extensive hydrogen field featuring at least five stacked reservoir intervals containing significant hydrogen that cover an estimated area well superior to 8 km in diameter. Results underline the potential economic interest of future natural hydrogen exploitation in continental onshore areas. A “hydrogen system” is presented with a kitchen of generation in the cratonic basement. The relatively pure hydrogen reservoirs are associated with traces of methane, nitrogen and helium. The geological stratigraphic accumulation of hydrogen is linked to the presence of multi overlaid doleritic sills and aquifers that seem to play a role to disable upward gas migration and leakage. The occurrence of a mixture of gas and water acting with an artesian activity confirms the presence of over-pressured fluids. This results in a geyser-type eruptive diphasic surface fluid in many of the wells. The “gas lift” system and the presence of traces of highly unstable carbon monoxide is linked to a recent hydrogen gas charge to reservoirs from underground aquifers, erupting with associated water. The Mali wells underscore the non-fossil source of hydrogen gas and presents features of a sustainable energy. The current estimate of its exploitation price is much cheaper than manufactured hydrogen, either from fossil fuels or from electrolysis.

Dyke Swarms in the Dharwar Craton: A Key to Understanding the Late Archean to Early Proterozoic Cratonic Correlations

Mafic dyke swarms are abundantly distributed in the Archean Dharwar craton. Previous studies have focused mainly on the major mafic dyke swarms in EDC; however, those in the WDC are yet to be studied in detail. Here we present preliminary geochemical data for the dykes in the Tiptur area, WDC and compare them with the dyke swarms in the EDC. Petrological studies indicate that the dykes in the Tiptur area fall into two distinct groups. The NW–SE trending dolerite dykes are unaltered, with characteristic ophitic textures and are geochemically comparable to 2.3 Ga EDC dykes. In contrast, the NE–SW trending meta-doleritic dykes showed high degree of alteration. The difference in petrography, major, trace and rare earth element geochemistry between the dolerites and meta-dolerites lead to a preliminary inference that these two suits of rocks might not be co-genetic. Meta-dolerites have not been reported from the EDC and it is possible to assume that they are a part of an earlier event, restricted in WDC, that might have emplaced prior to the amalgamation of WDC and EDC. In a global perspective, we compare our results with those reported in Archean cratons during late Archean to early Proterozoic around the world to constrain similarities that can lead to understanding the global scale magmatic activity and to aid in correlations between cratons.

Preliminary geochemical data of the mafic rocks from the Ovacik and Pülümür Ophiolite Zone (Eastern Anatolia, Turkey): implications for the geodynamic evolution of the Northern Neotethyan Ocean

The Izmir-Ankara-Erzincan-Sevan-Akera Suture Belt (IAESASB) stretches from the Aegean Sea to the Lesser Caucasus and includes the remnants of the Northern Neotethys. The Ovacik and Pulumur Ophiolite Zone (OPOZ) to the north of Tunceli is less-known members of this belt. In this study, preliminary geochemical data from the basaltic and gabbroic rocks from this zone are presented to provide the first insights into their geochemistry. The studied mafic samples exhibit geochemical characteristics of an intra-oceanic subduction system. Three types of samples were differentiated on the basis of their major, trace and rare earth element (REE) geochemistry. The first one is boninitic and displays highly depleted elemental pattern (relative to N-MORB) and concave REE profile. The second type is akin to back-arc basin basalts (BABB) and generally reflects normal-MORB (NMORB)-like high field strength element (HFSE) patterns coupled with negative Nb anomalies. The last type is relatively similar to the BABB-type, but it lacks the marked negative anomaly in Nb and can be ascribed to the mid-ocean ridge basalt (MORB). The overall geochemical features suggest that these mafic rocks were predominantly generated in a supra-subduction setting from a subduction-modified mantle source. It is further suggested that the Ovacik and Pulumur Ophiolite Zone was formed in an oceanic arc-basin system within the northward subducting Northern Neotethys.

Geochemical Characteristics of Mafic Rocks from the Xinlin Ophiolite, NE China

Located in the northern part of the Xinlin–Xiguitu suture zone, geochemistry and geochronology of the Xinlin ophiolite provide a unique opportunity to determine the the evolution of the eastern Xing'an–Mongolian Orogenic Belt. The Xinlin ophiolite was initially constrained roughly 21.5 km southeast of the Xinlin town by the First Regional Geological Survey Party of Heilongjiang Province. Subsequent work has shown that the mafic and ultramafic rocks in the adjacent Tayuan town was congenetic with the Xinlin ophiolite (Fig. 1). Over the past three years we have conducted a series of studies to the Xinlin ophiolite with the aim to better understand its characteristic and tectonic implications. The present work is is to provide our preliminary geochemical data of the mafic rocks of the Xinlin ophiolite and possible “congenetic” mafic rocks in the Tayuan town.The mafic rocks of the Xinlin ophiolite including the gabbro, diabase and metabasalt show flat REE pattern [(La/Yb)N = 0.68∼1.58] and no Eu anomalies, which are transitional between normal and enriched mid‐ocean ridge basalts (N‐MORB and E‐MORB). They also exhibit flat patterns from Ba to Yb in the trace elements spider diagram, which lie between those of typical E‐MORB and N‐MORB but closer to the former. The mafic rocks in the Tayuan town consist mainly of hornblende gabbro with alkaline affinity and are characterized by enriched in light rare earth elements and large ion lithophile elements, depleted in heavy rare earth elements and high field strength elements (Fig. 2). The obvious differences in the geochemical characteristics indicate that the mafic rocks in the Tayuan town may not be cogenetic with those of the Xinlin ophiolite. This was further corroborated by their different formation time. Our zircon U–Pb dating indicates that the gabbro in the Tayuan town was emplaced during the late Carboniferous (∼310 Ma; Fig. 3), significantly younger than the recently reported U–Pb ages for the mafic rocks of the Xinlin ophiolite (∼510 Ma; Feng, 2015). Therefore, the two units appear as independent bodies and their origin and tectonic implication need to be further examined.

Polymagmatic processes at monogenetic volcanoes: insights from Baossi monogenetic lava flows, Adamawa Plateau, Cameroon Volcanic Line

Abstract This paper presents principally the textural and geothermometric evidence of polymagmatic activity at a monogenetic volcano along the Cameroon Volcanic Line (CVL) in a bid to contribute to the global understanding of how such volume-limited basaltic magmatic systems operate. Preliminary geochemical data presented show that the lavas are alkali basalt and basanite (SiO2: 40.42 – 47.59 wt%, MgO: 7.61 – 9.13 wt%) and are petrographically indistinguishable. They outcrop as low mounds of columnar basalts associated with sparse pyroclastic materials. The lavas are fine porphyritic with phenocrysts of olivine and clinopyroxene set in a plagioclase microlite-dominated hypocrystalline groundmass. Olivine has two distinct populations: grains that are anhedral to glomerocrystic with spinel and plagioclase inclusions and usually have resorbed margins (olivine 1); and grains that are euhedral, sometimes skeletal, with a thin and well preserved rim (olivine 2). Similarly two clinopyroxene crystal populations are recognizable: clinopyroxene grains that are resorbed, dismembered, with sieve-textured cores and irregular core-rim margins (clinopyroxene 1); and euhedral clinopyroxene grains partially enclosing olivine phenocrysts with hour glass and sector zoning (clinopyroxene 2). These textural features suggest that the eruption was caused by an influx of a fresh batch of magma (that crystallized olivine 2 and clinopyroxene 2) into a fractionated crystal mush (olivine 1 and clinopyroxene 1) in the chamber. The crystal mush was re-heated by the intruding basaltic magma resulting in a broad pre-eruption liquidus temperature of 1040 – 1156oC calculated using mineral chemistry of equilibrium olivine-clinopyroxene pairs. Keywords: Cameroon volcanic line, geothermometry, monogenetic volcano, olivine-clinopyroxene pairs

Extremely Re-Rich Molybdenite from Porphyry Cu-Mo-Au Prospects in Northeastern Greece: Mode of Occurrence, Causes of Enrichment, and Implications for Gold Exploration

Extremely Re-rich molybdenite occurs with pyrite in sodic–calcic, sodic–sericitic and sericitic-altered porphyritic stocks of granodioritic–tonalitic and granitic composition in the Sapes–Kirki–Esymi, Melitena and Maronia areas, northeastern Greece. Molybdenite in the Pagoni Rachi and Sapes deposits is spatially associated with rheniite, as well as with intermediate (Mo,Re)S2 and (Re,Mo)S2 phases, with up to 46 wt % Re. Nanodomains and/or microinclusions of rheniite may produce the observed Re enrichment in the intermediate molybdenite–rheniite phases. The extreme Re content in molybdenite and the unique presence of rheniite in porphyry-type mineralization, combined with preliminary geochemical data (Cu/Mo ratio, Au grades) may indicate that these deposits have affinities with Cu–Au deposits, and should be considered potential targets for gold mineralization in the porphyry environment. In the post-subduction tectonic regime of northern Greece, the extreme Re and Te enrichments in the magmatic-hydrothermal systems over a large areal extent are attributed to an anomalous source (e.g., chemical inhomogenities in the mantle-wedge triggered magmatism), although local scale processes cannot be underestimated.

Cosmogenic 36Cl in karst waters: Quantifying contributions from atmospheric and bedrock sources

Improved reconstructions of cosmogenic isotope production through time are crucial to understand past solar variability. As a preliminary step to derive atmospheric 36Cl/Cl solar proxy time‐series from speleothems, we quantify 36Cl sources in cave dripwaters. Atmospheric 36Cl fallout rates are a potential proxy for solar output; however extraneous 36Cl derived from in‐situ production in cave host‐rocks could complicate the solar signal. Results from numerical modeling and preliminary geochemical data presented here show that the atmospheric 36Cl source dominates in many, but not all cave dripwaters. At favorable low elevation, mid‐latitude sites, 36Cl based speleothem solar irradiance reconstructions could extend back to 500 ka, with a possible centennial scale temporal resolution. This would represent a marginal improvement in resolution compared with existing polar ice core records, with the added advantages of a wider geographic range, independent U‐series constrained chronology, and the potential for contemporaneous climate signals within the same speleothem material.

Geoquímica de Gnaisses do arco magmático de Goiás na Região Sul do Estado de Goiás

Granitic to trondhjemitic gneisses from the Pontalina region in the southern part of Goiás State, Central Brazil, have calcic to calc-alkaline, metaluminous to peraluminous compositions. They have low concentrations of alkaline elements, and are enriched in Ba, Sr, K, Rb in relation to Nb, Y, Zr and REE, and have negative anomalies of Nb and Ti, features which are similar to those of magmas generated in magmatic arc environments. Those rocks were previously interpreted as part of the basement of the Brasília Belt, attributed to the Archean to Paleoproterozoic, but new Sm - Nd isotopic data indicate a neoproterozoic age (T DM = 0,9 a 1,2 Ga), and the preliminary geochemical data reveal compositions similar to the gneisses of the other regions belonging to the Goiás Magmatic Arc.

Mapping of Muslim Bagh ophiolite complex (Pakistan) using new remote sensing, and field data

TETHYS is a relational GIS database that combines geophysical, remote sensing, geochemical, and geochronological data, developed as a flexible resource for studying magmatic and geodynamic responses to continental collisions. In this paper, we demonstrate utility of our database by integrating field, remote sensing, and structural data, for detailed mapping and tectonic emplacement of Muslim Bagh ophiolite of western Pakistan. This ophiolite forms the uppermost part of a nappe pile which accreted onto the Indian continental margin during the closure of the Neo-Tethys during a pre-terminal collision that predated the final closure of Tethys during the major collision between India and Eurasia. Utilizing the TETHYS, Landsat, ASTER imagery, and a digital elevation model developed from the ASTER data are used to characterize the lithology and structure of the area. Use of image processing techniques improved the geologic map of the area, for a better understanding of the tectonic emplacement of the Muslim Bagh ophiolite. For the first time we report that the dikes in the Muslim Bagh ophiolite are cutting the metamorphic sole. Our preliminary geochemical data for sheeted dike complex suggest chemical affinities with arc-related rocks. This observation suggests that dikes were intruded in an island arc environment soon after the ophiolite was formed.

GEOCHEMISTRY, TECTONO-MAGMATIC DISCRIMINATION AND RADIOLARIAN AGES OF BASIC EXTRUSIVES WITHIN THE IZMIR-ANKARA SUTURE BELT (NW TURKEY): TIME CONSTRAINTS FOR THE NEOTETHYAN EVOLUTION

The Dagkuplu Melange in the Central Sakarya Valley represents the northernmost outcrops of the Izmir-Ankara Suture Belt in northwest Anatolia. In addition to blocks and slivers of serpentinite, gabbro, blueschist, neritic and pelagic limestones, it includes blocks of basic volcanic rocks associated with radiolarian cherts, pelagic carbonates and mudstones. The preliminary geochemical data revealed the existence of a variety of basaltic rocks with magma types ranging in composition of MORB, IAT, OIB and CAB, in the melange. The age of the radiolarian assemblage from a tectonic block of chert-mudstone alternation associated with OIB-type basalts within the melange is assigned to early Berriasian - early Hauterivian, based on the co-occurrence of radiolarian taxa as Angulobracchia sp. cf. A. (?) portmanni, Godia nodocentrum, Pantanellium masirahense, Thanarla brouweri, Pseudoeucyrtis hanni, Svinitzium mizutanii, Mirifusus dianae s.l., Tethysetta boesii. Another block of chert-mudstone alternation associated with MORB-type basalts includes the following Cenomanian radiolarian fauna: Thanarla pulchra, Novixitus mclaughlini, Pseudodictyomitra pseudomacrocephala, Pseudodictyomitra tiara, Stichomitra communis. New findings from the Central Sakarya area combined with previous data of the authors reveal that the Izmir-Ankara Ocean started to open already in the Late Triassic. The formation of OIB-type intra-plate seamounts within the Izmir-Ankara Ocean began in late Bathonian and persisted until early Aptian. The the intra-oceanic subduction and the generation of supra-subduction-type volcanism started in early Santonian and the spreading-ridge of the Izmir-Ankara Ocean plate was not subducted until the Cenomanian.

PRELIMINARY GEOCHEMICAL DATA ON VOLCANIC ROCKS FROM THE OLT NAPPE (TRANSYLVANIAN NAPPES, ROMANIA): IMPLICATION FOR THEIR ORIGIN IN THE TRANSYLVANIDES

The Transylvanian nappes (Rarau, Haghimas and Olt nappes - Fig. 1a) are interpreted as remnants of the Main Tethyan Suture in the Carpathians. According to some authors (Sandulescu and Russo-Sandulescu, 1981; Sandulescu, 1984; Russo-Sandulescu et al., 1983; Sandulescu in Ichim and Sandulescu, 1997), they consist of various types of sedimentary and Mesozoic magmatic rocks related to the ophiolites of Transylvanides (Fig. 1a), which in turn represent a Tethyan oceanic crust associated to the Vardar Zone (Bortolotti et al., 2002). The nature of volcanics from the Olt nappe (ON) is still poorly constrained. Cioflica et al. (1965) reported the occurrence of sub-alkaline and alkaline volcanics interpreted as evolved magmatic products of a tholeiitic ophiolitic series. Nonetheless the alkaline chemistry of volcanics is not consistent with the interpretation of these rocks as ophiolitic remnants linked with the Transylvanides ophiolites (Sandulescu, 1984; Sandulescu in Ichim and Sandulescu, 1997), since no alkaline rocks are found in these ophiolites. The aim of this paper is thus to present preliminary geochemical data on volcanic rocks from the ON in order to assess their possible genetic relationships with the Transylvanides ophiolites.

JD-QTZ-BEARING METAPLAGIOGRANITE FROM THE MONVISO META-OPHIOLITE (WESTERN ALPS)

In the Basal Serpentinite Unit of the Monviso meta-ophiolite (Val Varaita, Western Alps), a large jadeite-quartz-bearing metaplagiogranite body has been found, which shows magmatic contacts with eclogitic Pl-Cpx-FeTi oxide metagabbro. Primary texture and magmatic minerals in the metaplagiogranite are mostly obliterated by multistage Alpine re-equilibrations. The highest-pressure assemblage consisted of jadeite, quartz, phengite and rutile. Garnet and zoned ferroglaucophane developed immediately after the peak assemblage, and were followed by aegirine-rich clinopyroxene. Calculated phase compatibilities and phengite geobarometry indicate T = 575°C and P = 1.95 GPa as minimal temperature and pressure of formation for the peak assemblage in the metaplagiogranite, in agreement with garnet-clinopyroxene geothermometry from the associated FeTi-oxide metagabbro which yields T = 545±35°C at P = 2.0 GPa. Major, trace, and REE data are typical of oceanic plagiogranite, and similar to those of metaplagiogranite from other meta-ophiolite units of the Western Alps, Northern Apennines and Corsica. Petrological and preliminary geochemical data suggest that the Monviso plagiogranite likely represents late stages of the plutonic activity in this sector of the Western Tethys, and could have formed by fractional crystallization of ocean-floor tholeiite magmas in Late Jurassic times.

The petrology and petrogenesis of the North Motzfeldt Centre, Gardar Province, South Greenland

AbstractNorth Motzfeldt is an intrusive igneous centre within the Igaliku (formerly spelt Igaliko) complex, Gardar Province, South Greenland. A detailed field description of the centre is given, with preliminary geochemical and isotopic data. The North Motzfeldt centre is intruded into Ketilidian granitoid basement rocks, and current exposure retains parts of the original roof against earlier Gardar eruptives and clastic sediments of the Eriksfjord formation. The unconformity between the Ketilidian and the overlying Eriksfjord is believed to have been crucial in the siting of the centre. The centre is subdivided into two major units, NM1 and NM2. This was followed by significant peralkaline nepheline microsyenite in sheets, characterized by rare element-rich accessory minerals including rinkite-mł sandrite, pyrochlore and låvenite. A significant microsyenite body is called NM3. A preliminary Rb-Sr isochron of 1226±27 Ma indicates a far younger age for the centre than previously thought (~;1350 Ma). Cross-cutting relationships between North Motzfeldt and rocks of the adjacent Motzfeldt centre require a reappraisal of the chronology of Motzfeldt magmatism. We suggest that rocks previously believed to comprise the Motzfeldt centre represent the products of multiple phases of magmatism from Early- to Mid-Gardar times.

Sm[sbnd]Nd, Rb[sbnd]Sr and Th[sbnd]U[sbnd]Pb zircon ages of syn- and post-tectonic granitoids from the Axum area of northern Ethiopia

SmNd, RbSr and ThUPb zircon ages for four syn-tectonic and two post-tectonic granitoids from the Axum area of northern Ethiopia are determined. Two of the syn-tectonic granitoids (the Azeho and Deset) are intrusions into structurally southeast facing, predominantly tholeiitic arc metavolcanics and associated metasediments situated west of the central steep zone in the area. The other two syn-tectonic granitoids (the Chila and Rama) are intrusions into structurally northwest facing metasediments and calc-alkaline metavolcanics at the eastern part of the steep zone. One of the post-tectonic granites (the Sibta Granite) occurs west of the central steep zone and the second (the Shire Granite) cuts the central steep zone. Preliminary geochemical data of all the granitoids show that they are enriched in large ion lithophile elements, depleted in high field strength elements and have 1-type characteristics, similar to calc-alkaline granitoids in subduction-related volcanic arc setting. The geochronological data indicate three discrete intrusive events: ∼800 Ma to the east of the central steep zone, ∼750 Ma to the west and a post-tectonic intrusion at around 550 Ma. The ∼-750 and ∼800 Ma ages of the intrusives are interpreted as minimum ages of arc magmatism in the respective blocks across the central steep zone, and the ∼550 Ma age of the post-tectonic granites records the final magmatic event in the region. The contrast of age across the central steep zone emphasises that this zone is a major structural element that might have played a significant role during the accretion of structurally and lithologically contrasting tectonostratigraphical blocks.

Neoproterozoic zirconium-depleted boninite and tholeiitic series rocks from Adola, southern Ethiopia

In Adola, southern Ethiopia, mafic and ultramafic igneous rocks occur in narrow, 4–10 km wide, north-south-trending belts bounded by high-grade gneisses and migmatites. The mafic/ultramafic rocks are complexly deformed and metamorphosed in greenschist to lower amphibolite facies and are thought to be tectonically dismembered parts of an ophiolite complex. Preliminary geochemical and geochronological data highlight that the high-grade rocks in southern Ethiopia and northern Kenya include a significant portion of juvenile rocks that were accreted at the same time as ophiolitic rocks at 885-765 Ma. This is also the time of widespread oceanic magmatism and closure in the Arabian-Nubian Shield to the north. The Adola mafic rocks were previously described as island arc tholeiites and mid-ocean ridge basalts (MORB). New chemical analyses on the Megado belt rocks reveal the presence of boninites and related dacites interspersed with tholeiitic rocks. The Adola boninites are similar to the Cambrian boninites in western Tasmania in having relatively low Zr/Sm (≤32). Boninites with similarly low ratios have not been reported from elsewhere. The Adola tholeiites have high Ti/Zr (150–300). Mixing between tholeiite and boninite magmas may have resulted in elevated Ti/Zr (80–126) in some Adola boninites. Otherwise, Ti/Zr in the latter is low (20–40). Low Ti/Zr is characteristic of Tertiary boninites in the west Pacific. The fact that both Ti/Eu and Zr/Sm increase from the Adola and Tasmania type to the Tertiary boninites at constant Ti/Zr suggests that Ti might be an element that is also metasomatically added to the source of boninites and raises doubts about the role of amphibole in boninite petrogenesis.

The Deep Madden Field, a Super-Deep Madison Gas Reservoir, Wind River Basin, Wyoming: ABSTRACT

Madison dolomites form the reservoir of a super deep, potential giant sour gas field developed on the Madden Anticline immediately in front of the Owl Creek Thrust along the northern rim of the Wind River Basin, central Wyoming. The Madison reservoir dolomites are presently buried to some 25,000 feet at Madden Field and exhibit porosity in excess of 15%. An equivalent dolomitized Madison sequence is exposed in outcrop only 5 miles to the north on the hanging wall of the Owl Creek thrust at Lysite Mountain. Preliminary comparative stratigraphic, geochemical and petrologic data, between outcrop and available cores and logs at Deep Madden suggests: (1) early, sea level-controlled, evaporite-related dolomitization of the reservoir and outcrop prior to significant burial; (2) both outcrop and deep reservoir dolomites underwent significant recrystallization during a common burial history until their connection was severed during Laramide faulting in the Eocene; (3) While the dolomite reservoir at Madden suffered additional diagenesis during an additional 7-10 thousand feet of burial, the pore systems between outcrop and deep reservoir are remarkably similar. The two existing deep Madison wells at Madden are on stream, with a third deep Madison well currently drilling. The sequence stratigraphic framework and the diageneticmore » history of the Madison strongly suggests that outcrops and surface cores of the Madison in the Owl Creek Mountains will be useful in further development and detailed reservoir modeling of the Madden Deep Field.« less

The petrology of lower crustal xenoliths from the Executive Committee Range, Marie Byrd Land Volcanic Province, West Antarctica

Alkaline magmas from the late-Cenozoic Marie Byrd Land Volcanic Province, West Antarctica, have entrained lithospheric xenoliths which vary from spinel lherzolites to supracrustal rocks. Lower crustal xenoliths have been collected from the Executive Committee Range (Mounts Hampton in the north and Mount Sidley in the south) in central Marie Byrd Land, and their petrological characteristics together with preliminary geochemical data are discussed here. Granulite xenoliths include metaigneous gabbros and norites with varying proportions of clinopyroxene, spinel and either olivine or orthopyroxene. Pyroxenites occur together with granulites, which on the basis of their similar mineral assemblage, texture and composition are considered to be related to the granulites. The composition of xenoliths from Mounts Hampton and Sidley differ (e.g. Mount Sidley xenoliths have Mg# 32–80, are relatively LREE enriched and have 87Sr86Sr of 0.70286–0.70376 and 143Nd144Nd of 0.512864–0.512870, whereas Mount Hampton xenoliths have Mg# 68–78, are LREE depleted and have 87Sr86Sr of 0.70420–0.70458 and 143Nd144Nd of 0.512771–0.512819), defining a major lateral lower crustal discontinuity beneath the Executive Committee Range. Relict igneous textures and low abundances of incompatible elements indicate that the xenoliths initially formed as cumulates rather than as trapped melts. The xenolith suite differs in composition to the host rocks (Mount Sidley volcanics = 87Sr86Sr of 0.70300–0.70312 and 143Nd144Nd of 0.512814–0.512907) and cannot be co-genetic with them. They are interpreted here to represent the cumulates of mantle melts that evolved by crystal fractionation at lower crustal depths.

Neocomian Source and Reservoir Rocks in the Western Brooks Range and Arctic Slope, Alaska: ABSTRACT

Detailed (1:63,360) mapping of the Tingmerkpuk sandstone and associated rocks in the Misheguk Mountain and DeLong Mountains guadrangles of the western Brooks Range thrust belt documents potential hydrocarbon source and reservoir rocks in the northern foothills of the western Delong Mountains and adjacent Colville basin of northwest Alaska. Neocomian (?) to Albian micaceous shale, litharenite, and graywacke that overlies the Tingmerkpuk represents the onset of deposition of orogenic sediments derived from the Brooks Range to the south, and the merging of northern and southern sediment sources in the Colville basin. Both the proximal and distal Tingmerkpuk facies contain clay shale interbeds and overlie the Upper Jurassic to Neocomian Kingak Shale. Preliminary geochemical data show that in the thrust belt, these shales are thermally overmature (Ro 1.4-1.6), but are good source rocks with total organic content (TOC) that ranges from 1.2 to 1.8 percent. Shale in the overlying Brookian rocks is also thermally overmature (Ro 1.2-1.5 percent), but contains up to 1.8 percent TOC from a dominantly terrigenous source, and has generated gas. In outcrops at Surprise Creek, in the foothills north of the thrust belt, the Kingak (1.9 percent TOC) and underlying Triassic Shublik Formation (4.6 percent TOC) are excellentmore » oil source rocks with thermal maturity close to peak oil generation stage (Ro0.75-0.9 percent). These rocks have lower thermal maturity values than expected for their stratigraphic position within the deeper parts of the Colville basin and indicate anomalous burial and uplift history in parts of the basin. Preliminary apatite fission-track (AFTA) data from the thrust belt indicate a stage of rapid uplift and cooling at about 53.61 Ma.« less