Derivation of base-line geochemistry, petrography, and isotopic data for the host rocks to the Lucky Strike deposit and comparison with data from other alteration zones, Buchans Mining Camp, Newfoundland

The Buchans ore bodies of central Newfoundland represent some of the highest grade VMS deposits ever mined. These Kuroko-type deposits are also known for the well developed and preserved nature of the mechanically transported deposits. The deposits are hosted in Cambro-Ordovician, dominantly calc-alkaline, bimodal volcanic and epiclastic sequences of the Notre Dame Subzone, Newfoundland Appalachians. Stratigraphic relationships in this zone are complicated by extensively developed, brittledominated Silurian thrust faulting. Hydrothermal alteration of host rocks is a common feature of nearly all VMS deposits, and the recognition of these zones has been a key exploration tool. Alteration of host rocks has long been described to be spatially associated with the Buchans ore bodies, most notably with the larger in-situ deposits. This report represents a base-line study in which a complete documentation of the geochemical variance, in terms of both primary (igneous) and alteration effects, is presented from altered volcanic rocks in the vicinity of the Lucky Strike deposit (LSZ), the largest in-situ deposit in the Buchans camp. Packages of altered rocks also occur away from the immediate mining areas and constitute new targets for exploration. These zones, identified mostly by recent and previous drilling, represent untested targets and include the Powerhouse (PHZ), Woodmans Brook (WBZ) and Airport (APZ) alteration zones, as well as the Middle Branch alteration zone (MBZ), which represents a more distal alteration facies related to Buchans ore-formation. Data from each of these zones were compared to those from the LSZ in order to evaluate their relative propectivity. Derived litho geochemical data served two functions: (i) to define primary (igneous) trends and (ii) secondary alteration trends. Primary trends were established using immobile, or conservative, elements (i. e., HFSE, REE, Th, Ti0₂, Al₂0₃, P₂0₅). From these, altered volcanic rocks were interpreted in terms of composition (e.g., basalt - rhyodacite) and magmatic affinity (e.g., calc-alkaline vs. tholeiitic). The information suggests that bimodality is a common feature of all zones, with most rocks plotting as either basalt/andesite or dacite (or rhyodacite); andesitic senso stricto compositions are rare. Magmatic affinities are more varied and complex, but indicate that all units are arc volcanic sequences. Rocks from the LSZ/MBZ represent a transitional to calc-alkalic sequence, however, a slight shift in key geochemical discriminants occurs between the foot-wall to the hanging-wall. Specifically, mafic and felsic lavas of the foot-wall are of transitional (or mildly calc-alkaline) affinity whereas the hanging-wall rocks are relatively more strongly calc-alkaline as indicated by enriched LREE/HREE and higher ZrN, NbN and other ratios in the latter. The geochemical variations also serve as a means to separate the units (at least the felsic rocks) into hanging-wall and foot-wall sequences, therefore providing a valuable exploration tool. Volcanic rocks from the WBZ/PHZ (and probably the APZ) are more typical of tholeiitic to transitional suites, yielding flatter mantlenormalized REE patterns and lower ZrN ratios. Thus, the relationships between the immediate mining area (represented by LSZ/MBZ) and the Buchans East (PHZ/WBZ) and the APZ are uncertain. Host rocks for all zones consist of mafic to felsic volcanic rocks, though the proportion of pyroclastic and epiclastic rocks, is greatest at the LSZ. Phenocryst assemblages and textures are common in all zones, with minor exceptions, and are not useful for discrimination purposes. Felsic rocks from all zones are dominated by sericiteclay+/- silica alteration, whereas mafic rocks are dominated by chlorite- quartz- sericite alteration. Pyrite is ubiquitous in all moderately altered rocks and minor associated base metal sulphides occur locally. The exception is at Lucky Strike, where stockwork quartzveining contains abundant base-metal mineralization and barite. Rocks completely comprised of chlorite (chloritite) also occur in the LSZ foot-wall. In addition, K-feldspar alteration occurs in felsic volcanic rocks at the MBZ associated with Zn-Pb-Ba and, notably, without chlorite. This zone represents a peripheral, but proximal, zone of alteration induced by lower temperature hydrothermal fluids, presumably with little influence from seawater. Alteration geochemistry was interpreted from raw data as well as from mass balanced (recalculated) data derived from immobile element pairs. The data from the LSZ/MBZ indicate a range in the degree of alteration from only minor to severe modification of precursor compositions. Ba tends to show a strong positive correlation with K₂0, although most Ba occurs as barite. With respect to mass changes, Al₂0₃, Ti0₂ and P₂0₅ were shown to be immobile. Nearly all rocks display mass loss of Na₂O, CaO, and Sr reflecting feldspar destruction. These trends are usually mirrored by K₂0-Rb and MgO addition, indicating sericitic and chloritic alteration, respectively. More substantial gains ofK₂0 often occur in rocks with K-feldspar alteration, whereas a few samples also displayed excessive MgO enrichment and represent chloritites. Fe₂0₃ indicates both chlorite and sulphide formation. Si0₂ addition is almost always the case for the altered mafic rocks as silica often infills amygdules and replaces the finer tuffaceous material. The felsic rocks display more variability in Si0₂. Silicic, sericitic and chloritic alteration trends were observed from the other zones, but not K-feldspar, chloritite, or barite. Microprobe analysis of chlorites, sericites and carbonates indicate: (i) sericites from all zones are defined as muscovite and are not phengitic; (ii) at the LSZ, chlorites ranged from Fe-Mg chlorites (pycnochlorite) to Mg-rich chlorite (penninite), with the latter occurring in the stockwork zone and more proximal alteration facies; (iii) chlorites from the WBZ were typical of those from the more distal alteration facies of the LSZ, plotting as ripidolite to pycnochlorite; (iv) conversely, chlorite from the PHZ plot with Mg-Al-rich compositions (chlinochlore to penninite); and (v) carbonate species from each zone are also varied, with calcite occurring in each zone, in addition to dolomite and ankerite in the PHZ and WBZ, respectively. Lead isotope ratios for galena separates from the different various zones, when combined with data from older studies, tend to cluster into four distinctive fields. Overall, the data plot on a broad mixing line and indicate evolution in a relatively low-μ environment. Data from sulphide stringers in altered MBZ rocks, as well as from clastic sulphides (Sandfill prospect), plot in the Buchans ore field, as do the data for galena from altered rocks in the APZ. Samples from the Buchans East area are even more primitive than the Buchans ores, with lead from the PHZ plotting with the Connel Option prospect and data from the WBZ matching that of the Skidder prospect. A sample from a newly discovered debris flow-type sulphide occurrence (Middle Branch East) yields lead isotope ratios that are slightly more radiogenic than Buchans and plot with the Mary March alteration zone. Data within each cluster are interpreted to represent derivation from individual hydrothermal systems in which metals were derived from a common source.

Stable isotope ratios of planktonic foraminifera from North Atlantic surface sediments

We measured the oxygen isotopic composition of the deep-dwelling foraminiferal species G. inflata, G. truncatulinoides dextral and sinistral, and P. obliquiloculata in 29 modern core tops raised from the North Atlantic Ocean. We compared calculated isotopic temperatures with atlas temperatures and defined ecological models for each species. G. inflata and G. truncatulinoides live preferentially at the base of the seasonal thermocline. Under temperature stress, i.e., when the base of the seasonal thermocline is warmer than 16°C, G. inflata and G. truncatulinoides live deeper in the main thermocline. P. obliquiloculata inhabits the seasonal thermocline in warm regions. We tested our model using 10 cores along the Mauritanian upwelling and show that the comparison of d18O variations registered by the surficial species G. ruber and G. bulloides and the deep-dwelling species G. inflata evidences significant glacial-interglacial shifts of the Mauritanian upwelling cells.

Sedimentology, geochemistry and ostracodes from Lake Ohrid

Ancient Lake Ohrid, located in the southern Balkan Peninsula in Macedonia and Albania is characterized by a high degree of endemism and it is considered to be the oldest lake in Europe. But its exact age (between one and ten million years) and also its origin are so far not known. To unravel these uncertainties an ICDP (International Continental Scientific Drilling Program) drilling project (Scientific Collaboration On Past Speciation Conditions in Ohrid (SCOPSCO)), started in April 2013. In addition to the investigations about the age and origin, other paleolimnological studies, e.g., the reconstruction of past climate and of past lake level changes, should be performed with the drilled cores. Used proxies in such paleolimnological studies are, e.g., ostracodes because they respond sensitively to environmental changes but an accurate knowledge of their preferences and tolerances to specific environmental conditions is necessary for this purpose. So far, this knowledge about the, mostly endemic, Ohrid ostracodes was limited. Thus, within the framework of this thesis, ostracodes and a multiplicity of environmental data were collected in Lake Ohrid and its adjacent waters during four field campaigns. In a total of 47 ostracode species could be detected in the entire study area and 32 of them were found alive in Lake Ohrid. Multivariate statistic identified that water depth, salinity, conductivity, pH, and dissolved oxygen were the main determining factors for ostracode distribution in the entire study area. In Lake Ohrid, the distribution was mainly controlled by water depth, water temperature, and pH. Some ostracodes were identified as strong indicator species for important environmental variables, e.g., water temperature and water depth. A distinctive feature of Lake Ohrid was the finding of the ostracode genus Amnicythere whose species normally inhabit oligo-(meso-)haline waters and this could point to a marine origin of the lake. So far, the specialized endemic ostracodes show the highest abundances and the greatest spatial distribution in Lake Ohrid but during the sampling eight widespread species were found for the first time in the lake. They inhabited mainly the northern part of the lake, where two cities are located and industry and agriculture play a major role, and they were limited to water depths above 50 m and this could be an evidence for an increasing anthropogenic pressure because widespread ostracode species often replace endemic species. To unravel the human impact on Lake Ohrid during the last decades short sediment cores were taken and the multi-proxy study indicated that the lake productivity between the early 1920s and the late 1980s was relatively low. Diatom assemblages indicate a rising productivity in the southern part of Lake Ohrid since the mid 1970s and geochemical proxies and ostracodes point to an increasing productivity since the late 1980s in the southern and in the northern part. A slight increase in the productivity continued until 2009. Noticeable is the fact that since the early 1990s, the increasing productivity and the increasing concentrations of heavy metals correspond to a decreasing number of ostracodes in the northern part of Lake Ohrid. Perhaps, this indicates that living conditions in this lake part became less favorable for the mostly endemic ostracode species. Furthermore, the sediment samples from the cores show relatively high concentrations of arsenic, iron, and nickel. Fluctuations in ostracode assemblages from three longer sediment cores, the longest spans approximately 136 ka, taken in Lake Ohrid, correspond to fluctuations in the productivity, in the carbonate content, of the lake level, and of climate changes. Between the marine isotope stage (MIS) 6 and MIS 2 the number of ostracode valves is very low or the valves were completely absent. This corresponds to a low lake productivity, a low carbonate content, and a low lake level. At the onset of the Holocene, the number of valves increased markedly and this correlates with an increased productivity and carbonate content and a warmer climate. But during the Little Ice Age (LIA), the number of valves dropped again and species which prefer warmer waters disappeared completely. This drop corresponds also to a low productivity. After the LIA, the number of species increased again but since 1895 AD a strong and abrupt decrease is visible. A reason for this could be an increase in the heavy metal concentrations.

Geochemistry of sediment core HE337/001-1, North Sea

Reactive iron (oxyhydr)oxide minerals preferentially undergo early diagenetic redox cycling which can result in the production of dissolved Fe(II), adsorption of Fe(II) onto particle surfaces, and the formation of authigenic Fe minerals. The partitioning of iron in sediments has traditionally been studied by applying sequential extractions that target operationally-defined iron phases. Here, we complement an existing sequential leaching method by developing a sample processing protocol for d56Fe analysis, which we subsequently use to study Fe phase-specific fractionation related to dissimilatory iron reduction in a modern marine sediment. Carbonate-Fe was extracted by acetate, easily reducible oxides (e.g. ferrihydrite and lepidocrocite) by hydroxylamine-HCl, reducible oxides (e.g. goethite and hematite) by dithionite-citrate, and magnetite by ammonium oxalate. Subsequently, the samples were repeatedly oxidized, heated and purified via Fe precipitation and column chromatography. The method was applied to surface sediments collected from the North Sea, south of the Island of Helgoland. The acetate-soluble fraction (targeting siderite and ankerite) showed a pronounced downcore d56Fe trend. This iron pool was most depleted in 56Fe close to the sediment-water interface, similar to trends observed for pore-water Fe(II). We interpret this pool as surface-reduced Fe(II), rather than siderite or ankerite, that was open to electron and atom exchange with the oxide surface. Common extractions using 0.5 M HCl or Na-dithionite alone may not resolve such trends, as they dissolve iron from isotopically distinct pools leading to a mixed signal. Na-dithionite leaching alone, for example, targets the sum of reducible Fe oxides that potentially differ in their isotopic fingerprint. Hence, the development of a sequential extraction Fe isotope protocol provides a new opportunity for detailed study of the behavior of iron in a wide-range of environmental settings.

Sulphur mineralogy and geochemistry of serpentinites and gabbros from the Atlantis Massif

In-situ uplifted portions of oceanic crust at the central dome of the Atlantis Massif (Mid-Atlantic Ridge, 30°N) were drilled during Expeditions 304 and 305 of the Integrated Ocean Drilling Program (IODP) and a 1.4 km section of predominantly gabbroic rocks with minor intercalated ultramafic rocks were recovered. Here we characterize variations in sulfur mineralogy and geochemistry of selected samples of serpentinized peridotites, olivine-rich troctolites and diverse gabbroic rocks recovered from Hole 1309D. These data are used to constrain alteration processes and redox conditions and are compared with the basement rocks of the southern wall of the Atlantis Massif, which hosts the Lost City Hydrothermal Field, 5 km to the south. The oceanic crust at the central dome is characterized by Ni-rich sulfides reflecting reducing conditions and limited seawater circulation. During uplift and exhumation, seawater interaction in gabbroic-dominated domains was limited, as indicated by homogeneous mantle-like sulfur contents and isotope compositions of gabbroic rocks and olivine-rich troctolites. Local variations from mantle compositions are related to magmatic variability or to interaction with seawater-derived fluids channeled along fault zones. The concomitant occurrence of mackinawite in olivine-rich troctolites and an anhydrite vein in a gabbro provide temperature constraints of 150-200 °C for late circulating fluids along local brittle faults below 700 m depth. In contrast, the ultramafic lithologies at the central dome represent domains with higher seawater fluxes and higher degrees of alteration and show distinct changes in sulfur geochemistry. The serpentinites in the upper part of the hole are characterized by high total sulfide contents, high d34Ssulfide values and low d34Ssulfate values, which reflect a multistage history primarily controlled by seawater-gabbro interaction and subsequent serpentinization. The basement rocks at the central dome record lower oxygen fugacities and more limited fluid fluxes compared with the serpentinites and gabbros of the Lost City hydrothermal system. Our studies are consistent with previous results and indicate that sulfur speciation and sulfur isotope compositions of altered oceanic mantle sequences commonly evolve over time. Heterogeneities in sulfur geochemistry reflect the fact that serpentinites are highly sensitive to local variations in fluid fluxes, temperature, oxygen and sulfur fugacities, and microbial activity.

(Table 1) Interstitial water geochemistry at DSDP Leg 84 Holes

On DSDP Leg 84, drilling was conducted at three gas-hydrate-bearing sites on the Middle America Trench slope off Costa Rica (Site 565) and off Guatemala (Sites 568 and 570). At Site 569, on the mid-slope off Guatemala, hydrates may be present, according to the seismic profile (GUA-13), although the pore-water composition does not provide clear evidence. Sites 566 and 567, on the lower Guatemala Trench slope, appear to be free of hydrates, except in fractures of serpentinite at the bottom of Hole 566. Hydrate-bearing Sites 565, 568, and 570 show the effects of hydrate decomposition on pore-water chemistry that have been established during previous drilling at Sites 496 and 497 on the Guatemala Trench slope. These include a chlorinity decrease and d18O increase downsection. The new results, however, reveal more complex relationships between the chlorinity decrease and d18O increase than previously recognized. At Site 565, d18O values decrease in the middle section of the hole, whereas chlorinity continues to decrease from the top to near the bottom of the hole. Early diagenetic alteration of volcanic glass is suggested as a mechanism for the unexpected minimum in the O-isotope curve. Multiple fractionation by the pore-water/hydrate system is required to explain d18O-values greater than 2.7 per mil at the bottom of Hole 568, because with a fractionation factor of alpha = 1.0027, this is the maximum figure a single-stage fractionation could produce. In situ water samples from hydrate zones in most cases failed to display the elevated salinities expected for the residual pore waters not involved in hydrate formation. This is probably because the in situ sampling device still allows a systematic pressure drop sufficient to trigger hydrate decomposition in the immediate vicinity of the sample port.

Strontium isotope analysis of seawater and at a mixture of planktic foraminifera during the Paleocene and Early Eocene from different holes of ODP Leg 2008

We refined the strontium isotope seawater curve for the Paleocene and early Eocene by analysis of samples recovered from the Walvis Ridge during Ocean Drilling Project (ODP) Leg 208. The highest 87Sr/86Sr values occurred in the earliest Paleocene at 65 Ma and generally decreased throughout the Paleocene, reaching minimum values between 53 and 51 Ma in the early Eocene before beginning to increase again at 50 Ma. A plausible explanation for the 87Sr/86Sr decrease between 65 and 51 Ma is increased rates of hydrothermal activity and/or the eruption and weathering of large igneous provinces (e.g., Deccan Traps and North Atlantic). Strontium isotope variations closely parallel sea level and benthic d18O changes during the late Paleocene and early Eocene, supporting previous studies linking tectonic reorganization and increased volcanism to high sea level, high CO2, and warm global temperatures.

Geochemistry for sediment core MD02-2496

Variations in chalcophile and redox-sensitive trace elements are examined at high-resolution intervals from a ~50 kyr long sediment core (MD02-2496) from the Vancouver Island margin. Enrichments of Ag, Cd, Re, U, and Mo above lithogenous levels, signifying sedimentary suboxia and anoxia, occurred during the early Holocene and Bølling/Allerød, and during warm interstadial events of Marine Isotope Stage (MIS) 3. Down-core trace element profiles co-vary with productivity proxy records (opal, CaCO3, and marine organic carbon), and with sedimentary nitrogen isotope ratios, which reflect variably enriched nitrate upwelled from intermediate waters that were transported northward from the Eastern Tropical North Pacific. The similarity of the MD02-2496 record with records from the southern portion of the California Current System (CCS), and to the Greenland ice core oxygen isotope record during warm climate intervals, suggests that sedimentary redox conditions along the California Current responded to local productivity, to North Atlantic climate change and to tropical Pacific surface water processes via long-distance teleconnections. Concentrations of trace elements and productivity proxies were relatively depleted during the Younger Dryas, cool stadial events of MIS 3, and in two episodes of glaciomarine sedimentation from ~14.7 to 30.5 kyr BP (last glacial maximum, LGM), and from 44 to 50.4 kyr BP. Cordilleran Ice Sheet advancement onto the Vancouver Island continental shelf during the LGM led to intervals of increased terrigenous sedimentation and greatly reduced productivity not seen in the southern portion of the CCS, and along with ventilation of North Pacific Intermediate Waters, resulted in brief sedimentary oxic conditions.

Geochemistry of basement basaltic rocks of ODP Hole 131-808C

Major- and trace-element analyses, mineral chemistry, and Sr-Nd isotopic determinations were obtained on representative igneous rocks drilled from the Nankai accretionary complex (Site 808) during Ocean Drilling Program Leg 131. For the first time, the oceanic basement of the subducting plate below an accretionary prism has been reached. The Nankai Trough basement was encountered at a depth of 1289.9 mbsf and a total of 37.1 m of igneous rocks, middle Miocene (15.6 Ma) in age, was penetrated. Two main lithological units have been distinguished from the top downward; sill-like rocks (Unit I: Cores 105, 106, 107) and pillow lavas (Unit II: Core 108). Basalts are predominantly nonvesicular, hypocrystalline, aphyric to slightly phyric with intersertal to intergranular textures. Alteration is generally slight to moderate. All the basaltic rocks are cut by ramifying veins of varying widths. Secondary mineral assemblages (including vein fillings) are typical of submarine alteration and zeolite to low greenschist facies metamorphism. The order of crystallization of primary minerals is: olivine, plagioclase, clinopyroxene. This, together with mineral chemistry, characterized by forsteritic olivine (Fo 84-85), highly anorthitic Plagioclase (up to An 90), and in particular the composition of clinopyroxene, are typical of normal mid-ocean ridge basalts (MORB). In terms of Zr/Y (2.9-3.8) and Zr/Nb (21-58), all the analyzed samples plot in the normal MORB field. The chondrite-normalized REE patterns confirm the close affinity with normal MORB type (LaN/SmN: 0.6-0.8). Note that such magmatism does not reveal any evidence of subduction-related geochemical components. The 87Sr/86Sr isotopic ratios range from 0.70339 in pillow lavas to 0.70317 in the least-altered basalts of sill units (ratios reduced to 0.70265-0.70271 by HC1 2.5 N hot leaching), whereas 143Nd/144Nd ratios are 0.51314-0.51326. These values conform with those of normal MORB. Stratigraphy, petrography, and geochemistry of the basaltic rocks recovered at Site 808 appear very similar to those from the Shikoku Basin basement (particularly Sites 442 and 443, DSDP Leg 58), analogously identified as normal MORB.

Geochemistry in the mid Atlantic Ocean ridge basalts

The main objective of Leg 82 of the Glomar Challenger was to document mantle heterogeneity in the vicinity of, and away from, a so-called hot spot: the Azores Triple Junction. One of the geochemical tools that permits, at least in part, the recognition of mantle heterogeneities uses hygromagmaphile elements, those elements that have an affinity for the liquid. This tool is presented in terms of an extended Coryell-Masuda plot, which incorporates within the rare earth elements the hygromagmaphile transition elements Th, Ta, Zr, Hf, Ti, Y, and V. The extended Coryell-Masuda plot is used to summarize our knowledge of mantle heterogeneity along the ridge axis at zero-age. It is also used by choosing those hygromagmaphile elements that can be analyzed on board by X-ray fluorescence spectrometry to give preliminary information on the enriched or depleted character of recovered samples. Shore-based results, which include analyses of most of the hygromagmaphile elements measured either by X-ray spectrometry or neutron activation analysis, confirm the shipboard data. From the point of view of comparative geochemistry, the variety of basalts recovered during Leg 82 provides a good opportunity to test and verify the classification of the hygromagmaphile elements. Analyses from Leg 82 provide new data about the relationship between extended rare earth patterns (enriched or depleted) that can be estimated either by La/Sm ratio or Nb/Zr (or Ta/Hf) ratios: samples from Hole 556 are depleted (low Nb/Zr ratio) but have a high 206Pb/ 204Pb (19.5) ratio; in Hole 558 a moderately enriched basalt unit with a La/Sm (= Nb/Zr) ratio (chondrite normalized) of 2 has a high 206Pb/204Pb (20) ratio. One of the most interesting results of Leg 82 lies in the crossing patterns of extended Coryell-Masuda plots for basalts from the same hole. This result enhances the notion of local mantle heterogeneity versus regional mantle heterogeneity and is confirmed by isotope data; it also favors a model of short-lived, discrete magma chambers. The data tend to confirm the Hayes Fracture Zone as a southern limit for the influence of Azores-type mantle. Nevertheless, north of the Hayes Fracture Zone, the influence of a plumelike mantle source is not simple and probably requires an explanation more complex than a contribution from a single fixed hot spot.