Km Of Strike Research Articles

A Comparison of Granite Genesis in the Adelaide Fold Belt and Glenelg River Complex Using U–Pb, Hf and O Isotopes in Zircon

Abstract We present new U–Pb ages and Hf and O isotope data for zircon from I-, S- and A-type granites from both the western and eastern edges of the Delamerian Orogen in southeastern Australia. The I-type Tanunda Creek Gneiss contains zircon populations of 507 ± 4 and 492 ± 6 Ma inferred to reflect igneous and metamorphic ages, respectively. The I-type Palmer Granite yielded an age of 509 ± 3 Ma, and the Port Elliot S-type Granite has a magmatic age of 508 ± 7 Ma. Inherited zircon in these granites range from 1092 to 3343 Ma, probably derived from assimilation of Adelaide Group sediments. The Murray Bridge A-type Granite is 490 ± 2 Ma in age and lacks inherited zircon. In the Glenelg River Complex, an S-type migmatite from near Harrow contains a complex zircon population. It is most likely ~500 Ma in age and has inherited zircon of 550–700, 1000–1100 and 2437 Ma, hence matching those from the Kanmantoo Group. From this and detrital zircon ages, we infer that only the Kanmantoo Group extends across the Murray Basin into the Glenelg River Complex. The Wando Tonalite and Loftus Creek I-type granites yielded ages of 501 ± 2 and 486 ± 3 Ma, respectively. Zircon from the Dergholm Granite has suffered Pb loss, and the best age estimate for this granite is 488 ± 5 Ma. Combining all the granite data together, εHft and δ18O in the magmatic zircon range from 5.6 to −10.3 and from 5.8 to 8.1, respectively, and are well correlated. The zircon indicates the same temporal and compositional evolution of granitic petrogenesis across ~300 km of strike, reaffirming the notion that these terranes form part of the same orogen. Westward-directed subduction caused orogenic thickening, heating and increasing amounts of crustal contribution. This was followed by convective thinning of the thickened mantle lithosphere and a return to more primitive magmas lacking significant crustal contributions. It contrasts significantly with inferred granite petrogenesis and tectonic style in the younger Lachlan and New England Fold Belts further east that were not built upon extended cratonic lithosphere.

Compressional origin of the Aegean Orogeny, Greece

The Aegean Sea area is thought to be an actively extending back-arc region, north of the present day Hellenic volcanic arc and north-dipping subduction zone in the Eastern Mediterranean. The area shows extensive normal faulting, ductile ‘extensional’ shear zones and extensional S-C fabrics throughout the islands that have previously been related to regional Aegean extension associated with slab rollback on the Hellenic Subduction Zone. In this paper, we question this interpretation, and suggest the Cenozoic geodynamic evolution of the Aegean region is associated with a Late Cretaceous–Eocene NE-dipping subduction zone that was responsible for continent-continent collision between Eurasia and Adria-Apulia/Cyclades. Exhumation of eclogite and blueschist facies rocks in the Cyclades and kyanite-sillimanite grade gneisses in the Naxos core complex have pressures that are far greater than could be accounted for purely by lithospheric extension and isostatic uplift. We identify four stages of crustal shortening that affected the region prior to regional lithospheric extension, herein called the Aegean Orogeny. This orogeny followed a classic Wilson cycle from early ophiolite obduction (ca. 74 Ma) onto a previously passive continental margin, to attempted crustal subduction with HP eclogite and blueschist facies metamorphism (ca. 54–45 ​Ma), through crustal thickening and regional kyanite – sillimanite grade Barrovian-type metamorphism (ca. 22–14 ​Ma), to orogenic collapse (<14 ​Ma). At least three periods of ‘extensional’ fabrics relate to: (1) Exhumation of blueschists and eclogite facies rocks showing tight-isoclinal folds and top-NE, base-SW fabrics, recording return flow along a subduction channel in a compressional tectonic setting (ca. 50–35 ​Ma). (2) Extensional fabrics within the core complexes formed by exhumation of kyanite- and sillimanite gneisses showing thrust-related fabrics at the base and ‘extensional’ fabrics along the top (ca. 18.5–14 ​Ma). (3) Regional ductile-brittle ‘extensional’ fabrics and low-angle normal faulting related to the North Cycladic Detachment (NCD) and the South(West) Cycladic Detachment (WCD) during regional extension along the flanks of a major NW–SE anticlinal fold along the middle of the Cyclades. Major low-angle normal faults and ductile shear zones show symmetry about the area, with the NE chain of islands (Andros, Tinos, Mykonos, Ikaria) exposing the NE-dipping NCD with consistent top-NE ductile fabrics along 200 ​km of strike. In contrast, from the Greek mainland (Attica) along the SE chain of islands (Kea, Kythnos, Serifos) a SW-dipping low-angle normal fault and ductile shear zone, the WCD is inferred for at least 100 ​km along strike. Islands in the middle of the Cyclades show deeper structural levels including kyanite- and sillimanite-grade metamorphic core complexes (Naxos, Paros) as well as Variscan basement rocks (Naxos, Ios). The overall structure is an ~100 ​km wavelength NW–SE trending dome with low-angle extensional faults along each flank, dipping away from the anticline axis to the NE and SW. Many individual islands show post-extensional large-scale folding of the low-angle normal faults around the domes (Naxos, Paros, Ios, Sifnos) indicating a post-Miocene late phase of E–W shortening.

The Penikat Intrusion, Finland: Geochemistry, Geochronology, and Origin of Platinum–Palladium Reefs

The Palaeoproterozoic Penikat layered ultramafic–mafic intrusion in northern Finland is one of the most richly mineralized layered intrusions on Earth, containing at least six platinum-group element (PGE) enriched horizons exposed along >20 km of strike, amongst them the SJ reef, which at ∼3–7 ppm Pt + Pd over a width of ∼1–2 m is surpassed by few other PGE reefs globally in terms of its endowment in PGE. Important PGE enrichments also occur in the PV reef (average 2·6 ppm Pd, 4 ppm Pt over 1·1 m) and AP1 reef (average 6·2 ppm Pd, 1·7 ppm Pt over 0·7 m). Here we present new major and high-precision trace element and Nd isotope data from a traverse across the intrusion, and a new U–Pb age of 2444 ± 8 Ma for the intrusion. We show that the PGE reefs formed by predominantly orthomagmatic processes as, for example, reflected by well-defined positive correlations between Pt + Pd and Os + Ir + Ru contents. Late-magmatic fluids played no significant role in concentrating PGE. There are at least six cyclic units in the intrusion, displaying a progressive upward decrease in differentiation indices Mg# and Cr/V. Subdued stratigraphic variations in incompatible trace element ratios (Ce/Sm mostly 5–10) and Nd isotope compositions (eNd –3 to –1) indicate that mixing of magmas of distinct lineage, or in situ contamination with country rocks, was not required to form the PGE reefs. There is also no evidence for addition of external sulphur to the magma, based on S/Se ratios at, or below, primitive mantle levels. Instead, we propose that sulphide melt saturation at Penikat was reached in response to fractionation of a siliceous, high-magnesium basalt, and that the sulphides were concentrated through hydrodynamic phase sorting, consistent with bonanza-style PGE grades in large potholes.

The Kalahari Copperbelt in Central Namibia: Controls on Copper Mineralization

ABSTRACT The Kalahari Copperbelt stretches discontinuously for 800 km from central Namibia to northern Botswana (Figure 1; Borg, 1988; Borg and Maiden, 1989). In central Namibia, copper mineralization, hosted by slate and phyllite, is intermittently developed over more than 60 km of strike of the Kagas Member of the Klein Aub Formation. The strata-bound nature of copper occurrences led early explorers to the conclusion that copper was either syngenetic, i.e., emplaced during deposition of the host strata, or diagenetic, i.e., emplaced into the host strata during burial and compaction. As a result, initial exploration largely focused on “favorable” stratigraphic units as target horizons and for the most part ignored structurally controlled target areas of significant economic potential. Subsequent research at the (presently inactive) Klein Aub Mine and elsewhere in the Namibian part of the belt showed that, although copper concentrations are broadly strata-bound, the structural associations (e.g., the relation to a late reverse fault at Klein Aub) and detailed textural features (e.g., copper in veins, brittle fractures, cleavage-parallel lenticles, and tectonic breccia zones) indicate that copper mineralization was emplaced into structurally controlled sites late in the deformation history of the region. The conclusion of the present review is that economically viable copper accumulations resulted predominantly from one or more regionally extensive but locally structurally controlled hydrothermal events, mostly subsequent to formation of the dominant cleavage. As a result, modern exploration should focus primarily on favorable structures, particularly potential dilatant sites in tectonically complex zones.

Pennsylvanian coniferopsid forests in sabkha facies reveal the nature of seasonal tropical biome

Research Article| April 01, 2011 Pennsylvanian coniferopsid forests in sabkha facies reveal the nature of seasonal tropical biome Howard J. Falcon-Lang; Howard J. Falcon-Lang * 1Department of Earth Sciences, Royal Holloway, University of London, Surrey TW20 0EX, UK *E-mail: h.falcon-lang@es.rhul.ac.uk. Search for other works by this author on: GSW Google Scholar Nathan A. Jud; Nathan A. Jud 2University of Maryland College Park, College Park, Maryland 20742, USA Search for other works by this author on: GSW Google Scholar W. John Nelson; W. John Nelson 3Illinois State Geological Survey, Champaign, Illinois 61820, USA Search for other works by this author on: GSW Google Scholar William A. DiMichele; William A. DiMichele 4Department of Paleobiology, National Museum of Natural History, Smithsonian Institution, Washington, D.C. 20560, USA Search for other works by this author on: GSW Google Scholar Dan S. Chaney; Dan S. Chaney 4Department of Paleobiology, National Museum of Natural History, Smithsonian Institution, Washington, D.C. 20560, USA Search for other works by this author on: GSW Google Scholar Spencer G. Lucas Spencer G. Lucas 5New Mexico Museum of Natural History and Science, 1801 Mountain Road, N.W., Albuquerque, New Mexico 87104, USA Search for other works by this author on: GSW Google Scholar Author and Article Information Howard J. Falcon-Lang * 1Department of Earth Sciences, Royal Holloway, University of London, Surrey TW20 0EX, UK Nathan A. Jud 2University of Maryland College Park, College Park, Maryland 20742, USA W. John Nelson 3Illinois State Geological Survey, Champaign, Illinois 61820, USA William A. DiMichele 4Department of Paleobiology, National Museum of Natural History, Smithsonian Institution, Washington, D.C. 20560, USA Dan S. Chaney 4Department of Paleobiology, National Museum of Natural History, Smithsonian Institution, Washington, D.C. 20560, USA Spencer G. Lucas 5New Mexico Museum of Natural History and Science, 1801 Mountain Road, N.W., Albuquerque, New Mexico 87104, USA *E-mail: h.falcon-lang@es.rhul.ac.uk. Publisher: Geological Society of America Received: 17 Sep 2010 Revision Received: 15 Nov 2010 Accepted: 16 Nov 2010 First Online: 09 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 © 2011 Geological Society of America Geology (2011) 39 (4): 371–374. https://doi.org/10.1130/G31764.1 Article history Received: 17 Sep 2010 Revision Received: 15 Nov 2010 Accepted: 16 Nov 2010 First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Howard J. Falcon-Lang, Nathan A. Jud, W. John Nelson, William A. DiMichele, Dan S. Chaney, Spencer G. Lucas; Pennsylvanian coniferopsid forests in sabkha facies reveal the nature of seasonal tropical biome. Geology 2011;; 39 (4): 371–374. doi: https://doi.org/10.1130/G31764.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Pennsylvanian fossil forests are known from hundreds of sites across tropical Pangea, but nearly all comprise remains of humid Coal Forests. Here we report a unique occurrence of seasonally dry vegetation, preserved in growth position along >5 km of strike, in the Pennsylvanian (early Kasimovian, Missourian) of New Mexico (United States). Analyses of stump anatomy, diameter, and spatial density, coupled with observations of vascular traces and associated megaflora, show that this was a deciduous, mixed-age, coniferopsid woodland (∼100 trees per hectare) with an open canopy. The coniferopsids colonized coastal sabkha facies and show tree rings, confirming growth under seasonally dry conditions. Such woodlands probably served as the source of coniferopsids that replaced Coal Forests farther east in central Pangea during drier climate phases. Thus, the newly discovered woodland helps unravel biome-scale vegetation dynamics and allows calibration of climate models. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Discovery of the Moran massive nickel sulphide deposit using down-hole transient electromagnetic surveying

The Moran nickel orebody was discovered in 2008 when underground drill-hole LSU-152 intersected a 12.3 m thick interval of mineralisation with an average grade of 6.0% Ni. The discovery drill-hole had been designed to test a strong conductor detected by transient electromagnetic surveys read in drill-holes that had passed above and below Moran. Moran is located approximately 1.1 km south of the Long orebody. Both orebodies are interpreted to be hosted within the same komatiite lava channel. The exploration program was premised on the observation that fertile komatiite lava channels are often mineralised over significant distances. Diamond drill-holes were drilled from underground mine workings in order to test the target lava channel to the south of Long over 1.7 km of strike. Recognition of potential ore-hosting environments, integrated with conductor models developed from DHTEM data, were keys to success.

Zoning of platinum group mineral assemblages in the UG2 chromitite determined through in situ SEM-EDS-based image analysis

In situ scanning electron microscopy–energy dispersive X-ray spectrometry analysis of platinum group minerals (PGM) and base metal sulfides in the UG2 chromitite shows that this ore body is zoned along at least ∼6 km of strike. The uppermost part of the UG2 chromitite, referred to as the leader seam, is ∼16 cm thick and has a PGM assemblage that is dominated by PGE arsenides, sulpho-arsenides, and alloys (∼70 vol.% of all PGM), which are typical secondary PGM assemblages in other segments of UG2. This is the first time such laterally persistent secondary assemblages have been identified in the UG2 chromitite, as previously, they were only known to occur adjacent to transgressive fluid-bearing structures (e.g., pipes, faults). The underlying main seam is thicker (one to nine seams totaling ∼130 cm) and has a PGM assemblage that consists mostly of Pt sulfide, Pt–Pd sulfide, Pt–Rh–Cu sulfide, laurite, and Fe–Pt alloys (∼85 vol.% of all PGM), typically regarded as primary magmatic constituents of UG2 chromitite. There are, however, some subtle vertical changes in the PGM assemblages of the main seam that include the occasional presence of secondary assemblages in the top and bottom parts. The origin of these secondary PGM assemblages is related to alteration by hydrothermal fluids and/or fluid-rich melts that infiltrated during crystallization of the UG2 and may possibly have been derived from the UG2 chromitite itself.

Reconciling short- and long-term measures of extension in continental back arcs: heat flux, crustal structure and rotations within central North Island, New Zealand

Abstract Geodetic, palaeomagnetic and andesite-age data all point to a fan-like opening of central North Island, New Zealand, in the past 5 Ma. Palaeomagnetic rotation rates for the eastern North Island are c. 6° Ma −1 and this is accompanied by back-arc extension of up to 19 mm a −1 at the Bay of Plenty coast and lesser values further south. Although the geodetic observations only span a decade, they show a remarkable consistency with the palaeomagnetic and volcanic arc migration data that span 5 Ma. A fan-like pattern of extension in central North Island is implied, similar to that seen in other continental back-arcs. When rapid fan-like openings do occur, they are likely to be accompanied by extreme thermal events. Heat flux from the central North Island occurs at one of the highest continental rates recorded: c. 26 MW per km of strike of volcanic zone, or 4.3 GW in total. Prior to the Pliocene extension and rotation, central North Island had a c. 20 Ma history of compression and overthrusting. It is proposed that thickening, then subsequent detachment of mantle lithosphere during this phase had a role to play in the Pliocene back-arc opening and the consequent extreme heat flux.

Petrogenesis of contact-style PGE mineralization in the northern lobe of the Bushveld Complex: comparison of data from the farms Rooipoort, Townlands, Drenthe and Nonnenwerth

In the present study, we document the nature of contact-style platinum-group element (PGE) mineralization along >100 km of strike in the northern lobe of the Bushveld Complex. New data from the farm Rooipoort are compared to existing data from the farms Townlands, Drenthe, and Nonnenwerth. The data indicate that the nature of the contact-style mineralization shows considerable variation along strike. In the southernmost portion of the northern Bushveld, on Rooipoort and adjoining farms, the mineralized sequence reaches a thickness of 700 m. Varied-textured gabbronorites are the most common rock type. Anorthosites and pyroxenites are less common. Chromitite stringers and xenoliths of calcsilicate and shale are largely confined to the lower part of the sequence. Layering is locally prominent and shows considerable lateral continuity. Disseminated sulfides may reach ca. 3 modal % and tend to be concentrated in chromitites and melanorites. Geochemistry indicates that the rocks can be correlated with the Upper Critical Zone. This model is supported by the fact that, in a down-dip direction, the mineralized rocks transform into the UG2-Merensky Reef interval. Between Townlands and Drenthe, the contact-mineralized sequence is thinner (up to ca. 400 m) than in the South. Chromitite stringers occur only sporadically, but ultramafic rocks (pyroxenites, serpentinites, and peridotites) are common. Xenoliths of calcsilicate, shale, and iron formation are abundant indicating significant assimilation of the floor rocks. Sulfides may locally form decimeter- to meter-sized massive lenses. PGE grades tend to be higher than elsewhere in the northern Bushveld. The compositions of the rocks show both Upper Critical Zone and Main Zone characteristics. At Nonnenwerth, the mineralized interval is up to ca. 400 m thick. It consists largely of varied-textured gabbronorites, with minor amounts of igneous ultramafic rocks and locally abundant and large xenoliths of calcsilicate. Layering is mostly weakly defined and discontinuous. Disseminated sulfides (<ca. 3 modal %) occur throughout much of the sequence. Geochemistry indicates that the rocks crystallized mainly from tholeiitic magma and thus have a Main Zone signature. The implication of our findings is that contact-style PGE mineralization in the northern lobe of the Bushveld Complex cannot be correlated with specific stratigraphic units or magma types, but that it formed in response to several different processes. At all localities, the magmas were contaminated with the floor rocks. Contamination with shale led to the addition of external sulfur to the magma, whereas contamination with dolomite may have oxidized the magma and lowered its sulfur solubility. In addition to contamination, some of the magmas, notably those of Upper Critical Zone lineage present at the south-central localities, contained entrained sulfides, which precipitated during cooling and crystallization.

Northern Tonga Arc and Fonualei Rifts: initial results from the NoToVE (SS11/2004) Research Voyage

The SSI 1/2004 research voyage of the Marine National Facility to the northern Tonga Arc and adjacent Fonualei Rifts (backarc) discovered several new submarine, basalt-to-rhyolite volcanic centres (stratovolcanoes and massive caldera systems) in the Arc. Numerous sector and caldera collapses were likely tsunamigenic. Fonualei Rifts lavas are a global backarc end-member with markedly low Na contents reflecting a highly refractory mantle wedge source coupled with high degrees of partial melting. The southern Rifts have captured the entire supra-subduction zone magma flux over about 150 km of strike; adjacent Arc volcanoes are inactive and carbonate capped.

Discovery of seismites in Upper Sinian stratiform lead-zinc deposits in Dadu River valley, Sichuan, China: Implication for ore genesis

A number of lead-zinc sulphide deposits have been discovered in recent years in dolomitic rocks of the Upper Sinian Dengying Formation in the Dadu River valley region, Sichuan Province, China. The lead-zinc mineralization is closely associated with a black siliceous exhalite that has been traced over 50 km of strike. This makes Dadu River valley the most extensive stratiform non-ferrous sulphide horizon having been discovered to date in China. In a previous study by the first author (LIN), these deposits were recognised as belonging to the broad spectrum of lead-zinc sedimentary exhalative (SEDEX) deposits. In this paper, seismites and seismogenetic structures occurring in close association with the Dadu River valley mineralized zone are described. The close spatial and temporal relationship between the seismites and mineralization provides strong evidence for the hypothesis that the lead-zinc SEDEX deposits formed in direct response to earthquake activities around the Sinian-Cambrian boundary. The earthquakes are postulated to be an effective dynamic mechanism for triggering the upward migration of deep circulating ore fluids along synsedimentary growth faults and their exhalation onto the sea-floor to form the extensive deposits of stratiform lead-zinc sulphide ore.

The Auriga Nunataks shear zone: Mesozoic transfer faulting and arc deformation in northwest Palmer Land, Antarctica

The Auriga Nunataks shear zone places new tectonic and temporal constraints on the Mesozoic evolution of West Antarctica. The shear zone is a long‐lived, arc‐orthogonal, ductile transfer fault that preserves a history of regional Mesozoic compressional basement deformation and extensional arc pluton emplacement in the Antarctic Peninsula magmatic arc. It forms an east‐west trending positive flower structure 2.4 km wide, exposed along 5 km of strike. Marble, graphite‐bearing pyroxene‐granulite, granodiorite‐diorite, amphibolite, and gabbro are deformed to mylonite and marble‐hosted tectonic breccia. The Mesozoic history of the shear zone is interpreted as follows: emplacement of granodiorite‐diorite at circa 206 Ma during Early Jurassic dextral transtension and metamorphism that peaked at circa 188 Ma (D1), brecciation of marble and mylonitization of gneiss by Late Jurassic to Early Cretaceous sinistral shear during the peninsula‐wide Palmer Land orogeny (D2), Early Cretaceous dextral transtension with emplacement of gabbro and garnet leucogranite between 140 and 135 Ma (D3), and mid‐Cretaceous, ocean‐vergent thrusting and sinistral transpression between 125 Ma and 80 Ma, with a peak at circa 110 Ma that folded, mylonitized, and brecciated preexisting plutonic and metamorphic rocks (D4); this is responsible for the current geometry of the shear zone. The Auriga Nunataks shear zone transferred motion between arc‐parallel compressional and extensional structural elements and by hosting plutons appears to have acted like a leaky transform fault during episodes of regional extension.

Cryptic variations of minor elements Al, Cr, Ti and Mn in Lower and Critical Zone orthopyroxenes of the Western Bushveld Complex

AbstractCompositional variations with respect to minor elements A1, Cr, Ti and Mn, and major elements Fe and Mg, in orthopyroxenes along ca. 160 km of strike of the Lower (LZ), Lower Critical (LCZ) and Upper Critical (UCZ) Zones are reviewed on the basis of 1900 analyses by electron microprobe. AI increases with stratigraphic height and declining Mg/(Mg + Fe2) ratios (hereafter MMF ratios) through the LZ and LCZ, reaching peak values close to the base of the UCZ, where the first cumulus plagioclase appears in the succession. Above this, Al contents decline as MMF ratios decline. Through the same interval, subdued increase in Ti occurs through &gt;1000 m of ultramafic cumulates, but this increase accelerates within the ca. 450 m UCZ sequence. Mn increases linearly with declining MMF ratios through the entire succession, Cr levels are highest in orthopyroxenes of the ultramafic LZ and LCZ, and olivine norites of the UCZ, but decline in more evolved norites and associated anorthosites of the UCZ.This pattern of cryptic variations, displayed by a thick succession of cumulates, is consistent with the model of Bence and Papike (1972) and Grove and Bence (1977) for basaltic rocks, which links the levels of minor elements in pyroxenes with entry of plagioclase into the paragenesis.

Partitioning of the platinum group elements and gold between silicate and sulphide magmas in the Munni Munni Complex, Western Australia

The Munni Munni Complex in the Archaean Pilbara Block contains PGE mineralization in the form of disseminated sulphides within pyroxenite. The uppermost sulphide-bearing layer shows a characteristic vertical distribution of PGE and Au. Peak values for Pd, Pt, and the other PGEs are systematically offset by up to 5 metres below the horizon of maximum S, Ni, and Cu abundance. The Main Sulphide Zone of the Great Dyke of Zimbabwe, which occurs in an identical stratigraphic setting, shows very similar features. This distribution of PGEs with respect to the sulphide peak cannot be explained in terms of postcumulus mobilisation from an initially homogenous sulphide layer, for a number of reasons: the offset profiles are remarkably consistent along several km of strike; remobilization would be expected to affect Pd and Au more than the other PGEs; and underlying sulphide concentrations show coincident trends for all the PGE, Au, Cu, Ni, and S. The offset between PGE and Ni-Cu-S peaks has been interpreted by workers on the Great Dyke as the result of fractional sulphide segregation, combined with different values for sulphide liquid-silicate melt partition coefficient (SSD) for the various PGEs, Au, Ni, and Cu. A series of model calculations has been performed in an attempt to apply this interpretation to the Munni Munni case, with the object of making quantitative estimates of SSD values for the PGEs and Au. Results in general support the conclusions of Great Dyke studies that apparent SSDs decrease in the order Ir = Ru = Rh = Os ≥ Pd > Pt > Au > Cu = Ni. However, it proves impossible to duplicate some highly regular and persistent features of the Munni Munni offset sulphide layer with any model that assumes constant values for these SSDs. The inadequacy of the fractional sulphide segregation model implies that sulphide liquid is not the sole collector of the PGEs. The observed behaviour of the PGEs requires a sudden increase in the apparent affinity of the PGEs for the sulphide fraction at the base of the main sulphide peak, unattributable to any significant change in parent magma composition or crystallization conditions. Natural profiles can be modelled successfully by describing this apparent affinity of PGE for the sulphide fraction in terms of widely variable “virtual” partition coefficients. This behaviour can be explained if the silicate melt, and consequently the co-existing sulphide melt, is saturated with respect to solid PGE phases. The association of PGE with sulphides is necessary because of the extremely low concentrations of PGE in the silicate melt, which makes it kinetically impossible to nucleate solid phases even at substantial degrees of supersaturation. Pre-concentration by several orders of magnitude due to partitioning into sulphide liquid allows solid PGE phases to nucleate within sulphide droplets, giving rise to an apparently very strongly PGE-enriched sulphide fraction.

Oxygen Thermobarometry of Orogenic Lherzolite Massifs

The oxidation state has been determined for spinel peridotites from 13 orogenic lherzolite massifs including Beni Bousera, Ronda, and 11 smaller massifs in the French Pyrenees. The oxygen fugacity (fo2) was calculated for 67 samples from microprobe analyses using a set of secondary spinel standards to correct the ferric iron content in the spinels. The utility of this method is confirmed by the good agreement between the calculated values and those determined by Mossbauer spectroscopy on 28 samples. The spinel peridotites of Ronda and Beni Bousera are relatively reduced, averaging -11 and -1.5 log units relative to fayalite-magnetite-quartz (FMQ) respectively, which is in agreement with values from abyssal peridotites and mid-ocean ridge basalt (MORB) glasses. The Pyrenean massifs are relatively oxidized [ΔlogfO2(FMQ)=-0.4] and are intermediate between the abyssal peridotites and continental xenolith suites. No systematic gradients are observable. Instead, variations of up to 2 log units in fo2 occur at a localized scale. This type of variation is also observed for trace elements and radiogenic isotopes. The harzburgites at Beni Bousera record the most reduced conditions. Local oxidation coincides with the appearance of amphibolc, indicating that metasomatizing fluids or melts are generally oxidized compared with the host peridotites. Partial re-equilibration in the plagioclase peridotite fades has occurred at Ronda, causing the spinels to become Cr rich. Re-equilibration is extremely heterogeneous. Mild oxidation appears to attend the crystallization of fine-grained plagioclase. The similarity in fo2 values at Beni Bousera and Ronda indicates a fairly uniform oxidation state at a scale of ˜ 200 km. This scale of homogeneity is also observed in the Pyrenees, where no significant variation in fo2 is apparent along 200 km of strike in the Northern Pyrenean Zone.

The origin and significance of the stratabound, carbonate-hosted gold deposits at Tennessee Pass, Colorado

Numerous small manto deposits occur in the vicinity of Tennessee Pass, located between the Gilman and Leadville mining districts in central Colorado. These deposits are hosted in the Mississippian Leadville Dolomite, crop out over 7 km of strike, and average about 10 to 15 ppm Au. The ores are now heavily oxidized but at one time consisted of semimassive pyrite, fine-grained quartz, zeolite, and native gold. Most primary fluid inclusions in quartz homogenize at temperatures of 200 degrees to 220 degrees C, suggesting deposition temperatures of about 250 degrees C. Sulfur isotopes (delta 34 S pyrite = 1-4ppm) suggest an igneous sulfur source. The trace element geochemistry (high As, Sb, Hg) is similar to that in many other low-temperature hydrothermal systems. Oxygen isotope ratios in quartz (delta 18 O = 14-17ppm) indicate that the ore fluid was relatively heavy (delta 18 O fluid = 4-8ppm). Alteration around the Tennessee Pass deposits is stratigraphically controlled and most intense near the Leadville Dolomite. Two major areas (each 2 km diam) of pervasive alteration and veining affect the rocks 3 to 8 km east (Cooper Hill) and southeast (Buckeye Gulch) of Tennessee Pass. Fission-track geochronology in apatite and zircon indicates that these alteration centers are part of a large (5-7 km diam) paleothermal anomaly which has reset ages of 38 to 42 m.y. This suggests the presence of a large, unexposed stock.These geologic relations suggest the following mineralization model. Within the two major alteration centers, the hydrothermal flux was dominantly vertical, similar to that in the central part of the Leadville district. On the western margin of the thermal center (i.e., the Tennessee Pass area), updip fluid flow was concentrated in the Leadville Dolomite, resulting in few veins and stratigraphically controlled alteration (similar to the Gilman district). The Tennessee Pass district thus may represent the peripheral portion of a much larger manto-type system. The recognition of this potential was based in large part on the isotope systematics, which therefore may have important exploration applications for this type of ore deposit.

Stratiform barium enrichment in the Dalradian of Scotland

A stratigraphic zone of calcareous schist containing barite and/or barium silicates with minor sulfides occurs at the top of the Ben Eagach Schist near Loch Lyon in the metamorphosed and highly deformed Middle Dalradian rocks (ca. 600 m.y.) of the Grampian Highlands. The Ben Eagach Schist consists of graphitic schist, micaceous schist, and quartzite and is approximately 200 m thick on Beinn Heasgarnich. The mineralized zone is 1 to 3 m thick, containing 4.5 percent Ba and 0.2 percent Zn at its type section, and occurs over at least 4 km of strike. The main barium-bearing mineral is muscovite with up to 10.3 percent BaO; hyalophane is usually present as the coexisting feldspar. Though not everywhere present, barite comprises as much as 11 percent (modal) of the schist and is associated with manganoan calcite. At one locality the zone is celsian rich with pyrrhotite as the main sulfide. Pyrite and minor sphalerite are the most common sulfides, but traces of galena and chalcopyrite also occur.The newly discovered barium-enriched zone is the lateral equivalent of substantial strata-bound barite and sulfide deposits, with associated barium silicates, which occur in the Aberfeldy area some 45 km to the east-northeast. The zone is interpreted as having formed in a short-lived shelf area adjacent to a second-order basin (Easdale) immediately southwest of a similar basin centered on Aberfeldy, where the barite deposits are believed to define small third-order basins, the development of which was controlled by rift-related faulting. The Ben Eagach Schist and associated Middle Dalradian formations of the Aberfeldy and adjacent basins have considerable potential for further exploration.