The Coed-y-Brenin system is a diorite-hosted porphyry copper deposit of early Ordovician age (Tremadoc) situated on the south eastern side of the Harlech Dome: a thick sequence of Cambrian and Ordovician sediments and volcaniclastics that were subject to greenschist facies metamorphism during the Caledonian Orogeny. The copper mineralization occurs as disseminations, coatings on hairline fractures and in late-stage quartz-sulphide-calcite veinlets. Pyrite and chalcopyrite are the main opaque phases but sulphosalts, tellurides and electrum are also present. The copper mineralization is largely confined to the diorite and micro-tonalite dykes and sills associated with the Rhobell Fawr Volcanic centre. This study has shown that epithermal style precious-metal mineralization is also developed in the Coed-y-Brenin area in the shales and siltstones of the Maentwrog and Ffestiniog Flags Formations. It is characterised by pyritization, sericitization and silicification. The gold and silver occur as electrum in fractures and also as tellurides in late stage quartz-pyrite veins. Both styles of mineralization share a close spatial association and are characterised by propylitic and phyllic alteration. Argillic and potassic assemblages are not developed. Geochemical analysis of mineralized and unmineralized material has shown that the diorite-hosted mineralization is enriched in Cu, Au and Ag and depleted in Zn while the epithermal mineralization displays enrichments in Au, Ag, As, Sb and the base metals. Rb:Sr ratios increase with increasing alteration in both styles of mineralization. Sulphur isotope studies have shown that both styles of mineralization have a common, probably magmatic, sulphur source. Fluid inclusion data and 0 and H isotope studies indicate that following magma emplacement and development of the porphyry copper mineralization a low temperature-low salinity meteoric hydrothermal system was established. Fluid inclusion data and 0 and H isotope studies indicate that following magma emplacement and development of the porphyry copper mineralization a low temperature-low salinity meteoric hydrothermal system was established. This was responsible for the late-stage quartzsulphide-calcite veins in the diorites and for all the sediment-hosted mineralization. The association between epithennal and porphyry deposits has become apparent in recent years but the Coed-y-Brenin system is unusual due to its Lower Palaeozoic age and because both styles of mineralization are developed at the same structural level. This study of the Coed-y-Brenin porphyry system demonstrates the potential for the development and preservation of epithermal mineralization in porphyry systems and also in older orogenic belts.

A brief description of the geology of the Shingle Creek area southwest of Penticton B.C. together with a more detailed report on the mineralogy and petrology of the Shingle Creek Intrusion (granite porphyry) are given. The presence of flows and tuffs showing similar mineralogical features to the Shingle Creek: Intrusion suggest that this intrusion was shallow rather than deep-seated. Sanidine phenocrysts showing oscillatory zoning are described with a range in composition of from 40 to 70 percent orthoclase (based on a comparison of optical properties with those determined by Tuttle 1952). The zones often occur in pairs showing a gradation from an albite-rich inner zone to an orthoclase-rich outer gone, It is tentatively suggested here that pressure changes in the magma, by altering the liquidus - solidus relations, sight provide this reversed- zoning. Plagioclase phenocrysts from the intrusion show transitional to high temperature optics when compared with the results of Bowen and Tuttle 1950. Beta quartz phenocrysts showing rounded outlines and development of late halos are described, from evidence given it is suggested that these features are due to resorption of quartz in a magma chamber prior to intrusion, followed by late growth after intrusion and daring crystallization of the porphyry matrix, Petrographlc study of the largest dyke associated with the porphyritic intrusion has suggested that this dyke may have been intruded in several stages. / Arts, Faculty of / Geography, Department of / Graduate

Veins and pervasive wall-rock alteration composed of coarse muscovite +/- quartz +/- pyrite are documented for the first time in a porphyritic granite at Luhr Hill in the Yerington District, Nevada. Coarse muscovite at Luhr Hill occurs at paleodepths of similar to 6-7 km in the roots of a porphyry copper system and crops out on the scale of tens to hundreds of meters, surrounded by rock that is unaltered or variably altered to sodic-calcic assemblages. Coarse muscovite veins exhibit a consistent orientation, subvertical and N-S striking, which structurally restores to subhorizontal at the time of formation. Along strike, coarse muscovite veins swell from distal, millimeter-thick muscovite-only veinlets to proximal, centimeter-thick quartz-sulfide-bearing muscovite veins. Crosscutting relationships between coarse muscovite veins, pegmatite dikes, and sodic-calcic veins indicate that muscovite veins are late-stage magmatic-hydrothermal features predating final solidification of the Luhr Hill porphyritic granite. Fluid inclusions in the muscovite-quartz veins are high-density aqueous inclusions of similar to 3-9 wt% NaCl eq. and < 1 mol% CO2 that homogenize between similar to 150 and 200 A degrees C, similar to fluid inclusions from greisen veins in Sn-W-Mo vein systems. Our results indicate that muscovite-forming fluids at Luhr Hill were mildly acidic, of low to moderate salinity and sulfur content and low CO2 content, and that muscovite in deep veins and alteration differs in texture, composition, and process of formation from sericite at shallower levels of the hydrothermal system. Although the definition of greisen is controversial, we suggest that coarse muscovite alteration is more similar to alteration in greisen-type Sn-W-Mo districts worldwide than to sericitic alteration at higher levels of porphyry copper systems. The fluids that form coarse muscovite veins and alteration in the roots of porphyry copper systems are distinct from fluids that formed copper ore or widespread, shallower, acidic alteration. We propose that this style of veins and alteration at Luhr Hill represents degassing of moderate volumes of overpressured hydrothermal fluid during late crystallization of deep levels of the Yerington batholith.

The super-giant Oyu Tolgoi porphyry copper-gold deposits in the South Gobi desert, Mongolia, consist of multiple discrete porphyry centers aligned within a north-northeast trending, >6.5 km long, arc-transverse mineralized corridor. The porphyries are linked to a tectono-magmatic event at ~372 Ma within a Devonian to Carboniferous volcanic arc, and U-Pb (zircon) geochronology records magmatic activity from ~390 Ma to ~320 Ma. The Oyu Tolgoi district underwent at least three discrete periods of syn- to post-mineral shortening and there is evidence for at least three unconformities within the Paleozoic sequence. Although the deposits were formed in an active orogenic environment characterized by rapid uplift, their preservation is a reflection of climactic effects as well insulation from erosion by rapid burial under mass-wasted and pyroclastic material in the volcaniclastic apron of late-mineral dacitic volcanoes. The porphyry copper-gold deposits are spatially and temporally related to medium- to high-K calc-alkaline quartz monzodiorite (~372 Ma) and granodiorite (~366 Ma) intrusive phases that comprise the Late Devonian Oyu Tolgoi Igneous Complex (OTIC). Adakite-like wholerock compositions as well as zircon grains with high CeN/CeN*, EuN/EuN* and Yb/Gd in the sample populations from syn- and late-mineral porphyry intrusions are different from younger intrusions that are not related to porphyry Cu-Au deposit formation. Moreover, mixed zircon populations within OTIC intrusions indicate that efficient assimilation of material from different host rocks by a convecting magma chamber occurred. Mafic to intermediate volcanic units evolved from tholeiitic to calc-alkaline compositions, which is interpreted to be a reflection of marine arc maturation and thickening. Felsic rock suites are dominantly high-K calc-alkaline, regardless of age. Nd-isotopic geochemistry from all suites is consistent with magma derivation from depleted mantle in an intra-oceanic volcanic arc and lead isotopic compositions indicate that the sulfides in the porphyry Cu-Au deposits are genetically linked to the Late Devonian magmas. Magma mixing, adakite-like magmatism and rapid uplift and erosion in a juvenile marine arc setting differentiate the ore-stage geologic environment at Oyu Tolgoi from other settings in active and fossil volcanic arcs.

The Hopper property is located in the Yukon Tanana terrane in the southwestern Yukon. It is characterized by granodiorite-hosted copper and molybdenum mineralization that is related to propylitic alteration and shearing. The focus of this study is to develop a genetic model for the copper mineralization based on field, petrographic and geochemical analyses. The mineralization zone is roughly 500 m long and 20 m wide and is hosted in a granodiorite of calc-alkaline affinity interpreted to be part of the Ruby Range batholith which intruded the Aishihik metamorphic suite. The intrusion took place during the late Cretaceous based on U and Pb analyses of zircons with laser ablation inductively-coupled plasma mass spectrometer, placing it in the same time frame as the intrusion hosting the Casino mineralization, the largest porphyry copper deposit in the Yukon. The mineralization consists of chalcopyrite and molybdenite found along fractures, as disseminations and on shearing surfaces. Sulphur isotopes from chalcopyrite and pyrite range from -1.7 to -0.8 per mil suggesting it is ortho-magmatic in origin. The chemical compositions of biotite and amphibole of the Hopper intrusion resemble mineral chemistry of known porphyry systems. Biotites from both Hopper and Casino exhibit Mg-Cl avoidance and have similar values of MnO, TiO2, Al2O3, BaO, Na2O and K2O, and amphiboles range in composition between magnesio-hornblende and actinolite, similar to other porphyry copper type deposits. There are two distinct populations of titanite in the Hopper intrusion, magmatic and hydrothermal. Magmatic titanites are euhedral and are in sharp, planar contact with other magmatic phases, whereas hydrothermal titanites are anhedral and are associated with alteration minerals and the mineralization. Both populations show substitution between Ti and Al+Fe+Nb+Zr+Ce+Y. The hydrothermal titanites are richer in Ti, Cu, and Cr compared to the magmatic ones that are more enriched in Mo suggesting the mineralization is ortho-magmatic in origin. Sulphur isotopes analyses suggest Hopper could be a porphyry type mineralization. However, the mineralization was also found to be related with propylitic alteration, shearing and depletion in Si and K, indicating this is not a typical system. Therefore, two possible models can explain the copper occurrence at Hopper. According to the first model, the mineralization is ortho-magmatic and is part of a porphyry system, but it has been remobilized to the propylitic zone. A second model is that the mineralization is much younger than the intrusion and is related to shearing.

Thesis (Ph. D.)--University of Nevada, Reno, 2008. / "December, 2008." Accompanying CD-ROM contains appendices C and D. Appendix C: AutoCAD files with drill hole log; Appendix D: excel spreadsheet with copper mass balance calculations. Includes bibliographical references (leaves 206-215). Library also has microfilm. Ann Arbor, Mich. : ProQuest Information and Learning Company, [2009]. 1 microfilm reel ; 35 mm. Online version available on the World Wide Web.

No description available.

The biotite bearing Schultze Granite (Globe-Miami district) and the biotite-hornblende bearing Ruby Star Granodiorite (Pima district) compose two intrusive centers that produced multiple porphyry copper deposits during the Laramide orogeny. Both magmatic-hydrothermal systems were dismembered and tilted by Tertiary extension, as indicated by tilted Tertiary sedimentary rocks, paleomagnetic data, and geobarometry, thereby producing extraordinary exposures of these magmatic-hydrothermal systems: ~ 1 to ~10 km (Globe-Miami district) and &lt;1 to&gt;12 km (Pima district). Ages of emplacement range from 68 to 61 Ma for the Schultze Granite and 64 to 58 Ma for the Ruby Star Granodiorite. The plutons were formed by rapid accumulation of magma within short periods of time (~1 m.y.). The Schultze Granite is a high-silica granite and did not evolve chemically with time, except during formation of late porphyry and aplite dikes. Phases of the Ruby Star pluton range from granodiorite to granite, but appear to be distinct intrusive events separated in time by several million years. Each pluton is chemically homogenous with depth, probably due to convection. The low iron contents of biotites suggest that magmas related to porphyry copper deposits have higher oxidation states than typical granitic bodies. Hydrothermal alteration was associated with most phases of each pluton, with multiple alteration types overlapping to create complex centers. Veins persist to &gt;10 km beneath porphyry copper deposits. Deep styles of alteration differ in the two plutons. The Schultze Granite contains biotite veins and greisen veins (coarse-grained muscovite) (~10 km). The Ruby Star Granodiorite contains sodic-calcic alteration (4-8 km) and greisen veins (4-12 km). The sodic-calcic alteration is asymmetrically distributed on the eastern side of the Sierrita deposit and is interpreted to have been created by influx of external sedimentary brines from Paleozoic sedimentary rocks that only are present on the eastern side of the pluton. Greisen alteration occurs late in the hydrothermal history and may be the last fluids that were exsolved from the magma as the magma chamber completely crystallized. These deep alteration styles can be used to predict where porphyry copper deposition may have occurred, which can lead to discoveries in extended terranes.

We have conducted an experimental investigation of the solubility and speciation of Mo in HCl-, NaCl-, and NaOH-bearing water vapour, and of the partitioning of Mo between coexisting aqueous liquid and vapour at 300 to 370&deg;C and up to saturated pressure. Our results indicate that Mo concentration is enhanced in HCl-bearing water vapour at fHCl > 0.1 bar, and in NaOH-saturated water vapour, but is unaffected by the presence of NaCl. This suggests that Mo speciates as MoO3&middot;nH2O in water vapour at equilibrium with NaCl or fHCl &lt; 0.1 bar. The dependence of SigmafMo on fHCl at higher acidity points to the formation of Mo oxychloride (MoO2Cl 2). For the system MoO3-NaOH-H2O, log Sigma fMo increases with increasing fH2O, and with log SigmafNa in a ratio of 0.28+/-0.4, but Sigma fNa does not change with increasing fH2O. This suggests the formation of a small proportion of sodium molybdate (Na 2MoO4) in addition to MoO3&middot;nH2O. Our partitioning experiments show that at lower temperature and fluid density, Mo partitions more strongly into the liquid than the vapour, but the Mo concentration in the vapour increases as the temperature-pressure conditions approach those of the critical point of water (374&deg;C and 221 bar), surpassing that in the liquid at &sim;360&deg;C. The results of our experiments indicate that both the liquid and vapour phases may be important for the transport of Mo in porphyry ore-forming systems, and that vapour-phase solubility is enhanced in high fHCl magmatic gases.

The super-giant Oyu Tolgoi porphyry copper-gold deposits in the South Gobi desert, Mongolia, consist of multiple discrete porphyry centers aligned within a north-northeast trending, >6.5 km long, arc-transverse mineralized corridor. The porphyries are linked to a tectono-magmatic event at ~372 Ma within a Devonian to Carboniferous volcanic arc, and U-Pb (zircon) geochronology records magmatic activity from ~390 Ma to ~320 Ma. The Oyu Tolgoi district underwent at least three discrete periods of syn- to post-mineral shortening and there is evidence for at least three unconformities within the Paleozoic sequence. Although the deposits were formed in an active orogenic environment characterized by rapid uplift, their preservation is a reflection of climactic effects as well insulation from erosion by rapid burial under mass-wasted and pyroclastic material in the volcaniclastic apron of late-mineral dacitic volcanoes. The porphyry copper-gold deposits are spatially and temporally related to medium- to high-K calc-alkaline quartz monzodiorite (~372 Ma) and granodiorite (~366 Ma) intrusive phases that comprise the Late Devonian Oyu Tolgoi Igneous Complex (OTIC). Adakite-like wholerock compositions as well as zircon grains with high CeN/CeN*, EuN/EuN* and Yb/Gd in the sample populations from syn- and late-mineral porphyry intrusions are different from younger intrusions that are not related to porphyry Cu-Au deposit formation. Moreover, mixed zircon populations within OTIC intrusions indicate that efficient assimilation of material from different host rocks by a convecting magma chamber occurred. Mafic to intermediate volcanic units evolved from tholeiitic to calc-alkaline compositions, which is interpreted to be a reflection of marine arc maturation and thickening. Felsic rock suites are dominantly high-K calc-alkaline, regardless of age. Nd-isotopic geochemistry from all suites is consistent with magma derivation from depleted mantle in an intra-oceanic volcanic arc and lead isotopic compositions indicate that the sulfides in the porphyry Cu-Au deposits are genetically linked to the Late Devonian magmas. Magma mixing, adakite-like magmatism and rapid uplift and erosion in a juvenile marine arc setting differentiate the ore-stage geologic environment at Oyu Tolgoi from other settings in active and fossil volcanic arcs.