Дисертації з теми "Kanmantoo copper gold deposit"

The Troilus gold-copper deposit lies within the northeastern part of the Archaean Frotet-Evans greenstone belt, in the Opatica sub-province of the Superior Province, northern Quebec, and contains total reserves of 51 Mt at 1.08 g/t gold, 0.11% copper, and 1.4 g/t silver. The largest orebody, Zone 87, has been mined by open pit methods since 1993.
Rocks of the Troilus domain include a coarse- to medium-grained metadiorite, a finer-grained amphibolite, a rock with a brecciated texture and felsic dykes, which crosscut the metadioritic pluton, the amphibolite and the breccia. The amphibolite, breccia, and felsic dykes all locally host ore.
Previous researchers have proposed a porphyry-type model for the genesis of the Troilus deposit. However, evidence that the breccia unit is not hydrothermal but a product of magma mixing, that the felsic dykes predate mineralization, and that mineralization and associated alteration occurred as two discrete events separated by a major episode of regional metamorphism (amphibolite facies), requires that alternative genetic models for the deposit be considered, such as orogenic gold model. (Abstract shortened by UMI.)

Located in northwestern British Columbia within the Stikine terrane, the Red Chris Cu-Au porphyry deposit is hosted in the Late Triassic Red Stock (~203.8 Ma). The Red Stock is a quartz monzodiorite to monzonite intrusion hosted in the broadly contemporaneous volcanic rocks of the Stuhini Group. Red Chris has features that are characteristic of calc-alkalic and alkalic porphyry deposits and shares many similarities with the Ridgeway deposit of the Cadia district in New South Wales, Australia. A combined measured and indicated resource of 936 million tonnes at 0.374 % Cu, 0.385 g/t Au, and 1.224 g/t Ag has been outlined from the Main and East zones. Copper and gold are associated with bornite, chalcopyrite and lesser pyrite, hosted in quartz veins and stockworks as disseminations and fracture-controlled veinlets. High-grade mineralization is directly associated with high quartz vein density. Copper-iron sulphide minerals are laterally zoned, with a bornite > chalcopyrite core, grading outward to a chalcopyrite > pyrite shell and outward and upward to a pyrite > chalcopyrite halo. Five major groups of veins are recognized, of which the oldest two sets contain much of the copper and gold. Stable isotopic analysis indicates the presence of magmatic and mixed magmatic-meteoric hydrothermal fluids. Evidence from sulphur isotopes demonstrates a high temperature oxidized magmatic fluid was responsible for transporting and depositing much of the copper and gold. A vertical and lateral zonation in sulphur isotopes exists, whereby deep regions exhibit δ34S values between -1.9 to -0.9 % and transition to near-surface regions in the pyrite halo that exhibit δ34S values between +0.9 to +1.9 %. Isotopic analysis of oxygen and deuterium of hydrothermal alteration minerals provide evidence for a magmatic fluid (secondary biotite and muscovite) and a mixed magmatic-meteoric fluid (illite and kaolinite). Low temperature clay alteration (illite-kaolinite; intermediate argillic assemblage) significantly overprinted high temperature alteration (K-silicate, phyllic) in the upper levels of the system and gradually diminished intensity with depth. Carbonate veins and alteration also characterize the shallow levels and isotopic analysis of carbon and oxygen suggest a magmatic source with the possibility of minor mixing with an external meteoric fluid.

Reko Diq porphyry Cu-Au-Mo deposit in the western Chagai belt, Pakistan, is one of the world’s largest porphyry ore deposits, containing a global resource of 5,900 million metric tons @ 0.41 % Cu and 0.22 g/t Au. The Reko Diq volcanic complex hosts a cluster of eighteen porphyry centers within a NW trending, ~10-long mineralized corridor bounded by the Drana Koh fault system to the north and Tuzgi fault to the south. The western porphyry complex at Reko Diq is linked to a distinct tectono-magmatic event of middle-late Miocene (12.9-11.9 Ma) age, which formed four economic porphyry Cu deposits and remains the focus of this study. The Reko Diq western porphyry deposits are spatially and temporally associated with a series of medium-K calc-alkaline granodiorite and quartz-diorite intrusions forming H79, H15, H14 and H13 deposits, which are spatially distributed from north to south. High Sr/Y and low Y adakitic signature and petrochemical variations in the intrusive rocks suggest normal basalt-andesite-dacite-rhyolite magmas derived from a tholeiitic to calc-alkaline suite arc magma with significant upper crustal interaction. Combination of U-Pb-zircon and Re-Os-molybdenite geochronology and zircon mineral chemistry suggests that a short lived (~1 Ma) fractionated magmatic-hydrothermal system with sustained mafic recharge and efficient hydrothermal fluid flow was involved in the formation of the giant H15 and H14 porphyry deposits. Much of the high-grade (up to 2.0 % Cu and 1.5 g/t Au) Cu-Au mineralization is associated with intense hydrothermal potassic alteration and early quartz “A-type” veins in the early-mineral granodiorite and intra-mineral quartz-diorite intrusions and adjacent host rocks. The main ore-stage potassic alteration is typically associated with high temperature, hypersaline magmatic-hydrothermal fluids. Fluid inclusions with co-existing vapor and brine suggest a boiling phase of two immiscible fluids responsible for the copper ore precipitation. The intensity of potassic alteration and Cu-Fe-sulfide mineralization decreases with the emplacement of late-mineral and late-barren stage quartz-diorite intrusions forming a low grade core in H15 and H14 porphyry deposits. The decline in Cu-Au grades with time is interpreted as a manifestation of the underlying magma chamber depleted in metals and volatiles.

Located in the Late Triassic Galore Creek alkalic Cu-Au porphyry district in northwestern British Columbia, the Central Zone deposit represents the end-member of the silica-undersaturated class of alkalic porphyry systems. The deposit is hosted by volcano-sedimentary rocks of the Middle to Upper Triassic Stuhini Group that were intruded by a syenite-monzonite complex and hydrothermal breccias. Post-mineral tilt (45 to 60° W-SW) provides an opportunity to examine a vertically extensive depth range of the system, and the impact of host rocks and a redox control on the precipitation of sulfide and silicate alteration minerals. Early mineralization associated with potassic alteration is dominated by gold-bearing chalcopyrite + bornite (Cu:Au ~ 2:1). A second gold-poor mineralization event is associated with calc-potassic alteration and dramatically changes the Cu:Au ratio (5:1) in the core of the Central Zone. In general, greatest Cu-Au concentrations overlap lithological contacts characterized by contrasting ferromagnesian mineral content, thus forming redox gradients. Sulfur isotopic compositions emphasize the importance of fO₂ conditions in ore deposition. Sulfides in highly mineralized centers are characterized by moderately negative δ₃₄Ssulfide values (-10.66‰ to -7.84‰), whereas sulfides deposited distally show highly negative δ₃₄Ssulfide values (-17.13‰ to -4.03‰). These data suggest that the interaction of sulfate-rich (SO₄²-(aq)) fluids with varying amounts of Fe²⁺-bearing minerals in host rocks increased H₂S/SO₄²- leading to formation of reduced S, and precipitation of sulfide minerals. Trace elements such as V and As in host rocks and Eu²⁺ in hydrothermal garnet reflect the same redox influence. Vanadium and As are soluble under highly oxidizing conditions. The shift in oxidation state facilitates their incorporation in alteration minerals. Thus, highest V (>700ppm) and As (>40ppm) concentrations form halos distally to the redox gradients and ore bodies. Hydrothermal garnets near lithologic contacts contain excess Eu²⁺. In contrast to V and As, Eu²⁺ is soluble in reduced fO₂ conditions and precipitates close to the redox gradient. This study demonstrates that redox is the dominant control on ore deposition in the Central Zone. Recognizing redox changes may provide a valuable guide for future exploration in the Galore Creek district and perhaps other alkalic Cu-Au porphyry systems worldwide.

The Mt. Milligan deposit is a tilted (~45°) Cu-Au alkalic porphyry located 155 km northwest of Prince George, B.C., Canada. It is the youngest of the BC alkalic porphyry deposits, all of which formed between 210 to 180 Ma in an extensive belt of K-enriched rocks related to the accretion of the Quesnellia-Stikinia superterrane to ancestral North America. Mt. Milligan has a measured and indicated resource of 205.9 million tonnes at 0.60 g/t Au and 0.25% Cu containing 3.7 million oz. gold, and 1.12 billion lb. copper. Shoshonitic volcanic and volcaniclastic andesites host mineralization. These have been intruded by a composite monzonitic stock (MBX stock), and associated sill (Rainbow Dike). Early disseminated chalcopyrite-magnetite and accessory quartz veins are associated with K-feldspar alteration in the MBX stock. A halo of biotite alteration with less extensive magnetite replaces host rocks within a ~150 m zone surrounding the stock, while K-feldpsar alteration extends along the Rainbow Dike and permeable epiclastic horizons. Peripheral albite-actinolite-epidote assemblages surround the K-silicate zone. Albite-actinolite occurs at depth, and epidote dominates laterally. Copper and Au grade are maximal where the albite-actinolite assemblage overprints biotite alteration. Gold grade is moderate in association with epidote, whereas Cu is depleted. The post-mineral Rainbow Fault separates the core Cu-rich zone from a downthrown Au-rich zone. A similar zonation of metals occurs in the hanging-wall (66 zone), where a Cu-bearing, potassically-altered trachytic horizon transitions to a funnel-shaped zone of pyrite-dolomite-sericite-chlorite alteration with elevated gold. Sulfide S-isotope compositions range from -4.79 δ34S in the central Cu-Au orebody to near-zero values at the system periphery, typical of alkalic porphyries. Sulfur isotope contours reflect the magmatic-hydrothermal fluid evolution, and indicate late-stage ingress of peripheral fluids into the Cu-Au zone. Carbonate C- and O-isotope compositions corroborate the magmatic fluid path from the Cu-Au rich zone to Au-rich zone with decreasing depth. Strontium isotopic compositions of peripheral alteration minerals indicate a laterally increasing meteoric fluid component. Changes in major- and trace element composition of epidote and pyrite across the deposit are also systematic. These provide additional vectors to ore, and confirm the kinematics of the Rainbow Fault.

The Kansanshi Cu-Au deposit located in the Domes region of the North West province of Zambia is characterised by structurally controlled high angle veins and associated alteration halos. The northwest trending Kansanshi antiform flanks the Solwezi syncline to the north and hosts the Kansanshi deposit and consists of tillites and metasedimentary rocks. Mineralisation is associated with Neoproterozoic Pan African deformation events experienced during the formation of the Lufilian fold belt; however recent findings confirm that structures in the form of reverse and normal faults and drag folds are critical controls on mineralisation within the deposit, Main pit in particular. Low angle faults occurring below the current pit are believed to have served as major fluid pathways during mineralisation. Age dating data from the Kansanshi deposit suggest that mineralisation took place between 512 and 503 Ma indicating that the event was associated with metamorphism. Two types of alteration are dominant within the Main pit (Kansanshi deposit) with the type and intensity of alteration being largely controlled by lithological units. Albite alteration occurs dominantly in phyllites and schists whereas dolomitisation is prevalent in calcareous units. Alteration is associated with mineralisation, and therefore is used as a condition for predicting vein or disseminated mineralisation. The high Au tenor at Kansanshi can be attributed to gold grains occurring in association with melonite (NiTe₂) and microfractured pyrite intergrown with chalcopyrite in sulphide and quartz dominated veins and veinlets. Analysis of gold grade distribution within the Main pit shows a clear concentration of the element along the major north-south trending structures like the 4800 and 5400 zones, possibly through supergene enrichment in the oxide-transition-sulphide zones. It is imperative that exploration for Kansanshi-type deposits will require geochemical and geophysical studies, understanding of the geology of an area to identify the three lithostratigraphic units (red beds, evaporites and reducing strata).

Late-stage fissure-filling ore at the world class Bingham Canyon, Utah, porphyry copper deposit has long been recognized, but poorly studied. Physical and chemical characterization of the Pb-Zn-Cu-Ag-Au mineralized fissures in the porphyry-epithermal transition zone provides insight into the origin, timing, and controls of ore deposition. These sheared sulfide-rich fissures are dominated by pyrite and multiple generations of quartz, with lesser amounts of other sulfides and gangue minerals. Au (0.27 to 4.61 ppm) provides the most value to the ore in the transition zone. Host rocks include Eocene monzonite and Paleozoic limestone and quartzite"”all of which can contain economic ore bodies. Associated alteration is predominantly sericitic and argillic. Mineralization into the wall rocks is restricted, not exceeding 1.5 m from the fissure margins. Mineral assemblages vary with distance from the center of the main Cu-Mo deposit and the modal abundances are dependent on host rock. The appearance of both galena and sphalerite (and tennantite to an extent) mark the transition from a porphyry to an epithermal environment. This is accompanied by an increased concentration of chalcophile trace elements in sulfides as determined by EMPA and LA-ICP-MS. Significant hosts of Ag include galena and tennantite, while Cu is hosted primarily in chalcopyrite, tennantite, and sphalerite. Gold does not appear to be hosted in solid solution, but may be focused along fractures or inclusions in pyrite. δ3434S values of fissure pyrite has a narrow range (+2.3 to 3.4‰), while δ18O of quartz is more variable and high (+11.5 to 14.0‰) relative to typical hydrothermal quartz. This can be explained by increased fractionation at lower temperatures in the magmatic fluids, which could have additionally mixed with exchanged 18O-rich meteoric water. Ore grades improve with distance from the center of the deposit; however, this is accompanied by higher concentrations of elements (Pb, As, Bi, etc.) undesirable for downstream processing. The mineralized fissures were created sequentially throughout the formation of the deposit. Initial joints probably formed as a result of the intrusion of a barren equigranular monzonite. The NE orientation of the joints was controlled by the regional stress field, which is more apparent distal to the center of the deposit. A quartz monzonite porphyry then intruded, dilating the joints to allow precipitation of quartz and then pyrite during the Cu-Au-stage of mineralization in the main ore body. After dike-like intrusions of latite porphyry and quartz latite porphyry intruded, galena, sphalerite, and pyrite precipitated to form the Pb-Zn-Ag mineralization. This was followed by late precipitation of chalcopyrite and tennantite (and likely Au mineralization).

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The Kanmantoo Cu-Au deposit is located 55km east of Adelaide, on the eastern edge of the Mt Lofty Ranges, South Australia. It is of Delamerian age and is hosted in the Tapanappa series of the Kanmantoo Group, a pelitic turbidite sequence metamorphosed to amphibolites facies. Models for mineralisation vary from sedimentary exhalative system to epigenetic mineralisation. Despite recent work, the structural evolution of the deposit is largely unknown and this allows for the absence of a definitive model for mineralisation. Detailed face mapping of the 1190RL bench in conjunction with handheld X-Ray Fluorescence Niton gun was adopted to further investigate the relationship between key structural features and element distribution. Micro analysis by petrographic studies, Edax element maps and δ34S isotope analysis was completed to gain understanding into fluid-rock relationships and origin of mineralising fluids. The findings of this study strongly suggest timing of copper mineralisation was associated with the first phase of orogenic extension at 490 ± 3 Ma. The extensional reactivation of compressional D3 shear zones, along with the injection of partially oxidised igneous derived fluids interacting with Fe-rich sediments, allows for the formation of the Kanmantoo magmatic hydrothermal deposit. Sulphur isotope results, and the mapping of magnetite-pyrite-chalcopyrite bearing K-feldspar veins are a very strong evidence of an igneous influence. Cu precipitation is as a result of a cooling oxidised magmatic hydrothermal fluids reacting with Fe in metasediments, and partially interacting with a reducing environment, rather than being directly associated with Fe rich metasomatism. Broad unmineralised zones of chlorite alteration suggest circulation of magmatic hydrothermal fluid with copper mineralisation preferentially precipitating in veins within and adjacent to reactivated D3 shears and D3 antiformal zones.
Thesis (B.Sc.(Hons)) -- University of Adelaide, School of Earth and Environmental Sciences, 2012

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South Australia’s Kanmantoo Cu-Au deposit is currently operated by Hillgrove Resources and has an extended history of exploration and production dating back to 1846. However, there is little consensus on the paragenesis and structural controls of the deposit. Empirical work specifically on Au mineralogy and paragenesis has been completed. To investigate the mineralogical and geochemical associations between Au and host mineralogy, drill core samples, grab samples and ore concentrates and tailings have been collected from the East Kavanagh, Central Kavanagh, West Kavanagh, Spitfire and Nugent ore lodes. Petrographic analysis, Mineral Insights Goldsniffer analysis, secondary electron microscopy, mineral liberation analysis (SEM-MLA) and Laser Ablation (LA-ICP-MS) analysis observed and recorded evidence for four textural settings of Au. Two stages of Au development are proposed: early Au (associated with the main economic Cu-bearing hydrothermal fluids) and late Au (associated with retrograde Bi-rich hydrothermal fluids). Variations observed in major and trace element composition reflect changing input from a thermally-anomalous hydrothermal fluid source. The stability field for Au nanoparticles and the rarity of precipitated visible Au supports a late-peak to post-peak metamorphic origin. This study has implications about how Au can be recovered within the Kanmantoo Cu-Au deposit. The mineralogy and geochemical characteristics of Au at the Kanmantoo Cu-Au deposit can also be utilised as an exploration pathfinder within the greater Adelaide fold belt and the Delamarian-affected terrains at other exploration provinces within the Adelaide fold belt.
Thesis (B.Sc.(Hons)) -- University of Adelaide, School of Physical Sciences, 2018

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The Kanmantoo Cu-Au deposit, situated 55 km south-east of Adelaide, is hosted in the Tapanappa Formation of the Kanmantoo Trough. Recent evidence supports an epigenetic mineralising model for the deposit with respect to the Delamerian Orogeny of ~514 to 490 ±3 Ma. The Delamerian deformation event is the oldest portion of the Tasmanides, a 20 000 km orogenic belt along the eastern palaeo-pacific margin of Gondwana. Mineralisation of the Kanmantoo deposit has been linked with post-Delamerian multi-phase extension in east dipping normal faults. The final stages of extension resulted in non-mineralised north dipping normal faults and proximal discrete fracturing. Structural analysis of geology centred on the Kanmantoo deposit has classified a systematic set of extensional fracturing, developed in- the Kanmantoo deposit and in the region surrounding the deposit for >5 km radius. The fracture set trends east-west and dips steeply to the north with a recorded mean orientation of 75/359°. Fractures are characteristically not offset by shearing, strike for tens of metres, have variable frequency, and alterations influenced by fluid migration. Petrographic and geochemical analysis (SEM)in this study has defined a regionally distributed fracture-hosted albitic alteration, which is relatively enriched in Na, Ca, Al and depleted in Fe, Mg and K. A late stage extensional setting is supported for the development of the discrete sub-vertical fracturing.
Thesis (B.Sc.(Hons)) -- University of Adelaide, School of Physical Sciences, 2009

Appendices 5 and 6 are made available in CD-ROM format (See 04SuppMaterial).
Includes copies of articles co-authored by the author during the preparation of this thesis as appendix 7.
14 maps (some folded, some col.); inserted in back pocket.
Includes bibliographical references (leaves 242-264).
System requirements for accompanying CD-ROM: Macintosh of IBM compatible computer. Other requirements: Adobe Acrobat Reader.
[15], 264 leaves : ill. (some col.), maps ; 30 cm. + 1 computer optical disk (4 3/4 in.)
On the basis of the present study it is concluded that there is no firm evidence that the bulk of the mineralisation is pre-metamorphic, although the possibility has not been excluded.
Thesis (Ph.D.) -- University of Adelaide, Dept. of Geology and Geophysics, 2001

Appendices 5 and 6 are made available in CD-ROM format.
Includes copies of articles co-authored by the author during the preparation of this thesis as appendix 7.
14 maps (some folded, some col.); inserted in back pocket.
Includes bibliographical references (leaves 242-264).
System requirements for accompanying CD-ROM: Macintosh of IBM compatible computer. Other requirements: Adobe Acrobat Reader.
[15], 264 leaves : ill. (some col.), maps ; 30 cm. + 1 computer optical disk (4 3/4 in.)
Title page, contents and abstract only. The complete thesis in print form is available from the University Library.
On the basis of the present study it is concluded that there is no firm evidence that the bulk of the mineralisation is pre-metamorphic, although the possibility has not been excluded.
Thesis (Ph.D.)--Adelaide University, Dept. of Geology and Geophysics, 2001

The Early Jurassic Kerr copper - gold porphyry deposit is hosted within a northerly striking, highly deformed and metamorphosed alteration zone in Late Triassic Stuhini Group sedimentary and volcanic rocks. Mineralization is related to west dipping calc-alkaline monzonite, syenodiorite (197 ± 3 Ma, U-Pb zircon) and K-Ba-feldspar megacrystic plagioclase hornblende porphyry (194 ± 1.5 Ma, U-Pb zircon) dykes. The Late Triassic sedimentary rocks exposed along the footwall form an east dipping, upright sequence of well bedded mudstone with conglomerate lenses overlain by conglomerate, sandstone and argillite. Feldspar fragments in the conglomerate indicate a local volcanic provenance. Copper and gold mineralization is concentrated in a region of early, texturally destructive potassic alteration, which is flanked by lower grade propylitic alteration. The potassic alteration was cut by monzonite dykes that were subsequently altered to sericite - quartz - pyrite. Alteration of the monzonite dykes coincided with widespread retrograde alteration of the potassic assemblage during the development of a banded quartz vein stockwork followed by pyrite - chalcopyrite veins and finally anhydrite - quartz - siderite - pyrite - chalcopyrite veins. The retrograde alteration of the deposit resulted in a core of chlorite - sericite - quartz - anhydrite - pyrite - chalcopyrite - magnetite, which has a halo of yellow sericite - quartz - pyrite - rutile alteration. Structural fabrics in the deposit record southwesterly directed shortening coupled with southeasterly directed extension. A whole rock K-Ar age of 124 ± 4 Ma from deformed sericite - quartz - pyrite monzonite suggests that the deposit was deformed and metamorphosed in the Cretaceous. This age and style of deformation correlates with widespread deformation in the Skeena Fold belt during the Late Jurassic to Early Tertiary. The deformation caused widespread recrystallization of the deposit with only local redistribution into extension gashes. Boudinaged dykes and left lateral displacement of the fabric across westerly dipping faults suggest that this deformation was followed by additional southeasterly directed contraction. Undeformed kersantite dykes (51.5 ± 2 Ma, whole rock K-Ar) intrude these westerly dipping faults. Supergene alteration was focused where anhydrite occurred in the chlorite core of the deposit. Hydration of the anhydrite to gypsum caused the rock to fracture parallel to foliation, and resulted in a flaky rubble zone once the gypsum dissolved. Three supergene alteration zones developed in this permeable rubble: (i) leached hematite/jarosite, (ii) minor native copper and coatings of chalcocite/covellite, and (iii) stable chalcopyrite and pyrite without gypsum or anhydrite. The supergene alteration developed in the chlorite core mainly in response to the dissolution of the gypsum.

The Au-Cu-Bi- deposits of the Proterozoic Tennant Creek Inlier share geological and geochemical characteristics that indicate strong links in their genesis, yet the diversity in alteration assemblages, metal ratios and zonation patterns reflect variations in ore forming processes that previously have not been explained in detail. The West Peko deposit is representative of Cu-rich, pyrrhotite-bearing mineralisation with intermediate gold grades, in magnetite+ hematite-rich syntectonic ‘ironstones’. By contrast, the high grade Eldorado Au deposit contains minor sulfides and very low Cu grades, similar to several of the larger gold producers in the field (e.g. Juno, White Devil, Nobles Nob), and is also hematite-rich. Au, Chalcopyrite and Bi-sulfosalts were introduced into pre-existing ironside during progressive shearing, either late in the first regional deformation event (D1) or during a second phase of deformation. The occurrence of some Au zones outside ironstones suggests the ore fluids in part followed different flow paths to hose of the ironside-forming fluids. Three chemically and isotopically distinct fluids have been characterised. (i) Ironstone-forming fluids at West Peko and Eldorado were Ca-Na-Cl (-Fe?) brines containing 12-20 weight % total dissolved salts, and reached temperatures of 350-400°C during magnetite deposition. Oxygen and hydrogen isotope compositions of minerals formed at the ironside stage are consistent with an origin of ironstones from formation or metamorphic waters. (ii) The inferred Au-Bi+Cu transport fluid in he Cu- and sulphide-rich West Peko deposit was of low to moderate salinity (3-10 eq. wt. % Na Cl), ~300-350°C and N2 + CH4 – rich. Newly represented phase equilibria among the Fe-silicates stilpnomelane and minnesotaite, chlorite, biotite, sulfides, oxides and carbonates as well as fluid inclusion vapour compositions indicate that the Au-Bi+Cu transport fluid was relatively reducing with near-neutral pH and total dissolved sulphur contents of 0.001m to 0.01m. In the Eldorado Deeps Au- and hematite-rich deposit the Au-transporting fluid also may have been of low-moderate salinity, with Au deposition occurring at ~300°C. The reducing Au-Bi+Cu transport fluid at West Peko resembles primary magmatic or metamorphic water in oxygen and hydrogen isotopic composition. Carbon isotope ratios of Au-sulfide stage carbonates at West Peko point to involvement f organic carbon, probably sourced outside the host Warramunga Formation. (iii) A regionally distributed, oxidising Ca-Na-Cl brine with 20-35 weight percent total dissolved salts, was present prior to, after and probably during ore deposition. Mixing with lower salinity reducing Au-Bi+Cu transport fluid is inferred at West Peko and us suggested to have caused effervescence of N2+CH4 by ‘salting out’, relatively late in the Au depositional stage. An hypothesis of metal transport and deposition is proposed for the Tennant Creek deposits in which gold, copper and bismuth were transported in a reducing fluid and were deposited in the Cu- and sulphide-rich deposits dominantly by oxidation, desulfidation and initial pH increase as the reducing fluid reacted with magnetite+hematite ironstone. Mass transfer modelling indicates that relatively small amounts of ironstone are required to precipitate Au + Bi-sulfides, such as Eldorado, the oxidising brine may have played a significant role in ore deposition either by mixing with a reducing Au-Cu-Bi-transporting fluid, or by producing hematite oxidant additional to any already present in the ironstones. The greater extent of oxidation of the ore fluid in such deposits may have generally prevented saturation of copper minerals, resulting in low Cu grades. Gold is inferred to have been transported dominantly as uncharged bisulfide complexes, although biselenide complexes were potentially important. New thermodynamic data estimated for bismuth complexes are consistent with bismuth transport as uncharged S-H-O-bearing species in the Tenant Creek ore fluids. The existence of high grade Au-Bi deposits outside ironstones is predicted by chemical modelling of mixing between reducing and oxidising fluids, located where structures allowed focused flow of both fluids.

The Cerro Corona deposit is located in the Hualgayoc mining district of the Cajamarca province of northern Peru. The copper-gold mineralization is hosted by the Cerro Corona stock which is divided into two intrusive units: a pre or synmineralization phase (Quartz Diorite 1) and post-mineralization phase (Quartz Diorite 2). Intrusive phases are compositionally very similar and have fine grain crowded porphyritic textures. The quartz diorite is comprised of approximately 25 % plagioclase, 8 % biotite and 8 % hornblende phenocrysts and the remainder is a fine grained groundmass. ²°⁶Pb/²³⁸U analysis of zircon gives a Middle Miocene age (14.4 ±0.1 Ma) for the Cerro Corona quartz diorite. The Cerro Corona stock intruded the late Cretaceous Pariatambo formation which is composed of silty limestones. Skarning and mineralization of the Pariatambo formation is restricted to within twenty meters of the limestone diorite contact. Four hydrothermal alteration assemblages have been classified at the Cerro Corona deposit: 1) K-silicate, 2) Sericite-chlorite-clay (SCC), 3) Quartz-sericite-pyrite (QSP), 4) Orange clay (OC). K-silicate alteration is characterized by replacement of primary hornblende by hydrothermal biotite, replacement of plagioclase by potassium feldspar and the formation of potassium feldspar and leafy hydrothermal biotite in the groundmass and in veins. K-silicate alteration is interpreted to be the oldest alteration assemblage and it occurs commonly in the deeper levels of the Cerro Corona stock. SCC alteration is characterized by pale green colour and waxy texture due to the replacement of plagioclase and groundmass by sericite, chlorite, clay, and calcite. SCC alteration comprises a large area in Quartz Diorite 1 and is interpreted to have the occurred after K-silicate alteration. QSP alteration is characterized by the destruction of intrusive texture and the formation of massive quartz, sericite and pyrite. QSP alteration occurs in the upper levels of the stock in Quartz Diorite 1 and is interpreted to be the last stage in the hydrothermal sequence which is associated with mineralization. OC alteration is characterized by the replacement of plagioclase and groundmass by clay which is bright orange in colour where the alteration affects magnetite-hematite bearing rocks. OC alteration occurs throughout the Cerro Corona stock and may be related to supergene fluids. The main vein types classified at Cerro Corona are: 1) Biotite 2) K-feldspar 3) Magnetite 4) Quartz-oxide-sulphide (QOS) 5) Quartz-pyrite (QP) 6) Pyrite 7) Calcite. The majority of the copper mineralization occurs as chalcopyrite which forms in the Kfeldspar, Magnetite, QOS, QP and Pyrite veins. Two types of supergene mineralization occur at Cerro Corona: oxidized Au bearing rock and supergene Cu mineralized rock both of which occur in zones sub-parallel to the surface. Significant hypogene mineralization occurs throughout Quartz Diorite 1 above 3500m. Mineralization is interpreted to have occurred in two separate phases: 1) low Au/Cu ratio mineralization which occurs throughout Quartz Diorite 1 and is related to K-silicate alteration; and 2) high Au/Cu ratio mineralization which occurs in the center of the SCC altered zone and may be related to SCC alteration. The size, grade and mineralogy of the Cerro Corona deposit are characteristic of porphyry copper-gold deposits which occur throughout the world. The alteration and mineralization at Cerro Corona represents a magmatic-hydrothermal system which chemically changed and collapsed over time resulting in intense overprinting alteration assemblages and related mineralization.

The Olympic Dam Cu-U-Au-Ag deposit is a syngenetic orebody hosted by the Olympic Dam Breccia Complex, a high level, hematite-rich hydrothermal breccia system. The breccia complex occurs entirely within, and clearly post-dates, the Roxby Downs Granite. _Cu-Fe sulphide distribution within the deposit is zoned from pyrite-rich assemblages at depth, upwards to chalcopyrite-rich ores, ultimately to bornite-chalcocite ores. Mlnor proportions of paragenetically early magnetite are widely distributed. Felsic and mafic/ultramafic dykes are broadly coeval with ore deposition. The Roxby Downs Granite has an age of 1588 ± 4 Ma. Prior to this study the age of the breccia complex was unclear. SHRIMP U-Pb isotopic data for zircons from three igneous rock units constrain the minimum age for the bulk of the mineralisation. Twb autobrecciated felsic dykes that intrude hematite-rich sedimetitary rocks and·hydrothermal breccias have ages of 1592.± 8 Ma and 1584 ±- 20 Ma respectively. An ashfall tuff horizon from within a diatrerne that cross-cuts hematite-quartz breccias contains zircons with an age of 1597 ± 8 Ma. These three minimum age constraints are within error of the age of the host granite, meaning that the breccia complex has an age of -1590 Ma. This age determination allows confident correlation of ore deposition with a major regional magmatic episode, the Gawler Range/ Hiltaba-volcano-plutonic event. Pb isotopic measurements of hydrated U-Th-Y-REE~rich hydrothermal zircon-xenotime overgrowths on magmatic zircons yielded evidence of complex Pb mobility histories, and provided no strong evidence regarding the age of the deposit. SHRIMP analyses of Olympic Dam pitchblende indicate that U and Pb mobility has reset isotopic systematics heterogeneously, even on a microscopic scale. This open system behaviour is also apparent from the data of earlier workers. An earlier U-Pb concordia upper intercept "age" of 1400 Ma for Olympic Dam pitchblende, calculated by regression analysis of averaged isotopic analyses, should therefore be regarded with caution. Sm-Nd isotopic data indicate that the different hematite-sulphide assemblages share an initial ENd signature of -2.5. This suggests that these ore types are cogenetic at 1590 Ma. This signature also indicates that ore fluids received contributions of Nd from crustal rocks such as the host granite (£Nd -5) and from rocks or magmas derived from the mantle at 1590 Ma. In contrast, the volumetrically minor magnetite-rich assemblages have the same initial Nd signature as the host granite, suggesting that they are possibly cogenetic. The hematitic ores and breccia~ yield a fourteen point Sm-Nd isochron age of 1572 ± 99 Ma consistent with the age constraints provided by zircon geochronology. The least altered mafic/ultramafic dykes within the deposit have an initial isotopic signature of eNd +4 and are therefore a plausible source of the primitive Nd component in the hematitic rocks. With progressive alteration, these dykes have become depleted in several of the elements that are enriched in the Olympic bam ores and breccias, particularly Cu, Cr, Ni, V, Mn, Nb and Y. The trace element and Nd isotopic relationships strongly implicate the mafic/ultramafic dykes, or their plutonic or extrusive equivalents, as metal sources. Interpreted primary enrichments in incompatible elements suggest a strongly alkaline affinity for the mafic/ultraniafic rocks. Rb-Sr isotopic systematics of the Olympic Dam breccias and mafic/ultramafic dykes show evidence of open system behaviour after ore deposition. No firm conclusions regarding ore genesis are drawn from these data. In view of the data presented, plausible hypotheses of ore genesis include: a) fluid mixing involving a hypogene, granite-related fluid, and a saline meteoric fluid transporting metals of economic interest that have been leached from the Gawler Range volcanic pile, and b) the sequential activity of a granite-related fluid that produced magn-tite-rich mineralisation, and a fluid derived by late-stage exsolution of magmatic volatiles from an alkaline mafic or ultramafic pluton .

Kucing Liar is a large sediment-hosted Cu-Au mineralized system containing some 15Moz of gold and 5Mt of copper in ~500Mt of ore. It is situated in the Ertsberg Mining District in the Central Ranges of New Guinea, in the Indonesian province of West Papua. This study demonstrates that high sulphidation ore is continuous with typical porphyry-skarn style chalcopyrite ore and that both have formed from mixing of magmatic with meteoric waters within a zone of fault offset. Alteration and mineralization were localised within calcareous shale and thinly bedded limestone adjacent to the Grasberg Igneous Complex where they are zoned around fault offsets. Early phases of alteration are stratiform and are juxtaposed against the Idenberg Fault Zone, which has displaced host stratigraphy at least 600m vertically and possibly up to ~1,500m laterally. Four principal hydrothermal mineral associations are (1) calcic and magnesian skarn, (2) potassic assemblages including magnetite, (3) quartz-muscovite plus anhydrite and (4) locally massive pyrite. Cu and Au are associated with pyrite and occur discretely either as chalcopyrite ± bornite with an association of Cu-Au-Co (Zn-Pb) or as covellite ± enargite associated with Cu-Au (As-Sb-Hg). 40Ar/39Ar geochronology shows muscovite (3.18 ± 0.02Ma) was coeval with potassic-biotite assemblages (3.18 ± 0.02Ma and 3.20 ± 0.04Ma). Calcic and magnesian skarn were derived from magmatic fluids (_18OFLUID = 9-6‰), while potassic and magnetite alteration were derived from high temperature (>650°C), high salinity (>50wt%NaClEQUIV.) magmatic fluids (_18OFLUID = 6-12‰). Quartz infill crystals associated with voluminous silicification contain a variety of fluid inclusions that range from moderate temperature (TH<420°C) high and moderate salinity brines (35-55 and 15-30wt%NaClEQUIV.), to low density - low salinity vapour-rich fluid inclusions. Fluorite-hosted inclusions with lower TH (<300°C) and salinity (~5wt%NaClEQUIV.) are also related to quartz alteration. Quartz alteration, muscovite and anhydrite have estimated _18OFLUID ranging from 0-6‰. _D data from magnesian skarn suggest that the magma source was strongly but variably degassed during skarn formation while clustering of biotite and tremolite _D data may indicate ponding of fluids prior to exsolution, which was preceded by monzonite dyke emplacement that were emplaced during skarn and potassic stage alteration. Fluid infiltration was controlled by an active fault system characterised by strike-slip deformation overprinting a pre-existing reverse-slip fault. Periodic slip allowed infiltration of the magmatic fluids while a complex structural offset controlled the mixing of magmatic and meteoric fluids. Fluid mixing was augmented by phase separation which gave rise to brine and vapour-rich phases that migrated differently due to density contrasts. Ore deposition was related to mixing of magmatic and meteoric fluids, which resulted in an increase in H2S relative to SO2, causing intense sulphidation of magnetite and precipitation of sulphides, beginning with gold-rich chalcopyrite-dominant mineralization. High sulphidation covellite-style mineralization occurred by contraction of the vapour phase that had separated from quartz-forming brines. Au, As and Sb were partitioned away from the high sulphidation copper mineralization due to higher solubilities of these metals as bisulphide complexes and deposited in distal pyrite along with chloride-complexed Pb and Zn.

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The Ernest Henry deposit is situated within the Eastern Fold Belt of the Mount Isa Inlier, NW Queensland, and is the largest iron oxide copper-gold (IOCG) deposit in the Proterozoic Cloncurry district. The hydrothermal deposit is hosted in brecciated intermediate metavolcanic and metasedimentary rocks with a biotite-calcite-chalcopyrite-gold-magnetite-pyrite-quartz mineral assemblage. This study investigates the mineralogical, textural and geochemical association between gold and pyrite with samples collected from three drill holes (EH768, EH859 and EH864) at ~700m vertical depth within the ore body. Majority of the gold (~98%) at Ernest Henry is in the form of free gold, which is commonly observed in pyrite microfractures, associated with chalcopyrite infill. Free gold has been interpreted to have entered the system with the main economic Cu-Au mineralisation stage. We propose that the semi-conducting potential of pre-existing pyrite surfaces have acted as a catalyst for the precipitation of free gold. Implications from this study may assist in the improvement of gold recovery at Ernest Henry and provide a better understanding of the timing of the ‘C’ and ‘G’ in ‘IOCG’ deposits in the local Cloncurry District.
Thesis (B.Sc.(Hons)) -- University of Adelaide, School of Physical Sciences, 2017

Mount Polley is an alkalic porphyry copper-gold deposit of Lower Jurassic age within the Quesnel terrane in south-central British Columbia. The Mount Polley intrusive complex is assumed to be coeval with the regional Nicola Group volcanic rocks in which it is emplaced. Some volcanic rocks are silica undersaturated, contain feldspathoids and are chemically similar to the plutonic rocks. The Nicola Group rocks have trace and rare earth signatures of a volcanic island arc environment. The Mount Polley deposit is characterized by multiple intrusions that compositionally vary from diorite to crowded plagioclase porphyry to monzonite. Minor intrusion and abundant hydrothermal breccias are emplaced in a northerly trending diorite host. Hydrothermal breccias are the main host to mineralization and are associated with the highest concentrations of copper and gold. Hydrothermal breccias are subdivided into four distinct types based on the dominant hydrothermal mineral in the matrix. Actinolite breccia is developed in an elongate zone within the core of the system; it is parallel and lies east of a northnorthwest trending structure, the Polley fault. Actinolite breccia grades laterally and vertically into biotite breccia in the southeastern part of the deposit. Magnetite breccia is irregularly distributed and is relatively sparse. West of the Polley fault, albite breccia is dominant. Pervasive and vein-related alteration correlates with the breccia types. A zonal distribution of alteration minerals has been mapped. The core of the hydrothermal system at Mount Polley is subdivided into three zones: actinolite, biotite and potassium feldspar-albite. The actinolite zone is typified by development of actinolite-pyroxene-magnetite-sulfide veins with potassium feldspar envelopes. A biotite alteration zone is characterized by the formation of secondary, coarse grained biotite within open spaces of the hydrothermal breccias. Arcuate around these two zones is a region of pervasive potassium feldspar and locally intense albitic alteration, generally spatially related to hydrothermal breccias. The margins of the potassic zone are overprinted by a discontinuous zone of calc-silicate minerals. A complicated assemblage of garnet, epidote, albite, potassium feldspar, chlorite, magnetite and sulfides is present. This intermediate zone passes outwards into propylitic alteration. Mineralization is most prominent within hydrothermal breccias and is generally present as disseminations, blebs within the matrix and in abundant veins. Metals are outwardly zoned from a core of chalcopyritemagnetite- bornite to magnetite-pyrite-chalcopyrite. Constant copper-gold ratios indicate that chalcopyrite and gold are probably co-precipitating from the same fluids; the pyrite-dominated assemblage forming at lower temperature. Hydrothermal breccias are genetically related to the emplacement of the crowded plagioclase porphyry melt. Crystallization of the melt was probably accompanied by volatile/aqueous exsolution, foiming a water saturated carapace. Decompression of the chamber in response to magma withdrawal, fracture propagation and possible fault movement may have allowed hydrothermal brecciation to occur at the apex and margins of the intrusion. Alteration and mineralization appears to be controlled by fluids migrating away from the plagioclase porphyry.

The Telfer Au (+Cu) deposit is one of Australia's premier gold-producers, currently accounting for approximately 400,000oz. per year. It's discovery in 1972 heralded the recognition of the Proterozoic Paterson Province, in which it is located, as a polymetallic terrane with the potential for significant gold and base-metal mineralisation. The Paterson Province represents a small exposure (= 36 000km²) of the large NW -SE trending Paterson Orogen, a continental-scale orogenic belt that passes across northern Western Australia and into central Australia. Rocks in the Paterson Province have recorded a protracted Proterozoic history for this orogenic belt, which includes continent-continent collision at ~1250Ma (Watrara Orogeny) and late-Proterozoic collisional tectonism (the Paterson Orogeny) between 700 and 600Ma. The latter generated considerable mineralisation in middle- to late-Proterozoic metasedimentary rocks of the Paterson Province. Late-Proterozoic mineralisation in the Paterson Province is well developed in the NE region where numerous syn- and post-tectonic granitoids, gabbros and dolerites intruded the metasedimentary sequence. However, mineral exploration in this region is commonly hindered by extensive Phanerozoic and Tertiary cover that precludes observation of much of the Proterozoic sequence. Additionally, the province lies within the Great Sandy Desert, and is thus covered by extensive aeolian sand deposits. The large amount of younger cover has resulted in relatively small scattered "windows" that expose the mineralised Proterozoic rock sequence. The NE region of the Paterson Province provides one of the best exposures of this sequence and hosts the Telfer deposit. Sampling of gossanous and stratabound quartz veining in the Telfer Dome (a regional antiformal fold) in 1972 by geologists from both Day Dawn Minerals and Newmont Pty Ltd identified gold enriched horizons within the pelitic sedimentary sequence (Telfer Formation) exposed in the dome. Subsequent drilling of these horizons confirmed an initial resource of 1 Million ounces of gold and mining activity commenced in 1975. During the early 1980's mining concentrated on one particular reef that outcropped in Main Dome, a sub-dome of the Telfer Dome. This reef, the Middle Vale Reef (MVR) exhibited strong secondary enrichment of gold and has historically comprised a significant resource in the Telfer deposit (Dimo, 1990). During the middle to late 1980's a shift to high-volume low-grade mining was facilitated by the ongoing success of dump leach extraction of gold from rocks previously too low-grade to be milled. This incresased throughput caused production to expand to the second subdome (West Dome) as the E-Reefs were mined, and helped to make Telfer one of the top four gold producers in Australia during the late 1980's. More recently, deep diamond drilling in Main Dome (commenced in 1992) has led to the discovery of approximately ten to twelve new reefs at depth in the dome. Underground mining, which had commenced in 1990, has been extended through an exploration decline 10 the upper of these newly discovered reefs (MIO and M30). This decline is currently being extended to reach the deepest reef, the 130, at approximately 1 100m below the present ground surface. This should occur by the end of 1997. Another consequence of the deep drilling has been the further confirmation of an epigenetic genesis for the Telfer deposit. and particularly the identification of mineralisation in units other than the Telfer Formation. Initial research on the stratabound reefs in the Telfer deposit suggested that they had formed through syngenetic exhalative processes (Tyrwhitt, 1979; Turner, 1982). A variation on this, whereby the MVR was considered to have formed as an evaporite horizon that was subsequently replaced by quartzsulphide assemblages, was proposed by Royle (1985). However, the expansion of mining and deep drilling within the deposit provided increasing evidence for an epigenetic origin. This came both through structural observations and geochemical fluid-inclusion studies that indicated the reefs were locally discordant, had associated stockwork veining in both the foot- and hanging-walls, contained magmatic elements in the ore-assemblage and that the ore-fluids were of variable salinity and temperature (Goellnicht, 1987). These observations led to an epigenetic model whereby magmatic fluids from the regional granitoids had mixed with cooler connate/formational waters, and had precipitated in structurally controlled, and compositionally favourable, sites within the Telfer Dome (Goellnicht, 1987; Goellnicht et al., 1989). Subsequent research downgraded the role of the granites, suggesting that they acted more as heat sources to convectively circulate connate/contact-metamorphic fluids that scavenge elements from the sedimentary sequence (Hall & Berry, 1989; Rowins. 1994). The changing ideas on the genesis of the Telfer mineralisation are reflected in the changing focus of mineral exploration in the Telfer region and the Paterson Province. Initial exploration, utilising the syngenetic exhalative model, concentrated on locating further outcropping Telfer Formation. The inferred variable thickness of exhalative lenses was considered to have assisted the formation of the regional domal antiforms (Turner, 1982), and consequently those domes that exposed the Telfer Formation were targeted. However, recognition of an epigenetic genesis has focussed exploration activity towards targeting favourable host structures, such as regional folds and other ore-fluid traps. The change in exploration strategy means that a greater reliance is now placed on the structural geological setting of mineralisation in the Paterson Province. This study represents the first formal examination whereby the structural geological setting of mineralisation in the Telfer Mine is integrated with the regional- and orogenic-scale tectonic development of the Paterson Province.

Michael Calder recorded the hydrothermal alteration mineral assemblages to define alteration types at the Far Southeast porphyry copper-gold deposit, Philippines. Spatial extent and timing relations were recorded and two cross-sections through the high-grade zone were constructed. Alteration mineral assemblages were determined by drill core logging, an optical microscopy petrographic study, Short Wavelength Infra-Red (SWIR) spectral analysis, and a Quantitative Evaluation of Materials by Scanning Electron Microscopy (QEMSCAN) study. Timing relations of mineral assemblages determine a cooling of the system. Detailed study of mineral assemblages improve genetic models and industry exploitation efficiency.

The Marcona district of littoral south-central Perú represents the largest concentration of iron oxide-copper-gold deposits in the Central Andes. Hydrothermal activity occurred episodically from 177 to 95 Ma and was controlled by NE-striking faults. At Marcona, emplacement of massive magnetite orebodies with subordinate, overprinted magnetite-sulphide assemblages coincided with a 156-162 Ma episode of eruption of andesitic magma in the Jurassic arc, but mineralization is hosted largely by underlying, Lower Paleozoic metaclastic rocks. The magnetite orebodies exhibit smoothly curving, abrupt contacts, dike-like to tubular apophyses and intricate, amoeboid interfingering with dacite porphyry intrusions, interpreted as evidence for the commingling of hydrous Fe oxidic and silicic melts. An evolution from magnetite - biotite - calcic amphibole ± phlogopite assemblages, which are inferred to have crystallized from an Fe-oxide melt, to magnetite - phlogopite - calcic amphibole - sulphide assemblages coincided with quenching from above 700°C to below 450°C and with the exsolution of aqueous fluids with magmatic stable isotopic compositions. Subsequent, subeconomic chalcopyrite - pyrite - calcite ± pyrrhotite ± sphalerite assemblages were deposited from cooler fluids with similar δ34S, δ18O and δ13C values, but higher δD, which may record the involvement of both seawater and meteoric water. The much younger (95-110 Ma), entirely hydrothermal, Mina Justa Cu (-Ag) deposit is hosted by Middle Jurassic andesites intruded, on a district scale, by small dioritic stocks at the faulted SW margin of an Aptian-Albian shallow-marine volcano-sedimentary basin. Intense albite-actinolite alteration (ca. 157 Ma) and K-Fe metasomatism (ca. 142 Ma) long preceded the deposition of magnetite-pyrite assemblages from 500-600°C fluids with a magmatic isotopic signature. In contrast, ensuing chalcopyrite - bornite - digenite - chalcocite - hematite - calcite mineralization was entirely the product of non - magmatic, probably evaporite-sourced, brines. Marcona and Mina Justa therefore represent contrasted ore deposit types and may bear minimal genetic relationships. The former shares similarities with other Kiruna-type magnetite (-apatite) deposits. In contrast, the latter is a hydrothermal system recording the incursion of fluids plausibly expelled from the adjacent Cañete basin. Non-magmatic fluids are inferred to be a prerequisite for economic Cu mineralization in the Cu-rich IOCG deposits in the Central Andes and elsewhere.
Thesis (Ph.D, Geological Sciences & Geological Engineering) -- Queen's University, 2008-05-13 14:39:21.43

The North Fork porphyry Cu-Au deposit is located in the west central Cascades Mountain Range, Washington, U.S.A., and belongs to a belt of Eocene to Miocene porphyry Cu (Au) deposits that extend northward into the Coast Mountains of southern British Columbia. The deposit has a geological reserve of 80.4 million tonnes @ 0.44% Cu and 0.003 ounces (oz) Au (a 218,000 oz Au reserve) and is hosted in three main rock units. The oldest and most spatially extensive unit is the Mount Persis andesite (38.9 ±0.3 Ma). It is intruded by quartz monzodiorite (2 samples dated at 37.2 ±0.1 Ma and 37.0 ±0.2 Ma) and mafic latite porphyry (2 samples dated at 37.1 ±0.2 Ma and 36.8 ±0.2 Ma). Plutonic rocks are weakly to moderately peraluminous, calc-alkaline, I-type granitoids that have crystallized at uncommonly low oxygen fugacities (ƒO2's) ranging from the quartz-fayalite-magnetite (QFM) oxygen buffer to one log unit above (QFM+1). The older andesites are even more reduced and have crystallized at ƒO2's approximating QFM-1. Field relationships, age constraints, mineralogy, oxidation state, and whole-rock trace-element data indicate that plutonic and volcanic rocks are consanguineous. It appears that the reduced I-type granitoids have intruded into their own volcanic pile during construction of a late Eocene volcanic arc. Hypogene Cu-Au mineralization is associated with three stages of vein formation, but primarily occurs with banded and crustiform Main-stage quartz-actinolite-albite-chlorite-sulfide veins and accompanying sodic-calcic (albite-actinolite) alteration. Main-stage veins contain abundant hypogene pyrrhotite and lack primary hematite and sulphate minerals indicating formation from relatively reduced hydrothermal fluids. Studies of quartz-hosted fluid inclusions in Main- and Early-stage veins reveal that the North Fork deposit has formed from a thermally prograding system with Cu-Au sulfide deposition occurring at pressures of ~ 400 to 690 bars and temperatures between 348° to 576°C. These pressures are hydrostatic and correspond to depths of ~ 4 to 7 km because fluids were undergoing immiscible phase separation (boiling) into a dense aqueous brine (up to 51 weight % NaCl equivalent) and coexisting low-density vapor (1.4 to 3.4 weight % NaCl equivalent) at the time of trapping and Main-stage vein formation. These physicochemical conditions of ore formation are typical of porphyry Cu-Au deposits worldwide, and together with a direct genetic association with reduced I-type magmas, classify the North Fork deposit as a "reduced porphyry copper-gold" deposit. Measurement of 671 brittle structures (fractures and faults) define three main structural trends that are consistent with the various stress fields operative during the Eocene. The most important of these structures are the NNW-striking (320-340°) fractures and faults that have focused the intrusion of ~ 37 Ma mafic latite porphyry, hypogene Cu-Au mineralization, and related hydrothermal alteration. Repeated use of these structural conduits has resulted in overprinting episodes of magmatism, hydrothermal alteration, and Cu-Au mineralization. Argonargon dating of hydrothermal sericite from halos surrounding Late-stage quartz-sulfide veins yields an age of 35.5 ± 0.2 Ma, which is at least 900,000 m.y. younger than the emplacement age of the youngest mafic latite porphyry, but well within the time-frame that many porphyry Cu- Mo±Au deposits form. The δ34S values of hypogene sulfide minerals from all stages of mineralization lie between 2.3 to 3.0 %o, a range typical of magmatic sulfur and further support of a magmatic origin for the North Fork fluids. The recognition of reduced porphyry Cu-Au mineralization and related arc magmatism at -37 Ma, highlights the prospectivity of the Mount Persis andesites and raises the possibility that Late Eocene porphyry Cu-Au mineralization may be more common in the west central Cascades than has been predicted previously from the localized exposures of quartz monzodiorite and mafic latite porphyry at North Fork.