ABSTRACT The Bursa mineral district in northwest Anatolia (Turkey) is an emerging prospective area for porphyry Cu-Mo (Au-Re) mineralization along the Western Tethyan Eocene magmatic belt that links the Balkan to Lesser Caucasus regions along the southern Black Sea coast. Field observations as well as time constraints on mineralized magmas of the Bursa district are limited. Additionally, the tectonic setting of Eocene magmatism in northwest Turkey is controversial and includes either Neotethyan slab roll-back or break-off scenario. We show that the Bursa mineral district consists of porphyry Cu-Mo, skarn Cu, and rare epithermal Au-Ag deposits, prospects, and occurrences on the footwall of the Eskişehir fault, south of the İzmir-Ankara-Erzincan suture zone. The porphyry prospects are hosted within porphyritic dioritic, granodioritic, and granitic rocks that were altered by potassic and phyllic assemblages and quartz-sulfide vein stockwork zones. Our new CA-TIMS and LA-ICP-MS U-Pb and 40 Ar/ 39 Ar ages indicate that porphyry mineralization of the Bursa district formed between 51 and 46 Ma and thus before the porphyry and epithermal Cu-Au mineralization of the Biga (~43–39 Ma) and Rhodope districts (~35–31 Ma) and the Serbo-Macedonian belt (~36–22 Ma). Therefore, we interpret that Eocene magmatism and associated Cu-Au-Mo mineralization migrated westward along the western part of the Western Tethyan Eocene magmatic belt. This migration, which is specific to the Eocene period, represents a second-order and diachronous response to the first-order southward magmatic front migration and roll-back initiation of the Hellenic slab beneath the Balkan-Aegean-western Anatolian region since the Late Cretaceous.

Abstract The Nadaleen trend is a 25-km-long alignment of recently discovered Carlin-type gold prospects located along the northern margin of the Selwyn basin in east-central Yukon Territory, Canada. These prospects are among the closest analogues to the large, Carlin-type gold deposits found in Nevada. The Nadaleen trend is bound structurally to the south by the regional Dawson thrust and to the north by the Kathleen Lakes fault. The Dawson thrust marks the boundary between dominantly Neoproterozoic to Paleozoic slope and basin facies carbonate, siltstone, and clastic rocks of the Selwyn basin and strata of the Mackenzie platform. The Nadaleen trend contains numerous Carlin-type prospects, with the three largest being Conrad, Osiris, and Anubis. Carlin-type prospects of the Nadaleen trend are hosted in silty limestone and calcareous siliciclastic rocks along with isolated gabbroic dikes. Gold mineralization at Nadaleen is inferred to have accompanied decarbonatization of host limestone and subsequent silicification and/or brecciation. Typically, this was followed by late, open-space calcite, realgar, and orpiment. The prospects exhibit both structural and stratigraphic controls, with zones located near prominent fault and fold features. Gold is associated with elevated As, Hg, Sb, and Tl in mineralized zones. Several types of arsenian pyrite are found in mineralized zones, typically as rims around earlier barren pyrite cores or as <10- μ m disseminations and aggregates. Evidence from the Conrad zone suggests that Carlin-type gold mineralization occurred between 74.4 and 42 Ma. The tectonic and magmatic setting in this remote part of the Yukon during gold mineralization is poorly understood, with little or no evidence for contemporaneous regional magmatism or tectonism. While deposit-scale processes responsible for gold mineralization appear very similar for Carlin-type prospects in the Yukon and Carlin-type gold deposits in Nevada, whether the crustal-scale processes that formed these systems are similar remains enigmatic.

Abstract A magmatic and metallogenic framework for the northern Yukon-Tanana terrane of west-central Yukon and eastern Alaska is proposed, which contextualizes syngenetic, intrusion-related, and orogenic styles of mineralization in the region. The framework applies to bedrock gold and base metal enrichments in the Dawson Range, White Gold, Klondike, Sixtymile, and Fortymile districts, which are historically known for their placer gold endowment, but which host few significant bedrock mineral resources. New field and geochronological (U-Pb, 40 Ar/ 39 Ar, 187 Re/ 187 Os) data, along with contributions from exploration companies, provide the key constraints on this framework. Sedimentary exhalative Pb-Zn mineralization and porphyry-style Cu-Au mineralization are associated with Late Devonian to Early Mississippian (365-342 Ma) rocks of the Finlayson assemblage and Simpson Range plutonic suite, respectively—both of which formed in a continental arc built on pre-Late Devonian continental margin sediments (Snowcap assemblage) along the ancient Pacific margin of North America. By the Late Permian, these assemblages had rifted away from North America, and W-dipping subduction of the intervening Slide Mountain Ocean was initiated. Volcanogenic massive sulfide-style Pb-Zn-Cu-(Ag-Au) mineralization formed in subvolcanic to volcanic rocks of the Late Permian (269-253 Ma) Klondike arc assemblage that was built on the Devono-Mississippian arc. Together these assemblages make up the Yukon-Tanana terrane. Gold mineralization formed sparsely with syn- to postmetamorphic Late Permian (253-250 Ma) anatectic melts. Five metallogenic events are recognized that coincide with magmatic episodes superimposed on the Yukon-Tanana terrane: (1) Cu-Au mineralization formed during an Early Jurassic (200-179 Ma) pulse of magmatism and was accompanied by rapid crustal exhumation (e.g., Minto); (2) Au-mineralized breccia complexes, skarns, intermediate-sulfidation epithermal systems, and polymetallic veins are associated with mid-Cretaceous (115-98 Ma) magnetite-series arc magmas in the Dawson Range, whereas age-equivalent Au deposits in the back-arc region to the north are associated with ilmenite-series magmas (e.g., Pogo); (3) variably Cu and Au rich porphyry systems formed within the mid-Cretaceous arc in the early Late Cretaceous (79-72 Ma) (e.g., Casino, Nucleus-Revenue); (4) porphyry Mo and Cu systems and Ag-rich polymetallic veins, carbonate-replacement, and skarn bodies are temporally and spatially associated with NE-trending, sinistral oblique-extensional fault systems in the latest Cretaceous (72-67 Ma); and (5) examples of disseminated U, Cu-Pb-Ag skarn, and Au-Ag epithermal systems are associated with dominantly felsic but locally bimodal Paleocene-Eocene (60-55 Ma) magmatism, emplaced into zones of extension during early activity on the Tintina fault zone. At least two distinct orogenic Au-mineralizing events are recognized. Within a Middle to Late Jurassic hiatus in magmatism, gold mineralization formed at 163 to 155 Ma in brittle-ductile to brittle structures within sinistral fault zones (e.g., White Gold), high-angle reverse faults, and kink folds. A subsequent episode of mid-Cretaceous (96-92 Ma) orogenic gold mineralization formed in structures cutting Paleozoic metamorphic rocks and mid-Cretaceous granitoids (e.g., Moosehorn, Boulevard). Weathering and surficial preservation in this unglaciated region since the Pliocene resulted in economic placer gold endowments in the Klondike, Sixtymile, Fortymile, White Gold, and Dawson Range districts. The framework we describe for the magmatism and metallogeny of west-central Yukon and eastern Alaska provides a testable platform for regional exploration targeting and property-scale exploration in a region with demonstrated mineral potential.

Abstract Epigenetic gold deposits in metamorphic terranes include those of the Precambrian shields (approx 23,000-25,000 t Au), particularly the Late Archean greenstone belts and Paleoproterozoic fold belts, and of the late Neoproterozoic and younger Cordilleran-style orogens (approx 22,000 t lode and 15,500 t placer Au), mainly along the margins of Gondwana, Laurentia, and the more recent circum-Pacific. Ore formation was concentrated during the time intervals of 2.8 to 2.55 Ga, 2.1 to 1.8 Ga, and 600 to 50 Ma. Prior to the last 25 years, ores were defined by grades of 5 to 10 g/t Au in underground mines; present-day economics, open-pit mining, and improved mineral processing procedures allow recovery of ores of < 1 g/t Au, which has commonly led to the recent reworking of lower gradEzones in many historic orebodies. Most of these deposits formed synchronously with late stages of orogeny and are best classified as orogenic gold deposits, which may be subdivided into epizonal, mesozonal, and hypozonal subtypes based on pressure-temperature conditions of ore formation. A second type of deposit, termed intrusion-related gold deposits, developed landward of Phanerozoic accreted terranes in the Paleozoic of eastern Australia and the Mesozoic of the northern North American Cordillera. These have an overall global distribution that is still equivocal and are characterized by an intimate genetic association with relatively reduced granitoids. The majority of gold deposits in metamorphic terranes are located adjacent to first-order, deep-crustal fault zones, which show complex structural histories and may extend along strike for hundreds of kilometers with widths of as much as a few thousand meters. Fluid migration along such zones was driven by episodes of major pressure fluctuations during seismic events. Ores formed as vein fill of second-and third-order shears and faults, particularly at jogs or changes in strike along the crustal fault zones. Mineralization styles vary from stockworks and breccias in shallow, brittle regimes, through laminated crack-seal veins and sigmoidal vein arrays in brittle-ductile crustal regions, to replacement- and disseminated-type orebodies in deeper, ductile environments (i.e., a continuum model). Most orogenic gold deposits occur in greenschist facies rocks, but significant orebodies can be present in lower and higher grade rocks. Deposits typically formed on retrograde portions of pressure-temperature-time paths and thus are discordant to metamorphic features within host rocks. Spatial association between gold ores and granitoids of all compositions reflects a locally favorable structural trap, except in the case of the intrusion-related gold deposits where there is a clearer genetic association. World-class orebodies are generally 2 to 10 km long, about 1 km wide, and are mined downdip to depths of 2 to 3 km. Most orogenic gold deposits contain 2 to 5 percent sulfide minerals and have gold/silver ratios from 5 to 10 and gold fineness >900. Arsenopyrite and pyrite are the dominant sulfide minerals, whereas pyrrhotite is more important in higher temperature ores and base metals are not highly anomalous. Tungsten-, Bi-, and Te-bearing mineral phases can be common and are dominant in the relatively sulfide poor intrusion-related gold deposits. Alteration intensity, width, and assemblage vary with the host rock, but carbonates, sulfides, muscovite, chlorite, K-feldspar, biotite, tourmaline, and albite are generally present, except in high-temperature systems where alteration halos are dominated by skarnlike assemblages. The vein-forming fluids for gold deposits in metamorphic environments are uniquely CO 2 and 18 O rich, with low to moderate salinities. Phanerozoic and Paleoproterozic ores show a mode of formation temperatures at 250° to 350°C, whereas Late Archean deposits cluster at about 325°to 400°C. However, there are also many important lower and higher temperature deposits deposited throughout the continuum of depths that range between 2 and 20 km. Ore fluids were, in most cases, near-neutral pH, slightly reduced, and dominated by sulfide complexes. Globally consistent ore-fluid δ 18 O values of 6 to 13 per mil and δD values of –80 to –20 per mil generally rule out a significant meteoric water component in the gold-bearing hydrothermal systems. Sulfur isotope measurements on ore-related sulfide minerals are concentrated between 0 and 10 per mil, but with many higher and much lower exceptions, indicating variable sulfur sources and an unlikely dominant role for mantle sulfur. Drastic pressure fluctuations with associated fluid unmixing and/or desulfidation during water/rock interaction are the two most commonly called-upon ore precipitation mechanisms. The specific model(s) for gold ore genesis remains controversial. Although the direct syngenetic models of the 1970s are no longer applicable, the gold itself may be initially added into the volcanic and sedimentary crustal rock sequences, probably within marine pyrite, during sea-floor hydrothermal events. Gold transport and concentration are most commonly suggested to be associated with metamorphic processes, as indicated by the volatile composition of the hydrothermal fluids, the progressive decrease in concentration of elements enriched in the gold deposits with increasing metamorphic grade of the country rocks, and the common association of ores with medium-grade metamorphic environments. Gold deposits of typically relatively low grade, which formed directly from fluid exsolution during granitoid emplacement within metamorphic rocks, are now also clearly recognized (i.e., intrusion-related gold deposits), but there are limited definitive data to implicate such an exsolved fluid source for most gold deposits within orogenic provinces. The fact that orogenic gold deposits are associated with all types of igneous rocks is a problem to a pure magmatic model. Hybrid models, where slab-derived fluids may generate rising melts that drive devolatilization reactions in the lower crust, are also feasible. Although involvement of a direct mantle fluid presents geochemical difficulties, the presence of lamprophyres and deep-crustal faults in many districts suggests potential mantle influence in the overall, large scale tectonic event controlling the hydrothermal flow system.