This Atlas presents an extensive geoscience data set for the Late Triassic to Jurassic plutons that intrude the Intermontane terranes in Yukon. Plutons of this age are associated with significant porphyry Cu ± Au ± Mo deposits in British Columbia. In Yukon, Cu–Au–Ag deposits at Minto and Carmacks Copper are hosted by an Early Jurassic batholith but are not typical porphyry deposits. This project aims to provide the regional framework for Late Triassic to Jurassic plutons and associated mineral occurrences in southern Yukon. We systematically describe over 34 plutons and present new data for most of these, including 47 U-Pb zircon dates, 44 Ar-Ar dates, 187 whole-rock geochemical analyses, 30 whole-rock Sr and Nd isotope analyses, 27 feldspar Pb isotope analyses, zircon Hf isotope analyses for 22 samples and 26 pressure estimates from aluminum-in-hornblende analyses. Complementing these analytical data are extensive photomicrographs, photographs and maps.

Late Triassic to Early Jurassic age (~220-185 Ma) intrusions comprise one of the most widespread and volumetrically significant plutonic suites in central and western Yukon, and eastern Alaska, but have received very limited study thus far. A new research project has been initiated that will examine the temporal, geochemical and petrotectonic evolution of this magmatic event, and the nature and origin of associated Cu, Au and PGE mineralization. The intrusions are mainly hornblende- and biotite-bearing granodiorites and quartz monzonites, although granitic phases and rare ultramafic phases (as at Pyroxene Mountain) are also present. Several bodies of coarse-grained muscovite granite that are included within the suite have been recognized in southwestern Dawson, and central and western Stewart River map areas. Most intrusions give preliminary U-Pb zircon and titanite ages of ~195 Ma to ~185 Ma, although scattered bodies give ages up to 218 Ma. Geochemical studies completed thus far indicate that most intrusions are metaluminous and formed in a volcanic arc environment, although some of the muscovite-granite phases in western Yukon are peraluminous and trend into the anorogenic (within-plate) granite field on various tectonic discriminant plots. Dating studies at Minto and Williams Creek indicate that copper-gold mineralization in both areas is hosted in part by deformed intrusions dated at ~194 Ma and is crosscut by massive, post-mineralization Granite Mountain batholith dated at ~190 Ma. The mineralization is therefore intimately associated with the Triassic-Jurassic magmatism, and we tentatively interpret the deposits as deformed copper-gold porphyries.

Paleomagnetic and geobarometric results are reported here for the Early Jurassic Granite Mountain batholith, an ~600 km² granodiorite intrusion in the Yukon-Tanana Terrane. Paleomagnetic analyses of 331 specimens from 24 sites yield magnetite-borne characteristic remanent magnetization (ChRM) directions. Eight northerly and westerly sites display a mean direction A (D=337, I=69°; a 95 =7.6°, k=54). Another 11 sites appear to be lightning-struck, or exhibit unstable remanence, and 3 sites in a fault-bounded block to the southeast carry an anomalous northeast upwards ChRM direction B. Two sites in a Cretaceous andesite porphyry dyke and its contact zone give a westerly, steep-down ChRM direction, C. Aluminum-in-hornblende geobarometry at 10 sites defines emplacement depths of ~16 km in the north and west, and ~19 km in the southeast. The batholith has probably not been significantly tilted since its emplacement, but it may be subdivided into two separate intrusive phases or structural blocks that have experienced differential uplift. Ambient temperatures at 16-19 km were too high for magnetite to record an enduring remanence, so the observed ChRMs likely record uplift of the batholith through ~15 km depth, at 180-170 Ma. Both A and C ChRM directions are similar to those expected for Early Jurassic and Late Cretaceous reference poles, respectively, suggesting that the Yukon-Tanana Terrane is not far-traveled with respect to cratonic North America since Early Jurassic time.

The layered rocks in this area originated as continental shelf sediments overlain by volcanic arc successions. Now called Yukon-Tanana terrane, they tectonically over-rode the western edge of ancient North America beginning in Middle Jurassic time. Three elements are present in the map area. The west half comprises the Big Salmon Complex; the east half is a separate, in part contemporaneous succession composed of the Dorsey Complex and Swift River Group. Unconformably overlying both these elements are less metamorphosed Klinkit Group and Triassic sediments that are here interpreted as overlap assemblages. The unexposed contact between Big Salmon Complex and Swift River Group is inferred to be an east-side-down normal fault.

Granodiorite of the Coast Plutonic Complex intruded metasedimentary rocks in the Dezadeash Range of the northern Coast Belt in the late Mesozoic. Graphitic staurolite-biotite schist, associated with the Kluane Metamorphic Assemblage, underlies the western Dezadeash Range, whereas cordierite-biotite gneiss, previously correlated with the Late Proterozoic - Paleozoic Nisling Assemblage, is exposed in the eastern and southern regions. A terrane boundary was placed in the central Dezadeash Range. Recent petrographic studies reveal a southeastward increase in metamorphic grade. Prograde appearance of cordierite partly obliterated an older schistosity and caused a fabric change near the postulated terrane boundary. Furthermore, typical continental margin rocks, such as marble and quartzite, are not observed. This suggests that all metamorphic rocks in the Dezadeash Range can be correlated with the Kluane Metamorphic Assemblage, whereas Nisling Assemblage rocks occur in the Coast Mountains to the east. Therefore, the terrane boundary is located in the Dezadeash River valley, further southeast than previously thought.

Two distinct but undeformed suites of granitic plutons intrude deformed siliciclastic rocks in western Dorsey Terrane. A calc-alkaline hornblende-bearing gabbro to granodiorite stock has been dated at 181.5 Ma (by the Rb/Sr method). The second suite consists of highly evolved late-orogenic granites of the Thirtymile stock and Hake Batholith, which are approximately 100 Ma. The penetrative fabric of the metasedimentary rocks indicates generally eastward-vergent layer-parallel shear. The deformation of the siliciclastic rocks is thus constrained at older than 181 Ma. The absence of resetting of the Rb-Sr isotopic ratios of the Jurassic pluton indicates that the mid-Cretaceous magmatism was emplaced at a shallow crustal depth. Since the Jurassic pluton has both a 'juvenile' Sr isotopic ratio of 0.7045 and chemistry indicative of a largely mantle-derived source, a subduction-related setting for magma generation is likely. The spatial relationship of craton-derived clastic rocks and these plutons requires that subduction had an eastward polarity.

The upper Sixtymile River area is located approximately 128 km west of Dawson City, Yukon. Lithology in this area consists of Precambrian to Paleozoic metamorphic rocks, Paleozoic ultramafic rocks, Middle Jurassic pegmatitic and aplitic dikes, Upper Cretaceous porphyritic dikes and volcanic rocks with intercalated sedimentary rocks, Quaternary alkaline basaltic dikes and Quaternary alluvial sediments. Precious metal occurrences in these volcanic rocks are divided into two types, based on differences in local distribution, petrology and wall rock alteration: a gold-bearing pyrite-arsenopyrite type and a silver-bearing galena-sphalerite type. Both types are characterized by four stages of mineralization.

The central Pelly Mountains in southeastern Yukon Territory consist of imbricate thrust sheets, which have undergone syn and post-thrusting deformation and metamorphism. The local geology is further complicated by intrusion of Upper Cretaceous batholiths, and by strike-slip faulting related to the Tintina Fault, a major northwest-trending transcurrent fault of uppermost Cretaceous or early Tertiary age. This faulting disrupts the northeast edge of the study area. Upper Devonian and Mississippian strata are present in at least two of the thrust sheets, but the Mississippian volcanic rocks occur in only one of them. The volcanic rocks consist of volcaniclastic material with minor interbedded flows, and were deposited in a submarine environment. Several coeval and cogenetic syenite and trachyte domes and small stocks are the remains of vent areas. Although the volcanic rocks are all highly altered and show evidence of widespread chemical mobility, trace element data indicate that the rocks are meta-luminous trachytes, most closely resembling peralkaline volcanics generated in extensional environments. This suggestion of a predominantly extensional tectonic setting in mid-Mississippian time in the Pelly Mountains is consistent with recent tectonic syntheses for the area. Stratabound and stratiform massive base metal sulphide deposits that occur within the Mississippian volcanic sequence are similar in many respects to the Kuroko-type volcanogenic massive sulphide deposits of Japan. The Pelly Mountains deposits, however, are among the first known occurrences in the world of Kuroko-type mineralization in a rift environment. A copy of this thesis is available at the EMR library – QE195 M67 1979. This thesis is available online at http://hdl.handle.net/2429/22257.

This field trip crosses southwestern Yukon and examines the geology of the Intermontane and Insular terranes, and synorogenic sedimentary basins that developed during Mesozoic terrane accretion.

Between the Swift and Nisutlin rivers, unmetamorphosed granite to ultramafic intrusions of four ages (from Permian through Cretaceous) span the amalgamation of Cassiar Platform with Yukon-Tanana and Cache Creek terranes. The mid-Permian granitic Ram Stock and two plutons cutting the Sylvester Allochthon lie at the edge of the Dorsey Complex, a remnant of an ancient passive margin succession that underlies the volcanic arcs of Yukon-Tanana Terrane. Middle Jurassic, locally foliated granodiorite to gabbro intrusions are metaluminous, and high in Sr and low in Ti compared to the Cretaceous suite. These `I-type volcanic arc plutons may be the remnants of an overlapping arc correlative with the Quesnel Terrane. The Cretaceous (113 to 98 Ma) meta- to peraluminous granites are late orogenic incipient `A-type plutons from highly fractionated F- and Cl-rich magmas. These generated extensive hydrothermal systems that produced tin, tungsten, molybdenum and beryl occurrences.