ALBERT

Description: The Mesozoic magmatic history of the North American margin records the evolution from a more segmented assemblage of parautochthonous and allochthonous terranes to the more cohesive northern Cordilleran orogenic belt. We characterize the setting of magmatism, tectonism, and epigenetic mineralization in the western Fortymile mining district, east-central Alaska, where parautochthonous and allochthonous Paleozoic tectonic assemblages are juxtaposed, using sensitive high-resolution ion microprobe (SHRIMP) U-Pb zircon geochronology, whole-rock geochemistry, and feldspar Pb isotopes of Mesozoic intrusions and spatially associated mineral prospects. New SHRIMP U-Pb zircon ages and published U-Pb and 40 Ar/ 39 Ar ages indicate four episodes of plutonism in the western Fortymile district: Late Triassic (216–208 Ma), Early Jurassic (199–181 Ma), mid-Cretaceous (112–94 Ma), and Late Cretaceous (70–66 Ma). All age groups have calc-alkalic arc compositions that became more evolved through time. Pb isotope compositions of feldspars from Late Triassic, Early Jurassic, and Late Cretaceous igneous rocks similarly became more radiogenic with time and are consistent with the magmas being mantle derived but extensively contaminated by upper crustal components with evolving Pb isotopic compositions. Feldspar Pb isotopes from mid-Cretaceous rocks have isotopic ratios that indicate magma derivation from upper crustal sources, probably thickened mid-Paleozoic basement. The origin of the mantle component in Late Cretaceous granitoids suggested by Pb isotopic ratios is uncertain, but we propose that it reflects asthenospheric upwelling following slab breakoff and sinking of an inactive inner subduction zone that delivered the previously accreted Wrangellia composite terrane to the North American continental margin, after the outer Farallon subduction zone was established. Epigenetic Pb-Zn-Ag ± Cu prospects in the western Fortymile district are spatially associated with splays of the northeast-trending Kechumstuk sinistral-normal fault zone and with ca. 68–66 Ma felsic intrusions and dikes. The similarity between Pb isotope compositions of feldspars from the Late Cretaceous igneous bodies and sulfides from the epithermal prospects suggests a Late Cretaceous age for most of the mineralization. Fluid flow along the faults undoubtedly played a major role in mineralization. We interpret displacement on the northeast-trending faults to be a far-field effect of dextral translation along Late Cretaceous plate-scale boundaries and faults that were roughly parallel to the subsequently developed Denali and Tintina fault systems, which currently bound the region.

Description: There is a strong association between regions containing orogenic gold mineralization and exploitation of placer gold, although, in many cases, the nature of the source mineralization for these placer deposits remains unclear. This study describes a novel approach to evaluating the economic potential of in situ orogenic gold mineralization through characterization of both lode and placer gold mineralogy, followed by synthesis of this information with records of both mineral occurrences and historical placer mining. The northern Cariboo Gold District in east-central British Columbia, Canada, was chosen as a location for the study because of the gold endowment of the area (1.2 million ounces [Moz] of lode gold and between 0.5 and 3 Moz. of placer gold) and the information available from both placer and lode gold mining. Compositional analysis of 533 gold grains from 21 lode localities and 1,914 gold grains from 30 placer localities from throughout the Cariboo Gold District has identified four main compositional types in terms of their alloy compositions and associated suite of mineral inclusions revealed in polished section. A distinctive low (4–7%)-Ag gold that exhibits a strong Bi association in the mineral inclusion suite is geographically limited to the Wells area, where it is recorded in both lode mineralization and its placer expression. Regionally pervasive mineralization yields gold of binary Au-Ag alloy and a simple inclusion suite of sulfides and sulfarsenides. Gold in most large placers in trunk drainages was derived from multiple (mostly small) occurrences of this type. The nature of compositional variation between gold grains liberated from hypogene ore has informed the history of episodic mineralization and suggests multiple stages of gold emplacement at some localities, while others are dominated by gold deposited in a single stage. The new information from gold grain analysis has been considered in the context of other information. Classification of a placer as either allocthonous or autocthonous both informs interpretation of compositional characteristics of the detrital gold grains and provides information on distance to the hypogene source. Mineral inclusion assemblages observed in sample populations of placer gold grains have been correlated with reports of hypogene vein mineralogy described in mineral occurrence records to clarify the geographical extent of specific mineralization types. The compositional range of alloys of different gold types has been compared to historical records of gold production and gold fineness (Au-Ag ratio) to reconstruct the size and distribution of hypogene sources prior to erosion. Synthesis of gold compositional studies with other publicly accessible data sets provides a new generic approach capable of evaluating the most attractive targets for future exploration and highlighting compositional signatures of placer gold which relate to undiscovered in situ sources.

Description: The Middle Cambrian Mount Read Volcanics of western Tasmania, Australia, host several world-class volcanic-hosted massive sulfide (VHMS) deposits, representing a wide range of deposit styles. Although the deposits and their host sequences are variably deformed and locally preserve spectacular examples of primary textures and structures, rapid lateral and vertical facies changes, and faults with uncertain sense and magnitude of displacement, have made it impossible to correlate stratigraphy across the belt. Previous dating studies in the area have yielded relatively imprecise crystallization ages. We have employed the chemical abrasion isotope dilution thermal ionization mass spectroscopy (ID-TIMS) U-Pb zircon method to obtain highly precise crystallization ages for a total of 18 samples of volcanic and intrusive rock units from throughout the Mount Read Volcanics and underlying mafic-ultramafic complexes. The new data permit detailed resolution of age relationships within the belt. The study establishes an age of 516.0 ± 0.9 Ma for the McIvor Hill gabbro, which is part of a mafic-ultramafic complex interpreted to underlie the Mount Read Volcanics. Magmatism in the central Mount Read Volcanics lasted at least 12.7 m.y., from 506.8 ± 1.0 Ma for a massive dacite unit in the lower part of the Central Volcanic Complex to 496.0 ± 0.9 Ma for a welded ignimbrite in the lower Tyndall Group. Together with previous age constraints, results of the study provide a precise chronostratigraphic framework for magmatism and VHMS deposit formation within the Mount Read Volcanics. The precise age data indicate that, north of the Henty fault, magmatism occurred in three discrete pulses, at least two of which were separated by periods of sedimentation. We demonstrate that VHMS deposits in the Mount Lyell, Roseberry-Hercules, and Que-Hellyer districts, comprising the majority of the known, significant VHMS deposits in the belt, formed within a narrow time interval at ~500 ± 1 Ma, at a relatively late stage in the evolution of the belt. Some of the larger intrusions in the belt (e.g., the Bonds Range porphyry; 500.4 ± 0.8 Ma) were emplaced contemporaneously with VHMS deposit formation; however, other bodies such as the Murchison granite (497.3 ± 0.9 Ma) are younger than the deposits and are unlikely to have been involved in their genesis as has previously been suggested. These conclusions will aid explorers in targeting VHMS mineralization in the Mount Read Volcanics.

Description: The Xietongmen district is located 260 km west-southwest of Lhasa in the Tibet Autonomous Region, China. The district occurs within the Gangdese belt, which forms the eastern part of the Trans-Himalayan magmatic belt and is the product of complex magmatic activity that began during the Late Triassic or Early Jurassic and ended in the Eocene. The Xietongmen Cu-Au and Newtongmen Cu-Au-Mo deposits contain a total measured and indicated resource of approximately 610 million metric tons, with additional mineralization in the Langtongmen and Olitongmen Cu-Au prospects. Porphyry mineralization in the Xietongmen district formed during Middle Jurassic volcanic arc activity in the Lhasa terrane, prior to its accretion to the southern margin of Eurasia, and establishes that an economically important, but only recently recognized, metallogenic event is present in the region. Rock types in the Xietongmen district range from Early Jurassic to Eocene in age. Early Jurassic (~188-177 Ma) volcanic, volcaniclastic, and coeval intrusive rock types are crosscut by Middle Jurassic (176-171 Ma) hornblende diorite and quartz diorite porphyry dikes and stocks, including intrusions related to porphyry Cu-Au ± Mo mineralization. The Jurassic igneous assemblage was intruded by mafic dikes between the Late Jurassic and the Cretaceous, then by an Eocene (50-47 Ma) biotite granodiorite batholith and related dikes, and finally by, volumetrically minor lamprophyre dikes. The most important structures in the Xietongmen district are four E-striking, moderately N-dipping, sinistral-oblique thrust faults. Crosscutting and suturing relationships between the TSF-2 thrust fault, located in the south part of the district, and intrusions dated to between 174 and 180 Ma constrain the main stage of thrust fault activity to the Middle Jurassic. The Contact and Adit-1 thrust faults truncate the Xietongmen deposit and form the footwall and hanging wall to mineralization, respectively. Numerous zones of cataclasis deform the Xietongmen deposit between these bounding thrust surfaces. The strongly deformed Langtongmen Cu-Au prospect is located ~1.3 km west of the Xietongmen deposit and occurs in the immediate hanging wall of the Adit-1 thrust fault. The Newtongmen deposit and the Olitongmen Cu-Au prospect occur to the north in the hanging wall to the SBF thrust fault and are not strongly deformed. Mineralization and hydrothermal features in the Xietongmen district are fully compatible with porphyry Cu-Au ± Mo deposits. Alteration, vein types, and mineralization are zoned around quartz diorite porphyry intrusions. Early K silicate alteration and related veins occur within and proximal to the intrusions and contain the highest grade mineralization. In the Xietongmen deposit, the grade of mineralization decreases outward from a core of early biotite-rich K silicate alteration, through a transitional zone in which early K silicate alteration is partially overprinted by quartz-sericite-pyrite alteration, to a peripheral zone of poorly mineralized quartz-sericite-pyrite ± pyrrhotite alteration. Incipient sodic alteration occurs as albite alteration envelopes to quartz-sulfide veinlets in the deepest part of the deposit. Late polymetallic veins and veinlets contain sphalerite, galena, and other base metal sulfides and sulfosalts, occur throughout the Xietongmen deposit, and reflect telescoping during late-stage collapse of the hydrothermal system. Partially developed supergene mineralization forms less than 10% of the Xietongmen deposit. Underlying hypogene mineralization comprises ubiquitous pyrite, chalcopyrite, lesser and more erratically distributed pyrrhotite, and rare molybdenite. The characteristics of the Langtongmen prospect are identical to those found in the deeper parts of the Xietongmen deposit. Characteristics of the Newtongmen deposit are generally similar to those in the Xietongmen deposit but Newtongmen contains only minor supergene mineralization, is cut by very few late polymetallic veinlets, and contains zones of strong, weakly mineralized sodic alteration related to a relatively later stage of quartz diorite porphyry intrusion. The Olitongmen prospect has characteristics similar to the Newtongmen deposit. The Xietongmen deposit and the Olitongmen prospect were thermally recrystallized in a hornfels aureole to the Eocene biotite granodiorite batholith, whereas thermal effects are minor to absent at Langtongmen and Newtongmen. Copper and gold (Au/Cu; ppm / % ) are closely correlated within each of the two main deposits and ratios range from 1.5 to 1.2 in the Xietongmen deposit to between 0.8 and 0.6 in the Newtongmen deposit. Mineralization in the Xietongmen district formed in several coeval mineralized centers and the vein types, alteration and metal assemblages among these centers span a continuum in hydrothermal characteristics. The differences between the mineralized zones are interpreted to reflect exposure at different relative paleodepths as a result of displacement and deformation by posthydrothermal, sinistral-oblique movement on thrust faults. The Xietongmen deposit was transposed to the south by deformation on and between the bounding Adit-1 and Contact thrust faults. The Langtongmen deposit was separated from the deep hanging wall of the Xietongmen deposit and displaced approximately 600 m vertically and 1,300 m to the west by the Adit-1 thrust fault. The Newtongmen deposit and the Olitongmen prospect were uplifted relative to Xietongmen and Langtongmen in the hanging wall to the SBF thrust fault and were not significantly deformed. The genesis and relationship between porphyry deposits in the Xietongmen district can be reconciled by the combined effects of vertical and lateral displacement by thrust faults, preservation of the deposits at different relative paleodepths, and varying degrees of posthydrothermal mechanical and thermal recrystallization.

Description: Triassic rocks of the Western Canada Sedimentary Basin (WCSB) have previously been interpreted as being deposited on the passive margin of North America. Recent detrital zircon provenance studies on equivalent Triassic rocks in the Yukon have suggested that these rocks were in part derived from the pericratonic Yukon–Tanana terrane and were deposited in a foreland basin related to the Late Permian Klondike orogeny. Detrital zircons within a number of samples collected from Triassic sediments of the WCSB throughout northeastern British Columbia and western Alberta suggest that the bulk of the sediment was derived from recycled sediments of the miogeocline along western North America, with a smaller but significant proportion coming from the Innuitian orogenic wedge in the Arctic and from local plutonic and volcanic rocks. There is also evidence of sediment being derived from the Yukon–Tanana terrane, supporting the model of terrane accretion occurring prior to the Triassic. The age distribution of detrital zircons from the WCSB in British Columbia is similar to those of the Selwyn and Earn sub-basins in the Yukon and is in agreement with previous observations that sediment deposited along the margin of North America during the Triassic was derived from similar source areas. Together these findings support the model of deposition within a foreland basin, similar to the one inferred in the Yukon. Only a small proportion of zircon derived from the Yukon–Tanana terrane is present within Triassic strata in northeastern British Columbia, which may be due to post-Triassic erosion of the rocks containing these zircons.

Description: Gem-quality corundum (sapphire) occurs in scapolite-rich calc-silicate rock hosted in marble of the Lake Harbour Group near Kimmirut, southern Baffin Island. A deposit of blue and colorless gem corundum (Beluga occurrence) is compared to a similar calc-silicate pod generally lacking corundum but containing nepheline (Bowhead occurrence) and located 170 m to the SSW. Corundum formation was made possible by three equally important sequential metamorphic reactions: (1) formation of nepheline, diopside, and K-feldspar (inferred) at granulite facies peak metamorphic conditions; (2) partial retrograde replacement of the peak assemblage by phlogopite, oligoclase, calcite, and scapolite (Me 50 –Me 67 ) as a result of CO 2 -, H 2 O-, Cl-, F-bearing fluid influx at 1782.5 ± 3.7 Ma ( P-T 〈 720 °C, 6.2 kbar); and (3) retrograde breakdown of scapolite + nepheline (with CO 2 - and H 2 O-bearing fluid) to form albite, muscovite, corundum, and calcite. Late, low-temperature zeolite mineralization is common in corundum-bearing zones. Based on thermodynamic models, the corundum-forming reaction only occurs in a 〈100 °C window with an upper limit determined by scapolite-nepheline stability, and a lower limit determined by the formation of Al-silicate rather than corundum. The protolith is inferred to be dolomitic argillaceous marl with no evidence to suggest the initial presence of evaporites. The enrichment of trace metals V and Cr, and the depletion of Co, Ni, and Mn, suggest reducing diagenetic conditions in the initial sediment. Beluga calc-silicate rock is strongly depleted in REE (Total REE ~ 18 ppm). Oxy-dravite 11 B (+3.9 ± 0.7) is consistent with a marine boron source. The oxygen isotope composition of corundum ( 18 O VSMOW = 16.4 ± 0.1) is comparable to that of corundum in marble or desilicated pegmatite associated with marble. Phlogopite and muscovite 40 Ar/ 39 Ar ages and calculated closure temperatures (considered estimates) are ca . 1640 Ma ( T c = 455 to 515 °C) and 1510 Ma ( T c = 410 to 425 °C), respectively. In the Lake Harbour Group, the most prospective areas for gem corundum exploration are expected to be contiguous to the thrust fault separating the Lake Harbour Group and Narsajuaq terranes, where the retrograde, amphibolite facies overprint of the granulite peak assemblages was most pervasive.

Description: The Archean Hope Bay greenstone belt is located in the Bathurst Block of the northeastern part of the Slave Structural Province, a predominantly Archean, granite-greenstone-metasedimentary terrane. The greenstone belt is dominated by mafic volcanic rocks with less common felsic volcanic and volcaniclastic products and subordinate ultramafic bodies and metasedimentary rocks. Three main Au deposits have been defined in the greenstone belt, with a cumulative resource of over 10 million ounces (Moz) of Au, as of April 2007. The Hope Bay greenstone belt can be divided into a series of coherent panels of strata that tend to be fault bounded and collectively are used to construct a composite stratigraphic column of the belt constrained by U-Pb geochronology on felsic suites. The oldest felsic suite, the Flake Lake suite (ca. 2700 Ma), is a succession of felsic volcanic rocks that has a tholeiitic geochemical affinity, interpreted as a product of rift-related volcanism. Overlying this suite is a series of well-constrained calc-alkaline, mainly felsic volcanic rocks, which have ages of ca. 2690 Ma (Square Lake suite), ca. 2686 Ma (Windy felsic suite), ca. 2677 Ma (Koignuk suite), and ca. 2662 Ma (Clover Lake suite), all of which are interpreted as products of arc volcanism. The transition from rift- to arc-related volcanism divides the greenstone belt into upper and lower volcanic cycles with the transition occurring at about ca. 2690 Ma. The older cycle of volcanic rocks has a distinctive group of mafic rocks consisting of mafic pillowed flows which have an Fe-rich tholeiitic geochemical affinity which is not recognized in the younger volcanic cycle. These Fe-rich tholeiitic mafic volcanic rocks commonly show a spatial relationship to Au mineralization acting as host strata to the main Au deposits in the greenstone belt. The younger cycle (〈ca. 2690 Ma) of mafic volcanic rocks tends to have a tholeiitic affinity with some minor andesitic volcanic rocks near the top of the composite stratigraphic column. Detrital zircon work was carried out for all the main sedimentary successions recognized in the Hope Bay greenstone belt. The detrital zircon results show that in each case the sediments were derived mainly from local sources, as very few anomalously old zircons were recognized. A conglomerate outcropping in the north part of the belt is dominated by locally derived material, with detrital zircon work supporting this interpretation. The three main Au deposits at Hope Bay are all associated with the older cycle of volcanic rocks, interpreted as products of rift-related volcanism. The structural setting of each deposit is different but each is related to D 2 strain in structural and/or stratigraphic settings which provided dilational environments to localize auriferous vein-related mineralization or localized alteration and mineralization of favorable host lithologies. The older cycle of tholeiitic felsic and Fe-rich mafic volcanic rocks are rift-related volcanic products, which are time-equivalent to the Kam Group, in the Yellowknife Supergroup. Overlying these rocks are calc-alkaline felsic volcanic rocks and tholeiitic mafic volcanic rocks, which represent the transition to calc-alkaline arc-related volcanism, which correlates to the Banting Group of the Yellowknife Supergroup. Overlying the younger (ca. 2677 Ma) felsic volcanic rocks are a succession of conglomerates consisting mainly of locally derived clasts, representing collapse of the arc into a fluvial-dominated environment. This sedimentary succession is possibly equivalent to the regionally extensive Burwash Basin of the Yellowknife Supergroup.

Description: The Cerro Bayo low sulfidation epithermal district is located in the Aysén region, Chilean Patagonia, at the western tip of the Deseado Massif epithermal silver-gold province. Mineralization is hosted in the mainly rhyolitic fragmental successions of the Jurassic Ibáñez Formation. New biotite 40 Ar/ 39 Ar and zircon U-Pb ages together with published ages constrain the eruption of this unit to between 158 and 144 Ma. Rhyolitic domes and dikes, including Cerro Bayo proper, aligned on the prominent N-S–striking Cerro Bayo fault and intruding rocks of the Ibáñez Formation were dated by U-Pb on zircon at 146.5 ± 0.2 and 146.3 ± 0.2 Ma. Dacitic domes of 83.0 ± 0.2 and 82.6 ± 0.2 Ma (U-Pb on zircon) are present in the western part of the district at Laguna Verde. Silver and gold mineralization is hosted by steeply dipping, dominantly N to NW and subordinate W-striking quartz veins and, to a lesser extent, in breccias, which, on the basis of new adularia 40 Ar/ 39 Ar ages, were emplaced in three main episodes: (1) Mallines: ca. 144 to 142 Ma, (2) Bahía Jara and Brillantes: ca. 137 to 124 Ma, and (3) Laguna Verde: 114 to 111 Ma, overall spanning 33 m.y. Most Ag-Au has been produced from veins emplaced in the latter two episodes. Veins at Mallines and Bahía Jara are spatially related to the Cerro Bayo fault. The oldest episode of mineralization in the district is similar in age to the youngest epithermal deposits in the Deseado Massif, whereas the economically most important veins are age equivalent to skarn and polymetallic vein mineralization in the Patagonian Andes of southern Chile. The protracted history of epithermal processes at Cerro Bayo records the evolution from continental-scale extension related to the Gondwana breakup to the establishment of Andean-type arc and back-arc environments. The largely extensional tectonics throughout this period resulted in similar epithermal mineralization styles emplaced episodically over a large time interval.

Description: New isotopic ages have been obtained from euhedral, first-cycle zircon grains recovered from Rhaetian strata preserved at the Black Bear Ridge section in northeastern British Columbia. Two statistically significant populations are present: an older population ca. 224 Ma, and a dominant younger population ca. 205 Ma. The younger population includes a group of grains with a weighted average 206 Pb/ 238 U age of 205.2 ± 0.9 Ma, which is interpreted to represent the maximum depositional age of the sediment. Potential sources for the two populations are found in the Quesnel terrane in central British Columbia, implying close proximity between this terrane and the autochthonous North American margin during the Late Triassic. This is supported by geochemical and structural evidence. The implication that the Quesnel terrane was close to its present-day latitude during the Late Triassic is in conflict with older estimates of paleolatitude based on paleomagnetic and paleontological evidence. The age of 205.2 ± 0.9 Ma obtained from the youngest population of zircons is also consistent with recently published estimates for the age of the Norian-Rhaetian boundary.