Tectonic setting of synorogenic gold deposits of the Pacific Rim

Listwanite, a carbonate-altered serpentinite commonly associated with gold and mercury mineralization, also represents a natural analogue to CO2 sequestration via in situ carbonation of minerals. The reaction pathways and permeability structure controlling listwanite formation are preserved and exposed at Atlin, British Columbia, where listwanite extends tens of meters into surrounding wallrock. The overall mineralogical transformation is the same as that being considered for industrial sequestration of CO2. In nature, this reaction proceeds via subreactions that are fossilized as spatially distinct zones. Serpentine + olivine + brucite reacted with CO2 to form serpentine + magnesite, then magnesite + talc, and finally magnesite + quartz. These mineralogical transformations are achieved isochemically, except for the volatile species H2O and CO2. Although the first stage of the reaction only accounts for about 5‐15% of the carbonation potential of serpentinite, it is very widespread, and therefore may have sequestered a significant portion of the total bound CO2. Moreover, within intact bedrock, the progress of the magnesite + talc reaction generates fracture permeability, which appears to have locally enhanced reaction. The first two reactions combined account for about half of the carbonation potential for serpentinite and have a small associated increase in the volume of solids, which limits porosity loss. They thus hold the greatest promise for in situ mineral carbonation. The carbonation reactions are controlled by the activity of CO2 in the fluid phase. Proponents of industrial mineral-carbonation processes therefore may seek to control the composition of the input gas to preferentially drive carbonation reactions that minimize porosity loss and maximize permeability generation. Magnetite was progressively destroyed during carbonation, which allowed magnetic susceptibility to be used as a proxy for carbonation-reaction progress. It facilitated the mapping of the permeability structure of these systems and delineated subtle variations in reaction progress that might otherwise have gone unnoticed in the field.

https://people.ucalgary.ca/~gordon/InverseMassTransferProblems/Hansen_etal2005.pdf

Carbonated serpentinite (listwanite) at atlin, british columbia: a geological analogue to carbon dioxide sequestration

Exotic and far-traveled oceanic crustal rocks of the Cache Creek terrane (CC) are bordered by less exotic Quesnel (QN) and Stikine (ST) arc terranes to the east, north, and west. All of these terranes are enveloped by an arcuate belt of displaced continental margin rocks; the Kootenay (KO), Nisling (NS), and parts of the Yukon-Tanana (YTT) terranes, that have indirect ties to ancestral North America (NA). Initial 87Sr/86Sr isopleths conform to this arcuate pattern. Such a pattern of concentric belts presents a geological conundrum: How did the QN, ST, and CC come to be virtually enveloped by terranes with ties to NA? Past and current models that explain assembly of the Canadian Cordillera are deficient in their treatment of this problem. We propose that Early Mesozoic QN and ST were joined through their northern ends as two adjacent arc festoons that faced south toward the Cache Creek ocean (Panthalassa?). Oceanic plateau remnants within the CC today were transported from the Tethyan realm and collided with these arcs during subduction of the Cache Creek ocean. Counterclockwise oroclinal rotation of ST and NS terranes in the Late Triassic to Early Jurassic caused enclosure of the CC. Rotation continued until these terranes collided with QN in the Middle Jurassic. Paleomagnetic declination data provide support for this model in the form of large average anticlockwise rotations for Permian to Early Jurassic sites in ST but moderate clockwise rotations for sites in QN. Specific modern analogues for the Cordilleran orocline include the Yap trench, where the Caroline rise is colliding end-on with the Mariana Arc and the Banda Arc, located on the southeastern “tail” of the Asian plate, which is being deformed into a tight loop by interactions with the Australian and Pacific plates.

Cache Creek terrane entrapment: Oroclinal paradox within the Canadian Cordillera

The Cordillera of British Columbia, Yukon, and Alaska: Tectonics and Metallogeny

The Late Cretaceous Carmacks Group, a thick subaerial volcanic succession that once covered much of southwest Yukon, was deposited on an uplifted terrane and is divisible into a lower fragmental unit and an upper flood basalt unit. Coeval hydrothermal activity resulted in widespread alteration and gold mineralization. The lavas are shoshonites, enriched in large ion lithophile and light rare earth elements, but depleted in high field strength elements. Ankaramitic absarokite flows in the upper Carmacks Group range up to 15 wt% MgO, requiring a high liquidus temperature (1400 °C at 1 bar, dry). High K 2 O contents (>3%) of these magnesian lavas indicate that the potassic character of the volcanic suite was established in the mantle. Although previously interpreted as subduction related, the Carmacks Group was erupted during a Cordilleran-wide magmatic lull and lacks coeval calc-alkalic batholiths. The lavas are petrologically similar to plume-related Eocene to Pliocene potassic lavas of the western United States. New paleomagnetic collections, combined with previous work, place the Carmacks Group 17.2° ± 6.5° (1900 ± 700 km) south of its present position relative to the craton during deposition, near the paleolocation of the Yellowstone hotspot. The spatial coincidence, similarity of tectonic setting, and lithologic similarity of the Carmacks Group and Yellowstone volcanic successions suggest that the Carmacks Group is the 70 Ma effusion of the Yellowstone hotspot. Subsequent northward displacement of the Carmacks Group is attributed to coupling with the Kula plate. Correlation of the Carmacks Group and the Yellowstone hotspot fixes the paleolatitude and the paleolongitude of the terranes of the northern Intermontane belt at 70 Ma.

Yellowstone in Yukon: The Late Cretaceous Carmacks Group

New U–Pb and Rb–Sr isotopic and major and trace element geochemical data are reported for Late Triassic to Eocene granite bodies that intrude the Cache Creek and Stikine terranes in the Atlin–Benne...

Age of emplacement and basement character of the Cache Creek terrane as constrained by new isotopic and geochemical data

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Age of emplacement and basement character of the Cache Creek terrane as constrained by new isotopic and geochemical data