ALBERT

Description: The Whitehorse trough is an Early to Middle Jurassic marine sedimentary basin that overlaps the Intermontane terranes in the northern Cordillera. Detrital zircon dates from eight Laberge Group sandstones from various parts of the trough all display a major Late Triassic–Early Jurassic peak (220–180 Ma) and a minor peak in the mid-Paleozoic (340–330 Ma), corresponding exactly with known igneous ages from areas surrounding the trough. Source regions generally have Early Jurassic (ca. 200–180 Ma) mica cooling dates, and the petrology of metamorphic rocks and Early Jurassic granitoid plutons flanking the trough suggests rapid exhumation during emplacement. These data suggest that subsidence and coarse clastic sedimentation in the trough occurred concurrently with rapid exhumation of the shoulders. Isolated occurrences of sandstone and conglomerate units with similar detrital zircon signatures occur west and east of the trough, as well as overlapping the Cache Creek terrane, indicating that either the trough was once more extensive, or isolated basins tapped similar sources. Development of these sedimentary basins and accompanying rapid exhumation in the northern Cordillera were coeval with the onset of orogenic activity in the hinterland of the southern Canadian Cordillera, and subsidence in the western Canada foreland sedimentary basin. The Whitehorse trough is interpreted as a forearc basin that progressively evolved into a collisional, synorogenic piggyback basin developed atop the nascent Cordilleran orogen. Upper Jurassic–Lower Cretaceous fluvial deposits overlapping the Whitehorse trough have detrital zircons that were mainly derived from recycling of the Laberge Group, but they also contain zircons exotic to the northern Intermontane terranes that are interpreted to reflect windblown detritus from the Late Jurassic–Early Cretaceous magmatic arc that developed either atop the approaching Insular terranes to the west or southern Stikinia.

Description: Extensive pyrite-bearing, auriferous cobble and boulder conglomerates are present in the basal 30 m of the Mississagi Formation in Pardo and Clement Townships, Ontario, Canada. The sedimentology of the conglomerates, combined with regional geology, indicates limited fluvial transport in a gravel bed braided river with local hyperconcentrated flows, with material derived from a highly restricted catchment area. Postdepositional overprinting of the conglomerates is related to the Penokean orogeny at 1.85 to 1.5 Ga and alkali metasomatism at ~1.7 Ga. Several pyrite varieties, including detrital and postdepositional recrystallized and altered grains, are present in the conglomerates. Detailed in situ laser ablation inductively coupled plasma-mass spectrometry (LA ICPMS) analysis of pyrite revealed that the gold in the deposit is intimately associated to large detrital pyrite grains, as "invisible" Au, with lesser amounts of free gold. Postdepositional pyrite and pyrite overgrowths have very low Au content. Elements such as Pb, Bi, Sb, Te, Ag, and Hg, together with Au were mobilized during hydrothermal alteration and dissolution of detrital pyrite grains. Small-scale transport and reprecipitation formed part of the postdepositional pyrite and free gold in immediate contact with postdepositional grains. In contrast, Ni, Co, and As were not mobilized. External fluid infiltration is negligible and is indicated only by minor, compositionally distinct, late sulfide veinlets crosscutting the conglomerates. Sulfur isotope analysis of detrital pyrite yields generally positive{delta} 34S values (0.97-9.26{per thousand}). The{delta} 34S sulfur isotope composition of pyrite overgrowths and postdepositional grains overlaps the isotopic range of the detrital grains, suggesting a near-closed S system during postdepositional processes. However, the detrital pyrite tends to have slightly negative {Delta}33S while the postdepositional and overgrowth pyrite are either neutral or slightly positive. The S isotope composition of the detrital pyrite is compatible with an origin of S as dissolved sulfate in an ocean under a low oxygen atmosphere. The potential source of the Au-bearing detrital pyrite appears to have been an, as yet, undiscovered Archean deposit located within 1 to 8 km of the placer deposit.