ALBERT

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.

Description: The first-order importance of tectonic and environmental controls for terrigenous sediment supply has rarely been questioned, but the role of vegetation in the modification of ancient alluvial signatures has been observed since the mid-20th century (Vogt 1941). Studies of sparsely vegetated rivers (Schumm 1968) and alluvial stratigraphic variation (Cotter 1978; Davies & Gibling 2010) led to observations of (1) plant modulation of alluvial signatures and (2) Palaeozoic facies shifts (PFS): unidirectional changes to facies diversity and frequency, in stratigraphic alliance with the plant fossil record. One PFS is the Siluro-Devonian appearance of mud-rich, architecturally complex alluvium, traditionally ascribed to meandering rivers, and sedimentologically distinct from pre-vegetation strata (Davies & Gibling 2010; Long 2011). Using selected secondary data, Santos et al. (2017) dispute the correlation of these observations using three key points, as follows. (1) The mid-Palaeozoic was typified by orogenic assembly of low-gradient equatorial continents and elevated sea-level, which led to tropical weathering (abundant fine sediment) and extensive alluvial plains. This drove the PFS by promoting river meandering independently of vegetation. (2) Meandering does not require vegetation; this is shown by examples in Precambrian deposits, on other planets, and in ‘non-vegetated’ deserts. Meandering rivers were more abundant than the pre-vegetation rock record suggests, owing to selective bypass and deflation of fine material. (3) Early Siluro-Devonian (meaning Ludlow–Early Devonian) land plants were too small, their biomass and cover too limited, and their wetland habitat too narrow to have stabilized meandering channels, influencing landscape little more than earlier microbial communities. We contest the conclusions and method of the paper, and deal with each point in turn.