Moose River Basin: geology and mineral potential - Geology Ontario

Petrography of Mesozoic and Pleistocene Sands
15 to 30 percent hornblende and 25 to 45 percent pyrite, while in 75-02 the va
lues are O to 18 percent hornblende and about 21 to 75 percent pyrite, though
with much variation. Despite the variability in values, the reversal is striking.
Much of the hornblende, especially in the lower parts of the holes, is altered
and etched, and all pyrite appears to be authigenic.
Walker (1967) in considering Pliocene and Pleistocene red beds suggested
that red colouration was due to post-burial, diagenetic transition from "nor
mal" grey to hematite-stained red elastics resulting from groundwater process
es. He reasoned that, in an oxidizing environment, groundwater circulating
through porous sediments is capable of breaking down fairly unstable iron-rich
silicates (especially hornblende and biotite), releasing iron atoms, and leaving
montmorillonite clay as an alteration product. He proposed that iron could link
with oxygen after very local transport (to prevent saturation at the point of al
teration) in the solvent fluids and precipitate as fine hematite coatings on sand
grains. Intrastratal alteration can occur wherever grains are in contact with
suitable interstitial water and Walker's study indicates that normal ground
water chemistry straddles the Eh-pH fields of both iron solution and precipita
tion. Slight changes in either Eh or pH could change the overall effect on iron
within the sediments.
Rate of alteration depends on the amount of water moving through the sed
iments, though Potter (1968) noted that high clay matrix content inhibits in
trastratal alteration by reducing porosity, and Walker (1967) found that hema
tite pigment forms first along sand laminae in mud beds. In addition to
montmorillonite and iron oxides, a third product of iron silicate breakdown is
calcite, precipitated as a cement. Compared to non-red lateral equivalents and
surrounding beds, Walker found significant decreases in heavy mineral weight
percentage (30 percent) and hornblende percentage (60 percent) within redstained
beds. However iron oxide, montmorillonite, and calcite increase and
the remaining hornblende usually is badly etched and altered to clay.
Walker (1967) concluded that red beds could form in situ whenever the fol
lowing six conditions are met, regardless of climate: 1) occurrence of iron-bear
ing detrital grains, 2) post-depositional conditions favouring intrastratal alter
ation of those grains, 3) interstitial Eh-pH environment favouring iron oxide
formation; 4) absence of subsequent reduction of ferrie iron, 5) enough time for
the reactions to occur; and possibly 6) relatively warm temperature (+ 18 0C).
He regarded the authigenic hematite, the intrastratal alteration, and red beds
as useful indicators of interstitial Eh and pH rather than climatic indicators.
The presence of lignite in the Mesozoic sediments of the James Bay Low
lands, likely required reducing conditions to be preserved, and the occurrence
of authigenic pyrite, often in abundance, substantiate the idea of a reducing en
vironment. Siderite also normally is stable only under low Eh conditions par
ticularly when organic matter is present. These factors imply that at least
parts of the deposit may have originated under low Eh conditions and that this
character was maintained by the sediment after burial. Could a variation of
Walker's post-depositional process have occurred in a reducing environment?
Silica sands in the section are generally overlain by fine grained till, underlain
by clay or shale, and likely would be good groundwater aquifers. If sufficient
groundwater circulating through the sands was capable of diagenetically alter
ing the abundant hornblende under the proper conditions, then secondary
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