Moose River Basin: geology and mineral potential - Geology Ontario

Moose River Basin
clays, carbonate, and free iron could be produced, exactly as Walker found.
However, under reducing conditions with abundant organic matter present, ex
cess of carbon and sulphur ions, due to decaying organics and sulphate-reduc
ing bacteria, is expected. Bacteria withstand high metal contents of water by
rapid generation of sulphur ions from seawater sulphate which combines to
render metals non-toxic by precipitation of sulphide minerals in nearshore eux
inic environments (Stanton 1972). Sulphate-reducing bacteria are known to
rapidly produce much sulphide if connate marine or brackish water is avail
able. Most chemical sedimentologists now agree that these early diagenetic re
actions involving alteration of detrital iron silicates and the action of sulp
hate-reducing bacteria are the chief source of pyrite in sediments (Berner
1971). Therefore, instead of forming hematite as in Walker's oxidizing environ
ment, linkage of iron with abundant sulphur ions to produce pyrite is a possible
explanation for the hornblende-pyrite reversals so prominent in the studied
samples.
Unstable iron-bearing detritals occur as blue-green hornblende grains
which tend to have a higher than normal iron content. Fine size is noted by Ber
ner (1971) as increasing a grain's reactivity to alteration. These grains repre
sent an adequate source of iron and contact with intrastratal solutions after
deposition would allow alteration, as long as the hydraulic gradient allowed lo
cal removal of the soluble ions to prevent saturation at the point of alteration.
An interstitial Eh-pH environment favouring the formation of iron sulphides is
established by the presence of organics (low Eh), siderite (low Eh, likely low
pH) and the authigenic nature of the pyrite itself which grows in somewhat low
Eh, low pH environments (Guber 1972; Grim 1953). Preservation of organic
matter and fresh euhedral pyrite with striated faces indicates little subsequent
oxidation has occurred. Walker's (1967) study revealed that a few million years
may be sufficient for alteration and authigenesis to take place. Mesozoic sedi
ments therefore have had adequate time for reactions to occur. If warm temper
ature does enhance the reaction, the James Bay Lowlands lay considerably
closer to the proposed Jurassic and Cretaceous equators than it does to the pres
ent equator (Couillard and Irving 1975). It appears that conditions for intras
tratal alteration of hornblende and subsequent formation of pyrite can be met
by these deposits.
Evidence that solution alteration has taken place is provided by the min
eral grains. Hornblende and garnet display the surface features previously
mentioned, indicative of alteration etching (Rahmani 1973). These features do
not appear to be mechanically worn, hence are presumed to be post deposition
al. Garnet, though mechanically resistant, is chemically rather unstable and
etching by acidic solutions may be quite common (Bramlette 1929). Etched and
unetched grains often occur in the same sample, attributed by Rahmani (1973)
to the fact that only grain surfaces facing a pore can be attacked by fluids. Al
teration of pyroxene could be the result mostly of transport, though some
grains are quite fresh. Alteration coatings appear on some magnetite and il
menite grains, which normally are quite stable mechanically. The abundance
of kaolin matrix in sorted, fine to coarse Cretaceous silica sand may be a pro
duct of in situ breakdown of feldspars rather than detrital clay, though kaolin
is notably deficient in the lower part of drillhole 75-06. "Sawtooth" zircon over
growths were noted in drillhole 75-06, sample 115. Alteration features gener-
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