| Summary: | <p>The Big Fish River - Rapid Creek phosphatic iron formation,
in the Richardson Mountains, Yukon, is a unique sedimentary
deposit of lowermost Albian age. It had an unusual postdepositional
history which led to the development of a
spectacular phosphate occurrence.</p>
<p>Strata were deposited in an environment without currents
but of relatively shallow depth, probably just below storm-wave
base. The deposit formed on the west side of the penecontemporaneous
Cache Creek High. Fluctuations in sea level,
tectonic instability, or a progradational sediment system caused
coarsening-upward sequences capped by conglomeratic slump
deposits in the lower part of the formation in the Rapid Creek
area. The rest of the section in this area represents
relatively stable conditions. In the Big Fish River and
Boundary Creek areas, differential deposition of autochthonous
minerals has resulted in gradational mudstone-shale couplets.</p>
<p>Most rocks of the formation are texturally similar to other
phosphate and iron deposits. They are broadly categorized as
shale, mudstone, siltstone and sandstone. They are composed of
four basic components: pellets and granules, detrital quartz
grains, skeletal fragments, and siderite matrix. Mixed
phosphate-siderite pellets (as well as granules) and matrix
constitute a spectrum from sandstone to mudstone and comprise
the major part of the formation.</p>
<p>The rocks were originally composed of detrital quartz and
clay minerals and autochthonous siderite, pyrite, and a mixed
Ca-Fe-Mg phosphate of uncertain identity. Metamorphism altered
the phosphate minerals and remobilized the siderite. In
non-pelletal phosphate mudstone, the primary Ca-Fe-Mg phosphate
is altered to carbonate-apatite, which occurs together with
siderite as pseudomorphs in star-shaped concretionary bodies.
In coarser-grained rocks, the primary Ca-Fe-Mg phosphate is
altered to satterlyite ((Fe,Mg)<sub>2</sub> P0<sub>4</sub> (OH)) which in turn is
altered to arrojadite (K(Na,Ca)<sub>5</sub> (Fe,Mn,Mg)<sub>14</sub> Al(OH,F)(PO<sub>4</sub>)<sub>12</sub>).
Gormanite-souzalite ((Fe,Mg)<sub>3</sub> (Al,Fe)<sub>4</sub> (P0<sub>4</sub>)<sub>4</sub> (OH)<sub>6</sub> •2H<sub>2</sub>O) is a
common alteration (or replacement) in both.</p>
<p>The four major epigenetic, fracture-filling mineral associations
are categorized by the persistent occurrence or dominance
of one or two characteristic elements in one or more minerals.
They are related to particular host rocks: Ca-rich association
with phosphate mudstone, Ba-rich with conglomeratic slump
deposits, Fe-Mg-rich with siderite sandstone, and Na-bearing
with phosphate sandstone. Simple mineral associations (three
minerals, or fewer) are related to particular host rocks or
represent local accumulations of a restricted number of
elements.</p>
<p>The mineralization in the Big Fish River and Boundary Creek
areas is largely confined to spherulitic recrystallized replacements
of ammonites and pelecypods. Moreover, concretionary
phosphate nodules are present. The minerals in both comprise
pyrite, wolfeite ((Fe,Mg)<sub>2</sub>PO<sub>4</sub>(0H)), satterlyite, maricite
(NaFeP0<sub>4</sub>), vivianite-baricite ((Fe,Mg) <sub>3</sub>(P0<sub>4</sub>)<sub>2</sub>•8H<sub>2</sub>0), varulite
((Na,Ca)(Mn,Fe)<sub>2</sub>(P0<sub>4</sub>)<sub2</sub>), and their alteration products.</p>
<p>Nahpoite (Na<sub>2</sub>HP0<sub>4</sub>), a new mineral which occurs as a
white powdery alteration product of maricite in some nodules
at Big Fish River, was identified for the first time during
this study.</p>
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