Stratigraphic setting of some new and rare phosphate minerals in the Yukon Territory
Robertson, Benjamin Telfer
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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. 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. 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. 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)₂ PO₄ (OH)) which in turn is altered to arrojadite (K(Na,Ca)₅ (Fe,Mn,Mg)₁₄ Al(OH,F)(PO₄)₁₂). Gormanite-souzalite ((Fe,Mg)₃ (Al,Fe)₄ (PO₄)₄ (OH)₆ ·2H₂O) is a common alteration (or replacement) in both. 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. 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)₂PO₄(0H)), satterlyite, maricite (NaFePO₄), vivianite-baricite ((Fe,Mg) ₃(PO₄)₂·8H₂0), varulite ((Na,Ca)(Mn,Fe)₂(PO₄), and their alteration products. Nahpoite (Na₂HPO₄), 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.