HOST ROCK ALTERATION
Mild alteration of the Jackpile sandstone is not only associated with ore but occurs on a regional scale as well. The most obvious regional alteration feature is the partial or complete dissolution of feldspars and the precipitation of kaolinite in primary and secondary pore space. Alteration is most pronounced near the the Morrison-Dakota contact and is related to sub-areal weathering during the hiatus between deposition of the Morrison Formation and the Dakota Sandstone18. Additional regional diagenetic trends include local calcite, quartz, chalcedony, phyllosilicate, and hematite cements. The base of the Jackpile sandstone contains areas where plagioclase is partially or wholly replaced by albite and both quartz and potassium feldspar are overgrown. This typically occurs where the Jackpile sandstone overlies the zeolitic mudstone of the playa facies and the calcareous mudstone of the marginal mudflat facies. Thin sandstones interbedded within these facies have even more pronounced alteration of this type. Interstitial alkaline brine enriched in Na+, K+, and silica is responsible for these reactions in the sandstones which are interbedded with the mudstone. Brine also altered the base of the Jackpile sandstone where it was forced in from the compacting mudstone or where the Jackpile sandstone was dropped below the piezometric surface of the brine in the axes of the growing Jurassic synclines.
Adams et al.1 noted that albite overgrowths are present below ore at the Jackpile-Paguate mine. Their thickness and abundance increases with depth below the stratigraphically highest ore zones. In addition, mixed-layer illite-montmorillonite coats grains above, within, and marginal to ore. They also found that quartz overgrowths occur above and partially coincident with this illite-montmorillonite zone. Such occurrences mark zones where the saline alkaline brine mixed with fresh ground water in the Jackpile fluvial system1. Mixing of these two waters may have been caused in part by cyclic fluctuations in the water table on a seasonal or longer basis. Primary compaction of the saturated mudstones and the resultant expulsion of brine also contributed to mixing.
An alteration envelope of authigenic minerals containing reduced iron is present around the ore bodies. Its close association with humate cement suggests that a low Eh prevailed as the result of the slow oxidation of the humate. Pyrite which formed by the reaction of reducing pore fluids and early hematite is common within and immediately adjacent to ore1. Much of the mixed-layer illite-montmorillonite associated with ore1,6 may have formed by the alteration of early montmorillonite. Reduction of Fe+3 to Fe+2 in octahedral sites of montmorillonite results in an increased layer charge and if sufficient K+ is available, illite is the product phase10. With the exception of deeply buried sections, authigenic mixed-layer illite-montmorillonite is not common in the Brushy Basin Member. Ore zone chlorite6 may also have formed from the reaction of acidic reducing pore fluids with montmorillonite. Siderite is common where the downward percolating water associated with feldspar dissolution and kaolinite precipitation reacted with pyrite1.