A Link between Sediment Fabric, Mineralizing Processes, and Hydrologic Properties: Evidence from Vanadium Ores in the Morrison Formation, Utah
Richard B. Wanty, Neil S. Fishman, and Eileen Poeter
Fluvial sandstones of the Jurassic Morrison Formation, central Utah, contain excellent examples of sediment heterogeneities that influenced hydrology during V mineralization. Shortly after sandstone deposition, V ores formed at the interface between a basinal brine and an overlying, dilute meteoric water. VIII-oxyhydroxides precipitated where this fluid interface coincided with detrital organic material accumulations in the sandstones. There, brine sulfate was reduced to sulfide by bacteria; this sulfide reduced VIV carried in the meteoric fluid and precipitated VIII-oxyhydroxides. The dark V ore minerals highlight selected crossbeds by alternating with light (barren) layers. Petrographic observations, point counts, and grain-size measurem nts on ore and barren samples were used to determine the causes for the alternating character of the mineralization.
Size measurements of detrital framework grains revealed that ore formed in laminae with better sorting and larger average grain dimensions. These laminae had relatively greater original permeability. Ore-forming processes can be considered in two stages- the onset of mineralization and main-stage ore formation. In the first stage, the coarser, better-sorted laminae were preferred flow paths for continuous delivery of V and reductant. VIII-oxyhydroxides formed in these layers first In the latter stage, previously nucleated V minerals continued to grow, rather than forming new V minerals in the finer-grained laminae. Even though there must have been progressive occlusion of pores in the originally coarser laminae, the nucleation energy required to form new VIII-oxy ydroxides in finer laminae must have been great enough that it was energetically favorable to build on existing VIII-oxyhydroxides. Thus, original heterogeneities of crossbred laminae controlled loci of V mineralization by influencing the early hydrology of the host sandstones.
AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California