USGS Home
Contact USGS
Search USGS

Paragentic/Mineralogic Framework

Task Objectives

Understanding the processes that concentrate or disperse elements in fine-grained, organic-rich sediments requires knowledge the mineralogic residence of the elements and the stability of the host minerals. The dynamic nature of mineral assemblages further complicates the issue. While the mineralogical changes that occur during diagenesis and burial metamorphism are well known in general terms, the details vary based on the chemistry and physical conditions of the rock-water system (which change with time) and the fates of various trace elements are poorly known. In order to understand process that operated during the accumulation and burial of the sediments, it was necessary to trace the mineralogic evolution (paragenesis) of the rock back in time from what can be observed today and then draw inferences about what happened on the sea floor.

Similarly, in order to understand weathering processes it was necessary to trace the mineral paragenesis and mineralogical residence of elements through time. Emphasis was placed on ore-forming elements and environmentally sensitive elements (ESE), however in order to understand processes and possibly identify unrecognized problems, the mineralogic residence of most elements was determined. The small grain size of the rocks created a significant problem because their constituent minerals could not be readily separated nor could they be readily analyzed in situ for trace elements. The weathered nature of the near-surface rock and friability of soil posed sampling problems where the texture of the material needed to be preserved for paragenetic study.

Much of the work focused on documenting the various mineral assemblages, determining the elemental partitioning between coexisting phases, and estimating at what point some of the elements began to follow separate geochemical paths. This task, coupled with the task on clay mineralogy, determined the current mineral residence of a variety of elements and delineated the mineral paragenesis of selected portions of the Mancos Shale. The paragenesis and mineral residence of environmentally sensitive elements (ESE) and ore-forming elements in soils was also determined. Tracing the mineralogical evolution of the fine-grained rocks of the Mancos Shale and the partitioning of the ESE into their different residences as the mineral assemblages change in response to changing geologic conditions required several interrelated studies. In particular, the use of microbeam techniques (including SEM, TEM, electron microprobe, ion microprobe, LA-ICP-MS) in conjunction with optical microscopy was required. Deciphering the redox history and history of interaction with various water sources through modeling of equilibrium mineral assemblages and an examination of S, O, H, and C isotope systematics aided our interpretations of the mechanisms responsible for fixing/releasing several ESE. An attempt was made to establish an absolute chronologic framework in which to fit the paragenetic sequence. The approach to this difficult effort was dependent on identifying mineral systems that are appropriate for geochronologic study. K-Ar and Ar-Ar studies were conducted on a suite of clay mineral separates.

Highlights & Key Findings

Initial, in situ, chemical analysis in several of the trenches in the Elephant Skin area indicate that both selenium and silver have moved extensively in the upper 7 meters of the Mancos Shale (and its soil) and are at least temporarily concentrated on fracture surfaces. Selenium concentrations reach about 120 ppm and silver concentrations of up to 1200 ppm have been observed. These values represent enrichment factors of 100 and 1000, respectively.

Partial chemical analyses of shale and related soil from the Blue Gate, Upper Tununk/Lower Tununk members of Mancos Shale in the Hanksville, Utah area demonstrate that the Blue Gate and Tunuk members are chemically distinct. A trace metal suite comprising Se, V, Ba, Cr, Cu, La, Mo, Nd, Ni, Pb, Rb, Zn, and Zr has greater concentrations in the Blue Gate and appears to differentiate the members.

Mineralogic differences exist between the Mancos soils near Hanksville, Utah and those on the Gunnison Gorge National Conservation Area (GGNCA). Carbonates are the dominant Ca-bearing phases in Utah whereas sulfates dominate in Colorado, suggesting that the latter soils will more readily contribute salt to surface runoff. This is a significant finding in terms of land-use planning and allocation of remediation funds because of the need to control salt loading of the Colorado River and its tributaries.

Return to top of this page | Previous Task | Task List