Geology topics

G.H. Brimhall

Publications and source records attributed to G.H. Brimhall.

2 recordsLinked to original sources

USGS2005

Characterization of waste rock associated with acid drainage at the Penn Mine, California, by ground-based visible to short-wave infrared reflectance spectroscopy assisted by digital mapping

Prior to remediation at the abandoned Cu-Zn Penn Mine in the Foothills massive sulfide belt of the Sierra Nevada, CA, acid mine drainage (AMD) was created, in part, by the subaerial oxidation of sulfides exposed on several waste piles. To support remediation efforts, a mineralogical study of the waste piles was undertaken by acquiring reflectance spectra (measured in the visible to short-wave infrared range of light (0.35-2.5 ??m) using a portable, digitally integrated pen tablet PC mapping system with differential global positioning system and laser rangefinder support. Analysis of the spectral data made use of a continuum removal and band-shape comparison method, and of reference spectral libraries of end-member minerals and mineral mixtures. Identification of secondary Fe-bearing minerals focused on band matching in the region between 0.43 and 1.3 ??m. Identification of sheet and other silicates was based on band-shape analysis in the region between 1.9 and 2.4 ??m. Analysis of reflectance spectra of characterized rock samples from the mine helped in gauging the spectral response to particle size and mixtures. The resulting mineral maps delineated a pattern of accumulation of secondary Fe minerals, wherein centers of copiapite and jarosite that formed at low pH (<3) were surrounded successively by goethite and hematite, which mark progressive increases in pH. This pattern represents the evolution of acid solutions discharged from the pyritic waste piles and the subsequent accumulation of secondary precipitates by hydrolysis reactions. The results highlight the high capacity of the pyritic waste to release further acid mine drainage into the environment, as well as the effectiveness of the mapping method to detect subtle changes in surface mineralogy and to produce maps useful to agencies responsible for remediating the site. ?? 2004 Elsevier B.V. All rights reserved.

Chemical Geology↗

USGS1989

Paleohydrologic evolution and geochemical dynamics of cumulative supergene metal enrichment at La Escondida, Atacama Desert, northern Chile

Quantitative limonite mapping within the leached capping of the porphyry copper deposit at La Escondida, Chile, permits reconstruction of the paleohydrologic and chemical evolution of a well-developed supergene ore-forming system. The mineralogy, textures, and relative abundance of supergene limonite minerals (hematite, goethite, and jarosite) are used to reconstruct the former ratio of pyrite to chalcocite and the preoxidation copper grade based on empirical limonite sulfide correlations (after Locke, 1926; Blanchard, 1968; and Loghry, 1972). Estimates of preoxidation copper grades in surface exposures and tops of drill holes at La Escondida are significantly lower than actual copper grades in the underlying enrichment blanket at depth. This apparent inconsistency is explained by a progressive increase in the copper grade of the sulfide enrichment blanket as it descended to its present location in response to a descending water table. This systematic trend of reconstructed grades of supergene-enriched copper sulfide increasing with depth offers the first quantitative proof of cumulative downward enrichment in a supergene profile, as proposed by Locke (1926). The consistent trends of limonite mineralogy and abundance in vertical profiles indicate that water table descent at La Escondida was relatively continuous in space, although not necessarily in time.Evidence for cumulative downward enrichment in vertical profiles through leached capping allows quantitative analysis of chemical mass balance in dynamic supergene systems. Slopes of linear regressions for profiles of reconstructed enriched copper grades vs. depth indicate lateral fluxes into or out of a given vertical profile. This method provides independent verification of conclusions from a previous study of copper mass balance at La Escondida (Brimhall et al., 1985) which showed that lateral fluxes of copper were a significant factor during supergene leaching and enrichment.

Economic Geology↗