A remote sensing approach to alteration mapping: AVIRIS data and extension-related potassium metasomatism, Socorro, New Mexico
Abstract. Miocene crustal extension in the south-western United States was accompanied by potassium metasomatism, a type of alteration marked by addition of large amounts of potassium at the expense of other constituents, notably sodium. The fluids responsible for the potassic alteration may have played a role in triggering extensional fault motion; one way to determine this is to compare the geographic distribution of the alteration to fault and sedimentary facies maps. Despite the large impact on rock composition, however, the alteration is visually subtle and difficult to map in the field. In this paper we present a preliminary alteration map of the Lemitar Mountains near Socorro, New Mexico, within the Rio Grande rift. This map was created from Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) data. The potential to discriminate potassium metasomatism in Landsat Thematic Mapper satellite data is part of the motivation for this study, since such data are widely available. Although the potassic minerals do not exhibit spectral features in the visible and near-infrared, iron was mobilized during potassium metasomatism, and the distribution of haematite thus serves as a proxy for the potassium. Evidence supporting the proxy relationship between haematite and metasomatism include (a) published reports by previous workers stating that haematite is added to rocks during K-metasomatism; (b) limited chemical evidence that samples collected from areas displaying a distinctive spectral response are strongly metasomatized; (c) the presence of large amounts of haematite in metasomatized rocks, observed petrographically and with an EDAX microprobe; and (d) the coincidence of the southern edge of the K-anomaly in the Lemitar Mountains as defined by chemical analysis and the southernmost occurrence of the anomalous spectral response. No other significant source of haematite has been reported from this area. Manganese oxide, occurring as localized veins or as rock varnish, is black in colour and highly absorptive, resulting in a distinctly different spectral response than the metasomatism-related haematite. This work shows that K-metasomatism can be mapped using AVIRIS data. The spectral response is not subtle, and can be observed in Landsat Thematic Mapper data; it is likely that K-metasomatism can be mapped, at least to some significant degree, from the Landsat data. Ongoing work is focusing on calibrating the strength of the spectral response to the intensity of alteration.
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