XRD ANALYSIS OF MAGNETITE-BEARING VOLCANIC UNITS, KNIGHT RANGE, NEW MEXICO

The Knight Range in Grant County, New Mexico, exhumes magnetite-bearing Tertiary volcanic units (JPB Mountain Andesite, Malpais Latite and Andesite, and the newly mapped Juniper Dike), none of which are well described in literature. Investigation of the geochemical relationships between these units has been inconclusive. Further, some samples strongly attract a magnet and/or contain visible magnetite, but magnetite content has not been quantified. While these units have been classified using bulk geochemistry, the aphanitic matrix prevents modal estimates of matrix minerals via petrography. This study uses powder X-ray diffraction (XRD) to resolve these issues, with two goals: 1) determine mineral content and 2) estimate the amount of magnetite present. Samples were previously powdered for X-ray fluorescence analysis but still contained larger grains; these samples were sieved to 250 µm, the larger particles were again powdered using a ball mill, sieved again, then added back to the sample to ensure homogeneity for XRD analysis. After analysis, Bruker DIFFRAC.TOPAS software was used to analyze raw XRD data to determine minerals and modes. The Tertiary volcanic units consist of plagioclase (26-39%), pyroxenes (20-36%), and potassium feldspar (7-13%), with smaller amounts of quartz (0-6%), garnet (1-7%), hornblende (5-13%), muscovite (0-7%), zeolites (0-6%), and spinel (0-2%). Magnetite is present only as an accessory mineral (≤ 0.52%). Based on mineralogy, the JPB Mountain Andesite is classified as andesite, the Malpais Latite and Andesite plots as latite or andesite depending on the sample, and the Juniper Dike plots as latite. Future work includes XRD analysis of additional samples of the Tertiary volcanics, as well as magnetite-rich Mesoproterozoic diabase dikes that cross-cut the basement. Investigation will continue regarding the contrast between the XRD analysis of magnetite content versus the strong magnetism observed in outcrop and hand samples of Tertiary units that suggests a greater volume of magnetite present.