PETROGRAPHY AND HXRF GEOCHEMISTRY OF MAGNETITE-BEARING TERTIARY VOLCANIC UNITS AND DIKES IN THE KNIGHT RANGE, GRANT COUNTY, NEW MEXICO

Magnetite-bearing, intermediate to mafic volcanic rocks and dikes are part of a northeast-dipping package of Cretaceous-Tertiary volcanic, volcaniclastic, and sedimentary units making up the Knight Range of Grant County in southwestern New Mexico. The Knight Range is located in the Burro uplift, south of the Tyrone mining district, at the intersection of the Mogollon-Datil volcanic field and the southern Basin and Range province. These supracrustal rocks unconformably overlie the Mesoproterozoic Burro Mountain Granite, which is intruded by diabase dikes that contain ~12% magnetite. Field work was undertaken to investigate observations of magnetite in Tertiary units as well. In addition to collecting samples, new field observations include 1) a previously unmapped magnetite-bearing dike that cross-cuts the Cretaceous-Tertiary Conglomeratic Sandstone of Knight Canyon and 2) previously unreported magnetite in two mapped units (Latite and Andesite of Malpais Hills; Andesite of JPB Mountain). In these units, the magnetite is both disseminated and found in a foliated texture in areas of presumed fluid alteration near joints.

Samples of these and other units from the study area were made into polished thin sections for detailed petrographic analysis, and were powdered and analyzed using a handheld XRF (hXRF) to determine bulk composition. The samples are primarily subalkaline, and range from tholeiitic mafic rocks to calc-alkaline intermediate to felsic rocks. Proterozoic diabase samples are tholeiitic basalts, picrobasalt, and foidite, while the Tertiary magnetite-bearing samples are calc-alkaline andesites, basaltic andesites, and basaltic trachyandesites. Bulk geochemistry does not imply a genetic relationship between the Tertiary and Proterozoic magnetite-bearing rocks; however, similarities between the unmapped magnetite-bearing dike and other andesitic samples guide hypotheses that the dike may have been a feeder for the andesites, and/or that the andesite and latite units may be mismapped. The magnetite foliation suggests that much of the magnetite present in these units could be secondary. Petrography and bulk geochemistry will be used to further investigate these hypotheses.