Sedimentary fluorite in a lacustrine zeolitic tuff of the Gila Conglomerate near Buckhorn, Grant County, New Mexico

Abstract

Fluorite makes up 20-30% of a zeolitic tuff in a Pliocene or Pleistocene lacustrine facies of the Gila Conglomerate that has not been subjected to hydrothermal activity. The light gray zeolitic tuff is 40 cm thick and crops out over a 0.6-km 2 area about 2.5 km east of Buckhorn, New Mexico. This tuff is overlain by zeolitic mudstone that contains abundant and conspicuous Magadi-type chert. The fluorite occurs as prolate pellets and, rarely, as ooids that are mostly 0.14.3 mm in size. Broken pellets and ooids are extremely rare. Studies by X-ray diffraction and scanning electron microscopy show that the pellets and ooids consist mainly of submicrometer-size fluorite and quartz and that both minerals have poorly defined morphology. These pellets and ooids are embedded in a matrix that consists chiefly of micrometer-size mordenite and smectite and that has a vague vitroclastic texture. The pellets and ooids probably are the result of primary precipitation of fluorite and magadiite where dilute, calcium-bearing water from springs or streams mixed with the saline, alkaline lake water that had a high fluorine content. The pellets and ooids were then transported basinward and were incorporated with reworked vitric ash. During diagenesis, the magadiite of the pellets and ooids was converted to quartz, and the glass of the ash was altered to mordenite and smectite.