Diagenesis and Source of Coloration in Andesite-Derived Volcaniclastic Rocks of Datil Group (Late Eocene-Early Oligocene), West-Central New Mexico: ABSTRACT

Abstract

The Datil Group (32-39 Ma) is the basal unit of the Datil-Mogollon volcanic field of west-central New Mexico. Andesitic volcaniclastic deposits of the Datil Group exhibit a diverse assemblage of diagenetic minerals. Early-formed clay minerals (smectite and corrensite) are ubiquitous and occur as grain coatings and as a diagenetic component of matrix in sandstones. Silica minerals, opal-CT and tiny (5 to 20..mu..m) euhedral quartz crystals, formed next. Late-stage pore-filling cements include calcite, chlorite, and heulandite-group zeolites. Propylitization of Datil volcaniclastic rocks near intrusive centers produced pore-filling and replacement products of calcite, epidote, chlorite, and sericite. Coloration of unaltered Datil Group rocks is gray (average N7); such coloration is preserved in several parts of the outcrop belt. Buff colors (average 7.5 YR 7/2) result from diffuse limonitic staining produced during recent outcrop weathering. Pigment in volcaniclastic red beds (average 7.5 R 6/2) is derived from intrastratal dissolution of unstable mafic minerals, precipitation of hydrous iron oxides, and subsequent dehydration of these oxides to produce hematite. Propylitization of a neutral-colored protolith produces green beds (average 10 GY 5/1). Purple coloration (average 2.5 RP 5/2) in large areas surrounding mining districts in the Magdalena Mountains and Black Range results from incomplete propylitization ofmore » red beds. Gray and buff in red-bed sequences owe their color to selective, permeability-controlled dereddening.« less