New Mexico Geological Society Annual Spring Meeting — Abstracts

Investigations on the Ortiz Mine Grant have revealed the relationship of igneous activity and gold mineralization to the Tijeras fault system. The northeast-trending Tijeras fault system passes through the southeastern part of Ortiz Graben Mountains and is characterized by the asymmetric Ortiz Graben. Several fault blocks, containing up to 1,000 ft of Eocene Galisteo Formation, comprise the graben which has up to 4,000 ft and 2,000 ft of stratigraphic separation on its southeast and northwest sides, respectively.

Development of the Ortiz Graben is postdated by a late Oligocene (?) augite monzonite stock. It is postulated that the Ortiz Graben began forming in the late Eocene or early Oligocene.

Intrusions on the Ortiz Mine Grant occurred in four stages: 1) quartz-bearing latite-andesite sills, laccoliths, and dikes intruded Triassic to Tertiary strata at about 40 m.y. (Bachman and Mehnert, 1978). Recurrent fault movement makes the timing of latite-andesite intrusion relative to development of the Ortiz Graben obscure. 2) a Late Oligocene (?) augite monzonite stock with an extensive contact metamorphic aureole intruded the sill-and laccolith-intruded section. 3) quartz monzonite and syenite- latite stocks and plugs intruded and pipe-like breccia bodies developed near or along the northwest margin of the Ortiz Graben. 4) trachytic latite dikes intruded parallel to the northwest boundary of the Ortiz Graben and radial to intrusive plugs and breccia pipes.

Gold mineralization occurs five distinct environments on the Ortiz Mine Grant: 1) breccia pipes, 2) skarns, 3) veins and shear zones, 4) magnetite-chalcopyrite stockwords, and 5) placers. Lode mineralization postdates stage 3) intrusions and predates state 4).