Geology of the Vera Cruz mine and breccia pipe, Lincoln County, New Mexico
George E. Ryberg
The Vera Cruz mine occurs in a hypabyssal breccia pipe that intrudes a sandstone and shale sequence in the Upper Cretaceous Mesaverde Group. The sedimentary rocks are gently domed by mid-Tertiary alkalic sills and the pluton which forms Vera Cruz Mountain. Near the intrusive contact, the shale and sandstone have been metamorphosed to homfels and quartzite. Alignment of faults, alkalic intrusive rocks and dikes of the Capitan dike swarm suggest that a N20°E-trending structural zone exists. This zone is intersected by the east-west trend of the Capitan pluton and Jones Camp dike. The Vera Cruz breccia pipe formed at the intersection of local fault zones with trends similar to the regional structures. The breccia zone is 200 m long and up to 60 m wide on the surface. Distribution of breccia within the underground workings suggests the breccia zone widens with depth. Drill results suggest a southerly plunge to the mineralized portion of the pipe. Breccia fragments are angular to subrounded, clast supported and composed of highly altered fragments of sandstone, shale and intrusive rock. Much of the breccia zone is highly argillized with little pore space; however, the central mineralized portion is highly silicified and is more permeable. Gold mineralization occurs as fine grains in the upper, oxidized portion of the pipe. Gold values range up to 5.43 oz/ton from samples taken in the underground workings. The inclusion of altered intrusive fragments in the breccia together with the absence of crosscutting dikes and veins suggests that the formation of the breccia and subsequent mineralization were the latest hydrothermal-intrusive events in the Vera Cruz area.
- Ryberg, George E., 1991, Geology of the Vera Cruz mine and breccia pipe, Lincoln County, New Mexico, in: Geology of the Sierra Blanca, Sacramento and Capitan Ranges, New Mexico, Barker, James M.; Kues, Barry S.; Austin, George S.; Lucas, Spencer, G., New Mexico Geological Society, Guidebook, 42nd Field Conference, pp. 329-332.