The Precambrian terrane north of Pecos, New Mexico, contains metavolcanic and metasedimentary rocks. The subaqueous volcanic rocks are of bimodal composition (basalt-rhyolite). Volcanism was cyclic; three partial or complete cycles are exposed in the study area. A cycle consists of basal basalt, overlying rhyolite and a cap of banded iron formation. The chemical characteristics of the volcanic rocks are similar to those of several modern island arcs, suggesting the Pecos volcanic sequence may represent a portion of a similar Precambrian system.
Diverse structural elements in the study area. are related to four periods of deformation: 1) a Precambrian compressional event, 2) the intrusion of the Embudo granite, 3) the formation of the Pecos-Picuris fault and 4) a. late tensional event. Upper greenschist metamorphism occurred during folding.
The massive sulfide ore lenses of the Pecos Mine occur at the contact between rhyolite and overlying sedimentary rocks. The ore lenses lie on the flank of a rhyolite agglomerate dome an1d show several primary sedimentary structures. Replacement textures are lacking in the deposit.
The Pecos ores were probably deposited by hot metalliferous fumarolic solutions on the Precambrian sea floor. The nature and stratigraphic succession of various ore types are related to the declining temperature and density of the ore-bearing solutions. The sulfide ores have been regionally metamorphosed along with the contemporaneously deposited host rocks.
Weak surficial mineralization in the Jones Mine area southwest of, and possibly on the same time-stratigraphic horizon as the Pecos Mine, may indicate the presence of another massive sulfide deposit. Because massive sulfide deposits in other districts tend to occur in clusters and on the same time-stratigraphic horizon, the Jones Mine area appears to be an excellent exploration target.
Earth and Planetary Sciences
Level of Degree
Department of Earth and Planetary Sciences
First Committee Member (Chair)
Gary P. Landis
Second Committee Member
Jonathan Ferris Callender
Third Committee Member
Wolfgang Eugene Elston
Fourth Committee Member
Lee Albert Woodward
Fifth Committee Member
Philip J. Sterling
Riesmeyer, William Duncan. "Precambrian Geology and Ore Deposits of the Pecos Mining District, San Miguel and Santa Fe Counties, New Mexico." (1978). https://digitalrepository.unm.edu/eps_etds/283