New Mexico Geological Society Annual Spring Meeting — Abstracts

The Jones Camp dike magnetite deposits in eastern Socorro County are an example of a hydrothermal contact-metasomatic deposit. Other similar iron deposits occur in Lincoln, Grant, and Sierra counties. The Jones Camp deposits contain significant tonnages of high grade ore and are the subject of renewed interest in mining and development. The ore occurs as conformable pods of variable thickness, strike length, and grade. They are spacially associated with the central Jones dike and subsequent dikes and sills which provided the principal avenues for the ascending hydrothermal solutions as well as the heat source for the system. The intensely altered margins of the Jones dike are depleted in iron and are recognized as the principal source of the metal. The ore is non-specific with respect to host rock and occurs as replacements of carbonate, sandstone, and gypsum of the Pennian Yeso, Glorieta, and San Andres fonnations and lesser amounts are hosted by altered igneous rocks.

Fluid inclusion data obtained from secondary calcite cogenetic with magnetite mineralization yielded temperatures of 135-185 degrees centigrade. The system was determined to be non-boiling and under a hydrostatic pressure of about 154 bars. Primary fluid inclusions usually contained daughter crystals of anhydrite, and occasionally halite. Lack of experimental data in the literature with respect to systems high in sulfate precluded an estimate of the salinity in terms of NaCl equivalents; however, the inclusions were recognized as hyper-saline on the basis of extreme difficulty in freezing and the occurrence of immiscible liquids on heating (Roedder, 1984). The solubility of magnetite in high temperature saline solutions is pH dependent and decreases with temperature to a minimum in the range of 140-160 degrees centigrade. Magnetite is more soluble in low pH solutions. The principal chemical control on the replacement process is probably due to recipricol reactions between the dissolution of the sedimentary rocks by hydrolysis (hydrogen ion consuming reactions) and the precipitation of magnetite which produces hydrogen ions according to the reaction: Fe (divalent) + 2 Fe (trivalent) + 4 H₂O === Fe₃O₄ (magnetite) + 8 H (univalent).