Grant County, New Mexico
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Located east of I-25 and north of I-10, Grant County, New Mexico, rivals Taos in the diversity of its landscape. Home of the Gila Wilderness, Gila National Forest, and Gila Cliff Dwellings (all close to the Gila River), this county is also home to Silver City, and the world’s third largest open-pit copper mine (the Chino Mine).
Grant County's cultural and historical associations are rich, many related to the mineral deposits. The Gila Cliff Dwellings were the home of people of the Mogollon culture in the late 1200’s and early 1300’s. The mineral resources of the area, especially copper and copper minerals, were recognized by the indigenous peoples, then exploited by the Spanish when they arrived in the 1500’s. In the 1870’s, discovery of rich silver deposits attracted American prospectors; the arrival of the Santa Fe Railroad in 1886 spurred the growth of mining communities in the region. The county has ties with Billy the Kid, Butch Cassidy and the Wild Bunch, Geronimo, and others. To visually explore some of these sites in Grant County, click HERE.
Mining History of Grant County
Grant County still has a viable mining industry. Of the almost 600 mines in the county, about 380 of them are actively producing ores, predominantly of copper, lead, zinc, silver, gold, iron, manganese, and molybdenum.
The Chino Mine (aka the Santa Rita Mine, named after the village where the deposit was first commercially exploited) is the world’s third largest open-pit copper mine, located about 15 miles east of Silver City. There are two open pits with oxide and secondary sulfide mineralization. Chrysocolla, chalcocite, chalcopyrite and molybdenite are the dominant sulfide minerals extracted. Although known and exploited by native Americans and the Spanish, modern mining operations began in 1910 when the open pit was established. The mine has been through several periods of closure, reopening, and revamping of its facilities.
For a good overview of the mining geology of Grant County, click HERE.
Overview of Geologic Setting
Grant County geology is similar to that of Luna and nearby counties, typical of the Basin-and-Range region, an area of the southwestern US characterized by extensional faulting related to changes in plate tectonic movement that began about 35 million years ago with the formation of the Rio Grande rift. The extensional stresses result in fault block mountains with adjacent fault basins. Millions of years before the active faulting near Earth’s surface occurred, higher heat flows generated magmatic intrusions at depth, resulting in the emplacement of granitic and granodioritic plutons. Magmatism occurred throughout the region into the Tertiary and Quaternary periods.
The faulting of the Cenozoic period affected the whole sequence of rocks present in the once stable part of the continent. The fault block mountains expose sequences of rocks ranging from Precambrian through Mesozoic in age, largely sedimentary rocks, including shales, limestones, and sandstones. On top of these fault block mountains, and contemporaneous with them, more recent alluvial (stream-related) sediments are deposited, in-filling the fault block basins, and burying the fault block mountains in a sea of sand.
Economically valuable mineral deposits occur where these older rocks were intruded by granitic magma, and hot, aqueous solutions rich in elements such as gold, silver, copper, lead and manganese flowed around the bodies, flowing into cracks and veins where they later crystallized.
Other interesting but less economically valuable mineral deposits occur where surface waters percolate into the older volcanic and sedimentary rocks, dissolving silica and reprecipitating it as quartz, chalcedony and agate, either in seams or as nodules (geodes) in a host rock, or form zeolite minerals.
Copper Specimens and the Chino Mine District
The Chino Mine is found in the Cobre (“copper”) Mountains of Grant County, consisting of ancient metamorphic and igneous rocks covered by 4000-5000 feet of Paleozoic to Mesozoic sedimentary rocks. Igneous activity began in the Cretaceous with the intrusion of felsic to mafic magmas; eventually these magma bodies made it to the surface, erupting about 2000 feet thickness of lavas and tuffs. Late stage activity included intrusion of rhyolite dikes. Mineralization occurred in the tops of the intrusive igneous bodies. These older volcanic and intrusive igneous rocks and mineral deposits were later covered by volcanic rocks of the Mogollon-Datil volcanic field to the north. Basin and Range faulting later resulted in exposure of the deposits.
Azurite and the Hanover Fierro District
Mineralization in this district has a similar origin to that of the Chino District (see diagram above). Deposits are located in the Hanover-Fierro intrusive stock as fracture fillings. For more information click HERE.
Copper Roses and the Georgetown District
The Rose Mine (aka Copper Rose Mine, McGregor Mine, Copper Glance Mine, or Potosi Mine) is located in the Georgetown District of Grant County. This location is known for its beautiful copper "roses". These occur in oxidized bodies of the Fusselman Dolomite, and are related to intrusive dikes of a granodiorite. Deposits are localized along fractures, joints and bedding planes. The “copper roses” formed when azurite “roses” were oxidized and replaced by the copper oxide mineral cuprite.
Zeolites and the Alum Mining District
Excellent zeolite specimens are found close to the Grapevine Campground, located near Gila Hot Springs in Alum Mining District. These occur in vesicular andesites along the Gila River. For more information click HERE. Chabazite and mesolite are found, among other zeolites.
Fluorite and the Fluorspar District
Located southwest of Silver City, the Gila Fluorspar District is in the foothills of the northern Pinos Altos Mountains. Active mining of fluorite in the area intensified before and during World War II to satisfy the demand for steel manufacture which requires fluorite. Most of the fluorite occurs as veins in Precambrian granites and metamorphic rocks, some as veins in Tertiary volcanic rocks, and a few as veins in Paleozoic limestones. To learn more, click HERE.