Goodsprings District

The Goodsprings district lies in the Spring Mountains of southern Nevada. Although less famous than many of the other mining districts of the Great Basin it nevertheless ranks second only to Tonapah in total lead and zinc production from Nevada. During World War I the district was one of the most productive in the West, but by the end of World War II only a few mines remained in operation. By the late 1950's all operations had ceased. In recent years, renewed activity has centered on some of the gold mines in the district.

Stratigraphy

The sedimentary rocks in the district range in age from Upper Cambrian to Recent. The Paleozoic section includes the Cambrian Bonanza King and Nopah Formations, the Devonian Sultan, Mississippian Monte Cristo Limestone, Pennsylvanian/Mississippian Bird Spring Formation and Permian Kaibab Limestone (Carr, 1987). The Mesozoic section is comprised only of the Triassic Moenkopi and Chinle Formations and an upper Mesozoic unit of uncertain age termed the Lavinia Wash Formation. The Paleozoic rocks are dominantly carbonates while the Mesozoic units are continental clastics. Tertiary rocks include gravels and minor volcanic tuffs.

Only two varieties of intrusive rocks are known in the district. The most abundant is granite porphyry which forms three large sill-like masses (Hewett, 1931). The sills generally lie near major thrust faults and are thought to have been emplaced along breccia zones at the base of the upper plate of the thrust fault. Locally, small dikes of basaltic composition and uncertain age have been encountered in some of the mine workings. Mid Tertiary felsic volcanics, a ubiquitous constituent of most Cordilleran mining districts, are rare in the Goodsprings District.

Structure

The region reveals an amazing record of folding, thrust faulting and normal faulting. Folding began in the early Jurassic, resulting in broad flexures in the more massive units and tight folds in the thinly bedded rocks. The thrust faults in the district are part of a belt of thrust faulted rocks, the Foreland Fold and Thrust Belt, that stretches from southern Canada to southern California. Deformation within this belt began in the Jurassic and continued until Cretaceous time. Within the Goodsprings District thrust faulting appears to post-date much of the folding, but despite intensive study the actual age of thrusting continues to be the subject of contentious debate. Three major thrusts have been mapped; from west to east, the Green Monster, Keystone and Contact thrusts. Of these, the Keystone is the most persistent along strike having been mapped for a distance of over 50 kilometers. The stratigraphic relationships along the Keystone fault are similar to those for all the major thrusts in the area, Cambrian Bonanza King Formation has been thrust eastward over younger Paleozoic rocks.

Normal faulting has received much less study, despite its close association with many ore deposits in the district. Hewett (1931) suggested normal faulting began in the early Cretaceous and continued through the Tertiary. Albritton, et. al (1954) adopted the more recent theory that all normal faulting is related to Basin and Range extension and thus is no older than Miocene. Burchfiel and Davis (1988) also restrict normal faulting to the Tertiary, but concede some of the thrust faults may have been reactivated as low angle normal faults during early Tertiary.

Ore Mineralogy and Alteration

Ore deposits in the Goodsprings district can at best be characterized as enigmatic. They appear to fall into two distinct types which may or may not be related, gold-copper deposits and lead-zinc deposits. Gold-copper deposits are clearly related to sill-like masses of granite porphyry. All existing mines worked the contact between the intrusive and surrounding sedimentary rocks. Gold occurred in both the intrusive and the carbonate wall rocks. It appears any carbonate unit was a suitable host. The lead-zinc deposits are often distant from intrusives and occur as veins or replacements of brecciated rocks along fault zones, either thrust faults or normal faults. Unlike the gold deposits, the productive lead-zinc deposits are restricted to the Monte Cristo Formation.

Mineralogy of gold-copper deposits consists of native gold (often visible!), pyrite, limonite, cinnabar, malachite, azurite and chrysocolla. Lead-zinc deposits are comprised of hydrozincite, calamine, smithsonite, cerrusite, anglesite, galena and iron oxides. The rather unusual mineralogy of the district is due to the great depth of surface oxidation, exceeding 200 meters. Typical sulfides such as chalcopyrite, sphalerite and pyrite have been partially or completely altered to more stable hydrated carbonates and sulfates. Only the highly insoluble lead sulfide, galena has successfully resisted surface oxidation.

Primary alteration is difficult to characterize due to the supergene overprint, but again appears to differ for gold-copper deposits and lead-zinc deposits. Gold-copper ores have been extensively sericitized and kaolinized, altering the host pluton to a rock that can be mined through simple excavation with little or no blasting. The rock is so thoroughly altered it decrepitates on exposure to the atmosphere. On the other hand, lead-zinc deposits appear to be characterized by dolomitization and minor silicification.

The Red Cloud Mine

The Red Cloud Mine lies 4 kilometers northwest of Goodsprings, just to the east of the Wilson Pass Road. The mine was a small gold producer near the turn of the century and is typical of many of the older mines in the district. Several shafts and the remains of a small cyanide leach plant can still be seen on the property.

The underground workings, now largely caved and inaccessible, explored the contact between Bird Spring Formation and a small northwest trending granite porphyry dike about 20-30 meters wide. The dike lies largely under the alluvium to the east of the mine workings, but the contact relationship can be seen in the small open cut near the main shaft. It is also noteworthy that underground workings suggest a close structural relationship between the granite porphyry dike and the Potosi Thrust which lies in the alluvial wash to the east of the mine.

Gold was mined from the altered portion of the dike and a dolomitized zone of carbonate immediately adjacent to the dike. Atypically, gold from this mine not visible, but miners were able to rely on the presence of pyrite as an ore control. Examination of mine tailings reveals the presence of minor pyrite, copper carbonates, cinnabar and chert-like masses of silica. Total production of gold from this mine probably did not exceed $200,000.

Keystone Mine

The Keystone Mine lies 8 kilometers west of Goodsprings, but is only accessible via the Wilson Pass Road and a well graded dirt road leading to the east up Keystone Wash, a driving distance of about 20 kilometers. One of the larger gold producers in the district, total production in excess of $7,000,000, it has recently undergone renewed development and small scale open pitting.

The mine itself lies at the contact between a northwest striking sill of granite porphyry and carbonate units. The nature of the porphyry can be seen strikingly in the pit as can the associated alteration. More difficult to establish is the actual location of the Keystone Thrust. One branch should lie somewhere in the southwest pit wall, while the other underlies the graded area in the saddle. The general relationships here are the same as those better exposed further west in the wash, Bonanza King thrust northeastward over Bird Spring Formation.

Underground mine workings, as well as the open pit, explored the contact between the porphyry and carbonates. Mineralization consists of limonite pseudomorphs after pyrite, manganese oxides, copper carbonates and native gold. Reportedly, (Hewett, 1931) samples of green talcose clay from the altered porphyry if left to dissolve in the mouth yield masses of wire gold. Study suggests the intrusive is localized along the plane of the thrust and that mineralization postdates both the thrust and the intrusive, perhaps related to numerous northeast-trending normal faults mapped in the underground workings.

Geologic Map of Keystone Wash

REFERENCES CITED

Albritton, C.C., Richards, A., Brokaw, A and Reinemund, J., 1954, Geologic controls of lead and zinc deposits in the Goodsprings District, Nevada; U.S. Geological Survey Bulletin 1010, 111 p.

Burchfiel, B.C., and Davis, G.A., 1988, Mesozoic thrust faults and Cenozoic low­angle normal faults, eastern Spring Mountains Nevada, and Clark Mountains thrust complex, California in This Extended Land: Geological journeys in the southern Basin and Range, Field Trip Guidebook, Geological Society of America, Western Cordilleran Section, Las Vegas, Nevada, p. 87­10.

Carr, Micheal D. and Pinkston, John C., 1987, Geologic map of the southern Spring Mountains, Clark County, Nevada; U.S. Geological Survey Misc. Field Studies Map MF-1514.

Hewett, D.F., 1931, Geology and ore deposits of the Goodsprings quadrangle, Nevada; U.S. Geol. Survey Professional Paper 162, 172 p.