Platinum-group Element Assessment in Nevada

Fall 1989

Because of the need to reduce the United States' dependence on foreign sources for strategic metals like platinum, and because of the strong prices for several of the platinum group elements (PGE's), the NBMG Geochemical Laboratory is evaluating several different geological environments in Nevada for their PGE potential. PGE mineralization located in most of the environments under investigation in Nevada would represent "unconventional resources" to the extent that worldwide production comes mainly from orthomagmatic ores and placers and Nevada's potential lies mainly with PGE-bearing hydrothermal ores.

Of the 30 hydrothermal PGE occurrences known worldwide, three occur in Nevada. Two of these, both situated in Clark County, have know historical production: the Goodsprings district (Boss and Oro Amigo mines, approximately 400 ounces platinum and 600 ounces palladium) and the Bunkerville district (Key West and Great Eastern mines, 6 ounces platinum and 177 ounces palladium). The third occurrence is the Gibellini mine in Eureka County, recently recognized as being platiniferous by ongoing research efforts at NBMG. The Humboldt Lopolith in the Bolivia district east of Lovelock is also being investigated by NBMG.

The two hydrothermal PGE districts in Clark County, Bunkerville and Goodsprings, have been known since the mid-1800s. These districts define the approximate boundaries of a belt of Proterozoic metamorphic rocks (including mafic and ultramafic igneous rocks and exhalative polymetallic ores) extending in a northwesterly direction through central Arizona into southern Nevada. Although considerably younger, Clark County hydrothermal PGE mineralization appears to be spatially related to these Proterozoic rocks.

Mineralization in the Goodsprings district is confined to hydrothermal ores occurring in dolomitized Lower Mississippian limestone along the Ironside fault in the northwest portion of the district. The Boss mine was the principal producer of copper, gold, silver, platinum, and palladium with subordinate production from the Oro Amigo mine. PGE mineralization is most closely associated with bismuthian plumbojarosite, which may be a secondary (oxidation) mineral. Anomalous palladium abundances reportedly occur also in a rhyolitic dike proximal to the nearby Chiquita mine.

PGE mineralization at the Key West and Great Eastern mines in the Bunkerville district is related to hydrothermally altered amphibolite dikes and occurs with nickel and copper mineralization. Bunkerville PGE mineralization is more typical of the geologic settings of hydrothermal PGE deposits known from elsewhere in the world than is that of the Goodsprings district. In fact, until the recent discovery of platinum at the Gibellini mine near Eureka, Goodsprings-style PGE mineralization hosted by Paleozoic carbonate rocks was unique.

A recent discovery of Bunkerville-style PGE mineralization, reported in southern California, would extend the distribution of known hydrothermal PGE mineralization to the southwest along a possible PGE trend. This trend would be defined by the distribution of known hydrothermal PGE deposits in southern Nevada and would fit fundamental geologic and structural features in this region.

The Gibellini Mn-Zn-Ni-V hydrothermal mineralization occurs in Upper Devonian Devils Gate Limestone in the Fish Creek Range south of Eureka. Samples collected by NBMG researchers and analyzed in the Geochemical Laboratory exhibit platinum concentrations from 175 to 650 ppb. Uncharacteristically for hydrothermal PGE deposits, Gibellini mineralization is depleted in palladium, averaging approximately 30 ppb. Independent neutron activation analysis of select samples from this deposit verified the presence of platinum and indicated slightly higher abundances than the fire assay-atomic absorption method employed at NBMG.

The Humboldt Lopolith is a large layered gabbroic intrusion of Early Jurassic age. Layering consists mainly of lower picrite overlain by gabbro which is overlain by leucogabbro to diorite. Inactive nickel-cobalt mines in the Stillwater Range produced a small amount of concentrates in the late 1800s from hydrothermal mineralization hosted by gabbro and limestone.

Preliminary assessment by NBMG has shown primary nickel abundances as high as 800 ppm, cobalt up to 100 ppm, and chromium concentrations as high as 3,000 ppm in fresh picrite. Platinum and palladium abundances in both unaltered picrite and hydrothermal nickel-cobalt and copper ore in the Bolivia district do not exceed 20 ppb. The lack of anomalous PGE abundances in hydrothermal ores related spatially to these gabbroic rocks is surprising considering the close geologic similarity to hydrothermal PGE deposits known from several localities elsewhere in the world. The layered igneous rocks have not been carefully sampled and analyzed to date for discrete ortho-magmatic "reef" type mineralization and their PGE potential remains unknown.

Because of recent and historical discoveries of PGE-bearing hydrothermal deposits in Nevada and adjacent environs and because of the large number of hydrothermal deposits in Nevada, it is reasonable to expect additional discoveries to be made within the Great Basin. PGE assays are expensive and difficult to perform. Individuals involved in PGE exploration are advised to employ only well-known commercial laboratories having established reputations for providing reliable PGE assays when seeking analyses of exploration samples. There are several commercial laboratories in the Reno area. Results of ongoing NBMG investigations of the PGE potential of various areas in Nevada will be released regularly in NBMG open-file reports and elsewhere.

---Paul J. Lechler, Chief Chemist/Geochemist and L. C. Hsu, Mineralogist