New Mexico Mineral Symposium — Abstracts
A Comparison of Five Modern Gold Specimens - the Ibex Mine, Lake County, Colorado; Lokel Mine, Pershing County, Nevada, Eagles Nest Mine, Placer County, California, and the Colorado Quartz and Mockingbird Mines, Mariposa County, California
Robert B Cook
Dept of Geosciences, Auburn University, Alabama
Of the many gold occurrences known historically for fine specimens, only a few are productive today. Five of these are commonly represented by extraordinary gold specimens at most modern collector venues. Four are mines worked only intermittently and exclusively for specimen gold (the Lokel, Eagles Nest, Mockingbird, and Colorado Quartz mines) and one (the Ibex mine), although closed for decades, continues to yield fine gold specimens from its extensive dumps. Although within distinctly different physiographic and geologic settings, there are important features shared by several or all of the occurrences, as well as some striking differences.
The Ibex mine or group (including the famous Little Jonny) was the source of some of John Campion’s finest gold specimens. It is situated in the Leadville district three miles east of town near the top of Breece Hill. Based on the published literature, mine maps, and information gleaned from the mineralogy and petrology of dump samples, it is clear that the fine wire and ribbon gold for which the area is famous may be of both primary and supergene origin. The gold-rich area is within a well-defined 12.000 foot long, northeast-trending zone that extends at least from California Gulch on the south to the area of the Resurrection mine on the north. Within this zone are fault blocks that contain complex replacement ores hosted within the Leadville or similar limestones, as well as narrow but persistent veins or mineralized faults. Primary gold occurs as wires and sheets with sulfides, particularly sphalerite, and rarely carbonates and in white vein-type quartz, while masses of wire and ribbon gold occur with iron oxides and clays and may be supergene. Some of these masses have weighed up to 75 ounces. In addition to the Ibex mine(s) other nearby gold specimen producers include the Winnie, Fanny Rawlins, Big Four, Hopemore, and Donovan mines.
Some 600 miles west of Leadville and about ten miles from Winnemucca in the great Basin and Range Province is a relatively new discovery, the Lokel mine. The mine is in Humboldt County about one mile south of a site known as Pronto along Jungo Road. Here wonderful groups of coarse gold crystals, crystalline gold, and hackly gold masses weighing up to 95 ounces were picked up from the surface and in shallow dozer cuts by the lucky discoverers, Rod Pearce and Gene Baum. Exploration by several companies and specimen dealers quickly determined that the gold was float originating from a single north-trending, sulfide-poor quartz vein. Although the total production of gold specimens is unknown, it appears that many hundreds of ounces were cleaned and placed on the collector market both in the United States and abroad. Nearby gold specimen occurrences, all generally to the north, include prospects on Blue Mountain and mines of the Ten Mile district including the Mad Mutha and Golden Amethyst mines.
The Eagles Nest mine, 2.5 miles west of Foresthill, Placer County, California, is famous for its brilliantly lustrous, sharply crystallized gold specimens. The mine, operated by the Sykora family, is not a Mother Lode occurrence, lying 10 miles north of the termination of this belt. It exploits narrow, shallowly-dipping quartz veins in structurally complex metamorphics. Gold occurs at the intersection of these quartz veins with erratically spaced, vertical “control” structures that appear to be weakly mineralized joints. The host rocks have undergone carbonate alteration near productive veins and carbonate minerals occasional occur with gold. The site is relatively near a serpentinite body and there is geochemically anomalous nickel associated with productive veins.
The Colorado Quartz and Mockingbird mines occupy segments of the same occurrence, lying in the eastern pocket belt near the southern end of the Mother Lode in Mariposa County, California. Both mines exploit pockety, shallowly-dipping quartz veins and lenses within intensely silicified zones at the contact between schists and a vertically-dipping mafic dike. Exceptional crystallized gold, including the famous Dragon and Cobra specimens, occurs at and within a few feet of the dike contact. The host quartz is generally deficient in associated minerals although some pockets contain powdery black manganese oxides. Near-surface gold has likely been exhausted; however, both mines offer potential for continued specimen production with depth.
gold, Lokel, Eagles Nest, Mockingbird, Colorado Quartz, Ibex, ribbon gold, California Gulch, Ressurection mine, Winnie, Fanny Rawlins, Big Four, Hopemore, Donovan mines
36th Annual New Mexico Mineral Symposium
November 13-15, 2015, Socorro, NM