"Rock of the Month # 21, posted for March 2003" A specimen of rich ("bonanza-grade") gold ore from the Grass Valley district of the `Mother Lode' district in the western foothills of ther Sierra Nevada range in California, U.S.A. Sample 1444, collected at the Eureka mine and acquired from David New, displays abundant native gold and a trace of pale grey galena (PbS) in brittle, brecciated pyrite. Photomicrograph in reflected light, 160X magnification, long-axis field of view 0.7 mm.

The yellow precious metal occurs in a modest number of discrete mineral species, as well as occurring dissolved ("in solid solution") or in submicroscopic clusters within more common ore minerals, such as pyrite ("fool's gold"). The most common form by far is the native element, that is, native gold (Au), of variable purity, in masses (nuggets) ranging from a few microns (millionths of a metre) to tens of cm in diameter. Gold is cubic in crystal form, metallic yellow in colour with a distinctive lustre, soft (Moh's hardness 2.5-3) and very dense (specific gravity of pure gold is near 19.3, 70% "heavier" than lead).

The Grass Valley gold mining district lies in the northern part of the Mother Lode region of California, U.S.A., and is a major past producer of lode gold. Gold mining commenced here in 1851, scarcely two years after the famed placer gold strike that started the famed gold rush of 1849, and lasted for over a century. The Grass Valley quartz veins were generally thin. A timely report by Benjamin Silliman in 1867 noted that `two feet is probably a full average thickness, while some of the most productive ... have not averaged over a foot or possibly eighteen inches in thickness' (Silliman, 1867). He also noted that the sulphides were usually gold-rich, as in our present subject. Other early descriptions are those of Courtis (1890) and Lindgren (1895). The Grass Valley camp is well-documented by numerous reports, including a classic memoir by Johnston (1940).

The Mother Lode is some 120 miles (193 km) long and 1-4 miles (1.6-6.4 km wide). Dates on minerals such as sericitic mica in the alteration assemblage accompanying gold indicate mineralization in the Californian deposits occurred from latest Jurassic through mid-Cretaceous time. Thus dates quoted for Grass Valley, 144-141 Ma, are slightly younger than Quartz Hill in the Klamath Mountains (146 Ma) but older than the main Mother Lode, dated at 127-108 Ma along the Melones fault zone (Elder and Cashman, 1992).

References

COURTIS,WM (1890) Gold quartz. Trans.AIME 18, 639-644.

ELDER,D and CASHMAN,SM (1992) Tectonic control and fluid evolution in the Quartz Hill, California, lode gold deposits. Econ.Geol. 87, 1795-1812.

JOHNSTON,WD (1940) The Gold Quartz Veins of Grass Valley, California. USGS Prof.Pap. 194, 101pp. plus 5 sheets.

LINDGREN,W (1895) The Gold-Quartz Veins of Nevada City and Grass Valley Districts, California. USGS Ann.Rep. 17 part 2, 262pp. plus 2 separate sheets.

SILLIMAN,B (1867) Notes on the Grass Valley gold-mining district. Amer.J.Sci. ser.2, 44, 236-244.