Nevada County California Gold Production
The reports of Lindgren (1896) and Johnston (1940) have been drawn upon for the following discussion of the geology of the district.
At the Grass Valley camp, the oldest rocks are schists and slates of the Calaveras Formation, of Carboniferous age, and relatively unaltered clay slates of the Mariposa Formation, of Jurassic age. Igneous rocks in the area consist of large masses of diabase, porphyrite, and amphibolite schist of Carboniferous to Jurassic age and serpentine, gab-bro, diorite, granodiorite, and quartz porphyry, of Jurassic and Cretaceous age. Andesite flows of Tertiary age cover large areas east and southeast of the town of Grass Valley. The most characteristic geological feature of the Grass Valley camp is the elongate body of Cretaceous granodiorite which is 5 miles long and 14 to 2 miles wide. The ore deposits are gold-quartz veins in the granodiorite mass and in the serpentine, porphyrite, and diabase wallrock. The veins in the granodiorite, porphyrite, and diabase, in the central and southern part of the camp, strike north or slightly northwest, parallel to the contact of the intrusive. Veins in the northern part of the camp are north of the granodiorite and are in serpentine. These strike predominantly east. Quartz is the principal vein material, and it appears in several textural types representative of successive stages of mineralization. Comb quartz, milky quartz, ribbon quartz, and brecciated quartz are the most common varieties. Ankerite and calcite are common gangue minerals but are less abundant than quartz. The principal sulfides are pyrite, galena, and sphalerite; also present, but less common, are arsenopyrite and chalcopyrite. Gold occurs along cracks and grain boundaries in the sulfides and in brecciated quartz. Commonly bounding the ore shoots in the granodiorite are vertical or steeply dipping fractures, called crossings, that strike northeast, normal to the long axis of the granodiorite body.
The Nevada City camp, about 4 miles northeast of Grass Valley, is at the south end of a large body of granodiorite that extends northward into Butte County. This mass is separate from the stock at Grass Valley but is probably of the same age. A narrow belt, 400 to 1,500 feet wide, consisting of argillites, quartzites, and mica schists of the Calaveras Formation, is in contact with the southern end of the granodiorite and crosses the Nevada City camp in a northwesterly direction. Masses of porphyrite, diabase, and amphibolite schist, of Carboniferous to Jurassic age, form irregular lenses and bands southwest of the Calaveras Formation. North of the town of Nevada City are several ridges of Tertiary gravels capped by andesite lava flows. In general, the veins of Nevada City are miner-alogically similar to those at Grass Valley, the main difference being the larger amount of coarse gold in the Grass Valley veins. The Nevada City veins are concentrated in the vicinity of the granodiorite-country rock contact and they are arranged into two systems: (1) a system that trends west-northwest with steep dips to the north or south, and (2) a system that trends north with medium eastward dips and contains the most productive veins.
MEADOW LAKE DISTRICT
The Meadow Lake district, 35 miles east of Grass Valley, is a relatively minor lode district and has been inactive since 1905. Production, which began in 1863, totaled at least 10,000 ounces of gold, but records are fragmentary.
Bedrock consists of granodiorite intruded into and bordered on the east by diabase and eruptive rocks (Wisker, 1936, p. 192-194). Most of the veins are in the granodiorite, a few are in the diabase. There are two productive vein systems: the major one trends N. 25° W.; the other, composed of smaller veins, ranges from N. 45° W., to N. 85° W. Some of the larger veins are remarkably persistent and can be traced for as much as 6,000 feet on the surface. High amounts of pyrite, arsenopyrite, and chalcopyrite are present in the veins. Quartz is not as abundant as it is in the typical California gold-quartz veins. Free gold is associated with pyrite and rarely with quartz.
TERTIARY PLACER DISTRICTS
The Tertiary placer districts include the placer deposits in the following areas: San Juan Ridge, North Columbia, Sailor Flat, Blue Tent, Scotts Flat, Quaker Hill, and Red Dog-You Bet.
Most of the placer production of Nevada County before 1900 came from hydraulic mining of the Tertiary gravels from these localities. The production of individual areas is not known, but Lindgren (1911, p. 133) estimated that their aggregate production through 1909 was at least $60 million in gold. After passage of the California Debris Commission Act of 1893, many operators closed down, even though huge reserves of auriferous gravels still remain in these areas. In recent years, production from these areas has been sporadic and small scale.