Gold Hill Mine

Site Name

Primary: Gold Hill Mine

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Commodity

Primary: Gold
Secondary: Silver

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Location

State: California
County: Nevada
District: Grass Valley

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Land Status

Land ownership: Private
Note: the land ownership field only identifies whether the area the mine is in is generally on public lands like Forest Service or BLM land, or if it is in an area that is generally private property. It does not definitively identify property status, nor does it indicate claim status or whether an area is open to prospecting. Always respect private property.
Administrative Organization: Nevada County Planning Department

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Holdings

Not available

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Workings

Not available

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Ownership

Not available

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Production

Not available

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Deposit

Record Type: Site
Operation Category: Past Producer
Deposit Type: Hydrothermal vein
Operation Type: Underground
Discovery Year: 1850
Years of Production:
Organization:
Significant: Y

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Physiography

Not available

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Mineral Deposit Model

Model Name: Low-sulfide Au-quartz vein

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Orebody

Form: Tabular

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Structure

Type: R
Description: Wolf Creek Fault Zone, Gillis Hill Fault, Melones Fault Zone

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Alterations

Alteration Type: L
Alteration Text: Ankeritic, sericitic, and pyritic replacement of wall rocks adjacent to veins

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Rocks

Name: Diabase
Role: Host
Age Type: Host Rock
Age Young: Mesozoic
Age Old: Paleozoic

Name: Granodiorite
Role: Host
Age Type: Host Rock
Age Young: Early Cretaceous

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Analytical Data

Not available

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Materials

Ore: Gold
Ore: Pyrite
Ore: Galena
Gangue: Sphalerite
Gangue: Quartz
Gangue: Calcite
Gangue: Chalcopyrite

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Comments

Comment (Location): The location point selected for latitude and longitude represents the Gold Hill Mine symbol on Lindgren?s 1896 1:14,400-scale Grass Valley Special Map (contained in Lindgren?s 1896 Nevada City Special Folio) and transcribed onto the USGS Grass Valley 7.5-minute quadrangle.

Comment (Workings): No information is generally available regarding the underground workings in the Gold Hill Mine. Information may be available in the archives of the Empire Mine State Park since the Gold Hill Mine was acquired by and worked by the North Star Mining Co. prior to its acquisition by the Empire Mine.

Comment (Geology): An important structural feature in the district is a group of "crossing" vertical or steeply dipping fractures that strike northeast, about normal to the long axis of the granodiorite body. In places they are simple fractures; elsewhere they form sheeted fracture zones several feet wide. Some are tight, some are open and form watercourses, and few contain any quartz. Two main stages of primary or hypogene mineralization are recognized - 1) a hypothermal stage represented by one vein and one mineralized crossing, in which magnetite, pyrrhotite, pyrite, and specularite were deposited, and 2) a mesothermal stage, in which the gold quartz veins were formed. The mesothermal stage is further divided into two sub-stages - an older one, in which quartz is the principal gangue mineral, and a younger one, marked by the deposition of carbonates. Pyrite and arsenopyrite, deposited in the quartz stage, are the earliest sulfides of the gold-quartz veins. Sphalerite, chalcopyrite, and galena are somewhat later. No secondary or supergene minerals have been noted except limonite, calcite, and gypsum, which are being deposited in the oxidized zone. The distribution of gold in the ore shoots is extremely erratic and assays of adjacent vein samples commonly differ widely. Some ore shoots have a pitch length of several thousand feet, but most are much smaller. Adjacent to veins and crossing fractures, the wall rocks are generally highly altered. Ankerite, sericite, and pyrite have replaced the original rock-forming minerals. Lesser amounts of chlorite and epidote have been found. The wall rock has not been replaced by quartz. LOCAL GEOLOGY The Gold Hill and neighboring Massachusetts Hill (also called Rocky Bar) veins were among the earliest worked in the district. The veins strike generally to the north and dip 20? to 40 ? east or west. The veins lie either in granodiorite, porphyrite, or diabase. The Gold Hill vein crops out in diabase, but all of the lower workings are in granodiorite. The strike of the vein though irregular, is generally north-south and the dip is 28? E. The upper portion, near the outcrop, is much flatter. The quartz vein is generally narrow varying from a mere seam to as much as 6 feet and averages 2 feet. At 275 feet south of the shaft, the vein is said to have been cut off by a fault striking northwest and containing no ore. The hanging wall of the vein is strongly impregnated with pyrite. The vein is characterized by irregular pay shoots, at places being almost entirely barren, while at other places large pockets of coarse gold occur. North of the Gold Hill shaft, the vein splits, one branch extending north and the other northeast (Johnston, 1940). Gold occurred mainly as free gold, often as extremely rich masses of coarse mineralization. Both the Gold Hill and Massachusetts Hill veins were known for their heavy masses of gold (much more than the generally disseminated fine to coarse gold of the other veins). Gold in both veins averaged 850 fineness. Sulfides are generally poor grade.

Comment (Geology): REGIONAL GEOLOGY The Gold Hill Mine is within the Grass Valley District, home to California's two largest underground gold mines, the Empire and the Idaho-Maryland. The district is located in the northern portion of the Sierra Nevada Foothills Gold Belt. This belt averages 50 miles wide and extends for about 150 miles in a north-northwest orientation along the western slope of the Sierra Nevada range. The Foothills Gold Belt roughly coincides with the Foothills Metamorphic Belt, which can be subdivided into four major lithotectonic belts: Western Belt, Central Metamorphic Belt, Feather River Peridotite Belt, and Eastern Belt. The Grass Valley District lies within the Central Belt, where in the Grass Valley area it is marked by an 8-mile-wide north-trending assemblage of two accreted terranes that range from Late Triassic to Late Jurassic in age. The Central Belt is bounded on the east and west by regional-scale tectonic suture zones; the Wolf Creek Fault Zone on the west and the Gills Hill Fault/Melones Fault Zone on the east. The oldest rocks in the area are those of the Carboniferous-Triassic metasedimentary Calaveras Complex. Originally clastics, these rocks were converted to schistose or slaty rocks during the Late Paleozoic orogeny and locally into a contact-metamorphic biotite gneiss by intruded granodiorite during Late Mesozoic time. The slates of the Jurassic Mariposa Formation, which outcrop in a small part of the area, are relatively unaltered. Igneous rocks in the district include granodiorite, diabase, porphyrite, amphibolite schist, serpentinite, gabbro, diorite, quartz porphyry, and various dike rocks (Johnston, 1940). The veins of the Grass Valley and neighboring Nevada City districts are not connected with or continuations of the famous Mother Lode vein system to the south. The last veins of the Mother Lode end about 20 miles to the south. Also, the Grass Valley veins differ in general character from those of the Mother Lode. Generally, the Grass Valley veins are narrower and produce a higher-grade ore than those of the Mother Lode. The veins trend in two primary directions. One set trends N-S (dipping E or W), and the other trends E-W (dipping N or S). The major feature of the Grass Valley District is a body of Lower Cretaceous granodiorite and diabase five miles long from north to south and half a mile to two miles wide (probably the apex of a larger batholitic mass). It which is intruded into older sedimentary and igneous rocks, including diabase of the Mesozoic-Paleozoic Lake Combie Complex, and is itself cut by various dike rocks. Gold-quartz veins cut the granodiorite and diabase (and in some cases, serpentinite) throughout the district. Most of the veins strike generally north, parallel to the intrusive body, and display gentle dips averaging 35?. Others strike northwest, parallel to a diabase contact with the granodiorite. The veins fill minor thrust faults that occur within fracture zones of various width and degree of fracturing. The maximum measured reverse displacement is 20 feet (Johnston, 1940). In all veins, quartz is the principal vein material and occurs in four textural types: 1) Comb quartz that forms crustifications and lines vugs, 2) massive milky quartz with a granular texture that displays many sharp crystal faces and has not undergone deformation, 3) sheared quartz developed with little or no dilation of the vein fracture and commonly showing ribbon or shear-banding structures, and 4) brecciated quartz formed where vein movement dilated the interwall space (Johnston, 1940). Gold occurs in quartz and in sulfides, principally pyrite. Although specimen ore has been found, most ore from the district occurs as fine and coarse free-milling gold in ores averaging between 0.25 to 0.5 ounces per ton.

Comment (Commodity): Ore Materials: Free milling coarse and fine gold in quartz (850 fine). Auriferous pyrite and galena.

Comment (Commodity): Gangue Materials: Quartz, calcite, chalcopyrite, sphalerite

Comment (Development): Gold was first discovered in the Grass Valley District in quartz veins of Gold Hill on the outskirts of Grass Valley (within present-day Grass Valley) in 1850. These veins were the principal producers in the early days of the district. Almost immediately, the towns of Grass Valley and Nevada City were founded and went on to become permanent communities. Quartz mining was well established by 1857 and continued without interruption until the 1940s. The discovery at Gold Hill was followed by discoveries of veins at nearby Ophir Hill, Rich Hill, and Massachusetts Hill. And by 1867 most of the major mines of the district had been located. The Gold Hill Mine operated from 1850 to 1867 and is reported to have produced $4,000,000 during this period. In1903, the mine was purchased by the North Star Co., operators of the nearby North Star Mine. The North Star Co. did considerable development work in search of new ore bodies. No new ore was found, and the mine has remained idle ever since.

Comment (Economic Factors): The Gold Hill Mine has reportedly produced $4,000,000 in gold.

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References

Reference (Deposit): Clark, W.B., 1970, Gold districts of California: California Division of Mines and Geology Bulletin 191, p. 53.

Reference (Deposit): Johnston, W.G., Jr., 1940, The gold quartz veins of Grass Valley, California: U.S. Geological Survey Professional Paper 194, 101 p.

Reference (Deposit): Koschmann, A.H., and Bergendahl, M.H., 1968, Gold-producing districts of the United States: U.S. Geological Survey Professional Paper 610, 283 p.

Reference (Deposit): Lindgren, W., 1896a, Geologic atlas of the United States - Nevada City Special Folio: U.S. Geological Survey Folio 29.

Reference (Deposit): Lindgren, W., 1896b, Gold-quartz veins of Nevada City and Grass Valley: Seventeenth Annual Report of the U.S. Geological Survey, Part 2, p. 1-262

Reference (Deposit): MacBoyle, E.M., 1919, Mines and mineral resources of Nevada County: Sixteenth Annual Report of the State Mineralogist, California State Mining Bureau, p. 1-270.

Reference (Deposit): Additional information on the Gold Hill Mine is contained in File No. 339-7749 (CGS Mineral Resources Files, Sacramento)

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