Princeton Mine (New Princeton Mine; Princeton Extension Mine; Lewis Brothers Mine [Northwest Extension of Princeton Mine]), Mount Bullion, Bagby-Mariposa-Mount Bullion-Whitlock Mining District, Mariposa County, California, USAi

Regional Level Types
Princeton Mine (New Princeton Mine; Princeton Extension Mine; Lewis Brothers Mine [Northwest Extension of Princeton Mine])	Mine
Mount Bullion	Mountain
Bagby-Mariposa-Mount Bullion-Whitlock Mining District	Mining District
Mariposa County	County
California	State
USA	Country

A former Au mine located in secs. 13 & 18, T5S, R18E, MDM, 0.5 km (1,500 feet) WSW of Mount Bullion (town), on private land. Discovered in 1852. Operated by the Mariposa Commercial and Mining Co.. MRDS database stated accuracy for this location is 1,000 meters. The Princeton Mine was the most productive mine in Mariposa County. In its early days, the mine was part of the vast Las Mariposas land grant owned by John C. Fremont.

After its discovery in 1852, the Princeton Mine became the most productive mine in Mariposa County and at various times was the largest gold producer in California. The mine was idle between 1875 and 1900, and then resumed production until 1915. After another period of idleness, activity took place from 1921 to 1927. Small tonnages of ore and tailings were milled in the 1930's, but no sustained mining is known to have been carried on since then. Amalgamation processes were used at this mine.

MRDS file note: Location selected for latitude and longitude is a prospect symbol about one-half mile southwest of the town of Mt. Bullion on the USGS 7.5-minute Bear Valley quadrangle. This symbol may represent the main working of the mine, which was a shaft.

Mineralization is a quartz vein deposit (Deposit model code 273; USGS model code 36a; model name: Low-sulfide Au-quartz vein; Mark 3 model number 27, with a vein 4 to 8 feet thick, hosted in Late Jurassic slate of the Mariposa Formation. The vein system follows what is interpreted to be a reverse fault hosted in slate. The ore body is tabular. The ore is present in a fissure-filling vein system characterized by ribbon-structured milky quartz. The parts of the system that constitute ore carry considerable pyrite. The ore minerals are native gold and auriferous sulfides (pyrite). Alteration is local (none specifically reported; however, there is mention that the slate is severely bleached and hydrothermally altered near the vein in the New Princeton workings. Local rocks include Jurassic marine rocks, unit 1 (Western Sierra Nevada and Western Klamath Mountains) and/or ultramafic rocks, chiefly Mesozoic, unit 2 (Western Sierra Nevada and Klamath Mountains).

LOCAL GEOLOGY: The Princeton vein, which is believed to be part of the Mother Lode system, occupies a fault that cuts slate of the Jurassic Mariposa Formation. The slate contains thin beds of dark-colored graywacke and is cut by dikes described by Bowen and Gray (1957) as sheared, fine-grained granitic porphyry. The porphyry may be the same rock that Clark (1970) described as pyritic metarhyolite, which in places contains gold. Some of these dikes intersect the vein. The trace of the vein is not well-defined at the surface and lacks the massive, multiple characteristics of the Mother Lode at a few mines farther north in the county. The vein is en echelon with veins at adjacent mines; it is probable that the Mother Lode in this vicinity consists of several separated veins rather than one or two major persistent features.Wall rocks have invariably been hydrothermally altered, having been partially to completely converted to ankerite, sericite, quartz, pyrite, arsenopyrite, chlorite, and albite with traces of rutile and leucoxene (Knopf, 1929). The mineralization is generally adjacent to the veins in ground that has been fractured and contains small stringers and lenses of quartz. Locally, greenstone bodies adjacent to the quartz veins contain enough disseminated auriferous pyrite in large enough bodies to constitute what has been called "gray ore.? Altered slate wall rock commonly contains pyrite, arsenopyrite, quartz, chlorite, and sericite with or without ankerite (Zimmerman, 1983). Large bodies of mineralized schist also form low-grade ore bodies throughout the Mother Lode. This ore consists of amphibolite schist that has been subjected to the same processes of alteration, replacement, and deposition that formed the greenstone gray ores. The altered schist consists mainly of ankerite, sericite, chlorite, quartz, and albite. Gold is associated with the pyrite and other sulfides that are present. Pyrite comprises about 8 percent of the rock. The average grade of mineralized schist is about 0.1 oz per ton. The Melones Fault Zone separates the Mother Lode Belt from the East Belt. The East Belt is dominantly argillite, phyllite and phyllonite, chert, and metavolcanic rocks of Paleozoic-Mesozoic age. Carbonate rocks (marble) are also present locally. The phyllite and phyllonite are dark to silvery gray. The chert is mostly thin-bedded with phyllite partings. The Upper Paleozoic-Lower Mesozoic metasedimentary and metavolcanic rocks of the East Belt have been assigned to the Calaveras Complex by most investigators (Earhart, 1988). The Lower Paleozoic metamorphic rocks farther east have been assigned to the Shoo Fly Complex. More recently, some geologists have reinterpreted certain assemblages along and immediately east of the Melones Fault Zone as separate Jurassic units (Schweickert and others, 1999). The metamorphic complexes are intruded in places by Mesozoic plutonic rocks. Lode deposits of the East Belt consist of many individual gold-bearing quartz veins enclosed in metamorphic rocks of possible Jurassic age, metamorphic rocks of the Calaveras Complex, metamorphic rocks of the Shoo Fly complex, or in granitic rocks. Most of the veins trend northward and dip steeply. An east-west set of intersecting faults may be a controlling factor in controlling deposition of ore. Ore deposits of the East Belt are smaller and narrower than those of the Mother Lode, but commonly are more chemically complex, and richer in grade. Gold is generally associated with appreciable amounts of pyrite, chalcopyrite, pyrrhotite, galena, sphalerite, and arsenopyrite. LOCAL GEOLOGY The Princeton vein, which is believed to be part of the Mother Lode system, occupies a fault that cuts slate of the Jurassic Mariposa Formation. The slate contains thin beds of dark-colored graywacke and is cut by dikes described by Bowen and Gray (1957) as sheared, fine-grained granitic porphyry. The porphyry may be the same rock that Clark (1970) described as pyritic metarhyolite, which in places contains gold. Some of these dikes intersect the vein. The trace of the vein is not well-defined at the surface and lacks the massive, multiple characteristics of the Mother Lode at a few mines farther north in the county. The vein is en echelon with veins at adjacent mines; it is probable that the Mother Lode in this vicinity consists of several separated veins rather than one or two major persistent features.

Princeton vein system details: The deposit at the Princeton Mine consists of a typical gold- and sulfide-bearing hydrothermal quartz vein within metamorphic rock. The Princeton vein occupies a fault that cuts slate of the Jurassic Mariposa Formation. The slate contains thin beds of dark-colored graywacke and is cut by felsic dikes, some of which intersect the vein. The vein lacks the massive, multiple characteristics of the Mother Lode farther north in the county. It strikes N54-57W and dips 45-60NE. The enclosing host rock strikes N35W and dips 70NE. Vein matter, which is ribboned, milky quartz that carries numerous parallel sheets of included wall rock, varies from 4 to 8 feet wide. Parts of the vein that constitute ore carry a considerable amount of pyrite. Native gold is seldom visible, but minor amounts of galena, sphalerite, and tetrahedrite are commonly seen. Locally, the slate is severely bleached and hydrothermally altered near the vein. The richest ore was found within 100 feet of the surface, in places running about $70 gold/ton. Relatively high-grade ore continued to the 600-level. Ore below this depth to the 1,200-level was more typically low- to moderate-grade with one barren zone. Ore at the 1,400 and 1,600 levels was too low-grade for mining. The main ore shoot in the mine was distinguished from the rest of the vein by the prominence of pyrite. One ore shoot was reported to be 600 feet long and about two feet thick.

Workings include underground openings with a total length of 4,907.28 meters and an overall depth of 487.68 meters. Opened to a depth of 60 feet by several shallow shafts and connecting drifts. The shafts have 9 levels with over 11,500 feet of drifts, over 3,000 feet of crosscuts and raises.

The Princeton Mine was developed by standard shafts, crosscuts, drifts, raises, and stopes. Within Mariposa County, the extent of the workings is exceeded only by those at the Pine Tree-Josephine Mine several miles to the north. The main shaft, which was caved and inaccessible in the mid-1950's, is 1,660 feet deep on the incline and reaches a vertical depth of about 1,250 feet. It has 9 levels, the longest of which is about 2,100 feet. Drifts approximate about 11,500 feet, while there are over 3,000 feet of crosscuts and raises. Stoping extended from the lowest level to the surface. Lesser workings were developed elsewhere on the Princeton property, but these had insignificant production compared to the workings of the main shaft. Julihn and Horton (1940) provided a sectional view of the workings in the plane of the vein.

Production information: Clark (1970) reported a production value of $5 million (period values) for this mine.

Commodity information: Most of the ore mined averaged about 0.2 to 0.3 ounces of Au/ton. Except for ore produced at the shallowest depths, the Princeton Mine was characteristically one of large ore bodies of relatively low grade.

Early ore yielded $70/ton (period values), later it averaged $7/ton (period values).

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Link to USGS MRDS:	10310670

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References