BAM FIELD TRIP REPORTS

No. 1 - Garnet Hill

by J. F. (Fen) Cooper

LOCATION

Garnet Hill Mine, Calaveras County, California

DESCRIPTION OF AREA

The Garnet Hill Mine is located about 40 miles east of Jackson near the confluence of the Mokelumne River and Moore Creek. Garnet Hill is just south of the Mokelumne River and is accessible by Forest Service Roads from several points on Highway 88 to the point at which the road to the mine joins them. Garnet Hill is located in sections 6 and 7, T. 7N., R. 16 E, M.D.M. and at latitude 38^o 28' 23" North and Longitude 120^o 15' 23" West on page 99 of Delormeís Northern California Atlas and Gazetteer.

Map of the Garnet Hill Region. Click image to expand (392k).

Map of Garnet Hill & Vicinity. Click image to expand (332k).

Note the Garnet Hill Forest Service Campground on the Mokelumne River is located about one mile west of Garnet Hill.

The first recorded activity in the area was during World War 1 when the tungsten ores in the area were first developed but the fine garnet and epidot crystals from the area have long been noted. In 1896 the first reports on these crystals were published and over the years many papers have been published on the area. Mining activity has been sporadic and the mine is active only when tungsten prices permit. The most recent commercial activity was during the 1950s when tungsten prices were high but this activity was short lived and at present most of the activity at Garnet Hill is centered on the production of andradite and epidot specimens.

The workings are in two separate areas. At the summit of Garnet Hill several pits explored the contact zone and the bulk of the early production came from this area. Near Moore Creek two adits were driven to explore another contact zone and the bulk of the later production came from this area.

GEOLOGY AND MINERALOGY

Garnet Hill is located on the contact between granodiorite mass and line rich sediments. The rock to the south is composed of dark metamorphic slates that belong to the Calaveras formation. In the area of Garnet Hill the contact zone averages 20 to 30 feet thick with a north-south trend. The thickness varies considerably and in the area near the summit the contact zone is over 100 feet wide. The contact zone shows variations in mineralogical content with the Garnet Hill workings containing a greater preponderance of quartz and garnet while in the Moore Creek area epidot is more abundant though the crystals are smaller.

The Garnet Hill workings are weathered and sulfides are almost entirely absent in this area. Garnet is abundant in the area and most of the garnet is the iron rich variety andradite. The tungsten ore mined from this area consisted of massive garnet that contained small grains of scheelite scattered through it. In the Moore Creek area the veins contain a quartz-epidot rock. This area is not as highly weathered as the Garnet Hill area and sulfides are more often found in this area.

The minerals found in the area include a number of minerals common to the tactites found in the contact zones of the Sierra foothill belt.

ANDRADITE The Garnet Hill area produces the best garnet specimens. Garnet Hill andradite is commonly dark reddish brown to brown and most crystals found are opaque with semitransparent specimens being extremely rare. The crystals are dodecahedral and often show modifying faces with the largest crystals being the most highly modified. The garnet masses often contain pockets and these cavities are often lined with large garnet crystals. While material of this type will produce the largest specimens with crystals up to several inches across the crystals tend to be less perfectly formed. Occasional garnet masses also are made up of small individual crystals in loosely consolidated masses. While masses of this type produce large numbers of crystals they are commonly intergrown and crystals showing all the faces are rare.

Andradite on Epidot, Miracle Mountain Mine, Garnet Hill.       MORE HERE

Andradite is also found in quartz pockets as individual andradite crystals that may reach up to and inch. These crystals are generally tightly frozen in quartz but occasional areas have crystals that may be broken free without damage. Crystals from this type of material show bright faces and often have a textbook form.

CHALCOPYRITE Chalcopyrite has only been noted from the Moore Creek workings where it may be found as massive pieces embedded on quartz.

EPIDOT Epidot from Garnet Hill is also justly famous and large crystals are found here. The Garnet Hill workings have produced some exceptional specimens. The finest material from this area occurred as isolated pistachio green terminated prismatic crystals that reach several inches in length and are embedded in a pale green amphibole. Epidot also occurs as massive material. Pockets in this material can produce some large crystals that are well formed.

Epidote. Garnet Hill.       MORE HERE

While the Garnet Hill area produces the largest epidot crystals the Moore Creek workings produce it in the greatest abundance. It commonly forms pencil-like prismatic crystals embedded in quartz and masses of crystals are common.

FERROAXINITE Ferroaxinite has been found at the Moore Creek workings and some exceptional specimens were collected in the 1960s. The ferroaxinite crystals occurred as pinkish brown crystals that reached up to 2 inches across and groups of large crystals up to a foot long were found. The ferroaxinite crystals were often associated with small spinel crystals.

FERRIMOLYBDITE Ferrimolybdite is not common at Garnet Hill and has been noted only in small quantities from the Moore Creek workings. It occurs as bright yellow massive coatings on epidot. Only a few specimens were ever found and the mineral must be considered as rare at this locality.

ORTHOCLASE Orthoclase crystals are occasionally found in the contact zone near the contact with the granodiorite in small pegmatite like masses. The orthoclase crystals can reach several inches in size and are typically pink in color and untwined.

MOLYBDENITE Molybdenite is probably the most common sulfide found at the Moore Creek workings where it occurs as small isolated crystals and masses embedded in quartz and epidot. The molybdenite crystals found here are frequently well formed and locally abundant.

QUARTZ Quartz is ubiquitous at Garnet Hill but good specimens are uncommon and only rarely will crystallized specimens be encountered. Probably Garnet Hill produces the "best" quartz crystal groups but these tend to have a weathered surface and are commonly iron stained.

POWELLITE Powellite is not common at Garnet Hill and occurs in the Moore Creek workings as yellow massive material replacing molybdenite. It shows a bright yellow fluorescence under short wave ultraviolet light.

SCHEELITE Scheelite is ubiquitous at Garnet Hill occurring in both the Garnet Hill and Moore Creek workings. In both localities the scheelite occurs as small anhedral grains though crystals up to two inches in size were found during the early mining days. The scheelite is highly fluorescent under short-wave ultraviolet light but shows a yellow color due to its high molybdenum content.

SCHORL Schorl is found as black hair like masses in epidot and is most common in this form. Occasional "sunburst" groups of schorl are found on the contact with the granodiorite but material of this type is rare.

SPHENE Sphene is common as a rock forming mineral but specimens of ferroaxinite crystals often show small yellow crystals of sphene embedded in the crystal faces.

COLLECTING AT THE LOCALITY

Garnet Hill has been known as a source of excellent garnet and epidot specimens for many years. The area has been heavily picked over by collectors for many years. Finding good specimens requires a certain amount of luck and a great deal of rock breaking. Garnet Hill itself is probably the best source of specimens as large quantities of specimen material were discarded during mining and this material has the greatest specimen potential. The Moore Creek locality does not have the same potential for good specimens but has a greater variety of species present.

REMARKS

The Garnet Hills Tungsten Mines are patented ground and while no mining has taken place for many years the area is usually under lease for collecting garnet and epidot specimens. Currently Steve Perry controls the property and permission for entry must be obtained through him.

REFERENCES

Cooper, J.F., Unpublished field notes dated 10/10/60, page 23, 7/22/62, page 82.

Clark, W.B. & Lydon, P.A., (1962) Mines and Mineral Resources of Calaveras County, California, California Division of Mines and Geology County Report 2.