|Latitude & Longitude (WGS84):||38° 39' 58'' North , 119° 42' 12'' West|
|Latitude & Longitude (decimal):||38.66611,-119.70333|
|Köppen climate type:||Csb : Warm-summer Mediterranean climate|
A former Au-Cu-Pb-Zn-Ag-Sb-W mine located in secs. 31 & 32, T10N, R21E, MDM, 0.7 km (2,400 feet) SE of Colorado Hill (coordinates of record) and 0.6 km (2,100 feet) W of Loope, along Monitor Creek, on the lower slopes of the hill, on National Forest land (located claim). Discovered 1862. First operated 1864, through at least 1879. Two strikes were made (1867 & 1870), which stimulated production. Also operated during 1912; and intermittently during the period 1920-1968 (continuous production 1931-1941). MRDS database stated accuracy for this location is 100 meters.
Owned by Siskon Corp., Reno, Nevada, and leased to Claude Lovestedt, Minden, Nebraska (1977). Operated by the Western States Minerals Corp., Colorado (1990). Owned by the Siskon Gold Corp. (Centurion Gold Ltd.) (100.00%), Canada (1990). Owned and operated by the Western States Minerals Corp., Colorado (1997). Owned and operated by the Atlas Corp. (50.00%) (1983), and by the California Silver Ltd. (50.00%), Canada (1983).
Mineralization is a hydrothermal vein deposit hosted in andesite, rhyolite, tuff, basalt, and sedimentary breccia. The ore body is 120 meters thick, 15 meters wide, and 125 meters long. Ore body No. 1 is a tabular-shaped mass in a fissure vein. The No. 2 ore body is stockwork. The primary mode of origin is hydrothermal. Primary ore control was fracturing and the secondary was faulting. Wall rock alteration is moderate (sericitic, silicification and intermediate argillic).
The deposit is in silicified, altered rocks which are: 1.) dark gray, finely porphyritic, andesitic basalt; 2.) white and yellow to reddish-brown rhyolite locally showing abundant thin ribbons of gray quartz; and, 3.) some blocks of whitish-gray lapilli-tuff breccia, which apparently have been caught up in the rhyolite during its intrusion. The intrusive contact between 1 & 2 is exposed at the Colorado adit portal, & in parts of the Colorado, Advance and Upper Advance adits. Locally contacts sheared "ribs" of rhyolite & rhyolite breccia, to 50 feet thick outcrop on Monitor Pass Road. Underground, No. 2. is gray to white and contains disseminated sulfide minerals. Mineralized areas occur in soft clay pockets a few to to several feet in the longest dimension parallel to, and cross-cutting quartz banded zones and in soft, friable cores of anticlinal structures. Since 1960 these cores have produced the high-grade ore. Rhyolite cavities contain gray, euhedral quartz crystals up to ¾ inch long, on which crystals of rhodochrosite, pyrite, hübnerite and other Ag-bearing sulfites (?) (probably sulfides) may have developed.
The ore occurred in bunches and was associated with pink and black manganese minerals. Benstron, cited in Clark, 1977, felt that hübnerite was an indicator of high-grade ore. The tetrahedrite is Ag-rich.
Local rocks include Tertiary pyroclastic and volcanic mudflow deposits, unit 9 (Cascade Range).
From Wachter (1971): There are several types of ore in the underground workings. 1. Ore in lower levels consisted of disseminated sulfides in banded rhyolite and rhyolite breccia (grains, tiny pockets, and veinlets of quartz, pyrite, sphalerite (and exsolved chalcopyrite), galena, tetrahedrite, and silver sulfosalts (mostly pyrargyrite)). Values in silver equivalent usually no more than 14 ounces per ton silver. 2. High-grade veinlets (1 to 1/4 inch) appear in the Colorado workings. These veinlets have the previously-mentioned minerals plus polybasite and coarse native gold. There is no evidence of supergene enrichment in either type of ore. 3. Sericite bodies (approaching 100% sericite) in new workings just under the hill face have, in their deeper portions, sulfides similar to the previously-mentioned ore type 1. Here, the silver minerals are disseminated grains. Pockets of clay/sericite have grains of silver minerals and can be high-grade. Paragenesis of Zaca ore: 1. Quartz-rhodochrosite-pyrite. 2. Quartz-pyrite (+molybdenite?). 3. Quartz-sphalerite-galena (+huebnerite). 4. Tetrahedrite-polybasite-pyrargyrite. 5. Proustite-acanthite-covellite (supergene). Alteration: In altered rhyolite, sericite and quartz are pervasive. K-feldspar (microscopic aggregates) also usually present. "Jasperoid" pinnacles above Zaca are rhyolite that has been altered to 65-95% quartz.
Workings include surface and underground openings and operations with an overall length of 304.8 meters and an overall width of 182.88 meters, and an overall depth of 213.36 meters.
Several adits were driven at thousands of feet long. Extensive crosscut drifts and winzes, plus stopes (see Evans, et al (19966)). One adit was driven 700 feet below the outcrop, 660 feet from the portal the adit is forked. There are also over 2,500 feet of drifts and crosscuts. A caved shaft was said to be 300 feet deep at the portal of the adit.
Production data are found in: Clark (1977).
Production information: Small shipments estimated with a value of over $50,000 (period values). A small shipment in 1920 reported yields of 100 ounces Ag and $20 Au/ton (period values).
Peter Curtz operated the mine in 1879, employing metallurgist Ottokar Hoffman to apply the chlorination method to recover gold and silver. Ore was crushed and the sulfides were concentrated. Concentrates averaged $585/ton (period values), with approximately 38 to 43% of the value in Au and the remainder in Ag. Recovery was 91 to 95% (Clark 1977; Prenn and Merrick 1991).
Several thousand dollars worth of ore was taken out in 1912 (period values).
Claude Lovestedt operated the Zaca mine from 1961 to 1981. Approximately 20,000 tons of ore were mined and a flotation mill was used to concentrate the ore. Concentrates were shipped to smelters, including the Selby smelter. Typical ore grades (before concentration) were 5-7 ounces of Ag, 0.1 ounce of Au, and several pounds of Pb, Zn, and Cu per ton (Prenn and Merrick 1991).
More complete accounts of production from 1920-1968 are found Clark (1977).
Analytical data results: 1867: $150/ton Au, $800/ton Ag (period values).
Reserve-Resource data are found in: Randol Mining Directory, 1996/97: 170.
Reserves data: 1996: Demonstrated 8,532,979 metric tons ore at Au of 0.994000 grams/metric ton and Ag at 19.542000 grams/metric ton.
Proposed future development of this site is based on drill-indicated reserves (in situ), containing an estimated 6,620,000 metric tons averaging 1.03 Grams/metric ton of Au and 24.0 grams/metric ton Ag. Recoverable indicated reserves using a 20% dilution factor are estimated to be 7,944,000 metric tons at a grade of 0.86 grams/metric ton Au and 20.0 grams/metric ton Ag. All infrastructure costs have been included with mine cap total. Mine cap is estimated to be 30%, and mill cap 70% of working capital.
20 valid minerals.
This geological map and associated information on rock units at or nearby to the coordinates given for this locality is based on relatively small scale geological maps provided by various national Geological Surveys. This does not necessarily represent the complete geology at this locality but it gives a background for the region in which it is found.
Click on geological units on the map for more information. Click here to view full-screen map on Macrostrat.org
|Gelasian - Chattian|
1.806 - 28.1 Ma
|Tertiary pyroclastic and volcanic mudflow deposits, unit 9 (Cascade Range)|
Age: Cenozoic (1.806 - 28.1 Ma)
Stratigraphic Name: Alturas Formation; Bonta Formation; Cedarville Series; Delleker Formation; Ingalls Formation; Mehrten Formation; Nomlaki Tuff Member; Penman Formation; Reeds Creek Andesite; Spanish Canyon Formation; Tropico Group; Tuscan Formation; Valley Springs Formation
Description: Tertiary pyroclastic and volcanic mudflow deposits.
Reference: Horton, J.D., C.A. San Juan, and D.B. Stoeser. The State Geologic Map Compilation (SGMC) geodatabase of the conterminous United States. doi: 10.3133/ds1052. U.S. Geological Survey Data Series 1052. 
66 - 145 Ma
|Mesozoic intrusive rocks|
Age: Cretaceous (66 - 145 Ma)
Lithology: Intrusive igneous rocks
Reference: Chorlton, L.B. Generalized geology of the world: bedrock domains and major faults in GIS format: a small-scale world geology map with an extended geological attribute database. doi: 10.4095/223767. Geological Survey of Canada, Open File 5529.