The Mohawk Mine

The Mohawk Mine lies within the southwest portion of the Ivanpah Mining District, approximately 3 km east-northeast of the Cima Road exit from I-15. As we drive toward the mine workings, clearly visible above a series of tailings piles, note the "dirty brown" coloration of the rock immediately above the workings. This is the result of circulating groundwater which extensively oxidized the primary sulfide mineralization during Tertiary to Recent time. While the coloration is due to the presence of several different mineral phases, most are forms of hydrated iron and manganese oxide.

The Mohawk Mine was worked briefly during World War I (1916­1918). Three hundred tons of hand cobbed ore were shipped yielding 4 oz. of gold, 20,000 lbs. of copper and 250,000 lbs. of lead. The property lay idle until its acquisition in 1942 by the Ivanpah Copper Company. Production records are incomplete, but from 1942 to 1952 Ivanpah Copper reported the shipment of 16,700 tons of ore which produced 206 oz. of gold, 92,802 oz. of silver, 183,600 lbs. of copper, 2.9 million lbs. of lead and one million lbs. of zinc (Hewett, 1956). Mapping of the existing underground workings, which consist of seven adits, nine shafts and five prospect pits, suggests that significant unreported production may have occurred after 1952. The mine has been inactive since 1957 (Evans, 1958)

Stratigraphy

All productive mine workings on Mohawk Hill lie within the Bonanza King Formation. The Mesquite Pass thrust which marks the contact between the upper plate Cambrian Bonanza King and lower plate Devonian Sultan Formation lies 2 km east of the property. A small pod of Tapeats Quartzite (?) has been mapped in fault contact with the Bonanza King near the west edge of the mine property.

The Bonanza King consists essentially of two units; alternating thin bedded blue­gray and tan­white limestone and white dolomitic marble. Locally, a fault contact separates limestone from dolomitic marble. The Bonanza King strikes northwest and dips 20­30 to the southwest (Hewett, 1956 and Evans, 1958). However, along the crest of Mohawk Hill extreme local variation in both dip angle and direction can be noted.

A small outcrop of the Clark Mountain (Cretaceous?) stock occurs along the south flank of Mohawk Hill. Its contact with the Bonanza King strikes east­west and dips 30 to the south. Drilling indicates the intrusive is sill­like, averaging 40 meters in thickness (Wiebelt, 1949). The stock was mapped by Hewett (1956) as a quartz monzonite, but thin section analysis indicates a true granite.

A massive quartz "vein" was reported by Hewett (1956) at the intrusive­Bonanza King contact. The "vein" is clearly visible as a ridge of bull quartz a few meters thick trending northeast along the south flank of Mohawk Hill. Several smaller quartz veins lie to the east, wholly within the intrusive. Although the quartz vein is unmineralized at the surface, in underground workings it is locally replaced by ore mineralization. Joseph (1984) suggests that the quartz "vein" of Hewett is a skarn zone formed during intrusion of the Clark Mountain stock.

Structure

Three faults have been mapped on the mine property; all trend roughly north­northeast. The westernmost of the three faults was mapped initially by Dobbs (1961). He suggests the fault is a low angle west-to-east thrust with Tapeats Quartzite above Bonanza King. A second fault lies just to the west of the mine workings on the south slope of Mohawk Hill, its trace closely marked by the prominent quartz vein near the base of the ridge and a well defined zone of mylonitic deformation near the crest of the ridge. Both the footwall and hanging wall of the fault are Bonanza King Formation. Footwall carbonates have been weakly to moderately recrystallized to a medium grained marble. Burchfiel and Davis (1971) mapped this fault as a thrust, but more recent mapping suggests the last sense of movement on the fault was normal with the northwest side down. The easternmost fault has a nearly north­south strike, steep dip to the west and less than 10 meters of displacement.

Ore Mineralogy and Alteration

The ore mineralogy at the Mohawk Mine is complex. Cerrusite is dominant, however smithsonite is common. Minor malachite, azurite and chrysocolla can be seen on the mine dumps. Galena and sphalerite were reported by Hewett (1956), and native silver and gold by Evans (1958). Gangue consists of abundant iron and manganese oxides in a matrix of coarsely­crystalline quartz and calcite. Minor jarosite, plumbojarosite and pyrite were also noted.

Hypogene alteration consists of intense silicification, widespread recrystallization of the Bonanza King Formation and local sericitic and argillic alteration of quartz monzonite. Weathering has produced a prominent secondary gossan of iron oxides overlying the ore zone.

Ore mineralization lies within a zone of moderately to intensely silicified and recrystallized limestone and dolomite of variable thickness (2­10 meters). Previous workers (Joseph, 1984) have characterized this zone as a tactite or skarn adjacent to the intrusive. The only indication of the typical calc­silicate alteration associated with skarn mineralization is a small exotic block of talc­tremolite schist on the north side of Mohawk Hill, minor talc­serpentine alteration of calcite and scattered grains of epidote and idocrase in largely unaltered Bonanza King Formation several hundred meters from the intrusive contact. Quartz monzonite adjacent to ore zones has been subjected to moderate to strong sericitic alteration and weak argillic alteration. Biotite has altered to chlorite and a mixture of iron oxides while feldspar has altered to sericite + clay.

Copper World Mine

The Copper World Mine is located on the southeast slope of Clark Mountain, 1.5 km north of the Mohawk Mine. During the early history of the Ivanpah District the Copper World was among the most productive of the mines. Production was recorded from 1906 to 1908 and again from 1916 to 1918 (Hewett, 1956). Several thousand feet of underground workings are present although most are now inaccessible. The large adit near the present open pit was driven in 1977 to recover specimens of malachite and azurite for sale to mineral collectors.

Stratigraphy

The host rock for the ore deposit is again Cambrian Bonanza King Formation. Lithologically, the rock differs little from that at the Mohawk Mine to the south. Although, the area around the mine is structurally complex, the general strike is west­northwest with a gentle to moderate dip to the south­southwest. Near ore, the Bonanza King is strongly bleached and extensively recrystallized. Several thin sills of Clark Mountain stock intrude the Bonanza King along bedding planes. Generally, the sills dip to the south. Petrologically, the stock at this locality differs significantly from the outcrops on Mohawk Hill. The rock is noticeably finer grained, contains much less quartz and has abundant biotite and amphibole.

Structure

Local structure is complex and the following brief synopsis probably does not do justice to the structural picture. Numerous tight folds in the Bonanza King Formation can be seen in the hillside to the north of the mine. All trend roughly north­south and often have steep to overturned limbs. Two generations of faults were reported by Hewett (1956); one set cutting both the Bonanza King and monzonite strikes northwest and dips steeply southwest, while a second set of post-mineral faults strikes northeast and dips to the northwest.

Mine Geology

All mine workings lie within the Bonanza King Formation adjacent to contacts with quartz monzonite. Underground workings intersect two south dipping sills while a third crops out at the surface. The lower sill is 4 meters thick, the middle 15 meters thick and the upper 5 meters thick (Hewett, 1956). Post ore faulting has significantly complicated the geologic picture.

Mineralization generally lies at the intrusive­carbonate contact, but Hewett (1956) states that a series of northwest trending quartz veins within the intrusive and Bonanza King may be mineralized.

Mineralization and Alteration

Both mineralization and alteration exposed in the open pit are striking with the copper carbonates, malachite and azurite quite apparent. In addition, other secondary copper minerals such as chrysocolla and brochantite can be seen on the dumps. All these species are the consequence of supergene alteration of the pre­existing sulfide deposit. Our cursory examination of the dumps has revealed the presence of chalcopyrite, pyrite and chalcocite. Other primary copper sulfides, as well as galena and sphalerite, are undoubtedly present in small amounts.

Alteration consists of hypogene as well as supergene mineral assemblages. Biotite in the quartz monzonite is pervasively altered to chlorite. The host Bonanza King has been altered to epidote near the intrusive contact and silicified up to 30 meters from the contact. Hewett (1956) also reported the presence of serpentine(?) and brucite replacing epidote. Supergene alteration consists of the aforementioned copper carbonates and silicates as well as abundant boxwork aggregates of iron and manganese oxides.

REFERENCES CITED

Burchfiel, B.C., and Davis, G.A., 1971, Clark Mountain thrust complex in the Cordillera of southeastern California: geologic summary and field trip guide, in Geological excursions in southern California, Elders, W.A., Ed., University of California­Riverside, Campus Museum Contributions No. 1, p. 1­28.

Dobbs, P.H., 1961, Geology of the central part of the Clark Mountain range, San Bernardino County, California, Unpublished M.S. Thesis, University of Southern California, 115 p.

Evans, James M., 1958, Geology of the Mescal Range, San Bernardino County, California, Unpublished M.S. Thesis, University of Southern California, 118 p.

Hewett, D.F., 1956, Geology and mineral resources of the Ivanpah quadrangle, California and Nevada: U.S. Geol. Survey Professional Paper 275, 172 p.

Joseph, Steven E., 1984, Mineral land classification of the Mescal 15' quadrangle, San Bernardino County, California: Calif. Division of Mines and Geology Open File Report 84­2 LA, 52 p.

Wiebelt, Frank J., 1949, Investigation of the Mohawk Lead­Zinc Mine, San Bernardino County, California:U.S. Bureau of Mines Report of Investigations 4478, 7 p.