The
Mohawk Mine lies within the western portion of the Ivanpah Mining
District, five kilometers east-northeast of the Interstate 15
Cima Road Exit. The mine was worked briefly during World War I
(19161918). Three hundred tons of hand cobbed ore were shipped
yielding four ounces 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).
Hewett (1956) published a reconnaissance-scale map of the Ivanpah
district mentioning the Mohawk Mine in the context of ore reserves
and general geology. Evans (1958) mapped the Mescal Range to the
south, briefly discussing the Mohawk Mine. Dobbs (1961) mapped
Mohawk Hill in greater detail and recognized the presence of both
Tapeats and Bright Angel strata. Burchfiel and Davis (1971) prepared
the first comprehensive structural map of the Mescal Range, Clark
Mountains and Mohawk Hill. This paper presents a structural interpretation
of the ore deposits near the west end of Mohawk ridge. While relying
on previous work, we have based much of our interpretation on
detailed surface and underground mapping and thin section petrology.
We present a model for emplacement of the ore mineralization and
Mesozoic/earlyCenozoic tectonics of Mohawk Hill.
All productive mine workings on Mohawk Hill lie within the Bonanza King Formation (Geologic Map). The Mesquite Pass thrust which marks the contact between the upper plate Cambrian Bonanza King and lower plate Devonian Sultan Formation lies two kilometers east of the property. The Winters Pass thrust is thought to lie approximately three kilometers west of Mohawk Hill, but its presence has never been verified with certainty.
The Bonanza King, in the vicinity of the mine, consists essentially of two units; thin-bedded, bluegray and tanwhite limestone and brown to white, dolomitic marble. Hewett (1956) and Evans (1958) report that regionally the Bonanza King strikes northwest and dips 2030° to the southwest. Our mapping along Mohawk ridge suggests strikes from N 20°W to N 50°W with highly variable dips, generally to the southwest (see Geologic Map). Near the crest of the ridge, immediately above the West Adit, extreme variations in dip and strike were noted. Preliminary mapping suggests folding along a nearly east-west axis related either to intrusion of a sill-like mass of granite (discussed below) or drag folding accompanying faulting.
The thin bedded limestone, which is best exposed to the west of the mine workings, closely resembles outcrops of Bonanza King Formation (formerly Goodsprings) described by Hewett (1931) from the southern Spring Mountains. The dolomitic marble, well exposed on the high hill east of the East Adit, was examined in thin section (CNL) and found to consist of an equigranular (~2.0 mm) mosaic of subhedral to euhedral calcite (96%) cut by veinlets of opaque minerals (hydrated oxides of iron and manganese) (4%). Locally, the unit is both dolomitized and silicified. In general, the two units are in fault contact, but careful examination of the hillslope above the mine workings reveals small patches of thin-bedded limestone which are gradational to marble, suggesting the dolomitic marble represents recrystallized Bonanza King.
Dobbs, (1961) was the first to recognize the presence of a small outcrop of Tapeats Quartzite at the west end of Mohawk Hill. Since this juxtaposes Tapeats and Bonanza King, Dobbs concluded a fault must be present. Burchfiel and Davis (1971) mapped the fault as the Mohawk thrust. The present authors have remapped the area in detail and concluded that not only is the Tapeats present, but also the Bright Angel (Carrara). Furthermore, the Tapeats-Bright Angel contact is conformable.
The Tapeats is the more resistant of the two units. It has been extensively recrystallized into hard, blocky quartzite. Relict bedding indicates the unit strikes N 50-60°E and dips 40-45°NW. The Bright Angel is a gray-green, moderately fissile shale. Its attitude is similar to the Tapeats. The Bright Angel weathers readily making its contact relationship with the Bonanza King unclear. This is unfortunate since apparent outcrop thickness suggests the unit has either been remarkably attenuated near Mohawk Hill, or lies in fault contact with the Bonanza King. The latter interpretation is favored due to the difference in strike of the Bonanza King (northwest strike) and Tapeats-Bright Angel (northeast strike). One additional observation can be made regarding the Bright Angel. Near its contact with the Tapeats the unit is only weakly metamorphosed. To the east, metamorphic grade increases, to phyllite and eventually talc schist.
A coarse-grained, igneous intrusive, previously termed Teutonia batholith, outcrops along the south flank of Mohawk Hill. Thin section analysis (CNL) suggests the rock is a granite comprised of 45% quartz, 30% perthitic feldspar, 10% mica, 15% alteration minerals (talc, serpentine and sericite) and opaques. The contact of the granite with the Bonanza King strikes generally eastwest and dips 30° to the south. Drilling indicates the intrusive is silllike, averaging 40 meters in thickness (Wiebelt, 1949). Wise (1990) suggests the intrusive lies in fault contact with the Bonanza King. Our mapping (DWT, DRJ) indicates the contact is intrusive. Evidence includes extensive recystallization of the Bonanza King to the north and east of the intrusive; presence of minor calc-silicate minerals including epidote, chlorite, idocrase and diopside and hydrated calc-silcates such as talc and serpentine along the intrusive-Bonanza King contact, and a lack of features normally associated with a kinematically emplaced pluton such as brecciation or localized shearing in adjacent rocks. Further, Hazelton (1991) studied sill-like intrusives along the south margin of Clark Mountain and discovered several west-northwest trending, south plunging fold axes related to emplacement of the sills. Limited mapping (DRJ) suggests the presence of a comparable west trending, south plunging fold near the crest of Mohawk Ridge, possibly, related to emplacement of the Mohawk sill. It should be noted that a similar controversy regarding the contact relationship between intrusive and carbonate host has raged for over fifty years in the Goodsprings District, 25 kilometers to the east. The recent excavation of a gold prospect in upper Keystone Wash has created a spectacular exposure along 100 meters of the intrusive-carbonate contact, clearly showing the intrusive relationship of the pluton.
Recent research (Walker, Burchfiel, Davis, 1995) has resulted in a reinterpretation of ages for many of the intrusives in the Clark Mountains and Mescal Range. Crosscutting relationships near Pachalka Springs, seven kilometers northwest of Mohawk Hill, constrain initial thrusting to between 146 and 142 Ma and also provide evidence for a late Jurassic magmatic event. Since compositional and structural similarities exist between intrusives of the southern Clark Mountains and Mohawk Hill, it seems reasonable to postulate a late Jurassic age for the Mohawk granite.
A massive quartz "vein" was first reported by Hewett (1956) at the intrusiveBonanza King contact. The vein trends N 60°E along the south flank of Mohawk Hill. Several smaller quartz veins lie to the east, wholly within the intrusive. Sampling indicates the quartz is unmineralized. Joseph (1984) suggests that the quartz "vein" of Hewett is a skarn zone formed during intrusion of the Clark Mountain stock. Our mapping does not support this conclusion, but suggests the quartz vein may be related to more recent Tertiary extension. Evidence includes the proximity of the vein to a nearby normal fault and the presence of quartz crystals up to 15 cm in length within the vein, an open-space texture consistent with dilation during extensional tectonism.
Dobbs (1961) states that a low angle northeast-trending, northwest dipping thrust fault (Mohawk Thrust) has transported Tapeats Quartzite to the southeast onto Bonanza King. Our mapping reveals the presence of Bright Angel (Carrara) between Tapeats and Bonanza King removing the need for a fault. However, several aspects of the stratigraphic relationships have lead the authors to continue to map the Bright Angle and Bonanza King in fault contact. Both Tapeats and Bright Angel strike northeast and dip northwest while the Bonanza King strikes northwest and dips southwest. The Bright Angel section appears greatly attenuated at the west end of Mohawk Hill when compared to exposures five kilometers to the northwest. Furthermore, Walker, Burchfiel and Davis (1995 indicate the Bright Angel-Bonanza King contact is marked by a similar small thrust fault near Pachalka Spring.
A second fault lies just to the west of the West Adit mine workings on the south slope of Mohawk Hill. Its trace closely parallels that of the prominent quartz vein near the base of the ridge and a well defined zone of mylonitic deformation near the crest of the ridge. The fault strikes N 60-80°E and dips moderately to the northwest. Both the footwall and hanging wall of the fault lie within the Bonanza King Formation. Footwall carbonates have been moderately to completely recrystallized to a medium-grained marble while hanging wall rocks are unmetamorphosed. Mapping suggests dip-slip motion with the hanging wall (northwest) block down. The fault cuts recrystallized Bonanza King and a N 50°E trending splay offsets the prominent quartz vein. Additionally, the fault is unmineralized and appears to down-drop a portion of the west ore body. We conclude the fault is related to Tertiary extension.
Mapping of the West Adit has revealed a shear set trending N 30-50°W. The shears offset the intrusive causing brecciation of both the granite and the Bonanza King immediately adjacent to the contact. Intensity of shearing decreases eastward along the contact and outward from the intrusive. Ore body geometry suggests the shear zones acted as loci for initial hypogene mineralization.
Exact timing of the shearing is enigmatic. It postdates intrusion, but predates mineralization and Tertiary normal faulting. Hazelton (1991) mapping near the Copper World Mine recognized a series of fold axes related to east-directed thrusting. Such east-directed thrusting would cause ductile deformation of carbonates and brittle deformation of intrusives. Thus, the northwest-trending shear zones of Mohawk Hill may be a manifestation of thrust faulting. If the work of Walker, Burchfiel and Davis (1995) can be extrapolated to the south, the Mohawk granite is younger than the late Jurassic Winters Pass/Pachalka (Mohawk) thrust. Therefore, shearing might have occurred during Cretaceous compression related to the Mesquite Pass or Keaney-Mollusk Mine thrust.
In as much as the purpose of this paper is to detail stratigraphic and structural relationships at the Mohawk Mine, only a brief discussion of ore mineralogy and alteration is included. For a more detailed discussion the reader is referred to Wise (1990) and Fallis (1990). 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, and pyrite are also present.
Hypogene alteration consists of silicification, widespread recrystallization of the Bonanza King Formation, skarnification and local sericitic and argillic alteration of granite. Weathering has produced a prominent secondary gossan of iron oxides overlying the ore zone.
Ore mineralization lies within a zone of moderately 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. Indications of the typical calcsilicate alteration associated with skarn mineralization include a small block of talctremolite schist on the north side of Mohawk Hill and local grains of epidote, diopside and idocrase in extensively recrystallized Bonanza King. Granite 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. Locally, up to 3% of the granite is comprised of serpentine and talc. Since the latter are typical hydration products of calc-silicate minerals, it seems probable the granite was characterized by a zone of endoskarn mineralization which subsequently underwent hydration.
This paper presents a model for emplacement of mineralization at the Mohawk Mine. The first structural event was late Jurassic thrusting resulting in formation of the Mohawk thrust. Evidence for this event includes structural discontinuities between the Bonanza King Formation and juxtaposed Bright Angel/Tapeats; attenuation of the Bright Angle section; and the presence of a similar thrust fault to the northeast near Pachalka Springs. Thrusting was closely followed by intrusion of the granitic pluton. A similar granitic intrusive seven kilometers to the northwest yielded an age of 142 Ma (late Jurassic). The intrusive nature of the contact with the Bonanza King is supported by extensive recrystallization of the Bonanza King, calc-silicate alteration of the carbonate and intrusive, folding related to ductile deformation of the Bonanza King during intrusion and a general lack of a brecciation and shearing along much of the contact.
Cretaceous compression, perhaps related to formation of the Mesquite Pass or Keaney-Mollusk Mine thrusts caused the formation of northwest striking shear zones within the granitic sill and associated brecciation of the intrusive-Bonanza King contact. Mineralization occurred when pore waters were expelled from the sedimentary host rock into zones of low pressure (i.e. breccia zones) as a consequence of the compression. While the timing of mineralization is uncertain, it clearly postdates the intrusive and northwest shearing, but predates Tertiary normal faulting. The final event depicted is Tertiary extension which resulted in the northeast-striking normal fault that offsets mineralization, alteration and the quartz vein.. Not shown is the subsequent Quaternary denudation and oxidation of the ore deposits.
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.
Fallis, C. Nancy, 1990. Mineralogy and alteration associated with the ore deposits of the Mohawk Mime, San Bernardino County, California, Unpublished Senior Thesis, Calif. State Polytechnic University-Pomona, 83 p.
Hazelton, Garrett B., 1991, Structural and stereographic analysis of folds in the southwestern portion of the Clark Mountain Range, San Bernardino County, California, Unpublished Senior Thesis, Calif. State Polytechnic University-Pomona, 25 p.
Hewett, D.F., 1931, Geology and ore deposits of the Goodsprings quadrangle, Nevada; U. S Geol. Survey Professional Paper 162, 172 p.
__________, 1956, Geology and mineral resources of the Ivanpah quadrangle, California and Nevada: U. S Geol. Survey Professional Paper 275, 172 p.
Jessey, David R., and Fallis, C. Nancy, 1988, The Mohawk Mine: A base metal-silver deposit related to possible late Cretaceous normal-slip movement within the Clark Mountains thrust complex, San Bernardino County, California: California Desert Mineral Symposium Compendium, p. 163-176.
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.
Walker, J. Douglas, Burchfiel, B. C., and Davis, Gregory A., 1995, New age controls on initiation and timing of foreland belt thrusting in the Clark Mountains. Southern California: Geological Society of America Bulletin, June 1995, p. 742-750. Wise, William B., 1990, The mineralogy of the Mohawk Mine, , San Bernardino County, California: San Bernardino County Museum Association, vol. XXXVII, no. 1, 30 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.
