Mesquite Mine

Producer in Imperial county in California, United States with commodities Gold, Silver, Arsenic, Molybdenum, Lead, Antimony, Copper, Tellurium

Sections on this page

Identification information
Geographic coordinates
Site location context
Geographic areas
Public Land Survey System information
Commodities
Materials information
Alteration
Mineral occurrence model information
Host and associated rocks
Nearby scientific data
Geologic structures
Ore body information
Controls for ore emplacement
Economic information about the deposit and operations
Mining district
Land status
Ownership information
Bibliographic references
General comments
Reporter information

Geologic information

Identification information

Deposit ID	10310647
Record type	Site
Current site name	Mesquite Mine
Alternate or previous names	Big Chief Pit, Vista Pit, Rainbow Pit, Cherokee Pit, Lena orebody, Gold Bug orebody

Comments on the site identification

The Mesquite Mine is located in the historic Mesquite Mining District of Imperial County. Mexicans and railroad workers first worked the local placer deposits as early as the 1870s and intermittent attempts to mine the placers continued through 1968. The modern Mesquite Mine is an open pit, cyanide heap leach gold mining operation first opened by Gold Fields Operating Company in 1982 after discovery of the Big Chief orebody. Production commenced in 1986 after a pilot project demonstrated the feasibility of cyanide heap leaching. Currently, the mine is operated by Newmont Mining Corporation and ranks as the largest active gold mine in California. Operations comprise four open pits including the Big Chief, Cherokee (now closed), Vista, and Rainbow pits, seven large leach pads, and ancillary processing, recovery, maintenance, and administrative facilities. To date, the Mesquite Mine has produced in excess of 3 million ounces of gold.

Geographic coordinates

Geographic coordinates:	-114.9952, 33.06057 (WGS84)
Elevation	165
Location accuracy	100(meters)
Relative position	35 miles northeast of Brawley, 7 miles northeast of Glamis, California

Site location context

Political divisions (FIPS codes)

Imperial(county)

California(state)

United States(country)

North America(continent)

Land(continent)

USGS map quadrangles

Ninemile Wash(quadrangle 1:24,000 scale)

Trigo Mountains(quadrangle 1:100,000 scale)

Salton Sea(quadrangle 1:250,000 scale)

Hydrologic units (watersheds)

Salton Sea(hydrologic accounting unit)

Southern Mojave-Salton Sea(hydrologic subregion)

California(hydrologic region)

Geographic areas

Country	State	County
United States	California	Imperial

Public Land Survey System information

Meridian	Township	Range	Section	Fraction	State
San Bernardino	T013S	R019E	2-11, 15-21, 28-30, 33		California

Comments on the location information

The Mesquite Mine is located in the southeastern corner of Imperial County approximately 35 miles east of Brawley and 7 miles northeast of Glamis, California. While the mine originally involved approximately 2,250 acres when permitted in 1984, it has expanded over the years to almost 5,200 acres of public, private , and state lands comprising portions of sections 2-11, 15-21, 28-30, and 33, T13S-R19E, SBBM. To the north, the mine is bounded by the U.S. Navy Chocolate Mountain Aerial Gunnery Range. The location point selected for latitude and longitude corresponds to the historic Mesquite Mine shaft symbol in the southwest quarter of Sec. 5-T13S-R19E on the USGS Ninemile Wash 7.5 minute quadrangle. The mine is reached by taking State Highway 78 northeast from Glamis for about 4 miles. The private access road heads north for about 2.5 miles to the open pits. The east side of the access road is flanked by the mine's conspicuous heap leach piles.

Commodities

Commodity	Importance
Gold	Primary
Silver	Secondary
Arsenic Critical	Tertiary
Molybdenum	Tertiary
Lead	Tertiary
Antimony Critical	Tertiary
Copper	Tertiary
Tellurium Critical	Tertiary

Comments on the commodity information

Commodity Info: Supergene zone- Subhedral to anhedral gold grains generally less than 10 microns, but occuring in clusters of up to 100 microns (900+ fine).

Hypergene gold is coarser grained electrum (40-100 microns) high in silver (700-800 fine)
Ore Materials: Native gold, electrum, auriferous pyrite, silver
Gangue Materials: Muscovite schist, leucocratic granite, biotite gneiss, quartz, adularia, ankerite, sericite, limonite

Materials information

Materials	Type of material
Gold	Ore
Electrum	Ore
Pyrite	Ore
Silver	Ore
Granite	Ore
Gneiss	Ore
Quartz	Ore
Adularia	Ore
Ankerite	Ore
Sericite	Ore
Limonite	Ore
Schist	Gangue

Alteration

(Local) Weak to moderate seritizaton of mafic silicates in wall rock with biotite and plagioclase being altered to sericite and carbonate. Silicification of breccia materials.

Mineral occurrence model information

Model code	104
USGS model code	25a
Deposit model name	Hot-spring Au-Ag
Mark3 model number	45

Host and associated rocks

Host or associated

Host

Rock type

Metamorphic Rock > Gneiss > Felsic Gneiss > Biotite Gneiss

Rock unit name

Chuckwalla Complex

Stratigraphic age (youngest)	Jurassic

Host or associated

Host

Rock type

Metamorphic Rock > Gneiss > Felsic Gneiss > Biotite Gneiss

Rock type qualifier

Hornblende

Rock unit name

Chuckwalla Complex

Stratigraphic age (youngest)	Jurassic

Host or associated Associated

Rock type Metamorphic Rock > Schist > Mica Schist

Rock type qualifier Muscovite

Stratigraphic age (youngest) Jurassic
Host or associated Associated

Rock type Metamorphic Rock > Gneiss

Rock type qualifier Tonalite

Stratigraphic age (youngest) Jurassic
Host or associated Associated

Rock type Metamorphic Rock > Schist > Mica Schist

Rock type qualifier biotite

Stratigraphic age (youngest) Jurassic
Host or associated Associated

Rock type Plutonic Rock > Granitoid > Granodiorite

Stratigraphic age (youngest) Jurassic
Host or associated Associated

Rock type Plutonic Rock > Granitoid > Granite

Rock type qualifier pegmatite

Stratigraphic age (youngest) Jurassic
Host or associated Associated

Rock type Plutonic Rock > Pegmatite

Rock type qualifier Granite

Stratigraphic age (youngest) Jurassic

Nearby scientific data

(1)	-114.9952, 33.06057	ListMap

Economic information

Geologic structures

Type of structure	Local
Structure description	Singer Fault
Type of structure	Regional
Structure description	San Andreas Fault, Chocolate Mountains Thrust Fault

Ore body information

General form Tabular

General form	Tabular

Controls for ore emplacement

Mineralization in fault and fracture breccias within northwesterly striking strike-slip faults and intersecting northerly striking normal faults. High angle northeasterly trending faults offset original mineralized zone into discrete orebodies.

Comments on the geologic information

REGIONAL GEOLOGY

Crystalline basement units
Regionally significant basement lithologies are the late Mesozoic Pelona, Orocopia, and Rand Schists (collectively referred to as the POR schists) and older Jurassic gneisses and schists. The POR schists are units of highly metamorphosed and deformed greywacke, basalt, chert, limestone, and ultramafic rock stretching across southern California into Arizona, whose protoliths are considered to represent Triassic- Jurassic accretionary wedge deposits. The upper plate gneisses have been assigned to the Chuckwalla Complex, but locally are referred to as the Mesquite Gneiss.

During the late Mesozoic, earlier Jurassic gneisses and schists were thrust over the younger POR schists along low angle Vincent, Chocolate Mountains, Orocopia, and Rand thrusts. These thrusts have reportedly displaced the upper plate rocks as much as 30 miles to the northeast (Dillon, 1975). Regional studies indicate that metamorphism and thrusting were approximately coeval (Drobeck and others, 1986). In the vicinity of the Mesquite mine, the Orocopia schist forms the lower plate (footwall) of the Chocolate Mountains Thrust Fault (CMTF) and the Chuckwalla Complex gneisses form the upper plate (hanging wall).

The upper plate gneisses and granites host important gold deposits in several locales in the Chocolate Mountains and neighboring Cargo Muchacho Mountains. These include the Picacho, American Girl, Padre y Madre, and Oro Cruz (Tumco) mines. At the Mesquite Mine, a biotite gneiss unit within the Chuckwalla Complex hosts the bulk of the ore deposits.

Suprajacent rocks
Suprajacent rocks in the region consist of Tertiary volcanics and sediments unconformably overlain by Cenozoic alluvium, gravels, and lesser amounts of volcanics.

Tertiary volcanics were deposited on the older granitic and metamorphic rocks on an irregular erosional topography with considerable local relief. The earliest volcanics were basalt flows that erupted into paleovalleys. Basalt caps conspicuous mesas in the Chocolate Mountains. Fanglomerates, alluvial fan deposits, overlie the basalts and are in turn followed by several hundred feet of agglomerates, flows, and breccias of the Oligocene Quechan Volcanics. These volcanics are thought to immediately post-date the initiation of Oligocene extension in the region.

Deposition of alluvium on low land and pediment surfaces followed a period of extensive erosion. The youngest deposits occupy the washes that have dissected the older alluvium and cut into the older erosion surfaces. Desert pavement is conspicuous on the gently sloping surfaces covered with the older alluvium.

Regional Structure and Tectonics
Regionally, the Colorado Desert area has undergone a complex history of metamorphism, intrusion, volcanism and faulting. At least four important tectonic episodes have contributed to the structural complexity of the area: Jurassic-Cretaceous thrusting and metamorphism, Oligocene-Miocene extension with detachment and strike-slip faulting, Miocene-Pliocene Basin and Range normal faulting, and younger dextral strike-slip faulting associated with the evolution of the San Andreas Fault system. Structural ambiguities are many due to overprinting and fault reactivation. Late Cenozoic structural features (high angle normal faults, dextral strike-slip faults) overlap and sometimes obscure earlier Tertiary features (detachment faulting), which in turn overprint Mesozoic features (thrust faulting).
During the late Jurassic or Cretaceous, basement and supracrustal rocks across southern California and southern Arizona were folded and thrusted northward and northeastward during the Cordilleran Orogeny. The Cordilleran Orogen developed as the principal effect of oblique northeastward subduction of the Farallon plate, and to some extent the Kula plate, along the western continental margin (Atwater, 1989). This produced a large belt of deformation from Canada to Mexico. As the plates converged, allochthonous terranes were scraped from the descending plates and accreted to the continental mass including the Baldy and the Santa Lucia-Orocopia terranes in southern California. Jurassic gneisses, schists and intrusive rocks were then thrust over the Pelona and Orocopia schists along a regional system of mylonitic thrusts that included the CMTF. The exposed trace of the CMTF occurs about 3 miles northeast of the Mesquite Mine. Seismic data indicates that the lower plate schists dip at low angles to the southwest, but at moderate angles in proximity of later high angle Tertiary faults (Morris, 1986a) and that the CMTF itself deformed into a series of synforms and antiforms by high-angle normal faults and multiple detachment-style faults (Morris, 1986a,b, 1987).

During the early Tertiary, the Pacific Plate's relative motion slowed and became more northwesterly. Accordingly, convergence gave way to divergent plate motions with widespread volcanism and regional extension. Initial extension was occurred along low angle detachment fault systems which accommodated much of the Oligocene-Miocene extension with an anatomizing network of low angle faults throughout much of southern California (Frost and others,1997). Foremost among these were the detachment faults in the Colorado River Extensional Corridor (150 miles north of Mesquite) in which some of the best exposures of detachment faults in the western US occur. Displacement of upper plate rocks has been measured in tens of kilometers. Upper plate rocks are characteristically broken by numerous high angle northeast dipping normal faults and further deformed by rotation along these faults which flatten which with depth before merging with the main detachment fault plane. Important mineralization is associated with these detachment features at the Picacho Mine, 22 miles to the east. Geophysical studies have indicated that detachment-style extension was regionally pervasive and extended to mid crustal levels.

Volcanism and normal faulting swept from east to west across the Basin and Range and into southern California. Basin and Range extension continues to this day; however, extensional accommodation shifted from low angle detachment faulting in the Oligocene to high angle block faulting and strike-slip tectonics during the Miocene-Pliocene. By late Pliocene, the regional tectonic environment had became one dominantly of dextral strike-slip motion exemplified in the San Andreas Fault zone 12 miles to the southwest of Mesquite.

LOCAL GEOLOGY

The Mesquite ore is hosted by Jurassic qaurtzofeldspathic - mafic, amphibolite to greenschist grade gneiss (Mesquite Gneiss), subordinate schist, and granite of the Chukwalla Complex within the upper plate of the CMTF. As much as 200 feet of Tertiary conglomeratic sediments and Quaternary alluvium cover the orebodies, but a few bedrock outcrops are locally exposed where bedrock knolls pierce the alluvial cover. Contact with the underlying gneisses is via both depositional and fault contact. To the northeast of the mine, the Chuckwalla Complex is overlain by Mesozoic and Tertiary volcanics.
The ore deposits are confined to a west-northwest trending basement fault block about 1 mile wide and bounded on the north and south by steeply dipping faults displaying strike and dip-slip displacements. The Singer Fault, which bounds the block to the north, strikes N 60-70? W and dips 75-85? northeast. Strike displacement is unknown but approximately 1,200 feet of normal displacement is indicated by the juxtaposition of younger Oligocene volcanics against Chukwalla Complex gneisses. The south boundary fault, of unknown displacement and magnitude, does not crop out but is inferred from geophysical studies and exploratory drilling (Willis, 1988b). The intervening fault block itself is intensely broken by numerous intrablock faults, which are largely responsible for localization of the mineralization and segmentation of the orebodies.

In cross section, the mineralization displays an elongate northwest-southeast trend and gentle southerly dipping tabular form (Willis, 1988a). The zone of economic mineralization consists of an oxidized zone up to 500 feet thick that is 2.5 - 3 miles long and one mile wide with the higher grade ores dying out to the northwest and southeast. However, this geometry is complicated by extensive post mineralization faulting which has disjointed the original ore zone into separate orebodies.

Stratigraphy
Willis (1988a) and Manske et al (1988) defined a pseudo-stratigraphic sequence consisting of four primary divisions within the metamorphic basement at Mesquite. The lowermost unit is comprised of a deformed section of mafic gneiss, tonalite gneiss, amphibolite and epidote-biotite schist, and intrusions of granodiorite and pegmatite. It is commonly chloritic and is distinguished by its high mafic mineral content, schistose fabric, and the presence of tonalite. This unit generally exhibits little if any mineralization. The basal mafic gneiss unit is gradationally overlain by a unit of hornblende biotite to amphibolite gneiss characterized by its course texture, feldspar augen, and accessory sphene. This unit is the primary ore host in the lower grade Cholla and East Rainbow orebodies. An overlying unit of biotite gneiss is the primary host in the larger and higher grade Big Chief, Vista, and Rainbow deposits. It consists of 115 - 330 feet of a finer grained banded and more leucratic unit of "salt and pepper" quartz-feldspar-biotite gneiss exhibiting a bulk composition ranging from granite to quartz monzonite (Manske et al, 1988). Wills (1987) further divided the biotite gneiss section into a lower salt and pepper unit, a middle unit consisting of alternating layers of salt and pepper gneiss and biotite rich layers, and an upper quartzofeldspathic unit. The uppermost metamorphic unit consists of muscovite bearing leucogneiss grading to muscovite schist. In the Big Chief Pit, the biotite gneiss and the muscovite gneiss are cut by a sill of Cretaceous muscovite leucogranite and granodioritic to granitic pegmatite dikes. Smaller boudined or deformed dikes and sills radiate away from the main sill but are often so intensely faulted that their original geometry is questionable.

Absolute ages for the rocks are not well established. Lead - uranium (U-Pb) dates on zircon from the biotite and hornblende gneiss suggest a Jurassic to mid-Cretaceous age for the protolith (Manske et al, 1988). U-Pb dates from zircons within the leucogranite body yielded a date of 78-80 my (Frost, 1987).

Structure
The host rocks in the Mesquite Mine have been shattered by several sets of high angle normal and strike-slip faults (Grady and others, 1990). Several of these fault sets produced the conduits for hydrothermal mineralization, while the importance of others is unclear. There is ample evidence of both syn-and post-mineralization movement on most of these faults with the latest post-mineralization movement being Miocene age.
Gold mineralization is closely associated with three primary sets of high-angle faults and related fractures. Northwesterly striking high angle faults dominate within the district and the most highly mineralized. These faults, which cut all lithologies except the superjacent alluvium, parallel the bounding Singer Fault and regional strands of the San Andreas Fault system (Willis and Tosdal, 1992). Northerly and westerly striking faults are subordinate; the relative importance of the subordinate faults varying between the different orebodies. The northwesterly and northerly trending faults are most important in the Big Chief and Vista pits, whereas the northwesterly and westerly faults are more important in the Cherokee Pit. Movement on some of these fault sets is considered coeval because of mutually cross cutting relationships (Willis et al, 1989). Many, but not all, of theses faults have also experienced post-mineralization reactivation which is evidenced by reduction of the mineralized structures to finely ground gouge and microbreccia (Willis and Tosdal, 1992).

The best examples of orebody and fault geometry occur within the Big Chief Pit, where the orebody measures about 2,300 feet long by 1,200 wide and averaged 170 feet thick. The orebody is somewhat tabular and dips gently to the west at 20?. The two prevailing fault trends include a northwesterly striking set which strikes N 40?-50? W and an intersecting northerly striking set. Gold mineralization is associated with both sets but most mineralization occurs in fault breccias within the northwesterly faults which dominate the prevailing structural trend in the Big Chief and Vista pits (prevailing faults in the Cherokee, Rainbow, and East Rainbow orebodies trend more east-west). These faults undoubtedly provided the primary conduits for the gold bearing solutions and may have been contemporaneous with mineralization. Fault zones range from 400 to 650 feet wide, with individual faults exhibiting up to 50 feet of gouge and fault breccia. In plan view, these faults exhibit a complex braided geometry with opposing faults dipping steeply (up to 80?) to both the southwest and northeast. Where opposing faults intersect, the hingeline is subhorizontal. With depth, the faults converge into the stem of a flower structure indicative of strike-slip motion. Individual faults display steeply raking striations and dragged foliation suggesting less than 100 feet of normal or oblique-slip (Manske et al, 1988). Distinct generations of breccia indicate episodic fault movement and rehealing.

While some northwesterly striking faults occur in the Cherokee Pit, curviplanar westerly striking mineralized faults predominate. These faults also display opposing dips that diverge structurally upward in a flower structure. Most economic mineralization in the pit is confined to an area between 2 subparallel easterly striking braided high angle fault zones that dip inward toward the center of the pit and form the outermost branches of the flower structure (Willis and Tosdal, 1992). In the upper zones the structure and mineralization was 400 feet wide. With depth the mineralized stem of the flower structure narrowed to a 30 foot zone of fewer veins but of higher grade (Willis and Tosdal, 1992). As in the Big Chief pit, and the district as a whole, the representative fault and vein geometry in the Cherokee pit is consistent with that expected from dextral strike-slip faulting. A set of northerly striking, east dipping, normal faults, though not as well developed as in the Big Chief Pit, dip to the east. Both of these fault sets contain economic gold in carbonate veins and mineralized faults.
The Mesquite orebodies have an oxidized zone that extends as deep as 500 feet below the surface. The depth of oxidation depends largely on faulting intensity and amount of fault offset. Narrow zones of oxidation extend deeply into unoxidized primary sulfide ore along deeper faults that provide conduits for descending oxidizing surface waters (Willis, 1988a).

Postmineralization fault movement had an important influence on the Mesquite orebodies. Willis and Tosdal (1992) identified two main Miocene post-mineralization fault sets. The first involved reactivation of the steeply dipping northwesterly and northerly striking mineralized faults in the Big Chief, Vista, and Cherokee pits. Reactivation along these faults was relatively minor with displacements on the order of only tens of meters.

The second set, of mid - late Miocene age, strike N 40? - 50? E, dip 50? - 80? northwest, and exhibit sinistral oblique-slip motion. These faults are responsible for large-scale disruption and offset of original Mesquite orebodies. In the Vista Pit, these faults form a complex braided pattern with about 230 feet of strike separation (Willis and Tosdal, 1992), which offsets Miocene sedimentary rocks against mineralized gneiss. Movement along the northeasterly striking faults has affected the geometry of the mineralization in the Vista Pit much more than in the Cherokee or Big Chief pits.

Willis and Tosdal, (1992) proposed a reconstruction of the Mesquite orebodies prior to late Miocene faulting. Displacement on the northwesterly faults was ignored since the effect would only lengthen or shorten an otherwise northwesterly trending orebody. Removal of the sinistral slip components resulted in the realignment of individual orebodies into two sub-parallel northwest striking deposits. separated on two intrablock fault blocks and separated. The first aligned the Big Chief, Vista, and the intervening Lena-Gold Bug bodies into one large Big Chief-Lena-Gold Bug-Vista orebody. A second aligned the Cherokee and Rainbow orebodies into a single orebody. The reconstructed orebodies were located on two parallel fault blocks separated by northwesterly trending faults, leading them to conclude that the Mesquite mineralization was originally controlled by a complex of at least two dilational jogs or duplexes. The reconstruction also provided a viable explanation for the reactivation of the northwesterly trending intrablock faults to accommodate differential rotation in response to internal strain during left lateral oblique slip on the northwest trending bounding faults.

At least two shallow fault trends occur in the Big Chief Pit. These faults are not mineralized and poorly understood. One set strikes northwest and dips shallowly at 20?-40? to the southwest and does not persist to depth. Individual faults have an arcuate trace and are shovel shaped at their base (Willis, 1988a). In cross section, the arcuate system has a braided appearance which Adams et al (1983) suggested might be evidence for a tensional detachment faulting. Willis and Holm, 1987), on the other hand, identified localized evidence for low angle reverse movement. These low angle faults crosscut the steep northwesterly trending mineralized faults, but as the pit depth was increased, the shallow faults were removed exposing only the steeply dipping faults below (Willis, 1988a). These faults appear to have displaced barren rock on top of the underlying high angle mineralized system.

A second low angle fault zone, approximately 100 feet thick, occurs near the north end of the pit where a series of en-echelon faults strike east-west and dip northerly at 20?-30?. Both of these fault systems were active as recently as late Miocene-early Pliocene as they cut the overlying fanglomerates (Willis, 1988a).
Early workers surmised the Mesquite deposits were associated with detachment faulting, much like those in the nearby Picacho Mine. However, exposures produced during subsequent mine development did not corroborate this interpretation. Instead, structural relationships between the mineralized faults and fault kinematics within the Big Chief Pit lead later workers to suggest that dilatant jog associated with dextral simple shear along the two bounding faults and the northwesterly trending intrablock strike-slip faults controlled the structure and mineralization (Willis, 1988a, 1988b; Manske et al, 1988;Grady and others, 1990). This interpretation provided for the rocks between the faults to become intensely faulted and fractured due to dilatant extension between sets of parallel strike-slip faults. The mineralization process was cyclical and was probably linked to episodic slip on the larger strike-slip faults (Willis and others, 1989).

Willis and Tosdal (1992) observed evidence for dextral strike-slip control on all scales ranging from localized pit wall exposures on pit walls to the district as a whole. Most important was the presence of flower structures, indicative of basement wrench faulting. The vertical intersection of mutually crosscutting and conjugate faults and the subhorizontal intersections of the opposing dip faults within each fault set also fixed any triaxial stress and strain field to contain the maximum and minimum principle stresses and strains in a subhorizontal plane. Further, the northwest trending faults exhibited dextral strike-slip motion, while the northerly trending faults exhibited extensional normal dip-slip displacement, the expected direction and sense of faulting produced in a dextral strike-slip environment with north-south shortening and east-west extensional strains.

Mineralization
Gold mineralization is thought to be of Oligocene age. Potassium-Argon (K-Ar) ages of about 38 Ma have been obtained on biotite and feldspar from gneiss and pegmatite and are believed to indicate argon loss during hydrothermal mineralization (Willis and Tosdal, 1992). Willis (1988a) also reported K-Ar dates of about 32 Ma from fault breccia sericite.

Fluid inclusion studies have indicated the deposit is a typical epithermal gold deposit. Fluid inclusions within the siliceous ores indicates that the hydrothermal fluids comprised a dilute boiling NaCl-C02 solution, and that precipitation took place at about 220?C and at a depth of approximately 1,000 feet (Manske et al, 1988). Oligocene ages for the regional Quechan Volcanics and Mount Barrow quartz monzonite in the Chocolate Mountains suggests this magmatic activity may have provided the heat source for the hydrothermal activity.

Mineralizatiuon was largely controlled by the high angle northwesterly trending strike-slip faults and fracture sets in which several forms of quartz and carbonate mineralized veins and breccias were emplaced. The bulk of mineralization occurs within the oxidized zone along the northwesterly strike-slip faults and northerly normal faults. The highest grade ores occur at fault intersections. This is best displayed in the Big Chief Pit, where detailed fault mapping and blast hole assays confirm the correlation between mineralization and the northwesterly fault systems which are bounded by waste rock to the northeast and southwest (Willis, 1988a). Gold mineralization is greater in the south part of the pit where the fault zone is narrower, and to the north where the fault system widens , mineralization is spread over a wider area. Average ore grade in the south part is 0.1 opt, whereas in the northerly part it is 0.04-0.05 opt (Willis, 1988a)
While economic mineralization occurs in the mafic gneiss and leaucocratic granite, the biotite gneiss unit is preferentially mineralized. Mineralization is nearly absent in the overlying muscovite schist. The ore zone also has a rather distinct bottom and only rarely penetrates the basement mafic gneiss (Manske et al, 1988).
Willis (1988a) noted that faulting is most intense in biotite gneiss and attributed the enrichment to increased permeability to ore fluids within the schistose biotite rich layers.

Manske and others (1988) identified a paragenetic sequence involving 4 stages of vein filling. The earliest involved epidote and quartz veinlets within the unmineralized basement gneiss below and lateral to the orebody, but absent within the mineralized area. Siliceous matrix breccia veins ranging from microfractures to breccias 20 feet wide were the earliest to be deposited within the orebody and have been dated 32 Ma (Willis, 1988a). These veins consist of clasts of biotite gneiss and leucogranite in an assemblage of quartz, adularia, pyrite, electrum, and occasionally, visible gold. They are generally the highest grade ores in the mine (>0.100 opt), but are almost exclusively found in the upper 100 feet of the deposit and within the larger pits where they comprise the primary ores (Willis and Tosdal, 1992). A third stage of vuggy quartz veinlets cut the siliceous matrix breccias and unbrecciated wall rock. These are most commonly preserved in the unoxidized portions of the deposit where the vugs are filled with clay, electrum, and pyrite. A fourth ankeritic-dolomitic stage forms younger veins and breccias which cut both the siliceous matrix breccias and vuggy quartz veins. Clasts of siliceous matrix breccia, biotite gneiss, and leucogranite, within a pyritic matrix of ankeritic calcite and dolomite, are common and comprise a significant portion of the ore (Willis, 1988a). Weathering and oxidation within these veins has reduced the iron carbonate to a distinctive orange limonite. These veins generally occur as secondary deposits in the Big Chief Pit, but are the principal ores in the Cherokee and Vista Pits, where the siliceous breccia veins are minor (Willis and Tosdal, 1992). Together, the siliceous matrix breccias and ankerite-dolomite veins comprise most of the economic ore. Particularly rich parts of the orebodies display a history of repeated fracturing and mineralization and are characterized by an abundance of both these vein types. Willis (1988a) identified a fifth and final stage of white to clear unmineralized calcite veining that cuts across all other types of veins and attributed them to supergene deposition.

Electron microprobe analysis has identified two types of gold in the Mesquite ores. Native gold in the oxidized supergene zone occurs in subhedral to anhedral grains generally less than 10 microns, although clusters of up to 100 microns are common in the fault zones (Ferrell et al, 1988). This gold is relatively pure (900 + fine) (Gasparrini, 1983; Matlack and Springett, 1985). Mann (1984) suggested this gold might represent supergene refinement of the original hypogene electrum alloy. The gold is commonly associated with limonite, hematite, and goethite pseudomorphs after pyrite resulting from the oxidation of disseminated and vein pyrite dispersed throughout the ore (Willis, 1988a). Small amounts of unusually high-grade ore have been found in oxidized fault gouge. These zones are very limited being only 1 - 36 inches wide and only 3-50 long, but can average 1.0-2.0 opt. A sample grading as high as 35.9 opt was recorded in the Big Chief Pit (Ferell et al, 1988).
Unoxidized hypogene gold is coarse grain (40-100 microns) electrum and high in silver (700-800 fine). These ores tend to yield 65-78% free milling gold, 13% associated with sulfide minerals, and the balance associated with iron oxides and carbonates (Brierley, 1998; Wan, 1998). While the bulk of the deposit has been oxidized, auriferous sulfide mineralization has also been found within 200 feet of the surface. Pyrite is the dominant sulfide with traces of chalcopyrite, sphalerite, galena, and gersdorffite. There is no enrichment of gold at the oxide/sulfide boundary.

Hydrothermal alteration consists chiefly of weak to moderate sericitization of biotite and plagioclase to sericite and carbonate in the wall rock and wall rock clasts within silica matrix breccias. Silicification occurs mainly as siliceous envelopes around the siliceous matrix breccias and as matrix material. Both forms of alteration are controlled by the degree and extent of faulting and fracturing (Willis, 1988a). The deposit is conspicuously absent any pervasive clay alteration products. Thin sections of clay-like material in the fault zones have shown this material to be dominantly rock flour (Willis, 1988a).

Supergene oxidation of pyrite within the oxidized zone has resulted in the formation of abundant hematite, limonite and other oxide minerals. The Big Chief orebody is characterized by an unusual As, Mo, Pb, Sb,Cu, and Te association in the unoxidized zone (Manske and others, 1988). This association is not recognized in the oxidized zone having presumably been removed during oxidation.

Metallogeny
The association of the gold mineralization with fractured and brecciated basement rocks attributed to regional strike slip tectonics at the Mesquite Mine, and detachment faulting at the nearby Picacho mine, suggests that analogous deposits may be present throughout a large area of southeastern California and Arizona. While the Mesquite orebodies were first recognized in small basement exposures jutting above the valley floor alluvium, much of the southeastern California basement from the Mesquite Mine northward into the Colorado River Extensional Corridor is obscured by thick alluvial cover. Thus, this region likely harbors similar unexposed deposits. Further understanding of both strike-slip and detachment mechanics and the controls affecting ore deposition therein should advance our understanding of these orebodies. Application of these advances, in conjunction with geochemical and geophysical studies and exploratory drilling might lead to new orebodies being found.

Economic information about the deposit and operations

Operation type	Surface
Development status	Producer
Commodity type	Metallic
Deposit size	Medium
Significant	Yes
Discovery year	1876

Mining district

District name	Mesquite

Land status

Ownership category	Private
Area name	Imperial County Planning Dept.
Ownership category	BLM Administrative Area
Ownership category	State
Area name	California State Lands Commission

Ownership information

Type Owner-Operator

Owner Newmont Mining Corp.

Home office 6502 East Highway 78\nBrawley, CA 92227\n(928) 341-4653

Type	Owner-Operator
Owner	Newmont Mining Corp.
Home office	6502 East Highway 78\nBrawley, CA 92227\n(928) 341-4653

Comments on the workings information

The modern Mesquite Mine commenced commercial production in 1986. During that year the mine processed 8.7 million tons of ore. Since then, four open pits have been developed: the Big Chief, Vista, Cherokee, and Rainbow pits. The Big Chief Pit is the westernmost pit and develops the Big Chief, Cholla, and Lena-Gold Bug orebodies. About 5,000 feet southeast, the Vista, Bobble Ridge, and Panhandle orebodies are mined in the Vista Pit. About 2,00 feet northwest of the Vista Pit are the Cherokee and Rainbow pits in which their namesake orebodies are mined.

Presently, the operation involves approximately 5,200 acres consisting of three open pits, seven heap leach pads, ore crushing and ore storage facilities, overburden stockpiles, and ancillary repair and administrative facilities. The Big Chief (approx. 350 acres), Vista (approx. 335 acres), and Rainbow (approx. 77 acres) pits are active with the Big Chief and Rainbow pits recently approved for 222 and 126 acre expansions respectively. The Cherokee Pit was closed and backfilled in the late 1980s. The mine is permitted to produce a maximum 60 million tons of ore per year.

Opens pits are developed on 20 foot benches with pit walls maintained at 1:1 slopes. Pit depths range from approximately 450 feet to 570 feet. Pits are dewatered using sumps and shallow wells in the areas of active mining below the water table which ranges from 200- 325 feet below ground surface throughout the site.
Overburden is stockpiled adjacent to the pits. Initial strip ratios were 4:1 (waste:ore), but have since been reduced to about 3:1 on average. Ore is excavated using a 24 foot square grid of blast holes loaded with ANFO explosives and by mechanical ripping. Blast hole assays are used to flag the broken ore according to assayed grade. Until the late 1990s, the ore was hauled to the primary crusher before going to ore stockpiles or leach pads. The crushing stage has since been abandoned. Waste material is generally stored in waste rock dumps, however, some waste material is used for base material for leach pads, backfilling of prospect and mine pits, and construction of haul roads. As the open pits are exhausted, they are partially backfilled with excess waste material.

Until the late 1990s, produced ore was trucked to a conveyor and relayed to the crushing plant where a two stage open crushing circuit consisting of a 1070 x 1780 mm Nordberg gyratory crusher followed by a 2.3 m (7 foot) Symons standard cone crusher reduced the ore to less than 5/8 inch. Ore from the primary crusher was stored in a primary ore stockpile then transported to the secondary crusher for crushing and screening and loadout to the leach pads. Lime was added to the crushed ore at an average rate of 2.5 pounds/ton before being belt agglomerated and sprayed with a preliminary barren solution of about 0.02% sodium cyanide solution. Belt agglomeration increased the contained moisture to 8% which helps the fines adhere to the coarser particles and resulting in improved percolation. A sampling plant ahead of the agglomeration belts provided an ore head sample for the crushed ore heaps. The crushed ore had an average grade of 0.035 opt.

The heap leach process involves stacking ore in lifts on a prepared pad and percolating a cyanide solution through the material to dissolve the gold and silver which is later recovered from the leachate. To date, seven leach pads have been constructed; Pads 1 through 6, and the smaller Vista Pad designed specifically for lower grade ores. The primary leach pads measure approximately 4,200 by 390 feet at the base are hold more than 4 million tons of ore each.
Leach pads are constructed by grading and compacting the ground surface, then placing a finely screened thin layer of material over the prepared ground. The base grade is sloped toward leachate collection and storage basins located on the side of the leach pile and PVC lined interceptor channels constructed around the perimeter of each pad. Slopes are generally 1% - 2%. An impermeable 40 mil HDPE liner is placed over the ground surface and overlain with a 16 oz/yd. geotextile fabric for added protection (Haldane, 1990). A network of 3 inch perforated drainage pipes is then laid and connected to 8-inch trunk drains. Crushed ore is placed by truck end dump and spread by bulldozer into 20-foot lifts. Compaction created by equipment is not excessive and percolation rates average 0.003gpm per sq. foot (Haldane, 1990). Successive lifts are smaller than the preceding lift in order to shape the pile to resemble a truncated pyramid. Leach piles reach an ultimate height of 80 feet.

The top of each lift is graded level and series of berms is constructed to segment each leach pad into 20 sections to provide for metallurgical accounting and control. Typically, between 3-4 million square feet of pad are under leach at any time.

An average 0.025 % sodium cyanide solution is pumped from a barren solution ponds to the appropriate section and applied by drip irrigation utilizing 2 gpm emitters placed on a 30 x 24 inch grid. The cyanide solution percolates through the lifts, becoming pregnant with dissolved gold and silver. Percolation rates average approximately 0.003 gpm per sq ft. Each section is leached for an initial 120-125 day cycle then allowed to rest. The residual solution continues to extract gold in the resting leach pad that is recovered when the subsequent lift is placed and undergoes its primary leach. Leaching of a sector continues until the mineral values are stripped, then the leaching area is moved to another section. At the bottom of the pile, the leachate is collected in the perforated pipe collection system which drains to the PVC lined interceptor ditches which in turn drain to a pregnant solution pond. Pregnant solution is pumped to the recovery plant where it passes through a series of activated charcoal columns. The gold is adsorbed onto the carbon and the barren solution leaving the last column is pumped back into the barren solution pond for recycling. Make up chemicals are added to keep the solution at the desired pH and cyanide strength. Recovery through the adsorption circuit is usually 95% to 100%. Carbon desorption is accomplished by stripping the gold from the carbon with a hot caustic strip solution of sodium cyanide and sodium hydroxide. Gold is recovered from the strip solution by electroplating onto stainless steel wool cathodes (Haldane, 1990), after which, the gold is smelted from the cathodes and poured into dore bars (averaging 91.3% ?). The dore is shipped by armor car to an outside refinery for purification and marketing. On average, this process recovers as much as 85% of the gold from the oxidized ores (Higgins, 1990).

Exhausted leach piles are flushed with water to neutralized and detoxify them, then associated process ponds are backfilled, and the leach pile slopes are regraded to reduce slope gradients.
In late 1990s, the crusher circuit was deactivated and crushing was indefinitely discontinued in preference of leaching mine run ore. As early as 1989, Gold Fields experimented with dump leaching to supplement the crushed ore heap leach operations. By eliminating the crushing and belt agglomeration, lower grade ores could be economically competitive with crushed ore. More importantly, previously waste grade ore material could be processed. Whereas crushed heap leach ores required an average grade of 0.035 opt, mine run dump material grading as low as 0.016 opt cold be processed. Dump leach pad head grades are taken from blast hole samples. To further reduce costs, dump leach pads, such as the Vista Pad were placed placed closer to the lower grade ore sources to minimize haulage.

To accommodate the coarse uncrushed ore, dump leach pads included an initial 6-foot layer of crushed ore to protect the liner from puncture by large uncrushed mine-run boulders. The crushed ore layer was leached prior to placing the mine run material so as not to impact mine run recoveries (Haldane, 1990). Mine run material is also spread in thicker 40 foot lifts and allowed a longer primary leach cycle. Lime is spread on top of the lifts, then ripped into the heap using a bulldozer while the lift was leveled. Since this method is less efficient to belt agglomeration, more lime (5lb/ton) is used than in the crushed ore leach pads. Percolation rates on the dump leach pad is similar to that of the crushed heaps (0.003 gpm per sq ft.).

Comments on other economic factors

Despite the generally low average ore grades, which ranged about 0.01 to 0.055 opt, the Mesquite mine has produced over 3 million ounces of gold. Up to 1 million more ounces are estimated within the expansion area. The friable nature, low abrasion tendencies, and good metallurgy of the ore contribute to favorable cyanide heap leach recoveries which average approximately 80 - 85% of the gold in place for crushed ore and about 65% for the mine run ore (Haldane, 1990).

Average production costs have ranged from about $119/oz in 1987 to upwards of $220/oz in 2001.

Comments on development

Placer mining in the Mesquite area probably occurred as early as 1780, at which time Spaniards and Indians were mining in the neighboring Cargo Muchacho and Pothole districts. Almost all the dry washes in the region were mined using small scale dry washing methods such as small bellow washers and blankets (Clark, 1970).

The first recorded gold discovery occurred about 1876, when Felisario Parra located the placer deposits in the area of the current Mesquite Mine. Mexicans and railroad workers dry washed the "Mesquite Diggins" placers, recovering as much as $80-$90/day (Singer, 1969). Later, mineralized stringers were found in the scattered bedrock exposures and shallow shafts were sunk. By the Depression, the placers were again active with as many as 300 unemployed men working the deposits (Singer, 1969).

The first organized commercial operations commenced in 1941 when Reese Production Company prospected the area and conducted limited placer mining. Fifty-two borings were placed within their leasehold. Average value of the placers was $1.07/yard. Bank run gravels were run through a trommel and the concentrates were shipped to a mill where they were processed in a primary crusher and ball mill, then run over an amalgamation table. The concentrates averaged $10 per ton (Singer, 1969). In 1942, legislation closed the Mesquite District in order to provide General George Patton with a tank training ground.

After the war, the district was reopened and prospectors began to recognize the extent of mineralization. Limited attempts to mine the lode ores occurred in the early 1950s. The largest of these was the Big Chief Mine where a shaft was sunk to a depth of 150 feet with short drifts on three levels. In 1957, Richard and Ann Singer staked 27 claims and sporadically worked the placers until the 1980s (Horn, 1972). In 1966, Gold Placers Inc. reportedly identified over ten million cubic yards of placer deposits averaging $1.50/yard. In 1968, Alaska Mining and Exploration Company implemented an exploratory drilling program and concluded that a large low grade hard rock orebody lay beneath the surface alluvium (Singer, 1969). Neither of these operators commenced any mining operations.

The early 1970s brought renewed interest in the Mesquite orebody, but depth and ownership ambiguities caused most companies to quickly lose interest (Grady and others, 1990). Escalating gold prices in the late 1970s, however, again rekindled interest. By 1981, Gold Fields Operating Company had assembled a 16,000 acre position and commenced a comprehensive geochemical, geophysical, and boring program. Their first boring, located about 300 feet south of the old Big Chief Mine shaft, encountered mineralized Mesquite gneiss. Subsequent boreholes delineated a large disseminated gold deposit underlying approximately 200 feet of alluvium. This deposit proved to be the Big Chief deposit which was the first orebody to be developed.

Gold Fields announced its discovery in 1982 and reported a preliminary reserve estimates of 41 million tons grading 0.056 opt (2.3 million ounces). The same year, a use permit allowing pilot cyanide heap leaching was granted. A 10 x 10 foot decline was sunk to a depth of 220 feet to assess ore quality and continuity and to recover bulk samples for the pilot program. Twenty thousand tons of ore were produced. During 1982 and 1983, testing was conducted in small 1,600 ton lots (Wilkinson and Wendt, 1986). A permit to conduct full scale mining operations of the Big Chief orebody was issued in 1984.
Production commenced from the Big Chief orebody in 1986. During the first year of operation, 8.7 million tons of ore was mined at an average rate of 30,000 tons per day. At the same time, continued exploration efforts delineated additional reserves east of the Big Chief Pit in the nearby Vista, Cherokee, and Rainbow orebodies (Wilkinson and Wendt, 1986). As a resul, the mine was expanded in 1988. The expansion involved the enlargement of the Big Chief Pit and the opening of the Vista, Cherokee, and Rainbow pits south of State Highway 78. To accommodate the expansion, Gold Fields realigned 8.5 miles of the highway which previously passed through the center of the expanded area to the south side of the mine property.

In 1993 Santa Fe Pacific Gold Corporation acquired the Mesquite Mine, after which Santa Fe was acquired by Newmont Gold Company, the current operator.

Production increased to a peak of 132,000 ton/day in 1997 and has declined since. In response to the declining reserve base, in 1988 Newmont Gold applied for and another expansion of operations involving the expansion of the Big Chief pit to the northwest and southeast and the Rainbow Pit to the east. The expansion was approved in 2002 and is expected to add about 1 million ounces of reserves and extend the life of the mine until 2006 or 2007.

Comments on the environmental information

The Mesquite Mine is located in on the southwestern flank of the Chocolate Mountains in a barren region of the Sonoran Desert referred to as the Lower Colorado River Valley portion of the Colorado Desert. The landscape is dominated by an southwesterly sloping alluvium covered pediment of the Chocolate Mountains. Generally, property elevations range from approximately 550 - 800 feet above sea level, except for localized bedrock knolls which pierce the alluvium and reach elevations up to 1,025 feet. Slopes are composed of alluvium deposited in southwesterly sloping bajadas exhibiting desert pavement and cut by storm washes of various depth. Drainage is to the southwest along numerous unnamed ephemeral washes which drain toward the Imperial Valley. Vegetation is sparse and consists mostly of creosote bush, cholla cacti, and mesquite. A wide variety of small annuals appear when moisture is available in the spring months.

The climate is arid low desert. Average annual precipitation at the mine is 3.0 inches per year (Grady and others, 1990). Winters are mild and summers hot. Average summer high temperatures generally exceed 100? between May and October, but can reach as high as 120?. Daily temperature fluctuations can be extreme. Average winter lows in this part of the desert reach about 45? in December.

The area is sparsely populated. The mine lies approximately midway between El Centro to the southwest and Blythe to the northeast along State Highway 78. The nearest town of any importance is Brawley, California (pop. Approx. 20,000) approximately 35 miles to the west.

Reference information

Bibliographic references

Deposit

Atwater, T., 1989, Plate tectonic history of the northeast Pacific and western North America., in Winterer, E. L., Hussong, D. M., decker, R. W., editors, The geology of North America: The eastern Pacific Ocean and Hawaii, Geological Society of America, p. 21-72.
Deposit

Burchfiel, B.C., Cowan, D.S., and Davis, G.A., 1992, Tectonic overview of the Cordilleran orogen in the western United States: in Burchfiel, B. C., Lipman, P. W., and Zoback, M. L., editors, The Cordilleran Orogen: Conterminous U.S.: Boulder, Colorado, Geological Society of America, The Geology of North America, v. G-3. p. 407-479.
Deposit

Clark, W. B., 1970 Gold districts of California: California Divisions of Mines and Geology Bulletin 193, p. 49-50.
Deposit

Dillon, J. T., 1975, Geology of the Chocolate and Cargo Muchacho mountains, southeasternmost California: University of California Santa Barbara, Ph.D. thesis, 405 p.
Deposit

Dillon, J. T., Haxel G.B., and Tosdal, R.M., 1990, Structural evidence for northeastward movement on the Chocolate Mountains Thrust, southeasternmost California: Journal of Geophysical Research, v. 95, p. 19,953-19,971.
Deposit

Dillon, J. T., Haxel, G. B., and Tosdal, R.M., 1986, Field guide to the Chocolate Mountains thrust and Orocopia Schist, Gavilan Wash area, southeastern California, in Beatty, B., and Wilkinson, P. A. K., editors, Frontiers in geology and ore deposits of Arizona and the Southwest: Arizona Geological Society Digest, v. 16, p. 282-293.
Deposit

Frost, E. G. and others, 1997, Emerging perspectives of the Salton Trough region with an emphasis on extensional faulting and its implications for later San Andreas deformation: in Baldwin, J. and others, editors, Southern San Andreas Fault- Whitewater to Bombay Beach, Salton Trough, California, South Coast Geological Society Field Trip Guidebook N. 25, p. 57-98.
Deposit

Grady, L., Holm, R., and Brumit, P., 1990, Short term mine planning and grade control practice at the Mesquite mine, Mining Engineering, v. 42, no. 2, p. 187-190.
Deposit

Haldane, T. Q., 1990, Heap leaching and Dump leaching at the Mesquite mine, Mining Engineering, v. 42, no. 12, p. 1321-1322.
Deposit

Haxel, G. B., Jacobson, C. E., and Oyarzabal, F. R., 1996, Subduction and exhumation of the Pelona-Orocopia-Rand schists, southern California: Geology, v. 24, p. 547-550.
Deposit

Haxel, G. B., Jacobson, C. E., and Oyarzabal, F. R., 1997, Extensional reactivation of the Chocolate Mountains subduction thrust in the Gavilan Hills of southeastern California: Tectonics, v. 16, p. 650-661.
Deposit

Haxel, G. B., and Tosdal, R. M., 1986, Significance of the Orocopia schist and Chocolate Mountains thrust in the late Mesozoic tectonic evolution of the southeastern California-southwestern Arizona region: extended abstract, in Beatty, B., and Wilkinson, P. A. K., editors, Frontiers in geology and ore deposits of Arizona and the Southwest: Arizona Geological Society Digest, v. 16, p. 52-61.
Deposit

Higgins, C. T., 1990, Mesquite mine - A modern example of the quest for gold, California Geology, Califonia Division of Mines and Geology, v. 43, no. 3, p 51-67.
Deposit

Mann, A. W. 1984, Mobility of gold and silver in lateritic weathering profiles: some observations from western Australia, Economic geology, vol. 79, pp. 38-49.
Deposit

Manske, S. L., Matlack, W. F., Springett, M. W., Strakele, A. E., Watowich, S. N., Yeomans, B. Yoemans, E., 1988, Geology of the Mesquite deposit, Imperial County, California, Mining Engineering, v. 40, no. 6, p. 439-444.
Deposit

Merrill, F. J., 1916, Imperial County gold: California Mining Bureau Report No. 14, pp. 731-732.
Deposit

Morris, R. S., 1986a, Base of the Orocopia Schist as imaged on seismic reflection data in the Chocolate and Cargo Muchacho Mountains region of southeastern California and the Sierra Pelona region near Palmdale, California: Geological Society of America, Abstracts with programs, v. 18, p. 160.
Deposit

Morton, P. K., 1977, Geology and mineral resources of Imperial County, California: California Division of Mines and Geology County Report No. 7, p. 46-61.
Deposit

Skillings, D. N., 1984, Gold Fields evaluating mesquite gold property in southeastern California, Skillings Mining Review, v. 73, no. 15, p. 4-9.
Deposit

Willis, G. F., 1988a, Geology and mineralization of the Mesquite open pit gold mine, in Bulk mineable precious metal deposits of the western U.S., Geological Society of Nevada, p 473-486.
Deposit

Willis, G. F., 1988b, geology of the Big Chief orebody, Mesquite district, Imperial County, California: Society of Mining Engineers Preprint 88-16, 6 p.
Deposit

Willis, G.F. and Holm, V.T., 1987, Geology and mineralization of the Mesquite open pit gold mine, in Johnson, J.L., Bulk mineable, Guidebook for fieldtrips: Reno, geological Society of Nevada, p. 52-56.
Deposit

Willis, G. F., Tosdal, R. M., Manske, S. L., 1989, Structural control on epithermal gold veins and breccias in the Mesquite district, southeastern California, Fifth annual V. E. McKelvey forum on mineral and energy resources, USGS research on mineral resources, 1989 program and abstracts, p. 78-79.
Deposit

Willis, G. F. and Tosdal, R. M., 1992, Formation of gold veins and Breccias during dextral stike-slip faulting in the Mesquite mining district, southeastern California, Economic Geology, vol. 87, pp. 2002-2022.
Deposit

Ferrell, T., Harris, D., Loucks, D., Lozano, R., Mitts, R. and Ochs, M., 1988, Mesquite Mine, unpublished mine report, 30 p.
Deposit

Frost, D. L., 1987, Final report on U/Pb dating studies in the Mesquite pit and adjoining regions: unpublished report for Gold Fields Mining Corp.
Deposit

Gasparinni, C., 1983 Study of the gold distribution in 28 samples of core material from Mesquite: unpublished report for Gold Fields Mining Corp.
Deposit

Matlack, W. F., and Springett, M. W., 1985, Gold mineralization and trace element geochemistry of the Mesquite district, California: unpublished report for Gold Fields Mining Corp.
Deposit

Singer, A., 1969, Mesquite Diggings mining district and Quartz Peak Quadrangle district, unpublished report, 22 p.
Deposit

Miscellaneous information on the Mesquite Mine is contained in File Number 322-5647 (CGS Mineral Resources Files, Sacramento).

General comments

Subject category	Comment text
Deposit	The Mesquite gold deposit is a low grade epithermal disseminated and vein hosted gold deposit that has a complex metamorphic, plutonic and structural history. Mineralization occurs as steeply dipping open space vein fillings, brittle fracture fillings, and mineralized breccia zones of Oligocene age within a sequence of Jurassic quartzofeldzpathic to mafic gneisses and Cretaceous granite of the Chuckwalla Complex. The orebodies are unconformably overlain by a veneer of approximately 200 feet of alluvium on the southwest pediment of the Chocolate Mountains. Mineralization occurs in several discreet orebodies (Big Chief, Cholla, Lena, Gold Bug, Vista, Cherokee, and Rainbow orebodies) that have been developed in four open pits; the Big Chief, Vista, Cherokee, and Rainbow pits. Pre-and syn-depositional faulting is complex and currently interpreted to have occurred within an environment of dextral northwest-southeast trending strike-slip motion with north-south shortening and east-west extensional strain (Wills and Tosdal, 1992). Northeasterly post-mineralization faulting has fragmented the original mineralized zone into several distinct orebodies. Mineralization is confined to a northwest-southeasterly trending fault block approximately 6,000 feet wide. The block itself is characterized by mineralized intrablock northwest-southeast strike-slip duplexes and contemporaneous intersecting northeast trending mineralized normal faults. The larger orebodies occur within upwardly divergent series of veins, mineralized faults, and fault breccias characteristic of flower structures. Pre-Miocene palinspastic reconstructions indicate that the orebodies were originally deposited as two subparallel, trends which were later fragmented, and offset into separate orebodies by northeast post-mineralization sinistral oblique-slip faults related to the evolution of the San Andreas Fault system (Willis and Tosdal, 1992).

Subject category

Comment text

Deposit

The Mesquite gold deposit is a low grade epithermal disseminated and vein hosted gold deposit that has a complex metamorphic, plutonic and structural history. Mineralization occurs as steeply dipping open space vein fillings, brittle fracture fillings, and mineralized breccia zones of Oligocene age within a sequence of Jurassic quartzofeldzpathic to mafic gneisses and Cretaceous granite of the Chuckwalla Complex. The orebodies are unconformably overlain by a veneer of approximately 200 feet of alluvium on the southwest pediment of the Chocolate Mountains. Mineralization occurs in several discreet orebodies (Big Chief, Cholla, Lena, Gold Bug, Vista, Cherokee, and Rainbow orebodies) that have been developed in four open pits; the Big Chief, Vista, Cherokee, and Rainbow pits.

Pre-and syn-depositional faulting is complex and currently interpreted to have occurred within an environment of dextral northwest-southeast trending strike-slip motion with north-south shortening and east-west extensional strain (Wills and Tosdal, 1992). Northeasterly post-mineralization faulting has fragmented the original mineralized zone into several distinct orebodies. Mineralization is confined to a northwest-southeasterly trending fault block approximately 6,000 feet wide. The block itself is characterized by mineralized intrablock northwest-southeast strike-slip duplexes and contemporaneous intersecting northeast trending mineralized normal faults. The larger orebodies occur within upwardly divergent series of veins, mineralized faults, and fault breccias characteristic of flower structures.

Pre-Miocene palinspastic reconstructions indicate that the orebodies were originally deposited as two subparallel, trends which were later fragmented, and offset into separate orebodies by northeast post-mineralization sinistral oblique-slip faults related to the evolution of the San Andreas Fault system (Willis and Tosdal, 1992).

Reporter information

Type	Date	Name	Affiliation	Comment
Reporter	24-MAR-2003	Fuller, Michael and Downey, Cameron (Higgins, Chris T.)	California Geological Survey CGS (Formerly CDMG)
Editor	01-SEP-2007	Schruben, Paul G.	U.S. Geological Survey	Converted from S&A FileMaker format to Oracle. Edit checks on rocks, units, and ages with Geolex search, and other fields.

Beyond USGS

Supplemental information added by qvyshift.com. Not part of the original USGS MRDS record.

Operator history (post-MRDS)

MRDS records operators as of each record's last update (≤ 2019). Some of the operators listed here have since changed hands or dissolved:

Newmont Mining Corp. — renamed to Newmont Corporation (2019)
Renamed from Newmont Mining Corporation to Newmont Corporation in early 2020 (after the 2019 merger with Goldcorp).

Curated by qvyshift.com from publicly-reported M&A activity (SEC filings, press releases, USGS Mineral Yearbooks). Not authoritative — verify against primary sources before relying on it. The MSHA panel above is the current authoritative source for actively-permitted mines.

External references

Wikipedia: Mesquite Mine
Wikidata: Q6821690 (Mesquite Mine)

Maps centered on this site

Authoritative California resources

These are landing pages for further research — the state agencies don't currently expose per-mine deep links.