Picacho Mine

Past Producer in Imperial county in California, United States with commodities Gold, Silver, Antimony, Arsenic, Mercury, Copper

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	10310579
Record type	Site
Current site name	Picacho Mine
Alternate or previous names	Albert Polhamas Claim, Alycyon Claim, Alfonso Claim, Apache Claim, California Gold King Claim, Diablo Claim, Dulcina Claim, Eastern California Claim, Golden Casket Claim, Golden Dream Claim, Golden Crown Claim, Golden Hill Claim, Golden Sunshine Claim, Golden Rule Claim, Goshen Claim, Helen May Claim, Jayne Claim, Jita Claim, Mars Claim, Mars Extension Claim, Mina Rica Claim, Oriental Claim, Ponce de Leon Claim, San George Claim, Tierra Rica Claim, Venus Claim, Picacho Basin Claim

Comments on the site identification

The Picacho Mine is located in the historic Picacho Mining District which experienced a brief period of prosperity around the turn of the 19th century during which the mine reportedly yielded 150,000 ounces of gold. The mine really came into its own in the early 1980s when Chemgold, Inc. began exploiting the low grade ore reserves using open pit mining methods and cyanide heap leaching. During the next two decades, Chemgold mined mineralized brecciated gneiss and granitic ore within a detachment fault zone. Ore was mined from four open pits and the gold values were extracted via five heap leach pads. Chemgold ceased operations and completed reclamation of the site in May 2002. During their operations, Chemgold recovered an additional 650,000 ounces of gold.

Geographic coordinates

Geographic coordinates:	-114.64979, 32.96207 (WGS84)
Elevation	210
Location accuracy	100(meters)
Relative position	18 miles northwest of Yuma, AZ

Site location context

Political divisions (FIPS codes)

Imperial(county)

California(state)

United States(country)

North America(continent)

Land(continent)

USGS map quadrangles

Picacho Peak(quadrangle 1:24,000 scale)

Yuma(quadrangle 1:100,000 scale)

El Centro(quadrangle 1:250,000 scale)

Hydrologic units (watersheds)

Imperial Reservoir(hydrologic unit)

Lower Colorado(hydrologic accounting unit)

Lower Colorado(hydrologic subregion)

Lower Colorado(hydrologic region)

Geographic areas

Country	State	County
United States	California	Imperial

Public Land Survey System information

Meridian	Township	Range	Section	Fraction	State
San Bernardino	014S	022E	3, 10		California

Comments on the location information

The Picacho Mine is located in the southeastern corner of Imperial County approximately 50 miles east of El Centro and 20 miles north of Yuma, Arizona. The Colorado River is 5 miles to the north. At its maximum, the mine area consisted of 633 acres of patented and unpatented lode claims comprising portions of unsurveyed sections 2, 3, 4, 9, 10, 11-T14S-R22E, SBBM. The location point selected for latitude and longitude corresponds to the Picacho Mine symbol in the northwest quarter of Sec. 10-T14S-R22E on the USGS Picacho Peak 7.5 minute quadrangle. The mine is reached by taking Picacho Road north from Winterhaven, California for about 17 miles. The mine area is approximately 1,200 feet west of Picacho Road and is accessed by private road.

Commodities

Commodity	Importance
Gold	Primary
Silver	Secondary
Antimony Critical	Secondary
Arsenic Critical	Secondary
Mercury	Secondary
Copper	Tertiary

Comments on the commodity information

Commodity Info: Chemgold, Inc. produced approximately 650,000 ounces of gold via cyanide heap leaching of low grade run of the mine ore. Average grade was 0.05 ounces/ton. Average recovery was 73% of total contained gold. Gold dore avereged 85% Au and 12% Ag.
Ore Materials: Native gold, electrum, silver
Gangue Materials: Quartz, calcite, pyrite, hematite, goethite, magnetite, gneiss, granite

Materials information

Materials	Type of material
Gold	Ore
Electrum	Ore
Silver	Ore
Quartz	Gangue
Calcite	Gangue
Pyrite	Gangue
Hematite	Gangue
Goethite	Gangue
Magnetite	Gangue
Gneiss	Gangue
Granite	Gangue

Alteration

(Local) Minor hydrothermal chloritization of biotite; abundant post-mineralization, weathering related, sericitic alteration of plagioclase and clay alteration of placioclase and microcline.

Mineral occurrence model information

Model code	129
USGS model code	40a
Deposit model name	Detachment-fault-related polymetallic Cu-Au-Ag-Pb-Zn deposits

Host and associated rocks

Host or associated Host

Rock type Metamorphic Rock > Gneiss

Stratigraphic age (youngest) Jurassic

Host or associated

Host

Rock type

Plutonic Rock > Granitoid > Granite

Rock type qualifier

leucocratic

Rock unit name

Marcus Wash Granite

Stratigraphic age (youngest)	Cretaceous

Host or associated

Associated

Rock type

Volcanic Rock (Aphanitic) > Intermediate Volcanic Rock > Andesite

Rock unit name

Quechan Volcanics

Stratigraphic age (youngest)	Oligocene

Host or associated

Associated

Rock type

Volcanic Rock (Aphanitic) > Felsic Volcanic Rock > Dacite

Rock unit name

Quechan Volcanics

Stratigraphic age (youngest)	Oligocene

Nearby scientific data

(1)	-114.64979, 32.96207	ListMap

Economic information

Geologic structures

Type of structure	Local
Structure description	Chocolate Mountain Detachment Fault (CMDF), listric normal faults within the hanging wall of CMDF, late high angle northeast and northwesterly trending normal faults which cut haning wall and foot wall of CMDF
Type of structure	Regional
Structure description	Vincent-Chocolate Mountains Thrust Fault, Chocolate Mountains Detachment Fault, Chocolate Mountains Anticlinorium

Ore body information

General form Tabular, Wedge

General form	Tabular, Wedge

Controls for ore emplacement

Epithermal open space filling in intensely brecciated fault zones. Ore grade related to degree of shattering (ie: most open space), with best ore grades in brittle brecciated felsic gneisses and granitic rocks.

Comments on the geologic information

INTRODUCTION
The Picacho Mine is located within the Colorado Desert portion of the Basin and Range physiographic province near the southeastern end of the Tertiary Chocolate Mountains. The Chocolate Mountains are about 80 miles long and 210 miles wide and represent an anticlinorium developed along the Tertiary Chocolate Mountain Detachment Fault (Drobeck and others, 1986) above a regional system of Mesozoic thrust faults known as the Vincent-Chocolate Mountain Thrusts. Much of the region is overlain by either Tertiary volcanics or Quaternary alluvium.

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. These deposits were regionally metamorphosed during the Cordilleran Orogeny.

Ages for the gneisses is controversial. A Proterozoic age has been suggested based on the high degree of metamorphism and the similarity to regionally extensive Proterozoic porphyritic monzogranites. Tosdal and others (1985, 1986), however, have demonstrated that the gneiss may better correlate with units within the Jurassic Kitt Peak-Trigo Peaks Supergroup that are mineralogically similar to the hornblende-biotite augen gneiss at Picacho and have been U-Pb zircon dated at 165 Ma in several ranges to the north of Picacho.

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

The youngest crystalline basement rocks are the late Cretaceous-Paleocene(?) Marcus Wash Granite. This unit and its associated pegmatite dikes intruded both the lower plate Orocopia schist and upper plate gneisses and schists.

The upper plate gneisses and granites host important gold deposits in several locales in the Cargo Muchacho and Chocolate Mountains. At Picacho Mine, an augen gneiss is exposed through a low angle normal fault system, the Chocolate Mountain Detachment Fault (CMDF). Along the detachment fault zone, the gneiss and Marcus Wash Granitic rocks are highly brecciated and host gold.

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 relief. The earliest volcanics were basalt flows that erupted into paleovalleys. Basalt caps conspicuous mesas to the south of the Picacho Mine. Fanglomerates, alluvial fan deposits, overlie the basalts and are in turn followed by several hundred feet of agglomerates, flows and breccias of the Oligocene (32 Ma) Quechan Volcanics. These volcanics are thought to immediately post date the initiation of extension and detachment faulting 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 Picacho Mine 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 detachment faulting, Miocene-Pliocene Basin and Range normal faulting, and Pliocene and younger dextral strike-slip faulting associated with the evolution of the San Andreas Fault system. Structural ambiguities are many due to overprinting and possible fault reactivation. Cenozoic structural features (high angle normal faults, dextral strike-slip faults) overlap and sometimes obscure Tertiary features (detachment faulting), which in turn overprint Mesozoic features (thrust faulting).

Mesozoic
During the late Jurassic or Cretaceous, basement and supracrustal rocks across southern California and Arizona were folded and thrusted north/northeastward during the Cordilleran Orogeny. The Cordilleran Orogen resulted from the subduction of the Farallon and Kula plates along the western continental margin (Atwater, 1989) and 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 include the CMTF in the vicinity of the Picacho Mine and the Vincent Thrust Fault in the San Gabriel Mountains to the west. Collectively, these faults are called the Vincent-Chocolate Mountains Fault.

The CMTF is exposed in a number of outcrops near the mine where it separates upper plate gneiss from the Orocopia schist. In all exposures, it is clearly unmineralized, but the fault appears to largely control the location of the younger overlying mineralized detachment fault zones in which the Picacho ores occur. Seismic data indicates that the lower plate schists dips 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).
Tertiary

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. Low angle detachment faults accommodated much of the Oligocene-Miocene extension with an anatomizing network of low angle faults throughout southern California region (Frost and others,1997). Foremost among these were the detachment faults in the Colorado River Extensional Corridor (150 miles north of Picacho) in which some of the best exposures of detachment faults and footwall mylonites in the western US occur. Displacement of upper plate rocks relative to lower plate rocks has been measured in tens of kilometers. The upper-plate rocks are characteristically broken by numerous high angle northeast dipping normal faults. Upper plate rocks are further deformed by rotation along these faults which flatten with depth before merging with the main detachment fault plane. The repeated southwesterly dips that are common in the surface exposures of the Quechan Volcanics reflect the tilted blocks and grabens resulting from this rotation in the upper plate of the detachment fault. Syn-detachment sedimentary and volcanic rocks fill the half-grabens between these faults and record early-mid Miocene tilting. Mylonites display well developed northeast trending foliation. Geophysical studies have indicated that detachment-style extension was regionally pervasive and extended to mid crustal levels. Morris (1986a,b) observed that seismic profiles showed Tertiary clastic and volcanic sections that were preserved in half-graben basins displayed a distinct growth fault character.

The CMDF is the dominant structural feature in the Picacho Mine area is largely responsible for the present form of the Chocolate Mountains and Picacho Mine mineralization. Brecciation fabrics, low temperature mineralogy, and stratigraphic reconstructions in the mine area indicate the fault formed at a depth as shallow as ? mile. Most of the outcrops of the CMDF consist of small horsts formed by later normal faulting that further complicates the structural complexity and produces only isolated exposures.

Post-detachment, Miocene-Pliocene extension became rapid and occurred as an intense episode of block faulting on high angle normal faults and volcanism within the Basin and Range and extending into southeastern California. Normal faulting was pervasive along northeast and northwest trending normal faults which cut both the CMDF and CMTF and paralleled the upper plate syn-detachment normal faults. These are very obvious and important faults at the Picacho Mine and are responsible for offsetting originally continuous orebodies. These young faults display normal and oblique normal displacement of a few tens to hundreds of feet. By the late Pliocene, the regional tectonic environment became one of dextral strike-slip motion as represented by the Sand Hills Fault of the San Andreas Fault zone 22 miles to the southwest of Picacho.

LOCAL GEOLOGY

The Picacho Mine produces gold from intensely fractured and brecciated gneiss, schist, and granitic rocks within CMDF. The fault zone is structurally complex, and has been deformed by both syn-detachment normal faults in the hanging wall and post-detachment normal faults that offset both the footwall and hanging wall. (Losh and others, 1990) have suggested that that there may, in fact, be two detachment faults zones at Picacho, but most workers currently recognize only one. Underlying the mine, Jurassic gneisses are thrust over the Orocopia Schist along the CMTF. The CMTF is more than 500 feet below the ground surface in the mine area, but it is exposed about 5 miles northwest of the mine where Mesozoic gneisses are exposed resting on the younger Pelona Schist. Both Pelona Schist and the overlying gneiss complex have been intruded by the Marcus Wash Granite.
In the immediate mine area, the CMDF defines the contact between unmineralized Tertiary Quechan Volcanics of and fanglomerates of the hanging wall and pre-Tertiary footwall crystalline rocks. Drobeck and others (1986) identified the CMDF in outcrop several miles north of the mine and correlated it with the detachment fault underlying the Picacho deposit. In outcrop, the fault was not mineralized. Local arching and erosion through the hanging wall volcanics has exposed crystalline footwall rocks in a localized erosional window around the mine. Where present, the Quechan Volcanics consist of Oligocene andacitic to dacitic volcanic flows the brecciated and mineralized deposits within the fault zone, and the underlying undisturbed and unmineralized footwall gneiss, schist, and Marcus Wash Granite. Generally, lower plate rocks consist of chloritic biotite augen gneiss, schist and leucogranite - quartz monzonite of the Marcus Wash Granite. Gneiss and schist comprise about 80% of the exposed crystalline rock in the mine, the balance being Marcus Wash Granite.

The hanging wall is characterized by northwest striking, northeast dipping high-angle syn-detachment normal faults that cut and rotate blocks of hanging wall volcanic rocks 20-90? (Losh and others, 1996). Displacements, when measurable, are on the order of inches to yards. With increasing depth, these faults become listric and merge with the gently tilted detachment surface. Shattering and brecciation along these fault planes increases as they approach and merge with the detachment zone.

Steeply dipping northeast and northwest trending normal faults cut the CMDF and contribute to the mine's complexity in that they offset the detachment zone and associated mineralization into small horst and graben blocks, often obscuring the geometry of the CMDF. These offsets are generally a few meters to a few tens of meters.

THE ORE DEPOSITS
Four epithermal orebodies are known at Picacho and three main ore types are present. The orebodies include the San George, Dulcina, Apache, and Diablo ore deposits. Most of the ore material is localized along brecciated zones within the low angle CMDF and syn-detachment upper plate normal faults; however, some ores are accumulations of fault scarp talus breccias eroded from uplifted horsts containing mineralized portions of the fault zone. The main fault zone is generally characterized by a 3-10 foot thick cataclastic zone but can reach thicknesses of over 250 feet locally (Drobeck and others, 1986). Dips on the gentle dipping fault plane can reach 30?.

Originally a single contiguous orebody within the fault zone, post-detachment and post-mineralization normal faulting has dissected it into at least three discontinuous orebodies. Drobeck and others (1986) reconstructed an original orebody approximately 3,000 feet by 1,800 feet and 65 feet thick, and containing 27 million short tons. These faults exhibit displacements ranging from 3 - 325 feet and have so completely deformed the orebody that it is impossible to map the CMDF through the mine as a coherent fault zone (Drobeck and others, 1986). These faults are also responsible for much of the orebody being uplifted and eroded, thus forming the mineralized fault scarp talus breccias.
The three general ore types are silicified and unsilicified fault breccia in the CMDF and fault scarp talus breccia composed of both granitic and metamorphic rocks. All ores are highly brecciated and oxidized by weathering. The first two types of ore occur in the brecciated upper part of the hanging wall and are capped by unmineralized hanging wall Quechan Volcanics. These ores consist of intensely brecciated, weak to moderately propylitically altered cataclasite within the CMDF and the lower reaches of the syn-detachment high angle normal faults. The ores are characterized by abundant iron oxide. Ore streaks consist of parallel lenses dipping at about 45? (Merrill, 1916). Although generally low grade, the orebodies can be large. One measured 250 feet long and 160 feet wide. The grade of ore is directly related to degree of brecciation and amount of hematite with Marcus Wash Granite and felsic gneiss forming the highest ore grades owing to their higher tendency to shatter. Much of the hematite occurs as replacement of pyrite, but specular hematite veinlets are also common. Much of the hematite has been altered to goethite.

Liebler (1988) described the brecciated fault zone ores as quartzo-feldspathic cataclasites in which hydrothermal alteration and mineralization has removed or replaced most primary ferro magnesian minerals and has added quartz, calcite, hematite, goethite, pyrite and gold. Gold occurs as replacements of pyrite, in the native state in particles from 10-100 microns, in quartz veinlets and breccia filling and on late fracture surfaces. Liebler (1988) described microprobe results indicating native gold and electrum encapsulated within pyrite grains and concluded their close association indicated a common petrogenesis. The size range and distribution of the gold in brecciated and porous rocks makes it especially suitable for heap leaching. The rocks hosting the ore types are generally highly brecciated and porous. The uppermost surface of the fault zone is fairly smooth and dips shallowly, but it can dip as much as 30? where rotated by later normal faults. It consists a microbreccia grading downward into coarse breccia. The microbreccia is a dark brown, aphanitic layer that typically overlays a thin zone of reworked breccia in a matrix of rock flour. Precursor lithologies consisted of gneiss and Marcus Wash Granite. Below that, is a less intensely brecciated mineralized zone up to 250 feet thick. The lower part of the fault zone is gradational into unfractured and unmineralized gneiss, schist, and Marcus Wash Granite. Bladed or acicular grains of specular hematite occur as veinlets, masses in quartz breccia matrix, and replacements in biotite and chlorite along cleavage planes. Specular hematite and deep red earthy hematite is common as pseudomorphic replacements of euhedral-subhedral pyrite. Earthy hematite coated late fractures and is likely supergene, whereas the specular hematite is probably hypogene. Feldspars are altered to sericite and clays.

The third ore type occurs as fault scarp talus breccia at the foot of fault scarps. These orebodies were formed by the erosion of the mineralized gneiss and Marcus Wash Granite fault breccias. Post-detachment uplift normal faulting resulted in uplifted horst blocks which shed their talus along the adjacent fault scarps. The Talus breccias consist of clasts of unmineralized Quechan Volcanics and mineralized gneissic, and granitic clasts in a matrix of fine grained similar rock fragments.
The four known orebodies occupy a total area of approximately 160 acres. of the four, the Dulcina orebody is the largest, occupying the northeast portion of the mine complex. This orebody was extensively mined in the early 20th century using underground workings and a surface glory hole. It measured approximately 1,300 feet in a northeast-southwest direction by 500 feet wide and was between 50 - 130 feet thick. The main orebody consisted of a somewhat tabular, gently dipping conglomeration of quartzofeldspathic clasts of Marcus Wash Granite and Jurassic gneiss within the exposed CMDF zone and normal fault contacts with the fault plane. Mineralized fault scarp talus breccia also comprised a smaller portion of the orebody. Marcus Wash Granite comprised about 20-30% of the crystalline rock in the pit.

In the shallow portions of the deposit, northwest trending faults dip moderately to steeply to the northeast. Low angle normal faults are predominatet in the deeper levels (350 - 450 feet elevations). The CMDF zone is best displayed in a gently dipping sill of Marcus Wash Granite 25-50 feet thick and bounded on the top and bottom by low angle detachment faults, white cataclasite, and greenish fault gauge. The granite is highly brecciated and transected by numerous gently to moderately dipping normal faults that merge with the underlying CMDF. This sill is the deepest occurrence of Marcus Wash Granite in the mine and defines the base of the Dulcina Pit. The bottom of the ore zone is roughly planar, dips 10? - 20?, and displays a marked decrease in brecciation and gold content near its base before passing into generally unbrecciated and unmineralized gneiss. The orebody is overlain by unaltered and unmineralized Quechan volcanics.

The brecciated ore zone is mineralized with gold, pyrite, hematite and goethite. The degree of brecciation and the amount of hematite and goethite were regarded as good indicators of ore grade. Pyrite was oxidized to hematite with subhedral to euhedral pseudomorphs restricted to rock flour matrix and fractures within clasts. Pseudomorphs were unaffected by brecciation suggesting replacement during the a late period of brecciation or later (Drobeck and others, 1986).

The Apache orebody lies in the southwestern portion of the mineralized area. The ore rock is primarily fault scarp breccia composed of quartz, plagioclase, and microcline derived from leucocratic Marcus Wash Granite. The orebody was elongate in a north - south direction and dipped approximately 25? west. The bulk of the talus was shed from a large northeast striking, post-detachment normal fault on the north side of the Apache Pit. The upthrown wall consists of gneiss that presumably once held an orebody (Drobeck and others, 1986).

An old decline encountered in the pit followed a dike of brecciated Marcus Wash Granite enclosed by Jurassic gneiss. Drobeck and others (1986) used this exposure to illustrate the correlation between lithology, brecciation, and mineralization. The brecciated granite assayed 0.17 opt whereas the less brecciated enclosing gneiss assayed only 0.10 opt. Deformation of the brittle granite was primarily by intense shattering allowing easy access to the mineralizing fluids, whereas much of the deformation of the gneiss was accommodated by bending, recrystallization, and slippage along biotite cleavage planes.

Ore was characteristically reddish due to the abundance of earthy hematite. Goethite was subordinate to hematite. Quartz and later calcite filled fractures. Specular hematite and goethite pseudomorphs after pyrite were common and visible gold was abundant (Liebler, 1988). Apache ore averaged 0.05 opt. As with the other orebodies, hanging wall Quechan Volcanics overlie the deposit which grades downward into unbrecciated and unmineralized schist.
The San George orebody is the westernmost deposit in the mineralized area. The orebody was roughly circular with a diameter of approximately 820 feet and a thickness of 100 feet. The bulk of the ore was silicified brecciated felsic gneiss with leucocratic granitic clasts in a red-brown matrix of fine grained quartz and hematite. The degree of silicification is the major difference from other brecciated gneissic ores at the mine. The increased silicification formed resistant rubbly knobs of ore (Liebler, 1988; Drobeck and others, 1986) along the CMDF. This orebody was also overlain by unmineralized Quechan Volcanics. Ore samples from this body graded between 0.008 to 0.03ounce/ton.

The Diablo orebody is the smallest of the Picacho orebodies and is located in the northwest portion of the mine area. Ore consists of an east-northeasterly trending lens of brecciated gneiss, schist, and Marcus Wash Granite within the CMDF zone and is overlain by hanging wall Quechan Volcanics.

Mineralization
The Picacho Mine is mineralogically simple. Overall, there is only one stage of mineralization which took place between emplacement of the late Cretaceous Marcus Wash Granite and eruption of the Oligocene Quechan Volcanics. Drobeck and others (1986) concluded that ore mineralization occurred roughly contemporaneously with extensional faulting and volcanism, however some evidence suggests mineralization may have continued until shortly after detachment faulting ceased.

The age of mineralization and concurrent faulting is constrained by the age (>60 m.y.) of this brecciated and mineralized granite and the age of unconformable barren superjacent volcanics (32 m.y.)

Gold is associated with pyrite and specular hematite that overgrows fault plane cataclasite and fills voids and veinlets within the fault zone. Gold mineralization was syngenetic with pyrite and hematite. Generally, the gold occurs in grains up to 0.1 mm across, but usually is not visible. Euhedral to subhedral hematite pseudomorphs after pyrite are common. Supergene remobilization of gold may have contributed to the free deposits in fractures. The deposit is characterized by an Au-As-Sb-Hg trace element assemblage and is low in silver and copper (Losh and others, 1996).

Brecciation fabrics, low temperature mineralogy, minimal recrystallization, stratigraphic reconstructions, and fluid inclusion data all suggest the CMDF formed at a depth as shallow as ? mile. Fluid inclusions in ore-stage quartz yielded homogenization temperatures of 201-226?C with a salinity of 0.5-0.7 wt % NaCl equivalent (Liebler, 1988) suggesting that rising epithermal ore fluids were diluted by percolating meteoric water. Oxidation of the ascending ore fluids caused the syngenetic precipitation of gold, pyrite, and some hematite. Oxygen isotopes showed an O-18 shift indicating either significant rock /water interaction or the mixing of two distinct fluids (Losh and others, 1996, unpublished). The contemporaneous deposition of gold, pyrite, and hematite would suggest that oxidation conditions fluctuated during mineralization, possibly in response to oscillations of the water table (Liebler, 1988). Weathering contributed to continuing post-mineralization oxidation in the shallow portions of the orebodies after precipitation and is characterized by supergene red hematite staining.

Alteration of wall rock gneiss is generally weak to moderate propylitic. Plagioclase has been altered to sericite and/or calcite, and biotite to chlorite and/or epidote with minor titanite (Losh and others, 1996). Silicification is generally sparse, but is significant in the San George orebody.
Metallogeny

The association of the gold mineralization with the brecciated material within the CMDF suggests that analogous deposits may be present throughout the large area of southeastern California and Arizona. While the Picacho deposits were discovered by virtue of an erosional window exposing the orebodies at the surface, widespread areas of detachment tectonics including the Chocolate Mountain Anticlinorium and northward into the Colorado River Extensional Corridor may harbor similar, but unexposed, deposits. Further understanding of the of detachment mechanics within an extensional regime and the controls affecting ore deposition therein should advance our understanding of detachment related 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-Underground
Development status	Past Producer
Commodity type	Metallic
Deposit size	Medium
Significant	Yes
Discovery year	1880

Mining district

District name	Picacho

Land status

Ownership category	Private
Area name	Imperial County Planning Department

Ownership information

Type Owner-Operator

Owner Chemgold, Inc.

Home office 5190 Neil Road, Suite 310\nReno, NV 89502\n(928) 783-1891

Type	Owner-Operator
Owner	Chemgold, Inc.
Home office	5190 Neil Road, Suite 310\nReno, NV 89502\n(928) 783-1891

Comments on the workings information

Between 1906 -1910, the Picacho Basin Mining Company worked the Dulcina orebody through two shafts to depths of 250 and 450 feet, and by means of a glory hole measuring approximately 250 feet by 160 feet. Another shaft, the Diablo, was located 750 feet to the southwest and went to a depth of 450 feet. During this time the town of Picacho, located 5 miles to the north on the Colorado River, had a population of some 2,500 persons and 700 mine employees.

In 1980, Chemgold introduced the cyanide heap leaching process to the low grade Picacho ores. Operations involved open pit excavation and heap leaching to extract the precious metals. During the course of their operations, Chemgold operated 4 open pts and 5 leach pads, before reclaiming the site in 2002.

Ore was excavated using small closely spaced explosive charges and mechanical ripping with heavy equipment. Produced ore was trucked to the heap leach areas. Waste material was used for base material for leach pads, backfilling of prospect and mine pits, and construction of haul roads.

The early strip ratio was 2:1 (waste:ore), but was reduced to less that 1:1 for much of the operation. Secondary crushing and screening was not employed; instead, blasting and ripping were planned and controlled to provide a high degree of ore fragmentation. The brecciated and weathered nature of the ore was sufficient to produce the small particle size necessary for the wetting process and to maximize total recovery in the leaching process. Clays within the pervasive fractures also helped to naturally disaggregate the ore when wetted.

The leaching process at Picacho Mine involved stacking of the ore in lifts on a prepared pad and percolating a cyanide solution through the materials to dissolve the gold and silver. The minerals were recovered from the leachate by a carbon adsorption process, electrodeposition, and a melting process.

Leach pads were prepared by grading and compacting the ground surface, then placing an impermeable 20 mil HDPE liner, over which a grid of 4 inch perforated PVC leachate collector piping was installed. The base grade was sloped toward leachate collection and storage basins and PVC lined interceptor ditches constructed around the perimeter of the leach pile. An 18 inch pad of sand was placed over the piping and liner to serve as a filter media and to protect the leachate collectors and sheeting.

Heap leach piles were constructed in 10'-25' lifts with leach piles reaching heights of between 70-80 feet. Each lift was smaller than the preceding lift in order to shape the pile to resemble a truncated pyramid with slopes of 1.5: 1. Three pounds of lime per ton of ore was sprayed on the ore material (to maintain a pH of 10.5 - 11) as it was leveled by bulldozer into lifts.
The top of each leach pile was blocked into sections approximately 50 x 50 feet (2,500 feet sq.). The exterior of the top surface was protected by a haul road berm. Around each section, a berm one foot high and 2-3 feet wide was constructed to retain the leaching solution and to control of the area being leached. Only 100,000 tons of ore were leached at any one time. A 0.02 % cyanide solution was pumped from a barren solution pond to the appropriate section and ponded 2-4 inches deep. Sections were flooded each morning. After 2-3 hours the solutions percolated into the leach pile (on leach pads 4 and 5, a 0.03 - 0.04% cyanide solution and a drip irrigation system was used instead of ponding). The cyanide solution percolated through the lifts, becoming pregnant with dissolved gold and silver. At the bottom of the pile, the leachate was collected in the perforated pipe collection system which drained to the interceptor ditch. The solution flowed by gravity to a pregnant solution pond, from which it was pumped through an activated carbon filter and back into the barren pond for recycling. Make up chemicals were added to keep the solution at the desired pH and cyanide strength. Ponds were 20 feet deep with a capacity of approximately 1.9 million gallons. The mineral values were adsorbed from the solution onto the carbon filters. The gold and silver was stripped from the carbon filters and electroplated onto stainless steel cathodes. Gold and silver from the cathodes was smelted onsite and poured into dore bars.

Leaching of a sector continued until the mineral values were stripped, then the leaching area was moved to another section. As each leach pile was spent, it was neutralized and detoxified, its associated process ponds were backfilled, and the leach pile slopes were regraded to reduce slope gradients.

As each pit was mined out, it was backfilled as much as possible with available non-ore material. The final Dulcina pit was not backfilled so as not to preclude future mining of lower grade ores on site.

Comments on other economic factors

Early production figures for the Picacho are not recorded, but the mine is thought to have produced approximately 150,000 oz of gold (600,000 tons of ore averaging .25 opt) from the early underground workings by 1910 (Harris and Van Nort, 1985).

Despite the low grade of the gold ore, which averaged about 0.055 opt, Chemgold managed to make the Picacho Mine one of the lowest cost producers of gold in the US with costs as low as $125.00/oz. The success of this operation was based on the excellent heap leaching recoveries attributable to the brecciated and weathered nature of the ore, the fineness of the disseminated gold, and a very low stripping ratio. During Chemgold's operation of the mine, heap leaching recovered an average 73% of the total contained gold and the recovered gold dore averaged 85% gold and 12% silver. Before final closure of the mine in 2002, Chemgold produced approximately 650,000 ounces of gold putting the total recovery from the orebodies in the neighborhood of 800,000 ounces.

Comments on development

Placer mining in the Picacho area probably occurred as early as 1780, during 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. Winnowing of the dry deposits was done with small bellow washers and with blankets (Clark, 1970).

During the early 1890s, an English firm attempted to hydraulically mine the local placer deposits. After spending $240,000, low water pressures and an insufficient tailings grade caused the plan to be abandoned (Crawford, 1894). During the late 1890s, about 30 claims were consolidated and in 1902 the California Gold King Mining Company was organized (Merrill, 1916). It was promptly reorganized into the California King Gold Company and then into the Picacho Basin Mining Company in 1906. This company worked the Picacho Mine extensively between 1906 and 1910. Operations consisted mainly of underground workings and a glory hole centered on the Dulcina Orebody. Ore was shipped by narrow gauge rail to a 450 ton stamp mill about 5 miles north at the town of Picacho along the Colorado River. Picacho was a thriving town at the time, boasting a population of some 2500 persons and 700 mine employees. In 1908, the mill was relocated to the mine site. Between 1906 and 1910, the Picacho Mine reportedly produced 96,750 ounces of gold, worth $2,000,000. The early ores were considered low grade, yielding approximately 0.15 opt. Operations ended in 1910. During 1926, most equipment and facilities were dismantled and removed.

In 1937, Picacho Gold Mining Company acquired the mine and initiated a three-year long drilling program on the Picacho orebodies. Almost five hundred holes were drilled for a total footage of over 50,000 feet. The program identified 2.5 million tons of ore averaging 0.057 opt. In 1939, the property was optioned to the Nipissing Mining Company which planned to install a 500-ton cyanide plant at the mine. The outbreak of WWII, however, brought an end to these plans. After the war, most of these properties were acquired by the Picacho Development Company.

Chemgold, Inc. (a subsidiary of Glamis Gold Ltd) began operations in 1977 under lease from the Picacho Development Corporation. The first operations consisted of an experimental run of 8,000 tons of ore to ascertain the feasibility of heap leaching the Picacho ore. It was followed in May 1980 with the construction of the pilot scale Site 1 leach pad which involved leaching of a 55,000 ton leach pile over the following few years. The success of the Site 1 leach pad resulted in the construction and operation of four more leach pads (Sites 2-5 leach pads) and 4 open pit mining operations on the Dulcina, Apache, San George, and Diablo orebodies during the 1980s and 1990s. As each leach pile was spent, it was neutralized and detoxified, its associated process ponds were backfilled, and the leach pile slopes were regarded to reduce slope gradients. In March 2002, Chemgold completed reclamation of the Picacho Mine. At its peak in the mid 1990s, the Picacho Mine property consisted of 600 acres of fee lands and patented lode claims totaling 1650 acres. Total controlled land amounted to 2,250 acres.

Comments on the environmental information

The Picacho Mine is located in on the southern flank of the Chocolate Mountains in a barren region of low, northwesterly trending mountains rising out of broad desert basins. The mine site lies at the easterly foot of Picacho Peak (elev. 1,920 feet) and occupies a relatively level area bounded on the north by Picacho Wash.. The mine is located on a group of contiguous patented mining claims that are the only private lands in the area, and are surrounded by BLM administered public lands. Property elevations range from 520 - 980 feet above sea level. The site has been intermittently mined for over 100 years. Scattered relicts of historic mining operations remain including remnants of an old 450 ton stamp mill, mine shafts, prospect pits, and the old Picacho cemetery.

The landscape is dominated by volcanic plugs, and alluvial slopes. Lower slopes are composed of alluvium deposited in easterly sloping bajadas exhibiting desert pavement and cut by numerous storm washes. Vegetation is sparse and consists mostly of Creosote Bush on the flatter slopes and Ironwood and Palo Verde in the sandy washes. A variety of small annuals appear when moisture is available in the spring months.

Drainage is to the northeast into Little Picacho Wash which discharges into the Colorado River 4 miles to the north at the old ghost town of Picacho. Once a gold mining town with about 100 inhabitants, the town is today part of the Picacho State Recreation Area.

The climate is arid low desert. Average annual precipitation for the nearest station of record, Gold Rock Ranch, is 3.90 inches per year. Winters are mild and summers hot. Average summer high temperature is 107? in July and average low temperature 45.6? in December. Daily temperature fluctuations can be extreme.

The area is sparsely populated. The nearest town of any importance is Winterhaven, California and its neighbor, Yuma Arizona located 18 miles to the southeast.

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

Crawford, J. J., 1894, California Picacho mine, California Mining Bureau Report No. 12, p. 238.
Deposit

Crawford, J. J., 1896, Picacho Basin: California Mining Bureau Report No. 13, p. 343.
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

Drobeck, P. A., Frost, E. G., Hillemeyer, F. L., and Liebler, G. S., 1986, The Picacho mine: A gold mineralized detachment in 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. 187-221.
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

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

Jacobson, C.E., Dawson, M.R., and Postlethwaite,C.E., 1988, Structure, metamorphism, and tectonic significance of the Pelona, Orocopia, and Rand Schists, southern California: in : Ernst, W. G., editor, Metamorphism and crustal evolution of the western United States: Englewood Cliffs, New Jersey, Prentice-Hall, p 976-997.
Deposit

Liebler, G. S., 1988, Geology and gold mineralization at the Picacho mine, Imperial County, California, in Cooper, J. J., Schafer, R. W., and Vikre, P. G., editors, Bulk mineable precious metal deposits of the western United States: Symposium proceedings: The Geological Society of Nevada, p. 453-472.
Deposit

Long, K. R., 1992, Preliminary descriptive deposit model for detachment-fault-related mineralization in: Bliss, J. D., editor, Developments in mineral deposit modeling, U.S. Geologic Survey Bulletin 2004, p. 52-56.
Deposit

Losh, S., Jowett, E. C., and Sherlock, R. L., 1990, The detachment related Picacho gold deposit; structural setting and ore fluid controls, in Cuffney, B. editor, Geology and ore deposits of the Great Basin; programs with abstracts: Geological Society of Nevada, 110 p.
Deposit

Merrill, F. J., 1916, Imperial County, Picacho: California Mining Bureau Report No. 14, pp. 729-731.
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

Morris, R. S., 1986b, Crustal geometry of detachment faulting-structural analysis of seismic-reflection data in southeastern California: Geological Society of America, Abstracts with programs, v. 18, p. 160.
Deposit

Morris, R. S., 1987, Tertiary basin formation above middle-crustal shear zones in southern Chocolate Mountains, California: in Geologic Society of America, Abstracts with Programs, v.19, p. 434.
Deposit

Richard, S. M., 1993, Tertiary stratigraphy of the Middle and Chocolate Mountains of southwestern Ariz., in Nielson, J. E., and Sherrod, D. R., editors, Tertiary stratigraphy of the highly extended terranes, California, Arizona, and Nevada: U. S. Geological Survey Bulletin 2053, p. 193-197.
Deposit

Smith, D.R., Berger, B.R. Tosdal, R.M., Sherrod, D.R., Raines, G.L., Griscom, A., Helferty, M.C., Rumsey, C.M., and McMahan, A.B., 1987, Mineral resources of the Indian Pass and Picacho Peak Wilderness Study Areas, Imperial County, California, U.S. Geological Survey Bulletin 1711-A.
Deposit

Tosdal, R. M., 1986, Gneissic host rocks of gold mineralization at the Picacho mine, southeastern Chocolate Mountains, southeastern California, in Cenozoic stratigraphy, structure and mineralization in the Mojave Desert: Guidebook and volume, trips 5 and 6, p. 143-144.
Deposit

Miscellaneous information on the Picacho Mine is contained in File Number 322-5648 (CGS Mineral Resources Files, Sacramento).
Deposit

Losh, S, Sherlock, R. L., and Jowett, E. C., 1996, Geological and geochemical study of the Picacho gold mine, California: Gold in a low angle normal fault environmant, Unpublished report, 28 p.

General comments

Subject category	Comment text
Deposit	During the Oligocene-Miocene, southeastern California underwent a period of regional extension during which time several important gneiss-hosted gold deposits, including the Picacho deposit were formed. The Picacho deposits consist of four epithermal gold orebodies composed of intensely shattered fault zone breccias and cataclasites composed of Mesozoic augen gneiss and late Mesozoic Marcus Wash Granite. The orebodies are localized within the Chocolate Mountain Detachment Fault (CMDF) zone or as deposits of fault scarp talus breccias derived from uplift and erosion of portions of the mineralized CMDF zone. The CMDF is characterized by an upper plate of unmineralized Tertiary Quechan Volcanics and a lower plate of gneiss and schist. The upper plate is deformed by numerous syn-detachment northeast dipping normal faults which flatten with depth and merge with the detachment zone. The top of lower plate exhibits intense brecciation within the fault zone and grades downward into unbrecciated and unmineralized gneiss. Both plates are further deformed by Miocene-Pliocene normal faults which cut both plates, offsetting the CMDF zone and breaking it into separate discontinuous orebodies. Mineralization is simple, Gold occurs syngenetically with pyrite as disseminated grains, and as void and fracture fillings. Fluid inclusions geothermometry and salinity data indicate the deposits formed within the fault zone at shallow depth, in the epithermal range, and that ore fluids were diluted with meteoric waters.

Subject category

Comment text

Deposit

During the Oligocene-Miocene, southeastern California underwent a period of regional extension during which time several important gneiss-hosted gold deposits, including the Picacho deposit were formed. The Picacho deposits consist of four epithermal gold orebodies composed of intensely shattered fault zone breccias and cataclasites composed of Mesozoic augen gneiss and late Mesozoic Marcus Wash Granite. The orebodies are localized within the Chocolate Mountain Detachment Fault (CMDF) zone or as deposits of fault scarp talus breccias derived from uplift and erosion of portions of the mineralized CMDF zone. The CMDF is characterized by an upper plate of unmineralized Tertiary Quechan Volcanics and a lower plate of gneiss and schist. The upper plate is deformed by numerous syn-detachment northeast dipping normal faults which flatten with depth and merge with the detachment zone. The top of lower plate exhibits intense brecciation within the fault zone and grades downward into unbrecciated and unmineralized gneiss. Both plates are further deformed by Miocene-Pliocene normal faults which cut both plates, offsetting the CMDF zone and breaking it into separate discontinuous orebodies.

Mineralization is simple, Gold occurs syngenetically with pyrite as disseminated grains, and as void and fracture fillings. Fluid inclusions geothermometry and salinity data indicate the deposits formed within the fault zone at shallow depth, in the epithermal range, and that ore fluids were diluted with meteoric waters.

Reporter information

Type	Date	Name	Affiliation	Comment
Reporter	09-APR-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.

External references

Wikidata: Q49440277 (Picacho Mine)

Maps centered on this site

Authoritative California resources

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