Picacho Mine (Picacho Gold Mine; Goshen; Helen May; Tierra Rica; Mina Rica; St. George; California Gold King; Picacho Basin; Golden Rule; Venus; Jayne; Golden Casket; Dulciana; Mars), Picacho Peak, Picacho Mining District, Chocolate Mountains, Imperial County, California, USAi

Regional Level Types
Picacho Mine (Picacho Gold Mine; Goshen; Helen May; Tierra Rica; Mina Rica; St. George; California Gold King; Picacho Basin; Golden Rule; Venus; Jayne; Golden Casket; Dulciana; Mars)	Mine
Picacho Peak	Peak
Picacho Mining District	Mining District
Chocolate Mountains	Mountain Range
Imperial County	County
California	State
USA	Country

A former Au-Ag-Sb-As-Hg-Cu occurrence/mine located in the SE¼ sec. 3 and in the NW¼ sec. 10, T14S, R22E, SBM, 2.2 km (1.4 miles) ESE of Picacho Peak (coordinates of record), 7.4 km (4.6 miles) SSW of Picacho and about 20 miles N of Yuma, California, W of Burro Wash, on private (patented) land (located claims). Discovered before 1880. The property consists of 52 lode mining claims covering over 540 acres. 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, SBM. Owned by Glamis Gold Inc. (100%), Nevada (1996). Operated by Chemgold, Inc., Arizona (1996). Additional names which pertain to this locality: Mars Extension Mine; Eastern California Mine; Apache Mine. MRDS database stated accuracy for this location is 500 meters. The property may include the old Christie Gold Mine (Picacho Peak Gold project).

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 1980's 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.

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 ore bodies during the 1980's and 1990's. 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 1990's, 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.

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.

Environment: 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.

Mineralization is a polymetallic deposit (Mineral occurrence model information: Model code: 129; USGS model code: 40a; Deposit model name: Detachment-fault-related polymetallic Cu-Au-Ag-Pb-Zn deposits), hosted in Jurassic gneiss, Cretaceous Marcus Wash Granite, and Oligocene Quechan Volcanics andesite & dacite. The ore body is tabular, wedge-shaped. Controls for ore emplacement included epithermal open space filling in intensely brecciated fault zones. Ore grade is related to the degree of shattering (ie: most open space), with best ore grades in brittle brecciated felsic gneisses and granitic rocks. Local alteration includes minor hydrothermal chloritization of biotite; abundant post-mineralization, weathering related, sericitic alteration of plagioclase and clay alteration of plagioclase and microcline. Local rocks include Precambrian igneous and metamorphic rock complex and/or Tertiary volcanic flow rocks, unit 15 (Southern Mojave Desert).

Regional geologic structures include the Vincent-Chocolate Mountains Thrust Fault, Chocolate Mountains Detachment Fault, and the Chocolate Mountains Anticlinorium. Local structures include the 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 the hanging wall and footwall of CMDF.

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.

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 ore bodies are known at Picacho and three main ore types are present. The ore bodies 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 ore body within the fault zone, post-detachment and post-mineralization normal faulting has dissected it into at least three discontinuous ore bodies. Drobeck and others (1986) reconstructed an original ore body 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 ore body 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 ore bodies 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 ore bodies 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 ore bodies occupy a total area of approximately 160 acres. of the four, the Dulcina ore body is the largest, occupying the northeast portion of the mine complex. This ore body 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 ore body 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 ore body. 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 predominate 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 ore body 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 ore bodies, hanging wall Quechan Volcanics overlie the deposit which grades downward into unbrecciated and unmineralized schist.

The San George ore body is the westernmost deposit in the mineralized area. The ore body 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 ore bodies 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 ore bodies 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 ore bodies. Application of these advances, in conjunction with geochemical and geophysical studies and exploratory drilling might lead to new ore bodies being found.

Workings include surface and underground openings. The latest period of mining activity included the infrastructure for a heap leaching operation. Testing of each rock type on the property indicated non-acid forming. Simple neutralization is used. The process water circuit is closed. Actual consumption was 100,000 gallons per dy. Published heap leach costs of $125 per Troy ounce in 1985 (1986).

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.

Production data are found in: Glamis Gold Ltd. 1996 Form 10-K, pp. 11, 27; Glamis Gold Ltd. 1996 Annual Report, pp. 1, 12; Randol Mining Directory 1996-97, p. 164.

Production statistics: Year: 1991: Gold production (fiscal year 1991 - 07/1990-06/1991): 17,362 Troy ounces/year: Recovery percentage: Au = 540,019 grams. Year 1992: Gold production (fiscal year 1992 - 07/1991-06/1992): Recovery percentage: 540,019 grams. Year 1992: Gold production: 29,429 Troy ounces/year: Recovery percentage: Au = 915,344 grams. Year: 1993: Gold Production (fiscal year 1993 - 07/1992-06/1993): 27,578 Troy ounces/year: Recovery percentage: 857,772 grams. Year: 1994: Gold production (fiscal year 1994 - 07/1993-06/1994): 23,333 Troy ounces/year: Recovery percentage: Au = 725,738 grams. Year: 1995: Gold Production (fiscal year 1995 - 07/1994-06/1995): 25,270 Troy ounces/year: Recovery percentage: 785,985 grams. Year: 1995: Gold Production (fiscal year 1995 - 07/95-12/95): 14,434 Troy ounces/year: Recovery percentage: 448,948 grams. Year: 1996: Gold production: 34,621 Troy ounces/year: Recovery percentage: 1,076,834 grams. Year: 1996: Ore mined @ 0.036 ounces per ton: 1,631,600 short tons of ore: Recovery percentage: 179,853 metric tons. Gold avereged 85% Au and 12% Ag.

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 ore bodies in the neighborhood of 800,000 ounces.

Reserve-Resource data are found in: Glamis Gold Ltd. Annual Report, 1985; Glamis Gold Ltd. Annual Report, 1993; Glamis Gold Ltd., 07/95 - 12/95 Form 10-K: pp. 12, 25; Glamis Gold Ltd., 1996 Form 10-K, pp. 13, 26; Glamis Gold Ltd., 1996 Annual Report insert.

Reserve-Resource data: Type: In-situ: 1997: Demonstrated: 1,021,671 metric tons of ore; Indicated: 55,066 metric tons of ore; Measured: 966,605 metric tons of ore. Gold at 1.2 grams/metric ton (1997).

Other Regions, Features and Areas containing this locality

References