Cargo Muchacho Mining District (Hedges Mining District; Ogilby Mining District), Cargo Muchacho Mountains, Imperial County, California, USAi

Regional Level Types
Cargo Muchacho Mining District (Hedges Mining District; Ogilby Mining District)	Mining District
Cargo Muchacho Mountains	Mountain Range
Imperial County	County
California	State
USA	Country

Location: This district is an extensive area in the Cargo Muchacho Mountains in southeastern Imperial County, 7 miles N of Ogilby and 50 miles E of El Centre. The district includes not only the area known as the Cargo Muchacho district but also the area known as the Tumco or Hedges mining district.

History: Mining was first done in this region by Spaniards as early as 1780-1781, when placers in Jackson Gulch and oxidized ores in Madre Valley were worked. This is believed to have been the first gold mined in California. Later, mining was resumed under Mexican rule. The district received its name of Cargo Muchacho, or "Loaded Boy," when two young Mexican boys came into camp one evening with their shirts loaded with gold. American miners became interested in this district soon after the end of the Mexican War in 1848. Mining became firmly established in 1877 with the completion of the Southern Pacific Railroad to Yuma. Large-scale mining continued from around 1890 until 1916 and again from 1932 until 1941, with intermittent activity since World War II.

The Cargo Muchacho Mountains were first described by Spanish explorer Father Francisco Garcis who identified rich surface ores in 1776. By 1780, Spanish colonists had arrived and were working the placer deposits in Jackson Gulch and oxidized surface ores in Madre Valley (Van Wormer and Newland, 1996. This is believed to be the first gold mining in California. The district name, Cargo Muchacho, or Loaded Boy, refers to the legend that two Mexican boys returning to camp, arrived with their shirts laden with gold ore (Clark, 1970). American miners became interested in the area after the Mexican War of 1848. Gold was reportedly rediscovered in 1862 by members of a wagon train. The Cargo Muchacho Mining District was established that year, but was redefined several times in later years (Van Wormer and Newland, 1996). In the 1860s and 1870s small scale prospecting and mining flourished. The most important mines at this time were the Padre and Madre claims in the Madre Valley. The Padre y Madre deposits were first formally recorded in 1875 (Van Wormer and Newland, 1996). The completion of the Southern Pacific Railroad between Los Angeles and Yuma in 1877, brought an influx of American miners and additional strikes in the Madre Valley followed. Most of the early workings took place prior to 1890. The early Padre y Madre workings included several vertical shafts, the deepest of which was 325 feet. The extend of stoping is unknown but reportedly extended several hundred feet along strike. In 1880, a strike was made three miles further north in Gold Rock Canyon (later Tumco Wash). A small rush ensued and in 1884, the miners formally established the Ogilby Mining District and christened their settlement Gold Rock Camp (Van Warner and Newland, 1996). The principal claims were the Golden Cross, Golden Queen, and Golden Crown. The Gold Rock discoveries accelerated interest in the area. Rich ore samples were reported to grade as much as $9,000 to $12,000 a ton. In 1892, the Golden cross alone had produced $24,374 in gold from its small scale operation in which the ore was still hauled by wagon to Ogilby station and shipped by rail to the El Rio Mill. By the late 1880s, the rich surface ores were nearly depleted and many of the miners sold out, lacking the capital for underground mining. In April, 1893, having consolidated the Gold Rock claims and being adequately capitalized, William Hedges and Thomas Fuller formed the Golden Cross Mining and Milling Company. By October of the same year, the company had built a 20 stamp mill and was processing $15/ton ore from the Golden Queen shaft. The following year they installed a 12 mile pipeline to supply water from the Colorado River. By 1894, in anticipation of developing the Golden Cross and Golden Crown shafts, the company had added another 20 stamps and a Huntington Mill. The new mill crushed 100 tons a day and produced $1,000/day.

In 1894, the camp's name was changed to Hedges in honor William Hedges. By this time, some accounts place Hedge's population at several thousand. The same year, a new 100 stamp mill was added to supplement the 40 stamp mill whose operation had been curtailed for lack of tailings disposal space. By November 1895, 140 stamps were processing 500 tons a day of low grade $6 - $10/ton ore. Excessive expansion costs and lower than expected ore grades drove the company into debt. Within one year the company owed $125,822 and the owners agreed to a receivership with all mine proceeds going to retire the debt. However, the receivership also operated the mine at a loss. By 1897, the Golden Cross Mining Company sold their interests in the mines to the Free Gold Mining and Milling Company of Nevada. By 1898 Free Gold had the mill operating at full capacity and producing $43,000 a month. In 1901, a cyanide plant was added to process the accumulated mill tailings in. Cyanidization of tailings proved so successful that mining ceased and in 1902 the stamp mill was shut down. By 1905, all outstanding debts had been retired, the receivership was terminated, and the mine was closed. Shortly thereafter Hedges became a ghost town. The mines lay closed until 1909 when they were reopened by the United Mining Company (TUMCO) and Hedges was renamed Tumco. The venture was short lived and United Mining closed and abandoned the mines in 1911. Once again Tumco became a ghost town. All told, approximately 150,000 ounces were produced during the early years of operation of the Golden Queen, Golden Cross, and Golden Crown mines (Tucker, 1926; Morton, 1977). The original American Girl Mine in neighboring American Girl Valley was first opened in 1892 and was mined continuously until 1900, during which time an estimated 35,000 tons of $8/ton ore was produced. Little mining was done thereafter until the period 1913-1916 when 20,0000 tons of ore averaging $6.50/ton were milled. In 1920, the mine was patented. From 1916- 1936, the mine was idle but from 1936-1939 about 17, 750 ounces from 169,000 tons of ore grading 0.11 opt were produced (Henshaw, 1942). Limited intermittent activity occurred on the properties after WWII. Total estimated production was 205,000 tons valued at $1, 285,000. Development consisted of two single compartment inclined shafts 740 and 850 feet deep. The original American Girl working shaft was sunk in the footwall of a vein at an incline of 35? in the upper levels and at 25? in the lower levels. The Tybo Shaft, about 800 feet west of the first shaft was sunk at a similar inclination to 850 feet. Main levels were developed at 100 intervals to the 700 foot level, the lowest level at 740 feet. Total horizontal drifts exceed 8,700 feet. About 1 mile southeast of the Madre Valley, in Jackson Gulch where the earliest colonists had worked placer deposits, the Cargo Muchacho Mine was located in 1877, By 1882, 14,000 tons of ore averaging $12/ton had been produced by the Paymaster Company from an auriferous quartz vein. By the early 1890's, a 20 stamp mill and 12 mile pipeline from the Colorado River had been constructed. Unfortunately, a fault cut the vein and efforts to locate additional reserves resulted only low grade ore. By 1894, the mine was closed having produced only minor amounts of gold. Other periods of activity were 1936-1942, and 1949-1952(?). Cyanidization of previous tailings was conducted about 1940. Total production likely exceeded 25,700 ounces of gold valued at $852,000. The mine was developed by a 680 foot inclined shaft at the north end of the vein and a 200 foot vertical shaft about 1,100 feet south of the deep shaft Most of the stopes were below the third level. The area as far north as 800 feet from the main shaft was explored on the surface by several shallow shafts. consisted of a 550 foot main shaft and a 200 foot shaft south of the main shaft.

In the late 1930s and early 1940s, In Tumco Wash, the Sovereign Mine was developed along a shallow dipping quartz vein (Sampson & Tucker, 1942). The vein was followed to a depth of 200 feet on the incline and along a 350 foot strike length (Tosdal, 1999, in press). Exploration from 1941-1942 delineated a low grade resource in the hanging wall of the Golden Cross Mine (Calvocoresses, 1942). By 1950, an estimated 215,000 ounces had been produced from mines in the western and central parts of the range, largely from the mines in American Girl Valley, Madre Valley, and Tumco Wash (Morton, 1977). In 1979, Newmont Exploration, Ltd. began exploring the area of the old American Girl and Madre Valley mines. Exploration delineated nine economic zones (Guthrie et al., 1987) and placed original aggregate open pit mineable reserves at 6.4 million short tons grading 0.051 opt. Underground reserves in the American Girl Valley were originally pegged at 1.2 million short tons containing 0.232 opt (Tosdal, 1999, in press). In May 1986, Eastmaque Gold Mines Ltd., through their American Girl Mining Corp subsidiary, purchased the American Girl and Padre y Madre properties from Newmont Exploration Ltd. In 1989, Eastmaque Gold sold a 50% interest to Morris-Knudsen Gold Corp and formed the American Girl Mining Joint Venture (AGMJV) to operate the mines. Mining was implemented in a phased approach, starting with the Padre y Madre Mine and ending with the Oro Cruz operation. Due to topographic separation and different mining and development schedules, the mines were permitted and operated independently. By 1987, the pilot phase of the Padre y Madre operation was permitted for heap leaching of 200,000 tons of ore from the West Pit and an incline to access the underground B Zone had been driven at the American Girl Mine. After a successful pilot run, full scale development work in the West and East Pits was permitted later that year for 3.5 million tons of ore and 12.5 million tons of waste rock. Surface mining commenced at the American Girl Valley Mine in 1989 which was permitted for 8.5 million tons of ore and 400,000 tons of waste rock. Underground Mining commenced in 1990. While the American Girl Mine was originally planned to be the final phase of the American Girl project, in 1990, the AGMJV acquired the neighboring Oro Cruz properties. Previous exploration and historical mining had identified both surface and subsurface ores. In 1993 permits were approved to mine 2.5 million tons of ore and 8.5 million tons of waste rock from the Golden Queen and Golden Cross pits and underground ore bodies as a third phase of the American Girl Project. At the start of Oro Cruz surface mining, open pit mining at the American Girl and Padre y Madre operations was phased out. The operation was confined to mining with all processing done at the American Girl Mine. Operations at the Oro Cruz mine ceased in November, 1996. Mining operations at all three mines was completed by 1996, with reclamation completed in 1999. Currently, the mines are undergoing post-reclamation monitoring.

Mining placer gold deposits in the Cargo Muchacho Mountains were first worked in 1780 by Spanish colonists using primitive dry washing methods. Winnowing of the dry deposits was done with small bellow washers and with blankets (Clark, 1970). By the late nineteenth century, most placer deposits had been exhausted, and mining turned to rudimentary underground workings. Most notable were the Golden Queen, Golden Cross, and Golden Crown mines in Tumco Wash. Details of early mine workings are sketchy. The Golden Queen Mine reportedly followed a linear ore body extending 700 feet before being lost to faulting. The Golden Cross ore body was mined to a depth of 1,200 feet on the incline and in a glory hole along a productive strike length up to 500 feet long. The Golden Crown ore body was mined along its 1,100 foot length along a 20? incline (Frost et all, 1986). The neighboring American Girl Mine is reported to have followed an orebody along a 200 foot incline, from which drifts at several levels were driven. Details of later historic workings are unavailable. The modern American Girl Project involved three adjacent mines that were developed in phased stages by the American Girl Mining Joint Venture (AGMJV). This allowed a systematic and methodical approach to mining the several discrete ore bodies while minimizing the expense of duplicative equipment and facilities and minimizing surface disturbances. The mines involved both surface mining and cyanide heap leaching of lower grade oxidized surface ores and underground mining, milling, and cyanide recovery from higher grade ore bodies. With the exception of a pilot leach pad at the Padre y Madre Mine, all heap leach pads, milling equipment, processing and recovery facilities were maintained at the larger and more central American Girl Mine with ores from the neighboring mines being trucked in. Pilot scale operations commenced in 1987 at the Padre y Madre Mine. Mining at American Girl commenced in 1989, and the short-lived Oro Cruz Mine commenced operations in 1995. The Padre y Madre pilot operation involved surface mining and heap leaching of 200,000 tons of ore. Full-scale development was permitted for 3.5 million tons of ore and 12.5 million tons of waste rock from two pits within a 239 acre site. The leach pad facilities occupied 37 acres. The American Girl Valley Mine was permitted for 8.5 million tons of ore and 400,000 tons of waste rock (on a 347 acre site) and used both milling and heap leaching to process ores. Eight orebodies were mined; 4 in surface pits and 4 in underground workings. Lower grade oxidized surface ores were heap leached while the higher-grade underground ores and a small percentage of high grade surface ore was processed at the American Girl mill facility. While not originally part of the American Girl Project, the need for additional reserves to operate the American Girl Mine lead to the acquisition, exploration and development of the Oro Cruz Mine. The Oro Cruz Mine consisted of two open pits and underground workings permitted for 2.5 million tons of ore and 8.5 million tons of waste rock on the 191 acre site. Exploration and previous historical mining identified both surface and subsurface ores. This operation was confined to mining only with all processing and recovery done at the neighboring American Girl Mine. Like the American Girl Mine, it involved mining of different orebodies including low grade oxidized surface ores and higher grade underground ores.

Surface mining operations were conducted similarly at all three operations. Size and shape of the pits was determined by geometry of mineralized zone, economics, geological/geothechnical characteristics of pit areas, equipment limitations, and safety. The Padre y Madre Mine involved only surface mining in two open pits (West and East pits) approximately 1,000 feet apart on the floor of Madre Valley. The East Pit measured up to 1,100 feet wide by approximately 1,500 feet long in a northwesterly direction. The West Pit lay about 1,000 feet to the northwest and measured approximately 1,600 by 1,100 feet. The overall strip ratio for the Padre y Madre open pits was 4:1. The Padre y Madre leach pad was located immediately west of the West Pit at the mouth of the valley. The more extensive American Girl Mine involved separate orebodies occurring in a relatively continuous area about 400 feet wide by 3,000 feet long. Hence, they were mined in four interconnected pits. The four pits, the Tybo, West, Main, and American Boy pits were mined progressively up canyon with mining progressing from the west to east. The American Girl leach pads were west of the pits. The Oro Cruz Mine included 2 open pits, the larger Cross Pit of about 25 acres and the Queen Pit of about 16 acres. Oro Cruz operations commenced at the Queen Pit (northwest) where ore was available with minimal waste rock removal. Initial stripping of rock from the upper levels of the Cross Pit was conducted concurrently with mining of the Queen Pit. After depleting the Queen Pit, operations were moved to the Cross Pit and the Queen Pit was backfilled with Cross Pit waste rock. At the other surface pits, waste rock was used to partially backfill the pits. A small percentage of high grade surface ore produced at the Padre y Madre Mine and high grade underground and surface ore from the Oro Cruz Mine were hauled to the American Girl crusher mill and processed in the existing cyanide mill. The lower grade Oro Cruz ore was hauled to the American Girl crusher and leach pads. Open pits were developed on 40 foot benches with pit walls maintained at approximately 1:1 slopes. Ground water, where encountered, exhibited very low flow rates (<1GPM) which readily evaporated. Open pit ores were excavated using a blast hole grid loaded with ANFO explosives and by mechanical ripping. Blast hole cuttings assays were used to delineate ore-waste boundaries according to assayed grade. The ore was loaded with front end loaders into a portable crusher and crushed to minus 2.5 inches. Bulk lime and water was added during crushing for agglomeration, pH control, and to minimize dusting. The crushed ore was trucked to the heap leach pads or mill processing facilities depending on grade. To minimize haulage, overburden stockpiles and waste rock dumps were located adjacent to the open pits. Waste dumps were developed by end dumping over the active dump face. Dumps were developed on 40 foot lifts separated by 20 foot catch benches. Underground mining at the American Girl Mine involved the systematic development of four discreet ore bodies commencing with the B-Zone followed by the Southwest Extension Zone, after which the American Boy and C-Zone ore bodies were concurrently developed. Based on their proximity, the B, Southwest Extension, and American Boy zones were mined using the same surface facilities using the B-Zone surface portal. The C-Zone ore body and surface facilities were located 0.5 miles down valley to the west.

Geology and Ore Deposits: The Cargo Muchacho Mountains are composed of quartzites and schists that have been intruded by granitic rocks. In places there are andesite and dioritic dikes. The gold deposits are on the west side of the range and occur in both the metamorphic and granitic rocks. They are tabular bodies with a definite hanging wall or footwall but rarely both. The deposits consist of quartz, calcite, sericite, and chlorite, and the values are either native gold or auriferous sulfides. Appreciable amounts of silver and copper also have been recovered in the district. The deposits, usually striking west, with a few north-strike exceptions, are up to eight feet thick and have been mined to depths of as much as 1,000 feet. Appreciable high-grade ore was found here.

Comments on the geologic information
INTRODUCTION The Cargo Muchacho Mountains are an isolated northwest trending range approximately 8 miles long and 3 miles wide located 10 miles south of the Chocolate Mountains. The range consists of Jurassic metamorphic and crystalline rocks within the upper plate of the Chocolate Mountains Thrust Fault. In addition to the American Girl mines, other important hydrothermal gold deposits including those of the Mesquite and Picacho mines are located within similar rock sequences along the southern flank of the Chocolate Mountains. 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. Suprajacent rocks Suprajacent rocks in the region consist of Tertiary volcanics and conglomerates which, where preserved, unconformably overlie the basement metamorphic. The earliest volcanics were basalt flows portions of which are preserved as basalt caps on the more 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. 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. 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 Pliocene and younger dextral strike-slip faulting associated with the evolving San Andreas Fault system. Structural ambiguities are many due to overprinting and fault reactivation. Late Cenozoic structural features overlap and sometimes obscure earlier Tertiary features, which in turn overprint Mesozoic features.

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 accretionary terranes transported northeastward by these plates were scraped from the descending plates. In southern California, the allochthonous Baldy and the Santa Lucia-Orocopia terranes accreted to the continent between 60-40 Ma. Jurassic gneisses, schists and intrusive rocks were then thrust over the Pelona and Orocopia schists along a regional system of mylonitic thrusts including the Chocolate Mountains Thrust which underlies both the Cargo Muchacho and Chocolate mountains. In the southern Chocolate Mountains and Cargo Muchacho Mountains, the Orocopia schist forms the lower plate of the Chocolate Mountains Thrust Fault. Jurassic gneisses and schists and igneous plutonic rocks form the upper plate which host the Tertiary gold deposits. Thrusting has 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). 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 involved low angle detachment fault systems which accommodated much of the Oligocene-Miocene extension with an anatomizing network of low angle faults throughout southern California region (Frost and others, 1997). Important mineralization is associated with these detachment features at the Picacho Mine 12 miles to the northeast. 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 Oligocene detachment faulting to high angle block faulting and strike-slip tectonics during the Miocene-Pliocene. By late Pliocene, the regional tectonic environment had become dominantly one of dextral strike-slip motion as represented by the Sand Hills Fault of the San Andreas Fault zone (approximately 10 miles west). Similar to much of the east margin of the Salton Trough and southern pediment of the Chocolate Mountains, the Cargo Muchacho range is cut by a series of northwest striking dextral strike-slip faults (Dillon, 1975) that are thought to be inactive strike-slip strands of the San Andreas fault system (Willis and Tosdal, 1992). Metallogeny The association of gold mineralization with the AGSZ suggests that similar deposits may be present to the east and west of the American Girl, Padre y Madre, and Oro Cruz mines. Regional trends suggest the zone is part of a regional feature that likely plunges below the thick cover of flanking alluvium to the east and west of the Cargo Muchacho Mountains. While significant economic ores have not been identified within the range and east of the mines, this does not preclude the presence of ore bodies within the shear zone, perhaps at depth or fault separated. Further understanding of the shear zone kinematics, as well as the detachment faulting and strike-slip mechanics that control nearby Picacho and Mesquite mine mineralization, should advance our understanding of regional interactions and the complex relationships between tectonics and ore body mineralization. These advances will likely require a concerted effort employing sound geological interpretations, geochemical and geophysical studies, and exploratory drilling.

Local Geology: The Cargo Muchacho Mountains are comprised of Jurassic metamorphic and intrusive igneous rocks. The AGSZ, an east-northeast trending low angle Mesozoic ductile shear zone, bisects the range in the vicinity of American Girl Valley. The shear zone divides the rocks into an upper plate of mid-crustal Jurassic granitic plutonic rocks and a lower plate of supracrustal rocks consisting of granite gneiss and quartzofeldspathic gneiss of the Jurassic Tumco Formation. The granite gneiss consists of metamorphosed granitic rocks that intruded the supracrustal rocks before and during regional metamorphism. The kinematics and direction of shearing has not been adequately resolved, however, the inverted section indicates contractional deformation was dominant. The shears also parallel regional foliation and the underlying Chocolate Mountains Thrust Fault. American Girl Shear Zone The AGSZ is characterized by a series of parallel, gently south dipping, ductile shears that, in cross section, interleave a wedge of the Tumco Formation between two large sheets of granite gneiss. The sheets dip to the south, paralleling both foliation and shears. The top of the AGSZ generally corresponds to the transition from deformed gneiss to diorite and granitic rocks in the upper plate. In most places this corresponds to the contact between the uppermost granite gneiss and the overlying Araz Wash diorite. The upper granite gneiss sheet metamorphosed the overlying diorite, locally converting to schist. The contact metamorphism and ductile shear fabrics in the upper plate indicate that granitic intrusion and shearing were contemporaneous. Movement on the AGSZ is interpreted to have occurred as early as lower Jurassic and possibly continued to the early Tertiary Ductile microstructures within the granite gneiss suggest down to the south motion of the upper plate (Branham, 1988). However, down dip shear would imply an extensional environment whereas the superposition of older mid-crustal rocks over younger supracrustal rocks requires contraction. Tosdal (1999, in press) suggested this disparity might be the result of post-metamorphic tilting of the range which modified the original dip of the shear zone. The Upper plate of the AGSZ consists of the Araz Wash diorite, a composite intrusive unit of commingled granitic rocks (Hayes, 1989, 1992; Murphy et al., 1990). Hornblende-biotite diorite, monzodiorite and porphyritic monzodoiorite to granite are the dominant rock types. Hornblende geobarometry from the mafic phases of the Araz Wash diorite indicate it was intruded at depths greater than 20 km (Hayes, 1989, 1992). U-Pb ages from zircons within the diorite indicate a Middle Jurassic age between 170-173 Ma (Dillon, 1975). The Tumco Formation and the granitic gneiss compose the lower plate. The Tumco Formation is primarily a gray fine grained, gray quartzofeldspathic gneiss derived from silicic volcanic and sedimentary protoliths. Petrologically, the Tumco Formation resembles biotite gneiss of the same age which hosts significant gold reserves in the Mesquite Mine of the southern Chocolate Mountains. Upper greenschist to lower amphibolite facies metamorphism is indicated by a quartz-microcline-plagioclase-biotite-epidote-magnetite mineral assemblage (Dillon, 1976). U-Pb isotopic data indicates the Jurassic age protolith. Syn-and post-kinematic pegmatite and granitic sills intrude the Tumco Formation. The sills merge with the main contemporaneous masses of granite gneiss. U-Pb dating of zircons within the granite gneiss yields a late middle Jurassic age between 160-175 Ma (Tosdal, 1999, in press). Dikes of pegmatite and granite also form northwest and southeast striking swarms. These dikes are mostly concentrated in the Tumco Wash where they intrude auriferous quartz-magnetite-biotite ore bodies.

The AGSZ was subsequently impacted by younger Cenozoic brittle deformation. Dextral shear associated with the development of the San Andreas Fault system produced a regional fabric of right stepping, en-echelon northwesterly trending dextral strike-slip faults intersected by northerly striking normal faults. In the Cargo Muchacho Mountains, this deformation partitioned the range into several discreet blocks that may have undergone some counterclockwise rotation. This episode also reactivated parts of the AGSZ as shallow south dipping intrusive contacts and brittle low angle faults (Branham, 1988). Aluminosilicate mineral assemblages Feldspar, muscovite, and kyanite dominant mineral assemblages occur within the gneisses of the Tumco Formation. These assemblages form a zoned sequence that grades from the regional amphibolite facies quartzofeldspathic gneiss into locally occurring quartz-oligoclase-biotite-epidote-magnetite (feldspar zone), quartz-muscovite-biotite-magnetite-apatite-tourmaline (muscovite zone), and an aluminous quartz-kyanite-magnetite-rutile-apatite-tourmaline-lazulite (kyanite zone) assemblage (Owens and Hodder, 1993). The aluminosilicate assemblages are represented primarily by kyanite-quartz granofels and muscovite - biotite schists, that are laterally associated with all significant gold concentrations. The largest are about 0.5 - 1 mile west and on strike with the American Girl and Padre y Madre gold deposits along low angle faults. Henshaw (1942) concluded these rocks were products of regional metamorphism of sedimentary layers, but more recent interpretations favor a metasomatic origin (Wise, 1975; Dillon, 1976; Branham, 1988; Owens and Hodder, 1994). Owens and Hodder (1993) determined the zonation was consistent with hydrogen metasomatism by oxidizing slightly acidic magmatic fluids at 500-550? C and 400 kbar under mesozonal crustal conditions. Metasomatism was dominated by cation leaching and depletion, and the enrichment of immobile elements such as aluminum in the country rock. The oxidized fluids precluded gold deposition in the aluminosilicates, in favor of deposition in the more reduced peripheral chlorite and pyrite bearing assemblages (Owens and Hodder, 1993). Kyanite bearing inclusions within the intrusive rocks imply that metasomatism preceded or was coeval with the Jurassic intrusives The American Girl and Padre y Madre ore bodies Most of the ore in the Cargo Muchacho Mountains has been produced from mineralized zones in the vicinity of the American Girl and Padre y Madre mines where ore bodies are characteristically elongate southwest raking lenticular to sheet like bodies which are occasionally disrupted and offset by high angle faults. The primary deposits occurred within 1) sheared rock along 15?-45? south dipping faults within the AGSZ which served as conduits and host for mineralization (American Girl ore zone), 2) within fractured higher grade quartz veins along the these faults (B-Zone and Southwest Extension ore bodies), or 3) within fractured and permeable gneisses of the Tumco Formation which allowed the spread of mineralization away from the fault zones. The low angle faults strike generally northeast-southwest in the American Girl Mine and northwest-southeast in the Padre y Madre Mine.

Ore Genesis and Mineralization Gold mineralization is thought to have occurred during three superposed events (Henshaw, 1942; Guthrie et al, 1987; Branham, 1988; Borrastero, 1990). The first two events consisted of ductile deformation with invasion of syn-metamorphic or magmatic ore fluids and produced the major economically important deposits in the district. The last event was limited to brittle reactivation of faults, remobilization of gold, and the enrichment of existing deposits. In general, the highest gold concentrations occur along faults within shear zones, whereas lower disseminated concentrations occur in the surrounding country rock. The gold is closely associated with pyrite and occurs as <1 to 20+ micron size grains that adhere to surfaces, occupy fractures, or are enclosed within pyrite. The auriferous pyrite occurs in quartz veins and veinlets, in fractures lacking quartz, and as disseminated grains. The initial stage of mineralization is characterized by widespread metasomatic iron enrichment, formation of the aluminous assemblage rocks, and gold deposition. Mineralization occurred during metamorphism and tectonism along the AGSZ in the late middle Jurassic. Highly oxidizing, sulfide poor, and iron rich hydrothermal ore fluids formed lenticular auriferous quartz-magnetite-biotite veins that are widespread in the Tumco Formation but best developed in Tumco Valley where they reach ore grade and were mined in the Queen and Cross pits of the Oro Cruz Mine. Initial mineralization of the American girl orebodies is also attributable to this stage. Gold was submicroscopic. Increased magnetite concentrations crudely correlated with higher gold grades with gold concentrations between 1 - 5.5 ppm occurring only in rocks with greater than 5 weight percent total iron. Copper, lead, and zinc also tended to be elevated when where gold was present. Trace amounts of scheelite, cinnabar, pyrite, chalcopyrite and fluorite were also associated with mineralization (Tosdal, 1999, in press). These orebodies were on strike with the metasomatized aluminous gneiss and were abundant along the basal contacts of the upper granite gneiss, but also occurred along discreet foliation parallel horizons in the Tumco Formation. The second phase of gold mineralization occurred during retrograde greenschist metamorphism. High metamorphic grade minerals of the upper greenschist-lower amphibolite facies were recrystallized to lower greenschist facies mineral assemblages forming chlorite-quartz-sulfide bodies and hydrolytically altering the wall rocks. In contrast to the first stage, gold deposition was accompanied by deposition of sulfide minerals, primarily pyrite, from less oxidized and sulfur bearing hydrothermal fluids. Locally gold deposition was superposed on the older iron oxide rich horizons. Most of the second stage orebodies were concentrated in American Girl and Padre y Madre mine area where they occur within the faults of the American Girl, B-Zone, and C-Zones.
Mineralization developed along brittle-ductile shears that parallel foliation. In the Tumco Formation, orebodies were localized in areas enriched with biotite and magnetite, and occured as thin discreet shear zones rich in chlorite with disseminated sulfides and small quartz-pyrite veinlets. In the granite gneiss, the deposits occur as massive, ribboned, milky quartz veins and quartz stockworks with pyrite, chlorite, and sericite in and adjacent to the veins. Pyrite in both types of veins was associated with gold, and composed 3-5% of the veins, Chalcopyrite was locally associated with the gold. Tourmaline is an accessory (Tosdal, 1999, in press). Quartz-magnetite veins were associated with larger gold-bearing veins. Associated minerals included fluorite, scheelite, galena, sphalerite, and carbonates (Tosdal, 1999, in press). Fluid inclusion and stable isotope geothermometry from the quartz veins suggest deposition from magmatic fluids at a temperature of formation between 270- 300?C (Branham, 1988; Borrastero, 1990). Salinities were low, being less than 9 weight % NaCl. Some fluid inclusions contain CO2 which could indicate a contribution from metamorphic fluids. Timing of retrograde stage mineralization post dates formation of the aluminosilicate rocks and the initial stage of Jurassic gold mineralization Borrastero (1990) concluded it occurred during early Miocene extensional deformation due to similarities with retrograde metamorphics found under detachment faults of this age. Tosdal (1999, in press) suggested the mineralization was Paleocene since retrograde metamorphism and extensional faulting accompanied the Paleocene unroofing of the nearby Chocolate Mountains. The final stage of mineralization involved the brittle reactivation of older ductile and brittle shears zones and the remobilization of gold which locally upgraded existing ore bodies. The brittle nature of deformation associated with the final stage of mineralization suggests a mid Tertiary age coinciding with regional brittle deformation or to the development of the San Andreas Fault system in the Pliocene (Dillon and Ehlig, 1993). Fractures and microfaults that are typically marked by red brown clay gouge. Remobilized gold rarely, by itself, reached ore grade. The small Sovereign Mine in Tumco wash was developed along one of the late brittle shear zones. The Padre y Madre Mine ore body was highly fractured and faulted with some of the gold associated with a red clay gouge horizon along a gently dipping shear zone. Also, the B Zone in the American girl Mine was cut by mineralized listric normal faults that soled into a breccia zone at the base of the B-Zone. However, Borrastero (1990) noted that the basal breccia zones were ore grade only where they contained abundant second stage auriferous quartz vein material.

Production Data: Early production figures for the Cargo Muchacho District mines are sketchy at best. Hanks, (1886) reported that the "Cargo Muchacho mines" had worked 14,000 tons of ore and had produced $167,000 from discovery to June 17, 1882. The original American Girl Mine produced gold on a small scale between 1892-1916 and again between 1936-1939 (Branham, 1988). It produced 17, 750 ounces from 169,000 tons of ore at an average grade of 0.11 opt (Henshaw, 1942). Morton (1977) estimated 215,000 ounces had been produced from mines in the western and central parts of the range, largely from the mines in American Girl Valley, Madre Valley, and Tumco Wash. Commingled production from the American Girl Project mines between 1986 and their closure is estimated at slightly over 500,000 ounces yielding a total production estimate for the district of 700,000 ounces. While some of the early near surface oxidized ores and a select few quartz veins experienced higher grades, the average grade of ores mined in the open pits of the American Girl Project ranged 0.04-0.05 opt while richer underground ores averaged 0.2-0.5 opt.

Mines: American Boy, Amercian Girl ($1 million), Big Bear, Blossom, Butterfly, Cargo Muchacho ($100,000+), Coffee Pot, Colorado, Desert King, Golden Cross ($3 million+), Golden Queen, Guadaloupe, Little Bear, Madre and Padre ($100,000+), Ogilby group, Pasadena, Sovereign, Vitrafax and White Cap.

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References