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| Leaving Laramie, Wyoming and heading for Arizona (pencil sketch by the author). |
For those unfamiliar with porphyries, these represent root zones ofold volcanoes or magma chambers and rocks associated with them include multiple granitic stocks (more specifically granodiorite, diorite, quartz monzonite, etc) that include porphyritic rocks. A porphyritic rock is simply an igneous rock that has large crystals (such as feldspar and quartz) dispersed throughout a finer grained rock matrix or groundmass. When mineralized, porphyries exhibit disseminated sulfide blebs in their matrix. When they intrude reactive rocks such as limestone, the intruded rock is sometimes replaced by massive sulfides known as replacements or skarns. Other types of mineralization with porphyries include high-grade veins and low-grade networks of veinlets known as stockworks. The veins often provide excellent targets for prospectors as some contain very high values in gold and/or silver. A group of veins explored around the Kirwin porphyry by AMAX in the 1970s-80s, yielded channel samples that assayed >50 ounces per ton (opt) in silver and a few assayed >100 opt silver (Hausel, 1997)!
Veins may radiate from the intrusive center occupying fractures and mineralized breccias often lie adjacent to the porphyry. Even though porphyry deposits are considered to be low-grade and are mainly mined by open pit, they often contain a few $billion in metals that include copper, gold, silver, lead, zinc, molybdenum and gemstones. Porphyry deposits have distinct rock types, wallrock alteration, and mineralization.
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| Author prepares to go underground at the Resolution mine in Superior. |
Arizona is considered the place to visit to see classical porphyry copper-gold-silver and massive base metal-gold sulfide deposits. So many are found in Arizona that a book edited by Titley and Hicks (1966) was required reading in all mineral deposits and economic geology classes in geology, and many other books were published later.
When I worked for the Wyoming Geological Survey, I used this book as a guide to summarize similar porphyry copper deposits in the Absaroka Mountains in northwestern Wyoming (Hausel, 1982, 1997). The Wyoming deposits had been investigated by companies and university-related projects before being withdrawn by the Federal government and incorporated into wilderness, primitive, roadless and other withdrawals designed to build a giant border around Yellowstone National Park and to stop mining natural resources. I never could understand why we needed a boundary around one of the most caustic volcanic environments on earth that already included 3,472 mi2 of volcanic terrain (larger than some states) particularly when the boundary (not to mention the national park) included considerable precious and base metal resources.
Arizona has been an important source for copper, molybdenum, lead, zinc, gold and silver. In 2007-2008, miners extracted copper, molybdenum gold and silver at Morenci, Bagdad, Sierrita, Safford, Miami, Ray, Mission, Silver Bell, Pinto Valley, Mineral Park, Johnson Camp and the Carlota mine. In addition, exploration continued at Resolution, Rosemont and other properties (Niemuth, 2008).
The State’s gold deposits occur primarily in eluvial deposits, dry placers, stream placers, veins, faults and breccias along with gold associated with porphyry copper deposits (see video of Duval porphyry copper mine in Green Valley, Arizona below), massive sulfides and replacement deposits. Total historical gold production from Arizona has been about 16 million ounces sincethe 19th century and it could produce a lot more.
Much precious metal is recovered as by-product of copper from open pit porphyry, massive sulfide and replacement (skarn) deposits. Few primary gold lodes of major economic importance have been found. But notable are primary gold deposits at the Vulture mine, which recovered nearly 370,000 ounces of gold in the Wickenburg district north of Phoenix (central Arizona), and the Oatman and Katherine camps which produced >2 million ounces of gold in the San Francisco district near Toprock (Koschman and Bergendahl, 1968). Significant amounts of silver with by-product gold were recovered from mines in the Tombstone district in the past and other deposits of interest include the Lost Basin and Chloride areas in western Arizona, gold properties south of Tucson within the Arivaca and Oro Blanco districts near Bisbee, and in the Rich Hill-Weaver Mountains area near the center of the state. To me, one of the better deposits in the State besides Lost Basin, is Mexican Hat in the Gleeson district.
Much early mining was done by Spanish and Mexican prospectors in their search for silver. Gold was found by a Spanish priest in 1774 in the Quijotoa Mountains, 60 miles west of present day Tucson. The gold was found in gravel and in nearby veins associated with hematitic breccias. Over the next many decades, gold was identified in several districts described by Wilson and others (1969) and Wilson (1981).
Most gold placers in Arizona occur in fanglomerates on pediment surfaces or in adjacent gulches down-slope from these pediments. Similar to Australia, active stream placers are rare because of the lack of perennial streams. Over the decades, gold was identified in several districts. Only a few of the more prominent are discussed below and the reader is referred to Wilson and others (1969) and Wilson (1981) for more information.
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| Cartoon showing relationship of eluvial gold with lode |
Placers were found in nearly every county in Arizona. Most are fanglomerates deposits on pediments or in the adjacent gulches of the pediments due to the intense arid environment in Arizona. Similar to Australia, active stream placers are rare because of the lack of perennial streams. One of the few active stream placers is Lynx at the southern edge of Prescott. More than 108,000 ounces of gold have been recovered from this placer since discovered in 1863. The placers are reportedly found along the entire length of the creek. Gold occurs as fine flakes and nuggets up to 4 ounces. Portions of the placer were set aside for recreational gold panning by the Prescott City and the US Forest Service. Nuggets up to 11 ounces were found in the Copper Basin placers about 15 miles to the west. Another active placer was found in 1853 near the confluence of the Gila with the Colorado Rivers in southwestern Arizona. The discovery attracted more than 1000 prospectors in 1861, but the placer played.
Most Arizona placers are related to flash flooding, and the notable concentration of dry placers in the state associated with pediments suggest nearby source lode deposits. Research studies on minerals and the character of gold in these placers would likely lead to discovery of several lode gold and hidden porphyry deposits. The more productive pediment placers were found at LaPaz, Dome, Gila City, LaCholla, Weaver, Rich Hill, Greaterville, Quijotoa, King Tut and White Hills (Wilson, 1981). For the most part, these are dry placers: flash flooding events are important for gold concentration in pay streaks in these types of placers. Notable are the Greaterville placers in southern Arizona south of Tucson. This deposit was rich in gold, but water had to be transported upslope to the dry placers over a distance of more than 4 miles.
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| Match box reportedly made from a piece of gold-fracture-fill milky quartz from the Lost Dutchman mine located somewhere in the Superstition Mountains. I recently wrote about this in the ICMJ Prosepctors and Mining Journal. |
At the LaPaz placer west of Quartzite, gold was discovered in 1862 along the Dome Rock Mountains in western Arizona. Gold was recovered at Goodman Arroyo, LaPaz Arroyo, Ferrar Gulch, Garcia Gulch and Ravenna Gulch. The gold-bearing gravels ranged from a few feet to unknown depth and is distributed throughout the gravel and enriched on bedrock. The district became known for large nuggets that included a 26, 27, 47 and a 65 ounce nugget. Such large nuggets are often found near the lode source.
Rich silver lodes were discovered in Arizona. Some were found in the Bradshaw Mountains as well as at the Silver King, Signal, Globe and Tombstone districts. The silver boom was followed by a copper boom, which was stimulated by completion of the transcontinental railroads in 1881. Along with copper, came gold – as by-product of several lode mines and a few primary mines.
The majority of lode deposits are auriferous quartz veins with limited strike length and width, argentiferous veins and replacement deposits in limestone, and giant copper porphyry deposits and massive sulfides marked by multiple intrusive stocks with distinct zones of hydrothermal alteration and mineralization found as disseminations in the stocks and as veins are replacement deposits in the adjacent volcanic and sedimentary rocks. These large porphyries also contained several other base metals along with some gold and silver. Notably, several gold deposits that have considerable potential that has been mostly ignored are the Detachment Fault deposits all across Arizona and described by Hausel (2019, 2020). As a gold prospector, you should learn about these types of deposits because they are so under-explored and yet those that have been identified and cursory explored, have considerable gold and silver. And at one, known as Copperstone mine in LaPaz County, more than a million ounces of gold have been mined and identified through drilling.
Ajo district
Gold was recovered as a by-product of copper at Ajo, south of Gila Bend and 75 mi. south of Phoenix. Copper was initially recovered on a small-scale in 1750 AD by Spaniards and later by open pit at the New Cornelia mine in northwestern Pima County. By 1917, New Cornelia developed into Arizona’s first open pit mine. Total by-product gold through 1959 was 1 million ounces with the 6.3 billion pounds of copper. Operations ended in 1983. But like most mines, the deposit was never mined out.
The Ajo ore body is elliptical (6,400-feet long by 3,900-feet wide and 1,560-feet deep). It has veinlets and disseminated chalcopyrite, bornite and minor pyrite. The upper part of the porphyry is oxidized with copper carbonate and silicate and minor chalcocite in fractured and faulted Laramide-age quartz monzonite and quartz diorite. Mineralization extends into the intruded Cretaceous volcanics and controlled by fracturing and rock permeability associated with fractures, veins and breccia. Rock alteration includes sericitic, chloritization, kaolinization, silicification with oxidation down to water table (Joralemon, 1914). Gold is closely associated with copper-sulfide rather than pyrite.
The ore body was mined to a depth of 750-feet and total production from 1917 through 1972 amounted to 350 million tons of ore averaging 0.8% Cu, 0.05 opt Ag and 0.004 opt Au with some zinc (Zn) and lead (Pb)(Singer and others, 2008). Some copper minerals are gem quality and used in jewelry (ajorite, shattuckite and turquoise).
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| Lavendar Pit, Bisbee |
Copper was discovered in 1877 at Bisbee in the Mule Mountains of southern Arizona with by-product gold, silver, lead and zinc in massive sulfide and replacement deposits. Open pit operations were developed at the Copper Queen and Lavender mines and shafts sunk to access higher grade zones at depth. More than 8 billion pounds of copper, 2.9 million ounces of gold, 77 million ounces of silver, 305 million pounds of lead and 372 million pounds of zinc worth >$6.1 billion (1975 prices) were recovered. Mineralization occurs as high-grade copper carbonate with minor lead and zinc in irregular replacement deposits in the Martin Limestone (Devonian). Some mineralization was also found in the Escabrosa and Naco limestones. The ore was controlled by dikes, sills, faulting and associated brecciation. Much of the ore was bornite replacing pyrite. Malachite from the mine is used for gem material (Bryant and Metz, 1966).
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| Campbell Headframe, Bisbee |
The Campbell ore body sits adjacent to the Lavender pit. This was largely an oxidized copper deposit with minor lead and zinc as irregular replacements in the Abrigo (Cambrian), Martin (Devonian) and Escabrosa (Mississippian) limestones in close association with porphyry dikes and sills and developed by underground mining. The ore body is composed mainly of chalcopyrite with masses of bornite and chalcocite. Near the margin, sphalerite and galena are increasingly abundant.
Because of a significant oxidized and supergene enriched zone, new minerals were discovered in the Bisbee mines. One of these, known as shattuckite, forms a secondary copper-silicate-hydroxide that was discovered at the Shattuck mine in Bisbee in 1915. It produces brightly-colored, blue to green specimens and lapidary.
Because of a significant oxidized and supergene enriched zone, new minerals were discovered in the Bisbee mines. One of these, known as shattuckite, forms a secondary copper-silicate-hydroxide that was discovered at the Shattuck mine in Bisbee in 1915. It produces brightly-colored, blue to green specimens and lapidary.
Florence District
Not far from the Phoenix valley, a copper porphyry was discovered within the city limits of Florence. The deposit was apparently discovered a few decades ago, but is currently being developed for in-situ leaching. In-situ leaching has been around for many decades and is a relatively safe process by which injection wells are injected with a solution that will leach the ore and the pregnant solutions are recovered in other wells. The project is expected to produced 55 million lbs/yr for about 25 years from a 308 tonne orebody that averages 0.358% Cu. Most porphyry deposits have very low grade gold and silver and these precious metals will remain unmined. The project is only a short distance southeast of the Santan Valley along the Hunt Highway. The project will likely provide some valuable revenue to the town of Florence even though the deposit is located on State and Private land.
Gleeson-Pearce-Courtland-Turquoise District
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| The Commonwealth mine at Pearce has 20 miles of underground workings. The mine produced 16 million ounces of silver and 125,000 ounces of gold. You can bet they left some good ore, low-grade ore & likely missed ore bodies in the immediate area. |
Based on geology and past mineral production, significant silver, gold, copper and lead, semi-precious gems and lapidary material is found here. Lapidary includes copper, turquoise, agate, banded quartz, amethyst, prismatic quartz, opalized quartz, geodes and decorative stone occur here. World class specimens of wulfenite are reported at the Defiance mine (31°44'35"N; 109°49'21"W) (Larry Meyer, pers. com., 2013).
Past production includes 16 million ounces of silver, 200,000 ounces of gold, 887,000 tons of base metal ore, 250 tons of manganese ore, some turquoise and considerable quartzite mined for smelter flux. Based on modern metal prices, the value of past production is close to $1 billion. Based on recent drilling, a minimum of 1.0 million ounces of gold and 26 million ounces of silver remain in the district indicating that unmined ore at just a couple of properties is worth more than $2.5 billion based on metal prices at July 30, 2020 (Hausel, 2020; Stewart and Hoag, 2013; Volk and others, 2012)!
Copper was initially discovered here in 1890 (east of Tombstone). The initial mining camp was called Turquoise. Four years later, gold and silver was found on the Commonwealth property 11 miles to the north and the town of was Pearce was established. The Commonwealth has considerable unmined gold and silver estimated to include 17.6 million tonnes of indicated resources grade at an average of 0.016 opt Au with 1.4 opt Ag, along with and inferred resource of 1.089 million tonnes or ore at an average grade of 0.01 opt Au and 1.14 opt Ag. This suggests 300,000 ounces of gold (worth $585 million on July 30, 2020) and 26 million ounces of silver (worth $608 million) (Hausel, 2020; Stewart and Hoag, 2012) remain to be mined. Typically, when mining commences, resources and resources increase. The Commonwealth is also surrounded by several adjacent prospects that likely will increase the know precious metal resources (Hausel, 2019).
In 1895, a prospector named John Gleeson prospected near Turquoise and found copper and opened the Copper Belle mine. Other mines were developed and the new mining camp was named Gleeson. Mines in this district operated until copper prices declined resulting in mine closures in the 1930s (Hausel, 2019).
The geology in southeastern Arizona suggests the area has high potential for discovery of hidden gold, silver and base metal deposits. A variety of mineral deposits have already been identified in a broad, north-south, regional trend. Based on geology, this is one of the better districts in Arizona to prospect, and many other discoveries are likely. Hausel (2019, 2020) describes more than 2 dozen mines and prospects in this area providing GPS coordinates.
The mineralized terrain is in the southeastern Dragoon Mountains of southeastern Arizona: notable deposits are found at the Gleeson-Courtland mine, Commonwealth mine, Mexican Hat volcanic-hosted gold mineralization (Tertiary) and Gold Coin sedimentary-hosted gold project (Tertiary). Turquoise Ridge to the north is separated from the Gleeson Ridge to the south by a narrow gulch, Mexican Hat lies a short distance north of Turquoise Ridge. The ridges rise 900 to 1,200 feet above the adjacent plains. Quartz monzonite intrudes Paleozoic and older rocks, and granite cuts quartz monzonite and invades Cretaceous rocks.
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| Mexican Hat disseminated Gold deposit, near Pearce Arizona |
Mineralization occurs as (Hausel, 2020): (1) Irregular copper-blanket and replacement deposits where quartzite and limestone (Cambrian Bolsa Quartzite and Abrigo Limestone) have been thrust over Carboniferous limestone. (2) Irregular, tabular, pyritic lenses in Carboniferous limestone along a contact with quartz monzonite. (3) Manganese, lead, zinc, silver and minor copper and gold in irregular deposits associated with fault and fracture intersections in Pennsylvanian-Permian Naco Group limestones; (4) Near-surface turquoise in stringers in altered granite and quartzite; (5) Spotty base metals with gold and silver in veins in intrusive rocks; (6) Disseminated gold in Tertiary rhyolite and breccia; and (7) Structurally-controlled disseminated gold in faults and breccias in sedimentary rocks. At least 887,000 tons of base metal ore and 250 tons of manganese ore were produced with some turquoise and considerable quartzite for smelter flux in the past.
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| Gleeson-Pearce district near Tombstone one of the better gold and silver districts in Arizona. |
Two of the more interesting include a mineralized structure, 15- to 50-feet wide and 2,000-feet long at Mexican Hat. Gold in fractures is accompanied by hematite and limonite in Tertiary rhyolite and rhyolite breccia. In 1990, Placer Dome and Oneida Resources identified a geologic resource in 6 mineralized zones of 10.3 million tons of ore with average grade of 0.035 opt Au (362,000 ounces, worth $706 million. A similar deposit south of Mexican Hat, known as Gold Coin is an epithermal sedimentary deposit. Secova geologists collected composite chip samples from trenches on this deposit and included 45-feet of 0.11 opt Au: these samples began and ended in mineralization. In another trench, a composite chip sample yielded 60-feet of 0.06 opt Au (with an enclosed zone of 15-feet of 0.2 opt).
Soil geochemical anomalies show strong signatures of gold, silver, arsenic and antimony. Combined geochemical and geophysical anomalies outline a north-northwest trending zone 3,000-feet long by 1000-feet wide and two other anomalies of 1000-feet by 1500-feet and 2800-feet by 750-feet. The area is covered by thin veneer of soil making outcrop sampling difficult. Brecciated limestone in the mineralized zone has variable clay alteration and moderate to locally strong silicate veining with hematite veinlets. It is assumed Tertiary gold-enriched fluids were emplaced along N-trending structures and along subvertical to high-angle normal faults. Some Carboniferous limestones have extensive jasperoid alteration and strong limestone decalcification (Moore, 2010).
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| Gossan-stained mine dumps near Gleeson, Arizona |
The Northern Miner reported that Auracle Resources Ltd, picked up Mexican Hat.
Lone Star (Safford) district
Safford is 16-miles southwest of Morenci in southwestern Arizona, and is a porphyry copper district with the Dos Pobres, San Juan, and Lone Star open pits as well as other mineralized porphyry deposits that likely will be mined in the future. A group of mineralized porphyries were found along a northwesterly trend paralleling the Butte Fault. along the western flank of the Gila Mountains. The intrusives include the San Juan, Lone Star and Horse Shoe quartz monzonite stocks that intrude andesite. A distinct northeasterly shear is recognized in the volcanics.
Mineralization consists of pyrite and chalcopyrite as disseminations and veinlets with minor bornite, molybdenite, sphalerite, galena, magnetite, specularite, chrysocolla, brochanitite, cuprite, malachite, native copper and turquoise (Cook and Robinson, 1962). The deposits have associated gold and silver. Geophysical exploration at Safford suggests possibilities for some hidden copper-gold deposits at depth.
Globe-Miami-Hayden-Christmas
Copper was found at Globe in 1874. In 1904 development began on a large, low-grade, disseminated copper deposit, which by 1911 was mined on a large scale. Globe is 50 miles east of Phoenix in the foothills of the Pinal and Apache Mountains and includes a group of porphyry deposits at Miami, Hayden-Banner, Ray, Christmas and Superior. These operations produced copper, lead, silver, gold, and zinc worth in the $billions. Total gold production through 1959 included 191,801 ounces.
Mines, such as, Miami Inspiration, Castle Dome, Copper Cities and Cactus all produced by-product gold with copper, and some gulches peripheral to these, such as Castle Dome and Golden Eagle, yielded placer gold (Hausel, 2019).
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| The Ray pit in background behind author |
The Hayden-Banner district is 8 miles north of Winkleman and 22 miles south of Globe and includes the Christmas mine at the southeast end of the Dripping Springs Mountains (Boss, 1925). The Christmas was a surface and underground Cu-Au-Ag-Mo-Bi-Pb-Zn-Be-W-garnet abrasive mine discovered in 1880: mining began in 1905. Workings continued to the 908-foot level and mineralization continued to greater depth. The property included 5 shafts and an open pit that operated until closure in 1982. The deposit lies in a series of gently dipping Paleozoic limestones overlain by Cretaceous volcanics. A quartz diorite stock intrudes the succession.
The Christmas fault cuts limestone, lavas, and the quartz diorite intrusive along a NW-trend. Contact metamorphic and replacement deposits form an ore zone 4,900 feet long, 2,600 feet wide and 2,100 feet deep that were controlled by limestone-diorite contacts, favorable limestone beds, garnetized zones and fractures. The deposit has a pyrite-chalcopyrite core, chalcopyrite-bornite intermediate zone, and pyrrhotite-sphalerite-chalcopyrite margin. Past production focused on Naco limestone and a few smaller deposits were mined from the Escabrosa limestone. More than 55,340,000 pounds of copper (1905 to the end of 1943) and 300,000 ounces of silver were recovered. The ore had an average grade of 0.005 oz/t Au, 0.23 oz/t Ag and 2.04% Cu. Total gold production was only 26,000 ounces.
The Ray mine situated halfway between the Hayden and Miami camps and 15 miles south of Miami began as an underground mine prior to 1911 and yielded an estimated 4.5 million tons of copper. The operation was converted to open pit in 1955. Published reserves (1992) include 1.1 billion tons of ore averaging 0.6% Cu. The geology is complicated by faulting, host rock variation, two episodes of tilting, complicated enrichment history and hypogene and supergene alteration. Mineralization is controlled by rock type, faulting and enrichment. The deposit occurs in a variety of Precambrian rocks and Laramide igneous intrusives. Two large faults cross the ore body (Clark and others, 1998).
A major, hidden copper-molybdenum deposit was recently discovered in this district known as Resolution. Resolution was drilled and results indicate that the deep deposit has a minimum resource of 1.34 billion tons of ore averaging 1.51% Cu and 0.04 % Mo with gold and silver. The deposit is scheduled for production by 2020 and is located 3 miles east of the town of Superior. The deposit was discovered by drilling which interested mineralization at more than 4000 feet deep. One has to wonder how many similar deposits to Resolution occur in the states porphyry districts - probably several!
Tombstone District
The Tombstone district in extreme southeastern Arizona was primarily a silver district. From 1879 to 1932, >30 million ounces of silver, 36 million pounds of lead and >250,000 ounces of gold were mined with copper, zinc and manganese. Some mines included the Lucky Cuss, Bunker Hill, Herschell, Empire, Comet, Contention, Emerald, Grand Central, Ingersol, Luck Sure, Oregon, Old Guard, Prompter, Silver Plume, State of Maine, Toughtnut, Tribute and West Side. The ore was found as silver-lead mineralization and silver tellurides in (1) irregular replacements in Naco limestone along fissures and crests of anticlines, and (2) in altered porphyry dikes.
Rich bonanza ores were associated with tellurides, particularly hessite. Reported ore and gangue minerals included hematite, limonite, cerussite, cerargyite (AgCl) (known as horn silver), native gold, native silver, native copper, argentiferous galena, sphalerite, pyrite, alabandite (MnS), malachite, chrysocolla, psilomelane, tetrahedrite [(Cu,Fe)12Sb4S13], hessite (Ag2Te) and wulfenite (PbMoO4) (Wilson and others, 1969; Williams, 1980).
The main structure in the area is folded Paleozoic strata intruded by a porphyry dike. Mineralization occurs in quartz and vertical joints particularly along the edge of the dike, in limestone as replacement deposits and breccias, and as bedded deposits and fissure veins (Kemp, 1893). Many of the mines are located along the south end of Tombstone and northeast of Ajax Hill in Tombstone Hills.
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| Copper-stained limestone, Tombstone district |
The Emerald mine enclosed one of the largest ore bodies. The nearby Silver Plume shaft was located 1000 feet southwest of the Emerald where mineralization was hosted in Abrigo Limestone and Bolsa Quartzite and consisted of partially oxidized base metal sulfides with some wulfenite and horn silver in fault breccia. Ore control was related to a steeply dipping, N-S dike-fissure zone. The ore zone is 1,100 feet long and 10 feet wide. The Emerald shaft as sunk to a depth of 840 feet and the Silver Plume to 788 feet. Underground workings did not reach the limit of the ore. Work was discontinued because of flooding: the pumps were unable to handle the water influx below the water table.
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Brecciated Abrigo Limestone in Good Enough mine,
Tombstone
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Wickenburg district
Gold was discovered in the Wickenburg district, 50 miles northwest of Phoenix in 1863, after trace gold was found in nearby quartz vein. Over the next three years, high-grade gold was mined from the vein and milled in a primitive arrastre at nearby Hassayampa River. In 1866, a 40-stamp mill was constructed near the Vulture mine site. The ore was processed from 1867 to 1872, when the vein was discovered to pinch at shallow depth near the water table. Up to that point, the miners were high-grading ore that reportedly ran 1.2 to 4.5 opt Au.
In 1879, an 80-stamp mill was constructed, but in 1888, the vein was lost at a fault break on the 300-foot level and operations ceased. In 1908, a comprehensive geological study was conducted, and the vein offset was found. The mine was again operated until 1917 when the vein was lost again along another fault. In 1927, ore from mine pillars was treated. Drilling for the offset vein resulted in sinking a 500-foot deep shaft. But results were discouraging. The mine lies is in the Vulture hills formed of Tertiary andesites and rhyolites that lie unconformably on Proterozoic schist and gneiss intruded by Cretaceous granite and granodiorite.
The vein is fault controlled striking W-NW and dipping 45oN nearly parallel to foliation in the footwall schist. The hanging wall is granite dike and schist. Near the vein, the host rock is sericitized. The vein is approximately 32-feet wide at the surface, 47-feet wide on the 240-foot level and was traced on the surface for 1,000-feet. To the west, the vein splits into smaller veins and was mined to depths >1500-feet. The vein is cut by several faults including the Talmadge fault that sliced the vein above the 450-foot level, with 300-feet of vertical displacement. The amount of vein displacement along the lower Astor fault (intersected on the 950-foot level) is unknown.
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| Gold from Potato Patch region (photo from Bill Berridge). |
The property is the most productive gold mine in Arizona, next to Copperstone. Gold and silver are found in quartz veins and silicified and altered rock adjacent to the quartz porphyry dike that intrudes the Proterozoic metamorphics. The mine produced 366,000 ounces of gold and 260,000 ounces of silver up to 1959. The ore averaged 0.35 opt Au and 0.25 opt Ag (Spencer and others, 1989).
To the north of the Vulture mine, gold is also found along the southern edge of the town of Yarnell in the Weaver Mountains. This prospect is hosted by Precambrian granodiorite and has gold in a vein in the Yarnell fault. The gold grades are the highest in a quartz vein within the fault and values decrease away from the fault. The grades and width of mineralization appear to decrease long strike but are open down dip.
Jerome (United Verde)
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| Gossan in Proterozoic chlorite schist exposed in open pit wall at the United Verde mine. Take a look at Google Earth - this gossan can be followed about 10 miles along strike - this suggests there are very good possibilities for missed and hidden volcanogenic massive sulfide deposits in the area. |
The Jerome (Verde) district lies along the eastern slope of the Black Hills in central Arizona, 80 miles north of Phoenix and west of the Verde River. Gold and silver were recovered as a by-product from two major mines - the United Verde and the United Verde Extension. These are massive sulfide deposits in a Proterozoic greenstone belt.
Historically, copper was initially utilized by Indians for jewelry and dyes. The deposits remained undeveloped until found by the U.S. Army in 1875, and led to an influx of prospectors to the area in the following year. Prospectors found oxidized ore at the surface was rich in gold, silver and copper. These were mined in 1883-84, but by the end of 1884, the oxide ore was exhausted and the price of copper dropped, so work was suspended at the United Verde property until 1888. Mineralization occurs as a steeply-dipping, cylindrical ore approximately 700- to 800-feet in diameter, and extends down to a depth of 2,400-feet. This is perhaps the world's largest pyritic sulfide ore body.
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| Banded chert exhalite from United Verde. Such chemical sediments are distal to massive sulfides (copper, zinc, silver, gold) and represent submarine deposition near volcanic vents. Similar deposits were found in Wyoming by Conoco Minerals, and others. Because the Wyoming deposits had potential for commercial mineralization, the US Forest Service withdrew much of the area in the Sierra Madre Mountains of Wyoming, following each discovery. |
By 1899, the United Verde Extension was developed. In the early 1900's exploration to the southwest and east of the United Verde Extension produced few results. Finally in 1914, a chalcocite-rich ore body was found on the 1,200-level and in 1916, a much larger ore body was found. The company operated on a large scale until 1938 when the ore was mined out and the operation closed. The nearby United Verde mine continued underground operations until 1931, after which open-pit mining was expanded. Depletion of reserves finally forced this mine to close in 1953. Total gold production from 1883 through 1959 was about 1.6 million ounces of gold along with 2 billion pounds of copper and 34.6 million ounces of silver. However, it has been estimated that only 20% of the ore body had been exposed or excavated, and considerable ore likely remains in ground.
Bagdad district
The Bagdad open pit copper mine lies 80 miles northwest of Phoenix. The mine is an important source for copper, silver, gold, lead, zinc, molybdenum, titanium and rhenium. Copper was discovered in 1882 and production began in 1906 from a shaft that was sunk 465-foot deep. Operations were later converted to an open pit mine and continues to operate to the present. The mine is Arizona’s 3rd largest copper mine; and in 2007, the operation yielded 202 million pounds of copper and 10 million pounds of molybdenum. The operation is being upgraded to extract larger amounts of molybdenum along with the rare metal rhenium a very important metal used in jet engine construction because of its extremely high melting point. The operation produced more than 300,000 ounces of silver and a minor amount of gold.
Mineralization is classified as porphyry copper with blanket ore in the Brindle Formation. The largest quantity and highest grade ore occurs at closely spaced faults due to increased permeability.
San Francisco (Oatman) district, Mohave County
Mineralized terrains in Mohave county occur in the Northern Black Mountains, Mohave Mountains, Cottonwood Cliffs, Lost Basin, Gold Bug, Mockingbird, Pilgrim, Pyramid, San Francisco, Union Pass, Wallipai (Cerbat, Chloride, Mineral Park, Stockton camps), Weaver and Virginia districts.
In the Northern Black Mountains district, gold deposits are described at Gold Bug and Mockingbird. At the Gold Bug, a few thousand ounces of gold were mined in the past. Exploration work in the mid-1980s, by Ivy Minerals, discovered a small, high-grade, ore shoot, as well as low-grade, gold mineralization. One drilled zone shows the presence of 450,000 tons of ore at a grade of 0.025 opt Au, that is open in every direction. Another significant zone was identified with 3.2 million tons of inferred reserves grading at 0.015 opt Au (Vanderwall, 1988). These combined zones have a minimum of 59,250 ounces of gold worth $117 million, based on gold prices for August 3, 2020.
The nearby Mockingbird mine is associated with a detachment fault. Past gold production from this mine is reported at 15,000 ounces. In the 1980s, Mockingbird was explored by Gulf Resources, Anaconda and Hunt Resources and a 10-million ton ore deposit was outlined with an average grade of 0.05 to 0.1 opt Au (500,000 to 1,000,000 ounces) in minette host rock. The gold sitting in this deposit is worth a maximum of $1.98 billion at gold prices of August 3, 2020!
Search for 'Oatman, Arizona' on Google Earth. As you examine aerial photos, you will see dozens and dozens of mines and prospects. The photo to the right, shows the Oatman Camp, while northwest is the Katherine (Union Pass) Camp.
The San Francisco district in western Arizona encloses numerous mines in the Oatman and Katherine camps, 30 miles southwest of Kingman. This district produced more than 2 million ounces of gold from quartz veins in fault zones hosted by granite, rhyolite, latite and andesite. The veins are simple with many stringers and typically has andularia, quartz and calcite.
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| Exposed Gold Road vein in Oatman district. |
Most of the important veins were found on the western slope of the Black Mountains. Gold was discovered in in 1863, at the Moss vein between Oatman and Katherine. In 1901, rich gold ore was found in shallow prospects dug on the Tom Reed vein in the southern part of the district near the town of Oatman. This was followed by a rush in 1902 resulting in discovery of the Gold Road vein. Mines in the district include Hardy, Moss, Aztec, Oatman, Katherine, Tom Reed, Pioneer, Gold Dust, Gold Ore, Telluride, Ben Harrison, Black Eagle, Murdock, Leland, Sunnyside, Pyramid, Arabian, Treasure Vault, Gold Chain, Big Four, Red Lion, Big Jim, Frisco, Tyro, Time and others.
The Tom Reed vein is hosted by Oatman Andesite. The vein strikes northwesterly and has a northeasterly dip, and was controlled by faulting. The vein is offset at the Mallory fault near the Big Jim Mine. The ore body is lenticular and mildly pyritized, and has some bleaching and secondary kaolinite, calcite, and chlorite. South of the Tom Reed mine, most faults and veins trend E-W.
A 20-stamp mill was erected on site in 1904. The main ore shoot of the United Eastern and Tom Reed Extension claims had a shoot reported to be 750-feet deep, 950-feet long and 48-feet thick. This vein averaged >1 opt Au and produced 500,000 tons of ore: and ore grades declined with depth.
The Moss vein is faulted at several places and has a E-W trend. The vein can be traced over a strike length of more than a mile, and includes conjugate veins, a broad zone of silicification and stockworks, and offers considerable exploration potential. To date, drilling identified 360,000
ounces with another 129,000 indicated ounces of gold, along with 4,463,000 ounces of silver with another 1,375,000 ounces indicated. The property was recently explored by Patriot Gold Corporation (2013), and currently operated by Northern Vertex (2020).
At the Gold Road mine (35°2'45"N; 114°22'36"W), gold lies in a distinct mineralized zone of stringer ore with banded quartz-calcite. This active mine, has a vein that strikes N50ºW and dips 80ºNE occupying a fault mostly in latite. At shallow depth, Oatman Andesite forms the footwall of the vein while latite forms the hanging wall. Aerial photography suggests the Gold Road structure could be as much as 9,300-feet long.
The mine reopened in 1995, and 2.6 million tonnes of ore at an average grade of 0.355 opt Au were identified. In 1998, the mine closed due to low gold prices after producing 88,000 ounces. Mohave Desert Minerals reopened the mine in 2007 and poured its first gold bar in 2010 (Adams-Ockrassa, 2010). In 2011, mine operators reported 524,000 tons of reserves at a grade of 0.23 opt Au (Dodge, 2011). The mine is currently operated by Para Gold (Northern Miner, 2019, v. 105, no 24.)
Workings are accessed by a 6,200-foot decline, and the vein is open at depth and along strike (Northern Miner, 2019). Three ore-shoots include: (1) Number 1 shaft ore body, (2) Sharp orebody, and (3) Rice orebody. The principal area of stoping is around the No. 1 and No. 3 shafts on the Gold Road and Line Road claims. More than 316,000 ounces were recovered to the end of 1923. As of 2019, the mine is reported to have 978,000 tonnes at 0.24 opt (214,000 contained troy ounces) (Northern Miner, 2019, v. 105, no. 24), worth $435 million (8/7/2020 gold price).
The Katherine mine was developed on a vein in sheared granite near the Colorado River. This vein had a width of >60 feet at the surface and pinched at depth. It was traced for >1,700-feet along strike, and developed to at depth of 900 feet.
Many mineralized faults in the district are poorly exposed and some are hidden under gravel. Veins are simple, being tabular bodies of quartz and calcite with well-defined walls, while others consist of several vein stringers separated by barren rock. These consist of quartz, calcite, andularia, fluorite, gold and silver.
Ore shoots show an apparent decrease in grade with depth running half to a third of ore grades in the upper portions of the veins. For example, the Tom Reed extension was reported to average ~1 opt Au. Below the 800 foot level, the gold content declined to ~0.5 opt Au. At the Big Jim vein, ore grades were just under 1 opt Au to a depth of 600 feet. Below this depth, ore grades declined to ~0.3 opt Au.
Only minor placer activity was reported, primarily along Silver Creek (Lausen, 1931). This district offers considerable potential for new lode discoveries as well as exploration at depth. Many mine reports suggested the more prominent mines ceased production because of declining gold values at depth (at gold prices of about $20/oz). At today’s price, some of these are likely minable.
Wallapai district
The Wallapai is located near the center of the Cerbat Mountains, which extend north of Kingman in western Arizona and enclose the Chloride, Mineral Park, Cerbat and Stockton camps. The Cerbat Mountains are an eastward-tilted fault-block of Precambrian metamorphic and igneous rock intruded by Tertiary granite porphyry. Mineralization consists of copper in shattered granite porphyry and lead-zinc veins with associated by-product gold and silver. The veins are superimposed upon and grouped symmetrically around the porphyry copper mineralization. Turquoise deposits formed in the supergene enriched porphyry cap. A late fault cuts both bedrock and alluvium at the western base of the range. This fault and surrounding alluvial blanket contains chrysocolla at the Emerald Isle copper deposit.
The Chloride mining region was founded in the early 1870's and named from the character of its rich silver ore. Ores rich in gold and silver were mined in the 1870's, but activity declined with the collapse of the silver price in 1882. Base-metal ores below the oxidized zone were not mined extensively until the completion of a railroad from Kingman to Chloride in 1899. Thereafter, lead-silver ores were mined and subsequent improvement in milling led to exploitation of complex lead-zinc ores. Zinc-lead mining reached a peak from 1915 through 1917 owing to high metal prices during World War I. Production declined abruptly after 1917, and thereafter mining was confined to veins with relatively high gold content. Gold production increased in 1935 and reached a peak in 1937-38. At Mineral Park, copper, molybdenum, gold, lead, zinc, silver and gemstones were discovered in 1906. This porphyry copper deposit includes crenulate, tabular, and crescent-shaped ore-bodies hosted in the Escabrosa Limestone.
LaPaz District (LaPaz County)
Past, miners used picks and shovels to break up gold-bearing gravel, and recovered 50,000 ounces of gold in the first year of heavy mining in the Dome Rock Mountains. As much gold was recovered in each subsequent year until 1868. After 1868, production declined. Nuggets of 0.25 to 0.5 ounces, with a few in the 1 and 2 ounce range were mined (Jones, 1914). Larger nuggets weighed 26, 27, 47, 55, 60 and 65 ounces (Wilson, 1961)!
Eighteen miles north of LaPaz, gold was discovered at Copperstone. The mine was initially developed by open pit in the flat desert northeast of Blythe by Cyprus Minerals, who recovered 500,000 ounces of gold from 1987 to 1993. More recently, additional gold was found at the "mined-out" property: drilled proven and probable reserves include 911,367 tonnes of ore at average grades of 0.3 opt Au (256,430 contained ounces of gold). Measured and indicated resources include 941,357 tonnes of ore grading about 0.35 opt Au (313,183 contained ounces) with an additional inferred resource of 369,000 tonnes at an average grade of 0.4 opt Au (144,892 ounces) (Northern Miner, March 8-14, 2010).
The gold occurs in a N30oW striking, 30oNE dipping, tabular deposit controlled by brecciation along detachment and listric faults. Host rocks include metamorphosed volcanic rock (Jurassic) and overlying sedimentary breccia. The presence of quartz, hematite, and chrysocolla provide visual indicators for gold. The gold is in breccia above a fault, in quartz latite below the fault, and lesser amounts in basalt plugs. The mineralized contact zone extends horizontally for 3,000 feet and at least 1,000 feet down dip and is generally several tens of feet thick.
Although the property was deemed mined out by Cyprus Minerals, exploration in 2002 to 2006 by American Bonanza discovered additional gold mineralization in high-grade zones that were associated with favorable structures and geophysical targets. The mine lies along the Walker Lane mineral belt that extends to Nevada. Similar gold deposits in the vicinity of Copperstone are likely.
Dos Cabezas district
Eighteen miles southeast of Wilcox in the Dos Cabezas mountains of southeastern Arizona, are several Cu-Pb-Ag-Au mines. In addition to lodes, some arroyos, gulches, benches and terraces produced gold. The gold is described as flat, ragged and coarse masses.
Lodes in the district include LeRoy (a gold mine with lead and silver), Dives Mine (a rich gold lode), the Gold Ridge (Casey) Mine and the Gold Prince (Murphy) Mine. At the north foot of the Dos Cabezas mountains are the Teviston placers. The gold is often very coarse and includes nuggets. Production through 1959 was only 15,000 ounces.
Clifton-Morenci district (Greeley County)
The Clifton-Morenci district, in southeastern Arizona, lies immediately west of the New Mexico State boundary near the town of Clifton. It is principally a porphyry copper district with by-product gold and silver. Total gold production from 1882 through 1959 was 228,000 ounces.
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| Topographic map of the Ash Peak district |
A small amount of gold was also mined from underground silver lodes in the nearby Ash Peak district. Specimens of agate, opalized agate, geodes, amethyst, pyrite, and copper are sought by rock hounds. In addition, the BLM set up the Round Mountain rock hound area where fire agate, rose quartz, geodes and chalcedony can be found.
Copper was discovered at Morenci in 1872, but development was hampered by lack of transportation. The completion of the Southern Pacific Railroad in 1881 lowered transportation costs sufficiently to permit large-scale mining. Large, low-grade copper ores at Copper Mountain at Morenci assured a certain degree of stability in the district. Total gold production from the copper mine from 1873 through 1959 was about 203,000 ounces. Typical of porphyry copper deposits, Morenci also produced some molybdenum, silver, lead and zinc. The deposit was also anomalous in uranium, rare earths, gemstones and garnet. Mineralization iss accompanied by hydrothermal alteration including kaolinization, sericitization, calc-silicate hornfels and skarn.
The district is zoned with a chalcopyrite-rich core surrounded by a pyrite envelope which is surrounded by protore. Silver and gold are more abundant in less altered areas. The greatest molybdenum concentrations were in granite porphyry. Turquoise is found in the oxidized part of the deposit as was recovered as a gemstone. Massive andradite occurs in a skarn deposit at the southwest corner of the open pit.
At the nearby Ash Peak district, manganese, silica, silver gold and copper were mined from fissure veins in Tertiary andesitic tuffs and flows. Minerals found in some mines include quartz, argentite, pyrite, galena, calcite and rhodochrosite.
Lost Basin district
The Lost Basin district is located in northwestern Arizona near Mead City in the Lost Basin Range, northeast of Gold Basin. Much of the district is underlain by Proterozoic basement rocks. This district looks like it could potentially host a major gold deposit!
Gold was discovered in gravels and fanglomerates. The metal is visible and associated with pyrite and chalcopyrite. Gold is distinctly associated with a 7-mile long trend of fault breccia, dry placers, veins, pipes, fanglomerates and alluvium. Past work suggests zonation of mineralization with a central cupriferous belt (with gold) surrounded by a silver-lead-zinc halo enclosed by gold-dominant zones.
Highly anomalous gold-bearing iron formation, quartz veins & quartz breccia veins with a possible hidden porphyry copper deposit surrounded by auriferous veins and breccia veins are found north at the Climax mine. Gold-bearing iron formation south of the Bluebird mine is a pyritiferous iron formation with visible gold found along an E-W trend. Visible gold is reported in the iron formation and also associated host rock and continues to the east for nearly a mile.
Several prospects near Cu-blowout and Road Runner prospect are anomalous. At Cu-blowout, samples recorded by the US Geological Survey were opalized with chalcopyrite-bearing schist and visible gold in quartz veins. A magnetic low surrounding the anomaly is suggested to reflect a buried porphyry at depth. In addition, visible gold was identified in hundreds of hand specimens at the Golden Gate, Harmon, Gold Hill, Road Runner and Climax prospects. This district very likely not only hosts a hidden porphyry Cu-Au deposit, but also some major vein and iron formation gold deposits. The number of specimens containing visible gold and nuggets from this district are astounding (Hausel, 2019).
Summary
The presence of widespread gold mineralization in Arizona assures that additional gold deposits will be found in the future. It is highly recommended the State of Arizona establish a detailed geological mapping program to map all of the mining districts at great detail (1:24,000 scale) and all accessible abandoned mines at a scale of 1:120. This type of mapping forces the geologist to focus on detailed geology. Areas of greatest concern would be gold-bearing fanglomerates to determine the source of gold and to search for evidence of hydrothermal alteration patterns similar to that associated with porphyry copper-gold-silver-lead-zinc deposits. Arizona is in a budget crisis, by conducting such low-cost exploration, this will likely lead to several important discoveries and provide natural resources to assist in paying for the state functions. Based on reconnassisance of the Chlorite and Lost Basin areas, it is likely that significant primary gold and porphyry systems will be found.
GOLD- Field Guide for Prospectors and Geologists
Its here - at a Bookstone near you (2011). GOLD- Field Guide for Prospectors and Geologists. A book on gold, how to identify this metal and other precious metals, where to find gold and clues to look for. A book on the geology of gold in the Western US with emphasis on Wyoming. Written two geologists - W. Dan Hausel and his son Eric J. Hausel. W. Dan Hausel is arguably one of the better prospectors in the US. In addition to hundreds of gemstones, he also discovered more than 100 gold deposits, a whole new gold district with similarities to the world-class gold camps of Red Lake, Canada and Cripple Creek, Colorado combined (the Rattlesnake Hills greenstone belt). A member of a 7 man group who discovered the Donlin Creek, Alaska gold deposit in 1988 - a deposit with drilled and inferred resources of 41 million ounces with >$60 billion in gold! Reported by the Northern Miner as one of the largest undeveloped gold deposits in the world and the largest undeveloped gold deposit in North America! A deposit as large as the Homestake, and containing as much gold as mined during the entire history of Alaska! He will tell you how he found these and to look for.
A companion book to arm any prospector with information to find a gold mine, consider GEMS, MINERALS, and ROCKS of WYOMING - a no nonesense guide to mineral and rock identification also available at Amazon.
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