Wednesday, February 10, 2010

Gold Geology & Prospecting, Arizona

After moving from Wyoming to the land of the sun in 2006, I came across some reported gold anomalies that suggest the presence of hidden gold deposits in the state. Some reports even suggest possibilities for hidden, giant, copper-gold porphyry deposits. For those unfamiliar with porphyries, these are thought to represent root zones of old volcanoes or magma chambers and rocks associated with these are 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.

The author prepares to go underground at the
Resolution mine in Superior.
As an undergraduate in geology, we toured the Lark mine west of Salt Lake City in the Oquirrh Mountains. This was an underground operation developed in massive pyrite, sphalerite (zinc ore), and galena (lead ore). At that time, the mine was adjacent to the Bingham pit. As I understand, the Lark mine was later incorporated into the Bingham mine as the open pit expanded it boundaries into the largest open pit operation on earth. If you would like to see what several billion dollars and hundreds of high-paying jobs look like from space, just go to Google Earth and search for Bingham Copper Mine, Utah.

Arizona is considered a model for porphyry copper-gold and massive base metal-gold sulfide deposits because of the many deposits. So many were 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. When I later worked for the 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 all were 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 stop mining of our 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 square miles of volcanic terrain (an area larger than some states) particularly when the boundary (not to mention the national park) included considerable precious and base metal resources. To me, it was unnatural to ignore these resources.

Its about time we get priorities right!
I recently wrote to the Arizona governor hoping to excite interest and discussion about possibilities for discoveries of copper, gold and silver. Arizona, as many readers know, is in a financial crisis because our last governor left the state deep in debt before her expertise was needed in the Obama administration (makes one wonder why he needs so many politicians with expertise in tax avoidance, debt, gun running, global warming and other scams).

I’ve never liked politicians and find it a waste of time writing to them as most never respond unless they want donations. Politicians are all social-paths and only believe they have the right to freedom. But I could see a fix for our crisis and hoped the current governor would read my letter as the only way out of such debt is to dramatically cut taxes and government while producing natural resources (which Arizona has plenty to go around). What we need are dynamic leaders who will open the doors of Arizona to exploration, not to increased taxes. I don't know about you, but I pay too much in taxes. The first 4.5 months of each year, all of my wages pay for government. Then I finally get a chance to take home some money in late May.

A Happy Miner in Arizona
I don’t have all of the answers on how to attract mining companies to the State, but providing exploration incentives and making deposits accessible is a good start – why governors in the West can’t see this is beyond me. Take Arizona and California for instance. These two have nearly insurmountable debt, yet both have considerable natural resources that could be used to relieve this burden. When I worked in Wyoming, the economy was mostly positive and the citizens never paid state income taxes (similar to Alaska) simply because of the many natural resources that even the worse of politicians couldn’t eliminate (although they tried).

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 the Duval porphyry copper mine in Green Valley, Arizona), massive sulfides and replacement deposits. Total historical gold production from Arizona has been about 16 million ounces since the 19th century and it could produce a lot more.

Much of the precious metal was 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 (northwestern Arizona) (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. I tried and tried since 2006 to get a company interested in these deposits, but it seems no one wants to make money.  Oh well, its there loss.

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.

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 of the Arizona placers are related to flash flooding events. 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 of these was 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.

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
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 were found as 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.

Ajo district
Gold was recovered as a by-product of copper mining at Ajo, south of Gila Bend and 75 miles south of Phoenix. Copper was initially recovered on a small-scale in 1750 by Spaniard miners and later mined from a large-tonnage open pit at the New Cornelia mine. A carbonate cap with sulfide-rich primary ore was found and by 1917, New Cornelia was developed as Arizona’s first open pit. Total by-product gold through 1959 was about 1 million ounces recovered with 6.3 billion pounds of copper. Operations terminated in 1983. Like all mines, the deposit is not mined out and resources await better metal prices and economy.

The Ajo ore body is an elliptical deposit (6,400 feet long by 3,900 feet wide and 1,560 feet deep). It consists of veinlets and disseminations of chalcopyrite, bornite and minor pyrite. The upper part of the porphyry was oxidized and has copper carbonate and silicate with minor chalcocite in fractured and faulted Laramide-age quartz monzonite and quartz diorite. Mineralization extends into the intruded and altered Cretaceous volcanics. Deposition of mineralization was controlled by fracturing and rock permeability. Alteration included sericitic, chloritization, kaolinization, silicification with oxidation down to water table (Joralemon, 1914). Gold was closely associated with copper sulfides rather than pyrite. Local structures include NNW-trending fractures and a number of faults. The ore body was developed 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 and lead (Singer and others, 2008). Some rare copper minerals have been used for jewelry that includes ajorite, shattuckite and turquoise.

Bisbee district
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).

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.

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 pounds per year for about 25 years from a 308 tonne orebody that averages 0.358% Cu. Most porphyry deposits also contain very low grade gold and silver and these precious metals will likely remain in-situ for future generations. Although gold prices will have to rise considerably for the resource to have much value due to the low-grade nature of the ore.

The Florence 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 District

The Commonwealth mine at the town of Pearce contains at least 20 miles
of underground workings according to reports. 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, and there are likely missed ore bodies in the immediate
Copper was discovered 20 miles north of Bisbee and 15 miles east of Tombstone and a mining camp known as Turquoise was established in 1890. Four years later, gold and silver were found on the Commonwealth property 11 miles to the north and the town of was Pearce was established.

In 1895, a prospector named John Gleeson was prospecting in the Turquoise area when he discovered a copper deposit and opened the Copper Belle mine. Several other mines were developed and the new mining camp was named Gleeson. Mines in this district continued to operate until copper prices declined resulting in mine closures in the 1930s.

The geology of Gleeson (includes Pearce, Turquoise and Gleeson) in southeastern Arizona suggests the area has high potential for discovery of hidden gold 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 prospects in Arizona.

The district is located in the southeastern Dragoon Mountains: notable deposits are found at the Gleeson-Courtland mine, Commonwealth mine, Mexican Hat Mountain volcanic-hosted gold mineralization (Tertiary) and the 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 Mountain 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. Mark my words, if the right company works these properties, Arizona will have another gold mine. I had tried to get a CEO with a Canadian Company that has interests in the Bradshaw Mountains, as well as two of my former colleagues (Gordon and Paul) interested in these properties a few years ago, but no one was interested.

Mexican Hat with dozen cuts. This feature stands as positive structure due
to silification that accompanied disseminated gold.
Mineralization occurs as: (1) Irregular cupriferous 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.

Two of the more interesting occurrences in the district are epithermal gold deposits. Epithermal gold was identified in a mineralized structure 15 to 50 feet wide and 2,000 feet long at Mexican Hat Mountain in the northern portion of the district. The gold is described as free gold on fractures accompanied by hematite and limonite in Tertiary rhyolite and rhyolite breccia. In 1990, Placer Dome and Oneida Resources reported a geologic resource from six mineralized zones to be 10.3 million tons of ore with average grade of 0.035 opt Au (362,000 ounces of contained gold). A similar deposit south of Mexican Hat is known as the Gold Coin. This is an epithermal deposit hosted by sedimentary rocks. Secova geologists collected composite chip samples from trenches on this deposit that included 45 feet of 0.11 opt Au (enclosed 20 feet of 0.16 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 a 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 that Tertiary gold-enriched fluids were emplaced along northward-trending structures and along subvertical to high-angle normal faults. Some Carboniferous limestones have extensive jasperoid alteration and strong limestone decalcification (Moore, 2010).

Gossaniferous tailings at old mine at Gleeson.
Today, when I got up, I was reading the Northern Miner. Right there in print it was reported that another Canadian Company, Auracle Resources Ltd picked up the Mexican Hat property. My heart sank. Here was another great property that we could have had that is now in the hands of another Canadian company (so why can’t America get politicians off their dead …. And make it more favorable to raise money for mining companies and get rid of 99% of the regulatory agencies who do nothing more than sit around picking their noses and eating donuts all day). Yes I was angry – I had lost another chance at another good gold prospect.

In a press release on January 22nd, 2012 it stated that the company drilled 5 holes from a 19 hole drilling program at Mexican Hat. The gold mineralization was reported to occur in extensiverly fracture volcanic rocks with abundant limonite/hematite alteration and in areas of pervasive limonite/hematite (gossan) within the volcanic hosts. The rocks are highly fractured making drilling difficult.

            Drill Hole ResultsMexican Hat                           ________________________________________________________________________________________
Hole#                    From(ft)    To(ft)  Intercept(ft)       Grams/Tonne               Opt
            MH 11-001               267         305         38                           1.75                        0.056
                                             337         381         44                           2.16                        0.07
            MH 11-002               154         182         28                           2.49                        0.08
                                             273.2      349         66                           4.19                        0.13
                                             397         406.8     9.8                           2.53                        0.08

            MH 11-003               200.8     215.5     14.7                        138.30                   4.45

            MH 11-004               53.15     125          71.85                      0.41                        0.013
                                             170.9     194          23.1                        0.70                        0.023
                                             471         496         25                           1.34                        0.043

            MH 11-007               50           70           20                          13.40                      0.43

Lone Star (Safford) district
Safford lies 16 miles southwest of Morenci in southwestern Arizona, and is a porphyry copper deposit. Construction for the open pit mine began in 2006 with full production scheduled for 2008. A group of mineralized porphyries were found along a northwesterly trend paralleling the Butte Fault. 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 deposit has by-product gold and silver.Geophysical exploration at Safford suggests possibilities for hidden copper-gold deposits at depth.

Globe-Miami region
Copper was discovered at Globe in 1874. In 1904 development began on the large low-grade disseminated copper porphyry deposit, which by 1911 was mined on a large scale. Globe lies 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 more than a few billion dollars. Total gold production through 1959 included 191,801 ounces.

The Ray pit in background behind
The Hayden-Banner district, 8 miles north of Winkleman and 22 miles south of Globe, includes the Christmas open pit mine at the southeast end of the Dripping Springs Mountains (Boss, 1925). The Christmas was a former surface and underground Cu-Au-Ag-Mo-Bi-Pb-Zn-Be-W-garnet abrasive mine. The deposit was discovered in 1880 and mining began in 1905. Workings continued to the 908-foot level and mineralization continued to greater depths. 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. Mineralization is classified as contact metamorphic and replacement deposits that form an ore zone 4,900 feet long, 2,600 feet wide and 2,100 feet deep. Ore was controlled by limestone-diorite contacts, favorable limestone beds, garnetized zones and fractures. Hydrothermal alteration included epidotization, silicification, propylitic, K-silicate, quartz-sericite-chlorite, oxidation and carbonatization. The deposit has a pyrite-chalcopyrite core, chalcopyrite-bornite intermediate zone, and pyrrhotite-sphalerite-chalcopyrite margin. Past production focused on the Naco limestone and a few smaller deposits were mined from the Escabrosa limestone. Ore is confined to 11 distinct beds, which are consistently mineralized and constitute a zone 425 feet thick. 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.

Copper-stained limestone, Tombstone

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.

Brecciated Abrigo Limestone in Good Enough mine,
Wickenburg district
Gold was discovered at the Vulture mine in the Wickenburg district 50 miles northwest of Phoenix in 1863 after trace gold was found in a nearby quartz butte. Over the next three years, higher grade portions of the vein were treated by primitive arrastre at the nearby Hassayampa River. In 1866, a 40-stamp mill was constructed near the site of Wickenburg. The ore was processed from 1867 to 1872 when the vein was discovered to pinch at shallow depth near the water table. Up to this point, the operators 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 contact 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 the mine pillars was treated. Drilling for the offset vein resulted in sinking a 500 foot deep shaft. But results were discouraging. The mine is in the Vulture Mountains which is formed of Tertiary andesites and rhyolites that lie unconformably on Proterozoic schist and gneiss intruded by Cretaceous granite and granodiorite.

Mineralization and alteration occurred primarily within and adjacent to a north-dipping quartz-porphyry dike that extends eastward from a Late Cretaceous pluton intruding Proterozoic crystalline rocks. Mineralization was accompanied by sericitic alteration: gold occurs as native metal and associated with pyrite, argentiferous galena, and minor chalcopyrite and sphalerite. The mineralization and alteration occurred along and north-northeast-trending sub-vertical dike that projected upward from the structural top of the Cretaceous granite. The association of gold with the dike and gradation into granitic rocks indicate gold mineralization was related to Cretaceous magmatism and dike emplacement.

The vein is fault controlled striking W-NW and dipping 45oN nearly parallel to foliation in the footwall schist. The hanging wall is the granite dike and schist. Near the vein, the host rocks are 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 was intersected by several faults including the Talmadge fault that cuts the vein above the 450-foot level, with 300 feet of vertical displacement. The displacement of the vein along the lower Astor fault (intersected on the 950-foot level) is unknown.

Gold from Potato Patch region (photo from Bill Berridge).
The mine was the most productive gold mine in Arizona. 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 precious metal occurs as free gold associated with pyrite, argentiferous galena and minor chalcopyrite and sphalerite. 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, the Yarnell lies 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.

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 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, a massive sulfide deposit in a Proterozoic greenstone belt. Historically, copper was initially utilized by Indians in this area for jewelry and dyes. The deposits remained undeveloped until found by the U.S. Army in 1875 by led to an influx of prospectors to the area in the following year. Prospectors found that the oxidized ores at the surface were 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 was found as a steeply-dipping, cylindrical ore body approximately 700 to 800 feet in diameter, extending down to a depth of 2,400 feet. This was perhaps the world's largest pyritic sulfide ore body.

Banded chert exhalite from United Verde mine. Such chemical
sediments are found distal to massive sulfide (copper, zinc,
silver, gold) deposits and represent submarine deposits formed
near volcanic vents. Similar deposits were found in Wyoming by
Conoco Minerals, various UW grad students and myself. Because
the Wyoming deposits had potential for commercial
mineralization, the US Forest Service withdrew nearly all of the
area in the Sierra Madre Mountains of Wyoming.
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 little 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 mine closed. The 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.

Bagdad district
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 that continues to operate to the present. The mine is the 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 has produced more than 300,000 ounces of silver and only a minor amount of gold.

The deposit is a porphyry copper with a blanket ore-body in the Brindle Formation. Ore control, where the largest quantity and highest grade ore occurs, is where intersecting or closely spaced faults have increased permeability. Ore concentration was hypogene metallization and supergene enrichment to copper ores, oxidation. Alteration included sericitization and argillization.

Area structures include the fracturing of the stock at Bagdad, important to this porphyry deposit, appears to be the result of the intersection of dike swarms.

San Francisco district
The San Francisco district in western Arizona encloses numerous mines within 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 contain andularia, quartz and calcite. In many cases, veins show banding.

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 this region in 1863 at the Moss vein between the Oatman and Katherine mines. In 1901, rich gold ore was found in shallow prospects dug on the Tom Reed vein in the southern part of the district. This was followed by a rush in 1902 resulting in discovery of the Gold Road vein. Mines in the district included the 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. This vein strikes northwesterly and has a northeasterly dip. The vein was controlled by faulting and is offset at the Mallory fault near the Big Jim Mine. The ore body is lenticular and mildly pyritized, and has some bleaching with 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 reportedly averaged >1 opt Au and produced 500,000 tons of ore: the ore grades declined with depth.

The Moss vein was faulted at several locations and also exhibited an E-W trend. The vein can be traced on the surface for over a strike length of > a mile. The vein also includes nearby conjugate veins, a broad zone of silicification and stockworks and offers considerable exploration potential. To date, drilling has identified 996,000 ounces of gold and 11,357,000 of silver resources which will likely increase with additional drilling and exploration. It is the author's opinion that the Moss vein is one of the more important gold projects in the State. The property is being explored by Patriot Gold Corporation (2013).

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. The mine was developed to at depth of 900 feet.

Many mineralized faults in the district are poorly exposed and some are hidden under gravel. The 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 in the district show an apparent decrease in grade with depth sometimes 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 in the district, primarily along Silver Creek, because of lack of water and small gold grains in the lodes (Lausen, 1931). This district offers considerable potential for new lode discoveries as well as exploration at depth. Many old mine reports suggested that 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 to greater depth.

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. From 1950 through 1956 gold production was <100 ounces annually. From 1904 through 1956 the district produced 125,000 ounces of gold. At Mineral Park, copper, molybdenum, gold, lead, zinc, silver and gemstones were discovered in 1906. Porphyry copper mineralization included crenulate, tabular, and crescent-shaped ore-bodies hosted by the Escabrosa Limestone.

LaPez District
Gold placers and a few lodes were discovered in this district in southwestern Arizona. The placers included LaPez in the Colorado River basin and the adjacent LaCholla and nearby Plomasa placers. Eighteen miles north of LaPez, gold was discovered at Copperstone. The mine was initially developed as open pit in the flat desert northeast of Blythe by Cyprus Minerals who recovered 500,000 ounces of gold from 1987 to 1993. Proven and probable reserves drilled on the property 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).

Just a great photo along an Arizona Highway showing a group of faults cutting the strata.

The gold occurs as a tabular deposit with a N30oW strike and 30oNE dip. The orebody is controlled by brecciation along detachment faults. Host rocks include metamorphosed volcanic rocks (Jurassic) and overlying sedimentary breccias. The presence of quartz, hematite, and chrysocolla provide indicators for gold. The gold occurs in breccia above a fault, in quartz latite below the fault, and 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 to have been 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 is located within the Walker Lane mineral belt that extends into Nevada.

Dos Cabezas district
Proven and probable reserves recently drilled on the property include 911,367 tonnes of ore at an average grades 8.75 grams per tonne (256,430 ounces of gold) or about 0.3 opt Au. The reserves are based on a cutoff grade of 4.5 grams of gold with the highest assay value at 171 grams (6 ounces/tonne). Measured and indicated resources include 941, 357 tonnes grading 10.35 grams per ton (313,183 ounces) with an additional inferred resource of 369,000 tonnes at 12.21 grams per ton of gold (144,892 ounces) (Northern Miner, March 8-14, 2010).

The gold occurs as a tabular deposit with a strike of N30oW and dip of 30oNE. The ore is controlled by brecciation and detachment faulting. The wallrocks exhibit both sericitic and chloritic alteration. Host rocks include metamorphosed volcanic rocks (Jurassic) and overlying sedimentary breccias. The presence of quartz, hematite, and chrysocolla provide indicators of gold mineralization. The gold occurs in breccia above a listric fault, in quartz latite below the fault, and in basalt plugs. The basalt plugs have the least amount of mineralization. The listric fault is in the upper plate of a detachment fault. 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 to have been 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 is located within the regional Walker Lane mineral belt.

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 at the the Dos Cabezas placers. 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. These include all must gulches. The gold is often very coarse and includes nuggets. Production through 1959 was only 15,000 ounces.

In the Gleeson area to the southwest includes the Gleeson placers. These are dry placers with fine gold to medium size nuggets. The gulch west of the Copper Belle Mine has some coarse gold.

Clifton-Morenci district
The Clifton-Morenci district, in southeastern Arizona, west of the New Mexico State boundary, near the town of Clifton, is principally a porphyry copper district with by-product gold and silver. Total gold production from 1882 through 1959 was 228,000 ounces. A small amount of gold was also derived from the silver ores in the nearby Ash Peak district.

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. The discovery in 1893 of large low-grade copper ores at Copper Mountain at Morenci assured a certain degree of stability in the district. Total gold production from 1873 through 1959 was about 203,000 ounces. Typically 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 as 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.

Lost Basin district
The Lost Basin district is located in northwestern Arizona near Mead City in the Lost Basin Range in Proterozoic basement rocks. Now this district looks like it could potentially host a major gold deposit! Gold was discovered in gravels and fanglomerates. The gold is described as visible gold and associated with pyrite and chalcopyrite. Gold mineralization is distinctly associated with a 7-mile long trend in fault breccia, dry placers, veins, pipes, fanglomerates and alluvium. Past work in the district 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 the 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 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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