Wild West bonanza - the Jerome copper deposits
Michael Forrest visits the historic mining town of Jerome, Arizona, in search of one of the world’s richest copper deposits.
As economists know, a bonanza (from the Spanish meaning ‘fair weather’ and ‘prosperity’) does not last – and the same can be said for mining. In the past, high ore grades were required to compensate for expensive manual working, as well as limited mechanisation and processing ability. Those high-grade discoveries often resulted in a rush for development, attracting miners and others from across the world.
A prime example happened in 1883 with the opening of the United Verde mine in Jerome, Yavapai County, central Arizona, where exceptionally high grades of copper were found after drilling to great depths. The drilling aimed to test the hypothesis that a known fault (a planar fracture with significant displacement due to earth movement) may lead to the sources of copper oxide deposits that had been found coating gravels and boulders in nearby creeks, which had been worked for the previous seven years.
The mine was based on large and exceptionally high-grade sulphide mineralisation found adjacent to the Verde fault, which was seen to penetrate the underlying pre-Cambrian (>500Ma) basement. The initial view of the mineralisation was one of replacement of host rock with an ‘ore pipe’ of massive sulphides, primarily pyrite (iron) with secondary chalcopyrite (copper-iron) and sphalerite (zinc-iron).
The workings in the late 1800s extended down to around 245m from level ground adjacent to the town of Jerome. In the upper 50m of the drilled ore body, the sulphides are oxidised (azurite, chalcocite) with gold and silver visible in the ore. Between 1893 and 1975, 80% of the 42.3Mt of volcanogenic copper ore mined in Arizona came from Jerome. The overall metal yield of Jerome ores was 1.64Mt of copper, 44,000t of zinc, 314t of lead, 1,782t of silver and 49t of gold.
This remarkable yield attracted a raucous mix of men and women from around the world seeking fame and fortune following the first claims in 1874. This resulted in rapid expansion of Jerome from a tent city to a prosperous town, following the infl ux of wealth from the mine. During these boom days, miners from Mexico, Croatia, Spain, Italy, China and Ireland flocked to the bonanza town, accompanied by freighters, gamblers, storekeepers, prostitutes and preachers – giving the town a reputation as ‘the most wicked town in the west’. Soon it became the fourth largest town in Arizona, with a multitude of clapboard houses, hostels, bars and an attendant jail all founded on the steep slope of Cleopatra Hill. Such was the unplanned and ramshackle nature that between 1894 and 1899, four disastrous fires destroyed large sections of the town, only to be later rebuilt.
Faults and fortunes
The United Verde mine was based on mineralisation found within the Cleopatra formation – a thick, early Proterozoic quartz rhyolitic extrusive that marks the Jerome area and overlies basement rocks seen in deep canyons in the vicinity. The area is heavily faulted, displacing the pre-Cambrian basement into a number of fault blocks (grabens). One of these, the Verde fault, lies adjacent to the United Verde mine workings. The mineralisation is a steeply dipping, cylindrical body measuring around 215–245m in diameter, extending down to a depth of 730m (945m if calculated from the level of the Pre-Cambrian peneplain). Perhaps the world’s largest pyritic sulfi de ore body, it is marked by exceptional levels of sulphide mineralisation, ranging from 30-42% sulphur and 5-25% in the gangue (waste).
Working the mine required shaft-sinking and the development of horizontal adits into the mineralisation, which by 1907 were calculated as 4.5Mt grading 7% copper. By 1911, aggressive underground exploration raised this value to 10.5Mt, with new ore found in all levels of the mine.
The high sulphide level had an unfortunate effect on the mine, which first became apparent in 1894 when fire broke out in the upper levels (at 110m) that comprised 75% of the known reserves. Although the cause was never established, it is believed to have derived from spontaneous combustion. Sulphide minerals – especially pyrite – are known to be highly exothermic when ignited and continue to burn even when oxygen levels are depleted. These underground fi res posed a great problem in mining, raising the temperature of the ore to 49°C (in some cases up to 260°C) and generating considerable quantities of noxious gas – so much so that gas masks were issued to the miners. The mine developed the plenum method, which consisted of forcing air into the workings to push back the gases and reducing the temperature so that explosives (insulated in paper tubes) could be safely used. Although the plenum facilitated some ore extraction, it did not solve the problem and instead simply displaced the burning. The solution came in using steam shovels to covert the upper levels into an open-pit mine, by the removal of 6 million cubic metres of overburden following methods established by the Utah Copper Company and the Mesabi iron ore mines in Minnesota.
The ore body steeply dipping towards the Verde fault led the company to explore underground on its east side, in the hope of finding an extension to the Verde mineralisation. This is on the downthrow side of the fault, which is covered by basalts and limestones. In 1914 after considerable effort and expense, drills at a depth of 365m touched another ore body. Although a greater depth, this United Verde extension (UVX) was found by James S ‘Rawhide’ Douglas and his team, who had acquired the mine in 1912. Although not as large as that of United Verde, it was twice as rich in copper per tonne of ore. The assumption was that the extension was the eastern limb of a structural anticline hosting the Verde mine in the western segment, separated by the Verde fault. Douglas never claimed that he had great geological insight, other than following the downthrow of the fault to depths where a continuation of the mineralisation could be expected. After several disappointments, on the 20 December 1914 his perseverance paid off when 45% ore was encountered in an underground crosscut. The first shipment to the Verde smelter of 76t made US$75,000.
Despite the excitement, further crosscut extensions drew a blank. The following January, however, on the 430m level an orebody measuring 135m long with a maximum width of 80m was found, with 2.21 cubic metres releasing 1 tonne of ore. The upward termination of the ore was at 375m and the downward at 495m. In 1916 the mine yielded 16,516t of copper from 77,461t of ore, an average grade of 23.5% (consider that mines today run around 1–1.5%), 2,570 ounces of gold and 128,468 ounces of silver. The total value of US$9.95 million yielded a profi t of US$7 million.
In recent decades, subsequent researchers have investigated the exceptional mineralisation. The consensus is that the Cleopatra formation, originally considered to be an intrusive rock, represents a large, rapidly extruded submarine outpouring equivalent of a terrestrial ignimbrite (Geology and ore deposits of the Verde District, Jerome County, Arizona – PA Linberg, SME 1992). Others postulate that it is flow-dominated, subdividing the Cleopatra formation into upper-unaltered and lower-mineralised units. It is the lower unit that is economically most productive and the most important volcanic strata. Volcanogenic massive sulphides were formed on its fractured and altered surface from active submarine hot-spring activity.
More recently, ‘black smoker’ sulphides were discovered in the Verde open pit, remarkably similar to those dredged from modern ocean floor vents. The Verde fault does not in fact divide the same deposit and instead only tangentially touches the Verde United. And the UVX is actually a separate ore body, as two footwall alteration ‘root zones’ have been found within the Cleopatra formation.
Despite this reviewed hypothesis of the origin of the deposit, Douglas was right – albeit for the wrong reasons – in seeking the UVX orebody, probably one of the richest copper deposits in the world.