|Gold nuggets found in Arizona|
On the northwestern edge of Jerome is a large, partially reclaimed, open pit mine that is part of the United Verde mine. The rest of the mine lies underground and inclues 81 miles of tunnels and shafts hidden beneath the open pit, Cleopatra Hill and the adjacent valley. A large active quarry is visible near the northeastern edge of town that is actually the Clarkdale quarry and cement plant, which has little to do with the base metal mines. Evidence of the old smelter works is visible further east near the Verde River and expressed as black slag with some old buildings and foundations.
When I was a student in geology, the Verde district was considered as an exploration model for ‘submarinevolcanogenic massive sulfide deposits’. One geological dictionary defines massive sulfide very simply as: “an unusually large deposit of sulfide minerals”. But there is more to massive sulfide deposits.
Massive sulfides bring thoughts of copper and zinc to geologists. And for myself, I also think of precious metals. Sulfide minerals in many of these can enclose significant amounts of gold and silver. So much silver was found associated with these deposits at Jerome that the recovered silver essentially paid for the mining operations. And many banded cherts (known as exhalites) associated with ‘submarine volcanogenic massive sulfides’ (VMS) contain anomalous gold.
|White smoker sinter column. White smokers precipitate in hydrothermal|
vents and often have auriferous silica. Black smokers are sulfide rich
and associated with higher geothermal temperatures (specimen from the
United Verde mine, and on display at the Douglas Museum in Jerome).
In addition to volcaniclastics (mill rock), siliceous mounds or chimneys are formed by submarine fumaroles that are referred to as black and white smokers. Other distinct rocks include layered stratiform rock known as exhalites that formed during eruption of heterogeneous volcanic material in sea water: the layering was produced by sea water naturally separating minerals by specific gravity as they slowly settled in the ocean water.
The host rocks for the VMS deposits in Jerome are 1.7 billion years old and similar in age to VMS deposits discovered in Colorado and Wyoming in 1979 and the early 1980s. You might be wondering what submarine VMS deposits are doing in the dry deserts of Arizona and high mountains of Colorado and Wyoming? In the geological past, these were accreted to continents and buried by tectonic forces related to continental drift. After millions of years of erosion following mountain uplift, some have been exposed at the surface, while many more remain hidden and undiscovered. Those hidden at depth are referred to as ‘blind’ deposits. Still others continue to form in the oceans, particularly along active volcanic island chains (such as Japan) and along spreading centers such as the Mid-Atlantic Ridge.
When I was the Senior Economic Geologist and Deputy Director at the Wyoming Geological Survey, Conoco Minerals discovered VMS deposits in 1979 in the Sierra Madre Mountains west of Saratoga. But few have heard of these deposits because of the US Forest Service. Those that were found near the old Itmay mine, not far from the famous Ferris-Haggarty mine, were withdrawn from public use. Other VMS deposits were soon found in Colorado and Wyoming. The discoveries sent shock waves through the Forest Service which had little taste for mining; and the agency hurried to withdraw the new discoveries until all, or portions or most VMS deposits fell within or adjacent to the withdrawals effectively locking out mining companies and prospectors from public land.
Much of this region in Colorado and Wyoming was known as the Grand Encampment district, which in the 19th century, prior to government interference, was a major base metal district. But because of the withdrawals, no jobs were created because of Conoco’s discoveries, no valuable metals were mined, and the US continued out-sourcing jobs and importing base metals.
Today, the Verde district is idle and the town of Jerome is a tourist attraction built on the flank of Cleopatra Hill which rises more than 6,000 feet above sea level. Some buildings in Jerome were constructed on the Precambrian rock that forms Cleopatra Hill, while the rest of the town slopes downward on Tertiary to Recent fanglomerates and landslide debris. The landslide has been a historical problem and periodically slides following periods of high runoff. At the base of the slope, the surface flattens eastward to the Verde River valley at Clarkdale and Cottonwood 4 to 5 miles east of Jerome at an elevation of 3,300 feet.
Jerome was a major copper-gold-silver-zinc-lead district. According to the Arizona Department of Mines and Mineral Resources (2007), 48 of 70 known VMS deposits found in Arizona have produced ore - all of which are Precambrian age (1.7 to 1.8 billion years old). Production from these totaled more than 55 million tons. Most of these deposits are steeply plunging and deformed, such as the United Verde, which has been described as a rod-like ore body or pipe located within an axis of a major, steeply-plunging, fold.
About 98% of the past production in the Verde district was from the United Verde and United Verde Extension mines. Even though mining ceased in 1953, there is evidence that the miners left behind considerable ore. The district was considered to be a copper district, but at today’s metal prices, the district would be classified as a copper-gold-silver-zinc district. Considerable copper, gold, silver with some zinc and small amounts of lead were recovered from the two principal mines.
In 1989, the US Geological Survey indicated records from Phelps Dodge showed production from the United Verde Mine from 1884 to 1975 amounted to more than 2,926,900,000 pounds ($10.1 billion at 2012 prices) of copper, 97,891,000 pounds ($82 million) of zinc, 459,000 pounds ($390 thousand) of lead, 49,603,000 troy ounces ($1.4 billion) of silver, and 1,354,200 troy ounces of gold. Koschmann and Bergendahl of the US Geological Survey reported gold production was even higher at 1,571,000 ounces ($2.5 billion) over the lifetime of the district. Based on statistics from the U.S. Geological Survey and Phelps Dodge, the United Verde produced $14.8 billion in metals at 2012 prices.
A second company, (United Verde Extension [UVX]), was organized in 1899 to search for a suggested, down-faulted continuation of the United Verde massive sulfide. UVX began searching southwest and east of the United Verde property. Their efforts were fruitless until they were on the verge of bankruptcy in 1914, when a drift driven from a shaft east of the United Verde mine intersected bonanza grade ore rich in chalcocite on the 1,200 level. In 1916, another ‘blind’ deposit was found. The ore was so rich the company ended the year with a 74% profit after expenses. Later geological studies showed the UVX discoveries were not part of an off-set of the United Verde ore body, but instead completely separate, ‘blind’ ore bodies.
The UVX mine operated on a large scale until 1938 when much of its high-grade was mined out and operations ceased. The nearby United Verde mine continued mining. After 1931, much of their activity focused on open-pit mining. Depletion of ‘high-grade’ reserves finally forced the mine to close in 1953. This resulted in many people reporting that the deposit was mined out. However, the operations left behind a giant, low-grade, ore deposit and who knows how many ‘blind’ deposits.
|Banded Chert from the United Verde mine|
Sulfides are exposed at the surface down to the 4500-foot mine level, but in the geological past, the ore also extended upwards to the base of the Cambrian Tapeats Sandstone indicating at least 350 feet of vertical column of massive sulfide was removed by erosion. With this much erosion, drainages down-slope from the massive sulfide should contain detrital gold, but I could not find any report suggesting that gold had been detected in the nearby drainages. Based on the mode of occurrence, the gold would likely be very fine grained.
Ore minerals in the massive sulfide in decreasing abundance included pyrite, sphalerite, chalcopyrite, bornite, arsenopyrite, galena, tennanite and gold (electrum). Much of the electrum is thought to be present as microscopic inclusions in sulfide minerals. Silver is present in the electrum and in late-stage tennanite (Cu12As4S13) in quartz carbonate veins and in carbonate rich massive sulfide ore. The tennanite also contains gold as microscopic inclusions.
The Verde was the largest known volcanogenic massive sulfide deposit in the US with 33 million tons of mined ore, and 50 to 70 million tons of low-grade ore left in place. It is also considered to be one of the largest in North America. The massive sulfide was steeply-dipping, irregular- to cylindrical-shaped ore body approximately 700 to 800 feet in diameter that extended 2,400 feet deep. The United Verde mine reached a depth of 3,515 feet and included eight shafts. According to the US Geological Survey, proximal ore recovered from the mine averaged 4.77% Cu, 0.046 opt Au, and 1.65 opt Ag. Zinc and lead were not recovered during much of the operations. Another blind ore body was found in the lower levels of the mine 500 feet northwest of the roots of the main deposit.
Anomalous gold was detected in the pyritic and sphalertic massive sulfides and the chert rich ore: chloritic host rocks are poor in gold. The zinc ore contained as much gold as the copper ore. And because only the high-grade copper-rich portion of the massive sulfide lens was mined, a very large tonnage of low-grade ore remains in place and it was estimated that at least 75% of the mineralized deposit remains unmined because it was too low-grade to mine (<2% Cu) under historical economics. The estimates suggest about 115,000,000 tons of low-grade massive sulfide ore and 38,000,000 tons of mineralized black schist remain in the mine with grades of 0.5 to 1% Cu, 2 to 4% Zn, 0.01 to 0.015 opt Au and 0.5 to 1.0 opt Ag. This zinc-rich deposit is estimated to have 400,000 troy ounces of gold.
The UVX mine produced 3,879,000 tons of ore that averaged 10.23% Cu, 0.039 opt Au, and 1.71 opt Ag. Ore minerals included chalcocite, cuprite, native copper, malachite, chrysocolla and azurite hosted by felsic fragmental rocks, massive rhyolite and quartz porphyry – all units of the Deception Rhyolite.
Part of the UVX operation was developed by the Haynes Shaft due west the United Verde open pit. This deposit was another blind ore body found at 2,500 feet below the surface. The UVX decided to explore the ground under the Haynes shaft by driving a tunnel at the 3,000 level. A drift was started in 1930 and reached the Haynes massive sulfide pipe in 1931.
The Haynes proximal pipe was a steeply-plunging massive sulfide. It extended a short distance above the 2700-level downward to an undetermined point between the 3450 and 3700 levels of the United Verde Mine. The Haynes haft was sunk to 700 feet between 1907 and 1911; subsequently deepened to 1,200 feet, with 1,700 feet of drifts and crosscuts on the 700 level and 700 feet on the 1200 level.