Tuesday, January 10, 2012

Vulture Gold Mine, Arizona

Exposed gold-bearing gossan at the Vulture mine,
Vuture Mine. Gold was discovered 50 miles northwest of Phoenix in 1863 by Henry Wickenburg. The discovery led to the sinking of the Vulture mine shaft and development of an adjacent town named Vulture City. The town reportedly grew to a population of nearly 5,000. Today, Vulture City is a ghost town, but its ashes led the city of nearby Phoenix.

To visit the mine on Google Earth, search for ‘Vulture Mine, AZ 85390’ and this should take you 0.25 mile north of the mine. As you scan south, you will come across open cuts in rusty gossan at the mine site, remains of the ghost town to the east, the mill and assay office to the south of the gossan with extensive white leach pads.

The Vulture mine became known for high grade ore. History reports many people made a living from high-grading ore and those who were caught were hanged to stop theft at the mine (maybe similar opportunities provided to politicians and CEOs for high-grading public funds, taxes, and military secrets, would result in people who really represent our best interests).
Mine adit in fanglomerate at the Vulture
mine similar to many of Arizona's
 placer gold deposits.
Following discovery of gold, the precious metal was mined over the next three years from high grade ore milled in a primitive arrastre constructed to the east at the nearby Hassayampa River. In 1866, a 40-stamp mill was constructed on the Hassayampa River 12 miles to the northeast of the mine (other sources report the mill was a 20 stamp mill) and gold was recovered from high-grade ore reported to assay 1.2 to 4.5 opt Au. The mill site located at such a distance resulted in security problems: the ore had to be hauled by freighter wagon to the mill. And there are stories that suggest as soon as the wagons were out of site of the mine, high-grading was a common practice: so common that at least 18 men were hanged in Vulture City. Some suggest that freighting became more profitable than mining. In addition to high-grading by freighters, theft was common at the mill and by miners working underground.
In 1879, the mill was expanded to an 80-stamp mill. By 1888, the vein was lost along a fault contact on the 300-foot level and operations ceased. Twenty years later (1908), a comprehensive geological study led to discovery of the vein offset and the mine reopened and operated until 1917 when the vein was again lost along another fault.
View of the remains of Vulture City
behind gossan.

Exploration for the offset resulted in the sinking of a 500-foot shaft and the vein was rediscovered. The mine again operated until it was closed by a 1942 War Production Board order closing all non-essential gold mines in the US to ensure maximum effort was contributed to the war effort. Many mines that were closed at that time never reopened, suggesting that at today’s gold prices, many of these mines likely have commercial ore and potentially could be reopened.  The Vulture mine likely fits in this category; however, there is an effort to have the State of Arizona purchase the property for historic preservation, which would result in a valuable gold resource being preserved similar to the Carissa mine in Wyoming (like the Vulture, the Carissa mine was Wyoming’s largest historical gold producer and it was preserved by the State such that the gold resource is no longer available to the public: work by the author and past mining companies has shown the property likely has a very sizable gold resource that could be mined by open pit and underground [Hausel, 1991 and Hausel and Hausel, 2011]). The Vulture mine never reopened leaving one to wonder how much unmined ore and gold resources remain. The Vulture mine, the main incline shaft of the mine continues to a depth of 2,100 feet along a 35o incline.
Part of the ore zone at the surface of the Vulture mine -
highly silicified gossaniferous rock

A characteristic of much of the region in western Arizona is extensional faulting during the Miocene that produced numerous listric and planar normal faults. Such normal faults and associated fault blocks were rotated east to northeast and vein offsets are not uncommon.

The Vulture mine is surrounded by Miocene (23 to 5 million years old) andesites and rhyolites that lie unconformably on Proterozoic (2.5 billion to 600 million years old) schist and gneiss. These were tilted by rotational normal faulting such that the original bedding is now near-vertical to locally overturned, and faults exhibit gentle, westerly dips (Spencer and others, 1989).
Primary dipping vein at the Vulture.

Gold and silver were discovered in quartz veins and in silicified and altered host rock within and adjacent to a prominent north—northeast-dipping quartz-porphyry dike that intrudes Proterozoic basement rock and grades into a Late Cretaceous granite to granodiorite pluton to the west. The precious metal occurs in the form of native gold and electrum and is associated with sulfides including pyrite (fools gold), argentiferous galena (silver-bearing lead-sulfide) and minor chalcopyrite (copper-iron-sulfide) and sphalerite (zinc-sulfide). White (1988) reported a positive correlation of gold with abundance of secondary silica and sulfides. Pervasive wall rock alteration adjacent to the vein resulted in replacement of feldspar and mafic minerals to produce sericite, hematite and clay; thus the altered dike now consists of quartz ‘eyes’ in a fine-grained altered matrix. Gold is concentrated in quartz veins and in silicified and altered rocks within and adjacent to the dike.
Tightly folded Proterozoic basement
schist and gneiss exposed at the
mine site.
The mineralized zone is fault controlled and trends nearly parallel to foliation. The vein was traced for 1,000 feet and is 32 feet wide at the surface and swells to 47 feet on the 240-foot-level. To the west, the vein splits into smaller veins that are cut by several faults. Production figures are not complete, but the available reports indicate that the mine produced 340,000 ounces of gold and 260,000 ounces of silver from 1863 to 1942 from ore that averaged 0.35 opt gold and 0.25 opt silver (Spencer and others, 1989).  

The is a very interesting side to this mine. Apparently Jacob Waltz (known as the Lost Dutchman) worked at the Vulture mine for several years and legend suggests he likely high-graded considerable gold from the mine and his discovery of gold in the Superstition Mountains may actually be the gold high-graded at the Vulture (Miner, 1996-2011).
Today, the mine is inactive but the owners have made the available for self-guided tours for the public that are highly recommended.
Old headframe (right) with part of the exposed vein (left)
at the top of the hill
Just an interesting photo inside the assay lab.

Old assay furnace in the assay lab at the Vulture mine.
Old powerhouse at the Vulture gold mine.

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Minerals and Rocks

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