The great "Comstock lode" stretched its opulent length straight through town from north to south, and every mine on it was in diligent process of development. The (Gould and Curry) mine alone employed six hundred and seventy-five men … The “city” of Virginia roosted royally midway up the side of Mount Davidson, seven thousand two hundred feet above the level of the sea, and in the clear Nevada atmosphere was visible from a distance of fifty miles! It claimed a population of fifteen thousand to eighteen thousand, and all day long half of this little army swarmed the streets like bees and the other half swarmed among the drifts and tunnels … Often we felt our chairs jar, and heard the faint boom of a blast down in the bowels of the earth...
Nevada is called ‘The Silver State’ because it was founded while the Comstock silver-gold mines were being developed. The establishment of the state capital at Carson City, 19 kilometres southwest, reflected the influence of the mining district, which produced intermittently between 1859 and 1996. Using the prevailing metal prices during its boom years between 1860 and 1880, silver accounted for about 55 per cent (Rickard, 1932) to 60 per cent (Lindgren, 1913) of the total value of the ore, with the balance contributed by its gold output. If the value of its production was calculated today using November 2007 metal prices of $800/oz for gold and $15/oz for silver, the situation would be reversed. The value of the production would now be about 70 per cent from gold and the remainder from silver, Comstock would be called a gold-silver lode and the state might have a different nickname.
In terms of contained metal, the Comstock Lode ranks among the top 10 per cent of world epithermal districts and can be classified as world class. It was one of the first epithermal districts described in North America. Exact production records for the camp are unknown because record-keeping in the early years was incomplete. In addition, the unfortunate custom at that time of quoting the amount of silver and gold produced by dollar value, rather than by tonnage and grade, requires many assumptions to be made. The best available estimate of total production is about 257 tonnes (8.26 million ounces) of gold and 6,000 tonnes (193 million ounces) of silver from 16.35 million tonnes (18 million tons) of ore (Hudson, 2003). This equates to a relatively rich average grade of approximately 15.7 g/t (0.46 oz/ton) gold and 367 g/t (10.7 oz/ton) silver, a gold equivalent grade of about 22.5 g/t (0.66 oz/ton) at present metal prices.
Eighty per cent of the dividends paid from the camp came from just two pairs of adjacent mines — Con Virginia (California) and Crown Point-Belcher (Paul, 1963). The Con Virginia (California) oreshoot alone produced 1,131,900 tonnes of ore that averaged 87.4 g/t (2.55 oz/ton) gold and 1,834 g/t (53.5 oz/ton) silver (R.E. Kendall in Hudson, 2003). That is approximately 35 per cent of the gold and 31.5 per cent of the silver produced in the entire camp.
Two Mexican words commonly used in the Comstock camp to describe ore grades were adopted into the American lexicon — bonanza, meaning fair weather, was used to describe an especially rich precious metal lode and borrasca, meaning storm, was applied to an unproductive vein, mine or claim. Miners, promoters and investors spoke frequently about mine workings being in either bonanza or borrasca, and the richest orebody in the camp, the Con Virginia (California), was called the Big Bonanza. The promotional name ‘bonanza’ was subsequently given to deposits, claims or geographic features in almost every mining camp in North America. For example, it was given to one of the richest creeks in the Klondike Gold Field (Yukon Territory) in 1896.
Modern studies of the geology of the Comstock Lode are more difficult because the bulk of the mining took place before current research tools were developed, and the unstable ground conditions prevented later access to the richest and deepest parts of the lode. As a result, part of the research has, by necessity, been restricted to specimens collected by early mine foremen and/or is based on contemporary descriptions. The principal challenge throughout almost 150 years of study has been the complex structural deformation history, which is still partially unresolved. The main points of remaining controversy relate to the timing of mineralization with respect to transpressional deformation, regional extension and the local displacement gradient, and the localization of bonanza ore within a zone of strike-slip faulting linked by normal faults.
The Comstock fault zone, the main structure in the district, is traceable for more than 15 kilometres along strike and bounded by nearly parallel faults for almost its entire length. The ore-bearing part of the lode is about 4.2 kilometres long. The lode is comprised of about 50 small to large, lenticular oreshoots of irregular shape that occur within the fault zone. Two of the orebodies dipped 70° west near the surface, whereas the rest had easterly to near vertical dips. Mined widths were mostly in the 10 to 17 metre range. Individual orebodies rarely extended more than 150 metres vertically and most were less than 150 metres long. The margins of the orebodies were commonly marked by a narrowing and feathering into thin veins containing less sulphide minerals, or were sometimes cut off abruptly by clay seams.
In decreasing order of abundance, the major ore minerals were sphalerite, chalcopyrite, galena, pyrite, acanthite and electrum. Acanthite is a low-temperature polymorph of argentite and is the mineral that forms the tarnish on sterling silver. Stephanite was reportedly the main silver mineral in the Con Virginia orebody and was abundant in the Ophir. At least 12 other silver and copper minerals, including native silver, amalgam, covellite, chalcocite and chlorargyrite, were identified within the zone of oxidation, which extended to depths of 100 to 160 metres. No base metals were recovered from the Comstock ores because of the limitations of the treatment processes available at the time. Bastin (1922) reported that the ores were characteristically fine grained, with individual grains commonly less than one millimetre in diameter and only rarely attaining diameters of 5 to 10 millimetres.
A large variation in character was exhibited within and between orebodies, with some lacking precious metals, others containing intergrowths of base and precious metal minerals, and still others that essentially lacked base metals. The Ophir mine, for example, had a western vein rich in base metals and an eastern vein rich in both precious and base metals. Gold/silver ratios generally decreased with depth but varied widely in some oreshoots. Most of the richest ores reportedly contained appreciable chalcopyrite and were relatively poor in sphalerite. Massive calcite was deposited just below and in the lower parts of orebodies.
Pliocene to Holocene reactivation of faults disrupted the lode and affected the relative positions of many of the orebodies and alteration assemblages. Detailed studies have shown that the best (bonanza) mineralization resulted from the focusing of hydrothermal fluid flow into spatially restricted networks of interconnected fractures. These confined zones of high vertical permeability and hydraulic connectivity are interpreted as the result of complex deformational processes.
The orebodies are scattered within a quartz gangue composed of weakly mineralized massive veins, breccias and stockwork veins that have been complexly rearranged by post-mineral faulting. Stockwork veins were the most common, comprising up to 90 per cent of the lode, but usually 10 to 50 per cent. The next most abundant gangue material was white, massive to stockwork quartz, called ‘red quartz’ or ‘bastard quartz’ by the miners. It was usually mosaic and/or comb-textured, unbanded or poorly banded, and contained multiple generations of quartz breccia fragments containing crosscutting veins cemented by quartz. Massive quartz was found to the deepest explored depths, where it was hard, locally contained base metals, and consistently contained precious metals at sub-ore grades.
Early workers quickly recognized the difference between rich, ore-bearing quartz and barren, massive quartz. The ore-bearing quartz was milky white, very friable and apparently anhedral, with a sandy texture resembling sugar or table salt. The ‘sugar’ quartz was loose and crumbly except where it was cemented by ore minerals or later quartz. Some of the orebodies were completely shattered and broken, possibly by post-mineral faulting. The sandy texture and shattering contributed to the difficult mining conditions described in the previous chapter.
The Comstock Lode is an adularia-sericite epithermal system that can be divided into early and bonanza stages. Extensive studies over more than a century have identified 12 hydrothermal alteration assemblages or sub-assemblages. The main stage of Au-Ag-Zn-Cu-Pb mineralization was deposited towards the end of a late stage of deep, low-sulphidation alteration. The other type is intermediate-depth high-sulphidation alteration. The superposition of the two styles tends to obscure and conceal the low-sulphidation alteration within the much more obvious high-sulphidation alteration. A close spatial relationship between these two epithermal environments is quite unusual globally.
Richthofen (1868) first applied the term ‘propylite’ to certain rock units in the Comstock district. Becker (1882) realized that it was altered andesite, and this assemblage was later recognized as one of the most common hydrothermal alterations worldwide. Three sub-assemblages of propylite have been mapped, based mainly on the presence or absence of epidote. They form halos around the high-temperature parts of the lodes.
Tertiary volcanic activity centred in the vicinity of the Comstock district began at about 18.2 Ma with the eruption of andesitic lavas, and at least four superimposed Miocene hydrothermal events have been recognized. The oldest volcanic rocks are a suite of lavas, breccias, dacitic intrusions and minor sediments that were intruded by the 15.2 Ma Davidson diorite. The largest Davidson body forms the footwall of the Comstock fault zone and numerous dykes that are present in the hangingwall. The hydrothermal alteration and ore deposits have been dated at 14 Ma.
Mining at Comstock reached a depth of almost 1,000 metres and was exceeded only by the Adelbert shaft in the P?ríbram silver mine, Czech Republic, which became the deepest in the world in 1875, at 1,000 metres (see CIM Magazine, March/April, 2006, p. 65). The P?ríbram deposit, which consisted of relatively narrow veins hosted by strong wall rocks, encountered very little water and was completely dry below 800 metres, and reached an ultimate depth of almost 1,600 metres. Comstock, on the other hand, attempted to mine wide, soft oreshoots in unstable wallrocks and was extremely wet and unusually hot. These conditions required more extensive timbering than had been encountered in most mines in the world, as well as continuous pumping with the largest pumps available. To make matters worse, the mines required strong ventilation and were even cooled with ice because the inflowing water was so hot it made the working conditions at depth almost unbearable. The water temperature increased 3°F for every 100 feet of depth and reached 170°F (about 77°C) at a depth of 968 metres in the Yellow Jacket mine, where the rock temperature was measured as 167°F. Even with abundant ice, miners could only work alternate hours and, in some cases, only 15 minutes at a time. These adverse conditions eventually restricted further exploration when miners refused to work in such conditions.
The source of the hot water, and the impact of Comstock and the California Gold Field on American mining and exploration practice, and their effect on economic geology, will be discussed in the next chapter.
The geological information in this chapter is mostly derived from Berger, Tingley, and Drew (2003) and Hudson (2003), except where noted. Background historical information is mainly from Lord (1883), Paul (1963), Rickard (1932), Smith (1943), and Watkins (1971). In addition, ‘Barney’ Berger provided invaluable assistance, including two figures from his paper.