The gold-quartz veins of the Pacific Coast attain their greater development along the western margin of the Sierra Nevada batholith and appear again in southeastern Oregon… come to light in spots in British Columbia and are strongly represented on the shores of southeastern Alaska. Free gold, quartz gangue and scant sulphides are typical… They are common in and about minor intrusions of diorite and quartz-diorite; some are seen at contacts of serpentinite… Their proved vertical range is over 5,000 ft; their entire range probably over 10,000 ft, without marked change or zoning. Their simplicity is amazing.
This chapter has relied heavily on Ash (2001), who has prepared a fine summary of a complicated subject, and also kindly suggested improvements to my text. The other main source of information is Knopf (1929).
The Sierra Nevada region was one of the most intensely studied mineral districts in North America during the active mining period, and up until the 1980s. That was because of its important gold content as well as its complex geological setting. In fact, California gold is an excellent example of the evolution that occurred in economic geology theory over a span of about 150 years. Ash has summarized the three principal phases of understanding, beginning with observations made during the early classical stage, then describing how the origin of the host rocks had to be reconciled with plate tectonic theory, and finally concluding with an interpretation of the role played by the ophiolite model in the genesis of the gold deposits.
Gold-quartz vein deposits throughout the North American Cordillera, from California to Alaska, are often spatially associated with carbonate-sericite-pyrite-altered ophiolite rocks, both mafic and ultramafic, known as listwanites. Listwanite (after the Russian listvenity, from the type locality in the Ural Mountains) is a term that was used almost exclusively by Russian geologists until the 1970s to describe listwanite-altered rocks veined by hydrothermal quartz-carbonate. As used in California, the term describes an alteration suite composed of carbonate, mariposite/sericite, pyrite, and introduced quartz. This alteration assemblage was referred to by early California miners as "blue jay".
Ophiolites are obducted, usually dismembered, remnants of ancient oceanic lithosphere consisting mainly of crustal igneous and sub-crustal metamorphic mantle rocks. In orogenic belts like the North American Cordillera, ophiolitic assemblages occur either as allochthonous, dismembered and imbricated structural slices that were transported tectonically across former continental margins, or as imbricated and deformed slices in the central parts of orogens. The current view, which is well rooted in the development of plate tectonic theory, is that ophiolites are generated at oceanic spreading centres, either at mid-ocean ridges or in mantle slabs above subduction zones.
Mother Lode system
This was the bewildering geological setting for the gold deposits that faced economic geologists in the middle of the 19th century. The classical geological interpretation, as summarized by Knopf (1929), was that the Mother Lode system was a steeply dipping complex of metasediments, metavolcanics, and some serpentinized intrusive bodies. For obvious reasons, the knowledge base available at the time didn’t permit the various accreted and thrusted units to be differentiated. Now called the Calaveras Complex, it was originally divided into an older Calaveras formation (typically black phyllites with subordinate quartzite, limestone, and green schists) and a younger Mariposa formation of Carboniferous age (derived by metamorphism of augite tuffs and lavas and interbedded, in places, with phyllite). The complex is one of a number of litho-tectonic elements within the Sierra Nevada Metamorphic belt.
The early underground mapping recognized two principal types of gold deposits, quartz veins and bodies of mineralized country rock, of which the latter shows the most variety and complexity. The quartz veins generally occur as systems of parallel or acutely intersecting veins. As many as four veins were worked in individual mines but few could be traced for more than one kilometre. They usually intersected the cleavage of the enclosing rocks at an acute angle in both strike and dip, filling fissures that were formed by reverse faulting. In some cases, the displacement amounted to as much as 120 metres. The veins tended to pinch and swell abruptly, with quartz becoming more admixed at the edges of the lenses. Banding or ribboning was common in veins that were enclosed by slate or schist and invariably parallel to the walls of the vein.
Where the massive quartz frayed out into a stringer lode, it became poorer in grade. Although oreshoots tended to be wider than adjacent barren sections of the vein, large quartz bodies did not necessarily make ore. The oreshoots were generally short but persisted to depth, with a steep rake. Most of the known oreshoots cropped out at surface, although some blind ones were discovered whose tops were as deep as one kilometre. The only generalization that could be made about ore controls was that oreshoots could occur in any rock except serpentinite, that slate was a more favourable wallrock than greenstone, and that veins wholly enclosed in greenstone tended to be low grade. In the early years, ores grading as low as 0.1 to 0.15 opt were mined. The bullion ranged in fineness between 790 and 840, with the balance consisting mainly of silver.
Pyrite was by far the most abundant sulphide mineral in the ores, comprising one to two per cent of the quartz ore and up to two or three times that in the mineralized country rock. It was formed early in the initial quartz phase along with arsenopyrite, which was the second most abundant sulphide. Coarse arsenopyrite coincided with rich ore along the central part of the Mother Lode. Sphalerite and galena were generally contemporaneous with gold. Although the former was more abundant, galena was considered the best indicator for gold. The telluride mineral petzite, which was restricted to the portion between Jackson and south of Sonora, was also a good indicator of rich ore. Minor amounts of chalcopyrite and tetrahedrite were also present but were indifferent indicators. Stibnite was noted in one of the mines at the south end.
Two types of mineralized country rock were present, a so-called ‘grey ore’ (also referred to as replacement ore) and mineralized schist, occurring either adjacent to quartz veins or in broad zones of fissuring (stockworks). According to the classical interpretation, the grey ore, which is a mixture of ankerite, sericite, albite, quartz, pyrite, and generally some arsenopyrite, resulted from the hydrothermal alteration of mafic igneous rocks into listwanites. Many of the grey oreshoots were large, with an average grade of up to 0.4 opt and a sulphide content of three to six per cent. They generally adjoined thin quartz veins or occurred in the wedge between two intersecting veins. The grade of the fine-grained ore could only be determined with assays.
The mineralized schist oreshoots consisted of pyritic sericite-ankerite schist, generally ramified with quartz-ankerite veinlets, and were interpreted to be the result of alteration of amphibolite and chlorite schist. They were generally lower in grade, from 0.1 to 0.15 opt, although one of the richest ore bodies ever mined along the Mother Lode system, on Carson Hill, was of this variety. Many oreshoots of this type formed either the footwall or the hanging wall of large, thick, barren quartz veins.
In spite of the obvious limitations, pioneering advances had been made by 1929 in the study of the profound wall rock alteration that accompanied the ore-forming process of the Mother Lode veins. Ankerite formed as a replacement of fine-grained mafic igneous rocks, whereas magnesite formed from ultramafic rocks, usually serpentinite. Serpentinite and the augitic greenstones were the most susceptible to ankerite and magnesite replacement, forming belts many metres thick that are generally tinted a delicate green by the presence of the chrome-rich mica mariposite. Sericite, albite, pyrite, and arsenopyrite were also commonly introduced through chemical attack, while gold migrated into the wall rocks, where it is associated with sulphides. The alteration process was believed to involve the addition of great quantities of carbon dioxide and potassium-rich fluids to the wall rocks. At the same time, the removal of comparable amounts of silica from the wall rocks was thought to provide more than enough to supply the quartz in the Mother Lode veins.
Modern studies have shown that the gold-quartz vein deposits are preferentially associated with ophiolitic members of the complex. The late Paleozoic ophiolitic and chert-argilite members are interpreted to have formed as an oceanic basement along the continental margin that was disrupted by periods of basinal magmatism during the Late Triassic–Early Jurassic, and again in the latest Middle Jurassic to early Late Jurassic. Both events were followed by intervals of tectonism and deformation during the Middle Jurassic Siskiyou orogeny and the Latest Jurassic Nevadian orogeny. The post-Nevadian Sierra Nevada batholith intrudes the Sierra Nevada metamorphic belt along its eastern margin. Earlier 150 to 140 Ma magmatism resulted in small intrusive bodies, dykes and sills, whereas the bulk of the batholith was intruded between 120 and 80 Ma.
Clark (1970) calculated the total gold production from California up to 1969 as more than 106 million ounces (about 3,300 tonnes). The lode portion came from countless small mines plus a few large ones. Although detailed production records do not exist, he was able to document 40 that produced between about one and five million ounces, seven that produced between five and ten million, and two in the Grass Valley camp, Empire-Star and Idaho-Maryland, that were much larger.
Within the Mother Lode, about half the gold came from a 16 kilometre long section between Plymouth and Jackson, in Amador County. This portion also hosted most of the largest producers, including the Central Eureka and Kennedy mines, both of which yielded more than 46.5 kilograms (1.5 million ounces) and the Argonaut and Keystone mines (over 1.0 million ounces each). Grey ore was the mainstay of the Keystone Mine, which operated from 1852 until 1920. The only mine to produce more than 1.0 million ounces that was not situated in that part of the belt was Carson Hill. It was also the source of the largest piece of gold recovered from the Mother Lode, in 1854. It weighed 72.8 kilograms (2340 ounces).
Ash, C.H. (2001). Relationship Between Ophiolites and Gold-Quartz Veins in the North American Cordillera. Geological Survey Branch Bulletin 108. Victoria: British Columbia Geological Survey (available at http://www.em.gov.bc.ca/Mining/Geolsurv/Publications/Bulletins/Bull108/toc.htm)
Clark, W.B. (1970). Gold Districts of California. California Division of Mines and Geology Bulletin 193. Sacramento: California Division of Mines and Geology.
Knopf, A. (1929). The Mother Lode System of California. U. S. Geological Survey Professional Paper 157. Washington: Government Printing Office.
Lindgren, W. (1933). Differentiation and Ore Deposition, Cordilleran Region of the United States. In J.W. Finch (Ed.), Ore Deposits of the United States, Lindgren Volume, (p. 170). New York: The American Institute of Mining and Metallurgical Engineers.