About 120 million years ago, a metal-bearing, carbonate-rich hydrothermal fluid was channelled by a steeply dipping, major strike-slip fault zone which developed nearly 10 km beneath the contemporary land surface of the Sierra Nevada in California. As of today, about 100 million oz of gold have been extracted from deposits along this ancient fault, known as the Mother Lode Belt. It’s long been known that major fault zones are important controlling factors on certain types of gold deposits. But George Albino, who is with the University of Western Ontario’s Dept. of Geology, takes it a step further. He believes it is crucial to recognize what zone of an ancient fault is exposed at surface when planning and carrying out an effective exploration program. The most significant gold-produc ing belts, such as the Mother Lode Belt, are found where erosion has exposed fault rocks that were buried at moderate depths (7-10 km) at the time of mineralization. Deposits formed at greater depths (more than 20 km) are only locally of exploitable grade and are rarely capable of supporting sustained production. In deposits formed relatively close to the surface (less than 5 km), the fluids have already been depleted with respect to gold, and only deposits of metals such as mercury, antimony and arsenic will form. Local accumulations of high- grade gold may result, but probably not large ore deposits.
According to Albino, the levels of exposure, or erosional level, of the fault zone can be identified by the nature of the rocks and the metamorphic grade. Two other indicators are the alteration mineral assemblages and metal associations of the rocks. All these factors change as a function of the level of exposure. Once the level of exposure of the prospective fault zone is determined, the location of actual economic gold deposits is difficult and may require detailed structural and alteration studies and, of course, drilling.
The nature of the rocks in the fault zone will change in a predictable manner with depth and can be used to infer what level is exposed in an ancient fault zone. In the near-surface environment, non-foliated cataclastite and fault breccia will predominate. At middle depths, mylonites and phyllo nites will form and a steeply plunging lineation is normal. At great depths, gneisses or schists and sub-horizontal to moderately plunging lineations will mark the fault zone. The metamorphic grade also changes with the depth of the fault zone: sub-greenschist facies near surface, greenschist at moderate depths, and amphibolite facies at great depths.
Three gold-producing areas in North America, representing different erosional levels of a fault zone, were studied to show the characteristics of major faults at different depths. They are, from shallow to deep levels: Pinchi Mercury Belt, British Columbia; Mother Lode Belt, California; and; Dahlonega Belt, Georgia. The Larder Lake-Cadillac Fault is similar to the Mother Lode Belt, both structurally and in terms of alteration assemblages and associated ore deposits.
The Mother Lode Belt, in the foothills of the Sierra Nevada, is one of the largest Phanerozoic gold provinces in the world, with about 100 million oz of total production (including production from placer deposits) from numerous mines along a 200-km strike length. The Mother Lode Belt represents a middle erosional level (7-10 km) along a major fault. The Pinchi Mercury Belt in central British Col umbia has been, by far, Canada’s largest source of mercury production, almost entirely from Cominco’s Pinchi Lake mine (reserves are in the order of several million tons). Mercury has been the only metal recovered from the belt, but antimony occurs as stibnite in many areas and gold was present in the range of 5-120 parts per billion in the mercury ore at Cominco’s mines and in mineralized rock from other occurrences along the belt. The Pinchi Mercury Belt represents the little-eroded, near-surface zone (less than 2-3 km) of a major mineralized fault zone.
The Dahlonega Belt is in the Appalachians of Georgia and is of mainly historical interest as the site of the first major gold rush in North America. About 600,000 oz of gold have been produced in the area, mostly from hydraulic operation on saprolite. The mineralized zone of the Dahlonega Belt represents a deep level (more than 20 km) of a mineralized structure. The nature of the rocks within a fault zone changes in a predictable manner with depth as a function of two factors: strain rate, which is displacement along the fault, and recovery rate — essentially recrystallization. By assuming that the rate of displacement (strain rate) is constant at all depths on any individual shear zone, then the nature of the fault rocks will depend on recovery rate. The dominant factor governing recovery rate is temperature and, since temperature will increase with depth, the nature of the rocks in the fault zone will change in the predicted manner with depth.
High rates of strain relative to recovery are accompanied by the formation of non-foliated rock such as fault gouge (cataclastite if consolidated) and breccias. Moderate strain-to-recovery rates result in ductile faults in which deformation occurs through recrystallization; in this case mylonites and phyllonites mark the shear zone. These are rocks with reduced grain size, relative to their parent materials, and strong planar fabric parallel to the margins of the shear zones. They are often banded on a fine scale and, because of this strong anisotropy, they commonly are highly folded on a local scale. Development of more than one foliation is also a common feature of mylonites, and the relationship be tween these minor planar features and the main shear zone fabric can be used to infer sense of displacement along the fault.
If recrystallization is fast relative to displacement, the rocks along a fault will be similar in appearance to normal schists or gneisses, and the only clue to their origin as fault rocks will be their occurrence in narrow linear zones. The term “blastomylonite” has often been used for these rocks, but naming them as such is often dependent on the recognition of the structure. The alteration assemblages associated with mineralization also vary with depth and can be used to determine which zone of the ancient fault is exposed. Recognition of the differences in alteration assemblages be tween amphibolite and greenschist areas is important in helping geologists overcome “greenschist” biases, in which minerals like epidote (or zoisite) and hornblende are considered unfavorable signs for gold deposits.
In the Mother Lode Belt, ultramafic rocks provide the most sensitive indicators of the conditions of mineralization. There is a common pattern of alteration mineral zoning indicative of the moderate depths of the fault zone:
* serpentine
* serpentine-talc
* talc-calcite-serpentine
* talc-dolomite-chlorite
* magnesite-quartz-chlorite
* magnesite-quartz-mariposite (chromium-muscovite).
As in the Mother Lode Belt, the ultramafic rocks are the most sensitive indicators of alteration in the Pinchi Mercury Belt. The zones are:
* serpentine (-olivine)
* serpentine-magnetite
* serpentine-calcite
* quartz-chalcedony-magnesite.
The alteration assemblages of the Dahlonega Belt are quite different from those of most gold districts because of the much higher metamorphic grade/temperature at which they formed. Here the amphibolites of the belt are the most sensitive indicators of alteration intensity. The zones present include:
* hornblende-andesine
* hornblende-quartz-zoisite-oligo clase
* quartz-calcite-hornblende-zoisite
* quartz-calcite-biotite-hornblende * quartz-dolomite-muscovite- chlorite
Although absolute contents of ore metals vary along each belt, there are trends in metal associations with increasing depth of the fault zone. Gold is most abundant at intermediate depths (7-10 km), decreasing gradually with depth, and decreasing sharply in near-surface mineralized zones. The concentrations of antimony, mercury, tungsten and molybdenum decrease with increasing depth and, at the same time, there is a small increase in tellurium. The arsenic content varies little with depth. Ratios between metals also change in a regular way with depth, such that anti mony/arsenic and mercury/antimony are higher in shallow mineralized zones. Arsenic/antimony appears to be more sensitive to depth in the deeper portion of the systems, whereas anti mony/mercury is relatively constant.
If, in your next gold exploration program, you come across mylonites and phyllonites in greenschist facies rocks with a steeply plunging lineation, take a closer look at the alteration assemblages and metal associations. Who knows? You may very well have just discovered the next “Mother Lode.” REFERENCES Albino, G. V.: Jurassic History of the Central Sierra Nevada and the “Nevadan” Orogeny. Albino, G. V.: Large-Scale Vertical Metal Zoning in Mother Lode-Type Systems. Albino, G. V.: The Pinchi Mercury Belt, British Columbia: Near Surface Expression of a Mother Lode-Type Mineralized System. Albino, G.V.: Structural and Petrologic Expres sion of Shear Zones at Different Crustal Depths. Joyce Musial is a Toronto-based con sulting geologist and freelance writer.
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