Sierra Nevada Foothills
The gold belt of the Sierra Nevada foothills is the largest orogenic gold deposit in the North American Cordillera (Sillitoe, 2008). It extends about 270 km along a N-NW trending suture zone that adjoins accreted terranes to the North American plate proper. The gold belt is about 5 km wide, and is estimated to contain over 100 MOz of gold in lode-vein and placer deposits (Goldfarb et al., 2001).
Tectonic and Structural Setting
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Triasso-Jurassic accretion was responsible for continental growth and steeply dipping thrust faults (Melones, Sonora, Bear Valley) (Figure 8). Generally perpendicular convergence was accompanied by arc magmatism along the western North American margin in the late Triassic in the eastern reaches of the current Sierra Nevada Mountains (Chen and Moore, 1982). Intrusive activity related to the Sierra Nevada Batholith continued until about 140 Ma, when most arc magmatic activity came halt (Chen and Moore, 1982). This cease in magmatic activity coincides with a change in plate motion, resulting in sinistral-oblique convergence lasting until roughly 125 Ma (Figure 9) (Umhoefer, 2003).
This change in plate plate motion is critical for the formation of orogenic lode gold. Late Jurassic-Early Cretaceous convergence on the western margin became oblique, creating a transpressional stress regime (Umhoefer, 2003). This is recorded in the geologic record by sinistral strike-slip ductile deformation features in the western Sierra Nevada terranes (Glazner, 1991).
This period also overlaps with age ranges for greenschist-facies metamorphism and ductile deformation in the Sierra Foothills (Paterson et al., 1991). The age of hydrothermal mineralization in the Sierra Foothills belt has been constrained to 125 ± 10 Ma using Ar-Ar dating methods (Marsh et al., 2008).
Spatial relationships of the Sierra foothills gold belt along the Melones Fault zone seems to indicate that mineralizing fluids used the crustal scale fault system as a means of transportation during the Early Cretaceous (Goldfarb et al., 2008). Strike-slip reactivation of the Melones Fault allowed for the flow of deeply sourced fluids that led to gold mineralization folloeing the model of Sibson et al. (1988).
After 120 Ma, plate motion retured to a more westward movement (Umhoefer, 2003). This coincides with renewed magmatic activity in the arc (Stern et al., 1981). Gold mineralization ages predate the younger crystallization ages of the Sierra Nevada Batholith intrusives, which sits 20-30 km east of the major gold deposits. Figure 10 shows the timing of mineralization with respect to the change in convergence direction and rate.
The terrane bounding fault provides a mechanism for the release of deep-seated, metamorphic fluid with just a narrow range of mineralization ages. Likewise, the geologic evidence supports the interpretation of a reactivated fault system as a fluid conduit. However, for an accurate classification of ore deposit type, an analysis of fluid chemistry and isotopic signatures is required.
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As fluid travels along a fault, it crystalizes minerals to form a vein. These minerals, as well as the alteration mineral assemblage around the vein, record the isotopic character of the fluid from which they were precipitated. The δD and δ18O values should first be compared with changes in latitude along the Sierra Nevada foothills gold belt. By comparison of these with other orogenic gold in the North American Cordillera, latitudinal dependence will determine whether the fluid has any component of meteoric fluid. Jia et al. (2003) determined that the lack of latitudinal variation in δD or δ18O precluded the possibility of a meteoric component in these fluids (Figure 11).
In order to distinguish between deep seated metamorphic fluid derived from crustal dehydration and magmatic fluid from a deep seated intrusion, Bohlke and Kistler (1986) compiled graph based of existing isotopic data and added several samples from the Alleghany district of the Sierra Nevada foothills gold belt. Bohlke and Kistler (1986) plotted the data for muscovite samples on a δD- δ18O diagram and compared it to metamorphic and magmatic signatures developed by Taylor (1976) (Figure 12). These data points are then extrapolated based to show the isotopic signature of the fluid.
While it seems that the samples from this study are well within the range of metamorphic fluid, a more recent study on orogenic gold in the North American Cordillera has been conducted using the nitrogen isotopic system. Jia and Kerrich (2000) and Jia et al. (2003) have used the isotopic signature of nitrogen in ore forming systems, compiled with data from other geologic environments (e.g., schists, granites, shales, etc.) to create an isotope map (Figure 13). Fig 2.png)
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These data are from fluid inclusion samples in from the Juneau gold belt as well as the Sierra Foothills gold belt. An apparent increase in N concentration and a slightly lower δ15N values are apparent in the Phanerozoic orogenic gold belts. This is akin to a sedimentary component (Figure 7), or hypothetically, the hydrated lower crustal component (Jia et al., 2003). However this seems to preclude an I-type granite or mantle derived fluid source.
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The Sierra Nevada Foothills gold belt formed 125 ± 10 Ma during a period of transpressive deformation. Crustal scale, terrane bounding faults were reactivated as strike-slip faults allowing for fluid transport. Fluid was generated at lower crustal depths due to a combination of internal and external heating. This was coeval with a change in the convergence direction of the Farallon plate as it subducted under the North American plate. Mineralization occurs in between periods of major magmatic activity related to the Sierra Nevada Batholith.
Isotopic evidence supports the theory that the ore forming fluid was sourced deep within the earth. δD and δ18O data indicate no latitudinal correlations, eliminating the possibility of a meteoric water component to the system. Additionally, these both support a metamorphic fluid, as determined by the isotopic ratio data from previous studies. Subsequent δ15N data independently supports the δD and δ18O isotopic data, precluding a magmatic or magmatic source for the fluid.This isotopic data compares favorably with the model of Goldfarb (2001) for the Juneau gold belt ore genesis (Figure 14). This is th current model for orogenic gold deposits along the North American Cordillera, Sub-crustal heating and release of metamorphic fluids has been attributed to either slab break-off, slab rollback, or a slab-window, as the Farallon plate passed under the North American Plate (Goldfarb et al., 2005).
As more data becomes avaialble for the nitrogen isotopic systems (and others), there will be more insight into the process of gold genesis and the release of deep crustal metamorphic fluids. If orogenic gold occurs with subduction and accreted terranes, it may become possible to use geologic features representing former subduction zones as a target for gold exploration.