Introduction
Active subduction along the western margin of North America was extensive during the Mesozoic. Even today, while majority of motion along the plate boundary is tranform, subduction zones are still present (eg., Juan de Fuca subduction). The evidence for subduction is recorded by the accreted terranes and arc magmatism observed along the western coast of North America.
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Gold deposits around the world appear to outline present or ancient subduction zones. Figure 1 highlights gold deposits in the North and South American Cordillera. Although the deposits are of different types (e.g., orogenic, epithermal, intrusion related, etc.), they are all located in the arc environment above a subducting slab. Different types of gold deposits are located in different parts of the collisional arc setting. Gold deposit types associated with convergent plate boundaries include: Au porphyry, sediment hosted, Intrusion related, epithermal, and orogenic gold deposits. The western margin of North America exhibits all types of gold deposits, which can be divided by arc setting (Figure 2). Temporally, all of these deposits types occur after the accretion of the terranes that host them. Some of these deposits, such as intrusion related and porphyry type deposits, can be found in multiple locations in the arc, where others, such as orogenic deposits, are limited to continental fore-arcs (Sillitoe, 2008).
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 Figure 1.png)
The types of gold deposits, however, are not determined by their location in the arc setting. These spatial differences within the arc are also associated with differences in magmatic activity and fluid source. Gold deposits are classified based on the fluid source as well as the structural setting. In some cases, gold can be mobilized by convective movement of meteoric waters, or by the release of magmatic fluids. Sillitoes (2008) developed a model depicting several methods of fluid migration and gold deposition (Figure 3)
Orogenic Gold
Orogenic gold deposits occur in accretionary wedges in continental fore-arcs where crustal thickening and mantle-wedge heating cause the dehydration of a fluid-rich lower crust (Goldfarb et al., 2005). Strong structural controls require both first order, crustal scale faults, as well as smaller scale, second and third order splays. It is the crustal scale, terrane bounding thrust faults which are reactivated during post accretionary seismic events. When a convergent plate boundary enters a transpressional regime, these faults are reactivated and act as a conduit for the influx of metamorphic fluids (Goldfarb, 2008). It is likely that a strike-slip reactivation of a former thrust fault can cause a fluid release, allowing for transport of deep crustal, metamorphic fluid to shallower depths (Sibson et al., 1988). Although most of the fluid movement is along these crustal scale faults, the richest gold deposits are adjacent to these in smaller splays, forming gold-bearing quartz veins (Roberts et al., 2005).
The release of metamorphic fluid is due to the prograde dehydration, decarbonation, and desulfidation reactions that occur during subduction as temperature increases. Heating can occur internally, due to crustal thickening, or externally, due to some heat flux from the mantle (Stuwe, 1998). A combination of these types of heating produces the metamorphic fluids, which carry gold and other soluble metals upwards, precipitating mineralized veins during decompression and cooling (Stuwe, 1998).
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The economic value of these deposits varies with size and concentration of gold. One of the richest gold deposits is the Sierra Foothills belt in California, with an estimated 100 Moz, which is second to only the Carlin-trend (est. 105 Moz) along the western Cordilleran margin (Sillitoe, 2008). The Sierra Foothills belt is one of many Mesozoic-Cenozoic orogenic-type gold deposits along the paleo-plate boundary, but it is by far the largest (Figure 4). The Sierra Nevada foothills gold belt of California is one of the premier examples for orogenic deposits (Figure 5). Over 40 Moz have been extracted from the Sierra Foothills belt, with over 80 percent of that coming from the Mother Lode, Alleghany, and Grass Valley districts. These districts lie along the Melones Fault Zone, which is a major, crustal scale fault. During the Early Cretaceous, this reverse fault system was reactivated in a transpressive regime, resulting in gold mineralization around 125 ± 10 ma (Goldfarb et al., 2008). In order to distinguish the Sierra Foothills gold belt from other types of gold deposits, it is important to examine the structural setting, tectonic controls, and fluid chemistry of the system. To confirm the ore deposit type, the timing of transpressive deformation and ore mineralization must be coeval. Crustal scale faults, must act as a pathway for fluid flow, and fluid chemistry must indicate a crustal source.
 Figure 7a.png)
Orogenic gold deposits are characterized by low salinity, near neutral, high CO2 fluids; this differs greatly from intrusion related deposits (Ridley and Diamond, 2000). These properties can be determined from chemical analyses of fluid inclusions in quartz in gold bearing veins. These characteristics alone are not satisfactory, however, to discriminate between the types of ore genesis. Stable isotopic analysis of light, abundant elements in ore deposits is the primary method for determining the fluid source. Hydrogen, carbon, nitrogen, oxygen, and sulfur are the most common isotope systems used for fluid source identification. Each system has specific properties and characteristics for different geologic environments. The Methods page covers the isotopic systems relevant for determining fluid source, with a focus on the nitrogen isotopic system.