Earthquake-Induced Geochemical Anomalies

A recent review article by Cameron illustrates the importance of earthquakes in the generation of geochemical anomalies above buried mineral deposits.  The paper builds on previous work by Cameron and colleagues (e.g., Cameron et al., 2002; Cameron and Leybourne, 2005; Leybourne and Cameron, 2006; Leybourne and Cameron, 2010) and investigates how earthquakes generate faults, reactivate existing faults, change groundwater movement, and how these interrelated to form near surface geochemical anomalies above buried mineralization.

The paper reviews the hydrology and fluid movement as a function of proximity to earthquakes.  Cameron illustrates that primary earthquakes can trigger secondary earthquakes, reactivation of existing faults, and associated hydrologic changes both proximal (i.e., near field) and also at great distances (up to 1000s of km) from the earthquake foci (i.e., far field).  For example, the M9.2 Alaskan earthquake in March 1964 resulted in changes in water levels up to 12-23 feet in water wells in the Midwest United States.

The latter review sets the stage for three case studies from Chile (Spence deposit), Nevada (Mike, Gap and Pipeline deposits), and Saskatchewan (Athabasca Basin).  The Chilean example illustrates how subduction-related earthquakes resulted in the mobilization and expelling of saline groundwaters along faults above porphyry Cu mineralization. Prior to movement along faults the groundwaters interacted with porphyry mineralization and extracted metals from the deposits resulting in Cu, Mo, Re, As, and Se enrichments in groundwater (Figure 1).  The saline groundwaters travelled along faults several hundreds of meters above mineralization and deposited salts and metals (e.g., gypsum, Cu-oxides) along the fault surfaces and within the gravel near the fault surfaces (Figure 1). This was also accompanied by the discharge of metal-rich groundwater (Figure 1).

Figure 1.  Model for the development of anomalies above buried mineralization due to the movement of groundwater during seismic events.  Saline groundwaters interact with mineralization extracting metals from buried mineralization and are released upwards during seismic events.  The interaction of these fluids with meteoric (i.e., surface waters) results in fluids enriched in metals and the deposition of mineral salts and metals (Cu, Se, Re, Mo, As) in surficial gravels 100s of meters above mineralization.  From Cameron and Leybourne (2005) and Leybourne and Cameron (2010).

The second case study comes from the Mike and Gap-Pipeline Carlin-type deposits in Nevada. These deposits are associated with seismic activity associated with Basin and Range Province crustal extension; many faults have also been reactivated by far field effects from distal earthquakes (secondary earthquakes).  Mineralization in the Mike deposit is near faults and reactivation of these faults during seismic activity has resulted fault-proximal surficial materials being enriched in elements associated with mineralization (Cu-Au) and the supergene blankets that overlie the deposit (Zn-Cd).  Similarly, the Gap and Pipeline deposits have very strong enrichments in Zn, As, and to a lesser extent Au, in the surficial materials immediately above faults and mineralization (Figure 2). In both cases reactivation of faults due to seismic activity resulted in the upward migration of mineralization-related elements in groundwaters and subsequent deposition in the near surface environment (Figure 2).

Figure 2.  Enrichments in Zn, As, and Au in soils immediately above the Gap deposit, Nevada.  The anomalies are spatially associated with both mineralization and faults that intersect the surface and sub-surface mineralization.  Diagram from Muntean & Taufen (2011) and Cameron (2013).

The final case study involves unconformity-type uranium mineralization in the Athabasca Basin, Saskatchewan.  Unconformity-type deposits are spatially associated with faults in basement rocks and often occur either in faults within basement rocks or at the contact of these faults with overlying Proterozoic Athabasca sandstones (Figure 3).  These faults not only controlled the formation of mineralization, but were also reactivated numerous times following ore formation, likely by ancient (to modern?) sesimic activity.  In both the sandstones and surficial materials along these faults are anomalous enrichments in ²⁰⁶Pb and ²⁰⁷Pb derived from the breakdown of ²³⁸U and ²³⁵U in the ores, respectively (Figure 3).  Similarly, there are enrichments along both the faults and surficial materials above mineralization of elements associated with the ores, including U, Ni, V, Co, and As.

Figure 3.  Schematic diagram showing the dispersion of material, including radiogenic lead, from Athabasca uranium ores. Diagram from Cameron (2013) with geology based on Jefferson et al. (2007).

The above case studies illustrate how long-lived faults and repeated seismic activity can result in the transfer of metals in groundwater from the subsurface to the near-surface environment.  It represents a potential method for the targeting buried mineralization, a major challenge in modern mineral exploration.

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