New research published in Economic Geology by Tomkins provides insight into the importance of atmospheric evolution and bacteria in controlling the distribution of orogenic gold deposits (and resources) during the planet’s history. Orogenic (mesothermal) gold deposits form from gold-bearing fluids that are generated as a result of devolatilization of the crust during earthquakes, faulting, and metamorphism related to convergence of tectonic plates (see Figure 1; Goldfarb et al. (2002); and Instant Gold). The amount of gold liberated during tectonic convergence is controlled by numerous processes; however, one of the most important factors is the amount of gold present in the source rocks that the fluids are generated from (e.g., Large et al., 2011).
Figure 1. Orogenic gold deposits are associated with earthquakes, faulting, and metamorphism as tectonic plates accrete, often in a subduction zone tectonic environment. The combination of these processes results in the remobilization of gold-bearing fluids from the crust along major fault zones within the crust. Figure modified from Goldfarb et al. (2001).
Orogenic gold deposits have a very distinctive distribution through geological time (Figure 2). While there are abundant orogenic gold deposits in the Archean (>2500 Ma) and Early Proterozoic (Paleoproterozoic; 2500-1800 Ma), a significant proportion of orogenic gold deposits are in rocks that are Phanerozoic (younger than ~545 Ma); particularly so for orogenic gold deposits associated with sedimentary rocks (Figure 2). Tomkins argues that the association of sedimentary rock-associated orogenic gold deposits in the Phanerozoic is directly related to the pre-concentration of gold in sedimentary source rocks. Furthermore, he illustrates that the pre-concentration of gold in sedimentary sources rocks was due to oxygenation of the atmosphere and the oceans, and bacterial processes (Figure 2).
The Earth’s atmosphere was not originally oxygenated, but only became so in the early Paleoproterozoic (~2400 Ma) during the first great oxygenation event (GOE1), and became broadly similar to the modern atmosphere during the second great oxygenation event (GOE2) ~635-510 Ma (Figures 2-3; Holland, 2002; Canfield, 2005; Kump, 2008). This led to fundamental changes in the chemistry of oceans and they went from being originally Fe-rich in the Archean and Early Paleoproterozoic, to stratified mixed H2S-SO4 oceans in the Paleoproterozoic to Neoproterozoic, and oxygenated and SO4-rich with periodic stratified periods in the Phanerozoic (Figure 3; Farquhar et al., 2010).
Figure 2. Distribution of the tonnes of gold through geological time in relationship to the the variation in oxygen concentration in the atmosphere (light blue line) and growth of continental crust (dark blue line). Also show are the major oxygenation events during Earth history, including the first great oxygenation event (GOE1; ~2400 Ma) and the second great oxygenation event (GOE2; ~635-510 Ma). Diagram from Tomkins (2013).
Figure 3. Inferred evolution of atmosphere, surface ocean, and deep ocean and oceanic sulfate (SO4) concentration. Note the shifts in composition of the oceanic reservoirs with time. Diagram does not imply depth or geographic distribution. From Farquhar et al. (2010).
The shift towards oxygenated oceans in the Phanerozoic had a number of consequences. Firstly, the increase in oxygen resulted in an ocean that was oxygenated and not reducing, therefore allowing gold to be transported to deeper depths in the oceans (due to the solubility of gold in oxidized versus reduced solutions; Tomkins, 2013). Secondly, oxygenation allowed the developed of an ocean with abundant SO4, which allowed sulfate reducing bacteria to produce abundant H2S, which when complexed with Fe present in rocks resulted in abundant sedimentary pyrite formation (i.e., diagenetic pyrite). The coupling of gold transport to the deeper oceans and process of sedimentary pyrite formation resulted in minor amounts (e.g., ppb to ppm) of gold being “scrubbed” from the oceans and trapped within sedimentary pyrite (e.g., Figure 4; Large et al., 2011). Prior to GOE2, however, this process could not happen as gold could not be transported to great depths because of the low solubility of gold in reduced ocean waters. Oxygenation of the oceans also allowed the oxidation and hydration of oceanic crust, which when subducted at subduction zones resulted in the oxidation of post-GOE2 arcs and arc magmas. This in turn resulted in arc magmas that had greater abilities to carry gold (e.g., Mungall, 2002; Evans and Tomkins, 2011), which upon eruption and subsequent erosion resulted in sedimentary rocks with higher gold contents.
Figure 4. Diagenetic pyrite from black mudstone Roberts Mountain Formation, Carlin district, Nevada. Note the elevated gold content of the bacterial derived (diagenetic) pyrite and associated organic material in the sedimentary rocks. From Large et al. (2011).
This returns to the role of these processes in Phanerozoic sedimentary-rock associated orogenic gold formation. The trapping of gold in sedimentary pyrite and the increase in gold-rich detritus from magmatic arcs on the post-GOE2 Earth resulted in sedimentary source rocks with higher gold concentrations than sedimentary rocks on the pre-GOE2 Earth. When these sedimentary rocks were metamorphosed and dehydrated during accretionary orogenic processes the sedimentary rocks could yield gold-rich fluids (Figure 5) that could have formed the large deposits that common in the post-GOE2 Earth (Figure 2).
Figure 5. Generalized model for the generation of gold deposits in sedimentary-rich belts. The combination of metamorphic fluids (and potentially meteoric fluids) associated with faulting and tectonic accretion result in the dehydration and release of gold and other elements from sedimentary rocks. These fluids travel along faults in the Earth’s crust and deposit gold at suitable sites (traps) to form sedimentary rock-associated orogenic gold deposits. From Large et al. (2011).
A very interesting paper illustrating the interactions of the Earth’s atmosphere, oceans, biosphere, and tectonosphere, and how they ultimately played a role in current global gold resources!
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