Orogenic Gold and Magnetotellurics

I've been thinking a lot about orogenic gold systems recently, particularly in the context of geophysical imaging. I've been a bit surprised by how little attention magnetotelluric (MT) imaging specifically gets in the economic geology literature pertaining to orogenic gold (at least until very recently), since the technique is in fact highly sensitive to certain components of the metamorphic model for orogenic gold deposit formation. So, I figured I would lay out a few thoughts about what MT has to say, and potentially can say, about orogenic gold systems.

Executive Summary

To briefly summarize right upfront, here are a few key take-home points:

• MT imaging directly maps key source rocks for orogenic gold systems.
• MT imaging provides a fingerprint of orogenic processes, which are the geodynamic driver of orogenic gold systems.
• MT imaging may be able to track where aqueous-carbonic gold-bearing fluid moved through crustal shear zones via precipitated graphite, thereby pointing the way to orogenic gold deposits.

The Metamorphic Model for Orogenic Gold Formation

So to start out, a disclaimer: I'm strictly thinking about orogenic gold from the perspective of the metamorphic model here (see, for example, any paper by Richard Goldfarb on these types of deposits). I know there's still some controversy over the genesis of deposits identified under the "orogenic gold" umbrella, but I personally am convinced by the metamorphic model (and convinced against models focused on magmatic-hydrothermal fluids; see Goldfarb & Pitcairn, 2023).

Just to briefly review, the core idea behind the metamorphic model is that gold and transporting fluids are released during devolatilization and metamorphism of supracrustal rocks during an orogenic event (accretion of a microcontinent/island arc, continent-continent collision, etc.). The supracrustal rocks should contain pyrite in some capacity, as pyrite will be a primary repository for gold and as the desulfidation reaction of pyrite -> pyrrhotite + S under greenschist- to amphibolite-facies conditions will release and mobilize that gold. Metamorphism from greenschist to amphibolite facies will also lead to devolatilization of the primary sedimentary material in some capacity, thereby providing an aqueous-carbonic fluid for transportation of Au and associated elements (As, Sb, W, Te, Bi, released from other detrital/authigenic minerals). Although pyrite-bearing volcanic rocks can be gold sources in this model (and probably are key source rocks in Archean greenstone belts), sulfidic pelitic rocks and black shales in particular are identified as prime gold source rocks since they contain so much authigenic pyrite (see discussion in Goldfarb & Pitcairn, 2023).

The metamorphic model posits that these metamorphic H₂O-CO₂ Au-bearing fluids then migrate upwards through the crust along major crustal-scale shear zones. These fluids can deposit their gold anywhere along their flow path due to any number of physical and chemical factors. The resulting deposits can contain native gold and/or "invisible" gold within arsenic-bearing sulfides in quartz-carbonate veins, or gold in disseminated arsenic-bearing sulfides in replacement-style or skarn-like deposits at deeper crustal levels.

Orogenic Gold Districts and High-Conductivity Belts

Recently, some of my Australian colleagues have noted a statistically significant spatial correlation between orogenic gold deposits and high-conductivity belts imaged with MT (Heinson et al., 2021; Kirkby et al., 2022). There is in fact a very good, concrete (non-hand-wavy) physical explanation for this association, and it has to do with what is causing these high-conductivity belts. First of all, let's look at the spatial relationships between orogenic gold districts and electrical conductivity features in the conterminous U.S.

Excluding the districts in Idaho, Oregon, and California (where crustal conductivity signatures are greatly complicated by active tectonomagmatic processes), you'll notice that orogenic gold systems often occur along high-conductivity belts. For example, Homestake falls along the lineament I've labeled C1. The Dahlonega orogenic gold belt in Georgia falls along the southwestern end of the paired lineaments I've labeled C5.

The high conductivity values within these belts are due to sulfidic and carbonaceous metasedimentary rocks (likely now graphite- and pyrrhotite-bearing), which are highly electrically conductive, trapped within suture zones (see Murphy et al., 2023). The conceptual model for their formation is shown below; this is based off work that was originally done back in the 1990s (Boerner et al., 1996; Jones et al., 1997), which actually demonstrated the link between the sulfidic/carbonaceous lithologies and these high-conductivity belts in places where they extend to the surface (in the Penokean province, and in the Canadian portion of the Trans-Hudson).

Given this explanation, C1 represents conductive metasedimentary rocks trapped within the series of Tran-Hudson sutures, along which the Wyoming and Superior cratons were joined; C2 delineates the Cheyenne belt, the suture between the Wyoming craton and younger Paleoproterozoic island arcs; C3 represents the composite Great Falls tectonic zone, the multi-phase orogenic zone between the Medicine Hat block and the Wyoming craton; C4 marks the Penokean suture; and C5 has been interpreted as a composite Grenville-aged suture zone. (See Murphy et al., 2023 for more information.)

Sulfidic and carbonaceous metasedimentary rocks within suture zones... these are in fact the prime lithologies for sourcing Au in orogenic gold systems! And these conductors are specifically mapping out terrane sutures, along which orogenic metamorphism happened! So, this is a very important observation that I've noticed missing in most discussions of geophysical imaging for orogenic gold exploration: MT imaging in fact can directly map the source rocks for orogenic gold deposits and fingerprint the geodynamic settings responsible for their formation.

The Utility of MT Imaging in Orogenic Gold Exploration

In the mineral systems framework for ore deposit exploration, there are four key factors that one aims to understand: fluid and metal source, tectonometallogeny/geodynamics, lithospheric architecture/fluid pathways, and preservation (for example, see Groves et al., 2020). Clearly, MT imaging provides direct insights into two of these four components, as it directly maps out the source rocks for orogenic gold deposits and provides a signature of orogenic geodynamic processes. Consequently, the high-conductivity belts in the MT images above directly show the prime prospective regions in the conterminous U.S. for these types of deposits.

Let's consider the Wyoming craton more closely. The South Pass district is in an Archean greenstone belt, so it's not clear how well it fits into the conductivity-tectonic framework I've laid out above (although note that it does still fall along a weak conductive anomaly that may mark a portion of the suture between the Beartooth-Bighorn magmatic zone and the Southern Accreted Terranes, two components of the greater Wyoming craton; Frost et al., 2023). But note that Homestake (and the Black Hills in general) lies right along the high-conductivity belt that's part of the series of Trans-Hudson sutures. The high-conductivity values mark both the source lithologies for Homestake gold and provide a signature of the orogenic processes that created the Homestake deposit.

Note also the Jardine district (Gammons et al., 2021), which falls along one of the high-conductivity belts of the Great Falls tectonic zone (GFTZ). You might not have heard of the Jardine district before; it contains metamorphic-rock-hosted Au-W deposits that show many characteristics (fluid inclusion chemistry, mineralization style) of classic orogenic gold deposits. It hasn't been well studied, hence why it hasn't been included in discussions of orogenic gold deposits of the world (e.g., Goldfarb et al., 2001). But its characteristics and position along a suture-bound high conductivity belt demonstrate the prospectivity of that region for further exploration.

Of course, the conductivity images I'm showing here are very large-scale and blurry; in the form above, the MT information highlights ~100km-wide swaths of prospective territory along these belts. These images, though, are made from long-period publicly available MT data with site spacings of ~70km. Consequently, those data can only image high-conductivity belts at that sort of spatial length scale, with conductivity information blurred over ~50-100 km laterally. In reality, the high-conductivity metasedimentary units are confined to much narrower zones, and high-spatial-density broadband MT surveys would be able to provide a higher resolution (i.e., more precise) image of prospective corridors.

You may be wondering if there really is exploration value here. After all, prospectors and geoscientists have been crawling over every square kilometer of the conterminous U.S. for over 200 years, so one might then think that all the key orogenic gold systems in this region have already been found and that the MT images consequently provide no new information. Indeed, my observation has been that much orogenic gold exploration is in brownfields settings in North America. However, I would point out that only recently have we arrived at a point of actually understanding how orogenic gold systems work, and our structural-kinematic-tectonic understanding of many/all of the orogenic belts that I highlighted above are still evolving. For example, it took 150 years for a tectonic model of the Black Hills (Allard & Portis, 2013) to be developed that could explain the Homestake deposit and that could enable the F3 Gold folks to make new discoveries in that region. This is to say that only recently do we have models that can guide us in how to look at metamorphic rocks for orogenic gold. Consequently, the MT images provide fresh information that, in combination with our latest understandings of orogenic gold systems and of the structure/kinematics of these orogenic belts, can drive new discoveries.

Further Possibilities with MT Imaging

As I discussed above, MT imaging provides direct insights into two of the four pillars of the mineral-systems approach to thinking about ore deposits. I would contend that MT imaging can also provide direct insights into a third pillar, fluid pathways.

Orogenic gold processes mobilize aqueous-carbonic fluids under relatively reduced conditions. We can model such fluids under the simple system C-O-H(+/-S), and we end up finding that under certain P-T paths such fluids are expected to precipitate out large amounts of graphite as they cool (see discussion and references in Murphy et al., 2022). The figure below shows an example of this sort of calculation for the system C-O-H; the colors correspond with the amount of C remaining in the fluid, and decreasing C content in the fluid means that C is precipitating out as graphite.

Graphite is of course highly electrical conductive, so MT imaging is sensitive to even small amount of precipitated graphite. Consequently, if orogenic gold fluids precipitate graphite as they migrate upwards, as we might expect they do based on the aforementioned geochemical calculations, then we could use MT imaging to directly track fluid pathways. These conductive pathways probably don't lead directly to deposits (since graphite precipitation is generally expected to cease before the most common temperature/pressure field of orogenic gold precipitation), but they would point the way to potential deposits.

Although published high-resolution MT images may support these ideas (see, for example, the MT section shown in Groves et al., 2020), my ideas here are still somewhat theoretical. My conceptual model is based on geochemical processes that we certainly expect to happen in these types of systems, but publicly available datasets and currently published MT studies are insufficient to fully test these ideas. Anyone have a dense MT dataset over an orogenic gold system that they'd like to work on?

Why MT?

As just a final note, I should say that the reason I'm focused on MT imaging here is that it's the best technique to sense electrical conductivity deeper than a few kilometers over large areas (and sulfidic-carbonaceous metasedimentary units are of course highly electrically conductive). If you'd like to know more about the MT method, you can read my brief explanation here: http://rockingwiththerocks.com/mt.html

Thanks to J. Caleb Chappell and Anne Fulton for getting my mental wheels turning on orogenic gold systems.

References

Allard, S.T., Portis, D.H. (2013) Paleoproterozoic transpressional shear zone, eastern Black Hills, South Dakota: Implications for the late tectonic history of the southern Trans-Hudson Orogen. Rocky Mountain Geology. https://doi.org/10.2113/gsrocky.48.2.73

Boerner, D.E., Kurtz, R.D., Craven, J. A. (1996) Electrical conductivity and Paleo-Proterozoic foredeeps. Journal of Geophysical Research. https://doi.org/10.1029/96JB00171

Frost, C.D., Mueller, P.A., Mogk, D.W., Frost, B.R., Henry, D.J. (2023) Creating Continents: Archean Cratons Tell the Story. GSA Today. https://doi.org/10.1130/GSATG541A.1

Gammons, C.H., Korzeb, S.L., Hargrave, P.A. (2021) Metallic Ore Deposits of Montana. In Geology of Montana. Montana Bureau of Mines and Geology Special Publication 122. https://mbmg.mtech.edu/pdf/geologyvolume/Gammons_OreDepositsFinal.pdf

Goldfarb, R.J., Groves, D.I., Gardoll, S. (2001) Orogenic gold and geologic time: a global synthesis. Ore Geology Reviews. https://doi.org/10.1016/S0169-1368(01)00016-6

Goldfarb, R.J., and Pitcairn, I. (2023) Orogenic gold: is a genetic association with magmatism realistic? Mineralium Deposita. https://doi.org/10.1007/s00126-022-01146-8

Groves, D.I., Santosh, M., Zhang, L. (2020) A scale-integrated exploration model for orogenic gold deposits based on a mineral system approach. Geoscience Frontiers. https://doi.org/10.1016/j.gsf.2019.12.007

Heinson, G., Duan, J., Kirkby, A. et al. (2021) Lower crustal resistivity signature of an orogenic gold system. Scientific Reports. https://doi.org/10.1038/s41598-021-94531-8

Jones, A.G., Katsube, T.J., Schwann, P. (1997) The Longest Conductivity Anomaly in the World Explained: Sulphides in Fold Hinges Causing Very High Electrical Anisotropy. Journal of Geomagnetism and Geoelectricity. https://doi.org/10.5636/jgg.49.1619

Kirkby, A., Czarnota, K., Huston, D.L. et al. (2022) Lithospheric conductors reveal source regions of convergent margin mineral systems. Scientific Reports. https://doi.org/10.1038/s41598-022-11921-2

Murphy, B.S., Bedrosian, P.A., Kelbert, A. (2023) Geoelectric constraints on the Precambrian assembly and architecture of southern Laurentia. In Laurentia: Turning Points in the Evolution of a Continent. Geological Society of America Memoir 220. https://doi.org/10.1130/2022.1220(13)

Murphy, B.S., Huizenga, J.M., Bedrosian, P.A. (2022) Graphite as an electrically conductive indicator of ancient crustal-scale fluid flow within mineral systems. Earth and Planetary Science Letters. https://doi.org/10.1016/j.epsl.2022.117700