An aseismic alternative to Sibson’s fault-valve orogenic gold model

At the recent Structural Geology and Resources conference held in Kalgoorlie, Western Australia (18–20 October 2022), Prof Bruce Hobbs gave a paradigm-shifting keynote presentation where he produced an alternative to the 35-year-old Rick Sibson’s fault-valve model. But first, let me briefly explain the background on the fault-valve model.

In the introduction to a recent paper on the timing of orogenic gold deposits, Herzog et al (2023) summarised orogenic gold deposits as:

‘Gold mineralization in an orogenic setting preferentially occurs along high-angle reverse shear zones that channel fluids by repeated fault-valve processes during cyclic supralithostatic fluid pressure fluctuations’

Essentially, these authors describe what’s illustrated in the cartoon model above—that is, the shear-vein relationships that we expect to find in orogenic gold deposits (Dubé and Gosselin 2007, fig 8). This model is characterised by a steeply dipping shear zone (with shear veins parallel to the shear zone) and flat-lying extensional veins that occur at a high angle to the shear. The model is based on Sibson et al’s influential publication (1988) and has been widely accepted by the geological community since then. It has been so widely cited that the fault-valve fluid pressure cycles presented in their paper has become ‘the truth’—the standard explanation of how orogenic gold deposits form.

However, this fault-valve model has some serious technical problems that gold researchers have generally ignored. One is that the ‘shear zone’ illustrated in this model is high-angle to the horizontal shortening direction. This is a very strange orientation—you would expect the shears to be dipping at much lower dip angles (therefore thrusts) rather than reverse faults, but the Sigma gold deposit, on which this model is based, contains many steeply dipping veins as well as horizontal extension veins. When the shears are dipping at high-angle dips, layer-orthogonal compression would occur in the shearing plane, thus reverse shears at steeply dipping angles would be unlikely to even form. The authors recognised this and referred to these steeply dipping reverse shears to be ‘misoriented’.

So, given this very unlikely geometric configuration, how did Sibson et al 1988 explain the shearing on steeply dipping reverse shears? They got around the problem of shear initiation by proposing that the ‘shear zone’ was already present.

That is the shear zone was a reactivated pre-existing shear zone and they argued that these ‘misoriented’ shear zones could be reactivated through a series of fluid pressure fluctuations.

They proposed that the veins parallel to the shear zone formed during high fluid pressure peaks and extensional veins formed at low fluid pressures of the fluid pressure fluctuation cycle caused by repeated seismogenic failure.

This model was widely accepted by researchers and the mining industry, but everyone seems to have forgotten or failed to notice that the presence of the steeply dipping reverse shear was a requirement of the model and not based on actual observation in the field. More importantly, the reverse shearing could not be initiated by the fault-valve process described by the authors—this glaring problem didn’t seem to worry gold researchers for the past 35 years.

The most serious problem with the fault-valve model is that no credible evidence exists for one or more pre-existing shear zones at Sigma; this can be inferred from the original figure published by Sibson et al (1988, fig 1) shown below:

The authors proposed a pre-existing steep shear zone (shown on the right of the figure) that isn’t represented in the schematic of the Sigma deposit (left). If anything, the zone of veining has an enveloping surface that dips steeply to the north (left), not south. My own investigation into the drilling data and digitised veins by mine geologists from the Sigma deposit confirms the above sketch of the Sigma mine (Cowan 2020), with additional evidence from veins from the adjacent Lamaque deposit, which is south of the Sigma deposit. The following series of vertical sections shows the traces of veins mapped at the Sigma-Lamaque deposit along with bedding trace (S0) and S2 foliation trace reconstructed from drilling data. From the S0-S2 relationship we can see immediately see that the two deposits are located on the southern limb of an F2 antiform—a critical structural context that was never discussed by Sibson et al (1988).

There is no evidence at all for any pre-existing shears at the Sigma mine parallel to the veins coloured red (Cowan 2020). If anything the strains and the veins at Sigma-Lamaque can be simply be explained by strains from folding of the host rock package with no major pre-existing shear to be found. Many of the steeply dipping shears at Sigma can be explained by local slip sub-parallel to the lithological boundaries, so there is no real need for any pre-existing shears that were reactivated to explain their distribution.

It’s worth noting that the sub-vertical Cadillac tectonic zone, which is spatially associated with the Sigma-Lamaque deposit, is 5 kilometres south of the deposit; therefore, that tectonic zone cannot be used to explain the geometry of the mineralisation at Sigma-Lamaque.

The fault-valve model also does not explain why shear veins that are unfavorable in orientation initially formed, in preference to more favorable orientations, given that high fluid pressure would influence the formation of veins of any orientation and kinematics (S.F. Cox, pers. comm., 2019).

What this means, unfortunately, is that the widely accepted fault-value model, as illustrated and theorised by Sibson et al (1988) shown below, may not be as factual as most geologists believe it to be.

However, explaining how the steeply dipping veins under horizontal shortening can actually form in the first place remained an unresolved 35 year old problem, and this is where Bruce Hobbs’ talk takes over. Bruce Hobbs and Alison Ord (2022, 2023) have come up with a radical, but logical alternative to Rick Sibson’s fault-valve model that does not require:

1) reactivation of an earlier formed steep shear, or

2) seismogenic occurrence, or

3) a ‘seal’ that needs to be disrupted by an increase in fluid pressure.

In fact, the steeply dipping veins in this new model form not by an increase in fluid pressure, but a decrease—the exact opposite of the fault-value model of Sibson et al (1988). I’ll let Bruce Hobbs explain this in his keynote presentation given at the Structural Geology and Resources conference held in Kalgoorlie (18 October 2022).

References

Cowan, E.J., 2020, Deposit-scale structural architecture of the Sigma-Lamaque gold deposit, Canada—insights from a newly proposed 3D method for assessing structural controls from drill hole data. Mineralium Deposita, 55: 217–240.

Dubé, B., and Gosselin, P., 2007, Greenstone-hosted quartz-carbonate vein deposits, in Goodfellow, W.D., ed., Mineral Deposits of Canada: A Synthesis of Major Deposit-Types, District Metallogeny, the Evolution of Geological Provinces, and Exploration Methods: Geological Association of Canada, Mineral Deposits Division, Special Publication No. 5, 49–73. (article available in the folder here)

Herzog, M., LaFlamme, C., Beaudoin, G., Marsh, J. and Guilmette, C., 2023, U–Pb vein xenotime geochronology constraints on timing and longevity of orogenic gold mineralization in the Malartic‑Val‑d’Or Camp, Abitibi Subprovince, Canada. Mineralium Deposita, 58: 105–133.

Hobbs, B. E. and Ord, A., 2022, Failure modes in fluid saturated rocks: deformation processes and mode-switching. Geological Magazine.

Hobbs, B. E. and Ord, A., 2023, An alternative to the fault-valve model. Australian Journal of Earth Sciences.

Sibson, R.H., Robert, F., and Poulsen, K.H., 1988, High-angle reverse faults, fluid-pressure cycling, and mesothermal gold-quartz deposits: Geology, 16: 551–555. (article available in the folder here)