The Significance of the Auriferous Gravels

GG’s been noodling some on these deposits and has decided to share some thoughts in the hopes that either somebody else will see what is important, or GG will, having taken the time to write stuff down.

Quick background: Eocene river deposits in the Sierra Nevada are termed the Auriferous Gravels; these paleoplacers were a major source of gold and likely were also a major source for the modern placer deposits.  The channels themselves are of interest because they might (or might not) indicate that the northern Sierra has been tilted since then. Curiously, the aspect of these rocks that led to their name is the one aspect that hasn’t been used in trying to understand their origin and significance.

Observationally, most of the gold in these deposits is near the base.  Yeend’s 1974 USGS Professional Paper shows that the high concentrations of gold were limited to the bottom 80-100′ of these gravels. These concentrations were on the order of one part per million, while the average concentration in surrounding bedrock might be only one part per billion or even lower. The question is, why was there this 1000-fold concentration of gold?

A few ideas come to mind:

  1. Properly sized gold particles were only present early in the depositional history of these deposits
  2. Deeper gravels reflect a far longer period of winnowing of weathered material, concentrating gold better.
  3. Deeper gravels were coarser, creating more scoured spots downstream of fixed clasts that were primary spots for gold to settle compared with other materials.

(If you think of others, add a comment or let GG know).

Let’s start with (1). A study by Craw (Ore Geology Reviews, 2010) argued that large placer deposits needed “oxidative alteration of primary deposits to liberate gold from sulfide minerals and enhance secondary gold grain size.” A followup study described the geochemistry of this process (Craw et al., New Zealand J. Geol. Geophys., 2015); basically you need groundwater reacting with the sulphides, which creates an environment where the gold can be transported short distances, filling voids and creating larger flakes that are capable of being trapped in fluvial sediments once these deposits are eroded.

On one level, this is quite attractive: the deep weathering in the Eocene would seem an ideal environment for concentrating gold in this manner.  The only problem is that this environment lasted the whole time the Auriferous Gravels were being deposited. The upper gravels, which are far poorer in gold, are rich in quartz pebbles usually associated with the lode gold deposits and display the kind of deep weathering Craw and associates suggest is good for developing placer gold. Yeend speculated on the relative absence of gold in these upper gravels, suggesting that gold was dissolved, was broken into such small pieces that it was transported out into the ocean to the west, or it was trapped somehow in soils that were never eroded into the rivers.

So (1) isn’t terribly attractive; what of (2)?  This was more or less Yeend’s preferred means of accumulating so much gold.  In a simple way, if you have a fluvial system that transports nearly everything out to sea but still has nooks and crannies which don’t get fully flushed out, you could keep building up gold while processing large volumes of weathered material. The concentration of gold would in some sense reflect the amount of time spent accumulating a given layer of sediment.  So sediment accumulation rates would be low near the bottom and then gradually increase as you move up section.

But (2) kind of ignores all the work done trying to figure out how gold particles get trapped in sediment.  Slingerland and Smith (Ann Rev Earth Plan Sci, 1986) reviewed the physics of this, which mainly highlighted how little we know about important parts of this process.  Basically, you can either make it easier for dense grains to fall out or, more likely, make it harder for them to get entrained in the first place. A somewhat less technical review by Carling and Breakspear (Ore Geology Rev, 2006) tended to come to the same conclusion–lots of possibilities but few iron-clad rules.  One thing they noted, though, was the prevalence of large fluvial placer deposits to be in cobble deposits; gold tends to settle in the matrix around the cobbles, places where turbulent flow winnows out the lighter grains. So perhaps the flux of placer gold was about constant but the ability of the sedimentary system over time varied in its ability to catch the gold.  The most bouldery parts of the gravels are at the bottom.  Working against this, though, are higher gravels with very little placer gold.

Well so what? There is a huge difference in interpreting these deposits between options (2) and (3).For option (2) to work, presumably the low gravels represent a substantial period of time.  If this is the case, then the deposits of such high-grade placers have to be broadly coeval, which would tend to invalidate the argument of Cassel and Graham that these deposits are diachronous and thus individual river channels cannot be followed; this would tend to support older arguments about the tilt of the mainstem channels. However, if you can catch enough gold in a short time to match the placer deposits of the older gravels, then perhaps a diachronous origin makes sense (though this would presumably make a prediction of the age of sediments in different areas).

A diachronous origin seems difficult, though.  An attempt to calculate a mass balance of the gold in the placers by Loen (Econ. Geol., 1992) led to an estimate of 400-1400m of rock being removed all through the drainage in order to yield enough gold for the placer deposits. Given the fairly old U-Th/He ages reported in the foothills by Cecil et al. (GSA Bull, 2006), a lot of erosion in a short time seems a challenge. Maybe though this can be made to work (basically gold gets trapped at the upstream end of the sedimentary prism, so the relatively small amount of lowering of the surroundings is all pushed through a small area along the channel bottom).

There is another puzzle here.  Why are these gravels still present? If they really represent 400-1400m of average erosion over the watershed, why didn’t isostatic rebound rejuvenate the river and scour out the placer deposits? This seems to imply that either the rivers were systematically becoming less competent with time in just the right ratio to allow continued deposition, that the sediment load coming down the rivers was increasing, leading to steepening of the rivers, or that there was no isostatic rebound. Its not clear that there would be much of a change in the river discharge with time in the Eocene, though it is hard to rule out.  The paleo Yuba is pretty consistently described as being a braided stream over this time; if the water flow decreased, you might think that would have changed.  An increase in sediment in the rivers seems very unlikely; again, in the Eocene we aren’t looking at big climatic changes, and we aren’t seeing an increase in coarse sediment (that changes later, in the Miocene). So maybe the isostatic rebound was short-circuited? As odd as that sounds, it makes sense: the Sierra was cooling dramatically in the early Tertiary; at the top, this is seen in the old dates of K-Ar (Ar-Ar) in biotite and fairly old U-Th/He ages in even the youngest parts of the batholith, and at the bottom it seems evident in the very cold temperatures recorded in c. 10 Ma xenoliths in the southern Sierra. So perhaps thermal subsidence of the Sierra balanced out the exhumation of the region. Of course this doesn’t tell you if the Sierra was stable at its current elevation or much lower.

So maybe unravelling the reason for the high gold concentrations in the oldest Auriferous Gravels can tell us about the elevation history of the Sierra…

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