Thursday, January 29, 2009

Wedge #15: Ore Deposits of the Future

Exploration geologists occasionally speculate on where to find ore deposits in a future world, from 1 to 25 or more million years from now. I'm not sure why we do this; I think it's partly a natural outcome of some of the research we do, our generally or overall speculative natures, and the fact that we sometimes watch ore-deposit-forming processes in progress. It's a natural extrapolation to take processes that are occurring today, and think about what they will generate in the future.

There are many kinds of ore deposits that one could extrapolate into future times, in case anyone reading happens to plan on sticking around for a while. For example, certain erosional and tectonic processes could combine to create gold placers of the future, and here's how: take some gold veins not yet exposed at the surface, uplift them in a mountain range similar to the Sierra Nevada, and erode them into gold placers similar to the gold discovered at Sutter's Mill, California, in 1848, which led to the California Gold Rush and the influx into the west of The Forty Niners. All you need to do is identify some not-already uplifted and eroded, world-class mesothermal gold vein systems at just the right depth in just the right tectonic setting. [Mesothermal=moderate depth and temperature of formation.]

Because of all the currently active hot springs systems around the world, many of which are related to hot, molten magma at depth and cool or cooling volcanic rocks at the surface, it should be easy to put together a list of places to find gold sometime during the next 1 to 10+ million years, whenever the gold is done being deposited at depth, and whenever these future and hypothesized deposits have been uplifted or eroded to the future surface of the earth. Of course, not all our current geothermal and hot springs systems are necessarily now depositing gold - nor will they necessarily be doing so in the future - but some are and some will.

A few of the larger and better known hot springs, geothermal, and hydrothermal systems of the world come easily to mind: 1) Steamboat Springs, NV, 2) Yellowstone, WY, 3) Wairakei, New Zealand, 4) Lassen Peak, CA, 5) Iceland, and 6) Puchuldiza, Chile. Okay, well maybe Lassen isn't that huge, but it's relatively nearby if you happen to live on the west coast. Hot spring and hydrothermal waters at Steamboat Springs, NV, and Wairakei are known for active deposition of gold and related indicator elements like arsenic, mercury, and antimony.

So, what does that have to do with the future? Just this: I recommend drilling for gold in 1 to 5 million years in the Mendocino, CA, area, with 2 to 3 million years being my best estimate of the proper timing. For proper and exact placement of drill rigs, I would wait for the future, when faults and fractures controlling future ore deposits will be identifiable, and when rocks will be available for sampling. They aren't quite available for sampling just yet - they haven't formed!

The Clear Lake Volcanic Field, located north of Napa Valley, contains one of the world's largest (or the largest) producing geothermal fields, The Geysers. The volcanic field itself is very young, with volcanic rocks ranging in age from 2.2 million years old to 10,000 years old. A large felsic magma chamber sits beneath the volcanic field, and provides heat to the geothermal system. The area, I think, should be considered as potentially active as the Yellowstone area or the Long Valley caldera.

The Clear Lake Volcanic Field sits astride the famed and tectonically active San Andreas transform fault system at the northwestern end of a long chain of volcanic fields that may begin as far south as Pinnacles National Monument and the related or offset Neenach volcanic formations farther to the southeast. This series of volcanic fields become progressively younger to the northwest, except for offset portions of fields. The volcanic eruptive centers within the Clear Lake Volcanic Field have also migrated northward to northwestward, at least during the last 2.1 million years (Wood and Kienle, 1990, p.226-229).

The volcanic field in the Clear Lake area formed about 1 million years after the tectonic regime in the area switched from subduction to transform faulting, about 1 million years after what is now the Mendocino Triple Junction passed through the area. Hot spring systems around Clear Lake and The Geysers, active and inactive or "fossil" systems, have consistently deposited mercury as well as gold. In fact, the McLaughlin gold mine, located toward the eastern side of the Clear Lake Volcanic Field, is a known epithermal gold deposit formed by one of the hydrothermal systems of the area. [Epithermal=shallow depth and low temperature of formation; ignore the part of the link that says "occurring mainly as veins."]

Anyway, the short story is that the gold deposit at McLaughlin formed in a hydrothermal system that deposited gold in veins containing adularia and alunite from 0.5 to 1.0 million years ago (Enderlin, 2002). The gold deposit, therefore, formed about 2.5 to 2.0 million years after the Mendocino triple junction passed through the area, and about 2.0 to 1.5 million years after the onset of volcanic activity in the region. It was discovered and mined about 1.0 to 0.5 million years after it formed!

It stands to reason - if all variables continue to hold (and they did in some other volcanic fields along the San Andreas fault to the southeast) - that one should expect a volcanic field to begin forming in the Mendocino area in about 500,000 years, a field that might continue erupting and forming hydrothermal and geothermal systems for at least 2.5 million years after that. So I'd say - keep your eyes open! You should be ready to drill for a McLaughlin-type gold deposit in the Mendocino area in about 2 to 3 million years, give or take a couple million!

On another note, I don't know if the vineyards of Napa Valley will migrate to the Mendocino area, or not, so be sure to sample the wines now - that is, unless you want to predict the future coastal climate of the region and find gold.

A Few References:
Bailey, E.H., and Myers, W.B., 1942, Quicksilver and Antimony Deposits of the Stayton District: Geol. Survey Bull. 931-Q, 1942, p. 405-434.

Enderlin, D., 2002, Geology of the McLaughlin deposit: Homestake Mining Company published online, pages 1 - 5. Age dates cited on page 2.

Fox, Jr., K.F., Fleck, R.J., Curtis, G.H., and Meyer, C.E., 1985, Implications of the northwestwardly younger age of the volcanic rocks of west-central California: Geol. Soc. America Bulletin, vol. 96, no. 5, p. 647-654.

Leith, C.J., 1949, Geology of the Quien Sabe Quadrangle, California: Calif. Div. Mines, Bull. 147, 60 pages; including Bailey, E.H., and Myers, W.B., 1949, Quicksilver and Antimony Deposits of the Stayton District, p. 37-56.

Wagner, D.L., Fleck R.J., McLaughlin, R., Sarna-Wojcicki, A., Clahan, K.B., and Bezore, S., New constraints on the age and distribution of Cenozoic volcanics north of San Pable Bay, California: Implications for displacement along faults inboard of the San Andreas fault: [abs.]: Geol. Soc. America Abstracts with Programs, v. 37, no. 4, p. 83-84.

Wood, C.A., and Kienle, J., 1990, Volcanoes of North America: The United States and Canada: Cambridge University Press, Washington, DC, USA, 354 pages.

This post is a submission for The Accretionary Wedge, being hosted this month by BrianR at Clastic Detritus: "speculate about the future of the Earth within the context of geological processes/events." This is Accretionary Wedge #15, despite the post title.

Accretionary Wedge #15: Pondering the geological future of Earth


Suvrat Kher said...

very interesting perspective.

have you thought about what format your advice about future sites might be preserved for two million years?

On another note, I don't know if the vineyards of Napa Valley will migrate to the Mendocino area, or not, so be sure to sample the wines now -

You mean the current vintage won't age well over 2 my. I am disappointed :-)

Silver Fox said...

Suvrat - maybe this blog will last for 2 my. Today's wine won't last that long, but maybe if you pick out a really, really good Cabernet... I was thinking that with the climate changes in 2 my, there's no telling whether vineyards will move north from Napa to Mendocino, or maybe even farther north.

Andrew Alden, Oakland Geology blog said...

How wonderful that you found that report on the Quien Sabe quad! Unfortunately the map at the end is tantalizingly small.

Silver Fox said...

I can't seem to find too many old CDMG maps online - I suppose they are only in libraries. I just kind of stumbled onto that one, have never seen that format before. I think it's a neat format, except for the map!