Log in

View Full Version : What if? Different idea on structure enrichment.



overtheedge
10-05-2012, 07:19 PM
This is purely speculative geology from someone who is not a trained geologist.

Current tectonic theory has orogenisis being the outcome of plate collisions. Many, if not most, are the product of plate subduction with attendant volcanism.

The subducted plate partially melts and adds to orogenisis by up-welling. If this up-welling breaks through the crust, we get a volcano. Otherwise we get intrusives such as batholiths, lacoliths, dikes, etc.

These intrusives are linked to economical ore deposits through several mechanisms.

One thing that has struck me odd is that any form of enrichment comes from the subducted material and any chemical interaction with the overlaying crust. Effectively, you can only get what was there to begin with; elements and quantity.

I have no doubt that the current theories on ore deposition along converging plate boundaries is logical.

But what if?

What if we consider submarine hot springs aka black smokers as a possible primary enrichment zone? These black smokers are already being looked at as poly-metallic sulphide mineral deposits for potential submarine mining.

These occur along diverging plate boundaries and the attendant transform faults. They occur over large areas and the total mass is great. The chemical environment exists for deposition else they wouldn't be there. Once the portion of the plate moves far enough from the divergence zone, the hot spring gets cut off leaving just the deposit.

Now as the diverging plate carries these now dead black smokers into the subduction zone, the already enriched material gets remelted.

It seems to me that if you have an already enriched chunk of plate carrying saltwater into the trench, you set the stage for seriously enriched deposition. You have poly-metallic sulphides, saltwater and heat. I can't think of a better environment for further enrichment through the different processes.

Just thought I would toss this strange idea out for discussion.

Think of the possibilities.
eric

Bill Bohan
10-05-2012, 07:57 PM
Over,
I believe this is the model for the volcanogenic massive sulfide deposit. Google VMS deposits of Kuroko deposit

overtheedge
10-05-2012, 08:42 PM
Well don't I look the fool. Reckon that is one of the problems I face with not getting a degree in geology.

I was reading on black smokers and the potential for submarine mining and went "what if".

A few evening ago, I was reading on the different mineral provinces and their relationship to terrane boundaries and how Alaska is a hodge-podge of stitched together terranes. Then I recalled all the mines around the Valdez Arm and how there is/was a subduction zone offshore. I think it has become more of a slip/strike fault now.

Thanks Bill. I figure if I don't try to learn something new every day, I'm brain dead and the rest might as well follow.
eric

Lady
10-06-2012, 04:53 AM
We could do comparisons with this subject.

Seafloor massive sulfide deposits or SMS deposits, are modern equivalents of ancient volcanogenic massive sulfide ore deposits or VMS deposits. The term has been coined by mineral explorers to differentiate the modern deposit from the ancient.

SMS deposits were first recognised during the exploration of the deep oceans and the mid ocean ridge spreading centers in the early 1960s. Deep ocean bathyspheres and remote operated vehicles have visited and taken samples of black smoker chimneys, and it has been long recognised that such chimneys contain appreciable grades of Cu, Pb, Zn, Ag, Au and other trace metals.

SMS deposits are currently forming in the deep ocean around submarine volcanic arcs, where hydrothermal vents exhale sulfide-rich mineralising fluids into the ocean.

SMS deposits are laterally extensive and consist of a central vent mound around the area where the hydrothermal circulation exits, with a wide apron of unconsolidated sulfide silt or ooze which precipitates upon the seafloor.

Bill Bohan
10-06-2012, 10:04 AM
There is some pretty current data on young rock types on sea floor spreading . Most of the sea floor rock type is basalt with a sedimentary sequence of 3( layers) deposited over it. Younger basalts are at the rift margin and older basalts are toward the subjuction zones.
Dynamic spreading is caused by the rotation of the earth and centripital forces.

VMS deposits can be spread out over 80-200 miles depending on the regional faulting. The Bonnefield VMS on the north slpe of The Alaska Ranges is parted out from the Healy to Cantwell .

Another OCean flor sequence that would be of interest to you Michelle would be called the Ophiolite sequence model. These are premised "Obducted" seafloor accreationed that are exposed at the surface.

overtheedge
10-13-2012, 04:31 PM
Again my thanks on pointing out VMS deposits.

This month's ICMJ has an article on the Verde VMS deposits in Arizona and it stirred my interest a bunch.

Is it a reasonable assumption that VMS deposits are strictly localized to basalt based greenstone occurrences? Premise being start with basalt extrusions, end with meta-basalt/greenschist/amphibolite series greenstone.

Would it be a good assumption that because of the high silica content in these former smokers that there "might" be gold-quartz lodes above deeply buried greenstone basement rock? Again based on the premise that low-medium grade metamorphism and the quantity of entrained saltwater subducted with the oceanic plate would magmatically differentiate, remobilize and extrude the silicates as well as gold and miscellaneous pyrites into fracture zones associated with faults. The idea being that younger terranes haven't had enough erosion to expose the greenstone.

Part of this is extrapolated from the Nikolai greenstone-Chitistone limestone deposit of Kennicott copper fame. Granted that this is in the Wrangellia terrane, but I keep thinking about the Chugach terrane flysch and the gold mines of Prince William Sound. Running through and alongside these terranes is the Border Ranges fault system.

Another part of the idea coming from the Crow Creek gold deposit in the Chugach flysch with the Placer River fault that must be associated with the BRF, perhaps as an embedded transform fault.
eric

overtheedge
10-14-2012, 09:44 AM
Going back to the article in ICMJ, the Verde VMS sits on top of a rhyolite plug. This sort of baffles me. Oceanic crust is basalt. Rhyolite is just fast frozen granite if I understand this any at all. Granite is continental crust material. So it would seem that the subducting basalt had to get the 15-20% quartz by melting continental crust to alter to rhyolite. Then this now quartz richer material must have extruded beneath an inland sea. It this logical at all?

It would appear that this inland sea was salty, re: photo of copper ore in calcite-chert. Later the plug was buried under what became sandstone.

Then it would seem that the rhyolite must have fractured and the sulfide-rich material extruded under the sandstone.

Am I on the right track? I figure if I don't know how it happened, I'll never figure out what geologic structures to look at for anything.
eric

Bill Bohan
10-14-2012, 05:53 PM
Over,
The Chugach flyshe is interpreted as a sediment subducted zone wedge. Ore traps from volatiles presipitating metals (dissolution) against a non permeable layer of metashale is possible. You can google images of gas and oil tramps and next instead of modeling a hydrocarbon trap , you model a metamorphic trap within a temp and pressure range.

Rhyolites and granites can form within a gneiss body or from any composition that will distille a granite composition. There was some altered intrusive dike at Pogo that was mapped as (a.i.n.t. ie altered intrusive ingneous dike) but if you looked at the freshly blasted surface it appeared that the granite was deriving from the gneiss. To have a rhyolite form from this senario you would need a water component and a silica component and a hornblende inclusion of sort. ...the soup would have to become non viscous enough to surpass migmatiite texture, and then once the system was breeched by a fault, the hypabyssal goo would "plug" the strike of the fault. Hmm? Water plus fault gouge might form hornblende at depth? I don't know just thinking out loud.
Of course all of the above at the correct temperature and pressure.