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I think I read in one of your posts that "your" pluton is associated with a fault or near a fault. So which comes first, time wise, the pluton or the fault. Is a fault not associated with a pluton as likely to have valuable minerals nearby as a fault with a pluton or is there no correlation? Thanks. I am curious because the area I am working in AZ has a few faults. I assume (dangerous) all faults are not equal?

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Au+,

 

Natural geologists have that "fault" called assuming. :) But that is where we must begin our process of thinking. Geologists make certain assumptions and then attempt to validate what we assume to be true.

 

Geology is interesting because things have happened over many millions of years - even billions.

 

So, with reference to your first question, I haven't seen anything in the literature about the date of the Ophir Pluton. Other plutons have been dated with various degrees of accuracy.

 

The fault created a weak spot with fractures that provided a conduit for mineralization to move upward through the crust of the earth. Geologists have placed dates on the mineralization at 60 million years ago. (Late Cretaceous- Early Tertiary)

 

Minor faults around Arizona may have gold or silver mineralization depending on all of the above factors and a few more. The few more factors are depth of the fault zone and proximity to precious metal mineralization. Gold and silver are both rare as we know. The areas that have been mineralized with gold are few and far between. I have visited underground gold mines near Quartzite where gold could be seen in the quartz within fault zones in the mines.  

 

 

- Geowizard

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Intrusives like the one shown above are usually seen as a simple hill. Yet upon taking a closer look, you climb to the top of the hill and find that it is made of quartz. The intrusive may be mineralized with gold and silver. There-in lies the importance of carrying small light weight inspection tools like a magnifying lens known as a "loop".  

 

 

To be continued... :)

 

- Geowizard    

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Thank you, Geo.  Interesting reading and answers my question;  Are there plutons in AZ?  So much information and so little time to absorb it. :)

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There is a link to the granite pluton related to Vulture Mine geology in post #9 above.

 

Practically all of the mining districts in Arizona have plutons associated with copper, silver and gold mineralization. It would be a long list!

 

- Geowizard

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One thing it is important to point out is that there are many hundreds of plutons in Alaska. Most are unmineralized or only slightly mineralized. Some are mineralized but only enough that after millions of years of erosion and accumulation of the gold from these moderately mineralized intrusives will there be created workable placers. Only a very small fraction of Alaskan intrusives - less than 1% of them -  are suitable for direct, hard rock mining.

 

Porphyry type copper mineralization in Arizona with minor gold and silver or with richer gold and silver vein deposits distal to the porphyry intrusive are quite different from the mineralization style of Alaska. 

 

Plutons in a mineralized area can be pre- or post mineralization, meaning they may have existed long before any mineralization occurred. You can also have a situation where plutons are intruded long after mineralization has been completed.

 

Just because an intrusive rock is present, does not mean it is related to a deposit. The gold deposits of the Sierra Nevada Range in California are famous. The backbone of the Sierra Nevada is the Sierra Nevada batholith, a gigantic pluton of intrusive rocks of various types. The batholith did not cause the gold deposits, nor did the gold deposits cause the batholith. Both were caused by continental collision and subduction of the Juan DeFuca plate underneath the North American plate.

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Geo -

 

Do you know for certain that the intrusive mapped at the first post of this thread actually outcrops?

 

Determining if a body of intrusive rock actually outcrops just from a flyover geophysical survey can be difficult. It may be near the surface, but not actually outcrop.

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Chris brings up another good point that intrusives are not directly mineable for gold.

 

The intrusives provide a "conduit". The intrusive is a structural event that produces fracturing along side of the intrusive. The fractures in the crust of the earth offer a conduit for hydrothermal solutions to pass toward the surface. Hydrothermal solutions MAY contain gold or other precious metals. Without a conduit, hydrothermal solutions don't get to the surface. There must be a weak point caused by an eruptive event or structural event that breaks the basement rock. Exceptions are sedimentary zones that are soft and provide channels for solutions to escape from fractures in basement rock. IT's easy to get wrapped up in semantics of geology rather than looking at basic principles.  

 

Intrusives are easy to identify on geophysical contour maps.

 

The first step:

 

 Interpret the intrusive as an intrusive and then look at the immediate area for conductive mineralization.

 

Erosion of the intrusive will cause a concentration of heavy metals around it (if heavy metals exist in the intrusive).

 

If the intrusive caused hydrothermal venting to take place around it, those vents MAY be conductive. If local areas show conductive mineralization, that represents a target for further exploration.

 

To be continued... :)

 

- Geowizard

 

How are these geophysical contour maps different from regular topos and are they generally available or just for specialized areas?  I've searched and not been able to find any information.  Thanks and again great information!!!

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Thanks Rod.  I am in AZ and do not enjoy the benefits of all the different surveys, maps and info available in Alaska.  So I figured checking for intrusives on geophysical contour maps would probably be as close as I can come here,  That is if the maps are even available for AZ.  I guess I shoud have been more specific with my post. 

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Rod is correct. Thanks, Rod! :)

 

Arizona has not pulled together the resources needed ($$$) to fund geophysical surveys. Much of the problem surrounds the fact that only a small portion of the domain is owned by the state. Alaska on the other-hand has a big dog in the race for generating new resources to keep the money rolling in. Alaska and Wyoming are the only states (maybe Nevada) that has natural resources that fund the state coffers and have the cahones to develop them! 

 

- Geowizard

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Hi Stan, good to see you here.

 

You can get the complete Arizona Bouguer gravity maps HERE.

The complete Arizona Aeromagnitic Anamoly maps are HERE.
 

These are pretty coarse. There is a much better set of surveys available for the Phoenix area, including your area of interest but I seem to have misplaced the link to the original data. We considered using those but since the placers in your district are small and scattered with multiple sources we don't see much utility in their use locally.

 

The ones that are available could be a help for locating larger areas of potential higher mineralization but even the higher resolution ones come nowhere near something useful for the small prospector. For instance the area of higher mineralization associated with your fault and the local intrusives don't even make a significant blip on the better maps. The geology you already have is pretty detailed and probably the best we will see in the near future (years).

 

The White Picacho district near you has been studied rather extensively due to it's extreme mineralization and the truly monstrous crystals found in it's pegmatites. Sometimes you can get a small overlap from those studies into your region of interest but it takes a lot of digging through the huge amount of reports on the district.

 

I've seen much better surveys made by a few mining companies some with very good resolution, for aeromagnetics,  but the chance of them being shared with the public is just about nil.

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Here's another resource for Stan or anyone looking for geological information in Arizona.

 

The Arizona Geological Survey has been scanning and archiving their extensive collection online for public access.

 

AZGS Document Repository

 

We keep a local backup archive of all their scanned data. It really saves time when researching an area.

 

Besides the documents and maps available online if you give them a call about any particular document that is not yet available they will usually scan it for you. You will need to stop by their office with a USB memory stick to pick it up but there are some real jewels in their collection that are well worth the trip. Check out their mineral collection while you are there. They have some nice specimens.

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Thanks Clay. Appreciate the links. I had found some of the material you mentioned at AZGS Document Repository. I like it here and find a lot interesting information. Great stories too. The only negative is that the information makes me realize how little knowledge I have when it comes to prospecting/mining.

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