Contact Zone

[Dizzo]

What you guys say would be the best way to graphically describe a contact zone?

[Gilaoro]

From the dictionary of geological terms " The place or surface where two different kinds of rocks come together. Although used for sedimentary rocks, such as the contact between

a limestone and sandstone, it yet more especially employed as between igneous intrusions and their walls. The word is of wide use in western mining regions on the account of frequent occurrence of ore bodies along contacts. . In South America a lode of great length and between two kinds of rocks , one of which is generally an igneous intrusive. EC, ETC."

Geologic definition: AUREOLE "A zone surrounding an igneous

intrusion in which contact metamorphism of the country rock has taken place. SYM: Contact Aureole; Contact Zone

Hope this helps'

Max

[Dizzo]

That is a good verbal definition, but I was looking more for an experienced graphical representation, like what is a good graphical representation of a hot blonde? From my experience she is Caramel skinned, 5' 6" tall, 32 C, Green eyes and often occurs amongst or alongside not so hot short chubby girls, as to make her look hotter. Thanks Max

Dave

[Gilaoro]

How about this Dave?

[Steel Pan]

What you guys say would be the best way to graphically describe a contact zone?

The area between the knees and shoulders,....

[Dizzo]

Very good, Steel Pan! That would be my second guess All right Gillaoro! Would you say the gold would be between the vertical folds, between the vertical folds and horizontal folds on the right, in the horizontal folds in the middle, or after the horizontal folds to the left. I have read that it is more likely that gold will be on either either side of the zone of horizontal folds, but likely on one side or the other.

Dave

[scbane]

This reply may highlight my almost (but not total) ignorance of geology, but it would seem to me that since gold is deposited under incredible heat and pressure, gold veins usually have an upward diagonal trend if not straight up. Has anyone seen horizontal veins that were formed that way rather than tipped in some seismic event? Pressure must have an outlet, and that is usually up ward through weak spots. I would say that a gold deposit would be "Upward mobile."

[Dizzo]

My guess would be between the vertical and horizontal folds on the right, and the color of the soil looks pretty good as well.

[Steel Pan]

The path of least resistance. That's where an injection will flow, be it verticle or horizontal.

An injection deopsit is dif than a contact zone. Gold injection is usually in solution with quartz.

A contact zone is not necessarily an indicater of the presence of Gold. The Gold would have

had to be present BEFORE the creation of the contact zone. Such as ancient Gold bearing gravels being

overlaid with a basalt flow. This would create a contact zone between the gravels and the basalt, AND lock

the existing Gold in the gravels.

Go the other way,...

A basalt flow with overlying Gold bearing gravels. Now, you may find Gold at the contact zone but the presence

of Gold Is not due to the contact zone as it, the Gold, came in with the gravels.

[Dizzo]

Thanks Steel Pan, I would also imagine that if the contact zone was not bedrock prior to the folding and contact zone creation, the likely hood of gold would diminish. The injection aspect is a good conversation as well.

[Steel Pan]

Thanks Steel Pan, I would also imagine that if the contact zone was not bedrock prior to the folding and contact zone creation, the likely hood of gold would diminish. The injection aspect is a good conversation as well.

Not so.

The determining factors are, the Gold was either transported through errosion (of a Gold bearing extrusion) or deposited in a vein by intrusion.

Now, the trick is figuring your best candidate for Gold bearing material. This material can be below, above, or included in a vein in bedrock. Heck, you may not even find any bedrock. Bedrock is just a stopping place, so to speak, that stops the Gold from continuing to sink, which is what it does in sands, gravels, and even clays. That's why you hear so much talk about bedrock. That's where the Gold will settle. That's Placer deposition. Lode deposition is Gold deposited/injected into a vein in hard rock.(and the source of future Placer deposits )

Edited March 15, 2013 by Steel Pan

[Gilaoro]

Dave, I thought you where asking about contact zones, you didn't specify AU contact zones. In the picture the contact zone shows a lighter color, the caption on the original photo did not scan. Actually in the photo the ore body being mined was a lens deposit of a copper carbonate which was horizontal in the fore ground of the photo, (It was not successful due to thinness of the deposits) which was also a contact zone.

There are many theories about gold deposits , I accept the one about placer gold coming from "Hard rock" veins no matter what the host material but not that placer forms "Hard rock" deposits, not all of them are all "Hot deposits" some are low temperature. It has also been said that gold and silver does not form in sandstone but in Utah they have both in the same area, research "Silver Peak" Utah N of St George.

The little I know about which side of the vein gold forms on is they call one side the "Foot Wall" and the other the "Hanging Wall" but I could never figure out how because the few underground mines I have been into have had walls that went in all angles. The best one I was ever in was in the "Dale District" near 29 Palms, CA and it had vertical walls about 3 feet apart, the gold bearing material was cobbles on one side ( N on a E-W vein) and graduated to very fine stuff on the S, the last inch being a fine red powder and the richest but it had to be amalgamated to recover the values.

one half mile W this vein was cut by a deep wash that produced small nuggets. The vein did not continue having been eroded away .I was fortunate enough to be given 16- 5 gal buckets of the fine red material , It would have made good barn paint mixed with a base. Sorry to say this is all in the National Park and wilderness area now and has been many years, a number of beautiful old stone houses where destroyed and a number of shafts where filled in, its a shame what history in lost but they are "Saving the land for their kids" Notice the "THEIR" in those statements!

OK, I am off the soap box and gone, y'all have fun

Max.

Max

.

Very good, Steel Pan! That would be my second guess All right Gillaoro! Would you say the gold would be between the vertical folds, between the vertical folds and horizontal folds on the right, in the horizontal folds in the middle, or after the horizontal folds to the left. I have read that it is more likely that gold will be on either either side of the zone of horizontal folds, but likely on one side or the other.

Dave

[Dizzo]

I have heard the foot and hanging wall terms before and like you have a hard time finding the difference with little reference other than smooth verses coarse walls on a pitch or what have you. You sandstone comment was actually something I was going to state earlier, except it was going to be dolomite. I thought the AU part would have been assumed. Sorry about that.

Dave

[Steel Pan]

When it comes right down to it, "Gold is where you find it".

Seems like the rules change with every new discovery.

Meta-sedimentary that has solidified can contain Gold,

Only because Gold was deposited in the sedimentary strata

in some manner such as errosion deposits.

UNLESS, there was a more recent intrusive deposit.

That ought to make it clear as mud.

[Dizzo]

Good thing I like mud :wee:

[Patrick in Havasu]

Then I guess these color changes are not "contact zones", just pretty spots in the desert?

Patrick .....

Edited March 18, 2013 by Patrick in Havasu

[homefire]

OK! Not a Geologist but I know what I say here.

I see Intrusive Uplifts of Different Strata of Different Make up.

Obviously this area has gone through more then a few Million Years of Erosion and Breaking down of the the Basic Structures that made it up.

My guess is this was a Major Mtn Range at one time Long Long ago.

I would work the Areas between the Iron Red areas and the CONTACT ZONE of any other color.

A Old Geezer told me one time GOLD RIDES A IRON HORSE.

Iron and Quartz are Carriers of Gold.

Copper, Silver are formed in the same Areas Gold will and did Form.

Hell Most Gold has Copper and Silver within it.

Edited March 18, 2013 by homefire

[homefire]

I'm sure you already know this but this is how I would hunt.

I would hit anything like a Ravine, Gully or Wash between the Colors.

[Patrick in Havasu]

Thank you Homefire. I'll do just that in a few days. This is just outside of town.

Patrick .....