Real Science Friday: List of Not So Old Things

[gsweet]

Where were they found?

I would suggest it prudent to assume first that they formed where they are unless there is good reason why that would be impossible.

see points 1 and 2...:up:

Oh. That sounds reasonable.

At what depth are the deposits found today?

this type of system produces different deposit types at varying depths, so i'll outline them briefly below:
-porphyry-type mineralization (Cu-Au + or - Mo) occurs within and immediately surrounding the intrusive body. out in the field, if you happy upon a porphyry stock, it's very obvious. the mineralization is frequently contained within a series of stockwork veins within the intrusive phases. this reflects infill of the aforementioned hydrostatically-induced cracks (see point 1 above) by the depressurizing supercritical fluid. again, this requires a uniquely high pressure environment (3+ km depth) and would not be possible at surface pressures.
-epithermal mineralization (Au-Ag + or - Pb-Zn-Te and others) is still driven by the same heat source as porphyry mineralization, but is frequently distal to the magmatic body. the deposition of this metal budget (very distinctly different from the porphyry metal budget) is generally 1 or 2 km from the surface.
-hydrothermal and lithocap alteration/mineralization are the surface product

when exploration companies explore porphyry targets today, we frequently find a fairly predictable vertical variation through these deposit types and their associated mineralization/alteration. if you look at 1000 or 2000 meters of drill core (though it is rare that a company will drill that much), you'll see changes in metal budgets, alteration mineral assemblages, etc that coincide with a change in pressure. i guess what i'm trying to say here is:

yes, we do find porphyry-type mineralization at the surface today. however, we also find porphyry type deposits at much greater depths via drilling, and we find these other deposit types above them in the stratigraphic column. this type of mineralization is a system, and it varies laterally and vertically a great deal. because of the nature of porphyry mineralization and the inherent "demand" for high levels of lithostatic pressure, it's not possible for porphyry deposits to form at shallow depths, never mind the surface. when these fluid/brine rich magmatic bodies do get too close to the surface, they erupt forming a volcano or a diatreme (which one depends on a lot of factors).

How is this a problem? The water escapes toward the surface and leaves a deposit near the surface. That's exactly what I said. If you're saying that the process started at a few kms deep and left the deposit where it is then I have no problem. How does this make the deposits millions of years old?

see above for the explanation about vertical fluid movement with respect to porphyry environments. the fact that we have ore within the crystallized magmatic body indicates that this was at depth. shallow level mineralization does occur (hydrothermal and sometimes epithermal), but it is distinctly different in alteration assemblage (the minerals added to the rock as a byproduct of the fluid moving through them) and metal budget. that said, there's also a whole realm of porphyry study that centers around "telescoping" of a system; this refers to relatively rapid uplift/erosion of country rock above an active porphyry environment, resulting in the overprinting of epithermal and hydrothermal deposits ontop of formed or partially formed porphyry deposits.

and full circle to the age issue: it takes a lot of time for a large magmatic body to fully crystallize. Bob's citation of gold mineralization at lihir as an example of a young earth is negating the fact that in order to reach the point of Au-rich fluid production and deposition, many tens to hundreds of thousands of years must be accounted for.

The heat source would have had to have been contained and then ruptured. Agreed. That works at depth. But the deposits were formed where they are now .. near the surface. Right?

for a porphyry deposit, no; Cu-Au-Mo deposition is formed at depth. Lihir is a bit of an anomaly with respect to depth, though, as it's metal budget is atypical. Stuart Simmon's work on Lihir tends to lean towards a cataclysmic collapse of a side of a volcano above where the Lihir deposit currently is...hence rapid depressurization. however, this does not negate the fact that in order for those Au-bearing fluids to form, the deposit had to initially be highly pressurized at depth. i tried to illustrate this using a standard porphyry model above, as just jumping into the atypical Lihir deposit would produce a lot of confusion.

What if the pressure and heat were caused by something other than a plug of magma?

such as? i'm not sure i can think of something else that would produce the same effect as a magmatic body. you have to remember, this magma contains a lot of water that is isotopically distinct from the meteoric water that's already present in the surrounding country rock. we see this reflected in the lateral variation of isotopes within the alteration minerals...

if you have some ideas though, i'm all about it. this was what my previous comment to Jukia on "lack of processes" revolved around; if you have some alternative hypotheses, lets discuss them, test them and figure out whether or not they're valid!

Sure. It'd help a discussion if you could restate what's going on in more user friendly terms, but I'll read anything.

just let me know; i've got a collection of fairly good "intro to porphyry-style deposits" papers. some are a bit old and bits and pieces are outdated, but most are pretty good...

I think you're over-reacting.

perhaps. but i do enjoy these discussions, Stripe, and i don't like it when one side of the argument come's off as intentionally dishonest: it makes life harder for me as well, as i end up having to drag data back into the discussion. data "picking" is frowned upon in general, and i see no reason not to call someone on it...

Tasty?

indeed. though in my hastiness to emphasize my previous post the other night, i didn't really consider the gastronomic repercussions of eating a mace...:doh: it's been a rough day.

If you're going to pay attention to and derive material from Jokia then it's not going to help the discussion any. You have the background, qualifications and reputation to be able to state your case and be respected for it. Please don't descend to the typical Jokia tactic of belittling the opposition in order to try and win points.

my intention was not to belittle, but rather to call out an issue as i saw it. one of my major qualms with respect to YEC creationism has to do with methodology: where uniformitarianistic science compiles data, interprets and then provides models. YEC creationism tends to just attempts to disprove these models. any time one of the models is questioned or changed, it is declared a "success" (for example, all of Bob's points above). walt brown's work is the closest thing to creationist science that i've encountered. but with that said, i still have trouble with his theories as they are not based on data: from what i can tell, he speculates and produces a model. i've yet to see his multi-leveled evidence (i.e. microscopic, small and large scale evidence) and/or data that backs up his model. but i should say that i really haven't read into his work in detail, nor have i read his book. one day, Stripe, you and i will discuss Walt's theories at length...perhaps this spring? but now is not the time...gotta stay focused on this porphyry stuff!

If they keep coming out and telling us they were wrong how are we supposed to trust them, ever?

what PB said. these interpretations are based on data; as time goes by, the data set grows; gaps in the data are filled in and what were previously known as anomalies become explainable. this demands a change in our model, or even a new model all together. either way, it is a continued improvement towards an accurate explanation (something that we may reach or not, depending on the subject) via a higher and higher resolution of understanding...

 

[Lighthouse]

Trust the scientific process, its self-correcting nature, and the growth of knowledge, not "them"...
Science isn't about the people its about the facts and how they are applied and how new knowledge changes our understanding.

Seeing as how it keeps leading them to conclusions they later find to be false I think I'll trust in the Lord.

 

[Stripe]

see points 1 and 2...:up:

That's where they formed, 2-3kms deep. But the gold deposits are found near the surface, right?

and full circle to the age issue: it takes a lot of time for a large magmatic body to fully crystallize. Bob's citation of gold mineralization at lihir as an example of a young earth is negating the fact that in order to reach the point of Au-rich fluid production and deposition, many tens to hundreds of thousands of years must be accounted for.

I see. What if there were another means to produce and then release supercritical fluids? We both agree that the process that forms the metal bearing deposits is always quick so there's no real point in discussing that. But the conditions that lead to the process being able to proceed is where our debate is likely to reside.

for a porphyry deposit, no; Cu-Au-Mo deposition is formed at depth. Lihir is a bit of an anomaly with respect to depth, though, as it's metal budget is atypical. Stuart Simmon's work on Lihir tends to lean towards a cataclysmic collapse of a side of a volcano above where the Lihir deposit currently is...hence rapid depressurization. however, this does not negate the fact that in order for those Au-bearing fluids to form, the deposit had to initially be highly pressurized at depth. i tried to illustrate this using a standard porphyry model above, as just jumping into the atypical Lihir deposit would produce a lot of confusion.

Pressure occurs at depth, but depressurisation must include the uplift of the fluids. The whole process (pressure cooker to final deposit) works over a range of depth (0-3km) with the final deposits being found near the surface. Is that what you're saying or are you suggesting that many deposits require long ages of tectonic uplift in order to get them to where they are today?

such as? i'm not sure i can think of something else that would produce the same effect as a magmatic body. you have to remember, this magma contains a lot of water that is isotopically distinct from the meteoric water that's already present in the surrounding country rock. we see this reflected in the lateral variation of isotopes within the alteration minerals...if you have some ideas though, i'm all about it. this was what my previous comment to Jukia on "lack of processes" revolved around; if you have some alternative hypotheses, lets discuss them, test them and figure out whether or not they're valid!

perhaps. but i do enjoy these discussions, Stripe, and i don't like it when one side of the argument come's off as intentionally dishonest: it makes life harder for me as well, as i end up having to drag data back into the discussion. data "picking" is frowned upon in general, and i see no reason not to call someone on it...

You'll just have to believe that I'm not trying to be dishonest. When a certain feature is described as having to take millions of years, but then we find out the most easily understood part of it happened quickly I think it is fair enough to point out the distinction. It can only advance the debate to define exactly where we agree or disagree.

my intention was not to belittle, but rather to call out an issue as i saw it. one of my major qualms with respect to YEC creationism has to do with methodology: where uniformitarianistic science compiles data, interprets and then provides models. YEC creationism tends to just attempts to disprove these models. any time one of the models is questioned or changed, it is declared a "success" (for example, all of Bob's points above). walt brown's work is the closest thing to creationist science that i've encountered. but with that said, i still have trouble with his theories as they are not based on data: from what i can tell, he speculates and produces a model. i've yet to see his multi-leveled evidence (i.e. microscopic, small and large scale evidence) and/or data that backs up his model. but i should say that i really haven't read into his work in detail, nor have i read his book. one day, Stripe, you and i will discuss Walt's theories at length...perhaps this spring? but now is not the time...gotta stay focused on this porphyry stuff!

You might be surprised

I would agree with your analysis of Dr. Brown to an extent. There is not a lot of data to work with. But that is probably the curse of trying to rationalise a theory that is completely opposite to established thought. The data, even if available, is all couched in the wrong terms.

I think your objection, that there is no model, is uninformed. There is a model, it only requires a mind willing to separate the data from the established mindset.

The magmatic source/other source for porphyry depositsmight be a very good way to study a competition of the models.

 

[gsweet]

That's where they formed, 2-3kms deep. But the gold deposits are found near the surface, right?

yes and no. the term "porphyry" refers to both the magmatic product (a porphyritic stock in the crust) and the specific type of mineralization associated with that stock. my fault for not mentioning that earlier. with that said, porphyry mineralization occurs within the porphyry stock at depth. the depressurization that i referred to is relative: when the lithostatic pressure is exceeded by the hydrostatic pressure, the rock can no longer "contain" the fluid. the rock cracks and provides more space for the fluid...hence depressurizing it. however, this does not mean that the fluid is no longer under pressure, it just means that it is under less pressure. as the fluid migrates upwards, out and away from the magmatic body, it cools off and continues to depressurize, creating the other deposits/alteration i mentioned. technically speaking, the fluid isn't completely depressurized until it reaches the surface of the earth.

so yea, porphyry-type mineralization is at depth. there are examples of shallow crustal mineralization above porphyry stocks, but they are not porphyry style mineralization (Cu-Au-Mo), instead falling into the categories listed above (epithermal, hydrothermal, lithocap, etc).

I see. What if there were another means to produce and then release supercritical fluids? We both agree that the process that forms the metal bearing deposits is always quick so there's no real point in discussing that. But the conditions that lead to the process being able to proceed is where our debate is likely to reside.

again, yes and no. lihir is an example of mineralization that might have occurred rapidly. the porphyry deposits that i study/research are less definitive. i really haven't figured it out yet, as there are many, many "bursts" of mineralization associated with multiple intrusive phases to contend with... from the dating that i've performed (U-Pb, Ar-Ar, and K-Ar), i'm looking at the life of the porphyry (the intrusive history of the stock) to be about half a million years. so mineralization may occur quickly in these deposit types (lihir) or gradually.

as far as other methods of producing the supercritical fluid goes, i'm all ears. there'd be an awful lot of the same coincidences worldwide at every porphyry deposit though (i.e. similar tectonic setting, same local setting with respect to intrusive bodies, etc).

Pressure occurs at depth, but depressurization must include the uplift of the fluids. The whole process (pressure cooker to final deposit) works over a range of depth (0-3km) with the final deposits being found near the surface. Is that what you're saying or are you suggesting that many deposits require long ages of tectonic uplift in order to get them to where they are today?

see above; uplift of the fluids is not necessary. if they were completely depressurized at once, you'd see a very different deposit type (visually different, different metal budget, different alteration assemblage, etc). because of the gradual depressurization, we have deposits that range from deep (porphyry mineralization) to shallower (lithocap mineralization). tectonic uplift is not required, but when it happens rapidly (e.g. the Baguio Mineral District of the northern Philippines was uplifted a kilometer or so in under 500,000 years) you end up with the range of deposit types (porphyry through to lithocap) being overprinted on each other.

You'll just have to believe that I'm not trying to be dishonest. When a certain feature is described as having to take millions of years, but then we find out the most easily understood part of it happened quickly I think it is fair enough to point out the distinction. It can only advance the debate to define exactly where we agree or disagree.

agreed

You might be surprised

I would agree with your analysis of Dr. Brown to an extent. There is not a lot of data to work with. But that is probably the curse of trying to rationalise a theory that is completely opposite to established thought. The data, even if available, is all couched in the wrong terms.

I think your objection, that there is no model, is uninformed. There is a model, it only requires a mind willing to separate the data from the established mindset.

The magmatic source/other source for porphyry depositsmight be a very good way to study a competition of the models.

then discuss away; pick around for some other methods of producing porphyry deposits and we'll discuss that. i think from what i've heard of Brown's theories, the speculations are quite a departure from the general theories of geology. from a geological perspective, one would expect there to be an enormous amount of geological evidence (on both large and small scales) to account for this. i understand that you believe a lot of the sedimentalogical data today to be "misinterpreted", but i'm a bit skeptical. for example (and not to discuss this now), if one massive body of water, in one event, was to account for the massive surface erosion Brown gives it credit for, wouldn't we expect an enormous (and in sed terms, that could mean many miles thick), laterally continuous, homogeneous sediment package at the edge of continents? wouldn't the geochemical makeup of this material represent the a "blend" of the material removed?

 

[Jukia]

Ah, Walt Brown. I suspect you need not look to any geologic evidence. I think the real issue is basic physics. I am still waiting for someone to explain how the energy equivilent of 1.2 Million H-bombs/foot of mid ocean ridge. I have asked that question numerous times at this and other sites and have yet to find a response

 

[Stripe]

yes and no. the term "porphyry" refers to both the magmatic product (a porphyritic stock in the crust) and the specific type of mineralization associated with that stock. my fault for not mentioning that earlier. with that said, porphyry mineralization occurs within the porphyry stock at depth. the depressurization that i referred to is relative: when the lithostatic pressure is exceeded by the hydrostatic pressure, the rock can no longer "contain" the fluid. the rock cracks and provides more space for the fluid...hence depressurizing it. however, this does not mean that the fluid is no longer under pressure, it just means that it is under less pressure. as the fluid migrates upwards, out and away from the magmatic body, it cools off and continues to depressurize, creating the other deposits/alteration i mentioned. technically speaking, the fluid isn't completely depressurized until it reaches the surface of the earth.

Depressurisation triggers the minerals and metals to fall out of the solution. This will happen at a shallower depth than the fluid started at. I've got a picture in my mind of what's going on and I'm trying to get it into words .. the point being there will always be a difference between the depth of the source material and process and the final deposition with deposit shallower.

again, yes and no. lihir is an example of mineralization that might have occurred rapidly. the porphyry deposits that i study/research are less definitive. i really haven't figured it out yet, as there are many, many "bursts" of mineralization associated with multiple intrusive phases to contend with... from the dating that i've performed (U-Pb, Ar-Ar, and K-Ar), i'm looking at the life of the porphyry (the intrusive history of the stock) to be about half a million years. so mineralization may occur quickly in these deposit types (lihir) or gradually.

Your dating methods will only tell how long ago the deposits were formed. Not how quickly they formed, right?

as far as other methods of producing the supercritical fluid goes, i'm all ears. there'd be an awful lot of the same coincidences worldwide at every porphyry deposit though (i.e. similar tectonic setting, same local setting with respect to intrusive bodies, etc).

Mmm .. yeah. When I say it's an alternative theory .. it's an alternative theory for almost everything. So there is no reliance on coincidence.

then discuss away; pick around for some other methods of producing porphyry deposits and we'll discuss that.

I don't think there's another method. You require super hot water at depth under great pressure. This enables metals to go into solution. Then you need to transport the water upwards quickly enough so that the metals can fall out of solution in the way we find them.

I'd suggest that the deposits we find are more likely to be the result of a few large events rather than a slow, continual process.

The problem is I know of no way to place large amounts of water beneath that distance of rock. It's definitely there .. but it's almost worth approaching this with subterranean water as a presupposition rather than trying to explain its presence.

i think from what i've heard of Brown's theories, the speculations are quite a departure from the general theories of geology. from a geological perspective, one would expect there to be an enormous amount of geological evidence (on both large and small scales) to account for this. i understand that you believe a lot of the sedimentalogical data today to be "misinterpreted", but i'm a bit skeptical. for example (and not to discuss this now), if one massive body of water, in one event, was to account for the massive surface erosion Brown gives it credit for, wouldn't we expect an enormous (and in sed terms, that could mean many miles thick), laterally continuous, homogeneous sediment package at the edge of continents? wouldn't the geochemical makeup of this material represent the a "blend" of the material removed?

Did you want to discuss this now or not?