Greens Creek Mine
Geology Page

Special Note 12/15/07: An individual with extensive and unique knowledge of Greens Creek geology has made extensive comments. I am making changes and incorporating those comments on this page. The first iteration has been incorporated. More changes will be made.

Introduction

This is a page on the geology of the Greens Creek Mine. It includes--or will include when it is complete--references to and summaries of past work that is public.  But that is not really what this is about. The main thrust of this page is to present a structural model for the Greens Creek deposit--one that I think has application to many other volcanogenic massive sulfide (VMS) deposits in other parts of the world.

The ideas expressed here have their origin in an open-ended and free ranging examination of the Greens Creek deposit undertaken by me in 2004 and 2005. How or why I was chosen is not clear to me to this day. At the time I was a web businessman with a background as a lawyer and a more distant background as a mining exploration geologist.

The project began with a stint in the core shack logging core and learning the rocks. From there it went to reading all of the reports, public and private, on the property and creating an annotated bibliography. At this point I tried to incorporate all the recent literature on geologic basics and this type of deposit, in particular. Next it involved trying to solve a specific structural problem. And finally it morphed into a complete reevaluation of the model of the deposit.

Where this page is headed . . . This isn't an official Greens Creek page people at the mine occasionally look at at it. With that in mind, I will try to make it as useful as I can for their purpose. I have also had some comments from folks with a more scientific-academic bent. Some of that has been critical and I am now (12/15/07) in the process of making sure that the page is consistent with their objections to the extent that I recognize their validity, or that their point of view is at least discussed. While this page expresses a point of view and proposes a model, that point of view and model are not beyond question and discussion--and revision. This page should be able to advance the discussion.

Here is a table of some features observed at Greens Creek and a discussion of their past interpretation and a revised model. I will add pictures as well as entries in the future. (There is an issue of whether there is a structural model in use at Greens Creek currently. That will be addressed in the future.

There was no exploration goal in the assignment. It truly was free-form. When I asked how my work was described to management, I was told I was doing things the regular geologists didn't have time to do. At the time, I though the idea was inspired. (I was having fun, anyway.) Eventually someone disagreed and the project was ended in August of 2005. (This coincided with a visit of a group of outside academic "experts," but I am not sure this was the cause.)

Despite the company's ending of support, I continued the project on my own time. They loaned me thin sections to do some petrographic work.

So why is this important?  The Greens Creek deposit is explainable as a rather extreme seafloor collapse, a collapse that has created rocks that appear to be metamorphic in origin--in fact they are metamorphic, phyllites to be more precise.  But the paragenesis of the deposit indicates that the "metamorphism" is penecontemporaneous--that is it was formed at about the same time as the rocks were deposited.  This meshes with recent but halting advances in the understanding of pressure solution as a mechanism to create metamorphic rock textures in diagenetic or near diagenetic processes.  In short, Greens Creek geology is the best example I have right now to demonstrate pressure solution metamorphism.  A more theoretical page on the topic can be found here.

What is the significance of this for mining exploration?  In the case of collapse-related ore deposits, pressure solution metamorphism becomes a guide to ore.  Textures that were once considered post-ore can be recognized as related to the ore depositing event to guide exploration.  In addition, textures and features present at Greens Creek can be related to other deposits to aid in their understanding.  One striking example that has yet to be fully developed is the relationship of conglomerates and buckshot pyrite at Greens Creek to similar features in the Witwatersrand gold deposits--deposits that have produced about half of the world's total gold.  Features of banded iron formations such as those in Western Australia, uranium deposits, and stratiform gold deposits such as the Juneau Gold Belt deposits, Kennecott's former Ridgeway Mine, and many others are also explainable with these ideas.  And the implications go well beyond ore deposits. 

Much of this remains to be developed.  This page and the others are works in progress.  I add-to, revise and change what is here as I think about it and come across new information.  I invite comments and even participation, as you will see at the bottom of this page.   

A Note about Notes

To the extent possible, I have included sources for the information contained on this page. You can find more about these in an annotated bibliography at the end. I have also included notes information about the sequence of development of ideas. Those notes can be found here. Both of these are under construction.

On to the Substance . . .

Greens Creek is a polymetallic silver-rich volcanogenic massive sulfide deposit (VMS or VHMS for Volcanic Hosted Massive Sulfide deposit) located in they Late Triassic Hyd group. The Hyd contains sedimentary and volcanic rocks and is interpreted as having been deposited in a back arc basin.

This is a KGCMC-produced area geology map that was given to me before I did any work at the mine. If you click on it a more detailed pdf version is linked. (A. West presentation, 2004) This is a cross section that was given to me at the same time as the map. Green is the stratigraphic footwall phyllite and gray is the hangingwall mine argillite. By the trace of the contact, one can get a hint of the apparent complexity of the geology. (A. West presentation, 2004)

Here are some grade and reserve numbers from Freitag (2000). I expect that the total is about twice this now based on a presentation I heard last year. I have never been privy to grade and reserve information.

Below are two stratigraphic section cartoons showing the approximate relationship of the units. The first thing to note in these sections is the assumption that the Mine Argillite is stratigraphically higher. As can be seen in the cross section above, some of the section is overturned, some is right side up and some is on its side. I am not aware of definitive direction indicators in the rocks. Perhaps the assumption is based on the "normal" sequence where VMS deposits are found at the end of volcanic cycles--that is where volcanics stop and sediments begin.

This stratigraphic section shows the lithologic units using the mine's terminology and abbreviations. MA stands for"Massive Argillite" and SA for "Salty Argillite." MFB stands for "Massive Fine grained Base metal sulfide," WCA for "White CArbonate ore". There are other ore designations as well. In the Phyllite, SP is "Sericite Phyllite" and SPc is "Sericite Chlorite Phyllite." There are other designations as well. I will provide more information as this page is fleshed out. (Source, USGS 1) Immobile element geochemistry shows the footwall rocks to be mafic volcanics. (Newberry and Brew) This version of the section shows some of the structural style and features. As the map and cross section farther up and the screen clip below suggest, the deposit structure is apparently quite complex. The patterns shown on this section attempt to summarize my observations. Overall, deformation increases significantly down section with a radical increase just above the ore horizon. The conglomerate contains clasts with preexisting foliation and lithology similar to the underlying phyllite but enclosed in an apparently later foliated matrix. Serpentine chlorite clasts are found in some conglomerates. The right-angle fan of foliation and spaced cleavage pattern drawn in the phyllite is my attempt at a characterization of this unit's texture. It is folded as well. (Source USGS 1, Freitag)

Lithologic Descriptions

Lithologies are fairly well described in Freitag (2000) and in USGS 1. Below are is a table from Freitag describing the basic lithologic features:

Additional detail on the ore lithologies can be found in Freitag's Table 5:

Freitag's tables do not do justice to the very important conglomerate/breccia units that are found at and below the ore horizon. I will quote at length from USGS 1:

"The footwall sequence at Greens Creek contains the discontinuous presence of what are variably described in the drill logs as silicified conglomerates and breccias. They occur as 0-30 meter thick lenses of dense, white to gray, silicified quartz-carbonate, and cherty clasts in a siliceous matrix. The appearance of the clasts suggests that their protoliths were derived primarily from the underlying phyllites and less prevalently, from the footwall carbonates. Clasts are generally 1-10 cm in size and are either distinctly sub-rounded or sub-angular, suggesting that there may be at least two types of breccias. Those containing sub-rounded clasts tend to be polymictic and are likely conglomeratic breccias that formed as locally derived debris flows in response to the onset of late Triassic rifting. The sub-angular breccias that constitute the majority of these footwall accumulations are monomictic and are specially associated with the proximal footwall SR lithology. They are probably tectonic breccias produced by fracturing of the SR's after ore formation. Some of these are called foliation breccias by the mine staff, formed from the intersection of S2 and S3 foliations. A third less common type of breccia has been observed in a few locations underground. This is a polymictic breccia in the immediate hangingwall (or is it phyllite?) that consists of slightly smaller (1-5cm) clasts of predominately white quartz pebbles and argillite in a matrix of fine to coarse crystalline, subhedral pyrite. The best example to date of this type is located in the 720 ore access ramp of the upper Southwest orebody within about 20-30 meters of the ore haningwall contact. This breccia, here interpreted as a conglomeratic lens that formed as a debris flow during deposition of the hangingwall, is distinctly different in terms of its stratigraphic position, size and composition of the constituent clasts, and composition of its matrix.

"These breccias may have significance both as indicators of of the tectonic environment at Greens Creek and as a possible stratigraphic marker horizon. * * * "

(emphasis added)

List of Deformational Events under the Standard Greens Creek Structural Model (Source USGS 1):
  • S1 (D1) Foliation noted in conglomerate clasts. Conglomerate is noted as being mostly derived from the footwall phyllites. Foliation is attributed to late Paleozoic regional deformation.
  • S2, F2 (D2) Foliation and folding in all rocks. Foliation is mostly parallel to bedding except near folds identified as F2 folds. (Freitag (2000) describes the S2 foliation is "stylolitic" and of pressure solution origin.) Shearing is observed on S2 planes. This event is attributed to Cretaceous metamorphism.
  • Semi-Ductile Shearing (D2.5) A large shear zone predates later folding.
  • S3, F3 (D3) More open folds--the most recognizable in the mine. Spaced cleavage and crenulation cleavage associated. Attributed to later Cretaceous metamorphism.
  • Klaus Shear: Age uncertain, healed shear may correlate to the D2.5 event.
  • F4 (D4) Similar to F3 folds with parallel fold axes, F4 folds refold F3 axial planes. May be part of progressive metamorphism from D3.
  • Low angle faults: Healed faults truncate F3. Age relation to F4 not known.

Again, Freitag (2000) differs somewhat in her interpretation of the structure. She lists an additional deformation of early low angle thrusting between D1 and D2. She does not address D4. But perhaps most importantly, she attributes the very extensive ladder veining that is so important in the Mine Argillite unit--and as a guide to ore--as being diagenetic. A photo is shown below the questions section. The mine's model attributes ladder veining to a Cretaceous D2 event, I believe.

I am looking for public copies of diagrams of the structural interpretation of the geology. They are shown in public on a regular basis, but I don't have copies right now.

So as we close the "standard model" on structure structure and lithology, we have lithologies that indicate rather radical tectonic activity at the time the rocks were deposited--yet we have a structural model that does not reflect a bit of this. Can this be right?

Aside . . .

Is there an existing model that is used at Greens Creek or is it just a mapping scheme? Is it proper to use designations such as D1 and D2?

It has been pointed out to me that recent internal company reports don't use "D" designations at least in part because they suggest events that may not in fact be real. I agree with this change in use. Ds prejudice the system against consideration of progressive deformation. That said, Ds have become part of the public understanding of the deposit structure. They are used in Freitag's thesis and in the USGS summary. Not only that, they are a reasonable inference from the use of designations such as S2, F3, etc.

If "Ds" are out, is there a "standard model" at all? Or does this page just tilt at a straw-man model where none exists? Is it possible that the existing understanding is consistent with progressive penecontemporaneous deformation? Of course if a fold is simply measured in a mapping scheme, it is a fold whether it was formed around the time of deposition or in a much later metamorphic event. The mapping is mapping. To what extent does pigeonholing of a fold into a designation such as S3 imply a model? Does speculation in reports about Jurassic or Cretaceous accretionary metamorphic events make a difference to this? Or is it still just mapping? I am struggling with this right now and will make further revisions to address it.

 

A Second Aside—The unconformity issue

When one looks closely at the structure, metamorphism and stratigraphy at Greens Creek, it become apparent that whether or not there is an unconformity at the ore horizon is important. For example, are foliated clasts in the ore horizon conglomerate subject to a prior regional metamorphic event? That is, are they Paleozoic? Or are they Triassic suggesting penecontemporaneous foliation or emplacement as clastic intrusives? Or is there some other explanation? While some age dating has implied that the footwall is Triassic, questions have remained about the true stratigraphic placement of the samples and the reliability of the dates. This should be resolved when Patrick Sack publishes his PhD work on the footwall. My personal expection is that they will show Triassic dates.

On to some rocks . . .

An underground "Rib" map showing some of the complexity of the argillite just above the ore. (Freitag)

And another "Rib" map:


A photo--keep this in mind when you are reading below . . .

This is one complicated mess . . . or is it?

A Contention: the geologic understanding of the Greens Creek Mine is in a crisis. The current model--to the extent that there is a model--appears to rely on a large number of deformational events yet doesn't begin to explain the apparent complexity of the deposit. (Note 1)

A new model--or a model in the first instance--is needed and should simplify the understanding of the deposit, not add more layers of complexity. It should address the following . . .

Questions . . .

These are some questions that I think need to be answered if an understanding of the geology of Greens Creek is to be advanced:

  • Why does the metamorphic event that affected Greens Creek seem to be centered on the ore body while the enclosing rocks are less metamorphosed or even unmetamorphosed? (See Newberry et al., photo at bottom of this page)
  • If conglomerate units indicate tectonic collapse, why isn't this tectonic event reflected in the structure? Or is it?
  • Why does the deformation increase down-section with a big increase at the ore horizon? (See Newberry, et al)
  • What processes caused the intense deformation (including ladder veining) in the ore horizon? (See Freitag)
  • What processes formed the conglomerates? Are some or all of them pseudoconglomerates? Could they be mud volcanoes? (Note 2)
  • If the footwall is Triassic, then how can foliated footwall derived conglomerate clasts make any sense?
  • Why are delicate, yet undeformed textures such as aragonitic textures and olivine pseudomorphs found in the ore zone? (Note 3)
  • Why do minerals such as marcasite occur in mineralized zones? (Note 4)
  • What is the distribution and significance of the extensive carbonate alteration on the property?
  • Is the Serpentine Chlorite a stratigraphic unit? (Note 5)

Calcite after aragonite. (20mm wide.)

Above: Primary sulfide depositional textures, Upper Southwest Orebody.

Right: Photomicrograph of ladder vein and stylolite. Note author's interpretation that these veins are diagenetic. That is at odds with other workers.

From Freitag, 2000.

An Hypothesis: The deformation at Greens Creek is penecontemporaneous--it happened about the same time the rocks were formed.

As I attempt to elucidate below, if one recognizes that the deformation--meaning foliation, folding, and early faulting occurred at about the same time as the rocks were formed, the structure of the deposit is understandable as a simple but intense seafloor collapse. There are other factors in play, but the great bulk of the apparent complication is attributable to one event. The complex structure is suddenly simple.

 

So, now to support this--to answer the questions . . .

(or present the beginnings of answers--this is a work in progress . . . )

 

It May Look Like Soft Sediment Deformation, but Can That Be True?

A respected Alaskan geologist commented to me that he had taken a tour of Greens Creek in the 1980s and had seen "wild soft sediment folding" in the ore zone. (C. Freeman, personal communication, 2005) He isn't the only one to think that the deformation occurred while the sediment was still soft. Notwithstanding the appearance of the rocks, the available papers cited here uniformly state that this is not the case--or at least their conclusions hold that high P-T barrovian greenschist facies metamorphism is responsible for the deformation.

The issue of penecontemporaneous deformation in VMS footwalls is an old one. Many of the geologists that have worked at Greens Creek are familiar with the debates on these matters and I am told that the issue may have been considered. Perhaps that is why Curt Freeman came away with the impression he did. While there are vague reports of consideration--and reports that consideration of penecontemporaneous deformation is appropriate, I am aware of no mention of it in any report--certainly not in any public report.

I would like to back up for a bit. In order to recognize soft sediment deformation, one needs to understand what it is. It is not simple and it is not widely taught in the "hard rock" programs that mining geologists favor. Certainly--or unfortunately--it is not something that metamorphic geologists spend much time with. And it is also not something that happens just at the surface. The diagram and photos to the right show some aspects of soft sediment deformation--notably the continuum of conditions under which progressive deformation of sediments occurs. These come from Maltman (1994). Some might recognize the rocks as reflecting the structural and textural styles of Greens Creek.

There have been occasional undisputed reports of soft sediment deformation at Greens Creek. A truncated soft sediment fold was recognized in core above the deposit. (J. Proffett, personal communication 2006, A. West, personal communication 2007).

Here is an interesting historical vignette from Maltman on interpretation and misinterpretation of soft sediment deformation.

From Byrne, Figure 8.1, in Maltman ed.

From Byrne, Figure 8.6(a), in Maltman ed.

From Byrne, Figure 8.7(b), in Maltman ed.

From Martenson, Fig 5.42, in Maltman ed.

From Maltman Figure 9.41, in Maltman ed.


For comparison.

What about Foliation . . .

The intensity of the foliation in the footwall troubles some geologists with whom I have raised the idea of penecontemporaneous deformation. Surely such intense foliation must be caused by high grade metamorphism--or must it? First of all, the understanding of what causes foliations is gradually--I would say haltingly--changing. Now it is widely recognized that bedding plane foliations elsewhere--the S2 foliation at Greens Creek is a bedding plane foliation--are likely to be the result of diagenetic processes rather than isoclinal folding events. (See Passchier and Trouw 2005). Spaced cleavages and and crenulated foliation patterns similar to those found at Greens Creek are noted in rocks that have essentially no metamorphic recrystallization. (Borradaile, et al. 1982) Beginning in the late 1970s and early 1980s this style of deformation began to be attributed to pressure solution. (See Gray 1979)

The interesting thing about pressure solution as a cause of metamorphic textures is its independence from the normal constraints of pressure and temperature. Instead of P and T as facilitating factors, one is looking at porosity and permeability as important factors as well as such things as grain size, mineralogy, and stress and strain directions. And pressure solution is recognized as an important process in diagenesis. The idea of pressure solution metamorphism is developed further on this page.

So what about the application of this at Greens Creek? So far, the work done at the deposit seems to me to have bypassed the implications of large scale pressure solution metamorphism--including the very significant loss of volume it implies. (For example, Freitag describes the microscale deformation as being attributed to pressure solution, but applies seemingly constant volume ductile solutions to the larger scale features. Diagenesis is recognized on the micro-scale, but apparently ignored as a factor in larger scale deformation. The S2 bedding plane foliation is attributed to Cretaceous isoclinal folding, as are the associated soft sediment-appearing folds in the ore zone. This is a recipe for geological confusion and that is exactly what it has produced.

Simple and logical consideration of paragenesis may be able to confirm this. More work is needed on this point

When one recognizes that the foliation formed at about the same time the rocks were deposited and lithified, the geology of this apparently impossibly complex deposit becomes understandable and actually fairly simple. Occam's Razor has application here to be sure..

Above are two photomicrographs from Freitag (2000) of foliations in Greens Creek footwall rocks.

To the left are two photos from rocks of the Bull Formation in New York. Despite the foliation and crenulation, these rocks have undergone little if any metamorphic recrystallization. A portion of the author's comments relating to this are copied below the photos. From Borradaile, et al. 1982.

A closer look at the ore . . .

Like any mine, most of the study at Greens Creek has been concentrated on the ore zone. It is the best exposed in workings as well. In this section, I will look at some of the enigmas of the ore zones.

At least some of the ore at Greens Creek is reputed to be concentrated in fold noses. One conjecture is that it was moved there during metamorphism. Partly this is based on the prior conjecture that the folding is metamorphic, thus any mobilization must be metamorphic. On the other hand, Freitag reported that most of the textures in the Lower Southwest orebody were primary textures and described ore recrystallization as pre-deformation. Clearly there is some thinking that remains to be done.

Here is an example of a cross section from the Lower Southwest Orebody:

Below are some photos and diagrams relating to Greens Creek ore. With each, I have written some explanation to attempt to elucidate the issue.

As noted elsewhere on this page, by far the most thorough and useful study of Greens Creek ore is that done by Katja Frietag in her PhD dissertation from the Colorado School of mines. Though she did this work at the mine, it has not received much attention there. And much of it conflicts in detail with other work or working assumptions used by mine staff.

First and significantly, Freitag found that the majority of textures in the Lower Southwest Orebody were primary depositional textures. That is they were textures resulting from seafloor sulfide deposition. Freitag's figure to the left shows some of these features.

Here is an example of folded sulfide that I collected and a photomicrograph made from this piece. The rounded spongy looking minerals in the photomicrograph are framboids. And framboids are primary depositional features. (This rock is of interest because the folding resembles what one would expect from soft sediment deformation. If the rock was recrystallized--which it is not--then soft sediment deformation would be a less likely explanation.)
Freitag's study indicates that primary textures have been reworked. She attributes this to sedimentary reworking. Here is an example.

But is that the case?

Below, I will examine some examples.

To the right is a sample of ore from the 200S orebody. The appearance is that massive sulfides have been injected into cracks in veined, but not ductilly deformed, massive argillite. It doesn't look sedimentary and it doesn't look metamorphic either.

(Recall here that Freitag determined the ladder veining to be early--of diagenetic origin.)

Here us a closer photo of a chip sawn off of the piece above. The sulfides seem to be eating away at the argillite. They also seem to be banded.
Here is another larger scale example from the 5250 Orebody. Again the sulfide seems to be eating away at the argillite--and it looks as if it is flowing.
Here is a photo showing a sulfide band with apparent flame structures at the top (a soft sediment structure) and a row of dominos in line with the pencil. It appears that flow in the more fluid center part caused the dominos and folding in the lower layer.
To the right is a section of sawn massive fine grained base metal sulfide ore--MFB in the mine parlance. Next to it is a polished section taken from this piece. Of interest here are the pyrite grains (the bright grains in the photomicrograph). They are rounded. This was noted by Freitag as well. The small black patch in the lower left of the sawn sample is argillite that contains framboids. A halobia fossil was found in such a xenolith.

Based on the examples above, the simplest explanation for all of these textures is redistribution of sulfide mud in a near subsurface soft sediment environment. Progressive lithification may help explain some features too.

Typically at Greens Creek, massive sulfides are strong and competent in core or large faces while surrounding rock may be folded and broken. The photo at the right from the Upper Southwest Orebody shows some of this. The deformation in the argillite barely penetrates the massive sulfide. An explanation for this could be that progressive lithification of the sulfide caused it to no longer react as a mud. The D3 deformation evident at the top of the photo is a result of pressure solution. Since the sulfides are not amenable to pressure solution deformation, they have escaped the intense deformation of the surrounding rocks.
While flow of sulfide mud may seem attractive from what is presented above, it is by no means accepted. The more accepted explanation is that metamorphism moved the sulfides during a high P-T event. While I have expressed above that I think the logic for this is dubious, we should look at some of the theory. Under normal conditions, sulfides are among the most brittle and strongest minerals.

To the right is a graph from Cox, S.F., Ethridge, M.A., and Hobbs, B.E., 1981. The experimental ductile deformation of polycrystalline and single crystal pyrite. Econ. Geol., 76:2105-2117. It shows the very high temperatures and pressures required to deform sulfide minerals. An interesting thing to note is the inclusion of marble on the graph. Much of the surrounding rock, including the massive argillite, is in fact a carbonate rock. Above I have shown a piece of aragonitic textured carbonate. Clearly these have not been converted to marble.

So what did Freitag conclude?

The figure to the left is a summary of her interpretation of the sequence of sulfide formation. The essence of it is that almost all of it occurred prior to deformation--or at least D2 and D3, the pressure solution-based deformations. This is completely consistent with the idea of sulfide mud and soft sediment deformation. They only try sulfide deformation she observes is the bent galena cleavage planes. As can be seen in the graph above, Galena is the most likely to be impacted and the temperatures and pressures are attainable in sub-seafloor situations.

As an aside here, many of Freitag's findings were at odds with the "established models" for Greens Creek, yet she chose not to make any significant reinterpretation of the deposit structure. I expect this would have been very difficult for her considering the stature of the geologists who developed the models.

So what do I conclude? It seems apparent that the sulfides have moved. In some cases they moved as muds, in some they moved by soft sediment deformation of partly lithified rocks, and in some cases they were brittlly deformed. (There is evidence of duplexing in the sulfide masses that I have yet to document here.) In microscopic detail and in macroscopic form, the ore bodies are consistent with penecontemporaneous progressive deformation--that is deformation in a sea floor collapse.

 

 

 

Now to step back to the larger scale . . .

 

Above are two screen captures from a Hecla promotional video on the property. The brown is the East Ore--the first area mined on the property. SWB (or Southwest Bench) is misrepresented on this drawing. This is essentially the ore completely separated from the rest of the geology. None-the-less, it gives one an idea about the complexity--and the continuity--of the ore.

These models are the best I have but they do not reflect some of the morphology that is apparent when manipulating the mine's more powerful program. Apparently, there is a central keel that is almost flat lying just above the SW Ore and the 200S Ore.

To the right is a cartoon (to be revised) that shows the post collapse relationship of the units. To restore the cartoon to the current situation, one would rotate it 90 degrees in a clockwise direction. The central keel is the tongue of argillite in the center. What is not shown on the cartoon are the melange ores--the Upper Southwest Ore and the 5250 Ore. These were highly disturbed during lithification. Their location is in the center of the keel and they are separated from the argillite-phyllite contact in places.

 

Discussion . . .

This is a work in progress, so this section--as are the other sections--is subject to change.  I apologize if some of the sections above have not caught up to this one.  Above I contended that the geologic understanding of Greens Creek faces a crisis.  These are the points that lead me to that conclusion:
  • Because the structure and metamorphism observed in the immediate mine area are inconsistent with regional observations, regional metamorphism is unlikely to be the cause.
  • Textures and structures in the immediate vicinity of the mine are inconsistent with the dynamic greenschist metamorphism postulated as the cause of the deformation.
  • The postulated multiple metamorphic events--including a dynamic event with a principle stress direction perpendicular to bedding--are geologically unlikely.
  • Microscale observations of mine rocks are inconsistent with macroscale interpretations--e.g., diagenetic veining and dissolution in the Mine Argillite are given no place in interpretations; pressure solution is recognized on a microscale, but the volume loss implications of this process are not recognized on a macroscale.
  • Tectonic events suggested by lithologic units--i.e., conglomerates--are not reflected in the structural interpretations. 
  • Outcrops are simply unexplainable under the established model (see the Rib Maps above).
  • Evidence that the footwall is Triassic is ignored because this data conflicts with the structural model.
  • Evidence that the ultramafic rocks on the property are contemporaneous with deposition is ignored--probably because no mode for their emplacement can be imagined.
Solving Katja Freitag's Problem Using Penecontemporaneous Deformation . . .

In her PhD dissertation, Katja Freitag analyzed and "unfolded" the Southwest Orebody. She found that carbonate and sulfide deposits were separated--that is there was a mound of sulfide ore in one location and a mound of carbonate ore in a nearby location. A diagram is shown at the right. Her solution to this problem was to infer two feeders, on depositing sulfide and one depositing carbonate--probably at different times. Of course there was no real evidence of these feeders in the footwall--they were just inferred. There were little problems like bits of sulfide here and there in the wrong place, but those were ignored. The model is here:

A much simpler solution to Freitag's problem would have been to allow for the possibility of penecontemporaneous deformation. I don't have a diagram yet, but it is easy to imagine that the sulfide was simply pushed off of the carbonate shortly after deposition. This has been noted at other deposits, but is not considered at Greens Creek.


Above I suggested that the understanding can be advanced if it is recognized that the structure is penecontemporaneous.  Below, I will approach this hypothesis with two lines of logic, first, a deductive approach--what we would expect to see given geologic first principles: and second, and inductive approach--what we would conclude based on the detailed facts of the situation. 

What we would expect to see . . .

  • Beginning with the notion that Greens Creek is a VMS deposit, we know that this sort of deposit occurs at the end of a volcanic cycle--or at least the end of a sub-cycle.  In most instances, footwall volcanic rocks are overlain by sediments deposited in a quiescent environment.  (This inference is so strong at Greens Creek that it is the basis for assuming the stratigraphic direction.)
  • Collapse is to be expected at the end of a volcanic cycle. 
  • Collapse will manifest itself by gravity slumping at the surface and soft sediment deformation in unlithified and partly lithified strata below the surface.
  • Newly deposited partly lithified sediments and volcanics would be highly porous and permeable and would be very amenable to the effects of diagenetic pressure solution.  Shear stress and compaction stress would enhance pressure solution.
  • Dissolution of substantial quantities of rock by pressure solution would be expected to further enhance collapse and could cause folding and early stage faulting (now healed) in partly lithified rocks. 
  • Diagenetic pressure solution would result in foliation of phyllosilicate-bearing rocks, but would not foliate rocks with substantial proportions of minerals resistant to pressure solution.  E.g., sulfide-rich layers would not be subject to pressure solution and would appear relatively undeformed.  In addition, layers that have low porosity or permeability would escape pressure solution deformation.  Examples of this could be aragonite layers.  Any feature or structure that is resistant to pressure solution would be preserved.
  • Conglomerates and intraformational breccias may be formed.
  • Significant movement on and between beds would be expected.  Because rocks were in the process of induration when they moved, it may be difficult to discern the movement.
  • Folding would be coaxial since deformation would be progressive and related to a single event.  We would expect a fair divergence of attitudes given the nature of the event. 
  • Sheath folding would be expected in soft or partially lithified sediments.
  • Clastic dikes would be expected.
  • The collapse movement in combination with pressure solution could obliterate the larger textures of feeders, but might allow preservation of some remnants.
  • Because of the closeness in time, parts of the alteration system may be contemporaneous and overprint the deformation.

There are no doubt other effects that would be predicted--and I can't deny that my observations may have affected my list.

What we see . . .

  • The deposit is stratiform massive sulfide at the contact of volcanic rocks and sedimentary rocks suggesting that it is a VMS type deposit.  Abundant syngenetic depositional textures--e.g., more than half of the sulfide textures in the Lower Southwest ore body--confirm this.
  • Deformation increases down section.
  • In thin section, S2, the principle foliation, is stylolitic--i.e., caused by pressure solution.  S3 is also stylolitic and pressure solution related.  (Freitag 2000)  Pressure solution is a major factor in deformation of this deposit. 
  • D2, D3, and D4 are approximately coaxial, suggesting progressive deformation.
  • Metamorphism in the mine area is much more intense than surrounding areas suggesting either different processes or different effects for this area.
  • There are others too.  I will try to add them later.

So far, the penecontemporaneous idea seems to be winning, hands down.  So what about the counter arguments? 

The Evidence Against Penecontemoraneous Deformation

There are several lines of evidence that have been suggested as refutations of the ideas discussed here.  Primarily, these have to do with recrystallization--or dynamic recrystallization--suggesting P-T conditions in the greenschist metamorphic range.  Much of this remains unresolved at this point.  Freitag believed that a static annealing event may have recrystallized--or partly recrystallized--the massive sulfides.  Notwithstanding that, she found that most textures were syngenetic. 

There are occasional grains of actinolite approximately parallel to foliation in the phyllites.  I am not sure of the origin of these.  Several possibilities occur to me.  One might be that they are related to the annealing event.  Another is that they are remnants of propylitic alteration zones.  In general, the fine grained sericite and chlorite that are prevalent in the footwall phyllites do not have metamorphic overgrowths and are amenable to and interpretation of low P-T formation. 

Another objection relates to deformed quartz.  There is some undulatory extinction and even some chess board texture in quartz in the conglomerates.  At this time, I am not sure what to make of this.  Quartz deformation theory is controversial--at least so far as I am concerned.  Den Brok and others have suggested that low temperature deformation is possible.  And the conditions for such deformation may be especially favorable here where much of the quartz is calcedonic.  In any event, I do not see this objection--if that is what it is--as overcoming the very significant body of evidence suggesting penecontemporaneous deformation. 

In essense much of the objection remains amorphous. The idea will not be considered just because. Or it is asserted that it has been considered and rejected--even though there is no indication in the reports. Or the ideas are consistent with current understanding so we can move on. All of these are unsatisfying, of course. In some sense that is how science works. People consider what the want to consider.

Metamorphic geologists, in general, are fairly ignorant of the nature and extent of soft sediment deformation and diagenesis. Instead they have concentrated their studies on high temperature and pressure phenomena. This is starting to change, but the change is very slow indeed. Many very serious misconceptions remain--some have yet to be addressed in even the most cursory fashion. Greens Creek presents an opportunity to address fundamental issues of metamorphism, diagenesis, and soft sediment deformation . . . and to understand this world class ore body at the same time.

Explain this . . .

Is this sulfide recrystallized?

I don't think so.
Much of the pyrite is framboids.

Above is Triassic Hyd Group pillow basalt from the Gambier Bay area of Admiralty Island. Pillows are also found in the immediate area of the mine as are other primary depositional features that have somehow managed to escape metamorphic obliteration. But then again, metamorphic obliteration may not have happened. (Source: BLM) (It has been suggested to me that this locale is so far away from Greens Creek that it is irrelevant. The fact remains that regionally the Hyd is not a highly metamorphosed unit.)

Future work--other deposits

As I noted above, I believe that the observations on this page have importance well beyond Greens Creek. I hope to develop these themes:

I have collected some information from massive sulfide deposits in the Hyd Group--Windy Craggy and Woewodski Island. I hope to post information on these soon. I am also in the process of scheduling a trip to the Bathurst Camp to support and discuss the ideas expressed here.

Conclusion--A False Dichotomy

When I was looking at Greens Creek thin sections with Rainer Newberry, he loaned me the "Atlas of Deformational and Metamorphic Rock Fabrics" and suggested I consider the "metamorphic" explanation for the structure. (I had considered this some time before--after all, that's what I was taught when I worked there.) But I took the book and looked for textures that resembled those at Greens Creek. I found quite a few that were strikingly similar. I looked deeper. Some of the Greens Creek-like textures were from rocks that were not recrystallized--i.e., not metamorphosed at all in the sense that was being advocated. The references pointed to pressure solution as the cause of the foliations, spaced cleavages, and crenulations. I learned that pressure solution doesn't require recrystallization or even induration. In fact, because it needs fluids and fluid pathways, it may well do better in rocks that are less mature.

So the bitter debate about the nature of VMS footwall deformation--'soft sediment' or 'metamorphism'--may have been a false dichotomy. Both sides may have been right. But the crux is that the metamorphism was not what people thought. And, in a sense, the soft sediment deformation was not what people thought either. Because of the way pressure solution works, soft sediment deformation and metamorphism--or formation of the sorts of metamorphic textures we see at Greens Creek--can occur simultaneously and produce textures structures and fabrics characteristic of each simultaneously. When one recognizes this fact, this complex deposit becomes structurally relatively simple and the knotty problems that had be be ignored can now be explained.

Bibliography

[Under Construction]

The official company web page is here: http://www.greenscreek.com/default.html. There is virtually no geology here.

Borradaile, et al., Atlas of Deformational and Metamorphic Rock Fabrics, Springer-Verlang, 1982

Hecla Mining Co., Video promoting Greens Creek.

Freitag, Katja, 2000, Geology and Structure of the Lower Southwest Orebody, Greens Creek Mine, Alaska, PhD Dissertation, Colorado School of Mines. At this writing, Freitag's thesis is the most comprehensive source of basic information on the geology of the property. It includes most of the cross-sections, long sections and plans of the Southwest ore bodies. The work in interesting in its approach. Frietag assumes the validity of the standard constant volume, high P-T metamorphism in her work, but at the same time provides conflicting evidence in her detailed observations.

Gray D.R. Microstructure of crenulation cleavage: an indicator of cleavage origin // Am. J. Sci. 1979. Vol. 279. P. 97-128.

Maltman, A. (ed), The Geological Deformation of Sediments, Chapman Hall 1994.

Newberry, R.J., Crafford, T.C., Newkirk, S. R., Young, L.E., Nelson, S.W. and Duke, N.A., Volcanogenic Massive Sulfide Deposits of Alaska, in Economic Geology Monograph 9, Mineral Deposits of Alaska, 1997. This contains a brief summary of the deposit reflecting some information as it was known on the date of publication.

Newberry, R.J. and Brew, D.A., Chemical and Isotopc Data for Rocks and Ores from the Upper Triassic Greens Creek and Woewodski Island Volcanogenic Massive Sulfide Deposits, Southeastern Alaska. USGS Professional Paper 1614, pp 35-55.

Passchier and Trouw, Microtectonics, 2d ed, Springer 2005

M. Satre 2004 presentation. A presentation to the Anchorage convention of the Alaska Miners Association.

USGS1. The USGS has spent considerable time preparing a volume by Survey people, mine geologists and consultants that is expected to be more than 300 pages and be published in their Professional Paper series. Much of that work was completed a few years ago and I am told that it may be published soon. It will include almost all of the geologic data and interpretation as of about 2002. I look forward to publication of that "sanctioned" volume should assist this page to move forward as well. I have obtained the introductory chapter of this work from UAF. Points from that work are referenced as USGS1.

A West 2003 Presentation. This is one of many presentations made by mine staff.

The ideas expressed on this page are not in any way endorsed by Kennecott Greens Creek Mining Company, its staff, or its parent companies.

Your Comments Here--email to twelker@alaska.net:

I have some--hopefully they will be up soon.

Some folks have asked for more petrography. Here is a link to a report on the topic. It is a bit dated--I hope to fix some things in it soon. Still it might be worth looking at.

In response to comments, I have added more on foliations.


One person commented that the crystal structure of clays and micas could be important in understanding the deformation. This raises and interesting line of inquiry that needs to be pursued further. The catalyzing action of phyllosilicates is probably important in understanding the action of pressure solution. Boles, et al., recently described low temperature pressure solution of quartz and feldspar in contact with muscovite. Dissolution rates began in the range of 1mm per year in their experiment. This is very high indeed. The movement over a significant volume of rock over even a brief time would amount to huge change in the rock. At Greens Creek, the important phyllosilicate in the area of the ore is "sericite." Sericite isn't a mineral per se, but probably we are looking at fine muscovite today. Freitag described annealing that would have converted clay minerals such as illite to muscovite in any event.

With regard to the concept of penecontemporaneous deformation, there are a couple of things that might be considered in regard to Boles' findings. Was there muscovite formed in the alteration related to the hydrothermal event that formed the orebody? And would illite or other clay minerals have the same catalyzing effect that Boles found for muscovite?

I noted an interesting example of deformation related to mineralogy in the portal outcrop at the mine. Here a zone of very sericitic light colored phyllite cuts across darker chloritic phyllite at a high angle to the foliation. The light colored rock is very much more deformed than the dark. ET 3/14/07

 

Comments invited here too: http://www.erictwelker.com/coldspot.htm

Page by Eric Twelker

First posted December 10, 2006

Update December 15, 2007 added aside on mapping and metamorphic events plus other minor changes.

Update January 24, 2008 added aside on unconformity issue.

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