Learning Center What is a mineral?The most common minerals on earth Information for Educators Mindat Articles The Elements The Rock H. Currier Digital Library Geologic Time

Minerals by Properties Minerals by Chemistry Advanced Locality Search Random Mineral Random Locality Search by minID Localities Near Me Search Articles Search Glossary More Search Options

The Mindat Manual Add a New Photo Rate Photos Locality Edit Report Coordinate Completion Report Add Glossary Item

Mining Companies Statistics Users Mineral Museums Clubs & Organizations Mineral Shows & Events The Mindat Directory Device Settings The Mineral Quiz

Photo Search Photo Galleries Search by Color New Photos Today New Photos Yesterday Members' Photo Galleries Past Photo of the Day Gallery Photography

Discussions

💬 Home 🔎 Search 📅 Latest

╳Discussions

💬 Home 🔎 Search 📅 Latest

Groups

Recent Images in Discussions

GeneralPossible injection of fluidized sand into vesicles in basalt.

11th Jun 2017 15:36 UTCLarry Maltby Expert

I have puzzled over a group of agates and vesicle fillings that I collected some years ago. A possible explanation has emerged thanks to a professional abstract that Uwe recommended in a previous thread. Here is a photo of one of the agates that I recently posted. I am trying to figure out what the material below the diagonal line is and how it was deposited or injected into the vesicle.

Figure 1.

I have shown the entire abstract below and I have highlighted the words that may apply to this specimen. I have at least 20 more specimens like this showing various features that may help with a solution. Much more to follow.

Sequential opening and filling of cavities forming vesicles, amygdales and giant amethyst geodes in lavas from the southern Paraná volcanic province, Brazil and Uruguay

L, A. Hartmann, et-all, Journal: International Geology Review, 2012, Volume 54, Number 1, Page 1

“The opening and filling of cavities in rocks are the major processes related to the generation and sealing of porosity in ore deposits. This study documents three stages of opening and filling of vesicles and geodes in the basalts and rhyodacites of the southern Paraná volcanic province. Each step detailed here is actually part of a sequence of minor hydrothermal events. First, lava degassing at high temperature (1150°C) formed small (<4 cm) vesicles in the crusts of flow units. In sequence, these vesicles were partly to fully filled at low temperature (30–150°C) by hydrothermal minerals, particularly clays and zeolites; this process also sealed the porosity of the lava. Second, the injection of fluidized sand generated new cavities, which were partly filled with sand; the newly formed porosity was sealed by the low-temperature fluid. Third, intense alteration of the basalt or rhyodacite core into a claystone favoured the opening of small to giant protogeodes (0.1 mm to 4 m) by dissolution; cooling of the fluid led to the precipitation of hydrothermal minerals, particularly the spectacular amethyst, calcite, and gypsum-bearing geodes.”

During my research on this subject, I came across references to clastic dykes that are filled with sediments and intruded into other formations. Some of the dykes are filled with basaltic sand. All of the specimens that I will show here were found adjacent to a vertical dyke in the host basalt. Here is a reference: https://en.wikipedia.org/wiki/Clastic_dike

Please let me know what you think!

11th Jun 2017 23:32 UTCRalph S Bottrill 🌟 Manager

Interesting Larry. Clastic or sandstone dykes normally form in sedimentary rocks during soft-sediment deformation, usually in semi-consolidated clayey beds which can crack during earth movements, and the crack fill with less consolidated sand falling in from overlying beds. It's a bit more difficult in basalts, but these commonly form strong, deep vertical joints and columns, and you could envisage these opening up during earth movements and allowing unconsolidated sediment to fall in and fill them with dyke like structures. Any unfilled vesicles adjacent to joints could be filled with this sediment. Alternatively, vesicles can fully or partly fill with smectite clays deposited from slow groundwater infiltration along microfractures, and maybe this is what we see, now silicified? But if can can see sandstone dykes in basalts we would love to see the photos?

12th Jun 2017 09:25 UTCJason Bennett

The other possibility is that these sandy components could be sandstone clasts (possibly poorly consolidated?) that were picked up during eruption...

In line with Ralph's request for further photos, in-situ textural evidence would be really useful to help distinguish each hypothesis here...are the agates collected in-situ or weathered out somewhere?

12th Jun 2017 15:17 UTCLarry Maltby Expert

Ralph, Thanks for your comments.

This geological detail has not been professionally recorded in the Geology of Michigan. I need help to better interpret what we see and I would like to correctly describe these specimens in the Mindat data base.

Unfortunately I do not have a good photo of the vein in-situ. Like most field collectors we get so involved trying to read the terrain looking for a good spot to prospect that we forget we have a camera. Also, I did not realize the significance of these specimens at the time. I do think that I can supply enough photos to define the geology. The issue will likely be, is the material in the vein and in the vesicles sediment or a basaltic intrusion?

Jason, Thanks for your comments also. We look forward to your views on this.

Here are photos of the uncovering of the agate shown above. I was right in line with the vein but the vein was removed during excavation.

Figure 2.

This is the top of the agate rotated 180 degrees. The jagged edge has been cut away for the photos.

This is the top of the agate rotated 180 degrees. The jagged edge has been cut away for the photos.

This is the top of the agate rotated 180 degrees. The jagged edge has been cut away for the photos.

12th Jun 2017 17:12 UTCLarry Maltby Expert

This agate was attached directly to the vein. Other agates with partial filling of intruded material were not observed to be touching the vein but were near to it. Some vesicles were completely filled with the intruded material. Some have tiny micro-agates included in the filling making the analysis more complex. All will eventually be shown here.

Figure 3.

This is the exterior of an agate attached to an intrusive vein.

This is the exterior of an agate attached to an intrusive vein.

This is the exterior of an agate attached to an intrusive vein.

12th Jun 2017 18:27 UTCGregg Little 🌟

Larry, considering the heat and fluid around in the systems you are describing, this seems quite likely an injection mechanism would be plausible for your location. Is the "injected" portion as granular as it looks in the photo? This could point to it being clastic material. I think the key to this discussion is to determine the composition of the intruded material then the mechanism can be explored.

It appears the system was still very active after the "injection" as there is further agate(?) filling the void space inside the "injected" area. Also note that the agate part and the "injected" part could represent fault reactivation with the fill representing stages of fracture opening.

In the oil exploration field we see similar evidence where gravity-fill is not the sole operator. There are structures like sand dykes and even sand volcanoes. In these cases the sand bed is water saturated and confined above and below causing it to pressure up with compaction (settling, increase overburden, etc). Any fracturing or faulting allows a sudden release of water and sediment into adjacent beds or units either above or below.

13th Jun 2017 15:19 UTCLarry Maltby Expert

Thanks Gregg,

With your and Ralph’s comments I think that we will be able to come up with a reasonable explanation for these specimens. I have attached below a photo of the contact between the intrusive material and the host basalt. As you can see the contrast is significant. I plan to lap the specimen to 1200 grit and take micro-photos of each texture. Photos to follow.

Your comment on how sediment can be fluidized and pressurized probably explains how the vesicles near the vein were infilled. At the time that we were collecting we did not know about this and could have easily missed a feeder vein. Vesicles three or four feet away from the vein were completely filled with agate only. (8.5 x 5.0 x 8.0 cm)

Figure 4.

15th Jun 2017 16:15 UTCLarry Maltby Expert

Here is another specimen of interest. In addition to the agate, the remainder of the vesicle contents in the photo below is likely a fine grained siltstone. The nodule was found associated with what appears to be a clastic dyke with a system of veinlets that distributed sediments including silt, mud and sand into the adjacent empty vesicles in the host basalt.

The best way to see this is to enlarge the bottom portion of the specimen and note the texture and flow lines. There is even some indication of stratification. The porosity is likely the result of liquefaction that aided the transportation of the materials through the vein system. Some of the porosity was later filled with the deposition of quartz.

Figure 5.

17th Jun 2017 15:27 UTCLarry Maltby Expert

Ralph,

Here is a specimen that shows what I now think is a confirmed clastic vein passing right through a filled vesicle in basalt. It appears that when the basalt fractured the crack passed through an empty vesicle offsetting the two halves by about 0.5 cm. The view is of the slightly concave bottom with the left and right ends broken off. The face view of both ends shows the domed shape typical of a gas bubble in a viscus lava. As you can see the material in the vein looks like sand. (9.0 x 9.5 x 3.5 cm) The vein width is about 2.0 cm.

Figure 6.

The Lake Shore Traps are interbedded with conglomerate, sandstone and mudstone. These formations found on the beach of Lake Superior at one time covered the traps but were eroded away by glacial action on the inland ridges. It looks like a source of sand was available to fill the fracture and the color is similar.

Figure 7.

Jason,

I forgot to answer your question. Yes, all of the specimens were collected in-situ.

21st Jun 2017 16:51 UTCLarry Maltby Expert

The reference below shows a clastic dyke found in the Late Precambrian, Keweenawan, Osler volcanics that out crop on the northern shore of Lake Superior. It apparently was discussed on a field trip conducted by David Bee, a professor at Penn State University.

http://www.swanson-hysell.org/2012/01/20/featured-field-photo-bifurcating-clastic-dike/

23rd Jun 2017 23:35 UTCRalph S Bottrill 🌟 Manager

Very interesting material Larry. It's always hard doing long range geology, but it may be worth showing the samples to a geologist if you can find one locally?

24th Jun 2017 04:20 UTCPaul Brandes 🌟 Manager

I've been following this thread with great interest.

Very interesting to say the least, Larry! I have seen the clastic dykes of the Osler Group many years ago and we as a group marveled at how you could get a clastic (sediment) dyke in an igneous rock. The article you provided is one of two ways we postulated as to how you could possibly get this occurrence. One was indeed lava flowing over a wet sediment and boiling the water to steam. Once the steam pressure is high enough (think phreato-magmatic eruption), the slightly cooled lava cracks and sediments infill the fissure. The second way we thought of is a situation similar to sand blows you sometimes see during strong earthquakes whereby saturated sediments due to liquefaction are injected into any crack or fissure it can find in the already cooled basalt, then lithify over time.

As you stated earlier Larry, this is one of the many pieces of Keweenawan geology that has yet to be fully understood. To me, this begs for a research project to be initiated.

24th Jun 2017 06:19 UTCGregg Little 🌟

Larry;

As Ralph says it is difficult doing geology at a distance but I have some overall impressions to share. There appears, as seen in your June 15th post, to be at least two stages of dyke injection defined by the two areas of presence and absence of vesicles. The initial injection was the vesicle bearing sediment evident from formation against the fracture walls and its occurrence as semi-consolidated clasts floating in the vesicle free(?) second stage injection. The angular shape of the floating vesicle-riddled clasts reminds me of shale rip-up clasts in the sedimentary environment. Part of the basalt wall rock also appears to be ripped-up by the second stage injection.

I would also seem plausible that the first injection was more gaseous; higher temperature and greater pressure release (hence vesicles) followed by the second injection being slower(?) and probably lower pressure resulting in little or no vesicles and more of a laminar flow texture. Again this is just speculation from a remote perspective.

26th Jun 2017 17:25 UTCLarry Maltby Expert

Ralph,

I do hope to review these specimens with several geologists. I have about 30 specimens of this type. They are all very interesting and I will use this thread to show many more photos during this summer. I have added figure numbers to all of the photos to facilitate discussion.

Gregg,

Regarding your comment on figure 5, I see the “flow lines” that I think that you are referring to. I have added the photo below to illustrate some additional information. When I look at the two halves of the specimen, it appears that the fracture (4.5 cm wide) destroyed a preexisting vesicle. The dotted lines depicting the width of the vein can be seen as flow lines in figure 5. I also added my guess as to the direction of flow. (Particles of the host basalt were pulled into the vein.) The space in which the agate formed may have been preserved by compressed air/gas during the intrusion of the sediment.

Figure 8.

Paul,

I agree that more work is needed to define he mineralogy and the geology of the Traps. I have a lot of material that I am trying to get into the Mindat data base. I better hurry, I turned 80 years old this spring.

26th Jun 2017 18:46 UTCJolyon Ralph Founder

I think it's important to correct a possible misunderstanding in the title of this thread.

Larry refers to liquefied sand which, to me at least, suggests it is molten.

The reference cited refers to fluidized sand which to me, at least, suggests sand moving in a fluid-like manner (eg as it does when pouring through an egg-timer)

26th Jun 2017 20:29 UTCLarry Maltby Expert

Thanks Jolyon,

Point taken. Can I edit the title of the thread without messing it up?

Larry,

26th Jun 2017 21:14 UTCJolyon Ralph Founder

You can!

27th Jun 2017 15:26 UTCLarry Maltby Expert

I have just uploaded these photos to document the cross-cutting vein.

Figure 9.

The cross-cutting vein indicates that the alteration is secondary to the intrusion of the sediments. Laumontite is the most common zeolite found in the Lake Shore Traps ranging in color from white to orange and a dark brownish red.

27th Jun 2017 19:38 UTCGregg Little 🌟

Larry; Where you say "The space in which the agate formed may have been preserved by compressed air/gas during the intrusion of the sediment.", I think I might have to clarify my earlier comment on the stages of the fluidized sediment intrusion. I don't think it is necessary to invoke a preservation method for the agate nodule. Stages of formation as follows; 1) emplacement of basalt flow and vesicle formation, 2) agate deposition/solidification in vesicle(s), 3) fracturing and injection of first event fluidized sediment ("frothed" or sediment with abundant gas cavities), and finally for this specimen, 4) fracture reactivation and injection of second event of fluidized sediment (un-frothed or sediment with few or no gas cavities).

The first fluidized sediment injection might be relatively lower in viscosity due to its gassy character and possibly not have a tendency to mechanically include fragments of nodules and wall rock (basalt). The second fluidized sediment injection could be more viscous and with reactivation of the fracture would provide fragments of the previous sediment intrusion and weaken wall rock material to be included mechanically in the flow. I think the flow lines in the second intrusion indicates a slower rate of intrusion (just an impression). It would also be interesting to analyse these flow lines for composition and hence their formation. A cursory examination, to me, indicates a re-absorption of the included (first) fluid sediment injection fragments as there appears to be dark "tails" streaming from the fragments in the down stream direction, as you have indicated in the photo (figure 8).

Another observation; the gassy sediment fragment with your left-side arrow on it, may not have been transported but by-passed because the fracture wall formed as indicated by your dotted line.

If my ramblings seem a little "out in left field", please remember that fracture geometry is complex (not to mention the material that fills it), we're viewing a 2-D "time-slice" and all this speculation is from a photo.

27th Jun 2017 19:54 UTCGregg Little 🌟

Larry; I just had a small epiphany, if there is such a thing as a small one, from Jolyon's earlier comment. If the first sediment injection was due to gas (steam) then we have fluidization of the sediment. If the second injection was due to liquid water suspension then we have liquefaction of the sediment. That may explain the perceived viscosity difference of the two stages of injection as well as the second eroding the first. May be I am getting carried away here. Lets see what other have to say.

Jolyon; You are such a source of inspiration!

28th Jun 2017 14:06 UTCLarry Maltby Expert

Gregg,

Yes, when you hold the two halves of the large specimen shown in figure 5 and 8, and look at them in 3-D, it becomes apparent that the fracture pattern is very complicated. To really discuss this one we would need to be seated at the table together. The take away from this specimen for me is that the clastic material looks like lithified silt. In my younger days I was an avid trout fisherman and I studied the aquatic insects that trout feed on. This material looks just like the waterlogged silt that I have dredged up from the back waters of trout streams.

The next group of photos that I plan to show will illustrate what appears to be sandstone in veins and vesicles.

29th Jun 2017 19:11 UTCLarry Maltby Expert

Figure 10.

This nodule was found near a vertical clastic dyke and shows evidence of clastic veinlets on the surface. The veinlets contain sandstone that completely filled the vesicle in the basalt.

This is a close-up of the sub-angular grains that make up the sandstone in the veinlets. The veinlet is about 2.5 mm wide.

This nodule was found near a vertical clastic dyke and shows evidence of clastic veinlets on the surface. The veinlets contain sandstone that completely filled the vesicle in the basalt.

This is a close-up of the sub-angular grains that make up the sandstone in the veinlets. The veinlet is about 2.5 mm wide.

This nodule was found near a vertical clastic dyke and shows evidence of clastic veinlets on the surface. The veinlets contain sandstone that completely filled the vesicle in the basalt.

This is a close-up of the sub-angular grains that make up the sandstone in the veinlets. The veinlet is about 2.5 mm wide.

Figure 11

Inside the nodule the sandstone grains are clearly visible and show significant alteration due to silicification including the presence of micro-agates.

This is a close-up of the grains that make up the sandstone inside the nodule. The grains appear to be derived from the brownish red rhyolitic pebbles and cobbles that are very common on Keweenaw beaches and as clasts within the Copper Harbor Conglomerate.

Inside the nodule the sandstone grains are clearly visible and show significant alteration due to silicification including the presence of micro-agates.

Inside the nodule the sandstone grains are clearly visible and show significant alteration due to silicification including the presence of micro-agates.

The photo below shows some of the geological environment. The Lake Shore Traps are interbedded in the Copper Harbor Conglomerate. This photo was taken at the water’s edge. You can see the brownish red coloration in the loose gravel and also in pebbles imbedded in the exposed sandstone. The origin of the large clasts of what appears to be felsic amygdaloidal basalt is a mystery. All of the known flows are mafic and dark in color.

Figure 12

29th Jun 2017 23:12 UTCGregg Little 🌟

Larry;

I am not sure what is felsic, the amygdule mineralization or the ground mass of the basaltic clasts? Also these larger clasts don't seem to have much depth but I suppose that could be the wave erosion but if they are thin (when looking "into" the rock face), they could be the first phase of the sediment injection that coats the walls of the fracture, if the fracture orientation is in the plane of the photo, if you catch my drift? The red colour could be alteration from the warm/hot fluids of the second stage injection.

30th Jun 2017 15:36 UTCLarry Maltby Expert

Gregg,

Here is the definition of felsites/felsic in the Mindat data base. The term felsic is sometimes attached to basalt to refer to silica rich basalt as opposed to (mafic) iron rich basalt.

“Felsites (acid volcanic rocks) tentatively identified as rhyolite, with felsic minerals comprising >20% quartz and alkali feldspar/plagioclase 40-100%” (Ref. Mindat rock data base)

Another name that would fit these large clasts would be “amygdaloidal rhyolite” but I did not find that in the Mindat data base.

By the way this photo was taken on the beach shown in figure 7 above.

Mindat.org is an outreach project of the Hudson Institute of Mineralogy, a 501(c)(3) not-for-profit organization.
Copyright © mindat.org and the Hudson Institute of Mineralogy 1993-2024, except where stated. Most political location boundaries are © OpenStreetMap contributors. Mindat.org relies on the contributions of thousands of members and supporters. Founded in 2000 by Jolyon Ralph.
Privacy Policy - Terms & Conditions - Contact Us / DMCA issues - Report a bug/vulnerability Current server date and time: April 25, 2024 08:46:32

Go to top of page