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Author Topic: What does an epithermal precious-metal prospect look like?  (Read 24891 times)

Offline Bass

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I've been spending quite a bit of time (when not chauffeuring horses about the country) working on a new mining prospect.  Unique geology and potentially huge- could contain several million ounces of gold! :o

We are still just in the initial exploration stages on this project- it covers an area over 2 square miles in size and based on the steep relief the vertical extent is at least 1000 feet.  Unfortunately, or perhaps fortunately, the terrain is covered in dense brush with very little outcrop.  Unfortunate, because it makes interpreting the geology very difficult; fortunate, because it would have been found and mined years ago if it was obvious.  So we are in what I call the "Victorian Lady" stage of exploration- intrigued and excited by what we see on the surface, but until we remove all those bloomers, petticoats, bustiers and corsets, we have know idea what's really underneath all those layers.

I thought descriptions and updates on this project might be of interest to NSF, and any feedback or questions are welcome (some information can't yet be divulged, for obvious reasons). 

The setting is in the northern Rocky Mountains, forested with steeply incised stream valleys (no to little glaciation) and extremely brushy.  Best outcrops are along roadcuts:



What first attracted our attention was the presence of quartz veins (white) containing stibnite (black- antimony sulfide):
 
« Last Edit: 07/11/2008 22:21:41 by Bass »


 

Offline Bass

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Examination of the prospect revealed a huge area of epithermal (shallow hot water) precious-metal (gold-silver) mineralization and alteration (alteration of the original minerals in the rocks to something else due to the passage of hot water).  One of the characteristics of epithermal systems is open-space fracture filling, that is, the rocks are shallow enough that fractures remain open spaces which are then filled by minerals precipitated from the circulating hot waters.

Shown here is "quartz drip stone"- a rare occurrence (I've only ever found it one other mining prospect).  Similar to stalactites, the quartz is formed in open space fractures as the solutions containing high silica drip from the roof of the fracture.  Not only is this a sure indicator of epithermal deposition, it also shows the orientation of the vein when the quartz was deposited.



Open-spaced filling is also indicated by the "striped" appearance of the vein- as the fracture opens, quartz or stibnite is deposited, then opens more with additional deposition, and so on.




High grade gold and silver assays show that this is indeed a precious metal deposit.  So far, we have assays ranging up to 1200 ounces/ton silver and 0.61 ounces/ton gold.  (ounces of metal per ton of rock).  Almost everything we sample has low grade (0.03 oz/ton) gold and silver.

As time allows, I'll post more pictures with captions on one of the photo sharing web sites and will further reveal the geology, mineralization, alteration and project progress.
« Last Edit: 02/09/2008 22:56:49 by Bass »
 

Offline Bored chemist

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Very interesting.
Do you have a geiger counter?
The tin deposits in parts of Cornwall (UK) are associated with uranium deposits etc.

(I'm now waiting for someone to tell me that they Sn and U  are physically close to each other but not geologically connected or some such. I don't mind being wrong, just as long as I learn something.)
 

Offline Bass

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Both tin and uranium are found in highly fractionated granites- which are usually high in F and rare earths.  The intrusions here are more calc-alkaline in nature (little F). 
I suspect a geiger counter would show low-level U and Th radioactivity- but what really interests me is radioactive potassium.  These mineral deposits tend to be associated with high potassium granites- so a multi-channel spectrometer could be very useful in identifying potential targets by identifying areas enriched in potassium.
 

Offline JimBob

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Gosh, Bass, I'm impressed.

 

Offline Bass

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Background on the property:

A local prospector started investigating the area in the 1950's.  While prospecting in the creeks, he found cobble size pieces of quartz and antimony.  Following up the creek, he finally found an outcropping vein in the stream.  After his discovery, he staked claims and started a careful grid search of the area, which turned up more than a dozen additional veins.  He has been working the claims for over 45 years, in which time he has driven several tunnels, dug trenches, opened roads to new areas and drilled over 20 holes.

His accomplishments were no easy task.  His initial prospecting was into an area of no roads (the closest highway was about 7 miles away) in steep, brushy country.  The area was thick with downfall, and the creeks were overgrown with brush and Devil's Club (nasty stuff).  Even after his discovery, he had to hike in, lay out lines and meticulously try to cover the ground- all steep, brushy, full of downfall with very little outcrop.

Truly a remarkable story.  I don't know many people that could have done this- amazing job.

Over the years, several mining firms have examined his claims.   Everyone zeroed in on the high-grade (and very rich) antimony veins and the sporadic silver veins.  Basically no one looked for gold- but to be fair, at the time gold was less than $50 and of little interest except for high-grade bonanza type veins.  Most of the geologic mapping showed the veins, some scattered granites, and the "country rock", or ordinary rock surrounding the veins.  Very little attention was paid to the country rock- because it had no value.

Between the mining companies, the prospector continued uncovering more veins and would drill shallow holes to confirm their existence underground.  He eventually drove a couple of short tunnels and actually shipped good ore to a local mill, for which he was well paid.

A year ago the prospector hired me to try to put together the all the various bits of information he had accumulated over the past 45 years, and to help him with some state and federal permits.  He showed me some spectacular specimens of the vein and lots of pictures of mining, trenching and drilling- along with handing me a room full of maps, reports, assays and drill logs.

Finally got on the property in late spring this year.  During examination of one of the large veins, I wandered over into the country rock and was astounded to find hydrothermal breccia.  (Breccia is angular broken rock, hydrothermal refers to hot water (probably superheated in this case)).  And the breccia was full of goodies (metallic minerals and altered rock).

Astounded because nowhere in the mountain of data was there any mention of this breccia.  The prospector told me that was just the "country rock" and could be found everywhere on the claims.  Extremely skeptical, I asked if we could tour the rest of the property... only to find he was right.  Almost the whole property, about 2 square miles, is breccia, with veins and intense alteration (change in the rock from hot waters). 

I have now convinced him that he is on a massive gold-silver system, that the breccia is meaningful, and to be on the lookout for limonites (iron oxides) or "brown shlt" (as he calls it).

Next, pictures and descriptions of the breccias.
« Last Edit: 11/09/2008 04:11:38 by Bass »
 

Offline Bass

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Breccia: rock made up of highly angular coarse fragments, commonly cemented by a fine-grained matrix (Dictionary of Geologic Terms, AGI)

Altered fault breccia near a major fault.  Note the random orientation and mix of clasts (fragments).  Clasts include calcareous mudstones, quartzites and intrusive rocks (granitic).  This specimen shows intense alteration (or change due to the passage of hydrothermal fluids).  The matrix has been changed to a "felty" mass of mica and pyrite; the clasts have been changed to clay + quartz.

 

Ghost fragments in a quartz matrix.  Hydrothermal breccia- quartz replaced most of the fragments.  This rock contains low-grade gold.  Matrix is composed of quartz, pyrite and sericite (mica).


 

Offline Bass

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Breccia continued.


Intensely altered hydrothermal breccia.  The fragments are all completely silicified (changed to quartz).  Note the corroded edges of the fragments and a few round fragments.  Matrix is a mixture of hydrothermal alteration minerals- sericite, epidote, chlorite, actinolite, pyrite and quartz.  Also contains low grade gold.



Close up showing corroded fragments with sericite-epidote (green) filling in.  Note rounded fragment in upper left- rounding may be caused by rotation of the fragment or by replacement.

 

Offline Bass

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Fragments in this specimen have not been rotated and are close to in-place.  Green is mixture of diopside-sericite-epidote, white is quartz.  The original rock is a calcareous mudstone and sandstone.  I suspect that the green layers are mudstone, white layers are sandstone- both of which have been altered.  This breccia may have formed when the rock was deposited (intraformational).



Compare that to some of the unbrecciated (but still altered) country rock



As I said before, this breccia covers about 2 square miles- HUGE!  I believe the area is intensely faulted and was then intruded by shallow granitic rocks, which allowed the minerals to be deposited by superheated hydrothermal fluids.

One of the intriguing problems with such a large area, especially with little outcrop, is to find the ore zones, or centers of mineralization, which can be quite small in comparison.  Now, if I could just remember where I laid those x-ray glasses???
 

Offline Bass

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The geology of this project is still evolving.  Here's what we've pieced together so far:

Precambrian (older than 500 my) sedimentary rocks host the mineral deposits.  These rocks have been intensely faulted, intruded by a complex of shallow granitic rocks, and extensively altered.

The northwest boundary of the mineralization-alteration is a large (tens of miles) northeast-trending regional fault (gray).  I believe this fault caused a series of east-west oriented wrench faults (red) in the prospect area- I have identified at least 4 and possibly up to 8 of these faults, but with the limited exposures, there may be more.  These east-west faults caused the breccias to form and focused the intrusion of granitic rocks.  Northwest and northeast trending faults (green) are both concurrent with the east-west faults, and they crosscut those faults (indicating they also formed later).



Antimony, silver and gold mineralization is most noticeable along the e-w faults and is especially strong where there are intersecting faults.  The faults acted as conduits for the hot mineralizing fluids.  Alteration of the wall rock also gets more intense as one approaches the faults.

I have identified at least 4 different granitic rocks in the project- one of which, a white subvolcanic granitic rock known as aplite, may be responsible for the mineralization and alteration.

We have identified several gold anomalies, including one quartz-gossan zone with visible gold.  So far, we have been concentrating exclusively on the south side of the prospect, mainly because it has better access.  According to the owner, the north half of the property shows as many veins as the south half (next year's project).  Geologic mapping is continuing, along with magnetic surveys, radiometric surveys and several IP (induced polarization) lines.  The owner has a small backhoe, which is great- so we can dig small trenches to "create" outcrops in critical areas. 

note to bunny-huggers: the trenches are filled and reseeded immediately after examination and sampling
 

Offline Bass

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What does an epithermal precious-metal prospect look like?
« Reply #10 on: 07/11/2008 22:48:33 »
Weeks of lugging around a bulky gamma-ray spectrometer (the thing uses 12 D-cell batteries) was more than successful.  The spectrometer reads gamma ray emissions from radioactive elements and can distinguish between K, U and Th (potassium, uranium and thorium).  K-enrichment is a common side effect of mineralizing hydrothermal fluids, but also in granitic rocks (K-feldspar).  U and Th, while present in all rocks, can help distinguish igneous rocks, faults and weathered areas.

This worked better than my best hopes- K readings jumped in vein zones and in altered zones.  K was particularly high in rhyolite intrusives, which can be discriminated using U and particularly Th.  This was really useful in picking out faults and fault intersections- since they provide an excellent plumbing system for both minerals and granitic rocks.  We were able to confirm several suspected faults and find several new vein zones.

Assay results on a few grab samples of aplite are encouraging- .03 to .05 oz. Au/ton (1 to 2 g/tonne for you tea sippers). 

Also stumbled onto a couple of banded quartz-adularia veins.  These veins are associated with high grade Au and Ag in other low-sulfidation deposits; don't have any assays yet, but one mined out area ran >1000 oz/ton Ag.

Also, there is a distinct circular geomorphic feature surrounding the property.  This circle is no doubt to due to some sort of volcanic or subvolcanic process- but needs further investigation.

Will update when more assays arrive.
 

Offline frethack

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What does an epithermal precious-metal prospect look like?
« Reply #11 on: 08/11/2008 00:29:01 »
Is it rare to find deposits like this?  Seems like an awful lot of gold and silver.
 

Offline Bass

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What does an epithermal precious-metal prospect look like?
« Reply #12 on: 08/11/2008 01:30:02 »
Rare indeed, which is why finding a choice plum like this untouched is almost unbelievable.
 

Offline Alandriel

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What does an epithermal precious-metal prospect look like?
« Reply #13 on: 09/11/2008 21:18:41 »

This makes amazing reading.

Must be hugely exciting to be out there and getting more and more results.

Keep those updates coming  :)


 

Offline Karen W.

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What does an epithermal precious-metal prospect look like?
« Reply #14 on: 10/11/2008 02:35:33 »
I recognize mountainous areas just like those.. looks like a range of mountains up north of me a ways..

Bass This is a good thread.. Nice information... I have done a wee bit of panning and enjoyed it.. it is rather labor intensive and fun if only for a weekend... LOL Kudos to your friend he has spent a great deal of his life doing some very back breaking work.. I hope this all "Pans out" for him!! Nice work You are doing.. enjoy the scenery.. It is awesomely beautiful!

 Some of those rocks with all the quartz and brown and grey in them seem to me I have seen.. around this country.. Alot of miners up yonder....

I have a deed or Claim here somewhere For the three Sisters Mine..up Oregon way, that my grandma and cousins owned.. I still have the deed but do not know if family even own any of it these days as I thought one had to continue to work the mines in order to keep them going and hold their claims!

Good luck with your work.. I will enjoy further reading...
« Last Edit: 10/11/2008 02:42:26 by Karen W. »
 

Offline Bass

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What does an epithermal precious-metal prospect look like?
« Reply #15 on: 10/11/2008 04:56:32 »
I recognize mountainous areas just like those.. looks like a range of mountains up north of me a ways..

Bass This is a good thread.. Nice information... I have done a wee bit of panning and enjoyed it.. it is rather labor intensive and fun if only for a weekend... LOL Kudos to your friend he has spent a great deal of his life doing some very back breaking work.. I hope this all "Pans out" for him!! Nice work You are doing.. enjoy the scenery.. It is awesomely beautiful!

Placer gold is also called "free" gold- but, like your experience, I've found that there is nothing "free" about panned gold- backbreaking work for sure.

Quote
Some of those rocks with all the quartz and brown and grey in them seem to me I have seen.. around this country.. Alot of miners up yonder....

Keep track of where you see stuff like that... you're in some great mineral country- you just never know what you might turn up.

Quote
I have a deed or Claim here somewhere For the three Sisters Mine..up Oregon way, that my grandma and cousins owned.. I still have the deed but do not know if family even own any of it these days as I thought one had to continue to work the mines in order to keep them going and hold their claims!

If you have a deed, you may actually own a patented claim.  You should check it (there will be property taxes due if it is patented).  If the claim is unpatented, you would have lost all rights by now.  Do you know what was mined on the claim?
If you want to email me the particulars on the location of the claim (name of claim, claimant, township, range, section)- I'll be happy to try to check it for you.
« Last Edit: 10/11/2008 05:01:34 by Bass »
 

Offline Karen W.

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What does an epithermal precious-metal prospect look like?
« Reply #16 on: 10/11/2008 06:46:25 »
I saw some up on the mountain a few months ago where my friends funeral service memorial was held.. In a large river that is cut into the side of a canyon like wall naturally of course... about I would say easy 500 ft high Rock wall. In an area much like your picture at top!

I'll dig it out here tomorrow and send you the information.. double check the name too!
 

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What does an epithermal precious-metal prospect look like?
« Reply #17 on: 10/11/2008 15:27:02 »
Wouldn't bringing all this gold onto the market depress it's value?
 

Offline Bass

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What does an epithermal precious-metal prospect look like?
« Reply #18 on: 10/11/2008 19:44:53 »
Wouldn't bringing all this gold onto the market depress it's value?

Would be but a drop in the bucket!

Also, it takes years (decades) to permit, develop and mine something this large.
 

Offline Bass

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What does an epithermal precious-metal prospect look like?
« Reply #19 on: 24/12/2008 02:26:00 »
Thought a holiday update was in order.

As previously mentioned, this prospect is located on the northwestern side of a large (4 km radius) circular feature.  Further examination of the area shows 2 additional, adjacent, similar sized circular features along an E-W trend.  These circular features are undoubtedly volcanic in origin, one even shows classical ring and radial structures.  The E-W trend is not obvious on the surface, or in satellite photos- it probably reflects older basement fault structures that served as a locus for volcanic activity.  The E-W veins seen in the project area may reflect this basement structural orientation.

Part of our work next summer will involve exploring all three circular features for possible mineralization/alteration- as well as more closely inspecting the circular feature containing the project, especially the center and edges.

At least 5 different subvolcanic intrusive rocks have been identified in the project area.  Intrusives include:  1. true rhyolite, 2. quartz rhyolite porphyry (refers to larger crystals of quartz in a rhyolite matrix), 3.  quartz-orthoclase rhyolite porphyry, 3. quartz latite, 4. hornblende syenite (intrusive rock high in feldspar but low in quartz), 5. aplite (or alaskite where crystals are more coarse).  Additionally, there are fragments of earlier quartz monzonite, diabase and later lamprophyres.  The aplite is always silicified, with fine quartz veinlets and contains pyrite in veinlets and as disseminations- and runs 0.03 to 006 oz/ton Au everywhere we have sampled it.

Samples from quartz-adularia veins (adularia is a hydrothermal feldspar common in gold deposits) run between 0.16 to 0.30 oz/ton Au (ore grade already!) in the few places we have sampled it.  More assays are pending, and should help us more accurately locate some of the mineralized zones.  Gold appears to be associated with pyrite in the early E-W faults; silver and antimony are probably later and are found along both the E-W faults and along NW trending faults.  Ore is especially rich near the intersection of these fault trends.

Good news, results continue to be encouraging!!!

Have a merry Xmas!
« Last Edit: 24/12/2008 02:28:35 by Bass »
 

Offline LeeE

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What does an epithermal precious-metal prospect look like?
« Reply #20 on: 24/12/2008 22:53:29 »
Keep the reports coming Bass.  Following the development of this prospect is very enlightening.  It shows not only what is being discovered, but also the reasoning behind the conclusions.
 

The Naked Scientists Forum

What does an epithermal precious-metal prospect look like?
« Reply #20 on: 24/12/2008 22:53:29 »

 

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