Home PageMindat NewsThe Mindat ManualHistory of MindatCopyright StatusManagement TeamContact UsAdvertise on Mindat
Donate to MindatSponsor a PageSponsored PagesTop Available PagesMindat AdvertisersAdvertise on Mindat
Minerals by PropertiesMinerals by ChemistryAdvanced Locality SearchRandom MineralSearch by minIDLocalities Near MeSearch ArticlesSearch GlossaryMore Search Options
The Mindat ManualAdd a New PhotoRate PhotosLocality Edit ReportCoordinate Completion ReportAdd Glossary Item
StatisticsThe ElementsMember ListBooks & MagazinesMineral Shows & EventsThe Mindat DirectoryHow to Link to MindatDevice Settings
Photo SearchPhoto GalleriesNew Photos TodayNew Photos YesterdayMembers' Photo GalleriesPast Photo of the Day Gallery
Stunning gold specimen
Posted by Larry Maltby
Larry Maltby February 14, 2017 05:45PMThis stunning gold specimen was up-loaded this morning. Would some of our geologists please comment on its formation?
Is the center area quartz?
Is the gold deposit sequential?
Is the gold band a replacement like copper banded agate?
Holger Hartmaier February 14, 2017 08:21PMFrom the photo alone I can't tell if this is a nodular type formation (i.e. an "agate") or a tight folded vein. The gold mineralized zone looks remarkably consistent and conformable. Some close-ups of the gold mineralized zone would be interesting to look at to see what the bordering mineralogy consists of and the associated structure. Certainly very unusual. I've never seen anything like this before.
Kyle Beucke February 14, 2017 10:28PMI have been reading about the Goldfield district. See:
Much of the rich ore consisted of altered volcanic rock fragments coated with bands of fine-grained quartz, gold, bismuthinite, copper sulfosalts (famatinite, goldfieldite, etc.), etc. Each band represents a successive stage of deposition.
Based on the descriptions in the literature, I would guess that this specimen consists of a core of altered (silicified) volcanic rock coated with a band of gold (+quartz?), then that in turn coated with dark sulfides (bismuthinite, copper sulfosalts?), and possibly later bands as well.
The gold was described as being very fine-grained, and in rich ore was so dense that the quartz had the color of yellow paint (but without metallic luster, because the gold was in tiny grains within the quartz).
Kyle Beucke February 14, 2017 10:35PMTo reiterate, I think the deposition sequence is from the inside (immediately surrounding the presumed altered rock fragment) out. A closeup of the gray, inner core might show a trace of the texture of the volcanic rock (shapes of phenocrysts).
Gregg Little February 14, 2017 10:48PMReading the description for the sample indicates that the deposit is epithermal. This means that the ore formation occurs in relatively shallow (less than 1 km) and cooler (less than 300 degrees C) depths in hydrothermal waters associated with volcanic, and by inference, underlying plutonic terrains. The heat sets up a convection system the drives mineral/element bearing fluid through a plumbing system (fractures and veins).
There will be zonation of the minerals (deposited in layers) in the fractures, or any type of opening, as the characteristics of the ore bearing fluid changes; for example pH, sulfur activity, oxidizing to reducing, temperature, elements or compounds in solution, etc. The gold layer represents the fluid conditions at that time, the core material (quartz?) represents a different stage of the fluid's composition. The gold is probably not replacement but primary ore deposition.
Gregg Little February 14, 2017 11:27PMIf the core material is not mineralization as indicated by Kyle (silicified host rock) then the zonation development still holds but instead is coating the fragments (breccia?). The host rock and surrounding country rock also goes through various types of alteration of which quartz is prominent in the mineral assemblages.
I think Kyle is right and although the picture is blurry when zooming in, there appears to be relic structures (phenocrysts?) in the fragment as well as preserved fractures and an alteration halo (lighter rim) forming a border around the outer portion of the fragment.
Edited 1 time(s). Last edit at 02/14/2017 11:38PM by Gregg Little.
Dale Foster February 15, 2017 08:29AMPerhaps before making suggestions of 'fake', you should go and take the time to familiarise yourselves with the other 49 uploaded images of the gold ore from the Florence Mine.
Edited 1 time(s). Last edit at 02/15/2017 08:30AM by Dale Foster.
Dale Foster February 15, 2017 11:27AM
Some of the other pictures showing the Florence Mine ore - intriguing material but doesn't look fake to me.
Rory Howell February 15, 2017 02:21PMThe goldfield district was famous for this mineralization and was one of the most famous and richest gold districts. The ore was extremely rich when it was encountered (hundreds, sometimes thousands of ounces per ton), but was very discontinuous - it was described as "raisins in a pudding" where the high grade pods were small but extremely rich. High-grading by miners was a serious problem because of the grades, and a lot of the material that is on the market probably came from either company geologists collecting unique pieces, or from high graders. The material is sold by the gram and is quite expensive as most is made into cabochons for jewelry.
Goldfield is a very interesting semi-ghost town with many hundreds of shafts over a 4-5 mile area and at one time was one of the largest towns in Nevada - you would not know it now as much of the original town was destroyed by fires and flash floods.
Tony L. Potucek February 15, 2017 03:04PMThe Goldfield district produced an average of 1 troy oz. of Au per ton of ore mined over the life of the district. It is one of the richest gold districts ever mined in the world, based on average grade. Yes, the high grade did indeed look like those photos. The gold is intimately associated with the famatinite etc. sulfide rims around the altered volcanic fragments. In 1984, several geologists and I were allowed to enter the Florence Mine by the owner, Mrs. Martin Duffy, who still lived near the headframe. Our goal was to reach an area near the White Rock shaft off the 7th level of the Florence Mine. In 1948, Newmont mined that area and obtained 14,000 oz of Au from 28,000 tons of ore (0.5 opt average). The ground was taking weight and all timbers were cracked and snapped like broken match sticks, even where reinforced with steel and more lagging. In addition to samples taken, we retrieved Mr. Duffy's overalls, pipe, etc., at the request of Mrs. Duffy. During the Tucson show (Feb. 4), I gave a talk on this trip to an informal group of mining artifact collectors, and I also included underground trips into Delamar and Candelaria, Nevada. It was well attended, and I have to admit, a nice trip down my memory lane.
Uwe Ludwig February 15, 2017 04:37PMInteresting, I'm convinced now that this amazing specimens are natural.
However (may be I read it over), it is not clear for me how these pieces are generated.
Were there at first empty quartz geodes which have been filled with the gold layer and later completely filled with a next generation of quartz or were there quartz noodles which got a layer of gold and which are later empeded with quartz?
Kyle Beucke February 15, 2017 05:07PMGreetings Uwe,
This ore appears to be formed through deposition of gold, sulfides, and fine-grained quartz in layers over fragments of altered volcanic rock (that blotchy gray stuff in the "core" should be altered rock). Successive bands have different compositions, which is why the ore is so cool looking (black bands over, i.e. later than, gold bands, etc.).
Ransome (see link I posted above) wrote about these textures, mineralogy, etc. in great detail.
Edited 1 time(s). Last edit at 02/15/2017 05:08PM by Kyle Beucke.
Larry Maltby February 15, 2017 05:10PMVery interesting specimen! Great comments all.
Kyle, thanks for the reference to the book. I was able to read parts of it until I got timed out. I have added another photo with two arrows pointed to what looks like dissolution channels in the altered rhyolite. Particles of gold have penetrated the rhyolite in these channels. The overall appearance of the rhyolite looks bleached and significantly dissolved. In fact I wonder if the dissolution of the rhyolite created the space between the chunks into which the mineralization progressed. Possible replacement.
Some of the other photos showing the matrix seem to illustrate this point.
Edited 1 time(s). Last edit at 02/15/2017 05:14PM by Larry Maltby.
Kyle Beucke February 15, 2017 05:22PMHello Larry,
Good point. Not an expert, but this is a high-sulfidation epithermal deposit. The "high-sulfidation" part is significant. It implies a greater input of magmatic volatiles and a much more acidic alteration fluid. Porosity resulting from leaching is more significant in these deposits, and ore deposition (supposedly later than alteration) is supposed to take advantage of this open space resulting from leaching. "Vuggy silica" is the extreme product of the alteration; it is basically a sponge of residual silica after the feldspar, etc. is dissolved from volcanic rock. Gold, copper sulfosalts, etc. in high-sulfidation deposits are often concentrated in the "vuggy silica." Lots of deposits in Chile, etc. like this.
Rory Howell February 15, 2017 05:37PMQuickie geology of Goldfield district
Larry Maltby February 15, 2017 05:48PMKyle,
I follow your posts regularly because I know that you study hydrothermal deposits in geological settings that are much different from what we have in Michigan.
I have added a photo of an agate from the St. Louis Mine in Calumet, Michigan to illustrate the power of hydrothermal solutions to dissolve quartz. It would be difficult and dangerous to do this in a lab but nature seems to have no problem.
This is a Tom Rosemeyer agate that shows the dissolution of the outer quartz band and the replacement by copper suspended in recrystallized quartz.
Jim Gawura February 16, 2017 04:43AMThe state of Nevada has a very nice website that contains a lot of information on old mining districts. This is a link to the home page, http://www.nbmg.unr.edu/. Click, minerals and energy, mining, mining districts. type in Goldfield for district and your off. A lot of info. Anything listed that has a blue number on the far left is a hot link. USGS PP 66 can be found here https://pubs.er.usgs.gov/publication/pp66. Dive in and good luck.
Philip Persson (2) February 16, 2017 05:42AMWow, wonderful specimens. As other have alluded to, this is a classic low-sulfidation epithermal mineralization texture. These types of veins form proximal to silicic volcanic rocks like rhyolites and dacites and typically gold will crystallize at the 'boiling horizon' where precious metal-bearing fluids decompress and devolatilize to the point where gold-bearing complexes with chlorine, OH etc are no longer stable and thus native gold precipitates. The banded appearance is another hallmark of low-sulfidation epithermal systems and is caused by repeated sequential precipitation of minerals on walls of a faulty or fracture surface that is repeatedly opened, mineralized, closed, etc over and over. If you cut a cross-section of a pipe at a geothermal powerplant in Iceland or New Zealand, you will actually see identical textures and mineral assemblages with even some little black squiggles that are very rich in Ag, Au etc. What is not well understood now, however, is why gold (and often silver) will often form such discrete 'bands' in larger, otherwise barren epithermal veins... this will take more lab work and understanding of P-T conditions of ore fluids at time of crystallization, as well as reaction kinetics of au precipitation. Anyways, great ore, I'd love to own some at some point from this mine! :)
Kyle Beucke February 16, 2017 06:17AMHello Phil,
I am not a geologist, but Goldfield is considered to be a classic high (not low) sulfidation epithermal deposit. Abundance of alunite alteration (as opposed to adularia and sericite) and presence of enargite and luzonite/famatinite are some of the distinguishing characteristics. That being said, I have seen it described in the literature that the gold stage in high sulfidation deposits is often linked to a later, lower-sulfidation fluid than that responsible for the earlier alteration.
Again, not my field of expertise, but one hypothesis for sporadic Ag-rich bands in otherwise barren quartz (this time in low sulfidation deposits) was put forth by Wilkinson et al. 2013. They found evidence that pulses of magmatic brine are responsible for mineralization in a system otherwise dominated by meteoric water.
It is truly incredible ore, and not surprising it was often highgraded!
Edited 2 time(s). Last edit at 02/16/2017 06:21AM by Kyle Beucke.
David K. Joyce February 16, 2017 01:51PMUwe, if you look at the photo of the hand holding the banded fragment of high grade ore? This is the natural form of the ore. The mineralization is undulating. If you physically slice that specimen horizontally, through gold and examine the result, the slices will exhibit round or curved aggregates of mineralization. Most of the sawn samples shown in the pictures are sawn slices, that were sawn parallel to the wall of the vein to produce the dramatic rounded-curved-appearing slices of mineralization.
My thoughts, anyway!
David K joyce
Larry Maltby February 16, 2017 05:15PMI have up-loaded a photo from the USGS paper that Kyle provided with a link. For the purpose of conversation I have exaggerated the color and contrast.
I was interested in your comment that the area between the “islands” of primary rock, opened and closed multiple times in sequence. Meaning, I think, that the primary host rock fractured in a random pattern allowing hydrothermal solutions to enter the fractures and dissolve away some of the primary rock leaving a mineral deposit based on the chemistry of the solution. The process would then repeat with another sequence of dissolution and the precipitation of a different mineral based on the changing chemistry of the solution. Each time that this occurs, the gap between the “islands” widens and the “island” gets smaller.
This seems to be a reasonable explanation of formation of the above specimen. It also infers that the gold was deposited late in the sequence because the gold borders the “islands” in their final size and shape.
Gregg Little February 20, 2017 02:29AMI find the discussion around the sequence of mineralization somewhat confusing in that it appears to be opposite to the deposit description and samples pictures shown here. The literature describes, "Mined ledges are distinguished by nested heterolithic breccias that display upward clast displacement, and multiple encrustations of clasts by quartz, alunite, kaolinite, Cu-As-Sb-Ag-Bi-Sn-Te-S minerals, and gold.".
In the samples shown, the edges are sharp, corners angular and interiors nearly unmineralized by gold. I would contend that the country rock breccia fragments are altered (quartz, allunite, kaolin, etc.) and often have alteration selvages but in general not leached, dissolved, corroded or reduced to any extent. This would then indicate that the native gold, in contact with the clast, was early in the ore mineralization stage. Although there is some mention in the literature of clast replacement, the main mode of formation is encrustation which indicates the altered clasts were encrusted earlier rather than later by gold which was then followed by the latest stage void filling by gangue minerals.
Kyle Beucke February 20, 2017 03:51AMGregg,
I too think the gold looks like an early layer deposited on the rock fragments, with later minerals being deposited on that (for instance, some photos seem to show needles of what might be bismuthinite deposited on (later than) the gold). Ransome describes ore specimens with varying sequences of ore mineral deposition. In contrast to what we see in these photographs, he suggests that gold and quartz (without copper sulfosalts) was usually one of the later bands. I suppose fracturing (through tectonic means or hydrothermal explosions) at different times during the lifetime of the hydrothermal system could result in different sequences of mineralized bands appearing at different places.
Gregg Little February 20, 2017 09:46AMKyle;
I think you are right there as well, this is a complex ore deposit system. My point was that encrusting ore deposition was the primary mechanism as indicated by the literature. The samples from the Florence and Mohawk Mines seen in this discussion show the encrusting deposition as apposed to leaching of the breccia fragments exteriors making room for gold deposition at a later stage.
I did leave in that part of the abstract statement "nested heterolithic breccias that display upward clast displacement" ("AGES AND GEOCHEMISTRY OF MAGMATIC HYDROTHERMAL ALUNITES IN THE GOLDFIELD DISTRICT, ESMERALDA CO., NEVADA", USGS poster session by Peter Vikre, Robert Fleck and Robert Rye) as an acknowledgement of other modes of ore deposition. It seems "elevated fluid flow velocities" can be determined from the mineralization distribution indicating "episodic and dynamic degassing from a magma chamber directly beneath the main district". Other ore samples if they didn't go to the processing plant probably show the complexity with multiple encrustations and breccia clast replacement occurring in this very dynamic hydrothermal process. The poster also describes alunite and microcrystalline quartz as replacing the breccia clasts as well as the ore minerals. Note that words in quotations are from the USGS poster.
Edited 1 time(s). Last edit at 02/20/2017 10:23PM by Gregg Little.
Jon Aurich March 15, 2017 04:42AMI'm the owner of the specimen and the Mine that it came from, as I see by the questions raised on this specimen, not many are familiar with this species of ore. The ore is not enhanced or fake, if you go to my site, you will find many different species of Epithermal ore that has come from the Florence Mine. Jon Aurich Google Plus.
Jon Aurich March 15, 2017 05:14AMGenerally, on this ore from the Florence, the center kernel or brecciated piece that the band of gold is around will be either Dacite or Milltown Andesite, the gold is of a high fineness, (840 - 860) and the particle size is around 300 mesh and mixed with a Cryptocrystalline Quartz. The high grade ore of the Florence is almost always associated with Famatinite and Bismuthinite, some of the Bismuthinite has well defined Needles, some of the needles are around 3/8 of an inch long, some Specimens have large amounts of Alunite and others are sparce. This ore is of highly acidic Sulphide.
Edited 1 time(s). Last edit at 03/15/2017 05:15AM by Jon Aurich.
Johan Kjellman March 15, 2017 09:44AMJon,
I checked out your site and have a question. I see that you have many pieces in old jewelry. They are old right? If so, how come you know the different localities, is it something that has been in your family long time?
Edited 1 time(s). Last edit at 03/15/2017 11:23AM by Johan Kjellman.
Jon Aurich March 15, 2017 04:16PMHello, The watch fob and one ring are totally original, from around 1907. The other rings are antique, but I took out the original cabs and custom made the High Grade for them, I thought the specimens look better in antique settings. Most of the specimens that you see, I have found, they were found on old tailing piles or backfills inside the Mines themselves, since some of the old Lease era drifts were abandoned and the miners were using candles, their cobbing of ore from waste underground wasn't 100% efficient, so the abandoned backfills would carry High Grade in them. Some specimens that I have from restricted areas, ( Government sites) were found in a map drawer that belonged to The Goldfield Consolidated Mines Company, in it were numerous High Grade specimens that were labeled. High Grade Gold Specimens from Goldfield are very rare and the only property to obtain them currently is the Florence Mine.
Larry Maltby March 18, 2017 02:38PMJon,
The last specimen that you show above is one of the best to illustrate a possible mode of formation. I have been studying hydrothermal brecciation with regards to the hydrothermal alteration that I see in the basalts of Michigan’s Keweenaw Peninsula. I came across a professional paper on “Hydrothermal Breccia Pipe Structures”. Figure 1, shows an excellent illustration of a hydrothermal pipe structure in cross section. It favors the encrustation theory especially near the top of the pipe.
I spent a considerable amount of time going through your website. The combination of gold specimens and the historical artifacts that you have found was really interesting. My wife and I have always been interested in western history. Much or our travel in the west was to historical locations.
Jon Aurich March 19, 2017 03:28AMHi Larry, when you ever come through Nevada, call me ahead of time and I can maybe meet you here at the Florence Mine, I live in Vegas but come up regularly on the weekends. You may find it interesting to come up with your 10 power lense and maybe find some pieces of Epithermal High Grade Gold on the tailing piles yourself.......
Larry Maltby March 19, 2017 12:28PMHi Jon,
I would love to meet up and prowl the Florence with you but age related problems with my wife’s health has confined us to home. In our younger days we would be making plans to road trip to Nevada this summer to meet with you. We have always been big fans of the “road trip”. During our trips over a sixty year period, I have taken thousands of pictures mostly colored slides. Currently I spent my free time digitizing and restoring some of these old photos to upload them into the Mindat data base. It has been a good way to continue enjoying my hobby.
I have learned a lot from the comments on this thread.
Thank for the invite, Larry,
Jon Aurich March 20, 2017 06:34PMHi Larry, I'm sorry to hear that you and your wife cannot come out to Nevada, I guess that we should have met years ago, you would have enjoyed the mines here in Goldfield. I will continue to add more specimens to the site so that you can enjoy it that way..... Thanks for the interest in this type of ore that is unique. Talk soon, Jon.
Tony L. Potucek March 22, 2017 01:01PMHi, Jon,
In 1984, I sampled the Florence Mine with a couple of other geologists. Mrs. Martin Duffy was residing at the Florence Mine at the time and granted permission for us to enter the Florence and cross over into the White Rock Shaft ground via the 7 Level. We encountered two underground barricades, which we had to breach in order to gain access to ground last mined by Newmont in 1948. Mrs. Duffy requested that we bring up her late husband's work overalls and pipe, which we did. I would surmise that Mrs. Duffy has passed by now. Who lives in the house near the shaft nowadays? Thanks.
best regards, tlp
Jon Aurich March 22, 2017 05:54PMHello Tony, I remember when Ruth gave you access, she was very cautious of it and as I can remember. I wish that the barricades were up as I cannot tell if someone is accessing the seventh level of the Florence from what was known as White Rock, in the main shaft of the Florence above the seventh level about 100' of platforms and ladders have collapsed, so I have no (man way) access to the seventh level and White Rock has no access above the seventh.... my family and I Have two residences, the Florence is one of them, and we also have a friend that watches the place when we are not there. Ruth has been gone about 15 years now. The house and the Hoist House have been restored, the Rustler #2 Mine which are the headframe and buildings outside the gate are about 50% restored now. Go to: Jon Aurich Google plus. And you will see hundreds of Nevada historic items and current photographs of the Florence and Rustler mines. Thanks, Jon.
Sorry, only registered users may post in this forum.
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 1993-2017, except where stated. Mindat.org relies on the contributions of thousands of members and supporters.
Copyright © mindat.org and the Hudson Institute of Mineralogy 1993-2017, except where stated. Mindat.org relies on the contributions of thousands of members and supporters.