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Thread: Benchtesting Rocks & Minerals with a VLF Metal Detector

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  1. #1
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    Benchtesting Rocks & Minerals with a VLF Metal Detector

    Benchtesting Rocks & Minerals with a VLF Metal Detector

    Introduction

    From the earliest time when we were aware of our surroundings, most of us looked for pretty rocks. We wondered what interesting or valuable minerals might possibly comprise them. Now as adult hobbyists, I doubt if any of us hasnít benchtested an interesting rock from curiosity, and wondered what actually produced the signal.

    Although a sensitive benchtest usually has little in common with how marginally conductive rocks and minerals respond to metal detectors in the field due to ground effects, we can learn and become familiar with how rocks and minerals in our respective areas respond to metal detectors in a benchtest. A sensitive metal detectorís electromagnetic field penetrates rocks, usually generating either a positive or a negative signal in response to whatever material is in the rock. We can sometimes determine whether such signals should be investigated further, or whether worthless iron minerals produced them.

    Iíd generally describe my benchtest results as worthwhile and informative, but that notwithstanding, I look forward to doing a benchtest because I think it is an intriguing study on its own merit. That said, how do you conduct a benchtest? Iíll describe my methods and hopefully weíll see what you think about it.

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    Benchtest Requirements and Techniques

    Benchtesting ideally requires a visually displayed, fully calibrated, manually adjustable ground balance that covers the entire (soil) mineral range from salt to ferrite. As a minimum, the detector should feature a threshold-based true motion all-metal mode, and preferably an additional true non-motion all-metal mode for significantly improved sensitivity to borderline samples. Visual displays in either of the true all-metal modes are essential for target ID, Fe3O4 magnetic susceptibility and GB readouts.

    I prefer a small (concentric) coil to promote detector stability and improve sensitivity to the rock sample, to ensure uniform sample exposure to the coil, and to minimize EMI (electromagnetic interference) especially if benchtesting at home. Elevate the sensitivity control as high as possible while maintaining reasonable detector stability such that you can clearly hear changes to the threshold.

    To check for a target ID, move the sample back and forth across the coil at a distance that produces the best signal but does not overload the coil. To determine ground balance and Fe3O4 readouts, advance the sample toward the coil, back and forth to within an inch or two (depending on sample size and signal strength) of the coilís electrical sweetspot. Ensure your hand does not come within detection range of the coil to avoid creating false signals. If you extend your fingers to hold the sample, this is not an issue when testing larger samples. If necessary use a plastic or wood food holder that can firmly grasp small samples.

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    Benchtests should be conducted utilizing a minimum of two widely diverse GB control adjustments. Initially I prefer the same GB control adjustment that is typically required to keep my detector ground-balanced to the substrates in my prospecting areas. Itís a personal preference that works for me. Using the F75 as an example here, that particular GB control point (F75 / GB86) is more likely to improve any rock or mineral sampleís signal strength compared to using a more reduced (more conductive) GB compensation point.

    The next step is to use a dramatically reduced GB control adjustment (F75 / GB45) as suggested by Fisher Research Engineering. This setting ensures that (obviously weathered) oxidized samples do not generate a positive signal from any type of non-conductive iron mineral inclusions, particularly maghemite mineralization that may be present within such rocks. It follows that this second benchtest will, if anything, slightly subtract from the sample signal strength, particularly with low grade and otherwise marginally conductive samples, compared to the first step of the benchtest at GB86.

    As a general rule, I do not recommend the F75 / GB45 compensation point for benchtesting (non-oxidized) mafic samples that are dominated by constituents such as common magnetite or other black minerals that normally support highly (non-conductive) elevated GB readouts. Such samples can produce strong negative threshold responses at the reduced GB compensation point. It will be difficult or impossible for the signal from a marginally conductive substance to successfully compete with those negative threshold signals. For non-oxidized samples Fisher Research Engineering suggests using F75 / GB65 rather than the F75 / GB45 compensation point, since obvious iron mineral oxidation should visually be absent from such samples.

    With the above discussion in mind, extremely fine-grained, unweathered magnetite that occurs in pyroclastic material (for example volcanic ash) can drop into the GB45 range, but it is extremely rare. Unweathered volcanics do frequently drop into the GB70's due to submicron magnetite, but the recommended F75 / GB65 compensation point will eliminate those positive signals.

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    The arsenopyrite sample depicted above is a good example of a commonplace mineral that we encounter in the silverfields of northeastern Ontario. Generally field examples could be described as marginally conductive and many are low-grade. A good many react with only a mild positive signal, and sometimes not at all to a benchtest depending on which GB compensation point is used.

    The high-grade, solidly structured sample above produces a strong positive signal in either zero discrimination or true motion all-metal mode with the ground balance control adjusted to the GB compensation point required for our moderately high mineralized soils. As noted, thatís approximately F75 / GB86, although in the field, of course, it varies somewhat depending on location and coil type / size employed.

    The response is not as strong as a similar size and shape metalliferous sample would produce, but it does generate a surprisingly strong benchtest signal that would be readily detectable in the field. Even with the GB control dramatically reduced to more conductive values (F75 / GB45), to ensure that any positive signals produced by non-conductive iron mineral inclusions should now only produce a negative threshold signal, it is no surprise that this (non-oxidized) specimen continues to generate a strong signal.

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    For those readers unfamiliar with detector responses to such minerals, the same general response scenario described above with arsenopyrite applies to other marginally conductive minerals such as galena, pyrrhotite and to a lesser extent even iron pyrites. Ordinary iron pyrites is generally innocuous, but maghemitized pyrite, pyrrhotite, and the copper sulfide ores, particularly bornite and chalcocite, can be a real nuisance in the field due to magnetic susceptibility, magnetic viscosity, and / or electrical conductivity, just depending on what minerals are involved.

    Such variable responses from arsenopyrite and many other mineral and metalliferous examples clearly infer that signal strength and potential target ID depends on a sampleís physical and chemical characteristics, including the quantity of material within a given rock. These factors include structure, size, shape, purity (overall grade), and magnetic susceptible strength of iron mineral inclusions. Moreover, the VLF detectorís sensitivity, the GB compensation points employed, the coil type and size, and the sample profile presented to the coil further influence benchtest target signal strength and / or potential target ID readouts.

    Incidentally, neither of my PI units will respond to the arsenopyrite sample depicted above, even with a TDI Pro equipped with a small round 5Ē mono coil, the GB control turned off, and a 10 usec pulse delay to deliver its most sensitive detection capability. That result is typical of most, but certainly not all sulfides and arsenides that occur in my areas. Higher grade and solidly structured pyrrhotite, an unwelcome nuisance iron sulfide, and collectible niccolite, a nickel arsenide, are commonplace mineral occurrences here that do respond strongly to PI units, although their respective VLF target ID ranges are quite different.

    As a related but slight diversion, the photo below depicts a handsome example of the widely occurring mineral sphalerite. It forms in both sedimentary beds, and in low temperature ore veins. It is interesting to collectors because it possesses a dodecahedral cleavage which means that it breaks smoothly in twelve directions, and it is usually triboluminescent, meaning that it gives off a flash of light when struck sharply. Like many desirable minerals lurking in prospecting country, unfortunately sphalerite doesnít react to metal detectors.

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    A Final Word

    The foregoing is intended to illustrate that sensitive metal detectors can be utilized as a supplementary tool to assist with evaluating rocks and minerals. There is no question that the benchtest has serious limitations, particularly if trying to distinguish positive signals produced by some types of iron mineral inclusions from weak conductive signals.

    That notwithstanding, a positive signal that persists below the F75 / GB45 compensation point cannot be confused with iron mineral negative threshold signals produced at that same compensation point. Therefore a positive signal merits further investigation. Such signals are almost certain to be generated by a marginally conductive mineral or a metalliferous substance.

    On the more interpretive side of a benchtest, we need to point out that weak positive signals from lower-grade samples of minerals such as arsenopyrite, galena, pyrrhotite, chalcopyrite, and doubtless a few others, may disappear well before the GB control is reduced to the F75 / GB45 compensation point. We learn early that benchtests are frequently equivocal and require interpretation based on any further evidence that might support the benchtest result. Look for iron oxidation in addition to structural or other physical evidence as described above that could explain why a sample reacts as it does to a metal detector.

    Jim.
    Time, oh good, good time...where did you go?

  2. #2
    us
    Northern California

    Aug 2007
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    Thank you Jim for this interesting, thought provoking intelligent article though I must admit that it will take "ME" sometime to understand before I can put all of your information to good use especially as I am preparing to take a trip into the hills. But, thank you...............63bkpkr
    Out searching w/GMT & friend under my arm

  3. #3
    ca
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    Quote Originally Posted by 63bkpkr View Post
    Thank you Jim for this interesting, thought provoking intelligent article though I must admit that it will take "ME" sometime to understand before I can put all of your information to good use especially as I am preparing to take a trip into the hills. But, thank you...............63bkpkr
    Thanks for replying with those kind words Herb!!! It's been fairly quiet on this forum by and large for quite some time. And I didn't anticipate too much interest in this topic, but hoped it might make an enjoyable read.

    The GB compensation point for the F75 at GB45 essentially accomplishes the same thing as the Falcon 20 except that obviously it doesnít have the 300 kHz Falconís extreme sensitivity. Iron mineralizations will produce a negative threshold response, therefore conductive positive signals produced as the sample is advanced towards the coil should be investigated. Very weak positive signals that should respond on the Falcon may not respond at this GB45 compensation point.

    The potential issue is if the sample contains both a highly reactive iron mineralization and a conductive substance. Then it remains to be seen which will have the dominant signal. The mortar and pestle is undoubtedly a better solution for investigating suspect samples, detectors can only tell us so much.

    There were quite a few mineral samples that could not be included into this article so let's post specular hematite below. I donít know if you encounter this material in the southwest. We have some high production iron mines in several localities, including the renowned surface extraction facility at Marmora, Ontario. Unlike other types of hematite (that I know about) this material, although not exhibiting nearly the full magnetic susceptible strength of magnetite, does seriously react to VLF metal detectors.

    That's it for now Herb, good luck with your gold hunting trip, we'll look forward to hearing from you when you've returned. Have a great time!!!

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  4. #4
    us
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    Thank you Jim I will have a wonderful time even if the only Gold I find is the type that invites wonder and joy being out in those exciting mountains and canyons especially the ones with water running through them. At 74 years of age and recovering well from an April 06 shoulder rebuild I am So in need of a good amount of time "Out There". I was asked this evening by a dear sweet lady why don't you just live in the city? I responded to her that a part of me would die if I could not be in the out of doors and that is just the way it is. Keep on sharing good sir and I will do the same, more when I have it.........................63bkpkr (aka Herb)

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    Out searching w/GMT & friend under my arm

  5. #5
    ca
    Honorary Member of the Central Alabama Artifact Society (C.A.A.S)

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    You are more than welcome Herb, it is always a pleasure to chat with you. Youíve been involved with the hobby for as many years as I can remember, and you invariably have something sage or an insightful experience to share with us.

    Time does slip away, and before you know it weíre senior citizens!!! But itís probably more to do with your employment responsibilities, change in residence locations, and subsequent shoulder surgery that has kept you sidelined for so long. I donít doubt that a good long sojourn into remote wild places will do you the world of good. With or without finding gold, the trip will be a beneficial experience that ought to leave you feeling revitalized. Now just a reminder to not overdo things, because you're fresh off the convalescence, and it has been awhile since youíve shouldered a heavy load for any length of time through such rugged terrain.

    I should have mentioned in the above post that while the F75 / GB45 compensation point mimics the 300 kHz Falcon although obviously it is not as sensitive, the F75ís lower operating frequency and much larger coil size should improve electromagnetic field penetration when testing larger rock samples. Incidentally, I see no reason why other prospecting-capable VLF detectors cannot be used in the same manner providing that a ground balance compensation point is identified such that it eliminates positive signals from all iron mineral types.

    I donít remember exactly where the sample below came from, although itís undoubtedly from an abandoned site in the Temagami copper district just south of northeastern Ontarioís silver producing areas. Itís a more typical example of the nuisance chalcopyrite that we occasionally detect in the field as compared to the fine specimens used to illustrate mineralogy texts.

    Jim.

    PS: Thankyou for sharing that charming campfire photo above. It struck me as an ideal size and shape designed to keep the fire reasonably small and not require too much fuel to keep it good and hot. Absolute perfection Herb, throw another log on the fire for me!!!

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  6. #6
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    Thank you for sharing!

  7. #7
    ca
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    Hi Professor of EngineeringÖ thankyou for dropping around and commenting about the article. It only took a few hours to write, and I habitually photograph all my rock and mineral samples, so they were already available to use. It was motivated in part because lately Iíve been reading my Canada Geological Survey Reports entitled Rocks & Minerals for the Collector authored by Ann P. Sabina. These always get me fired-up about rockhounding and benchtesting rock samples.

    In reading about abandoned mines across Canada, there invariably is a detailed list of associated minerals that include the sulfide group. Sulfides occur everywhere because they form in hydrothermal veins, in magmas, and some such as iron pyrites and marcasite (iron sulfides) even form in sedimentary environs. It is not unusual to find several sulfides in close association, for example galena, sphalerite and frequently fluorite are commonly found together.

    Sulfides are the major ores of many metals such as lead, zinc, silver, iron, antimony, bismuth, molybdenum, nickel, and copper. Gold is also commonly found in sulfide deposits. Many sulfides and other indicator minerals are not detectable in the field due to ground effects, but sometimes do respond with a positive signal to the F75 / GB45 benchtest compensation point or equivalent on other VLF detectors. So while there are obvious shortcomings, the technique could prove handy either in the field or back at home for checking suspect rock samples. That's about it for now, thanks again for popping by!!!

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  8. #8
    ca
    Jul 2009
    Chatham, Ontario
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    Great article as usual Jim. I will be heading up to silver country in early September. As more and more sites become restricted, it may be necessary to venture further into Gowganda.

  9. #9
    ca
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    Quote Originally Posted by trainermick View Post
    Great article as usual Jim. I will be heading up to silver country in early September. As more and more sites become restricted, it may be necessary to venture further into Gowganda.
    Hi FredÖ thanks for dropping around and commenting. If access in that area becomes a deterring issue, there are plenty of other valuable and interesting minerals to be found elsewhere. The Bancroft area is a whole lot closer and the mineral diversity within a radius of say twenty miles of town is incomparable. The area literature indicates that 95% of all minerals known to man occur within fifty miles of the townsite.

    Iíll be heading north in early September and probably stay until the third week of October. Iíll be camped at Loon Lake as usual. I donít plan to trench or do any strenuous digging this year, so Iíll probably Ďscrape and detectí a few sites and hope for the best.

    Hope you are enjoying retirement Fred. Itís quite a change in lifestyle, but we do have extra time to indulge our interests at a more leisurely pace, which is niceÖÖÖÖÖÖÖÖ. Jim.

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  10. #10
    ca
    Jul 2009
    Chatham, Ontario
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    Thanks Jim. I grew up spending my summers in Bancroft. I have been a rock hound for over 50 years and have attended the Rockhound Gemboree almost every year.
    Jim Hemmingway likes this.

  11. #11
    ca
    Honorary Member of the Central Alabama Artifact Society (C.A.A.S)

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    Quote Originally Posted by trainermick View Post
    Thanks Jim. I grew up spending my summers in Bancroft. I have been a rock hound for over 50 years and have attended the Rockhound Gemboree almost every year.
    You’re welcome Fred, it’s always a pleasure to hear from you. I think the main benefit of these forums is the opportunity to develop contacts and friendships, based on sharing mutual interests in rocks and minerals. I hope that translates into exploring for silver with you this September.

    I’m hoping to get to the Bancroft area for at least a week or two later in the summer. If not then, it’ll have to be late October or early November. I have to find a campground that provides hydro and permits camping well into the late autumn season. Tory Hill (Gibson property) is a definite destination for apatite, titanite, and to a lesser extent feldspars, amphiboles, pyroxenites and possibly even richterite. If time permits, then it’s over to the Wilberforce area (Grace Lake) for tremolite and other interesting things. Do you still visit those areas to search for mineral-bearing fissures?

    The photo immediately below is a lithium aluminum silicate called spodumene. You are probably familiar with this mineral although the color variations are labeled differently. This colorless, opaque to translucent example is further identified as cymophane. It came to me years ago from a California mineral collector who wanted to trade for some native silver. He also exchanged some purple spodumene called kunzite, but it doesn't take a decent photo.................... Jim.

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    Last edited by Jim Hemmingway; Jul 14, 2018 at 07:47 PM. Reason: replace mineral titles that went missing????
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  12. #12
    Charter Member
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  13. #13
    ca
    Honorary Member of the Central Alabama Artifact Society (C.A.A.S)

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    Quote Originally Posted by Terry Soloman View Post
    Mr. Hemmingway,
    Hi TerryÖ thanks for dropping by to acknowledge the article. Iím really pleased that you took a few moments to look things over on this thread. I enjoy reading your knowledgeable posts, and appreciate that you are preeminent in the hobby, regardless whether gold prospecting in the wilds of Arizona or coin hunting potentially risky areas in New York City.

    The above write-up discusses a technique that could be useful to rock hunters / prospectors already possessing suitable metal detectors, such that no additional expense is required to benchtest rocks and minerals. Below are two photos of minerals you are doubtless familiar with in the areas you prospect for gold.

    All the very best and thanks again for stopping by for a quick howdy!!!

    Jim.

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  14. #14
    ca
    Jul 2009
    Chatham, Ontario
    102
    61 times
    Yes Jim, I still search the Gibson Occurance for gem quality apatite. I will be there for a week in early August. Looking forward to seeing you in September after I attend the gem and mineral show in Denver.

  15. #15
    ca
    Honorary Member of the Central Alabama Artifact Society (C.A.A.S)

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    Quote Originally Posted by trainermick View Post
    Yes Jim, I still search the Gibson Occurance for gem quality apatite. I will be there for a week in early August. Looking forward to seeing you in September after I attend the gem and mineral show in Denver.
    Hi FredÖ thanks for letting me know. Iím looking forward to visiting a few sites in the general area. Iím quite keen on becoming more proficient with searching for non-detectable minerals.

    With retirement youíd anticipate that there would be ample time to get up there. But somehow, with household chores, family commitments and other interests, not to mention some pretty good excuses, I just havenít followed through with any real resolve. I find it so easy to be complacent, but that has got to change. How are the bugs in that area in early August?

    Also Fred... I thought the Bancroft Gemboree was coming up shortly....................... Jim.

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