Silver Hunting with an F75 Metal Detector

Hello Everyone,

As you read this report, keep in mind that much of the silver found here in Ontario shares a similar conductive range to gold. This results mainly from non-conductive iron mineral and other low conductive mineral inclusions, or in short, purity.

Since writing this essay a year ago, I’ve added a new section entitled The Fe3O4 Bar Graph-What Does It Mean? There is a discussion of iron minerals and their relationship to both ground balance and magnetic susceptibility readings on the Fe3O4 bar graph.

Our forum allows for 15 photos, three more than in the original write-up. So, I’ve added three small subsections along with a suitable mineral photo for each…just to add more interest value. Please enjoy the read…


Rock hunting with a metal detector, for silver ores/nuggets as described below, is a fun pursuit for many hobbyists. If we set forth to hunt for silver, it is best to view it as an outing to enjoy the great outdoors and, with some patience and effort, perhaps locate a few nice specimens. Typical mining country surroundings, as depicted in the following photo, make such outings an enjoyable, relaxing adventure. I recently returned from a visit to silver country here in Ontario, where I had an opportunity to further evaluate the electronic prospecting capabilities of an F-75 metal detector.







General Searching

Let’s presume our research has been completed on an area where silver has been found in quantity in the past. We have arrived at a mine site that our research indicates may offer some electronic prospecting potential. Before unlimbering our detectors, let’s spend a moment to assess the site and consider a few search strategies.

We should look around the property and identify the location of former surface ore veins or shafts (mostly fenced-in nowadays in more accessible areas here in Ontario), headframe and storage building areas where we can be confident good silver was retrieved, graded, stored, moved, and sometimes inadvertently misplaced. Below is a photo of a typical headframe…where we know yesteryear’s miners handled considerable valuable silver.







Most frame buildings have long since collapsed or been removed, but on-site inspection usually will reveal evidences of former buildings, sometimes in the form of concrete foundations, remnants of building or roofing materials, and an unmistakable proliferation of iron debris.

As a result of misgrading of ores, good silver was occasionally dumped with waste rock onto the mine tailings. In some locals, tailings can be 25 to 30 feet high and occupy enormously large tracts and “runs” that snake off into the bush. These mine tailings were used to build local roads, storage beds, “driveways” into the mine sites, loading ramps, and routes to facilitate waste rock transport from the mine to the tailings disposal areas. All excellent prospects to search with a metal detector.

Both valuable ores and waste rock were transported with wagons, carts and oxen. Occasional breakdowns resulted in “spills” of high grade silver material. It’s easy to imagine under such primitive conditions and terrain…that some silver remained right where it fell. The specimen in the photo below was found a few years ago…in an overgrown ditch on the downhill side of a sharp bend on a mine site “driveway”…along with several other pieces.







As we survey the general mine tailing landscape, we will observe indications of previous metal detectorists activities. You will doubtless see many dig sites, mostly scattered randomly across the flat and other easily accessible areas. To increase our chance of locating desirable specimens, we should search those areas requiring a bit more effort to access and detect. Such opportunity exists out on the peripheral areas, down in the gullies, up on the benches, along the face of ridges/slopes, in under the trees and bushes, and finally…in the iron trash-laden areas wherever encountered.

Many detectorists will avoid trashy sites, preferring easier pickings. A suitable unit such as the F-75 that offers superior target separation, equipped with a DD or at least a sniper size coil can do well in such environs. Detect these trashy sites carefully, and exercise patience. The photo below exemplifies how a detectorist can increase the odds of locating a desirable specimen or nugget by making an extra effort to get away from the large, flat, accessible areas and on to more difficult to reach ground.







This particular ridge in the above photo rewarded me a few years back with this attractive piece, in addition to many other quality specimens…







The small but handsome silver nugget depicted below was one of my first finds with the F-75 this past season. It was found by getting in under the scrubby trees and “bushwhacking”… while fending off the heat and blackflies…







A final suggestion while hunting such areas. Do not be discouraged by any evidence of past digging in the area you select. Always check any unfilled holes you find. I was recently surprised that an electronic prospector did not dig just a few inches deeper to retrieve two specimens found in two separate holes within spitting distance. These two pieces gave off solid, tight, high conductive signals that could not possibly have been confused with large iron. Here’s one specimen and the other will follow further down…







Silver Ore/Nugget Target ID

When silver hunting, we are searching for more valuable coin size and larger pieces of silver. Silver purity, structure (dendritic, plate, disseminated, massive), inclusions of other conductive elements formed with the silver (such as cobalt, nickel, arsenic, copper), size and shape, iron mineralization within the target structure, and profile presented to the coil…result in a wide range of target conductivity. Most natural silver ores/nuggets will target ID in the lower conductive range as a rule, especially the middle foil to upper nickel coin range. There are plenty of exceptions, so be prepared to dig all conductive target ID’s. That includes upper iron range targets…especially in disturbed or loose ground, such as loose material at the base of slopes, and large areas where tailings have been moved around by mining or municipal authorities.

The F-75 in Rock Country

The F-75 is a deepseeking unit both in undisturbed and disturbed ground, with superb target separation ability. It demonstrates very good response to lower conductives. It operates at 13kHz, a good frequency for an all-purpose unit with real prospecting capability. The All-Metal mode and the JE disc mode (with/without small iron discrimination) excel in response to both large and tiny low conductives at very good depth. The threshold retune speed is fixed, but very fast. The unit offers either calibrated manually adjustable ground balance, or ground balance can be achieved using the auto grab trigger feature. It’s quick, accurate and convenient. The unit provides full range target ID digital readouts in all modes, plus four discrimination modes for coin/relic hunting pursuits. To compliment the manually adjustable ground balance, a stat all-metal mode is available to perform sensitive ore/mineralized rock bench tests. The motion all-metal mode’s adjustable threshold, fast threshold retune rate, and excellent sensitivity to tiny low conductives make it a very good choice for nugget hunting. The JE mode’s remarkable sensitivity and depth capability on small nuggets while operating with small iron discrimination…make it ideal for searching in trashy tailings.

The F-75 performed very well under all conditions encountered while hunting in mine tailings and adjacent natural environs using the stock 11” DD coil, even when using high sens/gain settings. Ground conditions ranged from a GB setting in the high 60’s up to the mid 80’s (GB scale range = 0 to 99) with a magnetite reading typically @ 0.1% or less. GB was usually in the low 80’s except where scattered areas of “rusty” iron mineralized rocks were encountered. In such areas the GB was 10 to 15 full units lower.

Despite nearby hydro wires and transmission towers in some areas, EMI was not an operating issue. The highly sensitive JE mode (used with small iron disc = a setting of 12) was occasionally affected by these power sources, thus requiring modest sens/gain reductions to re-acquire satisfactory stability without much impact on depth performance. If JE mode instability is an issue in some areas, switching over to the PF or DE modes utilizing small iron discrimination (disc setting = 6 or slightly higher) while still maintaining higher sens/gain settings offer less sensitive, but stable alternatives. Otherwise, use the highly stable all-metal motion mode wherever practical/feasible to capture faint target “whispers”. The visual target ID is available in this mode to help with identifying small iron trash.

In hard hit, quieter areas (less trash in the ground) where I felt only small silver remained that might be found with this unit, I enjoyed experimenting with and using the all-metal motion mode…a highly stable, deepseeking, smooth performing mode that is least affected by EMI overall. I made a point of using this mode adjacent to a nearby transmission tower at one site. The threshold retune rate was quite sufficient to maintain a smooth background “hum” under these search conditions. I could operate close to or at max sens settings in most areas…but then I prefer a little background chatter to ensure the unit is providing max depth performance. The photo below shows the 2nd silver specimen left in a partial excavation by a careless electronic prospector.







JE Mode

I prefer the JE mode for hunting smaller targets in trashy tailings. With the JE mode you can be confident of getting maximum depth in loose material (disturbed ground) despite employing small iron discrimination (disc set to “12ish” for JE mode). The other disc modes are much less responsive with any iron discrimination over disturbed ground. In measurable terms, even the small iron disc setting in JE mode will not sacrifice depth in disturbed ground, and you will not lose much if any measurable depth/sens compared to JE/PF/DE disc modes in zero discrimination. That applies to both nickels freshly buried at 10 inches or a three grain gold nugget freshly buried at 3 inches in my ground (GB=85ish; magnetite @ 0.1% and occasionally @ 0.3%). Keep in mind that whatever performance benefits can be derived in disturbed ground will improve in undisturbed ground.

JE mode demonstrates enhanced sensitivity to very small low conductive targets over the other disc modes, and in that respect the JE mode at higher sens levels might be a bit overwhelming for inexperienced operators in this type of hunting scenario. In that case, PF mode is a likely alternative for uneven mineralized ground.

Some considerations to keep in mind when using disc modes for searching mine tailings are as follows: (a) despite deeper silver targets in loose material/disturbed ground target IDing frequently as iron, the JE mode operating in monotone with small iron discrimination (disc = 12) will indeed eliminate small nails/screws and so on, but it will not discriminate deep silver targets that identify as iron. In my experience these targets respond with a solid, two-way signal. (b) using iron tone ID means that any target identifying within the entire iron target ID range will be assigned this low tone. This will result in the direct loss of good silver targets in disturbed ground/loose material. More, any co-located silver with iron (in any ground) that might average into the iron range, will also be lost to the iron tone. Stick with monotone for best results and signal interpretation, unless you are prepared to lose good targets. (c) while I’m not suggesting the JE mode is deeperseeking than the all-metal motion mode, in practical/measurable terms the unit will not respond any deeper in either mode to a three grain nugget buried at 3 inches in my ground. However, the JE mode even with small iron disc in place still yields a much cleaner, crisper signal response to the nugget (that reads with a target ID number ranging from 11 to 14 in the ground.…upper iron range) at that depth. That’s a fact…and it has very real implications for hunting small desirable targets in iron-infested tailing environs. Below is the first multi-oz native silver in calcite specimen found with the F-75.







Target ID and Discrimination

The F-75 responded to silver nuggets/ore with a stable readout, and "tight" audio signal. I was always fairly confident when silver was under the coil; a confidence that was enhanced by the fact that shallow, large iron nearly always gave a far more erratic target ID with different sweep directions. In that respect, target ID was a real benefit, and I was happy to have it psychologically too…as it encouraged digging those tough to retrieve signals that looked promising. This was my first experience with target ID electronic prospecting, it also added some “fun” factor to the experience.

Target ID and discrimination on large iron in the silver fields was quite different from the results obtained in my “disturbed ground” backyard test plot, as would be the case with many VLF detectors. Most small iron continued to identify and discriminate out well within the confines of the iron ID resolution…as did large iron in the home test plot. On location however, large iron (depending on size, shape, and profile presented to the coil) typically ID’d at much higher conductive levels very close to what you experience in an air test. I observed that some large iron items (drill bits for example), despite the higher readings, would resolve themselves into recognizable (but somewhat erratic) visual patterns, again, depending on sweep direction.

The usual audio indications were also evident with this unit such as harsh signal fringes on flat/wide iron, relatively widespread signals, uncertain pinpointing (iron moved around a bit), at times non-repeatable/or broken signals in one direction, and so on. However, I still had to dig most of it…just in case…not being entirely familiar with this new instrument. For any newcomers reading here, yes large iron can often be discriminated out at higher discrimination settings, but it’s self-defeating since most desirable silver will also be eliminated. Below is a photo of some of my beautiful iron finds....







The photo below depicts conductive iron sulfide in the form of massive pyrrhotite. Pyrrhotite is not the usual non-conductive iron mineralization commonly associated with hot rocks. Regardless, it causes similar headaches. It’s occurrence is widespread, it comes in all sizes, and there is nothing that can be done to mitigate its wide, blaring, masking iron ID signal other than to recognize and ignore it if possible. Pyrrhotite can render entire sites unsuitable for detecting. Many electronic prospectors in the area view it simply as a de facto positive hot rock.

It weathers to a rusty or very deep brown, almost purplish surface, but has a pale brassy metallic luster on a fresh surface. It’s somewhat similar in color/luster to iron pyrite (another iron sulfide with a different molecular configuration from pyrrhotite) when not weathered, but does not have pyrite’s cubic crystalline structure, and unlike pyrite… it is magnetic to varying degrees. The sample in the photo will discriminate out at a mid-iron disc setting on the target ID scale…but typical large samples with sufficient pyrrhotite concentration cannot be ground balanced. Pyrrhotite is the main ore body at the great nickel producing facilities of Sudbury, Ontario. It acts as a host ore for the nickel bearing materials embedded within it. Some collectors consider it worthy of specimen collection. I do too, but for quite a different reason.







The Fe3O4 Bar Graph-What Does It Mean?

What follows here is a more detailed look at ground minerals in relation to “black sand” meters than you will normally see in any manual. The manuals stay with simple, easily understood explanations that do not reveal as much information as may be desired by electronic prospectors.

There are many non-conductive iron mineral types that contribute in varying degrees to a soils magnetic susceptibility. Some common examples, aside from non-oxide / chemically reduced iron (Fe+2) found in clays and darker minerals of the silicate family that are normally weakly magnetic, include magnetite, maghemite, hematite, siderite, goethite and other hydrated iron oxides captured under the generic description of “limonite”. In similar quantity (amounts), many iron mineral types exhibit very modest/slight magnetic susceptibilities compared to magnetite, regardless of their ground phase (ground balance) measurements.

Magnetite followed closely by another non-conductive iron oxide, maghemite, have the most profound impact on the Fe3O4 readout, as both materials are highly magnetic susceptible (of the two substances, magnetite is the more highly magnetic). Despite this strong similarity, these two substances exhibit quite different ground phase measurements. Ground phase can be viewed as a ground "target ID" measurement based on phase shift similar to any other (phase shift) target ID measurement. On the F-75 ground balance scale, magnetite predominates from about 75 on up to the maximum of 99, whereas maghemite tends to occupy a range that spans the low/mid 40s up to about 60ish.

Where both magnetite and maghemite exist together (as is the case in many soils) the ground phase measurement falls between their respective ground phase ranges, subject to whichever substance is predominate. In this example, the soil’s magnetic susceptibility will be much more highly elevated than other soils that fall into a similar ground phase measurement range, but by comparison contain less magnetic susceptible iron minerals.

An example supporting the above statement is the hydrated iron oxide “goethite”, prevalent in brown soils and northerly latitudes. While able to generate a similar range of ground phase readings to goethite, the magnetite/maghemite mixture used in this example (or either substance individually) will raise the magnetic susceptibility many times more than a similar amount of goethite or other weakly magnetic iron minerals.

The Fe3O4 calibrated bar graph results (conveniently expressed as % volume magnetite) represent a measurement of magnetic susceptibility that results from any iron mineralization present in the soil. This measurement may or may not actually include magnetite (although the absence of any magnetite in most soils would be highly unusual), depending on ground iron mineral composition. The bar graph presents this data independently of the ground phase measurement (ground balance) feature. A result is that you can ground balance the soil’s non-conductive iron minerals, while still able to continue measuring its magnetic strength utilizing the Fe3O4 meter.

Increased levels on the Fe3O4 graph readout indicate more highly magnetic susceptible ground, and thus more difficult ground to achieve good target identification and depth performance results with VLF units compared to overall performance in relatively lower magnetic susceptible soils.

In summary, the Fe3O4 readout is a very convenient tool to identify the strength of soil magnetism quickly. Prospectors may utilize the Fe3O4 bar graph as an aid to locating shallow black sand deposits.

As a matter of interest only, below is a photo of a magnetite specimen that easily gives a maximum reading on the Fe3O4 meter in an air test.







Magnetite, a non-conductive iron oxide, is an abundant negative hot rock in my area that gives the well-known “boing” signal familiar to electronic prospectors using VLF units. If the ground balance control is advanced further into the ferrite end of the ground balance scale beyond magnetite’s ground balance setting, magnetite becomes a powerful, positive hotrock “zip zip” signal.

Bench Testing with the Stat All-Metal Mode

The manual notes the static all-metal mode featured on the F-75 should be used for seeking very large items at good depth. Beyond that specific task, many users do not have any further practical use for this mode. However, the stat all-metal mode is also necessary to properly perform sensitive ore/rock bench testing. The following are bench test observations, and a few thoughts that are open to correction by anyone. I am no expert, so don’t hesitate to add your two cents. We can all stand to hear new ideas.

Bench testing rocks using the all-metal stat mode has limited but definite value. It is primarily used to investigate low level detector responses to rocks that may result from iron minerals, or contain either conductive metal sulfides or tiny bits of (disseminated) precious metal. When testing rocks with the proper ground balance setting to ignore all non-conductive iron minerals, such iron minerals in some rocks can drive the detector deep into negative threshold response. Negative threshold response can overpower a slight, positive conductive response from tiny targets (even at the most optimal of GB test settings) within the rock structure.

Thus, the bench test as described below, is best suited for light to moderate iron mineralization where the potential valuable target size within a rock is at least a few grains. Fortunately in my area, most suspect rocks (mainly calcite or quartz samples) will fall into the “moderate” or less mineralization category. The stat all-metal mode bench test is considerably more sensitive and preferable to merely switching over to a zero discriminate mode.

Stat all-metal mode is much more sensitive to light/moderate iron mineralization or very low conductive material found in rock/ore samples than is the motion all-metal mode. The motion all-metal mode is not a suitable choice, since it's threshold retune rate is sufficiently fast as to eliminate many target responses. For example, lets test a fifty-cent piece diameter sized chunk of iron pyrite. Set the GB to "45" (scale 0 to 99). Use motion all-metal mode with sensitivity set as high as possible (which is max @ 99.. here in my house). Now bring the pyrite sample up to the electrical "sweetspot" on the coil...back and forth...no discernible positive/negative response is heard with this particular sample. Repeat the same test in stat all-metal mode (where sens level will likely need to be reduced somewhat in the house). The pyrite sample now gives a distinct and loud positive response…as should be expected from a sizeable piece, despite being a very low conductive sulfide.

For newcomers to the prospecting field, here is a photo of a much larger crystalline pyrite sample (fools gold) that, as a matter of interest, will give a good signal in the all-metal motion mode due to its larger size.







Ore/rock bench testing requires a manually adjustable ground balance (GB). The F-75 has this ability, and it also features a calibrated scale (with 5 détente steps between each full unit, on a scale that runs from 0 to 99 as noted above) that is very useful for making precise adjustments. To ensure that all non-conductive iron mineralization (this does not include conductive iron sulfides, common examples include pyrrhotite and iron pyrite as described above) will yield a negative threshold response, the ground balance should be set to “45” for “weathered” (including exposure to forest fires, campfires) rock samples. This setting ensures that positive hot rocks containing maghemite are captured with a negative threshold response. Maghemite is a magnetic red/red-brown iron oxide frequently contained in hot rocks that can exhibit GB compensation points down into the 40’s on the GB scale. Otherwise, the ground balance can be reasonably set to a higher, more sensitive GB setting of no more than “65” for testing “unweathered” rock samples.

I contacted Fisher Labs after running a few tests, and received a reply indicating the above ground balance settings would suffice. Fisher Labs recommended those specific settings. Except for rocks I’m familiar with, I doubt my ability to visually distinguish “weathering” especially wrt distant past exposure to fire in some instances. Moreover, some hotrocks containing maghemite may appear similar to many other typical rocks in a given area. This pretty much means that I stick with the “GB45” setting when testing suspect rocks.

A smaller coil is preferable for testing ore/rock specimens. The stock 11" DD coil test results are satisfactory provided the electrical "sweet spot" of the coil is used consistently throughout testing.

How do we benefit from ore/rock bench testing?

(a) Ore/rock testing using the stat all-metal mode increases our familiarity with the responses that can be expected from a wide variety of conductive/non-conductive mineral substances we encounter in the field. The full range target ID system is also fully enabled in the all-metal mode to aid with stronger target responses.

Bench testing allows us to distinguish which rocks in any area are most likely "hot rocks"...those non-conductive iron mineralized rocks that respond to a metal detector.

Explanation: In the field, hot rocks will either respond with a positive metallic-like target “zip zip” sound, or a negative “boing” sound due to autotune threshold “overshoot” reaction to a “negative” hot rock. A positive hot rock will have a ground balance setting below our current GB setting, whereas the negative hot rock will have a ground balance setting above our current GB setting. The point here is that any such rocks, regardless of ground balance setting or concentration of non-conductive iron mineral, will respond with a negative threshold response at a ground balance setting of GB45.

(b) Bench testing will reveal the minerals in your area that will not respond to a metal detector, and those will include some metal sulfides. You will come across samples that give neither a positive or negative response at the GB45 setting. A common example that comes to mind is sphalerite, an almost “glassy” appearing zinc sulfide found in close association with galena, a conductive sulfide that does give a positive response at the GB45 setting. Molybdenite is another metal sulfide example that gives no response at the GB45 setting, despite its galena-silvery appearance and high metallic lustre. Please see the test specimen in the photo below…







(c) At a setting of GB45, any rock that gives a positive response will either contain conductive metal sulfide, or native metal in sufficient amount to overcome any iron mineral negative threshold response. Not all non-conductive iron-mineralized rocks containing small amounts of conductive metals/sulfides will give a positive response. There may not be a sufficient quantity to produce a positive signal capable of overriding iron mineral’s negative threshold response…as described earlier. Magnetite gives such an overpowering negative threshold response at the GB45 setting (even at GB settings within a few units below it’s actual ground balance setting) that a small positive metallic signal simply cannot “breakthrough” the negative threshold part of the overall combined response.

(d) Such testing will indicate why we should really be digging all signals, even those small signals that can read as iron. For example, lets examine a pint-size chunk of quartz with visibly deep iron mineral staining into about half of the structure. It ground balances at GB70, and yields a modest positive response when using a higher ground balance setting (middle 80’s) typical in many mining areas. It does not identify on the target meter with an ID number. Please see the photo below.







This rock gives a good negative threshold response at the GB45 setting. Now place a small (in this test...a 3-grain nugget that ID's @ 17ish in the air) nugget in any position on the face of the mineralized quartz chunk and take another reading @ GB45 while rotating the sample. This combination gives a modulated positive response even when the nugget is behind the rock but still close enough to the coil to respond. It also responds with a slightly erratic target ID that remains mostly in the upper iron range.

(e) As magnetic susceptible (for example, “black sand”) mineralization levels increase, the “tolerance window” of the ground balance setting decreases. Therefore we need to monitor and adjust the ground balance more closely in higher magnetic susceptible mineral ground. Bench tests emphasize the importance of maintaining proper ground balance in tough mineralization if one is to have any chance at successfully detecting small nuggets. The following also underscores bench test limitations when dealing with extreme mineralized samples.

For example, lets test a chunk of magnetite, about 3/4" diameter with a GB = 89 plus 2 detente steps. At this setting or lower, the magnetite will not respond with an ID number. By adjusting to a slightly higher GB setting, the magnetite chunk becomes a positive hot rock, giving a powerful "zip zip" signal with a very stable ID reading of “14” (high iron range). However, reset ground balance on the magnetite chunk to give no signal...and then add the 3-grain nugget behind it. This combination will now also give a solid positive response with an ID reading of 14.

Finally, test this magnetite/nugget combination at the GB45 setting and it yields only a deep negative threshold response with no target ID.

I think that concludes this write-up for now. Thanks to everyone for taking the time to get through it.

Jim.