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Ontario is one of the most mineral-rich regions in North America, and for                                               collectors searching for world-class apatite, titanite, feldspar, amphibole,                                                   quartz, or rare carbonatite-related crystals, the key is understanding how to                                                 locate and interpret vein dykes.

 

If you collect in the Bancroft area, vein dykes are your bread and butter. There                                                 are at least a score of well known vein-dyke locations, all famous for one                                                   reason or another. Most attractive about vein dykes is that their crystals are                                                  more often than not eroded from the calcite and so are sometimes found in                                                 perfect condition. Try chipping a crystal from the rock to understand the                                                  damage that such extraction does.

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Much as they are publicized, vein dykes are hidden fissures—often narrow,                                                   often subtle—and the birthplaces of some of the province’s finest mineral                                              specimens. Beneath layers of moss, leaves, and forest soil lies a structural                                                  network of fractures that once served as conduits for super-heated, mineral-rich                                              fluids rising from underlying plutons. There are said to be at least 60 plutons                                                      in the land between Bancroft and Madoc and they consist of molten                                                       upwelling’s that powered the mobilization of the calcite infilling in our target                                                 vein dykes.

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These fluids arising from the depths and saturated in carbonate molecules were dissolved from sedimentary rocks. In the Bancroft area this means from the edge of the underlying Glamorgan formation which has been overlain by limestones and dolostones. When these super-heated dissolved fluids cooled, they deposited their entrained calcite load and a suite of associated crystals, forming what collectors now refer to as “vein dykes.”

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Mark and I are at this very minute scouring local geological maps for undiscovered vein dykes in our area. Our ability to locate vein dykes has been the reason for our success in the Dark Star project that some of you have been involved with. At this time we control the foremost crystal collecting localities in Ontario.

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What Vein Dykes Really Are

Vein dykes in the Bancroft and wider Ontario Highlands region are essentially carbonatite-like fissure systems—fractures filled predominantly with calcite that migrated upward from deep plutonic intrusions.

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We often wonder as to the cause of the fissures on our claim. Most are aligned along an orientation of 292 degrees. A visiting geologist suggested that the predictable cracking in our heavily masticated local rock could well be due to its being folded and then despite the metamorphosed rock, it broke with some regularity along the most extreme folds.

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Although each fissure is unique, most fall within a predictable geological expression: narrow linear depressions running across the forest floor, often aligned parallel to each other due to regional structural controls. And lets not forget the glaciers, these fissures would have formed deep within the rock, but glaciers removed the overburden and now vein dykes are exposed on the surface. Some trace of the glacier still remains in deep gouges on smooth rock surfaces, the shallow soils and water rounded cobbles that clog the upper reaches of the dykes and veins. If you are finding glacial boulders in your dyke, it means you are still above the level where crystals have weathered from the calcite.

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Where to Begin Your Search

Rockhounding success starts long before the first shovel hits the ground. The best way to locate promising vein dyke areas is to study geological maps and compare known mineralized occurrences with the structural patterns visible across neighboring terrain. Looking at the local geology on government maps, several vein dykes are marked near our current claim, but none appear prolific. I suspect that not being the country rock that appears to stretch like pulled taffy over wide areas, the vein dykes were simply noted by past prospectors for the appearance of a singular trench and then the full extent was never really measured. Ontario has been both scoured and deposited upon during the last ice age. I’m sure that there are plenty of vein dyke occurrences                                                  deeper down, but the ones on the map must lie beneath shallow soil, the remnants                                             of a retreating ice sheet.

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Now finding vein dykes without prior knowledge is a difficult task. Metamorphic                                              rock is a good place to start because of the folding and cracking, but there is no                                             real rhyme or reason as to where they pop up aside from the pluton below and the                                       folded metamorphic rock above. Save yourself some time and begin with a map.

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A single mark on the map might actually indicate a place where these vein dykes                                          stretch out over a sizable area. For us, we need to find our target geology                                                       (vein dykes) close to a road and it must harbor the kind of crystals that people                                                  would want to dig. Thus far our claims tend to be rich in the silicate crystals that                                            evolve from the Bowen’s reaction series; representative of the kind of elements                                                    granitic and syenitic gneisses hold.

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The calcite coming up in the fissure is also a big player in the crystal species that                                             our clients find – diopside, tremolite and edenite to name a few crystals that have                                              a calcite content. Seeing calcite on or near the surface is potentially a big give                                             away as to the location of vein dykes.And then there are the rare earths that come up with the calcite and are often entrapped within apatite or monazite ores. There is a new dyke complex that we are soon to investigate if the snow holds off. It has the promise of fluorite and that we think would be a hit amongst our clients, but again having spotted the location of a vein dyke marked on a map we now need to comb the land and find that elusive crevice and all of what we might surmise to be it’s cousins.

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Some notable local vein dyke occurrences in the Bancroft area:

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• The Schickler occurrence - is an excellent reference. If you examine surrounding territory with similar marble and gneiss geology, you can often project where additional fissures may lie. The schickler Occurrence is known for its purple fluorite cubs in matrix with apatite.                           

• Bear Lake Diggings - offers another model. The surrounding forest contains numerous parallel fissures, many partially filled with soil but still detectable as shallow trenches. This is possibly the most famous on Canadian vein dykes for the plorifiration of museum quality finds from here.         

• Titanite Hill – is known for its fluor-richterite and a few k’s from the Bear Lake area, these fissures are exposed along a hillside and it illustrates how heavy crystals might move when freed from calcite onto a hillside         (the tree roots are your friends).                                                                                                          

• Davis Hill – Just east of Bancroft and known for its nephyline and sodalite mineralogy.                

• Silver Crater Mine – a short hike brings you to an adit and tailings from which can be found various radioactives.                                                                                                           

• Richardson fissure Mine and the Dwyer Occurence – this area yielded the largest square uranite cube ever found and also purple smears of fetid fluoride in the rock.                                          

• Quirk Lake – fissures run beneath the Musclow Grenview Road and at one time it was a popular spot for hematite splattered quartz.                                                                                       

You see the thing is to recognize the proliferation of plutons in the Bancroft area and realize that a geological map shows country rock and yet the vein dykes intrude into the country rock in such a way that as much as they tend to swarm together, they are in no way predictably situated based on the areas rock formations. Alternately, the type of crystals that you find are somewhat predictable depending upon the elements in the local rock. The crystals grow along the contact margins and reflect the content of the country rock. Little aside from calcite comes up from deep below. This superheated fluid contributes calcite to the crystal makeup, but most importantly the calcite is a cooling medium that supports crystal growth. The less crowded the crystal and the slower it cools, the bigger and more impressive it is.

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Recognizing Fissures in the Field

Once you are on foot, begin by scanning the forest floor for elongated depressions. Shallow trenches, and long dips no more than a few feet wide are a start. In thin-soiled forests (as many are in the Shield), large trees often anchor themselves directly in these fissures, since roots can follow the softer, calcite-rich fill downward. Large trees lined up along the orientation of the fissure are a good indication of where the fissure leads and the probability of its size.

Tree roots are both a curse and a gift. A curse because you must hack through them as you excavate and a blessing because roots tell you where the fissure is. When it appears that you’ve reached the fissure floor a sly root might lead you on to a hole that drops down to a deeper level. Roots don’t go somewhere for no reason.

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Soils in a dyke that have evaded the purge of glaciations tend to be super-fine, almost dust-like and they seldom hold the heavier crystals which have slid down through these fine soils to the very lowest point in a dyke. A dam-like trap might catch sliding crystals and then that’s where they remain. Obvious channels of water flow might also suggest places where buried crystals might gather, but beware. Crystals are sometimes corroded from constant water contact (remember we are talking of things that take place over 100’s of millions of years).Where soils are slightly thicker, a thin metal pole can be used to prod for cap rock and the buried trenches that lie just below the surface.

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Understanding the Country Rock

On the Dark Star claim and similar regions around Bancroft, country rock consists of marble, granite gneiss, syenite gneiss, and mica schist. This heavily metamorphosed environment is prime for crystal growth—but one rock type stands above the rest.

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Granite gneiss produces the best crystal-forming material. When you peel away the topsoil, leaves, and moss along the fissure’s edge, seeing granitic gneiss in combination with abundant loose crystals—especially apatite, feldspar, amphibole, titanite, or quartz—means you are in the right place. Liquid calcite during the formative time will have drawn elements from within the country rock and in combination with calcite, formed the crystals that line the margin between the country rock and the calcite. Titanite illustrates this well as it seldom forms in the middle of the calcite melt, it  forms at the very edges of the calcite and as the calcite erodes the titanite breaks away from the wall and slides down into the fissure depths. It’s a rare titanite that does not have a scar along its edges where it broke away from the wall. Apatite is most commonly found in calcite, aligned to indicate the flow direction (of the once molten material)

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Digging Into the Fissure

Finding a fissure is only half the battle. The real crystals lie several feet below the surface.

A typical productive dig requires excavating 4 feet or more to reach the point at which crystals lie beneath the frost layer. The deeper you go, the closer you get to original crystal-growth zones where minerals formed undisturbed and have rested without the harmful effects of weathering.

Different country rocks control mineral behavior:

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• Mica schist commonly hosts titanite. Titanite crystals usually remain near the wall where they formed and over time they slide downhill along sloping fissure surfaces to the bottom of the trench.

• Apatite, however, can appear in any vein dyke because its chemical ingredients came upward with the calcite itself.

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Low-Temperature vs. High-Temperature Fissures

As is seen in the Bowen’s reaction series, crystals form according to temperature gradients and silica availability. We roughly group our fissures fissures along a spectrum between low- and high-temperature environments:

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Low-Temperature Fissures
Produce larger apatites, but they seldom have the color and clarity of the smaller high temperature apatites. You can also find orthoclase feldspar, amphiboles, quartz and biotite mica in low temperature fissures.

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High-Temperature Fissures
Are characterized by titanite, as commonly in matrix as as separate crystals, swarms of small apatite crystals still embedded in matrix, pyroxenite and plagioclase feldspar

Understanding this temperature spectrum helps you predict what minerals will occur before you even begin digging.

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Final Thoughts for Collectors

Ontario’s vein dykes remain unchallenged as a premiere collecting geology in Ontario. Most experienced rockhounds consider vein dykes as the most giving of Provincial rockhounding destinations.  With patience, geological awareness, and careful observation, rockhounds can uncover fissures that have not been exposed for thousands of years. Each one tells a unique geological story—and may hold spectacular mineral treasures just a few feet below the forest floor. The most significant determinant in your success is applying the knowledge that we have shared and digging with utmost determination. Sometimes you find nothing and at other times the treasure is just too amazing.