To all,
Kimberlites are often weakly magnetic and can therefore be detected by flying aeromagnetic surveys. This geophysical method is especially effective if the surrounding rocks are non-magnetic, such as most sedimentary rocks. If the surrounding rocks are also magnetic (( I believe the Marum drill cores descriptions of the Chinchaga area indicate the whole area is an old volcanic system at least 175 meters deep(depth of drilling to date,...likely is a lot deeper) and full of magnetic elements)), the magnetic responses will be noisy and the pipes will be hard if not impossible to detect.
The magnetic signal of these pipes (kimberlite or whatever) is fairly weak and is likely being overimprinted by the signature of the surrounding rocks. If the pipe is not kimberlite, but some other diamond bearing rock,...the signal might be even weaker.
As well, the magnetic sensor carried by the airplane measures the magnetic field, which decreases with the square of the altitude. Therefore the root of the pipe is not what the sensor is detecting in the airplane tests. It would be looking at how the higher portions of the pipe are deflecting the magnetic field of the surrounding rock.
A further problem,....at low latitude, magnetic bodies such as kimberlites will form dipoles (a magnetic high and low next to each other) that are wider than the actual body. At higher latitudes, the magnetic field of the earth can interfere with this signal making it smaller and harder to detect.
Now let's review Marums description of the drillcores,...
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Target 7-D, located approximately 600m east of Target 7 using an experimental technique, was drilled to a depth of 115 metres. Both the Target 7 and Target 7-D holes, at 30 metres, contain a 5-metre thick unit that is of undisputed primary volcanic origin. The unit is called the "P-1 Sequence" since it is the first true pyroclastic identified in the area. It is capped by a hard, sulphide-rich, carbonate-veined, dense pyroclastic that overlies a sulphide-rich rock described as a dark green, mica-rich, densely packed, accretionary lapilli pyroclastic with chlorite alteration, some serpentine textured alteration and as yet unidentified splotches of a dark blue metallic mineral. Additional pyroclastic units that have not yet been examined in detail underlie the P-1 Sequence. However, a quick microscope analysis of a grab sample taken from a core depth of 107 metres is described as a "dark green crystal pyroclastic with moderately packed lapilli, scattered mica and sulphides, containing fragments of crystal lithic pyroclastic and showing evidence of chlorite, slight serpentine alteration and occasional blue metallic splotches". All of the pyroclastic rocks are vesicular; that is, they contain slightly elliptical, slightly deformed bubbles formed by the de-gassing of the hot volcanic rock. The Target 7-D hole appears to contain a number of significant pyroclastic units over an interval of 75 metres below the P-1 Sequence.
Target 9 was drilled to a depth of 175 metres. Rocks with pyroclastic textures and minerals were encountered throughout the hole. The most important of these is a 10 metre thick unit encountered at a depth of 38 metres. It consists of a brecciated, somewhat fragmental, sulphide-rich, crystal pyroclastic that is underlain by a hard layer consisting of dark green vesicular, slightly pelletal pyroclastic rock with horizontal calcite veins and multiple horizontal and cross-cutting veins of sulphide minerals, including pyrite, pyrrhotite, possible chalcopyrite and splotches of a dark blue metallic mineral. This veined layer is underlain by a spectacular dark green and dark brown, massive, densely packed, pelletal lapilli pyroclastic with a fine-grained chloritized matrix. The dense pelletal unit contains disseminated sulphides as well as beautiful sulphide latticeworks that envelop the lapilli structures. The base of the main pyroclastic unit consists of a medium gray matrix with sparse lapilli and minor sulphides, including blue metallic minerals. The preliminary examination of samples has yielded numerous confirmations that the rocks are primary volcanic pyroclastics. One spectacular indicator is a grain of drop glass that was extracted from small sample of Target 9 core. It consists of a glass droplet with a delicate glass filament tail that formed as a drop of molten magma was thrown into the air and spiraled into the building mass of pyroclastic rock outside the volcanic vent. The droplet and tail, which could not possibly withstand any sedimentary transport without breaking, is perfectly preserved and contains gas bubbles. The glass and trapped gas may yield evidence concerning the composition of the original volcanic magma.
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It seems to me that this P1 layer is the spew from the pipes that may have brought the diamonds up. They emplaced in volcanic rocks with a lot of metals, and magnetite and other magnetic minerals in them, so the pipes magnetic properties are very close, if not the same as, the surrounding rock. The pipes will be under this P1 layer which seems to be under up to 30 meters or more of overburden. Perhaps my use of the word overburden is wrong here, but to me, anything over what I'm interested in is overburden,..and as I am interested in the pipes, that 30+ meters of P1 and what is over P1, is overburden. That overburden is going to make it very hard to see the pipes using aerial magnetic surveys considering the material is all volcaniclastic,.....all similar in composition,....and all giving off magnetic interference.
Jesse, if you are still with me here, it would be nice to know where RB thinks these pipes are (at what depth) and what those other tricks he mentioned he has up his sleeve for finding these pipes would be. Just an interested shareholder.
cheers, russett |
