Description of magmatic intrusions are defined pretty much by...

Description of magmatic intrusions are defined pretty much by their shape. Pipes are round to elongated, Dykes are long, thin and sub-vertical, sills are sub-horizontal. Dilatational structures are weak points magma will push through. Sub-vertical faults produce dykes from rising magma with very long strike to width ratio, pipes drill up in a more central focussed point, sills along stratigraphic boundaries usually.


Nb concentration is more related to how 'evolved' the magma is with elements that do not precipitate out in early minerals as the deeper magma chambers cool and solidify. The more feeder magma chamber solidifies, the more concentrated the immiscible elements become in the remaining magma/fluids. Often thick pipes, dykes and sills form with multiple intrusive events, with the later material being more enriched than earlier carbonatite, and because the later fluids scavenge even more of these immiscible elements from early intrusions on the way up... end result in detail can be very complex criss-crossing of intrusive features with a highly variable distribution of REE, Nb etc immiscible elements. Some thick carb dykes may be single event early fluids with very low Nb etc, some may be late with high concentrations of Nb... hard to know until enough drill holes are into it...

The beauty of laterite supergene REE, Nb enriched deposits (weathering enrichment post-intrusion) is that it creates more even layers of higher grade that are more easily defined by drilling and mined.