Greetings @stag11,
I apologise for not replying to your request of two days ago sooner. I was otherwise occupied with family commitments. Thank you for your enquiry. I shall attempt to give a brief summary to what you would like to know, but I am sure the link to the videos I attach will do a far better job than my explanations as this is an involvedtopic which encompasses many aspects of what happens in magma chambers, incorporating many very important topics of ore genesis, including composition of magma, temperature , pressure, and time . with the latter probably being the most critical.
Suffice to know that because we are talking about nickel and copper, lava carrying these elements and invading the overlying rocks, segregation and precipitation of these metals can happen at many levels within the invaded rocks, as is evident in the many well-known deposits in Africa, Russia, Australia, Canada and many other parts of the planet. But Ni/Cu deposits will also form in extruded rocks that flow along the surface as long as the heat within the flows is extreme and is retained. Also as long as material is being continuously added or replenished over several episodes and over time. Consider the Komatiite deposits of western Australia. Here temperatures played a very important role in the formation of these lava hosted deposits and are reflected in what is referred to as Spinifex Textures in the lava.
The other factor that is important in what seem to be massive sulphide deposits that lie close to the present surface is structural disturbance, and at times, combined with erosion, give the impression that the deposit was formed close to surface. Some of these Greenstone Belts which host the various Nickel/Copper or Gold deposits are usually bounded by major NNW trending major faults to form blocks that move relative to each other. Some or part of them have been subjected to major upheavals, have been upthrown, or downthrown or moved laterally. Intruding magma during these processes was digested and trapped in the intruded hosts. Subsequently post consolidation of the mineralisation, part of the overlying lithologies have been removed by erosion, thereby reducing the original depth of where these mineralized pods consolidated originally. There are many other factors which go into play to produce similar results. But, I will not go into detail as this is not the forum for such detailed discussions.
Now, I am not saying this is what happened at Mt. Alexander, but there are major faults like the IDA Fault that were active post mineralisation which could have contributed towards this phenomenon thereby partly answering your question.
I apologise that I cannot simplify it more, these are large broad-brush theories as I don’t know the detailed geology of the region. I suppose not many on the team would know the reasons either as it would require time consuming mapping and much more detailed exploration to decipher how the mineralisation in the Mt. Alexander and environs was emplaced. This will happen in time I’m sure.
Now that I have confused you totally, I attach an o audio-visual link for you to perhaps understand it a bit better. There are other topics on the link that you may be interested in.
https://www.bing.com/videos/search?q=kharalakh+nickel+ore+body&&view=detail&mid=45106CC109B560DA82A945106CC109B560DA82A9&&FORM=VRDGAR
Cheers, stag11
Helmenesh
(20min delay)