When looking through my acquaintance’s web photos from his visit to the Sudbury Neutrino Observatory, I came across this neat little image; a map really. In addition to showing an overview of the old mine in which the SNO is located, it seems to also indicate the location and distribution of the nickel ore (in yellow). And that gives rise to my question:
How is that done? How do ‘they’ determine the location of the ore, its orientation, and its twists and turns?
More specifically, it’s one thing to have a general idea that a site has an unexpectedly higher or anomalous concentration of a substance of interest. But this is different. It seems to be showing the length, breadth, and depth of the deposit, and the twists and turns it takes. I doubt such data could have been obtained by drilling for samples: it would take thousands of them to create the final image. Same goes for geochemical and geophysical analysis (I think). Again, it would take a huge number of samples and it’s not even clear to me that could even be done, i.e. how do you get downstream or on the other side of an underground deposit in order to check for anomalies.
In short (and as must be abundantly evident), I don’t have a clue how this vein (or any vein, in any mine, anywhere) is mapped out. Can anyone put a dent into my ignorance?
The ore IS all mined out now, so they now where they got it from..
The horizontals and verticals , black, are merely for safe movement.
The yellow (ore ) is also mined out, empty space, where the ore was removed !!!.
You might have thought it was like coal mines, where most of the horizontals are in coal.
Ground penetrating radar? Sonar? Nope. Those don’t work so good in solid rock mining.
You do it by drilling ahead of the current workings- extracting a hollow core - and analysing that for ore samples, faulting that could throw the ore layer up or down, etc.
I know because I was a geologist on a deep gold mine, and part of that job involved supervising teams of diamond drillers, determining where they’d drill, and then sitting in the core yard, hose in hand, logging metre after metre of core.
Note that there’s the initial, from-the-surface exploration drilling, which is bigger core and done before the ore body is ever exploited, and then there’s the daily, underground core drilling, which refines the picture for more precise work.
Worked as a stope and drift mapper for some years. The ore projection one derives remotely (remote sensing, drilling, geophysics, geochem, etc.) give you a workable estimate but will not provide the kind of precision one needs during actual mining. I’m talking about a gold vein that might only be a few inches thick but rich in content. So during development works and actual mining, the_diego has to creep through 12-inch thick crevices in 105 degrees humid heat and take out his mapping weapons.
Start with a known point (a surveyor’s spud embedded on the stone wall or ceiling of the tunnel. Take out your string and stretch it along the length of the vein drift you plan to map. Suspend your compass from that string to get the bearing from the known point and then measure the length. Next, at 2-meter intervals along that string, measure perpendicularly using a stanley metal tape the distance from the string to the tunnel wall at breast height. In doing so, you end up with a good plan view of the tunnel you are mapping (breast height, remember that.) Add permanent man-made structures like posts, pillars, doghole entrances, etc.
Having drawn the place you want to map, you start putting in the geologic features you see. Start with structures (like your faults and veins). These have to be projected at a plane at breast height. So a quartz vein parallel to the tunnel, visible at the center of the tunnel ceiling but slanting 60 degrees should appear slightly to one side. Horizontal structures (appearing at the walls) wll not appear on your plan so you have to make a special sectional map of it. Then you go into lithology. What rocks are there where these structures are found. Finally, in the case of copper and gold, you put in the rock alterations.
Mapping is an art and the really good ones aren’t just accurate to less than a foot on a 1:200 plan but aestheically pleasing (necessary for investors and consultants who want to see your maps.)
Yeah, the thing to realise is that it isn’t just the original exploration drilling (which is on a wide grid and just proof-of-concept stuff, really), it’s daily drilling ahead of the workings that constantly refines the ore body. Add in CAD software to map the extent of the geology and you can develop quite sophisticated models.
And then find one hair-fine fault has actually thrown everything off by 2m anyway, so the miners have to reblast the raises - boy, do they love you then
Also worth noting that not only is the core sampled for gold content, but there are dedicated teams of samplers (2 per geologist, on my mine) working the stope faces as well as all the new tunnel works, and it all adds up to a pretty well-delineated dataset. One of my jobs as a mine geologist was taking all my samplers’ logbooks and sketches, and the assay data, and plotting them on the official mine plans, then transferring all that to the CAD system for ore reserve calculations. And those are official financial documents on which the well-being of the mine are dependent. Big responsibility, it’s not all looking at pretty rocks. At least on a South African gold mine, the ore bodies are relatively tabular and easily recognizeable from the usual host rock. I’d hate to work hydrothermal veins, tricksy twisty things they are.
Also, quartzite core samples make wonderful pastry rolling pins.
Actually my toughest underground mapping job was not hydrothermal quartz veins. It was steeply-dipping coal seams that had a habit of disappearing every 50 meters or so by a lateral fault. I often had to be roused from sleep, asked to go underground, look at the faulted seam and tell miners right there and then where to swing their drift to intersect the displaced seam.
