Iron mines seem to generate more CO2 than most mines. Coal and shale mines generate their fair share and limestone based mines are less bad.
Anything that can decay will help produce CO2. I don't know the chemistry of the decay in iron mines, just that they are prone to it.
If there is little or no ventilation, as often happens in large pillar and stall workings, the CO2 just lays there from the floor upwards depending on gradients of the seam.
It is very easy to walk down into a 'lake' of CO2 and be overcome. That is one reason why you should move gently with your detector in your hand hanging down. No point having your detector clipped to the collar of your overalls in this scenario. I have been in steeply dipping drifts, waist deep in CO2, where you could lower your meter and get a massive reading of CO2 and little oxygen then lift it up and get very little CO2 and plenty of oxygen. The only real cure is ventilation. Given that large iron mines would take a long time to explore and navigation is often difficult without good plans, it is better to give up at that point. Using breathing apparatus is a skilled job and only for short duration, not meant for primary exploration.
Calculations for blackdamp are not straightforward add and subtract. Air has a fixed percentage of oxygen to nitrogen. So it follows that if you reduce the oxygen content there is still the required ratio of nitrogen to that lower oxygen plus some 'spare' nitrogen, since only the oxygen has depleted not the air. So you add CO2 into that and that reduces the volume of air too.
Blackdamp is actually a mixture of CO2 and the excess nitrogen. This is why basic add and subtract of percentages don't follow through.
Obviously from the practical point of view, nitrogen will kill you just as well as CO2.
The next issue is whether you are measuring a CO2 lake or CO2 in the general body of the air (ie all mixed up). Mostly in fairly level workings where your party has been moving about you will be reading the general body. Therefore you will have CO2 and excess nitrogen affecting the oxygen reading. If you have the CO2 lake situation, the lake will be high or pure CO2 and the air above the lake will be general body with the oxygen replaced by excess nitrogen. Stir up the lake and you have big trouble.
At Dragonby we found that the CO2 levels would increase by around 1.5% in the general body during our three monthly absences. Ventilating during daytime for 6 days resulted in lowering the main areas back to good limits of around 0.5% general body. However, the further reaches off the main vent circuit would remain around 2% most of the time.
With no ventilation the whole mine would have filled with CO2 over some years. Only the entrance roads remaining clear by expansion/contraction of the air mass.
In fact when we re-opened one section of the mine that was cut off from the rest, it was full to within 100metres of the surface as you descended the inclined drift. Mind you, it had been fully closed off for around 25 years.
Les