Complex sentence: It is a diminutive form of "la mine", and might be translated as "little mine, little colliery" or "little vein", referring to its relatively poor iron content of between 28% and 34%.
Split into simpler sentences: It is a diminutive of "la mine", and might be tranlated as "little mine, little colliery" or "little vein". This refers to its relatively poor iron content of between 28% and 34%.