Impact of chemical weathering on micro/macro-mechanical properties of oolithic iron ore
Résumé
This article concerns the problem of collapses of abandoned underground (room-and-pillar) iron mines in Lorraine (North-Eastern France). A study on the ageing of iron ore was carried out using different analytical techniques (SEM-EDS, EMPA, X-ray diffraction, Mössbauer and ICP spectroscopy). As iron ore is a reactive environment in an oxidizing atmosphere created by anthropic activity, this kind of weathering is triggered or activated by chemical processes, and differs from diagenetic evolution under reducing conditions. The weathering is characterized by the chemical alteration/oxidation of reduced minerals (siderite and berthierine) in the inter-oolithic cement, mechanical disaggregation of oolithes, recrystallization of berthierine and neo-crystallization of a honeycombed hematite crust (i.e., rust) on all mineral surfaces. This corrosion of iron ore was found to occur at a large scale in pillars, especially in very old pillars which have been exposed to the mine atmosphere for 70–100 years. The alteration of the inter-oolithic cement which ensures material cohesion was not found to have been compensated by the crystallization of new minerals, thus inducing the decrease in the micro/macro-mechanical properties of iron ore (micro-indentation tests at millimetric scale, micro-compression triaxial tests at centimetric scale and macro-compression triaxial tests at decimetric scale). This chemical weathering of anthropic-origin plays a large-scale and major role in the long-term behavior of pillars, and is probably one of the main causes of mine collapses that occured in Lorraine. Therefore, the decision to stop pumping of mine drainage, thus inducing flooding of iron mines (by groundwater), was the correct decision, since oxidation reactions in iron ore are necessarily less important under anaerobic condition than under aerobic condition. In addition, the knowledge of iron ore ageing allows for improved assessment of the long-term stability of iron mines which have yet to collapse (i.e., flooded iron mines and also iron mines which are still above the ground water-table).