The event geological maps: a new predictive tool to identify potential NOA (Naturally Occurring Asbestos) lithologies, Example from the Pyrenees (France).

The event geological maps consist in innovative numerical maps that were just designed and produced for the first time, as part of the RGF (“French Geological Referential”) mapping program in the Pyrenees. Rocks acquire their mineralogical, structural and textural characteristics through a complex geological history reflecting successive stages of transformation (i.e. metamorphism, deformation, alteration…), so called “geological events”. Classical geological maps can only represent some of these events.  In the Pyrenean orogenic belt, which results from a polyphase tectono-metamorphic history over 600 Ma (from Precambrian to present), 3400 geological events were identified. Such geological events were classified by types (e.g. deposit, volcanism, intrusion, metamorphism, weathering, hydrothermal alteration…) and time periods. They were referenced into a database and associated to mapped features (120,000 polygons and lines), coming from a compilation of 60 geological maps at 1: 50,000 scale.

In the Pyrenees, Naturally Occurring Asbestos (NOA) mostly occur in specific lithologies such as ultrabasic, basic and intermediate plutonic rocks, and meta-limestones. These rocks may be affected by different metamorphic events (i.e. hydrothermal alteration, greenschist and/or HT-LP regional metamorphism, contact metamorphism). We performed a GIS treatment to produce a predictive map of potential NOA hosting lithologies. This treatment crosses lithological and selected geological event informations (e.g different metamorphic and alteration events).

Subsequent geological field investigations with associated sampling and laboratory analyses (combining optical microscopy, microprobe and SEM analyses) allowed us to identify and characterize fibrous and asbestiform mineralogical species. Results of this work particularly emphasize: (i) the importance of actinolite-asbestos in doleritic rocks, and (ii) the occurrence of fibrous actinolite/tremolite in different marbles and skarns. Finally, we present a 1: 50,000 scale map of potential NOA occurrences in the Pyrenees.

Conversely, field observations allowed us to improve both the lithostratigraphic and the event geological maps, in particular with the identification of geological domains where intense hydrothermal alteration was not previously mapped. All the data (maps of potential NOA occurrences, field observations and results of laboratory analyses) are stored in a geospatial database, partly accessible to the public. This work illustrates a possible use of geological event maps as a powerful innovative and predictivity tool. This approach will be useful in the context of the evolution of French regulations now imposing the search for asbestos before all types of works in natural environments.