Context : Fornax is one of the most massive dwarf spheroidal galaxies in the Local Group .
The Fornax field star population is dominated by intermediate age stars but star formation was going on over almost its entire history .
It has been proposed that Fornax experienced a minor merger event .

Aims : Despite recent progress , only the high metallicity end of Fornax field stars ( [ Fe/H ] > –1.2 dex ) has been sampled in larger number via high resolution spectroscopy .
We want to better understand the full chemical evolution of this galaxy by better sampling the whole metallicity range , including more metal poor stars .

Methods : We use the VLT-FLAMES multi-fibre spectrograph in high-resolution mode to determine the abundances of several \alpha , iron-peak and neutron-capture elements in a sample of 47 individual Red Giant Branch stars in the Fornax dwarf spheroidal galaxy .
We combine these abundances with accurate age estimates derived from the age probability distribution from the colour-magnitude diagram of Fornax .

Results : Similar to other dwarf spheroidal galaxies , the old , metal-poor stars of Fornax are typically \alpha -rich while the young metal-rich stars are \alpha -poor .
In the classical scenario of the time delay between SNe II and SNe Ia , we confirm that SNe Ia started to contribute to the chemical enrichment at [ Fe/H ] between –2.0 and –1.8 dex .
We find that the onset of SNe Ia took place between 12–10 Gyrs ago .
The high values of [ Ba/Fe ] , [ La/Fe ] reflect the influence of SNe Ia and AGB stars in the abundance pattern of the younger stellar population of Fornax .

Conclusions : Our findings of low [ \alpha /Fe ] and enhanced [ Eu/Mg ] are compatible with an initial mass function that lacks the most massive stars and with star formation that kept going on throughout the whole history of Fornax .
We find that massive stars kept enriching the interstellar medium in \alpha -elements , although they were not the main contributor to the iron enrichment .