Tracking capable metal detector
You might consider this method: -


Coil A, B, C and D are 4 receiving coils and the larger outer coil is a transmit coil. The magnetic field disturbance caused by the metal will (if ferrous in nature) increase the flux coupling to the receive coil that the metal is located nearest to. If the metal is non-ferrous, the transmit coil will induce eddy-currents in the metal and the receive coil it is nearest to will therefore have a decreased flux coupling.

Increases or decreases in flux coupling will result in increases or decreases in the signal voltage on the relevant receive coils. All coils need to be ideally tuned to resonance with parallel caps but this is only necessary if the metal item is small. The transmit coil would ideally be parallel tuned for maximum efficiency of producing flux.

If the pieces of metal are small you will probably need to tune the rx coils to resonance. I'd look at an operating frequency of sub 50kHz and probably somewhere more like 10kHz as even large chunks of iron will still produce a net ferromagnetic effect despite the metal being conductive and also producing eddy-current (anti ferrous effects). As frequency gets towrds about 100kHz large-ish pieces of ferrous (circa 6mm and above) will start to look more "conductive" than ferromagnetic.

The circuitry is all fairly straight-forward. An analogue oscillator and 4 amplifiers but the trick is mapping the coil outputs against actual position of the metal. Good luck.