According to one theory, the region between Mars and Jupiter was once populated by several primordial planets with a combined mass of roughly the earth. These planets provided the material that made up the original asteroid belt. Since then, 99.9% of this material was lost. The current composition of the asteroid belt is about 10% metallic and 90% silicate/carbonaceous. Could we assume that the original asteroid belt had the same composition? Or, at least a very similar composition?
We can be pretty sure that the asteroid belt once held one or more sizable planetesimals,as there is evidence that it or they were large enough for gravity & internal heating to drag the heavy metals (mainly iron & nickel) to the centre,forming a metallic core. Bodes Law decrees that there should be a planet where the asteroid belt is now,but it seems that as this planet was forming it was shattered by an impact & the remnants scattered. Some reached Earth,which is how we know that their materials had differentiated into metallic core & rocky mantle. It is likely that some of the huge asteroids which have struck the Earth in the distant past were fragments thrown out of the asteroid belt & that some hit Mars & Venus as well. There may be more material in the asteroid belt than we think,because some fragments are so small & widely dispersed that they don't register in our telescopes. I think we can assume that the primordial planetesimals had a similar though not identical composition to the asteroid belt of today.