If your half gram was a single gold nugget that made it through...

If your half gram was a single gold nugget that made it through the sub-sampling process into the 30 gram fire assay charge it would return an assay of 16,667g/t

If your half gram was a single gold nugget that ended up in the unsampled half of the core and got put back in the core tray or it got split off (rejected) in the sub-sampling process after crushing, the fire assay would return an assay of 0g/t (zero).

or:

Half a gram of gold in the 3kg drill sample sub-sampled perfectly with zero nugget effect = 167g/t which is pretty close to 69g/t. 

In reality it's probably somewhere between the first example (extreme nugget effect) and the last example (zero nugget effect).