I would assume that infill is used to mean they have seen some 'irregularities' in either thickness of the gold bearing strata or in the concentration within the strata and they need more information on quantify the reserves.

I am a petroleum engineer and am simply assuming that the gold industry has similarities with the oil industry in how we quantify our reserves.

When quantifying reserves, you need (in simple terms) length, width, height and then concentration. The length and width you get by drilling outwards to define the boundaries. You drill infill to get additional data on the height or thickness and the concentration. You generally work on the length and width and then gather the thickness and concentration from the cores you drill to get the length and width.

Again, using oil as my model, I would guess that they have seen either some height or concentration irregularities which they want to better understand to better quantify the gold. As height will vary over the length and width, you simply extrapolate between the various points.

Again with oil, it is common that you find these irregularities in the way the strata was deposited (in oil's case by rivers, oceans, deltas, etc.). Oil's concentration is based upon porosity (voids between sand/rock particles) and then saturation (variation of oil, water and gas).

I hope this helps. Comments are invited as I certainly know more about black gold (oil) than yellow gold and even EVG.v