For LOW grade you would not see much in a 4mm chip of rock so unless its evenly disseminated so RC is hard to visually determine. To make matters tougher OXIDES means the sulfide has been erroded and the metals are mainly in a free state as staining that may or may not contain the metal you are after (it all looks the same).

Drill core with oxides will typically be fragmented and stained. The fresh sulphides are fairly easy to quantify in freash core with a considerable leeway for estimating. If the geologist starts to see galena (a lead sulphide)and sphalerite (a zinc sulphide) in the core there is no mistaking it. They will know roughly how excited to get. But again it will be an eyeball estimate and the mineralization will vary in intensity so some sections will more mineralized than others.

Below is a core sample with from the Black Angel Mine in Greenland. The grade is VERY VERY High. I included it just to show what Galena and Sphalerite looks like in drill core. 1-2% would look like the very left edge of the core vs the massive sulphides in the center area.