1) If a deposit only has an oxide resource, it is usually because the sulphide deposit beneath is too low grade to meet the resource cut-off. For example, there may be marginally economic copper porphyry deposits that have an oxide/supergene zone grading 0.4-0.5% Cu, but the sulphide is only 0.2-0.3% Cu, therefore is not economic to even quote as a resource. Geologically speaking, oxide deposits usually have sulphide "roots" to them. Laterite deposits are the exception (for example, a nickel laterite deposit is confined to only the lateritic portions).
2) Depends entirely on the size, recovery and geometry of the orebody. For large throughput operations (+200,000 Oz per annum), probably 0.3-0.5 g/t. For smaller operations or narrower orebodies (higher strip ratio), perhaps higher.
3) If we are talking about gold, oxide ore is usually free-milling and easy to recover (with cyanide and CIL). However, if it is encased in a banded-iron formation then perhaps it costs more to grind it (more abrasive, harder material). With oxide deposits, whether the material is free-dig, or if it must be blasted has a large impact on operating costs.
Apologies for the late reply. Any other questions, don't hesitate.
