Good info, but you are confusing 'high-grade' with 'economic'. You can have a high-grade intercept at depth which may remain uneconomic to extract as there is insufficient volume of resource, or poor metallurgical recovery, or access issues. Grade thresholds are not linked to depth below surface. Economic constraints are.
When it comes to lower grade intercepts, a wider zone of mineralisation, even at the lower grade, may be economic to extract...ONLY if there is a cost-benefit to the application of potential larger scale mining methods. You are not going to mine skinny (<10m) sub 2% Ni underground...and make money anyway.
1% copper is not high grade. In most underground mines an economic cut-off for copper would be in the order of 2-3% Cu. 1% Cu is OK for open-pits or large-scale (cave) mining.
Your calculations need to take into account mining cost also. This could be an operating cost of up to $80/tonne for skinny underground deposits. Processing at $10-25/t depending again on scale.
1% Zinc is really, really not interesting. In an underground scenario, if you mine and process (forget about costs of exploration and capital costs like: mill/plant...power...decline as these will kill your enthusiasm) for $100/t then you'll need ~7% Zinc ore to make a profit. Longgggg payback time on the plant though.
