the deeper the pit the longer and wider it needs to be at the top, think of an inverted cone. which is fine if you have wide spread mineralisation (google the super pit in kalgoorlie and the BHP expansion of Olympic dam in SA...big volumes that justify a large deep pit and moving a lot of earth)
However if you have narrow veins of high grade gold it may be uneconomical to mine them using an open pit even at shallow depths, better to sink shafts and chase the veins
as with most things there are no black and white answers it depends on a number of factors
