Fifteen of the 16 known porphyry copper deposits are known to be Laramide (72-54 m.y.), like their host plutons. Bisbee is older (170 m.y.). Erosion and supergene enrichment preceded burial by mid-Tertiary sediments or volcanic rocks (early Cretaceous rocks at Bisbee). This burial preserved them until their exhumation in the present erosion cycle. The base of these Tertiary rock sequences is therefore regarded as a favorable locus for exploration for new deposits.

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