RATES OF MINERALIZATION AND ORE DEPOSIT FORMATION

Modern geochronologic methods can distinguish geologic events that are separated in time by a few tenths of a percent of their age or less. As an example, a series of events, that occurred over 50 to a few hundred k.y. at 100 Ma, can be resolved. With this resolving power, duration of processes also can be precisely defined. If a single isotopic system is used to define the age of events, internal comparisons are straight-forward but the range of applications is restricted. If multiple isotopic systems are employed, the range of applications is broader but cross-calibration becomes the major factor that defines accuracy. If cross-calibrations are inaccurate, say by 1 percent, at best the high precision of the methods is obviated. At worst, the apparent differences in age of events are inaccurately recorded or missed. The 40Ar/39Ar isotopic dating technique is popularly applied in mineral-deposit research because of its natural versatility and experimental robustness. Because potassium-bearing minerals commonly form during processes associated with mineralization, a datable mineral is often present and that mineral can be used in determining the age and (or) cooling history of the mineralization event. In addition, the high precision of the method makes it especially applicable for defining duration and timing of events in complex ore deposits. For example, the once world-class 300-Ma Panasqueira, Portugal Sn-W deposit formed in several stages over 4 m.y. The 30-Ma Urad-Henderson Mo deposit formed during the multiple emplacement of plutons over a 3-m.y. period. The Butte Cu-porphyry hydrothermal system formed over a 2-m.y. period circa 65 Ma in two distinct stages. For broader applications, multiple isotopic systems must be employed. For the Cornish Sn-W system, England, cross-calibrated U-Pb, Ar/Ar, and Sm-Nd dates indicate more than 30-m.y. duration for 3 of 4 stages of activity. An example of inconsistent inter-calibration, which affects accurate resolution of events, is for the Western Australia Archean gold districts where 1-percent inaccuracy translates to more than 25 m.y. of apparent difference.