Abstract

Samples of chimneys (black smokers) and other volcanogenic massive sulfide (VMS) mineralization were collected from hydrothermal vent fields associated with submarine arc and back-arc volcanoes in the western Pacific. Each specimen was dated using radiometric techniques to establish the frequency and duration of hydrothermal activity at that site, information that is vital when establishing the time elapsed to amass an ore deposit. In each case, the radiometric dating relied on the radium isotopes 228Ra and 226Ra with half-lives of 5.75 and 1,600 years, respectively, which coprecipitate with barite and then decay.

We began dating VMS mineralization 15 years ago, knowing that the 226Ra/Ba decreases with time, with the Ba and Ra assumed to be from a common source such that the initial 226Ra/Ba value, i.e., the value at the onset of mineralization, was constant. Thus, the age of an “old” chimney (up to 15,000 yr) was calculated by comparing its 226Ra/Ba value with the initial value established from nearby “young” chimneys (<35 yr) that were active or still contained 228Ra. Since then, we have discovered that chimneys and other forms of sea-floor VMS mineralization can contain barite from two or more hydrothermal events. That is, older barite can be dissolved and reprecipitated (remobilized) from earlier mineralization deposited subseafloor, or earlier sea-floor mineralization overprinted (crosscut) by new. In many cases, the 226Ra/Ba dating did not provide an absolute age for the older material but, rather, a difference in age between two samples that had incorporated varying proportions of older barite that was low or “dead” (i.e., >15,000 yr) with respect to 226Ra, due to radioactive decay.

Remobilizing of older barite is consistent with zone refining of metals in large VMS deposits, but we have only considered this recently. However, our investigations show that this can affect the initial 226Ra/Ba values for VMS mineralization from arc and back-arc volcanoes, and also from mid-ocean ridges. Where sufficient samples contained 228Ra, we estimated an age for the remobilized barite by plotting a 226Ra/Ba versus 228Ra/Ba mixing line for the VMS at the time of mineralization and found that hydrothermal activity has occurred at some arc volcanoes for >15,000 years. Such long histories of hydrothermal activity indicate the propensity for producing large metal-rich deposits on the seafloor.

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