Mineralogy and U-Th-Pb age of a uranium-bearing jasperoid vein, Sunshine Mine, Coeur d'Alene district, Idaho, USA

摘要

The age of the Mesoproterozoic Belt Supergroup hosted silver and base metal vein deposits of the Coeur d'Alene mining district in northern Idaho has long been controversial. Evidence for Precambrian mineralization comes largely from many -1400 Ma Pb model ages for galena throughout the district, and several problematic 1100-1300 Ma ~207 Pb/~206 Pb ages reported in the 1950s on uraninite from the Sunshine Mine. Field studies, on the other hand, have generally related mineralization to Mesozoic and Cenozoic tectonic and igneous events. A possible explanation of the galena model age enigma is that lead from metal-enriched Belt rocks was remobilized with minimal isotopic modification into younger deposits. The availability of a new occurrence of the uranium-bearing jasperoid vein exposed in the 5700-foot level of the Sunshine Mine allows reinvestigation of the uraninite using modern electron microprobe and isotope analysis techniques. Most of the uranium occurs in a prismatic UTi_2O_6mineral commonlyin aggregates of 5-50 um long lath-like crystals, identified as originally being brannerite. Subsequently, the brannerite has exsolved into extremely fine-grained uraninite and a T_02 polymorph. Rare, small crystals of zircon discovered with the electron microprobe, however, proved to be an even more useful geochronometer in unraveling the vein's complex history. Three morphological varieties of zircon were identified: (1) rounded, purple grains of obvious detrital nature, (2) simple euhedral crystals inferred to have originated in an igneous(?) rock or quartziferous vein precursory to the present jasperoid vein, and (3) crystallographically complex crystals attributed to hydrothermal growth in the jasperoid vein itself. The first two varieties lend themselves to rather straightforward age interpretations, while the third held the key to resolving the problematic uraninite ages. Four detrital zircon grains give discordant but projected concordia ages of 1.5-1.8 Ga, which indicate derivation from Paleoproterozoic crystalline rocks that supplied sediments to the Belt basin. Two zircon crystals yield essentially concordant ages of -1350 Ma that significantly postdates deposition of the host St. Regis Formation, and appear to have crystallized during a widespread orogenic event occurring soon after deposition of the uppermost Belt Supergroup. Unlike the detrital and -1350 Ma-old zircon, six grains of the younger, presumably hydrothermal zircon have a relatively high common lead content and give no consistent age pattern. For example, a Coeur d'Alene-type initial lead (~206 b/~204 b =16.20, ~207 pb/~204pb =15.36~208PPb/~204PB = 35.90) yields highly discordant results that range in~ 207Pb/~206Pb age from 556-170 Ma and suggest variable 20-80% radiogenic lead loss. Similar discordance with an even greater range in ~207Pb/~206Pb age is shown by brannerite crystal aggregates, a euhedral pyrite crystal, and the whole-rock. Despite the uncertainty in initial lead isotopic composition, the rather tight clustering of ~206pb/~238U ages around 120-150 Ma combined with the much older ~207Pb/~206Pb ages strongly hints at an anomalous, old radiogenic lead inherited by younger crystals. Indeed, some choice of initial lead lying along a secondary isochron defined by radiogenic lead grown between 1350 Ma and -130 Ma with an original Coeur d'Alene isotopic composition could be selected for every mineral analysis so as to make it concordant at between 124 and 139 Ma. Alternatively, a 3-D plane can be fit to the mineral analyses on a ~238U/~206Pb ~207Pb/~206Pb ~204Pb/~206Pb plot, yielding concordia intercept ages of 1344 ± 77 Ma and 136± 2 Ma (MSWD = 0.54).