AGE AND GEOCHEMISTRY OF MULTIPLE GENERATIONS OF XENOTIME OVERGROWTHS FROM THE BELT SUPERGROUP: IMPLICATIONS FOR THE ORIGIN OF THE SPAR LAKE RED BED STRATABOUND CU-AG DEPOSIT

Xenotime (YPO4) develops as epitaxial overgrowths on detrital zircons in sedimentary rocks. Xenotime that forms during diagenesis yields ages approximating the time of sediment deposition. However, xenotime overgrowths can also form during subsequent hydrothermal or metamorphic episodes, thus creating multiple generations in many samples. Textural relationships, backscattered electron imaging (BEI), in situ U-Pb SHRIMP dating, and electron microprobe chemical analyses provide criteria for distinguishing these different types of post-depositional xenotime from each other and from igneous (detrital) xenotime.

At least three ages of xenotime overgrowths are common in the Belt Supergroup (BSG), a thick (15-20 km) sequence of clastic and carbonate metasedimentary rocks deposited between about 1.47 and 1.40 Ga. Detrital zircons from the Prichard Fm. (base of BSG) have well-developed xenotime overgrowths that show complicated zoning in BEI. These overgrowths show a range of U-Pb ages due, in part, to the 10-micron SHRIMP analytical spot having sampled more than one generation of xenotime, from about 1.46 Ga (diagenetic) to 1.38 Ga (metamorphic). In contrast, xenotime overgrowths from the Pilcher Fm. (stratigraphically just above the BSG) mostly have ages of about 1.05-1.15 Ga, suggesting a cryptic Grenville heating event. Limited data suggest deposition of the Pilcher occurred at about 1.35 Ga. Detrital zircons of the Revett Fm. (middle part of the BSG) have yielded xenotime overgrowths only at the Spar Lake red bed-associated stratabound Cu-Ag deposit. New SHRIMP age data suggest that most of this xenotime formed about 75 m.y. after deposition of the Revett. This xenotime probably was deposited from hot (hydrothermal) ground water that was driven upward across rock units, perhaps by regional heating associated with widespread granitic plutonism. Relatively high As content of this xenotime, plus textures permissive of coeval formation of Cu sulfides and xenotime, suggest the hydrothermal origin. An older generation of xenotime overgrowth is geochemically distinct from the hydrothermal generation, and is interpreted as diagenetic in origin. The occurrence of two generations of xenotime within the ore and peripheral zones suggests that fluid flow was localized at the deposit site repeatedly.