ZONING OF THE WILLOW CREEK PLUTON, HATCHER PASS
Figure 6: Independence Mine unit. Dark minerals are predominantly euhedral medium grain biotite.
the suite becomes progressively younger towards the center (Coleman et al., 2004). If the Willow Creek pluton is in fact a zoned pluton it would need to have these two characteristics: 1) Progression from more mafic outer rim to more felsic center. 2) Intrusions on the outer rim will be older than intrusions in the center.
Most of our newly-defined units were dated using U-Pb in zircon by Bleick et al. (2012), which offers further insight into the intrusive history of the pluton. The Fish Ridge unit includes two samples dated at 79.1 ± 1.0 Ma and 77.8 ± 0.8 Ma (Bleick et al., 2012). The discrepancy in these ages sug- gests that the Fish Ridge unit may contain more than one intrusive phase, however the deformation characteristic of this unit makes it difficult to map. The Archangel unit has one date from eastern margin at 76.1 ± 0.9 Ma (Bleick et al., 2012). There are no U-Pb dates from the Gold Bullion unit. The Independence Mine unit has one sample dated at 72.5 ± 0.4 Ma (Bleick et al., 2012). These dates support the idea that the Willow Creek pluton may be zoned; preliminary mapping and existing geochronology suggests that the Willow Creek pluton has a more felsic, younger interior, although it does not appear that the zoning follows a simple pattern. We find no correlation between our map units and mineralization within the Willow Creek pluton, which is expected since mineraliza- tion is believed to post-date intrusion of the pluton (Harlan et al., 2003). There also seemed to be little to no occurrence of mineralization in the neighboring plutons to the east and west.
Our mapping and existing geochronology suggest that the “eastern” quartz diorite pluton (the pluton east of the Willow Creek pluton) may be a part of the Willow Creek pluton (Figure 2). Previous work separated the two intrusions based on the abundance of mafic enclaves in the “eastern” quartz diorite pluton (Bleick et al., 2012). Our findings showed a similar abundance of enclaves throughout the Archangel unit in the Willow Creek pluton. Outcrops near the mapped contact between these two intrusions show a large variation in color index, texture, and grain size, as well as structures suggestive of magma mixing (Figure 7). The single U-Pb date from the “eastern” pluton (75.8 ± 0.7 Ma; Bleick et al., 2012) falls within the range of ages for the Willow Creek pluton (79.1 ± 1.0 Ma -72.4 ± 0.4 Ma-; Bleick et al., 2012). One major difference between the two units is the absence of any major mineralization found in the “eastern” quartz diorite pluton. Further study is warranted to investigate the connection between these intrusions and mineralization.
Figure 7: Evidence for mixing between the “eastern” pluton and the Archangel unit.
Future Research
Future work focusing on mapping, particularly along the eastern and western contacts, the interior, and in the northern region of the pluton would offer more insight into the zoned nature of the pluton. In addition, whole rock geochemistry and more detailed geochronology would allow a deeper understand- ing of the magmatic history.
Conclusion
The Willow Creek pluton is a Late Cretaceous intrusion that can be divided into four different units based on minerol- ogy: the Fish Ridge unit, the Archangel unit, the Gold Bullion unit, and the Independence Mine unit. These units represent a minimum of four intrusive episodes in the pluton. Field rela- tions and geochronology by Bleick et al. (2012) indicate that the outer rim of the pluton is relatively older than the center. The pluton also transitions compositionally from an intermediate rim to a more felsic at center, roughly following the pattern of a zoned pluton (e.g., the Tuolumne Intrusive Suite; Bateman and Chappell, 1979). However, further mapping and analyses are necessary to understand the pattern of this zoning.
Acknowledgements
The authors wish to thank the Alaska Department of Natural Resources, Division of Parks and Outdoor Recreation for granting permission to collect samples in the Independence
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