ZONING OF THE WILLOW CREEK PLUTON, HATCHER PASS
Plagioclase is partially altered to sericite. This unit is distin- guished from the other units within the pluton by the high level of alteration and may not represent a discrete intrusive phase (see Discussion). This unit also has areas of gneissic texture, likely due to its close proximity to the fault, however this texture is not continuous throughout the unit.
Archangel Unit – Kaa
The Archangel unit (Kaa) is located on the eastern margin of the Willow Creek pluton (Figure 2) and is in intrusive con- tact with the quartz diorite pluton (Kqd) to the east defined by Bleick et al. (2012). The “eastern” quartz diorite pluton
Quartz grains are clear to white in color and are anhedral. Plagioclase are white and anhedral. Small amounts of iron oxidization can be seen on fresh surfaces.
Gold Bullion Unit – Kgb
The Gold Bullion unit (Kgb) is located on the western half of the mapping area (Figure 2). The unit is made of coarse- grained tonalite and quartz diorite and has a color index of up to 30. The major minerals are biotite, quartz, hornblende, and plagioclase. Hornblende and biotite are present in equal abundance and represent all the dark minerals, both are subhedral to euhedral with the largest crystals being 1-3 mm. Quartz is grey to white in color and anhedral. Plagioclase is white in color and predominantly subhedral. This unit can be distinguished from other units by the presence of large euhedral mafic minerals (Figure 5).
Independence Mine Unit – Kim
Figure 3: Outcrop typical of the Fish Ridge zone showing the high degree of alteration of hornblende and biotite to chlorite (green) and plagioclase to sericite (white).
The Independence Mine unit (Kim) is located at the center of the Willow Creek pluton (Figure 2). The unit is made of
appears to be a younger intrusion into the Archangel unit and the apparent contact between these two bodies is mixed. The Archangel unit is made of medium- to coarse-grained tonalite and quartz diorite and has a color index of up to 30. Fine- grained elongate enclaves with quartz dioritic composition are present throughout and are characteristic of the unit (Figure 4). The largest enclave measured was 10 cm in length. Major minerals are hornblende, quartz, biotite, and plagioclase. Hornblende makes up roughly 60% of the dark minerals and is subhedral to euhedral. Euhedral crystals were measured up to 2 mm long in hand sample. Biotite crystals are subhedral.
Figure 5: Gold Bullion unit with coarse grained euhedral biotite.
medium-grained tonalite that has a color index of up to 25. Major minerals are biotite, quartz, hornblende, and plagio- clase. Biotite makes up roughly 60% of the dark minerals and is subhedral to euhedral. Hornblende is 40% of the dark minerals and is subhedral. Quartz is clear to white in color and anhedral. Plagioclase is white and dominantly subhedral, although some small euhedral crystals are present. This unit is distinguished by presence of euhedral biotite and relatively low color index (Figure 6)
Discussion
Plutons in the Willow Creek mining district post-date the majority of igneous activity in the Talkeetna range by nearly 100 m.y. and therefore are not part of that island arc event. This makes the area unique to the range and raises the question of what Late Cretaceous event these plutons can be associated with. These rocks have been associated with the Alaska Range (Hudson, 1979), but further study is warranted to confirm this connection.
Figure 4: Two examples of enclaves found in the Archangel unit. Both have fine grained hornblende, biotite, quartz, and plagioclase with phenocrysts of euhedral plagioclase.
The Tuolumne Intrusive Suite in the Sierra Nevada moun- tain range of California is a classic example of a zoned pluton. The suite is a quartz diorite on its outer rim and moves progres- sively more felsic into its center (Bateman, 1992). Additionally,
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