Zoning of the Willow Creek Pluton, Hatcher Pass State Historic Recreation Site, Talkeetna Mountains, Alaska.
David A. Meagher, SA-6485
geomeagher@gmail.com Erin Shea
eshea2@alaska.edu Abstract
The Hatcher Pass Management Area is located in the Talkeetna Mountains 60 miles north of the city of Anchorage, Alaska and includes the Independence Mine State Historic Park. The area has been known for its gold-bearing deposits since the early 1900s and has produced approximately 624,000 ounces of gold. The mining district around Hatcher Pass is historically Alaska’s third largest lode-gold producing region and continues to be explored for further mining possibilities. Mother Lode vein-style gold mineralization is largely found in the ca. 72 Ma Willow Creek pluton, which is intruded by a younger (ca. 70 Ma) non-mineralized pluton to the west.
New field-based mapping has subdivided the Willow Creek pluton into four distinct units based on variations in grain size, color index, and mineralogy: the Fish Ridge unit, the Archangel unit, the Gold Bullion unit, and the Independence Mine unit. The spatial relationships between the units suggest that the pluton may be zoned. Our preliminary field work, combined with previous geochronology on the pluton, indicates that the intrusions in the outer margin of the pluton are generally older and more mafic and that intrusions in the center of the pluton are generally younger and more felsic.
Key Words: Willow Creek pluton, Willow Creek mining district, Hatcher Pass Alaska, zoned pluton
Introduction
The Willow Creek mining district is located in southern region of the Talkeetna mountain range in southcentral Alaska, 60 miles north of the city of Anchorage. The Willow Creek mining district ranks as the third largest gold lode mining district in the state of Alaska, producing 624,000 oz. of gold (Harlan et al., 2003). The majority of lode gold is found in quartz veins in the Willow Creek pluton. Pelitic schist to the south of the Willow Creek pluton contains discontinuous gold-bearing veins that are economically insignificant (Harlan et al., 2003). There has been no economic mining of gold in the plutons to the west or east of the Willow Creek pluton. The region is now part of the Hatcher Pass Management Area which includes the Independence Mine State Historic Park. This management area is considered the Willow Creek mining district and there are currently active claims and exploration in the district. The purpose of this study was to map and classify differences within the Willow Creek pluton in an effort to learn more about its intrusive history and attempt to sub-divide the pluton into discrete intrusive units based on variations in
composition and texture. Field mapping was conducted over the summer of 2016.
Geologic History
Like much of the state, the southern margin of Alaska is an amalgamation of accreted terranes (Figure 1; Colpron et al., 2007). The Talkeetna Mountains span two of these terranes, Wrangellia and the Peninsular terrane, and the mountains are dominated by the remains of the Jurassic Talkeetna arc (DeBari and Coleman, 1989; Greene et al., 2006; Rioux et al., 2010).
Previous mapping in the Willow Creek mining district has defined three Cretaceous quartz diorite to tonalite plutons and a Cretaceous pelitic schist (Bleick et al., 2012). The Hatcher Pass fault zone separates the Willow Creek pluton and the pelitic schist (Harlan et al., 2003). The Willow Creek pluton is the oldest pluton in the immediate area; U-Pb in zircon ages range from 79.1 ± 1.0 Ma - 72.4 ± 0.4 Ma (Bleick et al., 2012). 40Ar/39Ar in hornblende ages range from 72.8 to 70.5 Ma and biotite ages range from 70 to 67.9 Ma (Harlan et al., 2003). Ore mineralization in the Willow Creek pluton post- dates intrusion: 40Ar/39Ar dates from sericite associated with the Au mineralization range from 66.9 to 65.6 Ma (Harlan et al., 2003). The pluton east of the Willow Creek pluton is a quartz diorite that commonly contains mafic enclaves and is also known to contain orbicular structures (Ray, 1952). This “eastern” pluton has a single U-Pb in zircon age of 75.8 ± 0.7 Ma (Bleick et al., 2012). The pluton west of the Willow Creek pluton is also a quartz diorite and has a single U-Pb in zircon age of 70.5 ± 0.2 Ma (Bleick et al., 2012).
Unit Descriptions
Mapping during the 2016 field season allowed the delinea- tion of 4 distinct phases within the Willow Creek pluton (Figure 2). These phases were distinguished from each other in the field based on composition, grain size, and texture.
Fish Ridge Unit - Kfr
The Fish Ridge unit (Kfr) is located on the southernmost boundary of the Willow Creek pluton (Figure 2). The unit is in contact with the pelitic schist (Kps) to the south via a fault defined by Bleick et al. (2012). This unit is made of highly altered medium-grained quartz diorite and tonalite and has a color index of up to 30. Hornblende and biotite have been almost completely altered to chlorite (Figure 3). Quartz and plagioclase are white in color, fine grained, and anhedral.
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