Petrology, Tectonic Setting, and Potential for Concentration of Rare Earth Elements (REE) and High Field Strength Elements (HFSE) in the High-K Darby and Kachauik Plutons, Seward Peninsula, Alaska

Science Center Objects

One of the geologic environments that host rare earth and other critical and strategic element deposits are alkaline intrusive rocks.

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Darby Mountain outcrop of quartz monzonite towards the contact between the pluton and Paleozoic marbles

Looking east past a typical Darby Mountain outcrop of quartz monzonite towards the contact between the pluton and Paleozoic marbles.
​​​​​​​(Public domain.)

Science Issue and Relevance

One of the geologic environments that host rare earth and other critical and strategic element deposits is alkaline intrusive rocks. The Kachauik pluton and nearby stocks and dike swarms on the eastern Seward Peninsula are some of the most strongly potassic intrusive rocks in Alaska, and among the most alkaline in the state. These plutons have not been assessed for rare earth or critical element deposits. A recent data-driven GIS analysis for critical minerals in the state of Alaska identified the eastern Seward Peninsula as having high potential with high confidence to contain concentrations of rare earth and high field strength elements, as well as other metals including copper, molybdenum, tin, and tungsten. These rocks are the best exposed alkaline igneous rocks in a belt that extends for 500 km from St. Lawrence Island through the eastern Seward Peninsula and along the foothills of the southern Brooks Range. Understanding the petrogenesis of the alkaline rocks and the sources and processes of concentrating the associated critical elements is important for evaluating mineral potential and conducting resource assessments in northwestern Alaska.

A structural corridor commonly referred to as the Kugruk fault zone bounds the eastern margin of the alkaline igneous complex on the eastern Seward Peninsula. The fault zone appears to be a juxtaposition of disparate structures and kinematic events in time and space. Faults within the zone displace rocks of different metamorphic grade and structural levels, indicating multiple episodes of activity, some of which predate emplacement of the alkaline igneous rocks. The timing, kinematics, and context of interaction of the fault structures with the alkaline intrusive rocks in the project area is unknown and will be investigated in the course of this project. The Kugruk fault zone is known to contain gold mineralization to the north of the project area but gold mineralization has not been identified in the southern part of the fault corridor. This project will investigate the southern Kugruk fault zone, and address implications of the structures in that area with respect to emplacement and exhumation of the alkaline igneous rocks, associated mineralization, and regional tectonics.

Rock with phenocrysts and hammer for scale

Stretched phenocrysts of potassium feldspar in syenite of the Kachauik pluton on the Bering Sea coast of the southeastern Seward Peninsula.
​​​​​​​(Credit: Susan Karl, USGS. Public domain.)

Research Activities

The project objectives are to investigate the petrogenesis of the high-potassium intrusive rocks, identify the sources and processes for critical element concentration in these rocks, and determine the tectonic setting of this belt of alkaline igneous rocks. Detailed geologic mapping of rock types, minerals, textures, and structures will help to evaluate the interactions of magmatic, deformation, and mineralizing events. Samples collected for detailed geochronologic, geochemical, and isotopic analyses will increase understanding of magma sources and petrogenesis, and the sources and mechanisms involved in concentration and deposition of rare earth elements, high field strength elements, and other potential commodities. The interplay of Kugruk fault structures with the eastern margin of the pluton will be investigated with respect to timing, kinematics, and possible fluid flow. This information will collectively help to define and constrain the tectonic conditions that fostered these alkaline rocks and associated types of mineralization, and contribute to a regional synthesis of the geology and tectonics of this mineral belt.