| Abstract |
We performed ground and airborne geophysical investigations of the Pilgrim Springs geothermal area (Seward Peninsula, west-central Alaska) to identify key structures controlling hydrothermal fluid flow. Studies included ground gravity and magnetic measurements, as well as an airborne magnetic and frequency-domain electromagnetic (EM) survey. Helicopter magnetic and EM data were acquired using a Fugro RESOLVE system equipped with a high sensitivity cesium magnetometer and a multi-coil, multi-frequency EM system sensitive to the frequency range of 400-140,000 Hz. The survey was flown ~40 m above ground along flight lines spaced 0.2-0.4 km apart. Pilgrim Springs is characterized by hot springs, nearby thawed regions, and high lake temperatures that are indicative of high heat flow in the region. The regional geologic and geophysical framework this work provides offers critical information for future development of this resource and is relevant more generally to our understanding of geothermal systems in active extensional basins. Depth to basement inversion of gravity data provide a regional model of basin geometry, while various derivative and filtering methods, including maximum horizontal gradient of the gravity data and pseudogravity transformation of the magnetic data, are used to locate faults, contacts, and structural domains. Gravity highs occur over crystalline basement of the Kigluaik, Marys, and Hen and Chickens Mountains. A local elongate gravity low extends from Pilgrim Springs, where it is ~4.5 mGal, southwestward where the lowest values (~10 mGal) occur ~4km southwest of the springs. The margins of the low are characterized by northeast-trending gradients that probably reflect the edges of fault-bounded structural blocks. The southeastern edge of the low near the springs, in particular, lies very close to the springs and may provide an important pathway conveying deep fluids to the surface. The maximum horizontal gradient of the pseudogravity transformation of the magnetic data shows a dominant northwest trending anomaly pattern across the northeastern portion of the survey area between Pilgrim Springs and Hen and Chickens Mountain that closely resembles the trend of primary basement structures mapped in outcrops north and south of the study area. The area south of the springs, however, is dominantly characterized by east-west trending, range-front-parallel anomalies likely caused by late Cenozoic structures associated with north-south extension that formed the basin. The area around the springs (~10 km2) is characterized by a magnetic high punctuated by several east-west trending magnetic lows, the most prominent occurring directly over the springs. The lows may result from demagnetization of magnetic material along range-front parallel features that dissect the basin. Models of the airborne EM data show a 1 ohm-m conductor around 10 m depth that extends to the north and east of the hot springs that likely reflect elevated heat flow and saline geothermal fluids associated with the hot springs. More moderate resistivities (50-200 ohm-m) characterize surrounding rivers and streams and are likely due to variations in sediment clay content. High resistivities (> 1000 ohm-m) associated with the mountain ranges reflect Precambrian metamorphic basement and overlying Paleozoic carbonates. The shallow EM models also agree closely with identified permafrost and thermokarst lakes. A highly resistive region between the range front of the Kigluaik Mountains and dense stream channels surrounding Pilgrim Springs likely reflects permafro |