Geophysical character of volcanic massive sulfide deposits and their geologic setting, Ketchikan Mining District, Alaska

by: Jon P. Doucette, Bruce D. Smith, Anne McCafferty, Jeff Wynn, and Peter E. Bittenbender

A helicopter electromagnetic and magnetic survey was flown in parts of the Ketchikan Mining District in 1999 with funding from local villages, Ketchikan Borough, and the Bureau of Land Management (BLM). Previously confidential helicopter electromagnetic data from 1000 square kilometers of contracted surveying in adjacent areas was donated by Sealaska Native Corporation. These data sets were combined and released in August of 1999 by the Alaska Division of Geological and Geophysical Surveys (DGGS). A helicopter electromagnetic survey of Annette Island was previously released by the Bureau of Indian Affairs (Division of Energy and Minerals) and the Metlakatla Tribal Government. These data will be open-filed by the U. S. Geological Survey (USGS) shortly. Sealaska Native Corporation and Abacus Minerals Corporation have allowed the USGS to examine ground geophysical data for selected volcanic massive sulfide deposits and occurrences. The USGS project to define, describe, and evaluate this (PreCambrian? Paleozoic?) volcanic massive sulfide belt is being done in cooperation with the BLM and DGGS as well as local governments.

A qualitative interpretation of the magnetic and electromagnetic data was made prior to field investigations carried out by the USGS in the summer of 2000. This interpretation identified geophysical terrains with specific electrical and magnetic properties that can be associated with lithologic units (e.g., Descon versus Wales group), intrusive rocks (such as Cretaceous dioritic intrusives), and mafic dikes. Field investigations concentrated on mapping lithologic units and measuring their magnetic and electrical properties. This information will be used in more quantitative modeling of the helicopter electromagnetic and airborne magnetic data to define terrains favorable for volcanic massive sulfide deposits.

An interesting aspect of the apparent resistivities computed from the electromagnetic data is that the lowest frequency (900 hertz, and thus the deepest penetrating data) have values well above 1000 ohm-meters over much of the survey area. This is in marked contrast to the helicopter electromagnetic survey data in the Wrangell and Admiralty Island areas where the 900 hertz apparent resistivity data are generally below the maximum detectable of 1000 ohm-meters. Geologic reasons for this difference are not obvious but may have implications for deep seated alteration that generally produces lower resistivities.

The USGS is preparing a compendium of geophysical signatures of volcanic massive sulfide deposits and occurrences in the Ketchikan Mining District. This will supplement the existing compilation by the BLM of geological information for various types of mineral deposits in the mining district. We have computed conductivity-vs-depth images from airborne electromagnetic data for much of the survey area and for all the flight lines that cover known volcanic massive sulfide deposits and occurrences. Many of the known occurrences are near the sea-shore, which makes interpretation of the electromagnetic data difficult because of proximity to conductive sea water. Generally the volcanic massive sulfide deposits/occurrences do not have distinctive low resistivity anomalies with the exception of the Khayyam-Stumble-On area. In part this might be due to non-conductive sulfide minerals (e.g. sphalerite) and in part due to lack of interconnectivity between stringer-type mineralization.

References

Wynn, J.C., 2000, Using ground geophysical signature studies to model airborne geophysical data and constrain geologic mapping: Alaska Miners’ Association Meeting, Anchorage, Alaska, November 2000.

Wynn, J.C., Karl, Sue, McCafferty, Anne, and Smith, Bruce, 2001, Developing geophysical signatures to constrain geologic mapping, southeastern Alaska: Proceedings volume of the SAGEEP-2001 conference, Denver, Colorado, March 2001, 14 pages, 10 figures.