Alaska Division of Geological & Geophysical Surveys Geophysical Report 2000-40 PROJECT REPORT OF THE AIRBORNE GEOPHYSICAL SURVEY OF PARTS OF THE ANIAK AND IDITAROD MINING DISTRICT, SOUTHWESTERN ALASKA by Mark Stephens Fugro Airborne Surveys October 2000 THIS REPORT HAS NOT BEEN REVIEWED FOR TECHNICAL CONTENT (EXCEPT AS NOTED IN TEXT) OR FOR CONFORMITY TO THE EDITORIAL STANDARDS OF DGGS. Released by STATE OF ALASKA DEPARTMENT OF NATURAL RESOURCES Division of Geological & Geophysical Surveys 3354 College Road Fairbanks, Alaska 99709-3707 PROJECT REPORT OF THE AIRBORNE GEOPHYSICAL SURVEY OF PARTS OF THE ANIAK AND IDITAROD MINING DISTRICT, SOUTHWESTERN ALASKA STEVENS EXPLORATION MANAGEMENT CORP. DIGHEMV SURVEY FOR THE STATE OF ALASKA DEPARTMENT OF NATURAL RESOURCES DIVISION OF GEOLOGICAL AND GEOPHYSICAL SURVEYS Quadrangle: Iditarod A-4, A-5, A-6, B-4, B-5, B-6 Sleetmute C-6, C-7, D-6, D-7 Mississauga, Ontario Mark Stephens July, 2000 Geophysicist R6006JUL.00 SUMMARY Introduction This report describes the logistics and results of a DIGHEMv airborne geophysical survey carried out under contract to Stevens Exploration Management Corp., Mining and Geological Consultants, for the State of Alaska, Department of Natural Resources, Division of Geological and Geophysical Surveys. The survey was flown from April 12 to May 8, 2000, over one block in the Aniak and Iditarod mining districts, southwestern Alaska. Total coverage of the survey block amounts to 5,709.6 miles (9186.7 km). Purpose This airborne geophysical survey is part of a program to acquire such data on Alaska’s most promising mineral belts and districts. The information acquired is aimed at catalyzing new private sector exploration, discovery, and ultimate development and production. The purpose of the survey was to map the magnetic and conductive properties of the survey area and to detect conductive mineralization. This purpose was accomplished by using a DIGHEMv multi-coil, multi-frequency electromagnetic system and supplemented by a high sensitivity cesium magnetometer. A GPS electronic navigation system ensured accurate positioning of the geophysical data with respect to the base maps. Visual flight path recovery techniques were used to confirm the location of the helicopter platform. Products Various maps depicting the survey results are provided at scales of 31,680 (1” = 1/2 mile) and 1:63,360 (1” = 1 mile). Some of the maps are presented on a topographic base. The data sets are processed and presented using Zone 4 of the Universal Transverse Mercator projection coordinates using the NAD27 datum. The following geophysical parameters are presented on the maps and/or on the digital archive: • Total Field Magnetics • Colour Shadow Total Field Magnetics • Apparent Resistivity – 900 Hz • Apparent Resistiviy – 7,200 Hz • Interpreted Discrete Electromagnetic Anomalies Geology The Aniak and Iditarod mining districts fall within the central Kuskokwim region in southwestern Alaska. The region is dominated by the Kuskokwim Mountains, which trend northeastward into central Alaska. The Kuskokwim River passes through the region from east to west in a gorge that transects the Kuskokwim mountains. The central Kuskokwim region lies at the centre of a mobile belt of mountain building and volcanic activity that trends east-west through central and southern Alaska (Cady and others, 1955). This belt was predominantly a zone of subsidence and geosynclinal deposition during the Paleozoic and early Mesozoic eras and became a zone of mountain building and intrusive igneous activity beginning with the Middle Mesozoic time (Cady and others, 1955). Geanticlines formed in the mobile belt during the first phase of mountain building and were intruded by granitic batholiths. Geosynclinal deposits, chiefly graywacke, formed in the intervening areas during the late Mesozoic. They were then folded and intruded by igneous rocks in the second phase of mountain building. The present mountain levels were produced by vertical uplift that began in the middle Cenozoic time and has continued to the present. Extrusion of basaltic volcanic rocks and intrusion of quartz monzonite stocks accompanied these later movements, and extensive faults formed where differential uplift was pronounced (Cady and others, 1955). The bedded rocks of the Kuskokwim region include both sedimentary and volcanic rocks that make up a very thick section of probably 60,000 feet or more deposited chiefly in marine water (Cady and others, 1955). Surficial deposits include residual soil and rubble, gravel that occurs on rock benches and terraces; glacial till and outwash deposits; windblown silt; and the flood plain deposits of exising streams. Geanticlines, geosynclines, folds and faults occur in the area. The Kuskokwim geosyncline, which occupies most of the region, is bordered to the southeast by the Alaska- Yukon geanticline and to the northwest by the Aniak-Ruby geanticline, both of which are of middle to late Mesozoic age. A system of folds, referred to as the Kuskokwim Mountain orogen, formed in the early Cenozoic, trends to the northeast. Faults strike east-northeast across the Kuskokwim Mountain orogen and intersect the Alaska-Yukon geanticline in the region to the notheast of the central Kuskokwin region. The faults are mainly of Cenozoic age and are still active (Cady and others, 1955). A major northeast trending fault, the Iditarod-Nixon Fork fault, occurs in the northern section of the survey area just south of Chicken Mountain. Latest Cretaceous and Tertiary right-lateral offsets of less than 150 km characterize the fault (Decker and others, 1994). Principal lode mineral deposits in the Iditarod Mining district consist of zoned, hydrothermal, mesothermal to epithermal, gold polymetallic deposits hosted in Late Cretaceous plutonic and volcanic rocks. The Golden Horn deposit, (located approximately 2.5 miles to the east of Flat, along Black Creek), consists of large amounts of gold with anomalous silver, antimony, ziconium, tungsten, and arsenic values. Additional significant bulk-tonnage polymetallic gold resources within the Iditarod mining district can be found within the drainage of Black, Granite and Boulder Creeks as well as on Chicken Mountain. The Iditarod Mining district is Alaska’s third largest placer gold district yielding 45,196 kg (1,453,000 oz) gold through 1990 and with byproducts of silver, tungsten and mercury (Bundtzen and others, 1992). In the central and southern section of the survey area, principal mineral deposits include mesothermal, boron-enriched, base-precious metal minaralization hosted in high- level cupolas and hornfels of the Horn Mountains volcanic-plutonic complex, mercury- antimony (gold) lodes in peraluminous granite porphyry and altered mafic dykes and heavy mineral gold placers (Bundtzen and others, 1999). Results and Discussion The geophysical results, in general, correlate well with the known geology in the survey area. The results confirm the general trends and serve to extend the mapping of individual geologic units beneath the surface. The total field magnetic and apparent resistivity data sets have successfully mapped the magnetic and oconductive characteristics of the lithologies in the survey area. Numerous faults and contacts have been inferred from the survey results. The discrete EM anomalies are interpreted to fall within one of four general categories. The first type consists of discrete, well-defined anomalies which are usually attributed to conductive sulphides or graphite. The second class of anomalies comprises moderately broad responses which exhibit the characteristics of a half space. Some of these anomalies may reflect conductive rock units or zones of deep weathering. The third class of anomalies consists of negative inphase responses which are indicative of magnetite. The fourth class comprises cultural anomalies. It is recommended that the survey results be reviewed in detail, in conjunction with all available geophysical, geological and geochemical information. Particular reference should be made to the multi-parameter stacked profiles which clearly define the characteristics of the individual anomalies in the identification of target areas. Image processing of existing geophysical data be considered, in order to extract the maximum amount of information from the survey results. CONTENTS 1. INTRODUCTION....................................................................... 1-1 2. SURVEY EQUIPMENT and FIELD PROCEDURES.......................... 2-1 Electromagnetic System............................................................. 2-1 Mobile Magnetometer............................................................... 2-2 Base Station Magnetometer......................................................... 2-3 Radar Altimeter....................................................................... 2-3 Barometric Pressure and Temperature Sensors................................. 2-3 Analog Recorder..................................................................... 2-4 Digital Data Acquisition System................................................... 2-4 Tracking Camera..................................................................... 2-4 Navigation System (RT-DGPS) ................................................... 2-5 Field Workstation Software........................................................ 2-6 3. PRODUCTS and PROCESSING TECHNIQUES................................ 3-1 PRODUCTS Maps................................................................................ 3-1 Other Products.................................................................... 3-2 PROCESSING TECHNIQUES Topographic Bases ............................................................... 3-2 Electromagnetic Anomalies..................................................... 3-2 Apparent Resistivity.............................................................. 3-4 Total Field Magnetics............................................................ 3-5 Multi-parameter Stacked Profiles.............................................. 3-5 Contour, Colour and Shadow Map Displays................................ 3-5 Digital Terrain .................................................................... 3-7 4. SURVEY RESULTS and DISCUSSION........................................... 4-1 Geology................................................................................ 4-1 Survey Results........................................................................ 4-4 Discussion............................................................................. 4-22 5. CONCLUSIONS and RECOMMENDATIONS.................................. 5-1 6. REFERENCES .......................................................................... 6-1 APPENDIX A.......................................................LIST OF PERSONNEL APPENDIX B............................................BACKGROUND INFORMATION APPENDIX C............................................................EM ANOMALY LIST LIST OF TABLES Table 2-1 DIGHEM System Specifications......................................... 2-1 Table 2-2 The Analog Profiles........................................................ 2-5 Table 3-1 Multi-parameter Stacked Profiles........................................ 3-6 Table 4-1 EM Anomaly Statistics .................................................... 4-5 Table B-1 EM Anomaly Grades....................................................... B-2 LIST OF FIGURES Figure 1-1 Location of Parts of the Aniak and Iditarod Mining Districts, Southwestern Alaska ....................................................... 1-2 Figure 3-1a Processing Flow Chart – Electromagnetic Data....................... 3-3 Figure 3-1b Processing Flow Chart – Magnetic Data............................... 3-3 Figure 4-1a Interpretation Sketch of Parts of the Aniak and Iditarod Mining Districts, Southwestern Alaska – Page 1 of 2 ......................... 4-8 Figure 4-1b Interpretation Sketch of Parts of the Aniak and Iditarod Mining Districts, Southwestern Alaska – Page 2 of 2 ......................... 4-9 Figure B-1 Typical DIGHEM Anomaly Shapes..................................... B-4 LIST OF MAPS 2000-40 Interpretation Map of Parts of the Aniak and Iditarod Mining Districts, Southwestern Alaska – Page 1 of 2.......................... map pocket 2000-40 Interpretation Map of Parts of the Aniak and Iditarod Mining Districts, Southwestern Alaska – Page 2 of 2.......................... map pocket - 1.1 - INTRODUCTION A DIGHEMV airborne electromagnetic/resistivity/magnetic survey was flown under contract to Stevens Exploration Management Corp., Mining and Geological Consultants, for the State of Alaska, Department of Natural Resources, Division of Geological and Geophysical Surveys (DGGS). The survey was flown from April 12 to May 8, 2000, over one block located in parts of the Aniak and Iditarod mining districts, southwestern Alaska. The survey was carried out in the Iditarod A-4, A-5, A-6, B-4, B-5, B-6, and Sleetmute C-6, C-7, D-6 and D-7 quadrangles. This airborne geophysical survey is part of a program to acquire such data on Alaska’s most promising mineral belts and districts. The information acquired is aimed at catalyzing new private sector exploration, discovery, and ultimate development and production. Survey coverage consists of approximately 5,709 miles (9,186 line-km), including 599 miles (964 line-km) of tie lines. The nominal line separation is ¼-mile (approximately 400 metres). Tie lines were generally flown perpendicular to the flight line direction with a separation of 3 miles (5 km). Tie lines were also flown parallel to the boundaries. The survey employs the DIGHEMV electromagnetic system. Ancillary equipment consists of a magnetometer, radar altimeter, video camera, analog and digital recorders and an electronic navigation system. Section 2 gives a description of the survey equipment and specifications and an outline of the field procedures. Section 3 describes the processing techniques and products. Section 4 describes the results, and the conclusions and recommendations for further work are given in Section 5. - 1.2 - Figure 1-1 Location of parts of the Aniak and Iditarod mining districts, southwestern Alaska. - 2.1 - SURVEY EQUIPMENT AND FIELD PROCEDURES The survey instrumentation was installed in an Aerospatiale AS350B2 turbine helicopter (Registration N214-EH) which was owned by Era Aviation Inc. A bird, which houses much of the electromagnetic and magnetic equipment, is suspended approximately 100 feet beneath the helicopter. The helicopter flew at an average air speed of 74 mph (120 km/hr). The EM bird was flown with an approximate terrain clearance of 100 feet (30 metres). Electromagnetic System The survey was flown with a DIGHEMV system which has a towed bird, symmetric dipole configuration and is operated at a nominal survey altitude of 30 metres. Table 2-1 lists the specifications for the DIGHEMV system. Five in-phase and five quadrature components are recorded for each of the five coil pairs. The sample rate of 10 per second is equivalent to 1 sample every 10.9 ft (3.3m) at the average survey speed of 74 mph (120 km/h). Table 2-1: DIGHEMV System Specifications Coil Nominal Actual Coil Separation Sensitivity Orientation Frequency (Hz) Frequency (Hz) (m) (ppm) Coaxial 900 1,049 8.0 0.06 Coplanar 900 855 8.0 0.06 Coaxial 5,500 5,600 8.0 0.10 Coplanar 7,200 7,155 8.0 0.10 Coplanar 56,000 55,700 6.3 0.30 The electromagnetic system utilizes a multi-coil coaxial/coplanar technique to energize conductors in different directions. The coaxial coils are vertical with their axes in the flight direction. The coplanar coils are horizontal. The secondary fields are sensed simultaneously by means of receiver coils which are maximum coupled to their respective transmitter coils. The system yields an inphase and a quadrature channel from each transmitter-receiver coil-pair. The DIGHEM calibration procedure involves four stages; primary field bucking, phase calibration, gain calibration, and zero level adjustment. At the beginning of the survey, the primary field at each receiver coil is cancelled, or “bucked out”, by precise adjustment of the position of five bucking coils.