The Airborne Geophysical Survey of the Greens Creek Area
By James Fueg
Chapter 5 ofGeology, Geochemistry, and Genesis of the Greens Creek Massive Sulfide Deposit, Admiralty Island, Southeastern AlaskaEdited by Cliff D. Taylor and Craig A. Johnson
Professional Paper 1763
U.S. Department of the InteriorU.S. Geological Survey
Contents
Abstract .......................................................................................................................................................109Introduction.................................................................................................................................................109Geology ........................................................................................................................................................109Physical Properties....................................................................................................................................109Airborne Survey Parameters ...................................................................................................................110Results .........................................................................................................................................................111Conclusions.................................................................................................................................................121Reference Cited..........................................................................................................................................121
Figures
1. Total magnetic field ..................................................................................................................112 2. Magnetics—calculated vertical gradient ............................................................................113 3. 800-hertz apparent resistivity .................................................................................................114 4. 4,000-hertz apparent resistivity ..............................................................................................115 5. Topography ................................................................................................................................116 6. Radiometrics—Potassium ......................................................................................................117 7. Radiometrics—Thorium ..........................................................................................................118 8. Radiometrics—Uranium ..........................................................................................................119 9. Radiometrics—Potassium over thorium...............................................................................120
Tables
1. Resistivity and chargeability of Greens Creek area rocks.................................................110 2. Airborne geophysical survey parameters ............................................................................110
AbstractAn airborne geophysical survey of the Greens Creek
massive sulfide deposit was flown in 1996 to collect magnetic, radiometric, and five frequencies of electromagnetic (EM) data. Results of a physical properties study conducted prior to the survey showed the mineralization to be strongly conduc-tive, and with the lithologies showing a wide range in both conductivity and susceptibility.
The magnetic response of the area was dominated by the highs associated with the ultramafic units, while the EM data were useful in mapping out the argillite/phyllite (low/high resistivity) contact. Large numbers of EM anomalies were picked from the data, but none of those checked corresponded to mineralization. The count rates of the radiometric data were extremely low, and the effects of topography dominated the response. The survey provided valuable structural and strati-graphic data, but selection of valid exploration targets in this environment cannot be done using geophysics alone.
IntroductionA combined helicopter-borne electromagnetic (EM),
magnetic, and radiometric survey was flown over the Greens Creek area in September 1996. The objective of the survey was twofold:
1. To attempt direct detection of additional Greens Creek-style mineralization.
2. To assist in the mapping of stratigraphy and structure throughout the area.
Results have shown that the magnetic and apparent resistivity maps produced from the survey were probably the most useful products produced. A large number of conductors were identi-fied in the survey, mostly formational conductors associated with the more graphitic argillite units.
GeologyGreens Creek is a high-grade massive sulfide deposit
with a global resource that is estimated to be 21.9 million
metric tonnes of ore at 13.9 percent zinc, 5.1 percent lead, 4.8 grams per metric tonne gold, and 599 grams per metric tonne silver at zero cutoff (chap. 2). The deposit can be regarded as a hybrid volcanogenic massive sulfide/sedimen-tary exhalative (VMS/SEDEX) deposit, as it exhibits char-acteristics of both categories of deposit (Taylor and others, 1999; chap. 15).
Host rocks to the Greens Creek deposit lie within a 600-km-long belt of Triassic-aged conglomerates, limestones, marine clastic sediments, tuffs and intercalated mafic pyro-clastics, and pillowed flows that form the eastern margin of the Alexander terrane. Deposits within this belt include Greens Creek and Windy Craggy (chap. 2). The deposit is located at the contact between a footwall sequence consisting predominantly of various categories of phyllite and a hanging-wall sequence of massive and slaty black argillites (chap. 6). Deformation in the area is extremely complex and involves at least four periods of folding, one of semiductile shearing, and two of faulting (chap. 7).
Ore lithologies at Greens Creek are broken into two main groups of massive sulfide ores and semimassive or gangue-rich “white” ores. The massive ores contain greater than 50 percent sulfides and consist of massive fine- to very fine grained ore in which base-metal sulfides are greater than pyrite and mas-sive medium- to very fine grained pyrite-rich ore in which pyrite predominates. The white ores are a group of mineral-ized lithologies that contain less than 50 percent sulfides and are defined by their principal gangue mineral. There are three types: white carbonate ore, white siliceous ore, and white baritic ore (chap. 6).
Physical PropertiesDuring 1996, 18 rock samples from the Greens Creek
area, including both drill-core and hand samples, were sent to Zonge Engineering and Research Organization for physical properties testing. Samples were tested for the induced polar-ization response (time-domain) and resistivity. The samples were vacuum impregnated with distilled water and the induced polarization measurements were made using an 8-second cycle time. The data are listed in table 1.
The Airborne Geophysical Survey of the Greens Creek Area
By James Fueg
110 Geology, Geochemistry, and Genesis of the Greens Creek Massive Sulfide Deposit, Admiralty Island, Alaska
Table 1. Resistivity and chargeability of Greens Creek area rocks.
[ohm-m, ohm-meters; msec, millisecond]
Description Source Resistivity(ohm-m)
Chargeability(msec)
Massive very fine grained base-metal sulfides Drill core 4.7 289.5
Massive fine-grained base-metal sulfides Drill core 2.1 202.0
Massive fine-grained base-metal sulfides Drill core 2.7 291.6
Massive fine-grained base-metal sulfides Drill core 4.7 174.3
Pyritic semimassive sulfide Hand specimen 20.1 44.0
Sericitic phyllite Drill core 134 74.2
Siliceous sericitic phyllite Drill core 191 83.0
Chloritic phyllite Drill core 200 110.0
Mineralized sericitic phyllite Drill core 1,446 121.8
Sericitic phyllite Drill core 1,470 10.9
Laminated black argillite Hand specimen 272 110.8
Slaty argillite Drill core 9.9 117.5
Slaty argillite Drill core 458 14.6
Slaty argillite Drill core 572 28.0
Contorted black argillite Hand specimen 862 65.5
Baritic sericitic schist Hand specimen 610 23.4
Chloritic serpentinite Drill core 1,604 6.3
Chloritic serpentinite Drill core 1,187 2.9
Analysis of the results shows that the massive sulfide sam-ples are strongly conductive (less than 5 ohm-m), the argillites vary from conductive to resistive (9 to 860 ohm-m) depending on the amount of shearing and carbon, and the phyllites range from resistive to highly resistive (130 to 1,600 ohm-m). See table 1.
These results initially suggested to us that not only should it be possible to directly detect Greens Creek-style zones of massive sulfide mineralization using airborne EM (provided the zones are large enough, not too deep, and not masked by other conductive material such as graphitic argillite), but there should also be sufficient resistivity contrast between many of the Greens Creek-area lithologies to allow them to be discriminated in the apparent resistivity maps produced from the EM data.
Susceptibility measurements on core and hand samples were also done at Greens Creek using a hand-held Exploranium KT–9 susceptibility meter. As expected, results showed that the more mafic rocks (serpentinite, gabbro, chloritic phyllite) have susceptibilities at least one or two orders of magnitude greater than the more silicic units (silicic phyllite, chert), the dolomitic units, the argillites, and also the massive sulfides. Clearly, the Greens Creek ore would not be directly detectable using aeromagnetic data, and the contrast between some of the more important lithologies in the area is not as well defined as in the resistivities. The results nevertheless suggested that aeromag-netic data would be of use in mapping lithologies in the area.
The possibility of elevated potassium levels in some of the altered volcanics around Greens Creek suggested that radio-metric data might be of use in mapping these units. Obvious concerns existed with the high rainfall, the dense nature of the vegetation cover, and the amount of swamp-covered areas that could potentially mask the radiometric response of the various lithologies. However, it was relatively inexpensive to add this technique to the planned survey.
Airborne Survey Parameters
Based on the previous analysis, it was decided that a combined airborne electromagnetic, magnetic, and radiometric survey should be flown over the property. The contract to fly the survey was awarded to Aerodat, and it was completed over a 5-day period in late September 1996. A total of 1,227 line kilometers of data were collected. Some of the data over the northern part of the Mansfield Peninsula remain proprietary and have not been included in this report. The survey param-eters and instrumentation are shown in table 2.
Table 2. Airborne geophysical survey parameters.
[m, meter; EM, electromagnetic; Hz, hertz; GPS, global positioning system]
Survey elements Parameter or instrumentation
Line direction 070°
Line spacing 200 m (100 m over immediate GC area)
Survey altitude60-m crystal, 45-m magnetometer,
30-m EM
Survey aircraft Aerospatiale SA 315B Lama
EM systemAerodat 5 frequency; coplanar 865,
4175, 32k Hz; coaxial 935, 4,600 Hz @ 10 Hz
Magnetometer Scintrex H8 Cs sensor @ 10 Hz
SpectrometerExploranium GR-820 (16.81) recording
4 channels @ 1 Hz (TC, K, U, Th)
Navigation Differential GPS
Results 111
ResultsFigures 1 through 9 are images of the various data sets
collected, including total magnetic field, calculated vertical gradient of the magnetic field, 4,000-Hz and 800-Hz apparent resistivity, topography, the potassium, thorium, and uranium spectrometer channels, and a ratio of the potassium and thorium spectrometer channels. All images were gridded at a 50-m cell size and sunshaded at an inclination of 45° from the northeast.
The magnetic response of the Greens Creek area is domi-nated by a series of circular and irregularly shaped magnetic highs (with amplitudes up to 500 nanoteslas) immersed within a relatively quiet background. Most of the highs correspond with mapped mafic and ultramafic rock units. A predomi-nantly northwest trending structural grain is visible in the magnetic data, and several structures offsetting magnetic features can be identified in the data. North of Hawk Inlet the character of the data changes substantially and consists primarily of northwest-trending linear magnetic features.
Comparison of the apparent resistivity data with mapped geology (chap. 4) reveals a distinct correlation between areas
of low resistivity and mapped argillite and more resistive areas and mapped phyllite. In the mine area the main phyl-lite/argillite contact can clearly be tracked in the resistivity data. The Greens Creek mineralization itself is not visible in the data, primarily due to the depth of burial of the bulk of the ore. Analysis of the EM profile data by Aerodat process-ing staff resulted in the picking of about 3,100 “EM anoma-lies.” The bulk of these occur within the low-resistivity zones associated with the argillites and probably represent more graphitic zones within the metasediments. The large number of anomalies made impractical the ground followups of all the identified bedrock conductors, but field visits to some of the best-ranked conductors did not result in any obvious targets being identified.
The average count rate for all four channels of the radiometric data is extremely low. The radiometric response is dominated by the effects of topography, with most anomalous areas correlating to topographic highs. These are zones of little or no vegetation, in strong contrast to the heavily forested slopes and muskeg-covered lowlands. Count rates are gener-ally so low that even ratio maps such as potassium/thorium show little consistency outside the topographic highs.
112 Geology, Geochemistry, and Genesis of the Greens Creek Massive Sulfide Deposit, Admiralty Island, Alaska
57087570265699856984569745696656960569565695156946569425693656932569275692256917569135690856904569005689456889568845688156878568755687256869568655686156858568535684956846568415683456825568135679256758
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Figure 1. Total magnetic field.
Physical Properties 113
Magnetics—calculated vertical gradient
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Figure 2. Magnetics—calculated vertical gradient.
114 Geology, Geochemistry, and Genesis of the Greens Creek Massive Sulfide Deposit, Admiralty Island, Alaska
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800-hertz apparent resistivity
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Figure 3. 800-hertz apparent resistivity.
Physical Properties 115
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4,000-hertz apparent resistivity
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Figure 4. 4,000-hertz apparent resistivity.
116 Geology, Geochemistry, and Genesis of the Greens Creek Massive Sulfide Deposit, Admiralty Island, Alaska
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Topography
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Figure 5. Topography.
Physical Properties 117
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Radiometrics—Potassium
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Figure 6. Radiometrics—Potassium.
118 Geology, Geochemistry, and Genesis of the Greens Creek Massive Sulfide Deposit, Admiralty Island, Alaska
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Radiometrics—Thorium
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Figure 7. Radiometrics—Thorium.
Physical Properties 119
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Radiometrics—Uranium
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Figure 8. Radiometrics—Uranium.
120 Geology, Geochemistry, and Genesis of the Greens Creek Massive Sulfide Deposit, Admiralty Island, Alaska
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Radiometrics—Potassium/Thorium
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Figure 9. Radiometrics—Potassium over thorium.
Reference Cited 121
Conclusions
The Greens Creek airborne geophysical survey was successful in providing valuable stratigraphic and structural information about the area. The most useful products of the survey were the apparent resistivity and magnetic maps produced, as may be expected given the range of physical properties in table 1. The radiometric data did not provide much useful information, primarily due to the effects on the data of ground cover, which dominated the response. The EM data produced a large number of anomalies, most of which are associated with graphitic metasediments. Identi-fication of valid exploration targets within these conductive zones cannot be done using geophysics alone.
Reference Cited
Taylor, C.D., Newkirk, S.R., Hall, T.E., Lear, K.G., Premo, W.R., Leventhal, J.S., Meier, A.L., Johnson, C.A., and Harris, A.G., 1999, The Greens Creek deposit, southeast-ern Alaska—A VMS-SEDEX hybrid, in Stanley, C.J., and others, eds., Mineral deposits—Processes to processing: Rotterdam, Balkema, v. 1, p. 597–600.
