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It is axiomatic that the distribution of gold deposits in Nevada is controlled by deep-seated geologic structures and plutons. These controlling features are not, however, usually evident on the surface. Small stocks and dikes may crop out, but they are often only minor expressions of larger intrusive bodies at depth. The primary controlling structures are rarely exposed; their presence in the deeper crust is commonly postulated only on the basis of the observed alignment of ore deposits. The Carlin and Battle Mountain-Eureka trends are classic examples. Small intrusive bodies are scattered along both trends, but, other than the linear alignment of the deposits themselves, there is no direct geological evidence of a single structure connecting the deposits along the trends. The danger in using such limited evidence to infer a genetic relationship to a series of deposits is that other geologic factors with no genetic association can be responsible for the apparent alignment. The "Getchell" trend may be such an example with the Preble, Pinson, and Getchell deposits roughly aligned because of the uplift of the Osgood Mountains during Basin and Range deformation. The alignment, while real, does not represent a genetic relationship.
Determination of the primary plutonic and/or structural controls, therefore, is fundamental to effective selection and ranking of areas for exploration. To do otherwise is to prospect at random or to follow a crowd. Unfortunately, the task is not easy; the controlling features are subsurface crustal features that cannot be directly observed. Through the integration of various types of data and a variety of techniques, indirect evidence can be used by the geologist to quantify and define controlling geologic parameters. These can then be used in a predictive way to guide exploration efforts into more prospective areas.
THE ROLE OF AEROMAGNETIC DATA
Aeromagnetic data reflect the varying rock types in the crust. As a general principal, low-level surveys are more influenced by variations in near-surface rocks, and higher level surveys "look" deeper, i.e. they map changes in rock-types at greater depth. Regional aeromagnetic surveys, then, are an appropriate - possibly the best - technique for mapping regional, subsurface geology. Quantitative interpretations of aeromagnetic data, integrated with the known surface geology, can delineate many structures and plutons that control the location of mineralization. Major, deep-seated structures are often manifested in the magnetic data by alignments of magnetic discontinuities. Plutons, particularly those of Tertiary age, frequently exhibit prominent magnetic anomalies.
THE NEVADA DATA SETS
Most of Nevada is covered by proprietary, non-exclusive aeromagnetic data. The surveys were flown between 1978 and 1988 on behalf of two consortia of petroleum companies and two consortia of mining companies. The four surveys were flown to uniform specifications - a 1.5-mile by 1.5-mile square grid at 7,500 feet barometric elevation. The data were acquired by high-sensitivity proton and cesium vapor magnetometers and are available in both digital profile and grid formats.
These aeromagnetic data are superior to those available in the public domain for several reasons. The surveys were flown to uniform specifications; the profile data are available and are in digital form; the surveys were flown on a square grid, not just east-west as were most of those in the public domain; processing of the data is state-of-the-art.
INTERPRETATIONS
PRJ has interpreted the data at two scales - 1:96,000 (or 1:100,000 for the western surveys) and 1:250,000. The geophysical analysis is quantitative. It outlines causative sources with depths, magnetic susceptibilities, and geometry; defines structural elements, faults, lineaments, and magnetic discontinuities; and delineates areas of similar magnetic response. The geophysical analysis is then integrated with the geologic data base, including radiometric age dates on plutons, volcanic rocks, and mineralization, to put the geophysical interpretation into a geologic context. Finally, the interpretation results are related to known mineral occurrences to identify the geological-geophysical parameters that appear to control the location of the mineral occurrences.
BENEFITS
Based on the interpretative work completed between 1987 and 1992, EDCON-PRJ has developed a tectonic model of Tertiary events in Nevada that provides a rational explanation for the location of many of the gold deposits. The model can be used in a predictive manner to select areas for exploration, to evaluate prospects and submittals, and to characterize possible targets in terms of probable structural controls, plutonic relationships, and minimum age of host rocks. Benefits include not only increased probability of exploration success but significant savings in costs by the elimination of exploration expenditures in lower probability areas.
DELIVERABLES and PRICE
DATA
Deliverables
Maps, on mylar, scale 1:100,000
Digital Data
Grids and profiles, on CD-ROM or diskette
Minimum Purchase
One 30-minute quadrangle, approximately 1,250 line miles
INTERPRETATIONS
Deliverables, scale 1:96,000
Deliverables, Scale 1:250,000
NORTHWEST NEVADA
Flight Date: November 1987 - January 1988
Line Mileage: 26,900
Flight Direction: East-West and North-South
Traverse Interval: 1.5 miles by 1.5 miles
Flight Altitude: 7,500 ft barometric with 1000-ft drape over the ranges
Location Systems: Loran navigation tied to photographic flight path recovery
Magnetometer: Cesium vapor, high sensitivity magnetometer
NORTHEASTERN NEVADA
Flight Date: May-July 1985
Line Mileage: 24,258
Flight Direction: East-West and North-South
Traverse Interval: 1.5 miles by 1.5 miles
Flight Altitude: 7,500 ft barometric with 1000-ft drape over the ranges
Location Systems: Teledyne-Ryan APN 220 Doppler System in conjunction with a 35mm flight path camera
Magnetometer: Geometrics Model G-813 Proton Magnetometer with 0.05-gamma sensitivity (sample rate at 70 meters)
Altimeter: Sperry RA215 Recording Radar Altimeter; Cyberstar Model 928-3 Recording Barometric Altimeter
Original Recording: HP 7100 Analog Recorder, Geometrics Digital Acquisition System (Model 714) and Geometrics Model G-826 Base Station Magnetometer (Digitally-recorded 0.25-gamma sensitivity)
EASTERN GREAT BASIN
Flight Date: November 1978 - April 1979
Line Mileage: 39,050
Flight Direction: East-West and North-South
Traverse Interval: 1.5 miles by 1.5 miles
Flight Altitude: 8,500 ft barometric with 1000-ft drape over the ranges
Location Systems: Singer SKK 1000 Doppler, Singer SKQ-601 Computer and Sperry C-12 Gyro Stabilized Compass
Magnetometer: Geometrics Model G-803 Proton Magnetometer with 0.5-gamma sensitivity (sample rate at 50 meters)
Altimeter: Sperry AA200 Recording Radar Altimeter
Original Recording: HP 7128 Analog Recorder, Geometrics Digital Acquisition System (Model 704) and Geometrics Model G-806 High Performance Proton Magnetometer Base Station (.25-gamma sensitivity)
WALKER LANE
Flight Date: June - July 1989
Line Mileage: 20,900
Flight Direction: East-West and North-South
Traverse Interval: 1.5 miles by 1.5 miles
Flight Altitude: 7,500 ft barometric with 1000-ft drape over the ranges
Location Systems: Loran navigation tied to photographic flight path recovery
Magnetometer: Cesium vapor, high sensitivity magnetometer