Off-the-Ice Placer Prospecting for Gold
Authors: Daily A.F. 
Published: October 2008

The feasibility of using sea ice as a drilling platform, and a high speed placer drill for valuation prospecting of unconsolidated marine sediments was demonstrated during a very successful project conducted by Shell Oil Company from February to April of 1964. The location was Norton Sound, at the northeasterly part of Bering Sea, in the vicinity of Nome, Alaska, adjacent to inland deposits that have been extensively mined since 1900. The area of interest extended from the beach to approximately a mile offshore in water depths up to 50 feet, where drilling from a floating structure would be extremely difficult at that locality.

A high speed placer drill was required that would deliver representative samples of material ranging from soft silt to semi-cemented gravel. The Becker placer drill with 3 inch ID and 51/2 inch OD pipe was selected, and proved satisfactorily. The drill with all auxiliary equipment was mounted in a steel sled. The dead weight was approximately 23 short tons and rated jacking force applied to the ice was 60 tons. The sled was moved by a D-7 dozer which also cleared snow and smoothed rough ice for roads and drilling sites.

In 110 shifts worked during 73 elapsed days, 568 holes were completed, totalling 24,600 lineal feet in sediment to an average depth of 42.9 feet. The average rate was 223.8 lineal feet per shift and 508 holes were completed to bedrock. The depth of water ranged from 6 to 44 feet. The thick-of ice formed after early December increased rapidly from 30 to 48 inches by latter February and was more in areas of older ice. Ambient temperature was generally from 0° to-38°F except for short periods near the end of the project.

Each increment of sample, usually from not more than 4 feet of drive, was put into a plastic bucket and transported by truck to a treatment laboratory situated three miles inland. Fine gold in visible grain sizes was extracted by washing, screening, panning, amalgamation, and parting in accordance with accepted procedures. A valuation for each hole was calculated on a basis of weight of free gold per unit of volume, applying appropriate corrections for excess core volume.

Safety of operations was assured by exposing the surface of ice along drill lines with a dozer. Examination of the ice was made by experienced Eskimos, who drilled thousands of holes to measure the thickness of the ice, and by maintaining an accurate record of the absolute position of sea level. It was found that conditions favorable for breaking of the ice and movement away from shore were predictable. The elevation of the sea also was required for adjustment of depths to the datum plane.

The total cost of the project was of the order of $11 per lineal feet drilled in sediment, which is about half the unit cost for many inland projects in North America.

INTRODUCTION

When plans were being formulated by the State of Alaska for granting of rights to prospect and mine for metalliferous minerals in offshore deposits, the area situated off Nome in Norton Sound, part of the Bering Sea, appeared promising to officials of Shell Oil Company.