Placer Deposit Exploration
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Placer Deposit Exploration
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annessa Ventures Ltd sponsored this investigation which formed part of an MSc. thesis project at the University of Victoria. GPR surveys characterized the fluvial depositional structure of a buried paleovalley at Maple Creek, Guyana. A combination of 50 and 100 MHz GPR data were acquired on more than 40 kilometers of line; the GPR data defined the local stratigraphy enabling creation of an organized structure for the fluvial valley-fill sediments.

Problem

The exploration project focused on developing the methodology to characterize stratigraphy of the valley and define the organization of fluvial elements within the valleyfill sediments. Prior to this study, little was known about the subsurface stratigraphy of the area and the suitability of GPR as an investigation method was unclear.

GPR Contribution to Solution

Two strong reflectors were observed over the area and these were interpreted to be major bounding surfaces. Trenching confirmed the lower surface (A) to be the bedrock–sediment interface or the boundary of the paleovalley. This strong reflector results from a contrast in the electrical impedance between the quartz–rich fluvial sand and the saprolitic kaolinized bedrock. The second major reflector (B) occurs at the boundary between the fluvial valley fill and overlying leached, white quartz sand.

The energy of fluvial deposition can be inferred from the detailed fine-scale structure of the foresets in the interval between the major bounding surfaces. This information, in turn, suggests the optimal locations for mineral deposition.

Two examples of the valley fill architecture are presented.

GPR interpretations were
obtained from a number of exploratory excavations
Example 1
Oblique parallel reflectors bounded above and below by laterally continuous reflectors represent a migrating feature such as a large scale mid-channel bar or pool fill. Potential buried diamondiferous and gold-bearing fluvial deposits are marked in red.

GPR interpretations were
obtained from a number of exploratory excavations
Example 2
Divergent fill that represents an incised channel that has been filled with sediment. Potential buried diamondiferous and gold-bearing fluvial deposits are marked in red.

Results & benefits

pulseEKKO 100MHz and 50MHz surveys defined the fluvial architecture of a buried paleovalley at Maple Creek, Guyana. This test pit shows the boundary between the surficial white sand and visibly stained underlying fluvial valley-fill sediments.

 GPR for placer exploration
Two major surfaces (A) bedrock-sediment interface and (B) fluvial valley fill –
white sand were clearly imaged in the radar survey.

This study demonstrates the value of GPR for placer exploration. Some key features of this study are:

  • The compact, portable and rugged pulseEKKO GPR was ideally suited for this type of survey
  • Freshwater coarse grained soils are ideal environments for use of GPR
  • GPR defines subtle variations in soil properties that identify depositional history
  • Operation of the GPR is simple and intuitive and users can be effective in acquiring high quality data with only a few hours of training
  • Digital data acquisition and post-survey analysis are critical to extracting the most from complex investigationsfanset & foreset bedding from a prograding delta

This project was the M.Sc. thesis of Adrian Hickin – the University of Victoria and we thank both Adrian and Vannessa Ventures for permission to share this interesting case study.

Download the case study: Placer deposit exploartion

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