By Dan Oancea

Geochemical methods have been used in hydrocarbon exploration since 1930s. They include soil and soil gas analysis as well as microbial counting.

The active soil gas sampling techniques are based on the fact that some sort of leakage occurs through a vertical or lateral seepage because of an imperfect seal or because of the presence of conduits like faults and fissures. The methods proved to be pretty much inaccurate, having a low sensitivity, testing for a reduced range of elements and having difficulties in differentiating noise from signal.

Here comes a well-known company even though not for geochemical services: W. L. Gore & Associates. They are the renowned manufacturers of Gore-Tex fabrics widely used for outdoor clothing. In collaboration with Ronald W. Klusman (Colorado School of Mines) they’ve developed a simple and precise tool for imaging of buried mineral and hydrocarbon deposits,

It is a new passive adsorbent sampling system which has small enough pores that allow only the gas to pass through. It is waterproof and could be placed in water up to a 10 m depth for sampling purposes.

The company states that:
“This works in much the same way as Gore’s famous waterproof, breathable GORE-TEX® Fabric. The module captures organic, inorganic, and sulfur gases from buried mineralized systems.”

The Gore modules are buried for weeks or months in order to sample an integrated geochemical signal less influenced by temporal variations. The small diameter hole is no longer than 1m and after the insertion of the module it is sealed again. The collected sample is then analyzed for over 150 compounds by accurate modern methods that could detect concentrations as low as 1 ppt.

Further on they write:

Buried ore bodies, with depositional environments that create oxidation, reduction, or electrochemical processes, can be imaged using GORE Surveys. Such ores include gold, copper, and polymetallic ores."

Methylated organic and inorganic compounds as well as other volatile compounds are often found directly over the mineralized areas with sulfur compounds either over the ore body or as a halo around the deposit depending on the state of oxidation of the ore.”

Three mineral exploration case studies are provided.

Here is my take on these cases:

- A copper deposit buried by 800 to 1,800 feet of volcanic cover is showcased and I think it represents their best shot.

- The ‘buried’ polymetallic vein case study: the vein is almost outcropping; its surface expression is clearly visible in the picture; and the ‘cover’ is just a thin veneer of colluvium, which certainly comprises at least some broken mineralized rock fragments. The surveyors knew the real location of the vein deposit.

- A buried gold deposit with limited outcropping was blind tested (Gore’s surveyors didn’t know anything about geology and mineralization). Target mineralization was buried by 10 – 100 feet of sand dunes. Sand dunes are difficult to be sampled because of the geochemical elements’ reduced mobility; therefore a soil gas sampling method is considered to be more adequate. The test proved to be pretty much accurate especially over the largest mineralized areas.

It appears that the method is really useful in targeting large buried mineral deposits. Because of its characteristics it could be successfully used in arid (areas covered by pediment) as well as tropical zones (totally water proof sampler). Just dig a hole, sink the sampler, get a GPS reading and forget about it for the next month or so.

It seems that a small mineral deposit or a steep vein could be easily missed by survey traverses. Another contra is the turnaround time: it takes a goodly month to retrieve your samples.

Since the focus of worldwide exploration has shifted to the discovery of large buried mineral deposits I am sure that the GORE survey will prove to be the tool of choice for many explorationists.

I let the company to have the last word:

The Gore Surveys Group offers the most accurate, comprehensive and cost-effective soil gas sampling technology available on the market today. These products, backed by comprehensive customer support, meet or exceed the needs of environmental and energy exploration professionals.”