Bark, leaves provide geology guidance

Exploration geologists searching for gold in Nevada these days pay close attention to plants as well as rocks.

Some analyze plant matter for tiny traces of precious metals that the vegetation absorbs through soils and groundwater a hint that gold deposits are nearby.

And the ever-pressing need for mining companies to discover and develop new gold deposits, coupled with advances in analytical techniques, could spell big business for one longtime Reno company.

Biogeochemistry, lately called phytogeochemistry by geologists, is one of many tools used by geologists working with exploration companies to get an idea of the geographical composition of a study area. Plant matter leaves, twigs, bark is analyzed using mass spectrometry to detect the presence of precious metals that may indicate the presence of gold.

Shea Clark Smith, head of Minerals Exploration & Environmental Geochemistry of Reno, has tramped Nevada's ranges and basin's for decades and has collected more than 100,000 plant samples for use in minerals exploration.

"Bark is an easy medium to take, and using that material we have come to realize that trace metals are incorporated in parts of the plant that can be easily sloughed off," Smith says. "Leaves also are a great place for metal accumulation, and leaves have been used in numerous studies."

Minerals Exploration & Environmental Geochemistry, founded in 1984, has used phytogeochemistry for years, but Smith says the practice has become more exact and more cost-effective because of advances in mass spectrometry.

"We can look at metal concentrations and see how they are elevated or depleted," he says. "We can use plant data to ascertain structures and we can almost map out the structural history of an area and determine if it is mineralized."

Jon Price, director of the University Nevada Bureau of Mines and Geology, says biogeochemistry is not used by every company, but many have used it successfully and the study results are pretty well accepted amongst geologists and the exploration community.

Price says the use of phytogeochemistry techniques could become more widespread with further advancements in analytical methods.

"As we are able to detect elements in much smaller quantities, that has opened up some possibilities for biogeochemical exploration," Price says.

Price's colleague at UNR, research geologist John Muntean, this month published a paper in the trade journal Economic Geology that examined and evaluated different geochemical survey methods at Barrick North America's Cortez Mine in Lander County. Plant samples taken from areas with known ore bodies showed elevated levels of gold and zinc in vegetative matter, Muntean writes.

Smith says biogeochemistry is primarily a tool to be used when prospecting companies want to determine where they should drill exploratory holes that tell them the extent of an ore body.

Other common exploration methods that strive for the same results include magnetic and gravity mapping. However, Smith notes, these geophysical techniques don't give an indication of the chemistry of an exploration area.

Smith says biogeochemistry is mostly used by Canadian-based junior mining companies to determine the scope of ore bodies on their claims in the state. The findings can lead to increased mining in Nevada.

Verde Resources of Mill Valley, Calif., for instance, recently hired Smith to conduct a biogeochemical study of its claims in Esmeralda County to determine if there is reason to proceed with further exploration efforts.

"It is very important that biogeochemistry and all other methods that are used continue to have a high profile in exploration activity," Smith says. "Mining in the state will cease if there isn't a good pipeline of prospective ground being discovered using these exploration methods."