By: Marlene Affeld
Gold, a precious metal sought for millennia, is not always found in obvious forms like nuggets or veins. Instead, it often resides deep within the earth, bound to minerals in soils, rocks, and even plants. The discovery that certain plants and soil types can serve as indicators of gold deposits has transformed how prospectors and geologists search for this valuable metal. Through a process called “biogeochemical prospecting,” researchers have identified specific plants and soil characteristics that signal the presence of gold beneath the surface.
How Gold Becomes Part of Soil
Gold is rarely distributed evenly across the landscape. Instead, it is often found in trace amounts, scattered throughout soils or bound to quartz veins and other minerals. Over time, weathering processes break down rocks and transport tiny particles of gold into soils, often concentrating in streambeds or areas where erosion has occurred. This is how placer deposits form, where fine particles or small nuggets can be found.
In other cases, gold remains locked within the soil, bound to clay particles or minerals like iron oxides. These fine particles of gold are not easily visible to the naked eye but can accumulate over thousands of years, making certain soil types excellent indicators of larger gold deposits beneath.
Plants in Northwest Montana that Indicate Gold and Other Mineralization
Northwest Montana, with its rich geological history, is home to various plants that can serve as indicators of gold and other mineral deposits. This region is characterized by its diverse terrain, from mountain ranges to river valleys, making it a prime area for gold prospecting. Certain plants, due to their unique ability to tolerate high concentrations of metals, can be valuable in identifying areas of potential mineralization.
1. Alpine Pennycress (Noccaea caerulescens)
• Associated Minerals: Zinc, lead, gold
• Growing Conditions: Pennycress thrives in mountainous and rocky terrains, which are common in northwest Montana. This plant is known to accumulate metals like zinc and lead from the soil, indicating areas with heavy mineralization. Pennycress is found in areas with disturbed or metal-rich soils, making it useful in identifying areas with potential gold-bearing ores.
• Indicator Role: As a hyperaccumulator of metals, pennycress can absorb and tolerate high levels of heavy metals, including gold. Its presence in mineralized soils, particularly around old mining areas or naturally rich deposits, can be a signal of underlying gold or base metal ores.
2. Western Yarrow (Achillea millefolium)
• Associated Minerals: Gold, copper
• Growing Conditions: Western yarrow is a hardy plant commonly found in dry, open areas in Montana. It can tolerate poor, mineralized soils, often growing where other plants struggle. Yarrow’s ability to thrive in mineral-rich soils makes it an indicator species for gold and copper.
• Indicator Role: In regions with historical mining activity, yarrow is often found in soils with higher concentrations of metals, including gold. Its resilience in harsh conditions makes it a useful marker for mineralized ground.
3. Lodgepole Pine (Pinus contorta)
• Associated Minerals: Gold, silver, copper
• Growing Conditions: Lodgepole pines are abundant in northwest Montana, thriving in a range of soil types from sandy, well-drained soils to more acidic, rocky areas. These trees are often found in regions with mineral deposits, as they can grow in soils rich in gold and other metals.
• Indicator Role: Lodgepole pines have deep root systems that can draw trace metals, including gold, from the soil. Researchers have used tree core sampling to study the metal content in lodgepole pines, finding that elevated gold concentrations in the tree’s tissues can indicate gold deposits in the soil below.
4. Douglas Fir (Pseudotsuga menziesii)
• Associated Minerals: Gold, mercury
• Growing Conditions: Douglas fir trees are widespread in the forests of Montana, often growing on well-drained, rocky slopes where mineralization may be present. These trees can tolerate soil conditions that contain trace amounts of heavy metals, such as gold or mercury.
• Indicator Role: The presence of Douglas fir in areas with past or present mining activity can indicate mineralization. Like lodgepole pines, these trees can absorb metals through their root systems, allowing geologists to test for the presence of gold or other metals by analyzing the tree’s tissues.
5. Sagebrush (Artemisia tridentata)
• Associated Minerals: Gold, mercury
• Growing Conditions: Sagebrush is often found in drier regions and foothills of Montana, preferring well-drained soils with low organic content. It is a key indicator plant in mineralized areas due to its ability to survive in soils containing high levels of metals.
• Indicator Role: In northwest Montana, particularly in areas with historical mining, sagebrush can indicate the presence of gold and mercury. This plant is often found growing near gold-bearing deposits, especially in dry, mineral-rich soils, making it a valuable tool in prospecting for metals.
6. Serviceberry (Amelanchier alnifolia)
• Associated Minerals: Copper, silver, gold
• Growing Conditions: Serviceberry plants are commonly found in mountain valleys and open forest areas in Montana. They thrive in well-drained soils and can grow in locations with high mineral content.
• Indicator Role: In areas with metal-rich soils, serviceberry plants have been known to absorb trace metals. Their presence, particularly in regions near historical mining activity, can be an indicator of underlying copper, silver, or gold deposits.
7. Indian Paintbrush (Castilleja spp.)
• Associated Minerals: Selenium, gold
• Growing Conditions: Indian paintbrush grows in dry, open grasslands and rocky slopes across Montana. It prefers well-drained soils and can grow in regions with poor organic content, often near gold-bearing deposits.
• Indicator Role: Indian paintbrush is a hyperaccumulator of selenium, and its presence can also suggest soils with elevated gold concentrations. This plant is often associated with mineral-rich areas, making it a potential indicator of gold-bearing ground.
8. Viola calaminaria (Zinc Violet)
• Associated Minerals: Zinc, lead, cadmium
• Growing Conditions: The zinc violet is native to Europe and prefers calcareous or zinc-rich soils, often growing on mine tailings or in areas naturally rich in heavy metals. This small violet thrives in mineral-rich soils where other plants would struggle to survive.
• Mineral Indicator Role: Known as a hyperaccumulator of zinc and lead, this plant is often found in areas with significant metal deposits. Its ability to tolerate high levels of heavy metals makes it an important indicator in the search for mineralized soils.
Biogeochemical Prospecting in Montana
In Montana, biogeochemical prospecting—the use of plants to detect underlying minerals—has become an increasingly valuable tool. Due to the state’s rugged terrain, conventional prospecting can be challenging, and these natural indicators offer a low-cost, non-invasive way to identify potential mining areas. Plants like lodgepole pine, sagebrush, and Indian paintbrush are particularly useful in signaling mineralization, including gold and other precious metals.
Using the chemical composition of plant tissues (such as leaves, bark, or tree rings), researchers can analyze the concentration of metals like gold, copper, or silver in the plants growing in mineralized soils. This method allows geologists to pinpoint areas of interest before committing to more costly and invasive exploration techniques like drilling.
Conclusion
The unique flora of northwest Montana offers important clues for prospecting gold and other metals. Plants like sagebrush, lodgepole pine, and Douglas fir can serve as natural indicators of mineralized ground, drawing trace amounts of metals from the soil and concentrating them in their tissues. Understanding the relationship between these plants and the underlying geology can significantly aid in gold exploration efforts, particularly in remote and difficult-to-access regions.
Soil Characteristics That Indicate Gold
Apart from plants, certain soil types can also signal the presence of gold. Geologists often look for specific soil characteristics and mineral associations when prospecting for gold.
1. Iron Oxides and “Gossans”
One of the most important indicators of gold is the presence of iron oxides in the soil. These oxides, often reddish or yellowish in color, form when iron-bearing minerals weather and oxidize. In areas where gold-bearing sulfide minerals (such as pyrite) are present, these oxides can form a cap, known as a “gossan,” over the deposit. Gossans are highly visible on the surface and are often found in areas with rich gold deposits beneath.
2. Clay-Rich Soils
Clay soils can also signal the presence of gold, as fine particles of gold can become trapped in the clay. In areas with placer deposits, gold often concentrates in clay layers due to its high density. Prospectors sometimes dig through clay-rich soils to recover the gold particles mixed within.
3. Quartz Veins
While not technically soil, the presence of quartz veins in the ground is a strong indicator of gold. Gold is often found alongside quartz in hard rock deposits, and areas with abundant quartz veins are likely to contain gold. Quartz and gold form together under high pressure and temperature, and prospectors often search for quartz outcrops when hunting for gold deposits.
Biogeochemical Prospecting: Using Nature to Find Gold
Biogeochemical prospecting is the practice of using plants and soil indicators to locate valuable metals, including gold. This technique has become increasingly popular in recent years due to its non-invasive and cost-effective nature. Instead of drilling or digging, geologists analyze plant tissues, soil samples, and the chemical composition of surface minerals to detect trace amounts of gold.
This approach is particularly useful in remote or difficult-to-access areas where traditional exploration methods may be impractical. It allows researchers to narrow down areas of interest before committing to more intensive exploration techniques, such as drilling.
In mineral prospecting, certain plants, often referred to as geobotanical indicators, have evolved to tolerate high levels of metals in the soil, such as gold, copper, nickel, or zinc. These plants not only signal the presence of mineralized ground but can also thrive in harsh soil conditions that would be unsuitable for most other plant species. Below is a list of additional plants, alongside those mentioned earlier, that can indicate the presence of various minerals, as well as the specific conditions in which they grow.
The presence of gold in soil and plants has opened up new avenues for gold prospecting, transforming how we search for this precious metal. Certain plants, alongside soil characteristics such as iron oxides and quartz veins, serve as natural indicators of hidden gold deposits. With ongoing research into biogeochemical prospecting, the use of nature to find gold is likely to become an even more valuable tool for future exploration.