Magmas to Metals

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No one wants to have an active volcano in their backyard (just ask Dionisio Pulido), but ancient eroded volcanoes can sometimes be literal goldmines for mineral ores.

Here at USGS, one way we’re studying how molten earth cools into mineable ores is by looking at something called melt inclusions. Although it may sound like a particularly fancy hot sandwich, melt inclusions are actually tiny pockets of magma that get trapped in the crystals of growing igneous rocks.

Traditionally, volcano scientists study melt inclusions because they give us a snapshot of the conditions which drove explosive eruptions. Today, USGS scientists are looking at melt inclusions when studying mineral deposits. Thus, the study of how we get from magma to metal, as the saying goes, is an important one for learning where large mineral deposits might be found.

Image shows a scan of a zircon on a black background
Here's an image of a zircon grain in an igneous rock called rhyolite. The scan shows a melt inclusion in an inherited core that is about 100 million years older than the age of the host rhyolite, which can show us the conditions of the molten mix that would later give rise to the rhyolite. (Public domain.)

Snapshots in Time

So how do tiny hot pockets of melted rock help us learn about how and where mineral deposits might form? Just like insects getting trapped in amber, these melt inclusions give us a snapshot of what conditions were like when the rock was first forming.

Igneous rocks, which are where many hardrock mineral ores are found, take thousands, even millions of years to form. So when they’re studied, they only show the final product of all those years of development. Melt inclusions, on the other hand, remain mostly unchanged. By giving us an idea of what the original melt composition looked like, melt inclusions help us understand why that particular set of minerals formed as the magma cooled.

Image shows a scan of an apatite grain on a black background
Meanwhile this image shows a spectral cathodoluminescence map of apatite grains that host inclusions and intergrowths of the rare-earth element-bearing minerals, monazite and xenotime.(Public domain.)

For instance, the hot mess of molten rock and magma can be thought of as a box of Legos. The box of legos (like the molten mix) contains the different colors and shapes of blocks to build structures (the mineral ores). As we use up legos from the box (melted rock cools and minerals begin to form) there are fewer legos remaining to choose from to build new structures. Similarly, when magma crystallizes underground, elements are removed from the mix to form some minerals, and they are not available to form others.

Melt inclusions can lead us to a better understanding of how and where metals like gold, copper, tin, zinc, and tungsten form.

Image shows a scan of an apatite grain on a black background
This is another spectral cathodoluminescence map of apatite grains that host inclusions and intergrowths of the rare-earth element-bearing minerals, monazite and xenotime.(Public domain.)

Mining the Magma

The next question is, how do we use the data we get from melt inclusions? Primarily, we use it to help refine our models of what conditions allow mineral deposits to form, particularly metal deposits. One of the most exciting areas of research for using melt inclusions is the study of how porphyritic rocks form.

Porphyritic rocks are a type of igneous rock that typically have large crystals and form when rising magma columns cool in a particular way. Porphyritic rocks are important, because they’re where we often find mineable concentrations of critical metals.

These metals, along with many other mineral commodities, are critical to the Nation’s economy and security, so learning as much as we can about how and where they form is an important goal of the USGS Mineral Resources Program.

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