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Porphyry Copper Deposits: Portable X-Ray Analysis of Clay Minerals Provides Clues for Mining

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Porphyry copper deposits

Porphyry copper deposits are an important resource for copper and other valuable metals or minerals. They are typically mined using large-scale, mechanized open-pit methods because of their large reserve, low grade, and shallow burial depth.

An efficient method to discover porphyry copper deposits is portable X-ray analysis. In this post, we dig into the ways portable X-ray analysis tools can give engineers the clues they need to mine these valuable resources and extract hidden copper ore.

What Is a Porphyry Copper Deposit?

Porphyry copper deposits are a type of hydrothermal deposit. Characteristics of porphyry copper deposits include:

  • Their mineralization is temporally and spatially related to neutral-to-acidic porphyry bodies
  • Their formation is somewhat intrinsically related to volcanic-intrusive activities
  • They are characterized by certain alteration and mineralization zonation
  • Their ore is in disseminated veins

As contemporaneous and post-period products of porphyry copper ore formation, clay minerals are important guides to discover, explore, and extract copper ore deposits.

Portable X-Ray Analysis Methods for Testing Clay Minerals

Field engineers can use various portable X-ray analysis tools to gather real-time geological information from the clay minerals. For instance, handheld X-ray fluorescence (XRF) analyzers can identify their chemical elements, while portable X-ray diffraction (XRD) analyzers can show their mineralogical information. This on-the spot data helps geologists make faster and more accurate decisions on-site. Consider the following example.

Example: Analyzing Clay Minerals from a Porphyry Copper Mine

Olympus recently worked with engineers from a mining company to complete an analysis of clay minerals obtained from a porphyry copper mine in Myanmar. The engineers took three samples of minerals from the mine and named them high clay, moderate clay, and low clay (Figure 1).

Clay samples from a porphyry copper mine

Figure 1: Three clay samples collected from the porphyry copper mine.

To prep the clay minerals for analysis, the engineers crushed the samples to –2 mm and reduced them to about 100 g each (Figure 2).

Crushed clay samples from a porphyry copper mine

Figure 2: The clay samples after being crushed.

The engineers then analyzed the crushed clay samples using an Olympus handheld XRF analyzer and portable XRD analyzer to identify their chemical elements and obtain their mineralogical information respectively.

Testing the Clay Minerals with Handheld XRF

The XRF test was conducted using the Vanta™ C Series analyzer (VCA model) equipped with a silver (Ag) target X-ray tube. Designed to operate in harsh field conditions, the rugged analyzer is IP55 rated and is built to pass the drop test required by the US Department of Defense standard (MIL-STD-810G). The built-in silicon drift detector (SDD) combined with Olympus’ proprietary Axon Technology™ enables ultra-high-speed detection and lower limits of detection (LODs).

XRF gun for copper mining

Figure 3: Two engineers use a Vanta handheld XRF analyzer on-site.

Another advantage of using the Vanta analyzer is that the sample can be analyzed immediately without any special processing. In fact, qualitative and quantitative information about the sample’s elemental composition can be obtained within 20 seconds.

In contrast, on-site laboratories typically take hours or even days to provide similar analysis results. With the Vanta analyzer, users can quickly obtain accurate laboratory-level analysis data.

Sample Al Si P S Cl Ca Fe Cu Pb
% % ppm % ppm ppm % ppm ppm
Low Clay 1.19 3.66 311 3.72 1087 3378 3.70 3.70 210
Moderate Clay 0.83 3.94 341 0.90 2921 3771 2.34 1332 102
High Clay 0.98 3.95 422 0.90 4324 4062 1.42 821 48

Table 1: Elemental analysis data produced by the Vanta handheld XRF analyzer. The table shows the elements present and the amount of each element in the sample. Elements include aluminum (AI), silicon (Si), phosphorus (P), sulfur (S), chlorine (Cl), calcium (Ca), iron (Fe), copper (Cu), and lead (Pb).

Testing the Clay Minerals with Portable XRD Analysis

The XRD test was performed using the TERRA™ II portable XRD analyzer equipped with a cobalt (Co) target X-ray tube. The analyzer’s XRD testing capabilities combine proprietary technologies from both Olympus and the National Aeronautics and Space Administration (NASA).

The TERRA II analyzer features a unique sample loading system. It requires only about 15 mg of 100-mesh-sieve (<150 μm) powdered sample to collect both structural (mineralogical/crystallographic) information and approximate elemental (chemical) information in less than 10 minutes.

With XPowder software installed, the TERRA II analyzer can collect and process data quickly and easily. Both Vanta and TERRA II analyzers don’t require a large sample size or any special sample prep. They can perform tests even on blast hole samples.

Sample ID Quartz Kaolinite Pyrophyllite Illite Alunite Pyrite Total
Low Clay 62.9 - - - 30.3 6.8 100
Moderate Clay 49.6 8.3 9.7 18.8 10.3 3.2 100
High Clay 45.5 18.7 - 29.8 6.0 - 100

Table 2: XRD analysis data produced by the TERRA II analyzer. The table shows the minerals present and amount of each mineral in the clay samples.

XRD analysis of clay minerals

Figure 5: Comparison of diffraction patterns obtained by the TERRA II portable XRD analyzer

By analyzing the three samples (low clay, moderate clay, and high clay) from the porphyry copper mine using the Olympus handheld XRF and portable XRD analyzers, we found that:

  • The amounts of clay minerals (mainly illite and montmorillonite) in the three samples increased from low clay to high clay
  • The amounts of quartz, alunite, and pyrite decreased from low clay to high clay

This data corresponded to their mineralogical characteristics generally observed in the field.

The discovery of pyrophyllite formed by special alteration is also important for exploring and extracting porphyry copper ores. The elemental data (e.g., Ca, Fe, etc.) from the XRF analyzer and the mineral composition data from the XRD analyzer could corroborate each other, helping to confirm accurate analysis results on this batch of samples.

This is just one example of how portable XRF and XRD instruments can be used in the geoscience field. To learn about other applications for geochemistry, mineralogy, and mining, check out our brochure, Online Geology Resources for Portable XRF and XRD.

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Application Specialist, Analytical Instruments

Howard Yu is an application specialist for analytical instruments at Evident based in Beijing, China. He has been with Evident since 2019 and supports the Evident X-ray fluorescence (XRF) product lines to provide enhanced solutions to customers. After obtaining a bachelor's degree in geology, he received a master’s degree in mineralogy, petrology, and mineralogy of deposits from the Hefei University of Technology. Howard works with customers from a variety of backgrounds, including oil and gas exploration, maintenance, research, medical, forensics, mining, and mineral processing.

June 1, 2021
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