Quantitative D-Sims Analysis of Sub-Microscopic Gold in Sulphide Minerals

Request for analysis:
A refractory feed ore sample containing pyrite and arsenopyrite mineral phases was to be analysed for the presence of “invisible” sub-microscopic gold.

Objectives:

  • To quantify the sub-microscopic gold content in pyrite and arsenopyrite mineral phases by dynamic SIMS.
  • To map the distribution of sub-microscopic gold in pyrite and arsenopyrite mineral phases by dynamic SIMS

Methodology:
Polished sections from specific size fractions of the ore sample were prepared and grains representing both mineral phases were selected and marked for analysis. These mineral particles were analysed using the Cameca IMS-3f SIMS instrument and concentration depth profiles for Au, As, S and Fe were produced. The quantification of sub-microscopic gold and arsenic in pyrite and gold in arsenopyrite was established using internal mineral specific standards.

Results of the study:
Three different morphological types of pyrite (coarse, porous and fine) and arsenopyrite (coarse, porous and fine crystalline) were studied. The quantified sub-microscopic gold and arsenic (for pyrite only) concentrations in arsenopyrite and pyrite are listed in Tables 1 and 2.  Concentrations measured in each grain and their average values per mineral type with corresponding 95% confidence intervals are included in the tables.

Major findings:

  • The gold in both the arsenopyrite and pyrite mineral phases occurs exclusively in sub-microscopic form.
  • The arsenopyrite is the principal gold carrier in the ore sample. All morphological types carry high gold content: coarse (86.39ppm), porous (100.98ppm) and fine (55.5ppm) arsenopyrite, Table1.  All SIMS concentration depth profiles show the presence of solid solution gold, Figure 3a.
  • The determined gold concentration in the second mineral carrier, pyrite, is in the low ppm range. Different morphological types carry increasing amounts of gold in the following order: coarse (0.35ppm), porous (1.39ppm) and fine crystalline (6.03ppm) pyrite, Table 2. Statistically, 85% of the SIMS concentration depth profiles in pyrite show the presence of colloidal-size gold, Figure 3b.
  • Comparison by mineral type between measured mean values of sub-microscopic gold concentration is shown on Figure 1.
  • There is a positive correlation between the measured gold concentration and arsenic content in pyrite, Figure 2.
  • The mapped distribution of sub-microscopic gold in a pyrite grain is shown on Figure 4.

Table 1 – Measured concentrations of sub-microscopic gold in arsenopyrite

Table 2 – Measured concentrations of sub-microscopic gold and arsenic in pyrite.


Figure 1. Comparison by mineral type of the measured mean values of sub-microscopic gold concentration.


Figure 2. Scatter plot of gold and arsenic concentration in different morphological types of pyrite grains.


Figure 3. Concentration depth profiles of sub-microscopic gold.
A) coarse arsenopyrite grain containing solid solution gold: Au=158.72ppm
B) coarse pyrite grain containing colloidal gold: Au=1.18ppm


Figure 4. Sub-microscopic gold and arsenic distribution in a coarse pyrite grain.