Mineralization

The mineral comes from the Inkaracay, and Mina Sanca 2 veins of the gold deposit located in the Paucartambo province of Cusco-Peru, belonging to the Qero's Gold mining company (Figure 1).

Figure 1 Site location 

The Inkaracay vein (Figure 2a) presents an outcrop of approximately 130 m and 15 to 20 cm power, with a depth that reaches 60 cm. The assembly presents quartz (SiO₂, Qz), pyrite (FeS₂, Py), calcium carbonate (CaCO₃, Ca), and gold (Au) represented by Qz-Py-Ca-Au, carbonates as well as coarse and fine gold are observed in the pyrite microfractures and disseminated gold in quartz with an average grade of 20 g/T; the Mina Sanca 2 vein (Figure 2b) presents an outcrop greater than 350 m, with an average power of 17 cm, and a depth that reaches 34 cm, the assembly shows quartz, pyrite, calcium carbonate, and Au (Qz- Py-Ca-Au), thick gold is appreciated in the form of native gold trapped inside a gray quartz matrix, they also appear in the form of leaves when in contact with quartz and slate, fine gold is shown as filler in pyrite microfractures with an average grade of 45 g/T.

Figure 2 a) Inkaracay Vein with gold dissemination (dipping mantle, pyrite, and gold trace) and b) Mina Sanca 2 Vein 

In the stockwork zone are the Inkaracay and Mina Sanca 2 veins (Figure 3a and 3b, respectively), corresponding to a mineralized structure with the presence of several families of milky quartz veinlets; the predominant veins have a SE-NW direction, the rock assemblage is of the Qz-Py-Apy-Au-Ag type which presents high mineral pyritization encapsulated between quartz and pyrite, some veins exceed 250 g/T, and are thus exploited in a global area of approximately 1,000 m² with a grade of 8 g/T having proven mineralization of 40,000 T at 15 m depth and potential mineralization of 120 m in depth.

Figure 3 Stockwork area and veins 

Mineralogical analysis

X-ray diffraction analysis of the two samples shows that the main composition in the Inkaracay vein (Figure 4a) is quartz (SiO₂), muscovite (KAl₂Si₃AlO₁₀(OH)₂), and dolomite (CaMg(CO₃)₂) with minor composition, in the Mina Sanca 2 Vein (Figure 4b) it presents quartz (SiO₂), muscovite (KAl₂Si₃Al O₁₀(OH)₂) and rutile (TiO₂) to a lesser extent.

Figure 4 X-Ray diffraction analysis: a) Inkaracay vein, b) Mina Sanca 2 

The mineralogical analysis of the Inkaracay vein (Table 3) was carried out at different granulometry +100 (149 µm), +200 (74 µm), and -200 m mesh by microscope of polarized light to determine the degree of release of the mineral. It can see mixed binary gold grain (Au) and gangues (GGs) at +100 m mesh (Figure 5a), ternary mixed grain of chalcopyrite (py), goethite (goe), and gangues (GGs) at +100 m mesh (Figure 5b), native gold free grain (Au) and pyrite (py) mesh +200 m (Figure 5c), a free particle of native gold (Au), mixed grain of pyrite (py) and goethite (goe) mesh + 200 m (Figure 5d), mixed binary grains of arsenopyrite (apy) with goethite (goe) and pyrite (py) with goethite (goe), free grains of sphalerite (ef) and pyrite (py) in a fine mesh of -200 m (Figure 5e), a mixed particle of native gold (Au) with pyrite (py) showing occlusion of the gold in a pyrite matrix at a fine mesh of -200 m (Figure 5f).

Figure 5 Inkaracay vein photomicrographs at mesh +100 m (a, b), +200 m (c, d) and -200 m (e and f) 

The mineralogical analysis for the Mina Sanca 2 Vein (Table 3), in Figure 6a, at a granulometry of +100 m, a binary mixed grain of non-metallic gangs and native gold (GGs-Au) can be seen, in Figure 6b at a mesh +100 m mixed grains binaries of pyrite and goethite (py-goe), for Figure 6c, in a granulometry of +200 m, free grains of non-metallic gangue minerals (GGs), mixed binary grain of gold and goethite (goe/Au) can be appreciated.

Figure 6 Mina Sanca 2 vein microphotographs at +100 m (6a, 6b), + 200 m (6c, 6d) and -200 m (6e, 6f) mesh 

At +200 mesh observe mixed binary grains of goethite and gold (goe/Au, Figure 6d); it can be seen that the finer the mineral (-200 m), the higher the degree of liberation, the gold is seen separated from the gangue free grains of native gold (Au, Figure 6e), finally it can be observed that at a -200 m there is the presence of mixed ternary grains of pyrite (py), hematite (hm) and goethite (goe) and free non-metallic gangue particles (GGs) in Figure 6f.

A) Grindability

From the grinding tests, the longer the residence time in the mill, the finer granulometry is obtained (Figure 7a and 7b); for example, for sample 1 at 16.4 min, there is a P₈₀ of 110 µm and with a longer residence time. The P₈₀ decreases to 65 µm, unlike sample 2, significant variability in the granulometric curve concerning time is appreciated. At the P₈₀, to achieve a granulometry of 100 µm, a time greater than 31.20 min at 65 % -200 m must also be reached at 43.60 min; it achieves 68 µm in 85 % -200 m.

Figure 7 Time vs grindability of the samples 1, 2 and 3 

Sample 3 shows similar behavior to sample 1, because for 17.50 min, 115 µm is obtained at 65 % -200 m and 85 % -200 m to 27.60 min. An average of 19.37 min was determined to obtain a P₈₀ of 100 microns, which is the optimal granulometry for the metallurgical processes selected with our type of mineral, for the flotation tests, and worked as a granulometry 85 % -200 m to evaluate the effect of particle size in all tests.

B) Gravimetric concentration

With this concentration test, it is possible to recover 49.19 and 13.40 % for gold and silver respectively. Sample 2 (Table 6), and Figure 8 show the photomicrograph obtained from the concentrate obtained. For sample 3, recovery of 58.61 and 14.63 % of gold and silver respectively (Table 7), the low degree of recovery is related to the occlusion of gold within the pyrite as shown in (Figure 5f), and the encapsulation in the gangue (Figure 6a). On the other hand, sample 1 was not considered in this test due to the low concentration of gold (0.16 g/T).

Figure 8 Gravimetric concentrate of sample 2, a) 80X and b) 160X 

Figure 9a shows the presence of oxidized pyrite with the presence of oxides such as hematite and goethite. In Figure 9b, arsenopyrite is present that causes problems in concentration; according to Figure 5, the release of this type of species depends on the size of particle obtained in the milling.

Figure 9 a) Altered arsenopyrite matrix, b) Matrix with arsenopyrite and pyrite 

Table 8, presents a summary of the species found in the concentrate of sample 2 that has a higher content of gold (89.78 g/T) in the head, where there is a higher content of quartz (63 %), followed by pyrite and arsenopyrite (18 and 5 % respectively), a low amount of gold is recovery from the presence of gold as electrum at 0.1 %; the tailings obtained go to the next stage of concentration by flotation.

C) Rougher flotation

Table 9 presents the recovery of gold and silver as elements of interest and arsenic and iron as harmful elements to evaluate the process. For sample 1 by flotation, 87.90 and 16.36 % recovery of gold and silver are obtained respectively in the concentrate for a gold head grade of 0.24 g/T, and less than 0.5 g/T of silver with a granulometry of 85 % -200 m pH of 7.7 this pH value prevents the flotation of pyrite instead of gold (^{Mu et al., 2020}), CuSO₄ acts as a pyrite depressant in the conditioning stage, in addition the collectors A-404 and Z-11 are used in conditioning and flotation as well as in foaming the MIBC, this procedure is repeated for samples 2 and 3.

Table 10 shows a recovery of 95.54 and 50.74 % of gold and silver recovery, respectively, with a grade in sample 2 of 2.91 g/T and less than 0.50 g/T for gold and silver. The presence of arsenic in this sample is of 0.87 and 26.98 % of the iron in the concentrate. This behavior is related to the higher content of arsenic species in the form of arsenopyrite (Figure 9).

Table 11 presents the results obtained with sample 3; this has an intermediate recovery compared to samples 1 and 2 (90.43 and 29.44 %) for the recovery of gold and silver, respectively with a gold grade of 1.96 g/T and less than 0.50 g/T for gold and silver respectively with a mixture in similar compositions of the two veins tested (49.12 % of the Inkaracay vein and 50.88 % of the Mina Sanca 2 vein).

For the three samples presented, the gold recovery was high with 87.9, 90.43, and 95.54 % for samples 1, 3, and 2 respectively. Arsenic and iron are closely linked to gold recovery because their recovery is more significant, 78 % for As and 22 % Fe, respectively; this behavior is related to the presence of arsenopyrite; it is essential to mention that arsenopyrite has a collateral effect on the flotation due to hydrophobicity tends to float with valuable minerals despite depression with sodium aluminum silicate so the concentrates obtained went through the grinding, amalgamation, and retort process for the final recovery of gold.

D) Leaching tests

The objective of this test is to evaluate the behavior and metallurgical response of sample 1 (63.64 and 36.36 % of the Inkaracay and Mina Sanca 2 veins, respectively), pH natural=6.5 was measured by shaking the sample in water for 24 h. To maintain process leaching stability the pH is controlled with lime, small amounts were added, it was stirred for two hours, then the pH value is measured, and this is continued until reaching the cyanidation pH equal to 11.

Figure 10 shows the percentage of recovery of gold and silver with respect to a time of 72 hours when the cyanide is dosed from the milling and the direct leaching of the mineral; despite these conditions, it shows a low recovery of gold and silver (lower that 19 %) which is related to the preg-robbing effect which occurs in the presence of carbon (^{Sibrell & Miller, 1992}), the sample used presents a content of 0.83 % of carbon in sample 1 (Table 4), it is also refractory to the process of leaching due to the fact that the gold is occluded in a pyrite matrix (Figure 4f), presents contents of arsenopyrite and altered arsenopyrite (Figure 9a and 9b). A similar behavior is observed if the cyanide concentration is increased to 2000 ppm CN^- and some alternatives were evaluated like as use prefloat and chlorine process with less performance and elevated consumption of reagents, these results are not shown in this research work.

Figure 10 Gold recovery, a) direct cyanidation b) cyanidation from grinding 

Finally, in sample 1, the presence of the gold and silver alloy was found, known as electrum, which constitutes one of the leading gold minerals present in the sample (Figure 11) and presents chemical refractoriness (^{Yannopoulos, 1991}), the other peaks were not identified because for the matter of these analysis only gold and silver are the objective, for this type of minerals there are several methods for their treatment such as the use of cyanide alternatives such as thiosulfate (^{Aylmore & Muir, 2001}), thiourea (^{Deschênes & Ghali, 1988}; ^{Li & Miller, 2006}) and others (^{Haque, 1987}). However, these agents have limited chemical stability against certain pH and potential conditions (^{Calla & Nava-Alonso, 2020}).

Figure 11 Presence of electrum (Au-Ag) 

Of the three samples tested, the contend in sample 1 presented has and intermediate concentration (0.83 %) and presents preg-robbing for gold, so it was not evaluated leaching test for samples 2 and 3 with contents of 1.29 and 0.76 % carbon total, respectively (Table 5).