The Importance of Mineralogical Knowledge in the Sustainability of Artisanal Gold Mining: A Mid-South Peru Case

Abstract

Mineralogy and gold processing techniques from several mining areas of the Nazca-Ocoña gold belt, Mid-South Peru, were investigated to assess the efficiency of gold extraction methods in relation to their mineralogy. The deposits from this belt are intrusion gold related to mineralization in quartz veins. Native gold occurs as micrometric grains encapsulated in pyrite and in minor amounts in other sulfides and quartz. Electrum is found mainly in fractures of pyrite and attains up to 35 wt. % Ag. In addition to these occurrences, gold tellurides also occur and they are abundant in San Luis. Gold processing is carried out by amalgamation with mercury and/or cyanidation. The comparison of the gold grade in the mineralizations and in the residual tailings indicates that a significant amount of gold is not recovered using the mercury amalgamation process and also, in the case of the gold recovery by cyanidation, except when cement was added to the cyanide solution. This was due to an increase in the pH that favours the dissolution of the gold matrix. In the cyanidation process carried out in tailings previously treated with mercury, part of the mercury retained in them is released to the atmosphere or to the cyanidation fluids.

Ore Mineralogy, Trace Element Geochemistry and Geochronological Constraints at the Mollehuaca and San Juan de Chorunga Au-Ag Vein Deposits in the Nazca-Ocoña Metallogenic Belt, Arequipa, Peru

“…Based on the information above and our observations, we suggest that San Juan de Chorunga, Misky, and San Cristóbal deposits have the same ore mineralogy and occurrence in the veins. Previous studies in Cháparra deposit showed that gold mineralization occurs in quartz veins related to a high quantity of iron oxyhydroxides and sulfides, mainly pyrite [16]. Our microscopy observations suggest that Cháparra and Mollehuaca deposits have similar vein mineralogy.…”

“…The San Cristóbal veins are located close to San Juan de Chorunga and have similar Au and Ag contents. Figure 8 also includes EMP analyses of electrum grains from the Misky, Santa Filomena, San Cristóbal, Cháparra, and San Luis deposits [16]. Similar to these deposits, we observed that electrum from Mollehuaca and San Juan de Chorunga maintained a negative linear trend in Ag vs.…”

“…The observed ore and gangue minerals, textures, compositions, and the field relationships with host rock, structure, and alteration at San Juan de Chorunga and Mollehuaca are consistent with these possible genetic models but do not distinguish among them. Alfonso et al (2019) published a brief description of five gold deposits in the Nazca-Ocoña belt. Similar to these deposits, the mineralization at Mollehuaca and San Juan de Chorunga occurs as electrum and native gold, although those studies also documented calaverite, sylvanite, and petzite, which were not identified at Mollehuaca or San Juan de Chorunga.…”

“…In the Misky and San Cristóbal deposits, the ore minerals are sulfides, dominated by pyrite followed by chalcopyrite, sphalerite, galena, and arsenopyrite. The gold-bearing minerals are native gold and electrum [16]. Previous studies [24] showed that in Misky, native gold occurs as inclusions in pyrite with sizes less than 80µm, while electrum fills fractures in pyrites.…”

Gold Characterization of the San Luis Alta Mine, Ayacucho, Perú

“…The mineralization consists in veins hosted in diorites and granodiorites from the Tiabaya Super-Unit. This deposit belongs to the intrusive-related type [2]. Mineralization is mainly found in NW-SE in veins of some tens of decimetres wide and dipping about 70 °.…”