Summary:
Deposit Types
The Zancudo deposit is a high-grade gold-silver-quartz epithermal vein deposit. Epithermal gold and silver deposits of both vein-style and bulk-tonnage style are grouped into high sulfidation (HS), intermediate sulfidation (IS) and low sulfidation (LS) types based on the sulfidation states of their hypogene sulfide assemblages (Simmons et al., 2005).
Vein mineralization is deep epithermal type with temperatures extending above 300°C into the mesothermal field, with an evolution from low to intermediate sulfidation type. This type of mineralization is considered to be porphyry related. The Cerro Vetas porphyry Au-Cu deposit occurs 3 km southwest of the Zancudo deposit and may have been the source of the hydrothermal fluids that formed the Zancudo veins. However, the veins lie to the east of the porphyry deposit at a lower altitude and dip to the east, suggesting that the fluid source may be another, undiscovered porphyry deposit at lower elevation to the east of Zancudo.
t is reported that Zancudo also has potential for porphyry gold-copper mineralization related to porphyry intrusions, and there are occurrences of porphyry-style alteration and veining in the western area (Redwood, 2012; Gallego, 2014), which has not been the focus of the current MRE.
Mineralization
Mineralization at Zancudo occurs in multiple steep or flat lying vein/structures that have been exploited over a strike length of 2,500 m. The known vertical extent of mineralization is at least 650 m.
Gold mineralization at Zancudo occurs in two different types of structures. Mineralization in flat-lying veins and dissemination in conglomerates and sandstones at or near the base of the sedimentary sequence at the unconformity with the underlying schists. These were historically mined in sub-horizontal structures called “mantos” near surface at Zancudo and Otra Mina and most of the historical gold production came from these. A near continuous zone of flat lying veins occurs west of the Zancudo Fault over a strike length of 1,600 m with a strike of N30°W and a width of 400 m in the northern zone (PPM, 2002, 2003).
Mineralization in higher angle structures is hosted by N-S striking, steeply dipping veins in chlorite schists. These were mined by several long crosscuts with the levels defined in meters above the La Independencia level, namely: Chaverra (+269 m level), Castaño (+230 m), Sucre (+189 m), Palma (+189 m), Troya (+140 m) and La Independencia (0 m level, at 923 m altitude). These were all made before 1923 and were described by Botsford (1926) and Grosse (1926). The Independencia Mine has a crosscut 740 m long in direction 254°, and four veins (Colmena, Platanal and splays) were exploited over a strike length of about 300 m and to 125 m above the Independencia level.
The veins have early stage, base metal sulfides (pyrite, sphalerite, galena, arsenopyrite) infilled by quartz or quartz-carbonate gangue, with banded textures that are typical of epithermal veins. The vein minerals, in order of decreasing abundance, are pyrite, galena, arsenopyrite, sphalerite, silver-sulfosalts, bournonite, boulangerite and jamesonite, with minor chalcopyrite, pyrrhotite, native gold or electrum, and native silver. The gangue minerals are quartz, calcite and clay minerals. The clay minerals identified are kaolinite, muscovite and sericite. Wall rock alteration is sericite, carbonate and disseminated sulfides.
All three types of structure have halos of argillic alteration. The vein textures are massive sulfide with grain size up to 20 mm; banded quartz-sulfide with wall rock clasts; and quartz veins with cockscomb banding, colloform banding, druses, bladed quartz replacement of calcite, and banded textures. The principal sulfides are arsenopyrite, pyrite, galena, sphalerite, and chalcopyrite.
The Santa Catalina structure follows an andesitic dike that may have strong argillic alteration with a stockwork of narrow sulfide veinlets (IAMGOLD, 2020). The wall rocks may be mylonitized in schists, brecciated in schists and sedimentary rocks, or have strong argillic alteration with quartz veinlets. Disseminated sulfides are common in the matrix of sedimentary wall rock and along the foliation of schists.
The low angle structures or mantos typically have a hydrothermal breccia texture with clasts of quartz and wall rock with sericite alteration, and a quartz-sulfide matrix (IAMGOLD, 2020). Another important texture is a stockwork of narrow quartz-sulfide veinlets.
The sub-vertical structures commonly have quartz veins with low sulfidation epithermal textures as described above.
There are occurrences of porphyry-style magnetite ± quartz (M type) and quartz ± pyrite (A and B type) veining accompanied by potassic alteration overprinted by propylitic alteration in andesite porphyry, diorite and basalt in the western area of the property.
Gold/electrum occur as inclusions in sphalerite, pyrite, arsenopyrite, and may also be partially surrounded by pyrite, arsenopyrite, sphalerite, and tetrahedrite. About 80% of the gold/electrum grains are below 30 microns in size. Much of the gold/electrum occurs as small inclusions of less than 10 microns in pyrite and arsenopyrite, or intergrown with other minerals. A small proportion of gold occurs in fractures in other minerals. A small percentage is coarse grained (>100 microns). The average Au/Ag ratio is 72/28, and varies from 67/33 to 74/26 (Gallego et al., 2005). Native silver occurs in minor amounts as small grains in contact with silver-rich sulfosalts. The silver-bearing sulfosalts identified are argentian tetrahedrite ((Cu,Fe)12As4S13)- freibergite ((Ag,Cu,Fe)12Sb4S13) solid solution, andorite (PbAgSb3S6), miargyrite (AgSbS2), diaphorite (Pb2Ag3Sb3S8) and owyheeite (Pb10Ag3Sb11S28). The lead-antimony sulfosalts identified are bournonite (CuPbSbS3), jamesonite (Pb4FeSb6S14), and boulangerite (Pb5Sb4S11). The FeS content of sphalerite varies from 0.91 molar percentage (mol %) in the early generation to higher FeS in the later stages that show zoning from 4 to 20 mol %, with a dominant range of 9 to 16 mol % FeS.
Significant Mineralized Zones
Based on the exploration completed to date drilling has identified the presence of both steeply dipping structures or veins and shallow manto style vein material. The vein textures are massive sulfide with grain size up to 20 mm; banded quartz-sulfide with wall rock clasts; and quartz veins with cockscomb banding, colloform banding, druses, bladed quartz replacement of calcite, and banded textures. The principal sulfides are arsenopyrite, pyrite, galena, sphalerite, and chalcopyrite.
The low angle veins or mantos typically have a hydrothermal breccia texture with clasts of quartz and wall rock with sericite alteration, and a quartz-sulfide matrix (IAMGOLD, 2020). Another important texture is a stockwork of narrow quartz-sulfide veinlets.
The sub-vertical veins commonly have quartz veins with low sulfidation epithermal textures as described above.
During initial test work on potential stope targets for mining to aid in with exploration planning it was noted that a number of potential explorations targets could be noted.
The current strike length of the known mineralization approximately 2.5 km, with a dip ranging from 40 to 60º for the steep to sub horizontal for the manto vein mineralization.