Summary:
Mineralization at the Cisneros Project is structurally controlled, and gold is associated with shear zones, breccias and quartz-sulphide veins. The host rocks are mainly granodiorite, quartz-diorite and tonalite.
Hydrothermal alteration of the host rock is narrow around the veins, with carbonates, chlorite, sericite and potassium feldspar being the main alteration minerals. There is a late phase of post-mineral alteration with chalcedony, calcite and zeolite.
The sulphide minerals present are gold, pyrite, chalcopyrite, bismuthinite, galena, sphalerite and molybdenite. However, the order of events (paragenesis) is not yet well-defined.
Native gold and electrum (alloy of gold and silver) show anhedral, subangular to subrounded forms and sizes between 0.001 to 0.082 mm. Gold appears as inclusions in pyrite and chalcopyrite or in veinlets associated with chalcopyrite and bismuthinite.
Based on mineral associations, gold mineralization occurred in two stages. In the first stage, gold is associated with chalcopyrite and in pyrite veinlets, in the second stage, veins of chalcopyrite, bismuthinite and gold cut the pyrite veinlets.
The following deposit descriptions are based in part on an internal report for AGD (Pratt, and Attewell, 2022).
Nus Shear Zone
The Nus shear zone is a composite zone made up of thin ductile shear zones and wider zones of brittle fracturing. The zone varies from approximately 10 m to 45 m in horizontal width and strikes west-southwest with dips ranging between 65° and 80° to the north-northwest.
Sulphide and gold mineralization are associated with chlorite-calcite-sulphide veinlets and an anastomozing array of sulphide-rich faults. Gold mineralization is known to occur over a strike length of 400 m and a distance of 450 m down-dip. The mineralization is open along strike to the west-southwest at depth.
Guaico Vein System
The Guaico Veins are a series of north-northeast striking, steeply west-dipping extensional (tension gash) veins branching from the Cano structure to the south. The veins have jogs indicating dextral strike slip (consistent with antithetic Riedel shears), where dilational vein breccias have formed high-grade mineralized shoots with a subvertical plunge within the plane of the veins.
Mineralization is associated with quartz-sulphide veining along a strike length of approximately 200 m and over a vertical interval of 400 m. The Guaico, Chachos, FW3 and FW5 veins show a braided, anastomosing pattern. The FW3 and FW5 veins form an east-dipping linking set of veins with a dip of around 70° to the east.
Guayabito Vein System
The Guayabito mineralized zone comprises a swarm of north-northeast striking, subvertically dipping veins. There is a second set of northeast striking mineralized structures with quartz veins. The northeast striking vein set shows a late, post-mineral alteration event with zeolite, calcite and clay alteration produced by late, post-mineral fault movement. The main veins are the G1, Fuente, C, V2, X and Bolo.
Higher-grade mineralized shoots are located at the intersections of the two vein sets where there are wider zones with quartz-pyrite veining and breccias. These are named Cuerpos Masivos in the mine (massive bodies). There are four Cuerpos Masivos; Caucasia, Amaga, Santiago and Jardin. The plunge of the mineralized shoots is subvertical within the plane of the veins.
Deposit Type
The Guaico and Guayabito deposits have some characteristics of Intrusive-related gold deposits and some characteristics of mesothermal orogenic vein deposits.