Summary:
The mineralisation observed at the Alpala and the Tandayama-America deposits is considered a classic porphyry Cu-Au system. These mineralised systems are hosted within a linear belt (Andean Porphyry Belt) that extends from southern Chile right through to Ecuador and Colombia to Panama. The Andean Porphyry Belt hosts the largest concentrations of copper in the world, including numerous deposits with active mining operations.
Alpala Deposit
Major host rock types of the deposit consist of gabbroic and basaltic basement rocks, overlain by Cretaceous siltstones and minor sandstones that are unconformably overlain by a sequence of Tertiary volcano-sedimentary and andesitic lavas. The volcanic and volcano-sedimentary sequences that host the deposit were previously mapped as the Macuchi Unit (Vallejo 2007), a volcanic arc of tholeiitic and calc-alkaline composition which was formed on oceanic crust during the Eocene.
This sequence has been intruded by a series of Middle to Late-Eocene (Bartonian) hornblende-bearing diorites, quartz diorites and tonalities that form plutons, stocks, and dykes. Intrusions have been emplaced episodically such that each subsequent intrusion has introduced mineralising fluids (notably as porphyrytype quartz and quartz-sulphide veins) into the Alpala porphyry system, and/or remobilised existing mineralisation or contributed to localised overprinting and destruction of the pre-existing mineralisation.
Thin-section petrography reveals the presence of very fine-grained quartz in the groundmass of the intrusions, which suggests compositions that range from quartz diorite to tonalite. However, the intrusive rock types have been classified on observations made by the field geologist using a 20 times magnification hand lens (Garwin et al., 2017).
Intrusions are typically emplaced with a stock-like geometry that is moderately elongate in a northwest direction. Intrusions often hold typically vertically and laterally extensive northwest trending, steeply dipping dyke extensions beyond their stock margins.
The equigranular to sub-porphyritic, hornblende-bearing intrusions at Alpala are narrow, taper upwards, and are geometrically similar to grade models of Cu, Au and Ag mineralisation. Mineralisation occurs as a prolate body approximately 2,400 m northwest by 1,200 m northeast and 2,800 m in vertical extent, defined at a Cu equivalent (CuEq) cut-off criteria of greater than 0.15% CuEq and/or greater than 0.55% B-type quartz veins.
The porphyry-related vein types and mineralisation paragenesis at Alpala indicate a systematic progression in time and have been described by SolGold using the nomenclature originated by Gustafson and Hunt (1975).
Understanding the intrusive phase relationships within the Alpala Deposit was advanced following a detailed review of the drill core in 2014, which focused on identifying the different vein types and their paragenesis. This work resulted in advances in the recognition of inconspicuous intrusive contacts and clearly showed that each intrusive phase contains a specific set of veins and resultant Cu and Au grades.
The geometry of the intrusive units and porphyry style B-type quartz vein abundance zones correlates with Cu-Au grade distributions.
Planar and pervasive, B-type quartz veins crosscut the early vein types and consist of quartz- magnetite-chalcopyrite. At least two stages of B-type veins have been recognised, B1 and B2, with magnetite more abundant in early B1 veins and chalcopyrite more common in the later B2 veins. B-type veins contain the majority of the Cu and Au in the deposit.
Scanning Electron Microscopy (SEM) techniques indicate that the primary Cu minerals are chalcopyrite and bornite. Gold occurs as discrete grains of electrum (typically 65% to 85% Au) that range from one to 50 microns in diameter. The electrum grains occur within chalcopyrite, bornite, pyrite and rarely quartz and anhydrite. Grains of low-Ag electrum (greater than 90% Au) that are 1 to 3 µm in diameter are associated with sulphide grains and occur locally within silicate minerals.
Chalcopyrite-rich, C-type veins contain rare to minor bornite and cross-cut earlier vein types. C-type veins contain significant amounts of metal but constitute a small volume of the drill core. B-type and C-type veins are spatially associated with intrusions that show variable feldspar destruction and sericite-chlorite-clay overprinting of biotite-actinolite and chlorite-epidote alteration mineral assemblages.
Late-stage, pyritic D-type veins with quartz-sericite-pyrite selvedges contain chalcopyrite, minor bornite and, locally, molybdenite. Many of the later vein stages exploit and re-open earlier vein stages, as does anhydrite. Transitional to late stage, anhydrite-bearing veins are inferred to form a halo to the deposit core. Late-stage hydrothermal matrix breccia bodies and volumetrically small igneous matrix breccias, including pebble dykes, typically postdate sericite – chlorite ± clay alteration and are locally cut by pyritic D-type veins and anhydrite veins. The breccia bodies cut the volcanic host rocks and the premineralisation, early-mineralisation, and intra-mineralisation intrusions.
A Re-Os age date determined by a commercial laboratory on molybdenite in a D-type pyrite-chalcopyrite bearing anhydrite-quartz vein that cuts a late mineralisation D20 diorite dyke indicates an age of 38.6 ± 0.2 Ma (2 s). The age dates of the QD10, QD20, and late-stage molybdenite are no different statistically.
The relationship between B-type quartz vein (B-vein) abundance and Cu, Au and CuEq grades throughout the deposit show considerable scatter. However, a linear relationship has been defined with selected examples of 0.15% CuEq, 0.70% CuEq and 1.50% CuEq, equating to 0.55% B-veins, 4.1% B-veins and 9.4% B-veins respectively.
The Alpala deposit geological, structural and mineralisation all display a clear anisotropy which is prolate in a sub-vertical sense, dipping approximately 78° towards the northeast and oblate in plan (striking northwest). Recognition of this geometry in the field and various datasets has played a critical role in the path to the discovery of the Alpala deposit. During initial exploration, quartz vein orientation analysis from the Alpala Central area assessed over 400 quartz vein measurements, the majority of which were from B-type quartz veins.
Tandayama-America Deposit
Major host rock types of the Tandayama-America (TAM) deposit consist of a sequence of Tertiary volcano- sedimentary and andesitic lavas of the same age as those at the Alpala deposit. This sequence has also been intruded by a series of Middle to Late-Eocene (Bartonian) quartz diorites and diorites that form plutons, stocks, and dykes.
The mineralisation style and trend at Tandayama-America is essentially the same style as that seen in Alpala.