Summary:
The Loma Larga property is located between the Gañarin fault to the northwest and the Girón fault to the southeast. A collapsed caldera structure, four kilometres in diameter, the remnant of an eroded stratovolcano, lies along (and probably emplaced and controlled by) the Gañarin fault and 400 m west of the main Loma Larga mineralization. The caldera is underlain by late felsic domes and is cut by a multi-phase diatreme. The north-south trending Rio Falso fault, which appears to be a conjugate fault linking the Gañarin and Girón faults, is the locus for alteration and mineralizing fluids.
The property and immediate surrounding area is mostly underlain by Upper Miocene volcanic and volcaniclastic rocks, of the Turi, Turupamba, Quimsacocha, and Tarqui formations. These formations are flat lying to gently dipping and usually do not outcrop on the property. The outcrops that are exposed form a radial pattern around the caldera and gently dip away from it to the south and east.
The property is largely underlain by the Quimsacocha Formation which hosts the Loma Larga deposit and consists of alternating andesitic banded lava flows with phenocrysts of fresh plagioclase and andesite tuffs and breccias, distributed radially only around the outside of the caldera. The Quimsacocha Formation overlies the Turi Formation which consists of tuffaceous breccias, conglomerates, and sandstones with a high content of andesitic clasts and occasional clasts of tuffaceous breccia.
At Loma Larga, like in most typical high sulphidation epithermal systems, alteration is characterized by multiphase injections of hydrothermal fluids strongly controlled by both structure and stratigraphy. The alteration-mineralizing event is characterized by an early alteration phase caused by a strong inflow of volatile, acidic fluids which cooled progressively and were neutralized by their reaction with country rock, leading to the formation of silicified layers surrounded by alteration halos of clay minerals while the sulphides and gangue minerals associated with the mineralization were deposited by later fluids inside the silicified bodies (IAMGOLD, 2008).
The majority of the limited amount of outcrop exposed at Loma Larga exhibits silica alteration, due to its resistance to weathering. In epithermal environments the silica alteration displays evidence of hot acidic leaching. Multiple types of silica alteration occur at Loma Larga, including vuggy, sugary, banded, fracture fill, and hydraulic-breccia (MacDonald, 2010).
Alteration can be seen to be structurally controlled as it typically occurs as silica ribs mimicking fault locations and orientations. The most significant alteration zone, host to the deposit, is coincident with the north-trending Rio Falso fault, extending for over eight kilometres northsouth, along the eastern edge of the collapsed caldera. This long, linear zone contains multiple large pods of silica alteration ranging up to two kilometres in east to west width. The location of the Rio Falso fault suggests that it was coeval with or postdates the caldera collapse (MacDonald, 2010).
The high sulphidation epithermal gold-copper-silver mineralization in the Loma Larga deposit is also stratigraphically controlled as it occurs at lithological contacts between Quimsacocha Formation andesitic lavas and tuffs and reaches greater thickness in the more permeable tuffs. The deposit is a flat lying to gently western dipping (less than ten degrees), north-south striking, cigar shaped body, which has a strike length of approximately 1,600 m north-south by 120 m to 400 m east-west and up to 60 m thick, beginning approximately 120 m below surface. It also dips slightly to the north, such that the mineralized zone is closer to surface at the south end. Resources are defined as a smaller, higher-grade subset within this mineralization.
Mineralized zones are characterized by multiple brecciation and open-space filling events and sulphides such as pyrite, enargite, covellite, chalcopyrite, and luzonite or, at lower sulphidation states, tennantite and tetrahedrite. Higher grade intervals typically coincide with increased amounts of enargite, minor barite, and intense hydraulic brecciation that contains subrounded to rounded silicified fragments. Visible gold is rare. Gold mineralization is found, for the most part, in one of the following mineralogical assemblages: (a) vuggy silica plus fine grained pyrite and enargite; (b) massive pyrite, including a brilliant arsenical pyrite; or (c) vuggy silica with grey silica banding, sulphide space-filling and banded pyrite. Very fine grained pyrite is dominant in semi-massive to massive zones, and is interpreted to have formed earlier than coarser fracture and vug-filling pyrite (MacDonald, 2010).
The silica alteration is surrounded by varying widths of a halo of argillic alteration, grading from higher to lower temperature mineral assemblages including pyrophyllite, alunite, dickite, kaolinite, illite, and smectite.
The high sulphidation epithermal gold-copper-silver mineralization in the Loma Larga deposit is also stratigraphically controlled as it occurs at lithological contacts between Quimsacocha Formation andesitic lavas and tuffs and reaches greater thickness in the more permeable tuffs. The deposit is a flat lying to gently western dipping (less than ten degrees), north-south striking, cigar shaped body, which has a strike length of approximately 1,600 m north-south by 120 m to 400 m east-west and up to 60 m thick, beginning approximately 120 m below surface. It also dips slightly to the north, such that the mineralized zone is closer to surface at the south end. Resources are defined as a smaller, higher- grade subset within this mineralization.
Mineralized zones are characterized by multiple brecciation and open-space filling events and sulphides such as pyrite, enargite, covellite, chalcopyrite, and luzonite or, at lower sulphidation states, tennantite and tetrahedrite. Higher grade intervals typically coincide with increased amounts of enargite, minor barite, and intense hydraulic brecciation that contains subrounded to rounded silicified fragments. Visible gold is rare. Gold mineralization is found, for the most part, in one of the following mineralogical assemblages: (a) vuggy silica plus finegrained pyrite and enargite; (b) massive pyrite, including a brilliant arsenical pyrite; or (c) vuggy silica with grey silica banding, sulphide space-filling and banded pyrite. Very finegrained pyrite is dominant in semi-massive to massive zones and is interpreted to have formed earlier than coarser fracture and vug-filling pyrite (MacDonald, 2010).
The focus of mineralization occurs at approximately 3,610 m (± 30 m) elevation, where structural feeder zone(s) intersected a permeable tuff horizon that was acid leached. There is an upper barren silicic lithocap, locally indicative of steam heated alteration, which is typically barren of mineralization, although there is an outcrop exposure of this zone that contains minor, fine visible gold (MacDonald, 2010). Silica textures within the upper zone range from sugary, to two-phase, massive, vuggy, and laminated, while the main body centred at 3,610 m exhibits either massive or vuggy silica with intense brecciation in the core and pervasive veinlet and vug infilling alunite alteration. A third lower silicified horizon described below is primarily vuggy in nature (MacDonald, 2010).