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Geology and Mineralization
The project consists of two main zones (Figure 2) displaying epithermal precious metal mineralization associated with silica-(clay)-sericite alteration emplaced in Miocene volcanic and volcanoclastic rocks and in adjacent sedimentary strata of Late Cretaceous and Tertiary age. These zones have potential to host economic gold deposits.
Regional Geology
The Aguas Calientes property is located in the central part of the Puna Region and is underlain by volcanic, pyroclastic and epiclastic rocks of dacitic composition. These rocks represent one of several closely spaced volcanic centers of Miocene age which occur between San Antonio de los Cobres and Olacapato (Figure 3C). Epithermal-style-precious and base-metal-bearing alteration zones, commonly associated with spectacular colour anomalies, are closely related to these centres.
The Aguas Calientes property is located on the northern margin of an extensional bend in the El Toro fault system, defined by a left-stepping inflection in the trace of the major structure (Figure 3A). This zone of dilation coincides with recent hydrothermal activity in the Tocomar area and may have controlled earlier volcanism and hydrothermal activity to the north and south of the El Toro Lineament during Miocene-age east-west shortening.
The resulting configuration of the Miocene volcanics suggests a northeast-trending belt that straddles the extensional bend along the Olacapato – El Toro fault zone (Figure 3B). Further, this northeast trend, together with the more obvious northwesterly trending faults, is reflected on all scales of observation, from lineaments visible on the landsat images, to the orientation of dykes, faults, alteration zones and individual veins, recorded in the Oculto and Aguas Calientes areas. It is the intersections between these northeast and northwest-trending structures that appear to represent the fundamental structural control on localization of volcanic and hydrothermal centres. It is unknown whether the circular structure centred on the Aguas Calientes property, as outlined on the landsat images, is formed by caldera-related ring structures or by the intersection between NW and NE trending faults or both.
The basement to the Miocene volcanic rocks consists mostly of variably foliated and regionally metamorphosed igneous and sedimentary rocks, assigned to the Ordovician Faja Eruptiva and Formacion Parcha, respectively (Blasco et al., 1996). However, in the western part of the belt, folded (but not penetratively deformed) continental red-beds of the Late Cretaceous to Early Tertiary Salta Group constitute the basement for locally emplaced igneous complexes.
These volcanic centres and their associated alteration zones occur close to the Olacapato – El Toro Lineament. This lineament defines a zone of discrete but discontinuous, northwest trending sinistral transcurrent faults that can be traced from the Salta Basin in the southeast to at least as far as the international boundary with Chile in the northwest (Allmendinger et al., 1983). This fault system has only been active in a strike-slip sense for the past 2 million years (Marrett et al., 1994). However, major tectonic differences across the Olacapato – El Toro lineament indicate that it was active, probably as a normal fault, down to the north, prior to the Cretaceous (Richards, 1998). Moreover, the distribution of mineral occurrences and alteration zones of differing ages on or close to this fault system demonstrate its importance as a deep-seated conduit for hydrothermal fluids, at least throughout the Neogene tectonic history of northwestern Argentina.
Local Geology
The Aguas Calientes project is located in the same province as operating mines Pirquitas (Ag, Silver Standard), Aguilar (Pb-Ag-Zn, Glencore), Olaroz (Li, Orocobre).
The property is underlain by dacitic flows and breccias, with minor, locally preserved fine-grained epiclastic rocks. The volcanic rocks are composed of fine to medium grained quartz, feldspar and biotite phenocrysts set in an aphanitic matrix. Some flow units are also hornblende-phyric. The flow breccias are monolithic, comprising angular to sub-rounded clasts texturally and compositionally identical to the matrix. These breccias commonly exhibit a weakly defined planar fabric that dips at shallow to moderate angles toward the north and is interpreted to reflect primary layering. On the northeast side of the property, the volcanic breccias are found in fault contact with flat-lying ash-flow tuffs. Paleochannels within these tuffs indicate a source to the south and southeast. The contacts between the Tertiary aged volcanics and the Ordovician “Faja Eruptiva” units are not exposed but field relationships suggest that most are high angle faults.
The area appears to have undergone a complex history of horst and graben style block faulting that has controlled the distribution of the rocks now currently exposed and also rocks that may have been eroded away. The oldest lithological unit recognized in the property corresponds to an Ordivician sequence with abundance of foliated volcanic rocks and quartz veins. Unconformably overlying those Ordovician rocks there are brown coloured pyroclastic volcanic rocks of undetermined age. Unconformably overlying both the Ordovician rocks and the old volcanic rocks there are younger volcanic rocks of undetermined age. These younger volcanic rocks appear to cut-off the source area for mineralized boulders or the original paleo-drainage of the colluvial fan deposits.
It has been noticed that alluvial fan deposits have a lot of fresh-looking, volcanic flow rocks and numerous rusty weathering blocks and boulders. The source of all of these blocks and boulders is likely uphill. The topographically high areas are mostly covered with fresh-looking, volcanic flow rocks which appear to be unconformably overlying the older volcanic rocks and breccias that host the mineralization encountered. The suggestion is that in both cases the talus colluvial deposits are paleo-colluvial deposits whose source areas have now been either partly eroded away or covered by younger volcanic rocks.
Structure
Many of the northwest-trending faults that occur in the area of the project were originally interpreted as splays to the Olacapato – El Toro Fault System. However, no evidence of strike-slip movement was found along these faults and many are apparently older than the Late Pliocene when sinistral movement began along the Olacapato – El Toro Fault System (Marrett et al., 1994). The oldest faults are west-verging thrusts, which only affect the basement rocks to the Miocene volcanics. Some of these are truncated by an angular unconformity between steeply dipping red beds of the Salta Group and overlying shallow-dipping sedimentary and volcanic strata of Miocene age. Other northwest and northeast trending faults were active or reactivated during the Miocene when they controlled volcanism, sedimentation and hydrothermal alteration zones. It is likely that the northwest orientation of these faults in the area is in part a result of rotation during sinistral movement along the Olacapato – El Toro Fault System, because they become progressively more northerly trending toward the north, with increasing distance from the major structure.
A zone of northeast-trending high angle faults that were probably active during Miocene volcanism and sedimentation extends from the southwestern margin of the project area, to the central part of the property, where it forms the northwestern margin of the East Oculto volcanic complex. Further northeast on the property, faults of a similar orientation are inferred along the contact between Miocene volcanics and the Faja Eruptiva, and along the Silon Creek. Several other trends are evident on the property, including toward the north-west and north-south, as well as possible ring structures defined by faults with an arcuate trace southeast of the Silon zone (Figure 5).
Alteration and Mineralization
The principal alteration zones in the area are not confined to the Miocene igneous rocks but extend into adjacent sedimentary sequences of Cretaceous and Tertiary age. They are both lithologically and structurally controlled, mostly along and adjacent to northwest and northeast-trending faults. The main zones of hydrothermal alteration on the property are hosted by flow breccias of dacitic composition. In the Silon zone (previously called the Ambrose zone), silica-sericite(-clay) alteration occurs in three distinct areas adjacent to northeasterly trending faults. The altered breccias probably dip at a shallow angle toward the southeast, below unaltered to weakly propylitically altered massive flows. Gold values obtained from grab samples on silica-altered volcanic breccia, are generally less than 2 g/t.
On the other hand, in the Boulder zone there are angular boulders up to 1.5 m in diameter, of limonitic and silicified volcanic breccia which occur along a north-south trending belt, about 350 m in length. Silicification comprises grey opalescent quartz. Gold values are commonly in the 2 to 5 g/t range but are locally as high as 41 g/t Au. These mineralized boulders occur mostly along the creek but extend as much as 20 m up the eastern slopes of the valley. Given their distribution, size and angularity, it is most likely that they reflect the underlying bedrock. Their association with angular float of Faja Eruptiva suggests the existence of a north-south fault along the Boulder zone main creek. The southern end of the Boulder zone is defined by an east-west gully that is probably structurally controlled. Here, the boulder train continues to the east, with the most extensive area of mineralized boulders at the intersection between the north-south and east-west creeks.
About 350 m to the east of the North Boulder zone, there is a north-south trending belt of weak sericite(-clay) alteration that is anomalously high in both arsenic and antimony. Narrow, discontinuous northwest-trending zones of silicification with erratic gold values occur within the broader zone of weak alteration. This belt extends for a strike length of 2 km and is bounded on both sides by the Faja Eruptiva, the contacts with which are not exposed but are probably faults. The South Boulder zone, at the southern end of the belt, occurs along two creeks that define the trace of these faults. The boulders from this zone are lithologically identical to those from the North Boulder zone and are similarly anomalous in gold. However here, the boulder trains are much narrower, indicating a more limited potential.