“Multiple zones of shallow, multi-commodity copper mineralization have been intersected by RC drilling”
Los Mantos is situated within an early Cretaceous shoshonitic volcanic arc. The host sequence comprises 2,000-3,000m of early Cretaceous volcanics and sediments (andesite, volcanic breccias and pyroclastic rocks with intercalated and variably calcareous sedimentary lenses).
The Los Mantos region is cut by the Romeral Fault system which separates distinct geological domains within the Coastal Range. This large structure is believed to have initiated as a steeply-dipping normal fault that was inverted during oblique convergence in the early-mid Cretaceous (Moreno & Gibbons, 2007). The Romeral Fault is part of the Atacama fault system. Mineralisation on the Los Mantos project may be associated with splay faults linked to the Romeral Fault.
The Los Mantos project is structurally located along a N-S trending fault zone which splays off the regionally significant Romeral Fault, thought to represent a southward extension of the Atacama Fault Zone. The Los Mantos project is located at the contact of Cretaceous diorite-monzodiorite and Arqueros Formation andesites.
At Los Mantos, skarn alteration of andesites has occurred with magnetite-actinolite-tourmaline development associated with hypogene copper (cpy). Locally significant ENE-trending cross fault structures are evident at surface and act as feeders to copper mineralisation. These structures define an ENE trending corridor of structural disruption extending to the large Andocolla copper mine, approximately 15km ENE of Los Mantos. Mineralisation is characterised by multiply-stacked tabular mantos horizons that dip moderately toward the east. At surface, mantos zones locally display widths up to six metres and extend along strike semi-continuously over several hundred metres. Mineralisation is evident at surface in numerous pits, trenches and adits.
The following geological observations were documented by J. Beeson during his 2009 field reconnaissance review of the project.
Traversing of the Los Mantos project has allowed compilation of additional geological information along the main mineralised zone. This work shows that the mineralisation at Los Mantos is hosted within specific units of a sedimentary sequence within the Arqueros Formation. The mineralisation is evident as a series of copper-rich mantos horizons (at least three mantos are evident) as well as fault-controlled disseminations, breccias and semi-massive pods that trend generally ENE to WSW. Intersections between the numerous cross faults and the sedimentary horizon may well have assisted the mineralisation process. In addition, a large-displacement sinistral fault juxtaposes the host sequence against a strongly-altered granodiorite body to the south. This association may also be significant as a mineralisation control with the granodiorite possibly acting as both a heat and fluid source. Other granitoid rocks (monzonite and feldspar-crowded andesitic porphyry) intrude the host sequence and show a spatial relationship to the mineralised cross-faults. This relationship provides a further link between magmatic activity, faulting and mineralisation.
The Los Mantos project is a well-mineralised hydrothermal system with strongly elevated copper grades, together with significant gold grades. The temperature, metal and alteration zonation evident along strike of the hydrothermal system suggests that the potential to locate significant mineralisation under the hill remains high. It is possible that the combination of fault-related disseminations and semi-massive pods with copper mantos may extend from exposures along the southern slope of the hill northwards under the hill. Based on the nature of the along-strike zonation it is suspected that the mineralised system plunges gently to moderately northwards.
The alteration patterns associated with mineralisation appear to change in style quite significantly from north to south. Alteration in the northern area is typically magnetite (haematite)-silica (albite)-epidote-carbonate-chlorite with minor K-feldspar. Towards the south the intensity of K-feldspar alteration increases significantly, and tourmaline becomes more evident (in lieu of chlorite). This southwards transition to K-feldspar-tourmaline dominated Fe-oxide alteration assemblages is particularly evident towards the base of the hill at the southern end of the project.
Structural Controls & Observations
In detail the mantos horizons show changes in strike within the underground developments. These changes in strike appear to relate to both subtle kinks in the mantos horizon as well as the influence of cross-cutting (ENE- to WSW-trending) faults. The cross faults are likely to cause these subtle strike changes via brittle-ductile drag and re-orientation of mantos horizons immediately adjacent to fault zones.
Numerous cross-cutting faults were found to cut the copper mantos, with the majority of cross-faults trending between ENE and WSW. In many cases these faults were themselves host to significant copper mineralisation. Where copper minerals are present in the fault zones they typically take the form of fault-zone replacements, breccias and veins. In most cases, copper mineralisation is best developed along fault zones in proximity to the copper mantos. Mineralisation extends along mineralised fault zones for several metres each side of copper mantos.
Mineralisation Styles & Controls
Mineralisation comprises a number of mantos-style copper lodes, together with broader-scale disseminations and more localised semi-massive pods, both of which appear to be associated with cross-cutting faults. The location of the mantos is influenced by intersections between ENE-WSW trending faults and favourable lithological units. The mantos appear to be largely concordant, but may transgress stratigraphy in detail.
The style of the mineralisation changes along strike, and arguably with depth. In the northern part of the Los Mantos project the mineralisation is clearly associated with distinct mantos horizons and localised upgrades along fault zones. Towards the south the mineralisation appears to change in style to include copper mineral disseminations as well as copper-rich pods and breccias more obviously associated with faults. The mantos style also appears to continue southwards but it is more obscure in character. Copper mineralisation is the most obvious style of mineralisation observed at surface. However, there is also significant potential for other poly-metallic mineralisation, especially to the south and at depth.
Towards the south, particularly towards the base of the Los Mantos hill, the significance of the cross-fault zones as hosts for copper mineralisation appears to increase markedly. In the southern areas copper mineral disseminations and semi-massive pods of copper minerals are evident along the fault zones.
Geological Conclusions & Recommendations
Sedimentary units within the Arqueros Formation are the preferred hosts for mantos-style mineralisation at Los Mantos, with the mantos focusing within arkosic units within the sedimentary intercalations. The hosting sedimentary sequence can be traced through the project area for at least 1300m and is associated with semi-continuous copper mantos mineralisation along this N-S trend. Several mantos are evident across strike, with at least three and possibly more mantos being evident. The mineralisation appears to have a depth extent exceeding 200m based on occurrences of high-grade copper deposits from the top of the hill to the base of the hill. The style of the copper mineralisation changes down-dip and southwards, in proximity to a large and potentially fault-bound granitoid body.
Cross faults have acted to both host and potentially upgrade mineralisation where they cut across the mantos.
The potential to delineate stacked copper-ore lenses over a significant strike length and a significant cumulative width is considered reasonable as demonstrated by extensive previous mine workings.
A good analogue for the Los Mantos mineralisation is the Candelaria system of the Punta Del Cobre district, Chile.