The western part of Salta Province is underlain by mid to late Tertiary continental volcanic arcs and related sedimentary rocks of the Andean cycle. The Andean volanic arcs are concentrated along the north trending axis of the high Andes and along several northwest trending “structural transverse zones”. The Rio Grande area consists of two overlapping andesitic volcanic centres, as well as numerous flanking shallow intrusive plugs, dikes, and sills. Both are constructed of dacitic to andesitic flows, sills and dikes, intruding and flanked by volcaniclastic rocks, including breccias, agglomerates, and lahars, generally dipping away from the volcanic centres. Alteration is roughly concentrically zoned and is strongly influenced by rock type. The occurrence of veining and mineralization at Rio Grande is associated with the development of several distinctive hypogene events during the evolution of the deposit. In addition, supergene types of mineralization at Rio Grande were developed during the uplift and erosion of the deposit in younger stages and up to the present-day time.
The Rio Grande deposit has been the subject of much debate concerning the origin of the mineralization and deposit type. Different styles of copper-gold mineralization with associated alteration have been recognized. There is an early mineralized system with affinities to IOCG (iron-oxide-copper-gold) type deposits, and a later mineralized system with affinities to porphyry style copper-gold deposits.
The Rio Grande property is a large porphyry Cu-Au prospect with alkaline porphyry affinities. Mineralization covers a large area measuring at least 4 sq. km. and consists of disseminated and veinlet-controlled magnetite-chalcopyrite. Primary copper sulphides have been weathered to secondary, black and blue-green copper oxides to depths in excess of 200 m. A thorough review of work previously completed by Teck in 2000-2001, as well as new geological and structural mapping by Regulus, has identified a series of target areas with potential for higher copper-gold grades within a distinct 2 km diameter ring of IP chargeability and anomalous surface and subsurface copper and gold geochemistry. The 200 m average estimated depth to the top of the IP anomalies together with drill hole information suggest that there is potential for an extensive area of oxidized mineralization that may be amenable to SX-EW recovery of copper.
Cu-Au-Ag mineralization at Rio Grande occurs within a distinct 2-km diameter ring-shaped fracture zone defined by IP chargeability, as well as Cu- and Au-soil geochemical anomalies. Systematic surface trenching shows the majority of the ring structure is mineralized in several zones/targets; moving clockwise from the north these zones are (a) North, (b) Sofia, (c) Discovery, (d) Southwest, and (e) #7. Trenching and drilling in the central part also shows presence of mineralization.
Rio Grande is located within a partially eroded, earliest Middle Miocene (16.5 Ma), andesite-dominated volcanic-intrusive complex of high-K, alkaline affinity. The Rio Grande complex intrudes through granitic basement rocks into a continental back-arc basin filled with a thick (>1,500 m), oxidized sequence of continental red bed sandstones. Sections within this sequence contain sulphate evaporites and possible halite in the upper parts of the section. Host rocks include a wide variety of dacite to andesite, sub-volcanic, hypabyssal intrusive rocks and dykes.
The Rio Grande complex is located along the regional ESE-trending Archibarca lineament; where it is intersected by NNE-trending “East Fissure” parallel structures. The faults vary from large, property-extensive structures to narrow, outcrop-specific structures. The character of the faults measured is quite variable, ranging from single, narrow fault surfaces to complex fault/fracture zones exhibiting gouge and broken rock. Structural mapping indicates there are two principal orientations of faults; a prominent ESE-trending set (“Archibarca Transverse Zone”) and lesser prominent NE-trending set (“East Fissure Fault Zone”), which together define the rhomb-shaped, annular (i.e. ring-like) geometry of the mineralization. The mineralized zones are steeply, inward-dipping (i.e., towards the centre of the Rio Grande complex).
The alteration developed in rocks from Rio Grande project is the result of extensive hydrothermal activity. Five main alteration types have been identified, including:
- Potassic K-feldspar (PK),
- Calcic-Iron-Sodic (CaFeNa), (commonly diopside ± magnetite ± K-feldspar ± scapolite ± actinolite)
- Potassic Biotite (PB),
- Chloritic / Propylitic (CHL/PRO),
- Iron Oxide / Clay (FeOx / CLAY).
Zonation of the alteration is roughly concentric, with a strong to intensely altered central core area, where mainly intrusive subvolcanic rocks are located, grading outward to less altered rocks in the surrounding areas. Rocks in the outlying areas are dominated by intermediate volcanoclastic rocks and red bed sedimentary rocks which often exhibit clay and/or chloritic or weak propylitic alteration.
Near the core of the Rio Grande system, the rocks exhibit variable degrees of potassic, K-feldspar alteration. The innermost zone of the main igneous centre contains broad zones of moderate to intense calcic-iron-sodic (CaFeNa) alteration that commonly consists of variable amounts of diopside, magnetite, actinolite and scapolite. Also limited areas of potassic biotite altered rocks are more developed in the central area of the Rio Grande system.
Mineralization covers a large area measuring at least 4 sq. km. Four distinct mineralization styles have been identified at the Rio Grande property:
- Near surface, fracture-controlled copper oxides ±sulphides (at depth) with gold
- Structurally controlled copper-gold style mineralization with magnetite-chalcopyrite-pyrite-magnetite, possibly related to deeper porphyry copper-gold style mineralization
- Copper-gold style mineralization with chalcopyrite-pyrite-molybdenite associated with anhydrite veinlets, likely related to deeper porphyry copper-gold style mineralization
- Gold-copper mineralization associated with quartz-magnetite stockworks in diorite intrusive rocks similar to the nearby Lindero deposit.
Within the upper oxidized and mixed ore zones, the main copper-gold mineralization at Rio Grande occurs as fracture-controlled copper oxides and sulphides, which form an annular ring around the main zone of alteration. Mineralization occurs as blue-green and black copper oxides in the near surface environment (i.e., upper ~100-150 m) and mixed oxides and sulphides in the deeper environment (i.e., below ~150 m); both with associated gold.
The principal copper oxide minerals are chrysocolla, malachite, and traces of azurite. The principal sulphide minerals are chalcopyrite and pyrite, which are typically associated with magnetite. Chalcopyrite (with gold) occurs as coarse-grained clots, disseminations, stringers, and fracture-fillings. In the near surface environment the chalcopyrite is commonly oxidized to a dark brown coloured, translucent “copper limonite”, as well as chrysocolla and other blue-green and black oxides (i.e., Cu-bearing Mn-oxides and neoticite). Mineralization of minor quartz-magnetite sheeted veinlets which contain gold and little copper occurs in a satellite target named Cerro Cori, located approx. 2 km northeast from the main Rio Grande mineralized area. In addition to the mineralization described above, there are more classic, quartz-pyrite-magnetite veins and stockworks associated with K-feldspar alteration; which occur in the southwest area.