John Muntean, Dept. of Geological and Environmental Sciences, Stanford University, Stanford CA 94305-2115
The Maricunga belt is a region containing abundant gold-silver prospects located in the Andean Cordillera of northern Chile between latitudes 26 30' and 28 00' S and longitudes 69 00' and 69 30' W. The Maricunga belt covers a north-northeast chain of andesitic to dacitic volcanoes that was part of a late Oligocene-Miocene continental margin plutonic-volcanic arc. Gold in the Maricunga belt occurs predominately in epithermal acid-sulfate gold-silver deposits and porphyry gold deposits. At Refugio, a geologic resource of approximately 250 million tonnes at an average grade of about 1 g/t gold occurs in the Verde and Pancho porphyry gold deposits. Gold is associated with quartz veinlets hosted by subvolcanic porphyry stocks of intermediate composition. The mineable oxide reserve at the Verde deposit contains 94.7 million tonnes at an average grade of 1.01 g/t for a total of 3.1 million ounces of gold.
The Verde deposit, comprised of Verde West and Verde East, is hosted by a subvolcanic intrusive complex, approximately 1.6 x 1.2 km in plan view, that was emplaced into probably cogenetic volcanic rocks consisting dominantly of volcaniclastic breccias. The earliest intrusive phases are medium-grained quartz diorite porphyries that contain 25-50% plagioclase phenocrysts, 3-7% hornblende phenocrysts, 0-5% biotite phenocrysts, and 1-5% quartz phenocrysts set in a microaplilitc groundmass of K-feldspar, quartz, and plagioclase. The early quartz diorite porphyry at Verde West is slightly coarser grained and has more abundant, conspicuous quartz phenocryts than the early quartz diorite porphyry body at Verde East. Time relations between these two bodies are not understood. The Verde West quartz diorite porphyry is intruded by bodies of fine-grained diorite and medium-grained diorite porphyry that appear to have been emplaced simultaneously. The fine-grained diorite is equigranular to weakly porphyrytic with a grain size generally less than 0.5 mm. The fine-grained diorite contains very ill-defined "fragments" of diorite porphyry that commonly increase in abundance towards contacts with the diorite porphyry. The diorite porphyry is very similar to the Verde East quartz diorite porphyry except that quartz phenocrysts are rare and there are local miarolitic cavities. The final intrusive phase that hosts mineralization are bodies of microdiorite. The microdiorite is equigranular, is generally less than 0.1 mm in grain size, and has an aplitic texture. Microdiorite occurs as matrix to intrusive bodies of breccia and as irregular, indistinct dikelets (cm's thick) that cut all rock types described above. The intrusive bodies of breccia consist dominantly of well- and ill-defined, cm-scale, matrix-supported fragments of diorite porphyry with lesser fine-grained diorite in a matrix of microdiorite. Local fragments of microdiorite indicate episodic activity. The composite body of fine-grained diorite/diorite p orphyry, the microdiorite dikelets, and the intrusive bodies of igneous-matrix breccia are cross-cut by gold-quartz veinlets. Veinlets cut-off at intrusive contacts and the presence of veined fragments in the igneous-matrix breccias, however, indicate that the period of formation of gold-quartz veinlets overlapped the emplacement of the composite stock, microdiorite, and breccias but predated the emplacement of three small porphyry bodies. The eastern half of Verde East is covered by unaltered, post-ore, dacitic volcanic rocks. This unconformity appears to have been an important control on the depth of oxidation.
The orebodies at Verde have a general annular shape, but in detail appear to have a strong structural control. The highest grades are located in zones of increased quartz vein density, especially at Verde East. The dominant veinlet type at Verde consists of quartz-magnetite (0-5%)-pyrite (0-5%). The veinlets are < 2 cm wide (mostly < 0.5 mm) and have slightly wavy walls, indicating possible formation during the ductile/brittle transition. The quartz is commonly laminated with dark quartz along the margins and locally vuggy quartz in veinlet centers. The veinlets lack selvages and are not similar to "A", "B", or "D" veinlet types typical of porphyry copper deposits. Sulfide content in the Verde deposits, in general, is less than 2%, whereas magnetite content commonly ranges from 2 to 5%. Hypogene copper grades vary from about 0.01 to 0.08% with an average of about 0.03%. Alteration at Verde is typified by complete replacement of mafic minerals to fine-grained aggregates of magnetite and green sericite/chlorite. Shreddy-textured secondary biotite has been observed only in a few locations. Plagioclase is largely fresh. The orebodies are generally surrounded by quartz-sericite-kaolinite-pyrite (2-3%) alteration that locally hosts barren quartz-alunite-pyrite ledges.
In contrast to Verde, the Pancho deposit has features similar to typical porphyry copper deposits. It appears to be a strongly telescoped porphyry system exposed over a 400 m vertical extent, in a cross-section fashion, along a steep hillside. Gold grades are similar to those at Verde, but hypogene copper grades are commonly > 0.1%. The major lithology is a diorite porphyry, similar to the diorite porphyry at Verde, which intruded volcaniclastic breccias. The diorite porphyry, in turn, is intruded locally by microdiorite dikelets and igneous-matrix breccias that are similar to those seen at Verde. As at Verde, the microdiorite dikelets and igneous-matrix breccias were emplaced during mineralization. At the lowest elevations hornblende is altered to secondary biotite. Plagioclase is largely fresh. Discontinuous quartz-magnetite-chalcopyrite (0-5%) and biotite-magnetite veinlets, resembling "A"-type veinlets in porphyry copper deposits, and sugary quartz-chalcopyrite (0-5%)-magnetite (0-5%)-pyrite (0-5%) veinlets, resembling "B" type veinlets in porphyry copper deposits, predominate at the lower elevations. The amount of pyrite and the pyrite:chalcopyrite+magnetite ratio increases outwards. At lower elevations gold grades are generally low (c.a. 0.5 g/t) but fairly evenly distributed. At higher elevations, evidence for secondary biotite is less apparent, and mafic minerals are altered to aggregates of magnetite (commonly martitized) and sericite whereas plagioclase is moderately altered to sericite. Overlying volcanic rocks are strongly altered to quartz-sericite-kaolinite-pyrite(2-3%)-(tourmaline) with common barren quartz-alunite-pyrite ledges. The "A" and "B" type veinlets are much less common at higher elevations (absent in the volcanic rocks), where banded quartz veinlets, very similar to the quartz veinlets seen at Verde, predominate and cross-cut the "A" and "B" type veinlets. Also at higher elevations, gold grades are higher (commonly > 1 g/t), but are more irregularly distributed than at lower elevations.
Faults mapped in the deposits and at the district scale have dominantly NNE and WNW/ENE trends. The NNE faults may be related to a major NNE reverse fault located west of Refugio that places Permian-Triassic rhyolites in apparent fault contact with unmineralized quartz diorite porphyries similar to the porphyries seen at Refugio, The NNE faults, including the reverse fault to the west, are commonly segmented by the WNW/ENE faults. Post-mineral fault movement, however, appears to be minimal.
Field mapping has been concluded and petrographic studies continue. Over 7500 m of road-cuts and over 1500 m of drill core were mapped and logged at scales between 1:100 and 1:1000. Planned fluid inclusion work coupled with phase equilibria studies will be aimed at understanding why the porphyry gold deposits at Refugio differ from typical porphyry copper deposits. Also, high-precision Ar-Ar dating is planned for Refugio and other key locations in the Maricunga belt in order to test whether porphyry-related veinlet mineralization and spatially associated advanced argillic zones that locally host epithermal acid-sulfate mineralization represents a possible genetic link between these two potential ore-forming environments.