Summary:
The Maricunga belt hosts numerous alteration zones which are associated with appreciable precious metal mineralization. Marte, Lobo, and Aldebaran, is dominated by gold-rich porphyry deposits and prospects (>0.3 ppm Au) associated with eroded andesitic stratovolcanoes.
Marte and Lobo are rich in gold (1.43 and 1.6 ppm) and poor in copper (0.05 and 0.12%) and molydenum (46 and ~ 10 ppm), and may be designated as porphyry gold deposits. However, gold contents are lower (0.6-1 ppm) and hypogene copper contents probably higher at Refugio and Casale Hill.
In the Maricunga belt, nine examples of well-developed porphyry-type mineralization carrying gold and variable amounts of copper have been recognized to date: Marte, Lobo, Valy, Escondido, A1-Aldebaran (Casale Hill sector), La Pepa, Refugio, Amalia, and Santa Cecilia. Weakly developed quartz stockworking similar in style to that constituting the porphyry-type mineralization, and anomalous in gold, was observed within the alteration zones at Esperanza, La Coipa, and Pantanillos. As described above, porphyry-type mineralization is present in both the western-early Miocene-and eastern-middle Miocene-metallogenic subbelt.
Intrusive complexes
Subvolcanic porphyritic intrusive complexes are recognized locally in the Maricunga belt and appear to be located principally in the nuclei of eroded compound volcanoes, exclusively in association with areas of hydrothermal alteration. Some petrographic differences are detected between intrusions in the western and eastern subbelts, although the absence of unaltered intrusive rocks precludes meaningful wholerock analysis. Intrusions in the eastern subbelt are all dark-colored porphyry stocks and dikes of predominantly dioritic composition. The textures of the least altered rocks are fine- to medium-grained porphyritic, locally with flow features around large seriate phenocrysts of zoned plagioclase. Marlcs comprise relicts of biotite, hornblende, and very subordinate pyroxene, all altered largely to chlorite, sericite, and clay. Surface mapping and core logging of the Lobo and Marte systems reveal a strikingly similar intrusive evolution, with pre-, syn-, and late mineralization phases distinguishable. The sequences begin with relatively coarsegrained diorite porphyry followed by at least one finer grained porphyry of similar composition, and finally by fine-grained and only weakly porphyritic microdiorite displaying miarolitic cavities. At least three microdiorite phases spanning the mineralization interval are recognized in the Lobo deposit. Intrusive contacts are generally abrupt and clearcut, although microdiorites tend to have brecciated and partially assimilated earlier intrusions along their roofs at Marte and Lobo to form intrusion breccias consisting of diorite porphyry xenoliths in a microdiorite matrix.
Intermediate argillic alteration is the term employed to describe quartz-chlorite-sericite-clay assemblages that accompany most gold-bearing stockworks elsewhere in the Maricunga belt; they are best studied in diorite porphyries at Marte and Lobo. Clay has been subjected to only limited X-ray diffraction analysis and is therefore poorly known. However, much of it in oxidized rock is suspected to be kaolinitc of supergene origin. Smectite is also present in sulfide-bearing rock and is considered hypogene. Plagioclase phenocrysts underwent conversion to clay, sericite, and minor chlorite, mafics to chlorite and subordinate sericite and clay, and groundmass silicates to clay-sericite-chlorite mixtures. Gypsum is ubiquitous, especially at Marte, as crosscutting veinlets and impregnations and is believed to result in part from the hydration of anhydrite.