Gold-rich porphyry systems in the Maricunga Belt, northern Chile

Abstract

The Maricunga belt is a linear metallogenic unit defined by at least 14 zones of gold and/or silver mineralization between latitudes 26 degrees and 28 degrees S in the Andean Cordillera of northern Chile. After 10 years of exploration, three precious metal deposits are in production, two more have proven reserves, and others are being drilled. Total geologic reserves are 420 metric tons of gold and about 14,000 metric tons of silver divided between intrusion-hosted porphyry-type deposits and volcanic-hosted epithermal deposits of high sulfidation, acid sulfate type.Precious metal mineralization is related to a belt of Miocene volcanic rocks, most of which constitute a series of large compound stratovolcanoes of calc-alkaline composition. The volcanic rocks overlie unconformably and largely conceal an igneous and sedimentary basement of Paleozoic and Mesozoic age. Volcanic rocks and contained alteration and mineralization are subdivided on the basis of K-Ar dating into partly overlapping, western early Miocene (24-20 Ma) and eastern middle Miocene (14-13 Ma) subbelts. High-angle reverse faulting occurred between these two epochs in response to regional compression induced by subduction zone flattening. A northwest alignment is also prominent in the belt as reflected by the strike of several components of the alteration and mineralization zones.Nine zones of porphyry-type mineralization have been discovered in the Maricunga belt, where they are present in both western and eastern subbelts. The porphyry-type mineralization was generated beneath andesitic-(dacitic) stratovolcanoes, which were dissected more extensively in the western than in the eastern subbelt. Volcanic rocks were intruded by isolated, composite porphyry stocks which are dioritic in the eastern subbelt but include quartz dioritic phases in the western subbelt. Weakly porphyritic microdiorite and associated intrusion breccia are prominent stock components. Minor inter- to late mineralization hydrothermal breccias are commonly present.Gold-copper mineralization is believed to have been introduced with K silicate alteration, which is well preserved only at the Amalia, Refugio, and Casale Hill (Aldebaran) prospects. K silicate alteration is overprinted and commonly obliterated by sericite-clay-chlorite assemblages of intermediate argillic type. Much of the gold is present in quartz stockworks. Iron oxides, both early magnetite and late hematite, constitute 5 to 10 vol percent of mineralized zones. Sulfides are dominated completely by pyrite but include minor chalcopyrite and trace bornite and molybdenite. Supergene leaching of copper is developed to various degrees, but enrichment is developed only incipiently.Several porphyry-type stockworks are overlain by pyrite- and alunite-rich advanced argillic alteration, which carries barite, native sulfur, enargite, and at La Pepa, high-grade, vein-type gold mineralization of high sulfidation, epithermal type. The quartz stockworks and advanced argillic caps are telescoped at Marte, Valy, Santa Cecilia, and La Pepa but are separated by a chloritized zone transected by a swarm of gold-poor, polymetallic veins with quartz-alunite selvages at Aldebaran.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 approximately 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. Zinc tends to occur as a geochemical halo to the Au-Cu-Mo mineralization, which is always deficient in silver (<0.1 ppm). Arsenic and mercury are enriched in the advanced argillic caps, and along with lead, are also weakly anomalous at Marte in the underlying, shallow-level quartz stockwork.The depth of erosion of Maricunga porphyry-type systems is believed to decrease from the K silicate zones exposed at Refugio and in the Casale Hill sector at Aldebaran, through Marte, Valy, Santa Cecilia, and La Pepa where remnants of advanced argillic caps are present, to the highest, mercury-rich part of the Cathedral Peak sector at Aldebaran and zones higher than and west of Marte which comprise advanced argillic alteration rich in native sulfur. Reconstruction of the porphyry systems suggests that gold-bearing quartz stockworks were generated 600 to 1,000 m beneath paleosurfaces.Gold-rich porphyry mineralization in the Maricunga belt is closely comparable to that in the Philippines because of the association with dioritic to quartz dioritic porphyry stocks intruded into andesitic-(dacitic) stratovolcanoes, the widespread distribution of intermediate argillic assemblages as partial or complete overprints to K silicate alteration, the abundance of hypogene iron oxides with ore, and the common preservation of advanced argillic caps. The Maricunga porphyry-type mineralization stands in contrast to the porphyry deposits else-where in northern Chile, which are associated with more felsic intrusions lacking coeval volcanic rocks, contain large volumes of sericitized rocks, lack abundant ore-related iron oxides, are molybdenum rich, gold poor, and more than ten times larger, and include major supergene enrichment blankets. However, both the gold-rich Maricunga belt and the molybdenum-rich deposits in northern Chile are underlain by thick continental crust, whereas the Philippine copper-gold province is in an island-arc setting.