Abstract From the first issue in 1905 onward, Economic Geology has been the main publication for those who study mineral deposits; indeed, it is now difficult to imagine economic geology without Economic Geology. It is interesting to ask, therefore, Who were the farsighted people who founded the journal, and Why did they think a specialized publication devoted to mineral deposits was needed?

Abstract Geophysical data relating the dynamic processes of plate motion and subduction to Andean orogenesis are interpreted in terms of a new model for magmatic and tectonic development of the central Andes. The model is based on changing subduction geometry—from normal to flat to normal—and the attendant magmatic and tectonic effects of slab dewatering, continental lithospheric hydration, and asthenospheric flow during closing and opening of the subduction zone mantle wedge. The model includes five stages: 1. Normal subduction extended into Eocene time. 2. A slab transition from normal to flat subduction occurred in late Eocene-early Oligocene time, coincident with extensive crustal deformation in the eastern Altiplano and Eastern Cordillera. 3. Flat subduction during much of Oligocene time was accompanied by a volcanic null throughout the central Andes, when water from the slab infiltrated and hydrated the overlying continental lithosphere, resulting in advective cooling and abnormally low heat flow values. Lithospheric hydration was concentrated not only in the usual fore-arc region but also within the inner arc, in the zone of resubduction where amphibole is presumed to break down and the slab dips steeply into the mantle. 4. The transition from flat to normal subduction in late Oligocene-earliest Miocene time brought about an influx of asthenospheric material from depth into the growing mantle wedge above the slab. Hot asthenospheric mantle in contact with hydrated lithosphere of the inner arc produced widespread melting of both mantle and crust beneath the eastern Altiplano-Eastern Cordillera and ushered in a period of ductile deformation associated with oroclinal formation. The magmatic activity and orogenic uplift that began in the inner arc broadened westward as hot asthenospheric material flowed into the mantle wedge above the sinking slab. 5. The westward broadening of volcanic activity culminated in a resumption of calc-alkaline volcanism all along the main volcanic arc by at least 20 to 15 Ma. The crust beneath the main arc, probably thickened by previous magmatic and deformational events, was further thickened and uplifted by the intrusion or underplating of massive volumes of mantle-derived magmas. Eruptive activity in the inner arc, much of it anatectic and correlated with periods of crustal deformation, gradually waned, with migration of minor magmatic centers eastward almost to the present day. The thermally thinned and weakened lithosphere of the Eastern Cordillera and sub-Andean belt formed a ductile block in which compressive stresses have been concentrated in Neogene time. The tectonic collapse of the inner arc is correlated with the Neogene formation of the Bolivian orocline. By contrast, the Western Cordillera and the western Altiplano, underlain by the Arequipa-Antofalla cratonic block, underwent relatively minor deformation.

Abstract The distribution and chemistry of late Oligocene to Recent central Andean magmatic rocks and mineral deposits between 22° and 33° S latitude reflect changes in the dip of the subducting Nazca plate and the thickness of the overlying lithospheric mantle and crust. Correlations of major magmatic and tectonic events at ca. 18 to 16 Ma, ca. 10 Ma, ca. 7 to 5 Ma, and ca. 2 Ma with previously proposed Andean-wide pulses support external causes for major events and regional geometric control on local style. Evolving magmatic and tectonic patterns indicate that the slab has shallowed beneath the modern Chilean flat-slab region (28° to 33° S), steepened beneath the modern northern Puna plateau (ca. 25° and 22° S), and remained in a transitional state beneath the intervening modern southern Puna. Shallowing in the Chilean flat-slab region is indicated by eastward migration of subduction-related magmatism and deformation, termination of main-arc andesitic volcanism by ca. 10 Ma, and the virtual cessation of volcanic activity by ca. 5 Ma. Shallowing was accompanied by crustal thickening, lithospheric thinning and hydration, and substantial loss of the asthenospheric wedge. Steepening of the slab below the northern Puna is indicated by widespread deformation and basin formation associated with virtual volcanic quiescence in the late Oligocene to middle Miocene, followed by westward contraction of the middle Miocene to Recent volcanic arc. A westward-shifting focus of giant late Miocene to Pliocene ignimbritic eruptions reflects massive melting caused by the introduction of a thickening asthenospheric wedge above a steepening subduction zone and below a thinned hydrated lithosphere. A contemporaneous eastward shift in the major zone of thrusting to the sub-Andean belt can be explained by compressional collapse of the hot, ductile crust beneath the plateau. Lithospheric thickening accompanied deformation above the steepening slab. A persistent, intermediate dip of the slab beneath the intervening southern Puna is supported by the lack of a volcanic gap, and by a transitional, magmatic, and tectonic history compared to that of the north and south. Extreme crustal thickening over the intermediately dipping slab resulted in instabilities in eclogitic lower crust that led to Pliocene continental lithospheric foundering (delamination). Evidence for delamination comes from Pliocene to Recent eruptions of the Cerro Galán ignimbritic center, a concentration of primitive mafic lavas associated with normal and strike-slip faults, high average regional elevation, and seismic evidence for a thin underlying lithosphere and an abnormally hot subducting slab. Temporal variations in mantle-derived mafic magma chemistry indicate Neogene mantle enrichment by introduction of crustal material during the subduction process. Within this framework, major central Andean Neogene Au and Cu deposits in the greater El Teniente (ca. 32°–34° S), greater El Indio (ca. 29°–31 ° S), and Maricunga (26°–28°) belts formed as crustal thicknesses reached 45 to 50 km over the shallowing and cooling subduction zone. The general southward younging of these deposits reflects a southward pattern of crustal thickening. Emplacement of the deposits took place in the waning stages of arc volcanism as the arc front migrated eastward or extinguished. Mineralization occurred as geochemically inferred, hydrous, hornblende-based, residual mineral assemblages that were in equilibrium with erupted magmas dehydrated to yield high-pressure, garnet-bearing assemblages.

Abstract The Peruvian segment of the Andean Cordillera represents the paradigm of the Andean type of subduction, whereby the oceanic Nazca plate subducts the ensialic South American plate. This plate has developed along its western margin a considerable crustal thickening of as much as 70 km, leading to an attendant cordilleran uplift of nearly 4,000 meters above sea level (m.a.s.l.). The Andean Cordillera is the result of three major geodynamic cycles: Precambrian, Paleozoic to Early Triassic, and Late Triassic to present. The last cycle commenced with the opening of the South Atlantic in the Triassic and includes a first phase of Late Triassic to Early Senonian, Mariana-type subduction, which was basically extensional and of crustal attenuation. During this phase, the cordilleran belt was the site of major shelf sedimentation, bordered on the west by island arc volcanism or a marginal volcanic rift. In the Early Senonian, a profound geodynamic change led to the Andean-type of subduction, marine withdrawal, and emergence of the Cordillera. This phase was characterized by the recurrence of compressive pulses and the presence along the continental margin of a magmatic arc with intense plutonic and volcanic activity. During this phase, a sequence of compressive episodes: Peruvian (84-79 Ma), Incaic I (59-55 Ma), Incaic II (43-42 Ma), Incaic III (30-27 Ma), Incaic IV (22 Ma), Quechua I (17 Ma), Quechua II (8-7 Ma), Quechua III (5-4 Ma), and Quechua IV (early Pleistocene) formed three major, successive, and eastward-shifting fold and thrust belts: Peruvian (Campanian), Incaic (Paleocene-Eocene) and sub-Andean (Neogene). In general, the compressive pulses affected the entire mobile belt, but were particularly focused on the fold and thrust belts. They resulted in crustal thickening and uplift which was followed by periods of relative quiescence when well-developed erosional surfaces were formed, the most distinctive of which is the Puna surface, generated about 17 m.y. ago. The compressive pulses interrupted longer periods of extension during which the magmatic arc was particularly active, and which were also characterized by the development of fore-arc basins, intermontane grabens, and the great eastern foreland basin. All along this process, however, there were some persistent features, such as the continued presence of the magmatic arc, the Marañón arch, and the eastern foreland basin. The western margins of the Incaic and sub-Andean fold and thrust belts are considered to represent megafaults, deeply rooted into the ductile region, and along which the shortening experienced by the compression of the belt was absorbed.

Abstract The <10,000-yr-old volcanoes of the Central Volcanic zone (15°-27° S) form a 50-km-wide belt that widens locally to 100 to 150 km and has three outliers 100 to 200 km east of it. The locations of over 1,800 radiometrically dated igneous rocks and hydrothermal ore/alteration minerals between 6° S and 33° S are plotted for 25 time intervals (varying between 2 and 65 m.y.), from the Precambrian to the Holocene. For short time intervals, these locations define 25- to 75-km-wide belts that widen occasionally to 75 to 125 km and have local outliers of volcanic tuffs or ignimbrites. Nonmagmatic stretches, such as the current Northern (2°-15° S) and Southern (27°-34° S) Nonvolcanic zones, probably occurred at various times and locations in the past, but were distinctly subordinate in strike length and duration to the magmatic zones. There are two roughly parallel belts that are 200 to 400 km apart (locally separated by only 125 km or up to 500 km). Over the chosen time intervals, both magmatic belts were often active. However, judging from presently active magmatic belts, they were probably seldom coeval over geologically very short time spans. The western belt corresponds to the conventionally envisaged magma generation by a subducting oceanic plate at 100- to 125-km depth. The eastern belt is akin to a back arc in an oceanic setting, except that it occurs in a continental plate. The apparent parallelism of both belts suggests that they were generated by linked mechanisms. Pegmatites, granites, rhyolites, and rhyodacites occur in both belts, but as a group are more common in the eastern belt. Calc-alkaline igneous rock compositions also occur in both belts, but as a group predominate in the western belt. Although basaltic rocks occur in both belts, as a group the mafic igneous rocks appear to be largely restricted to the western belt. The two phonolites and the nepheline syenite dated are in the eastern belt. In many areas, the location of the magmatic belt did not change significantly over a long period of time. The location of the magmatic belt gives the appearance of essentially continuous magmatism accompanied by occasional hydrothermal activity that resulted in the formation of ore deposits. Significant changes in the activity and locations of magmatic belts can occur in about 5 million years. As magmatic belts shift eastward or westward, individual magmatic centers may be dragged along. Integrated over a long time, this process may give rise to transverse magmatic alignments or transbatholiths with associated hydrothermal ore deposits of different ages that appear to be controlled tectonically. The relatively straight magmatic belts have local deflections. These deflections can be interpreted as smooth changes in the dip of the subducting plate or as faulting of either the oceanic or the continental plate. Oceanic plate subduction below the central Andes has occurred since the Cambrian. Folding and overthrusting in the continental plate did not significantly disturb the geometry of the magmatic-hydrothermal belts.

Abstract The Miocene metallogenic belt of central and northern Perú, extending for at least 900 km along the Western Cordillera and the adjacent high plateaus province, is defined by a large number of hydrothermal mineral deposits of different types that formed between about 6 and 20 Ma. The belt, centered east of the Mesozoic and early Paleogene Coastal batholith, is on mature continental crust that has undergone multiple episodes of compressive deformation from at least middle Paleozoic to latest Neogene time. Mineralization began before the early Miocene Quechua I compressive event and spanned later Quechua II tectonism. Mineral deposits are mostly hosted by shelf carbonates and other sedimentary rocks of Late Triassic,Jurassic, and Cretaceous age and by volcanic and intrusive rocks mainly of Neogene age. Base metal and precious metal mineralization was intimately associated in time and space with the eruption of calc-alkalic volcanic rocks of intermediate composition and the emplacement of mineralogically and chemically similar dikes and stocks. These igneous rocks are moderately potassic and the few available data suggest relatively nonradiogenic Sr, Nd, and Pb isotope compositions. Mineral deposits range from porphyry and associated proximal skarn deposits to polymetallic, precious metal, and mercury deposits formed at relatively lower temperatures. Porphyry deposits include the La Granja Cu porphyry, the Au-bearing Michiquillay Cu porphyry, the Mo-bearing Cu porphyritic rocks of Toro Mocho, Pashpap, and Páraq, the Mundo Nuevo-Tamboras-Compaccha Mo-W porphyry system, and the Cerro Corona, Minas Conga, Collpayoc, Laguna Chamis, Carhuacayán, and Puy-Puy Au-Cu porphyry deposits. Many of the classic base and precious metal deposits of central and northern Perú are within zoned polymetallic districts, some with one or more porphyry centers. Many districts have veins or replacement bodies containing enargite in their central parts, and a number are characterized by deposits of both vein and limestone replacement type. At a number of polymetallic districts, for example, Julcani, Yauricocha, Morococha, Casapalca, Huarón, Raura, Antamina, Pasto Bueno, Quiruvilca, Algamarca, and Hualgayoc, stocks containing high-salinity fluid inclusions are exposed, known from drill-hole data, or can be confidently inferred from fluid-inclusion or isotope data. Vein and limestone-replacement Pb-Zn ± Ag ± Cu deposits are common, and range from vertically persistent, high-temperature deposits, such as the veins of Casapalca, to largely stratabound deposits such as Cercapuquio and Azulcocha, that were formed at temperatures below 200°C. Although certain writers have interpreted some manto deposits to be diagenetic or syndiagenetic, field relations and lead isotope compositions argue strongly for an epigenetic origin. Vein systems or epithermal paragenetic stages in which silver is the economically most important metal, such as those of Milluachaqui, Millotingo, and Colqui, typically contain appreciable amounts of base metals and can best be considered a variant of the polymetallic vein group. The Huancavelica mercury deposit represents an extremely large geochemical anomaly, perhaps developed at the top of a polymetallic system. High-sulfidation-type Au-Ag deposits, such as Pierina and those of the Yanacocha district, are economically important. At Tantahuatay and Colquijirca, oxidized Au-bearing, vuggy silica rock occurs at higher elevations than surrounding, zoned, enargite-cored Cu-Pb-Zn-Ag veins and strata-bound replacement deposits. In contrast to the association of precious metals with enargite, tetrahedrite, and barite at Julcani and other reduced-type deposits, in moderate- to high-grade ores at Pierina and probably certain deposits in the Tantahuatay and Yanacocha districts, most of the gold is very late, following initial quartz-alunite-pyrite alteration, the destruction of alunite to form vuggy silica rock, and the subsequent deposition of pyrite and enargite accompanied by small amounts of gold. Gold and silver in economic quantities were then introduced by compositionally distinct, late fluids that oxidized pyrite and enargite, leached Cu, Zn, Se, Te, Tl, and other elements, and introduced Hg, Pb, Bi, Sb, and large amounts of barite. An analogous case for a distinct, compositionally different Au-Ag mineralizing pulse perhaps can be made for the sedimentary rock-hosted gold deposits of Purísima Concepción in Yauricocha. The ubiquitous presence of enargite, and the spatial and temporal association in several districts of pyrite + enargite, with modest gold content, and oxidized Au-rich ores, support the interpretation that bulk-mineable, volcanic-hosted gold deposits of a high-sulfidation type represent one of the many types of deposits related to the general class of porphyry-related, zoned polymetallic systems. The sandstone-hosted gold deposits of northern Perú also appear to be related to subjacent magmatic systems, although there are certain geological, mineralogical, and chemical differences from both volcanic-hosted, high-sulfidation and Purísima-type gold deposits. High W and Sn content of many of the sandstone-hosted ores of the Angasmarca district suggest that they are high-level manifestations of subjacent W-Mo ±Au systems such as are exposed at the nearby, more deeply seated Mundo Nuevo-Tamboras-Compaccha and Pasto Bueno districts. Several subsidiary belts are recognized within the Miocene metallogenic belt. A group of deposits in northern Perú, including the polymetallic deposits of the Quiruvilca district, the several Cu-Mo porphyry systems at Pashpap, and the Pierina high-sulfidation Au deposit, defines the 13 to 15 Ma or older Quiruvilca-Pierina subbelt in the western part of the metallogenic belt. The provisional Michiquillay-El Toro subbelt, including the Michiquillay Cu porphyry, the El Toro Au prospect, and probably the Au-Cu porphyry systems of the Minas Conga district, appears to have formed in northern Perú along the eastern margin of the metallogenic belt between about 18 and 20 Ma. A narrow, late Miocene subbelt that comprises a number of deposits dated at less than about 10 Ma, including Huachocolpa, Yauricocha, San Cristóbal, Morococha, Puy-Puy, Carhuacayán, Huarón, Raura, Huanzalá, Antamina, Pasto Bueno, and Angasmarca, extends from the Huachocolpa district at the southern end of the belt to the latitude of Santiago de Chuco in northern Perú. Deposits of the late Miocene subbelt postdate the 9 to 10 Ma Quechua II compressive pulse, and the initiation, location, and narrowness of the subbelt may have been related in some manner to this tectonic event. Intersections of successive, magmatic mineral axes with northeast-trending and other fault systems of probable crustal scale may have combined to influence the location of individual mineral deposits or clusters of deposits. Mineralization had ceased, and possibly was terminated, by the 5 to 7 Ma Quechua III compressive event. The emplacement of the 5.2 Ma late phase of the Cordillera Blanca batholith and the eruption of approximately coeval units of silicic ash-flow tuff and lava in northern and central Perú may reflect the subsequent relaxation of compressive stress, leading to the switching of axes of least and greatest principal stress indicated by 4 Ma north-south-trending dike systems in central Perú. Four important older districts within the Miocene metallogenic belt (Quicay, ca. 37.5 Ma; Uchucchacua, ca. 24.5 Ma), or bordering it on the east (Atacocha and Milpo, ca. 29-30 Ma), are related to older, and perhaps in part less intense, periods of magmatic activity. Although gold deposits may prove to be more important in northern than in central Perú, there is little indication that the concentrations of other metals vary markedly along or normal to the Miocene metallogenic belt. For example, porphyry molybdenum deposits are found in both the eastern and western parts of the belt. Moreover, particular types of deposits do not appear to be preferentially restricted to a given time period: several sandstone-hosted gold deposits in northern Perú have yielded ages ranging from less than 9 to greater than 18 Ma, and Au-bearing porphyry systems include examples of early, middle, and late Miocene age. Local geology and depth of erosion may be more important controls of deposit type. If future work shows that individual subbelts are as narrow and continuous as the present data suggest, areas within the narrow subbelts may prove to be the most prospective for mineral exploration.

Abstract Ore lead isotope provinces in the central Andes between 6 ° S and 32 °S correspond in part to broad differences in the ages and types of rocks exposed in each area. If these provinces reflect scavenging oflead from upper crustal rocks and reconcentration into ore deposits, ore lead isotope ratios reflect the average upper crustal composition in each region. If the ore metals have a deeper source, the provinces instead reflect differences in magma sources or generation processes among the provinces. Ores from province II (the high Andes of Perú) show steep lead isotope arrays indicative of source mixing. The igneous rocks in the Hualgayoc district in northern Perú overlap with the radiogenic end of the province I array and are representative of the nonradiogenic end of the province II mixing trends; exposed supracrustal rocks are candidates for the radiogenic end member. The origins of both isotopic signatures are investigated to examine the relationship between hydrothermal metal budgets and magma sources. The exposed crust in the northern Peruvian Andes consists of middle Cretaceous platform carbonates, sandstones, and shales that overlie a similar Jurassic sequence and probably a Precambrian to early Paleozoic metamorphic basement. The metamorphic basement and overlying sediments have broadly similar Pb-Sr-Nd isotope systematics. Whole-rock Pb isotope and U/P systematics of the sediments suggest U/Pb fractionation in the sediment source at approximately 1800 Ma, followed by evolution with elevated U/Pb ratios. ∊ Nd values of the metamorphic basement and Cretaceous sedimentary rocks range from −11.6 to −16.5, with T DM equal to 1.43 to 2.06 Ga. Northern Perú basement rocks have much higher 206 Pb/204Pb values than metamorphic basement terranes in eastern Colombia, southern Perú, and northern Chile, and their isotopes more closely resemble basement terranes to the east in Brazil. The sedimentary rocks were intruded in the middle to late Miocene by numerous felsic igneous bodies associated with hydrothermal Ag-Zn-Cu-Pb mineralization. The intrusive rocks are intermediate to high K andesitic intrusions and rhyodacitic volcanic domes. Fresh igneous rocks have rare earth element (REE) abundances less than 100 times chondrites, lack significant Ce and Eu anomalies, and are relatively depleted in Ti and Nb. The isotopic compositions and homogeneity of the igneous rocks with respect to Pb, Sr, and Nd suggest that they assimilated little shallow crust and were derived largely from deeper sources in the upper sub-Andean mantle or the lower sub-Andean crust. Because no exposed Andean basement rocks resemble the compositions of province I ores, and because subducted sediment has recently been shown to be an important source of lead in arc magmas, the role of subducted sediment in producing a province I-like signature is evaluated. A simple numerical model for the enrichment of a possible mantlewedge source region by subducted sediments is presented. The model suggests that subducted sediment can account for the lead isotope signature of province I ores, and that the quantity of subducted material along the Perú-Chile trench could produce a mantle source with this signature within a few million years of the onset of subduction.

Abstract The El Indio district, Chile, contains two types of high-sulfidation, precious metal deposits hosted in intensely altered Tertiary rhyodacitic volcanic rocks: El Indio, with enargite-pyrite and gold-quartz mineralization in complex vein systems, and Tambo, with alunite-barite-gold, mainly in tectonic breccia pipes. This single, world-class district contains more than 10 Moz of gold, 100 Moz of silver and 1 Mt of copper. At El Indio, the banded alunite and enargite + pyrite veins, peripheral to the main copper and gold veins, suggest alternating fluid conditions prior to the spectacular high-grade gold mineralization accompanied by sericitic-argillic alteration. The δ 34 S, δ 18 O, and δD ratios indicate the 250° to 300°C, moderate to low salinity (<5 wt % NaCl equiv), weakly acidic (pH = 3.5–4.5), reduced mineralizing fluids for both El Indio ore types had a dominantly magmatic water component (>60%), with no evidence of boiling. Copper deposition is attributed to decreasing temperature while precipitation of large quantities of gold is ascribed to mixing with an acid-oxidized fluid. At Tambo, gold was deposited with early barite and again after intermediate-stage alunite, from 200° to 250°C, low-salinity, intermittently boiling fluids. The δ 34 S ratios at Tambo indicate that the barite fluids were mildly reduced (sulfide/sulfate ratio of 10–25) and contained disproportionated magmatic SO 2 . The δ 18 O and δD ratios indicate that alunite formed from condensed, δD-depleted magmatic vapor between gold stages. Calculations show that, within a limited range of dissolved sulfur and pH conditions, a single magmatic fluid could have evolved to produce the multiple mineral assemblages seen at both El Indio and Tambo, with the former in a deeper, more reduced, hydrothermal environment and the latter in a near-surface setting.

Abstract Gold-bearing quartz veins in the Parcoy mining district occupy brittle shear zones in the Pataz batholith, which we have dated at 329 ± 1 Ma by U-Pb in zircons. The batholith is emplaced in metamorphic rocks of the Proterozoic Marañón complex. The veins contain paragenetically early quartz, pyrite, and arsenopyrite, and later quartz, sphalerite, galena, and chalcopyrite. Both paragenetic stages contain important gold mineralization. Wall-rock alteration consists of quartz, sericite, and pyrite, with envelopes of propylitic alteration. In the Gigante vein, between 3,900 and 4,200 m, the early and late ore assemblages filled an en-echelon fault-fracture system of limited sinistral, oblique thrust slip. Higher grades of mineralization lie in dilational inflections in the fault system. The vein is offset sinistrally and normally to the north by east-west-striking faults and by minor normal faults parallel to the veins themselves. Lead in galenas from the Parcoy district is isotopically homogeneous. Estimated corrections for in situ decay of U and Th in the batholith and the metamorphic basement suggest that the Pataz batholith provided most of the ore lead in the system. 208 Pb/204Pb of Marañón complex samples are too high for the basement to have been a major lead source; however, lead isotope ratios of Pataz batholith samples are not greatly different from the basement rocks. Marañón complex metamorphic whole-rock samples have values of -8.9 to -12.3, with unusually high Nd contents (34-66 ppm). Depleted mantle model separation ages for the metamorphic rocks range from 2.06 to 1.43 Ga. Initial values of the Parcoy district granodiorites vary from -4.7 to -6.1, which indicate an addition of 35 to 70 percent ancient crustal material to a depleted mantle-derived parental melt, depending on the characteristics of the contaminant. Coarse-grained hydrothermal muscovite gives a K-Ar age of 286 ± 6 Ma, suggesting that mineralization greatly postdated the emplacement of the host batholith, and was therefore unrelated to cooling of the batholith, as previously proposed. However, the batholith is clearly a composite feature; undated quartz monzonite porphyry intrusions that cut the Pataz batholith and felsic dikes that cut both the batholith and the mineralization indicate that magmatism occurred well after the batholith was emplaced. Further geochronology will be needed to explore any possible genetic link between these later intrusions and the Pataz gold mineralization.

Abstract Detailed logging of core from drill holes in the Sur-Sur and La Americana breccias in the Andina portion of the Rio Blanco-Los Bronces porphyry copper deposit, supported by petrography, geochemistry, and study of fluid inclusions, has documented zonal and temporal patterns in ore breccias over 1,600 vertical meters. Breccia textures progress from incipient crackle breccia with tourmaline veining to increased rounding of fragments and filling of open spaces by mineralization products and rock flour, reflecting many repeated pulses of brecciation. Both angular breccias and rounded clast breccias with rock-flour matrix have been mineralized primarily by the quiescent flow of hydrothermal solutions between pulses of brecciation. Sharp contacts mark boundaries between pulses separated by a period of thorough cementation, while diffuse, gradational contacts mark boundaries between more closely spaced pulses. Refracturing of the rock mass continued after consolidation of rock-flour matrix breccias, as documented by local angular rebrecciation and ubiquitous postbreccia veining. The angular, tourmaline breccia with chalcopyrite-pyrite at Sur-Sur grades downward, with decreasing tourmaline and increasing biotite in the matrix, into breccia with biotite-alkali feldspar alteration and chalcopyrite-bornite. Variably tourmalinized rock-flour breccias at La Americana extend about 400 m higher than Sur-Sur, with upward increasing ratios of specularite/tourmaline and pyrite/chalcopyrite, and decreasing grades of Cu and Mo. Minor dikes of porphyry have intruded the breccias but are themselves fragmented and appear to be contemporaneous with brecciation. Dikes that have intruded deep, high-grade Sur-Sur breccia display intense biotite alteration and disseminated chalcopyrite-bornite. A high-grade interval of La Americana breccia has been intruded by a dike with intense sericite-chalcopyrite alteration. Dikes intruding poorly mineralized La Americana breccias are barren and are not biotized. The Sur-Sur breccias were formed contemporaneously with early-stage porphyry copper mineralization at depth, and are cut by a sequence of quartz-molybdenite and sulfide veins with sericitic halos that are typical of the evolution of veining in porphyry copper systems. In both the Sur-Sur and La Americana breccia matrices, highly saline fluid inclusions, saturated with NaCl (over filling temperatures from 225° to 500°C), coexist with vapor-rich inclusions and with fluid-rich inclusions from 150° to 450°C and 2 to 30 wt percent NaCl equiv. High-salinity fluids are most abundant at depth and with higher copper grades, while liquid-rich and vapor-rich fluids are dominant near surface, particularly at La Americana. This, and prior stable isotope evidence, is compatible with the interpretation that magmatic fluids, derived from magma giving rise to porphyry dikes and the PΔV energy for brecciation, were primarily responsible for mineralization of the ore breccias. Intermixed meteoric water, however, may have been responsible for the huge volume and complex reworking of the breccias, and for apparently wide fluctuations in temperature, pressure, f o 2 , and salinity, which are suggested by fluctuations in magnetite-hematite and anhydrite saturation in the breccias. This district represents nearly an end member in the wide range of variations that are characteristic of porphyry copper mineralization. The breccias are copper ores because they were formed and mineralized by intrusions derived from a differentiated magma chamber which became saturated with typical porphyry copper-ore fluids. Location, complexity, and geochemistry offer the explorationist clues to the relatively rare tourmaline breccia, which may be ore bearing.

Abstract The magnetite deposits at El Laco, Chile, have been widely cited as the type example of an iron deposit formed from direct consolidation of a magma. This study presents field, petrographic, and rare earth element (REE) evidence that shows that magnetite deposition was largely the result of hydrothermal activity. Hydrothermal activity at El Laco is clearly manifested in alteration assemblages observed in host-rock andesite: magnetite occurs in association with a series of hydrothermal alteration assemblages. The earliest-formed alteration assemblages include weak propylitic (chlorite-sericite-clay), sodic-potassic mineral phases (scapolite-albite-K feldspar-sphene), and silicification by cation leaching. Alteration of andesite to calcium-rich pyroxene (diopside) ± magnetite followed. Bulk iron-oxide mineralization then occurred, producing an assemblage of magnetite ± apatite ± quartz ± calcite. The final stages of hydrothermal alteration of andesite included argillic alteration (kaolinite-sericite-alunite), silicification associated with anhydrite, and retrograde alteration (sericite-clinozoisite-talc-illite/smectite-biotite) of sodic and calcic alteration phases. Oxidation of magnetite to hematite (±jarosite ± goethite) postdated magnetite emplacement. Textural evidence also supports the interpretation of a metasomatic-hydrothermal origin: 1. Abundant layers of magnetite mimic the morphology of andesite flow layers, suggesting direct replacement of andesite by magnetite. 2. Porous textures in magnetite resemble andesite breccia fragments that were partially or completely replaced by magnetite. These textures are associated with bladed diopside and pyroxene casts, confirming that, prior to iron mineralization, andesite experienced calcic alteration. Porous regions in magnetite that occur on a larger scale than typical breccia fragments (>10 cm) indicate direct replacement of unbrecciated andesite. 3. Microscopy shows direct replacement of diopside by both magnetite and apatite. 4. Abundant hydrothermal magnetite textures include coarse-grained magnetite octahedra encrustations, magnetite veins occurring with drusy quartz, brecciation, geyserlike magnetite terraces, and fumarole-like tube structures. 5. Magnetite veins crosscut diopside-rich altered breccia fragments, indicating that hydraulic fracturing of footwall andesite was followed by hydrothermal precipitation of magnetite. REE patterns of magnetite and andesite alteration assemblages show negative Eu anomalies during early alteration (diopside and scapolite) and magnetite-apatite phases, and positive Eu anomalies in later alteration. This likely reflects a change in oxidation state from Eu 2 + to Eu3+ as the system developed. REE concentrations in magnetite are the same as REE concentrations in unaltered country rocks, suggesting that the total REE content of magnetite was inherited from precursor andesite during replacement processes. These results are most consistent with the interpretation that magnetite-apatite formation represents a hydrothermal-alteration event, in which iron- or phosphorous-rich fluids reacted with and replaced host-rock andesite and formed by hydrothermal open-space filling. Silica that was removed during replacement was later deposited as quartz veins and as siliceous alteration associated with argillic and sulfate deposition.

Abstract Oxygen isotope analyses of iron oxide at El Laco, Chile, were conducted in order to test whether magnetite formed either by a combination of magmatic and hydrothermal metasomatic processes or by a single hydrothermal-metasomatic process alone. If magnetite formed from two distinct sources—one magmatic, the other hydrothermal—then a contrast in oxygen isotope compositions between magnetite that crystallized in a magma and magnetite that precipitated from a hydrothermal fluid should be expected. In fact, δ 18 O values in magnetite at El Laco show no significant variability between textural types (mean = 4.1 ± 0.49‰). Perhaps more important, δ 18 O in wall-rock andesite shows a distinct increase in oxygen isotope values—from 7.2 to 24.2 per mil—with increasing degrees of hydrothermal alteration. This observation strongly suggests that oxygen exchange occurred with an isotopically heavy fluid that was distinct from what might have been generated from a magma. Diopside separated from altered andesite, apatite separated from magnetite, and quartz separated from hydrothermal magnetite-quartz (± apatite) veins all have heavy δ 18 O values (7.1–8.9‰, 7.8–8.0‰, and 7.1–27.9‰, respectively) relative to values typical of igneous rocks. The quartz values are among the heaviest reported in the literature and are exceedingly variable both between and within individual samples. In contrast to the values for magnetite, apatite, diopside, and quartz, δ 18 O values of hematite-bearing iron oxide samples have much lower δ 18 O values, down to a minimum of −8.9 per mil, and the δ 18 O value decreases as the hematite content increases. These results strongly support the theory that the bulk of the magnetite at El Laco formed by metasomatic replacement and did not form by direct consolidation from a magma. The hydrothermal fluids that reacted to form magnetite were isotopically heavy in oxygen composition. These 18 0-rich fluids were also responsible for hydrothermal alteration of wall-rock andesite and the formation of quartz veins. Heated, closed-basin water that experienced significant evaporation, or deep-seated fluids (possibly magmatic) that interacted with buried evaporite deposits, may be the source for these isotopically heavy hydrothermal fluids. The extremely high δ 18 O values and isotopic variability of the quartz suggests that quartz veins formed as the hydrothermal fluids boiled, perhaps losing large quantities of volatile components. The much lower δ 18 O values of hematite-rich samples suggest that isotopically lighter meteoric fluids reacted with, and oxidized portions of, the magnetite deposits at temperatures ranging from approximately 65° to 150°C in the near-surface or surficial environment. However, these fluids were volumetrically minor.

Abstract Geophysical data relating the dynamic processes of plate motion and subduction to Andean orogenesis are interpreted in terms of a new model for magmatic and tectonic development of the central Andes. The model is based on changing subduction geometry—from normal to flat to normal—and the attendant magmatic and tectonic effects of slab dewatering, continental lithospheric hydration, and asthenospheric flow during closing and opening of the subduction zone mantle wedge. The model includes five stages: 1. Normal subduction extended into Eocene time. 2. A slab transition from normal to flat subduction occurred in late Eocene-early Oligocene time, coincident with extensive crustal deformation in the eastern Altiplano and Eastern Cordillera. 3. Flat subduction during much of Oligocene time was accompanied by a volcanic null throughout the central Andes, when water from the slab infiltrated and hydrated the overlying continental lithosphere, resulting in advective cooling and abnormally low heat flow values. Lithospheric hydration was concentrated not only in the usual fore-arc region but also within the inner arc, in the zone of resubduction where amphibole is presumed to break down and the slab dips steeply into the mantle. 4. The transition from flat to normal subduction in late Oligocene-earliest Miocene time brought about an influx of asthenospheric material from depth into the growing mantle wedge above the slab. Hot asthenospheric mantle in contact with hydrated lithosphere of the inner arc produced widespread melting of both mantle and crust beneath the eastern Altiplano-Eastern Cordillera and ushered in a period of ductile deformation associated with oroclinal formation. The magmatic activity and orogenic uplift that began in the inner arc broadened westward as hot asthenospheric material flowed into the mantle wedge above the sinking slab. 5. The westward broadening of volcanic activity culminated in a resumption of calc-alkaline volcanism all along the main volcanic arc by at least 20 to 15 Ma. The crust beneath the main arc, probably thickened by previous magmatic and deformational events, was further thickened and uplifted by the intrusion or underplating of massive volumes of mantle-derived magmas. Eruptive activity in the inner arc, much of it anatectic and correlated with periods of crustal deformation, gradually waned, with migration of minor magmatic centers eastward almost to the present day. The thermally thinned and weakened lithosphere of the Eastern Cordillera and sub-Andean belt formed a ductile block in which compressive stresses have been concentrated in Neogene time. The tectonic collapse of the inner

To the reader: During review it became apparent that two groups of authors in this volume had drawn different and incompatible conclusions concerning the origin of Sherman-type deposits. R. J. Johansing and T. B. Thompson, in their paper “Geology and origin of Sherman-type deposits, central Colorado,” concluded that the deposits formed during the Tertiary from basinal brines moving along thermal gradients controlled by igneous intrusions. An entirely different conclusion was drawn in the two-part paper “Late Mississippian karst caves and Ba-Ag-Pb-Zn mineralization in central Colorado,” in which Part I on “Geologic framework, mineralogy, and cave morphology” is by R. J. Tschauder, G. P. Landis, and R. Npyes; and Part II on “Fluid inclusion, stable isotope, and rock geochemistry data and a model of ore deposition” is by G. P. Landis and R. J. Tschauder. These authors concluded that Sherman-type deposits formed during the Mississippian from regional brines moving under a hydraulic gradient. The differences between the two conclusions are so great it seems likely that at least one of the genetic interpretations is based either on an incomplete set of facts or on erroneous data. It is not apparent to the reviewers, to the editors of this monograph, or to me where the resolution may lie. Therefore, in order to assist readers and to help those who may carry out research on the deposits in the future, a series of questions have been addressed to both groups of authors. The questions are designed to explore the key differences between the two papers, and both groups of authors graciously agreed to respond and to allow their responses to be published. In addition, because some of the observations that each group considers to be facts are in dispute, both groups of authors have been asked to identify the points of factual disagreement in the other’s work. Both groups responded with a list of data they consider questionable, and these follow their replies to my questions.

Abstract When the price of gold rose from about $200 (U.S.) an ounce in 1979 to nearly $700 an ounce by the end of the same year, the gold rush of the 1980s was under way. Gold production in the western world rose dramatically; from 1981 to 1986 production increased by 300 to 1,282 metric tons per year. Annual production may reach 1,500 to 1,600 metric tons by 1990 (Woodall, 1988). The major contributors to the increased stream of gold have been Australia, Canada, Brazil, and the United States together with other circum-Pacific countries. The increased price of gold and new methods of extraction have allowed many older deposits to be reopened, but the most important factor has been the high success level of exploration. This success has resulted in large part from the application of new genetic models and from the development of new exploration techniques.

Abstract The number of mineral deposits, large and small, discovered in North America over the last half millennium has not been tallied, but it must be in the hundreds of thousands. Most discoveries are small and of no more than local importance, but some are giants. As a result, Canada, Mexico, the Caribbean region, and the United States have at one time or another each been the world′s leading producer of one or more mineral resources. The complex diversity and the remarkable plenty of the deposits played a vital role in the development of North America′s vibrant societies. From the time of the first European settlers to the present, our use of, and dependence on, mineral resources has grown steadily larger. Mineral consumption in North America has now reached enormous proportions. For the region as a whole, the mass of mineral resources used directly or indirectly in 1988, by every man, woman, and child, weighed 14 metric tons. There are ap-proximately 360 million people in the region. In Canada and the United States, annual per capita consumption is approximately 16 metric tons; in Mexico and the Caribbean, per capita consumption is lower—closer to 10 metric tons—but still considerable. Nonmetallic resources, such as crushed stone, sand, gravel, clay, cement, and plaster, account for a significant fraction of the consumed mass. Such materials tend to be locally produced, and the sources are so large and widespread that there does not seem to be any reason to suspect that supply limitations lie ahead. But even though supply problems may not be cause for concern, continued massive use of mineral resources does raise environmental concerns. The magnitude of mineral products mined each year now exceeds the magnitude of the sediment transported annually to the sea by streams. Inevitably, use of mineral resources is changing the global environment.