Ulrich Petersen

Episodic Cenozoic volcanism and tectonism in the Andes of Peru

Abstract Radiometric and geologic information indicate a complex history of Cenozoic volcanism and tectonism in the central Andes. K-Ar ages on silicic pyroclastic rocks demonstrate major volcanic activity in central and southern Peru, northern Chile, and adjacent areas during the Early and Middle Miocene, and provide additional evidence for volcanism during the Late Eocene. A provisional outline of tectonic and volcanic events in the Peruvian Andes during the Cenozoic includes: one or more pulses of igneous activity and intense deformation during the Paleocene and Eocene; a period of quiescence, lasting most of Oligocene time; reinception of tectonism and volcanism at the beginning of the Miocene; and a major pulse of deformation in the Middle Miocene accompanied and followed through the Pliocene by intense volcanism and plutonism. Reinception of igneous activity and tectonism at about the Oligocene-Miocene boundary, a feature recognized in other circum-Pacific regions, may reflect an increase in the rate of rotation of the Pacific plate relative to fixed or quasifixed mantle coordinates. Middle Miocene tectonism and latest Tertiary volcanism correlates with and probably is genetically related to the beginning of very rapid spreading at the East Pacific Rise.

Metallogenic provinces in South America

ZusammenfassungDiese Arbeit beschreibt die allgemeine Verteilung einiger Erzlagerstättentypen in Südamerika und untersucht ihren Zusammenhang mit großtektonischen Einheiten. Sie betrifft nur eine Auswahl von Lagerstätten, für die es genügend Daten gibt und die uns einen Überblick gestatten über verschiedene genetische Typen, aber auch zugleich eine gute geographische Verteilung haben und es ermöglichen, Lagerstätten zu studieren, die entweder häufig oder spärlich in Südamerika vorkommen. Inbegriffen sind Porphyry Copper, zonierte Kupfer-Zink-Blei-Silber, kontakt-metasomatische, Red-bed Kupfer-Uran-Vanadium, Blei-Zink (einschließlich Mississippi-Valley-Typ), Quecksilber, Zinn, pegmatitische, karbonatitische, Diamant, Nickel-Kobalt-Chromit-Asbestos-Talk, Eisen, Bauxit, Wolfram, Titan, Uran, Gold, Kali und Phosphat-Lagerstätten.Die Verteilung dieser Lagerstätten wird mit der in anderen Kontinenten verglichen, speziell in Nordamerika und Afrika. Daraus ergeben sich allgemeine Gedanken über Lagerstättengenetik. Die meisten hier beschriebenen Lagerstätten werden hauptsächlich durch Prozesse in der oberen Erdkruste bedingt (mit Ausnahme von den Karbonatiten, Alkalischen Komplexen und Diamantschloten). Weiterhin spielt die relative Tiefe, bis zu der ein Orogen abgetragen worden ist, eine wichtige Rolle in der Verteilung der heute bekannten Erzlagerstättentypen. Die Beobachtung, daß bestimmte Lagerstättentypen in engen, aber langen Streifen entlang der Anden verteilt sind und die Richtungsänderungen der Kordillere mitmachen, sich ihre chemische Zusammensetzung quer der tektonischen Richtung ändert und die Verhältnisse in Nord- und Südamerika ähnlich sind, deutet darauf hin, daß sie durch die Verteilung von Eugeosynklinalen und Miogeosynklinalen bedingt sind oder ihre Entfernung von batholitischen Komplexen bzw. sub-vulkanischen Stocks und Vulkanen, maßgebend ist. Ihr anscheinender Zusammenhang mit Sedimentcharakter und Gips/Anhydrit-Folgen spricht für die modernen Anschauungen, daß hydrothermale Lösungen hauptsächlich meteorische Grundwasser sind, die durch eruptive Massen aufgeheizt und bewegt werden. Wie es oft der Fall ist, hat dieses Studium mehr Fragen als Antworten geliefert, und es ergeben sich mehrere Probleme, die weiterer Untersuchung bedürfen.Nach der praktischen Seite hin deutet diese Arbeit auf mehrere Möglichkeiten für zukünftige Prospektionen und für die weitere Entwicklung von natürlichen Vorräten in Südamerika. Mehrere Länder könnten wahrscheinlich ihre Erzproduktion erhöhen oder mannigfaltiger gestalten und damit den Lebensstandard ihrer zunehmenden Bevölkerung bessern. Während in Nordamerika sowohl Kanada wie die USA den Kontinent von der Ostküste bis zur Westküste umfassen und dadurch einen weiten Bereich geologischer Bedingungen einschließen (kratonische, geosynklinale und orogenische Zustände), liegen in Südamerika die Länder entweder entlang der Anden oder hauptsächlich im kratonischen Bereich. Dieser Unterschied hat natürlich wichtige geopolitische Auswirkungen betreffs Erzvorräten.AbstractThis paper discusses the broadscale distribution of some ore deposit types in South America, trying to discern possible relations with major geotectonic features. It does not cover all ore deposits but rather a selection based on available information, genetic types, geographic distribution, and a representation of deposit types which are either abundant or very rare in South America. Included are porphyry copper, zoned copper-zinc-lead-silver, contact-metasomatic, red-bed copper-uranium-vanadium, lead-zinc (including Mississippi Valley type), mercury, tin, pegmatite, carbonatite and alkalic complex, diamond, nickelcobalt-chromite-asbestos-talc, iron, bauxite, tungsten, titanium, uranium, gold, potash and phosphate deposits.Comparisons are made with the distribution of these deposit types in other continents, especially in North America and Africa. This leads to some speculations bearing on theories of ore genesis. The evidence suggests that upper crustal processes predominate in the formation of the majority of ore deposits considered (excepting carbonatites, alkalic complexes and diamond pipes). Also, the relative level of an orogenic belt exposed by erosion seems to be an important factor bearing on the presently known distribution of many ore deposit types. Furthermore, the fact that ore deposits of given types tend to be aligned along the Cordillera in more or less defined belts with changing chemical composition accross the Andes, which partake in all its inflections, and in patterns which are similar in North and South America, indicates that they are either dependent on the distribution of eugeosynclinal vs miogeosynclinal sediments in the original geosynclinal prism, or that their character is related to distance from the major batholitic masses or to shallow stocks and volcanic activity. The indicated correlation with sediment character and evaporites speaks in favor of modem concepts viewing hydrothermal fluids as mainly meteoric but heated and circulated largely by igneous activity. This study raises more questions than it can answer and calls attention to a number of problems requiring further research.From a practical point of view, this investigation reveals several possibilities for future exploration, as well as opportunities for further development of mineral resources in South America. Several countries could probably improve or diversify their mineral production significantly, thus raising the standard of living of their increasing population. Whereas in North America both Canada and the U.S.A. span from east to west the whole range of geological environments, from cratonic to geosynclinal and orogenic, in South America the distribución of countries is essentially either along the Cordillera or over shield areas, a fact of obvious geopolitical implications with regard to mineral resources.ResumenEl presente artículo describe la distribución a grán escala de algunos tipos de yacimientos minerales en América del Sur, tratando de discernir su posible relación con las principales unidades geotectónicas. Se trata solo de una selección de yacimientos para los cuales disponemos de suficiente información, escogidos de manera de tener una buena representación de diferentes tipos genéticos y una distribución geográfica adecuada, teniendo además cuidado de incluir por un lado yacimientos que parecen abundar en Sud América y por otro lado tipos de depósitos que parecen escasear en esta parte del mundo. Se incluyen yacimientos del tipo diseminado de cobre, zoneados de cobrezinc-plomo-plata, metasomáticos de contacta, cobre-uranio-vanadio en capas rojas, plomo-zinc (incluyendo el tipo Mississippi Valley), mercurio, estaño, pegmatitas, carbonatitas y complejos alcalinos, diamantes, niquel-cobalto-cromita-asbesto-talco, fierro, bauxita, tungsteno, titanio, uranio, oro, potasio y fosfatos.Se hacen comparaciones con la distribución de éstos tipos de depósitos en otros continentes, especialmente en Norte América y Africa. Esto conduce a algunas especulaciones fundamentales referentes a la génesis de yacimientos minerales. La evidencia sugiere que la mayoría de los yacimientos considerados (con excepción de las carbonatitas, los complejos alcalinos y las chimeneas de diamantes) deben su formation principalmente a procesos que tuvieron lugar en la parte superior de la corteza terrestre. Más aún, parece que uno de los principales factores que afecta la distribución conocida de los diferentes tipos de depósitos es el nivel hasta el cual la erosión ha denudado una cadena de montañas. Finalmente, observamos que diversos tipos de yacimientos están distribuídos en franjas a lo largo de la Cordillera (participando en todos sus cambios de dirección), que su carácter quimico varia de Este a Oeste y que las relaciones entre éstas franjas son similares entre Norte y Sud América. Esto indica que dependen de la distribución relativa de sedimentos eugeosinclinales versus miogeosinclinales dentro del prisma geosinclinal o que su naturaleza depende de su distancia de los principales complejos batolíticos o su relación con stocks cercanos a la superficie y centros volcánicos. La correlación que se sugiere con respecto a carácter general de los sedimentos circundantes y con evaporitas sería un argumento en favor de las teorías modernas que consideran que las soluciones hidrotermales son predominantemente aguas meteóricas calentadas y puestas en circulación por actividad ígnea. Como ocurre a menudo, éste estudio revela más interrogantes que las respuestas que dá y llama la atención a varios problemas que requieren mayor investigación.Desde el punto de vista práctico el presente trabajo revela varias posibilidades para exploraciones futuras, así como oportunidades para un mayor desarrollo de los recursos minerales de Sud América. Probablemente varios países podrían aumentar o diversificar considerablemente su producción minera, levantando así el nivel de vida de su creciente problación. Mientras que en la América del Norte tanto el Canada como los E.E.U.U. de Norteamérica van de costa a costa abarcando con ello una grán variedad de ambientes geológicos (desde cratónicos hasta geosinclinales y orogénicos), en Sud América los países están distribuídos principalmente a lo largo de los Andes o sobre los escudos antiguos. Este último hecho tiene consecuencias importantes desde el punto de vista geopolítico en lo que se refiere a recursos minerales.Краткое содержаниеОписаны отдельные ти пы рудных месторожде ний Южной Америки и рассмативается воз можная связь их с боль шими тектоническими структурными единиц ами. Напр.: Porphyry — Copper, месторо ждения меди, цинка, свинца, сер ебра, контактно-метасомат ические месторожден ия, типа “red bed” с медью, урано м и ванадием, свинцовоцинковые (типа Valley, Миссиссиппи), рт ути, пегматитов, карбо натов, никель-хромит-кобаль т-асбесто-тальковые, о ловянные, железнорудные и бокситовые. Эти рудн

Mineralogy, geochemistry and alteration of the Cherry Hill, California hot-spring gold deposit

Abstract At Cherry Hill, California, gold-precipitating hot springs are associated with an economically significant body of gold mineralization. The deposit is about 0.56 Ma old and consequently unaffected by structural disruption, metamorphism, or excessive weathering and erosion. The mineralogy of the deposit is simple: gold-quartz-adularia-carbonate-iron sulfide microveinlets dominate the ore. The paragenetic sequence consists of twelve stages: early adularia (I), an alternating sequence of silica and carbonates (II–XI) and a late argillic event (XII). Gold deposition occurred during stages V–VI and IX. Early adularia-sericite alteration of the host mudstones was restricted to the central part of the deposit. Following this initial phase, fluids spread to the northwest and southeast creating anomalies of gold, silver, mercury, antimony, thallium and arsenic. Late in the history of the system, strong argillic alteration developed to the northwest of the central zone. Adularia-sericite alteration usually, but not always, correlates well with gold; argillic alteration does not correlate well with the strongest gold anomalies. Gold anomalies correlate broadly with those of silver, mercury, antimony, thallium and arsenic, though highest values do not always coincide. The lack of complete correspondence between the presence of gold and a given alteration or geochemical anomaly illustrates that the use of geochemical or mineralogical associations as guides to ore is greatly improved by understanding the paragenetic relations at a deposit.

The Society of Economic Geologists 2000 Awards R.A.F. Penrose Gold Medal for 2000 Citation of Alberto Benavides de la Quintana

Dear colleagues, ladies and gentlemen: There are moments in life when there is a confluence, a juxtaposition—if you wish—of memories, of friendships, of experiences, of ideas, and of ideals. This is one such moment. Perhaps I should start by narrating how I met our honoree, Ing. Alberto Benavides, because this tells something about his human qualities. It was in the late 1940s, when I studied at the Peruvian School of Engineers—a few years after Alberto had done the same. Students were required to work in a mine at least six months spread over three summers. I chose Colquijirca, because it is unique among Peruvian ore deposits in that the ore is in well defined mantos (as opposed to in veins or disseminated). Furthermore, nobody else wanted to go to this mine because the pay was miserably low and you were charged for the hard hat and rubber boots. Worse yet, you were housed in a room without hot water—something you certainly miss after having worked hard every day for a shift and a half at an elevation of about 13,000 feet! So, every Saturday I climbed on …

Synthetic and natural phases in the system Au-Ag-Te

Petersen discusses various details of Cloke9s criticism (Econ. Geology, v. 58, p. 1163-1166, 1963) of Markham9s paper of the above title (Econ. Geology, v. 55, 1960) and attempts to reconcile the viewpoints.