Precious metal deposits in the Neogene-Quaternary Volcanic complex of the Central Andes

The Neogene-Quarernary volcanic complex of thc central Andes contains hundreds of hydrothermally altered zones, severa! of which are associated with world-dass cpichcrmal precious-metal deposits, but most of which are either unexplored or inadcquately explorcd. Thcse young volcanic rocks, chiefly andesitic to dacitic lavas and rhyolitic ash-ílow tuffs, ·extenct· over an area of about 300,000 km2 in the Andean Highlands of southern Perú, northcrn Chile, western Bolivia, and northwestern Argentina. At least a dozen majar deposits have been discovered in che young volcanic complex since the late 1970's by exploration programs stimulated by the phenomenal rise in gold and silver prices. Not only are eruptive centers in che complex prime exploration targets, but thc volcanic complcx covers older rocks and associated mineral deposits of one of the world's greatcst metallogenic provinces. Future exploration should reveal the presence of more deposits both within and beneath the volcanic rocks. The volcanic-hosted epithermal precious-metal dcposits are spatially and temporally related to eruptive centers--stratovolcanoes, calderas, and domes--within the volcanic complex. Included are both adularia-sericite and acid-sulíate types of deposits for which distinct sub-types can be distinguished and corresponding descriptive modcls constructcd. Adularia-sericite-type deposits include (1) silver-rich polymetallic base-metal veins, (2) polymetallic tin-silver-rich veins, and (3) stockwork porphyry deposits. Acid-sulfate-type deposits, which are charactcrizcd by che presencc of enargite and magmatichydrothermal alunice, includc (1) silver- and gold-silvcr-rich polymctallic base-metal veins and (2) lowgrade vuggy silica and breccia gold-silver dcposits. Radiometric ages show that most of the precious-metal deposits are of middle Miocene age (17-9 Ma), but severa! are as young as Pliocene and Pleistocene (5-1.2 Ma), and others may be forming today in association with active thermal spring systems and fumarolic-srage stratovolcanoes. Grade and tonnage data íor both high-gradc (gencrally mulli-vein) dcposits and low-grade (generally stockwork or vuggy silica) dcposits show that most contain bctween 500 and 5,000 metric tons (t) of silver and lcss than 60 t of gold. Cerro Rico de Potosí, a polymctallic tin-silver deposit, is the world's largest silver deposit, having produced an cstimatcd 30,000 t of silver from ores that probably averaged more than 500 grams per metric ton (g/t) silver. A comparable amount of silvcr is estimated to remain in Cerro Rico in veins and stockworks, disseminated into the wallrock, and in waste dumps, ali having grades of 150-250 g/t silver. Silver-rich base-metal ores generally contain 500 or more g/t silver bue less than 2 g/t gold; ore shoots in sorne veins may contain 100 or more g/t gold. Thc richcst gold ore reccntly found in the central Andes has bcen the 3500 vein of the El Indio district, Chile, which has yielded about 30 t of gold from ores averaging about 250 g/t gol d. Significanc amounts of the ore in this vein contain more tan 1,000 g/t gold. Low-grade stockwork, vuggy silica, and brcccia dcposits, of which at least a dozcn have been discovered during the 1980's and 4 or 5 are currenLly in operalion, havc average grades on the arder of 1.5-2 g/t gold and 20-100 g/t silver. Resources of such deposits rangc from 25 to 60 t of gold and to as much as 3,000 t of silver

Cochlear neuropathy and the coding of supra-threshold sound

Many listeners with hearing thresholds within the clinically normal range nonetheless complain of difficulty hearing in everyday settings and understanding speech in noise. Converging evidence from human and animal studies points to one potential source of such difficulties: differences in the fidelity with which supra-threshold sound is encoded in the early portions of the auditory pathway. Measures of auditory subcortical steady-state responses (SSSRs) in humans and animals support the idea that the temporal precision of the early auditory representation can be poor even when hearing thresholds are normal. In humans with normal hearing thresholds (NHTs), paradigms that require listeners to make use of the detailed spectro-temporal structure of supra-threshold sound, such as selective attention and discrimination of frequency modulation (FM), reveal individual differences that correlate with subcortical temporal coding precision. Animal studies show that noise exposure and aging can cause a loss of a large percentage of auditory nerve fibers (ANFs) without any significant change in measured audiograms. Here, we argue that cochlear neuropathy may reduce encoding precision of supra-threshold sound, and that this manifests both behaviorally and in SSSRs in humans. Furthermore, recent studies suggest that noise-induced neuropathy may be selective for higher-threshold, lower-spontaneous-rate nerve fibers. Based on our hypothesis, we suggest some approaches that may yield particularly sensitive, objective measures of supra-threshold coding deficits that arise due to neuropathy. Finally, we comment on the potential clinical significance of these ideas and identify areas for future investigation

Association of Physical Activity with Hormone Receptor Status: The Shanghai Breast Cancer Study

Evidence exists that breast tumors differing by estrogen receptor (ER) and progesterone receptor (PR) status may be phenotypically distinct diseases resulting from dissimilar etiologic processes. Few studies have attempted to examine the association of physical activity with breast cancer subtype. Such research may prove instructive into the biological mechanisms of activity. Consequently, this investigation was designed to assess the relationship between physical activity and hormone receptor-defined breast cancers in a population of Asian women in which the distribution of receptor types differed from traditional Western populations. Participants, ages 25 to 64 years, were recruited into this population-based, case-control study of breast cancer conducted in Shanghai, China from August 1996 to March 1998. Histologically confirmed breast cancer cases with available receptor status information (n = 1001) and age frequency-matched controls (n = 1,556) completed in-person interviews. Polytomous logistic regression was used to model the association between measures of activity with each breast cancer subtype (ER+/PR+, ER−/PR−, ER+/PR−, and ER−/PR+) using the control population as the reference group. Exercise in both adolescence and the last 10 years was associated with a decreased risk of both receptor-positive (ER+/PR+) and receptor-negative (ER−/PR−) breast cancers in both premenopausal and postmenopausal women (odds ratios, 0.44 and 0.51 and 0.43 and 0.21, respectively). Sweating during exercise within the last 10 years was also associated with decreased risk for receptor-positive and receptor-negative breast cancers among post-menopausal women (odds ratios, 0.58 and 0.28, respectively). These findings suggest that physical activity may reduce breast cancer risk through both hormonal and nonhormonal pathways

Quantitative mineral resource assessment of undiscovered porphyry copper resources in South America

A quantitative resource assessment to be published this fall identifies and evaluates 26 tracts in South America where the geology isconsidered permissive for the occurrence of porphyry copper deposits. For each tract, information is provided on: (1) the rationale fordelineating the tract, (2) examples of important deposits in the tract, (3) the rationale for choosing the mineral deposit model used forthe assessment, (4) exploration history, and (5) expected spatial distribution of undiscovered deposits in the tract. The scale used toevaluate geologic information and define tracts is 1:1,000,000. There are about 600 million tonnes of copper in known porphyry copper deposits in South America. This study estimates there areapproximately 720 million tonnes of additional copper in undiscovered porphyry copper deposits, yielding a combined endowment ofabout 1.3 billion tonnes of copper. There are about 69 known porphyry copper deposits using the criteria adopted here to define a well-explored deposit. This study estimates that a mean of about 140 deposits remain to be found. In other words, about twice as manynew deposits might be found as have already been found. Overall, deposit densities in South America are comparable to those in therest of the world but differ in important details that are reflected in mapped distributions of deposits, metal densities, and percentagesof undiscovered deposits in each tract. The deposits in the tracts that include Chuquicamata and El Teniente are significantly larger intonnage and grade and are reported in a separate model that is more representative of their characteristics. The results of theassessment afirm that not all porphyry copper deposits in South America are located in the Andes. Geologic indications support thepresence of undiscovered deposits in Patagonia as well as the Amazon of Brazil. A preliminary 1:4,000,000 map of the 26 tracts will bedisplayed

The Effect of Social Desirability and Social Approval on Self-Reports of Physical Activity

The purpose of this investigation was to examine social desirability and social approval as sources of error in three self-reported physical activity assessments using objective measures of physical activity as reference measures. In 1997, women (n = 81) living in Worcester, Massachusetts, completed doubly labeled water measurements and wore an activity monitor for 14 days. They also completed seven interviewer-administered 24-hour physical activity recalls (PARs) and two different self-administered 7-day PARs. Measures of the personality traits “social desirability” and “social approval” were regressed on 1) the difference between physical activity energy expenditure estimated from doubly labeled water and each physical activity assessment instrument and 2) the difference between monitor-derived physical activity duration and each instrument. Social desirability was associated with overreporting of activity, resulting in overestimation of physical activity energy expenditure by 0.65 kcal/kg/day on the second 7-day PAR (95% confidence interval: 0.06, 1.25) and overestimation of activity durations by 4.15–11.30 minutes/day (both 7-day PARs). Social approval was weakly associated with underestimation of physical activity on the 24-hour PAR (−0.15 kcal/kg/day, 95% confidence interval: −0.30, 0.005). Body size was not associated with reporting bias in this study. The authors conclude that social desirability and social approval may influence self-reported physical activity on some survey instruments

Using social media to support small group learning

Abstract Background Medical curricula are increasingly using small group learning and less didactic lecture-based teaching. This creates new challenges and opportunities in how students are best supported with information technology. We explored how university-supported and external social media could support collaborative small group working on our new undergraduate medical curriculum. Methods We made available a curation platform (Scoop.it) and a wiki within our virtual learning environment as part of year 1 Case-Based Learning, and did not discourage the use of other tools such as Facebook. We undertook student surveys to capture perceptions of the tools and information on how they were used, and employed software user metrics to explore the extent to which they were used during the year. Results Student groups developed a preferred way of working early in the course. Most groups used Facebook to facilitate communication within the group, and to host documents and notes. There were more barriers to using the wiki and curation platform, although some groups did make extensive use of them. Staff engagement was variable, with some tutors reviewing the content posted on the wiki and curation platform in face-to-face sessions, but not outside these times. A small number of staff posted resources and reviewed student posts on the curation platform. Conclusions Optimum use of these tools depends on sufficient training of both staff and students, and an opportunity to practice using them, with ongoing support. The platforms can all support collaborative learning, and may help develop digital literacy, critical appraisal skills, and awareness of wider health issues in society

A preliminary quantitative mineral resource assessment of undiscovered porphyry copper resources in the Andes mountains of South America

Quantitative information on the probable locations and amounts of undiscovered porphyry copper resources of the world is important to international exploration managers, land-use and environmental planners, economists, and policy makers. The U.S. Geological Survey is organizing and facilitating a cooperative assessment, in collaboration with interested geological surveys and geological organizations, of the likely global distribution, quantity, and quality of selected undiscovered nonfuel mineral resources. This report on undiscovered porphyry copper deposits of the Andes Mountains was produced jointly with the geological surveys of Argentina, Chile, Colombia, and Peru, and is a summary of the more extensive report (>300 pages) that will be published soon. Reports on undiscovered copper resources of other regions of the world are being prepared and will be followed by assessments of the global undiscovered resources of platinum-group minerals and potash

Challenging notions of gendered game play: teenagers playing `The Sims`

This paper challenges notions of gendered game playing practice implicit in much research into young women\u27s involvement with the computer gaming culture. It draws on a study of Australian teenagers playing The Sims Deluxe as part of an English curriculum unit and insights from feminist media studies to explore relationships between gender and game playing practices. Departing from a reliance on predetermined notions of &ldquo;gender&rdquo;, &ldquo;domestic space&rdquo;, and &ldquo;successful game play&rdquo;, it conceptualizes The Sims as a game in which the boundaries between gender and domestic space are disturbed. It argues that observing students\u27 constructions of gender and domestic space through the act of game play itself provides a more productive insight into the gendered dimensions of game play for educators wishing to work computer games such as The Sims into curriculum development.<br /

El complejo volcánico cerros Bravos, región de Maricunga, Chile: Geología, alteración hidrotermal y mineralización

El Complejo Volcánico Cerros Bravos (Oligoceno-Mioceno) es un conjunto de lavas y domos dacítico-andesíticos calcoalcalinos de gran extensión (450 km2) ubicado a unos 140 km al este de Copiapó, Chile, que integra la Franja de Maricunga, cadena volcánica neógena que representa un arco pluto-volcánico de margen continental. Los análisis radiométricos K-Ar señalan que el complejo volcánico Cerros Bravos estuvo activo entre los 26-18 Ma (Oligoceno-Mioceno inferior) en tanto que el complejo La Coipa registra actividad hasta el Mioceno medio (15 Ma), y desde allí hacia el sur los centros volcánicos presentan edades entre 16 y 6 Ma. El complejo hospeda la zona de alteración hidrotermal del Prospecto Esperanza, sistema epitermal del tipo sulfato-ácido con mineralización de oro y plata controlada estructuralmente y asociada a una intensa silicificación y argilización con calcolinita, alunita y jarosita, con edades radiométricas K-Ar entre 20-18 Ma. Los estudios isotópicos de Pb realizados en rocas y sulfuros de los depósitos de la Franja de Maricunga indican que las rocas ígneas del Oligoceno-Mioceno son una mezcla de manto subcortical máfico o material cortical y material cortical radiogénico. El Pb en los depósitos de metales preciosos de la franja es dominado por una fuente como las rocas ígneas huéspedes y, localmente, otras rocas huéspedes (sedimentarias triásicas) pueden haber contribuido con Pb durante la circulación hidrotermal en ambiente cercano a la superficie. La aplicación de las metodologías de la teledetección en imágenes Landsat TM en la zona de Maricunga demostró ser una herramienta eficaz para la exploración ya que permitió destacar perfectamente las zonas de alteración hidrotermal presentes en el área y la elaboración de mapas de alta confiabilidad a escala regional