The role of thermochemical sulfate reduction in the origin of Mississippi Valley‐type deposits. I. Experimental results

The kinetics of thermochemical sulfate reduction (TSR) were studied in experiments at 205–250°C, with determination of both sulfate and sulfide at intervals during the runs of up to 430 h. Analysis of the data indicates the reaction to have first‐order kinetics, and extrapolating these data, plus data from the literature, to 150°C gives a range of possible rate constants from 10⁰ to 10^?4 year^?1. Although the rate law has not been well established, a reasonable estimate allows calculation of the amount of sulfide ore formed as a function of flow conditions and time. It is here concluded that TSR could happen during the formation of Mississippi Valley‐type ore deposits, subject to several caveats.     

The role of thermochemical sulfate reduction in the origin of Mississippi Valley‐type deposits. II. Carbonate–sulfide relationships

The two‐dimensional (2D) reactive transport model of Corbella et al. [Min. Deposita 39 (2004) 344] of the classic ‘mixing hypothesis’, in which a solution containing reduced sulfur and a solution containing aqueous Zn are mixed, has been extended to the case where one brine contains sulfate rather than reduced sulfur, and H₂S is produced during the mixing process by thermochemical sulfate reduction (TSR), using a possible rate law and experimentally derived rate constants. It is shown that TSR can provide sufficient reduced sulfur to form an ore body in geologically reasonable times, using rate constants in the upper part of the experimental range, in agreement with the results of a constant stirring tank reactor model ( Thom & Anderson in press ). The results of the 2D model are also compared with the results of titration (zero dimensional) and one‐dimensional models, showing that these models fail to adequately show the amount of carbonate precipitation. Although the 2D model qualitatively shows the distribution and amounts of carbonate precipitation and dissolution, these results are quantitatively inaccurate due to several factors that are intrinsically hard to define.     

METROPOLITAN AND NON-METROPOLITAN POPULATION TRENDS IN CANADA, 1966–1982

This paper discusses broad population movements within and between metropolitan and nonmetropolitan regions in Canada during the recent past. It is based on an annual series of aggregate migration at the metropolitan level. Three complementary approaches are used in order to distill information from this vast array of interregional flows. The first is based on net migration. The second, disequilibrium analysis, is based on the difference between the observed population shares of regions and a set of calculated steady-state population shares which would result if the currently observed set of interregional migration rates remained constant. Both approaches indicate short-term effects of migration on the evolving pattern of regional growth and decline. The third approach, based on aggregate interregional flows, allows one to observe some changes in migration behaviour which are not made evident by the first two approaches.     

INTERNATIONAL MIGRATION AND REMITTANCES TO A LEBANESE VILLAGE

Remittances are an important source of income for many labour-exporting Third World countries, especially for rural communities. Based on a theoretical framework developed by Lucas and Stark (1985) and Stark and Lucas (1988), a number of hypotheses concerning remittances from foreign emigrants are developed and tested using questionnaire survey data from Lala, a Lebanese village. According to the theory, a rural family engages in an implicit insurance-type contract with the potential migrant because both are averse to risk. A family interested in adopting new but risky agricultural technology invests in the emigration of one of its members and insures him or her against possibilities of unemployment or other uncertainties. Once established abroad, the migrant is expected to reciprocate by remitting, and thus insuring the family against unforeseen agricultural adversities. An emigrant is deterred from defaulting on the implicit contract because of a desire to inherit the parent's wealth and a need to maintain contacts with residual family members who help to administer and oversee a migrant's investment in the village, to maintain a sibling's reputation, and thus facilitate his or her return and reintegration into the community. Results from testing the hypotheses are somewhat consistent with the proposition that, after a period of adjustment, migrants do begin to remit, thereby providing their families with an insurance against the uncertainties of life in the country of origin. However, the level of remittances does not appear to depend on the potential to inherit from a family's wealth. Moreover, families do not seem to invest in their migrants so as to improve their farm lands and to undertake risky agricultural techniques. The long-term remittance commitment of migrants to their residual families suggests that migrants’ altruism may be a more significant motivating factor than self-interest. ?envoi ?argent par des émigrants dans leur pays ?origine est une importante source de revenu pour plusieurs pays exportateurs de main-?oeuvre du Tiers Monde, spéialement pour les communautes rurales. Basées sur un concept théorique développé par Lucas et Stark (1 985) et Stark et Lucas (1988), plusieurs hypothèses au sujet de ?envoi ?argentpar des émigrants à?étranger sont développées et testées en utilisant les données ?un questionnaire soumis à Lala, un village libanais. Selon cette théorie, une famille de milieu rural s'engage avec un éventuel migrant dans un contrat implicite de type assurance pour minimiser les risques des deux parties. Une famille intéressée à adopter une nouvelle technique agricole pleine de risque investit dans ?émigration ?un de ses membres et ?assure contre les possibilités de chomage et contre ?autres incertitudes. Une fois ?immigrant établi à?étranger, il se sentira obligé de leur rendre la pareille en leur faisant parvenir de ?argent pour les garantir contre des événements agricoles imprévisibles. ?émigrant est dissuadé de manquer à ses engagements à cause de son desir ?hériter de la fortune de ses parents et du besoin de garder contact avec la famille restante qui ?aidera à administrer ses investissements dans son village, et à maintenir sa réputation au sein de sa famille, facilitant ainsi son retour et sa réinsertion dans la communauté. Les résultats du test des hypothèses sont plutǒt compatibles avec la proposition qu'après une période ?ajustement, les migrants cornmencent à envoyer de ?argent dans leur pays ?origine, ce qui fournit à leur famille une assurance contre les incertitudes de la vie dans le pays ?origine. Par contre, le niveau ?envoi ?argent ne sernble pas dépendre des possibilités ?hériter. De plus, les farnilles ne sernblent pas investir dans le migrant dans le but ?arnéliorer leurs terres agricoles et ?acquérir des techniques agricoles risquées. ?engagement à long terme des émigrants à leurfarnille au pays ?origine suggère que ?altruisme est peut-ětre un facteur plus rnotivant que ?intéret personnel.     

1. DECOLONIZING HISTORIES IN THEORY AND PRACTICE: AN INTRODUCTION

This is a reflection on the close relations of the writing of postcolonial histories and recent decolonial critiques, and on the tensions between them. Postcolonial historical analysis often has been preoccupied with hybridity and mixture, conjugation and adaptation, exchange and interaction—with subversions of sovereignty in contact zones, borderlands, and on the beach. As a structuralist formulation, decolonial historical binarism in contrast echoes Indigenous politics of self-determination, even suggesting at times an ontological decoupling of settler and Indigenous histories and practices. Stringent decolonization of historical inquiry—implying the sabotage and superseding of settler colonial linguistic, narrative, and temporal conventions and the disturbing of standardized assumptions about evidence, agency, and authorship—would give us an epistemic assemblage perhaps not recognizable as “history.” Even if desirable, is that imaginable now except as metaphor or ideal?