Metasomatic Norms and Mass Balance Chemico-mineralogic Models of Hydrothermal Alteration System

Abstract Norm calculation procedures originally designed principally for igneous rocks are very rigid in their application and, in general, do not utilize important alteration minerals. For instance,
volatile components are essential constituents of many metasomatic rocks but either are not used or are not used effectively in determining normative minerals by the CIPW norm or Niggli-Barth norm procedures. Our approach to determining norms for metasomatic rocks allots all major, minor and volatile constituents to a set of standard normative minerals which are commonly observed as products of hydrothermal alteration. A useful procedure is to have a metasomatic norm approximate the mode as closely as possible. Petrographic information helps to provide the necessary mineralogical constraints. With the recognition of an immobile component and knowledge of sampling and analytical errors in lithogeochemical data, it is possible to recast norms into amounts of alteration minerals relative to a given amount (100 grams) of parent rock. Combining these results with the calculations of absolute loss or gain of individual chemical constituents and considering errors (e) for an appropriate con?dence level, an overall (net) material exchange equation can be presented as follows:
SMineralparent rock ± e + SConstituentgained from solution ± e
= SMineralaltered rock ± e + SConstituentlost from wall-rock ± e
where all items have extensive units (e.g., grams). The model includes the initial and ?nal mineralogy of rocks (evident in the ?eld) as well as gains and losses of speci?c elements. Only those constituents whose calculated losses and gains are signi?cantly different from zero are retained in the above model, and only the normative minerals with abundances signi?cantly different from zero are included in the model (equation). One advantage of the chemico-mineralogic model is that parent rock and product are modeled in mineralogical terms that are familiar to and easily usable by exploration geologists.
Keywords: Hydrothermal alteration systems, Igneous rocks, metasomatic rocks, Petrography.
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Summary: The McDougall-Despina fault set in the Noranda district of northwestern Quebec de?nes the eastern margin of the Despina cauldron, nested within the district-scale Noranda cauldron. The faults are stratabound in that they displace stratigraphy of the cauldron ?ll, but do not offset the cauldron cover. The faults are occupied by dikes that acted as eruptive centers to volcanic members of the cauldron-?lling sequence. Measurable displacement across the McDougall-Despina fault set is greater than...
Publication: Exploration & Mining Geology
Author(s): Department of Earth Sciences, University of Western Ontario, London, Ontario, Canada, N6A 5B7, H.L. GIBSON, Mineral Exploration Research Centre, Laurentian University, Sudbury, Ontario, Canada, P3E 2C6, J.J. WATKINS P.O. Box 3127, Courtenay, British Colu
Keywords: Fault-controlled Volcanism, Hydrothermal fluid flow, Stratisgraphy, synvolcanic faults.
Issue: 4
Volume: 4
Year: 1995
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Summary: The size, morphology, surface texture, per cent mineral inclusions, and bulk chemistry
(Ag and trace elements) of placer gold particles vary systematically according to transport distance from source rocks, as indicated by a case study from the Pioneer district in Montana, and data from the literature.
Placer gold samples show a rapid decrease in mean and maximum particle length, and a gradual
increase in ?atness index (F.I. = length + breadth/2 thickness) from about F.I. = 2 in bedrock
depo...
Publication: Exploration & Mining Geology
Author(s): JEFFREY S. LOEN
Keywords: Gold, Gold Characteristics, Placer Gold, Mineralization
Issue: 4
Volume: 4
Year: 1995
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Summary: The Murray Brook precious metal (Au, Ag) gossan deposit is one of nine supergene mineral deposits that have developed over polymetallic massive sulfide deposits in the Bathurst Camp of New Brunswick. Reserves consist of 1.9 Mt of gossan containing 1.53 g/t Au and 65.9 g/t Ag. The supergene zone at Murray Brook consists of six distinct units: a) altered massive sulfide, b)
pyrite-quartz sand, c) massive sulfide gossan, d) disseminated sulfide gossan, e) ferruginized wallrock, and f) leached...
Publication: Exploration & Mining Geology
Author(s): D.R. Boyle
Keywords: Geochemistry, Genesis, Murray Brook, Precious metal deposit, Gossan deposit, Mineralogy, Metallurgy, Gold concentration
Issue: 4
Volume: 4
Year: 1995
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