1Geol 481: Earth ResourcesEl-Shazly, A. K., 2004Mineralogical and Geochemical techniques used to study ore depositsI- Mineral parageneses, paragenetic sequences and zoningAccording to the International Association on Genesis of Ore Deposits, a "paragenesis" isan association of minerals having a common origin. A "paragenetic sequence" is thesequence of "mineral assemblages" or associations developed over a certain time period inan area or ore body (Fig. 1). Therefore, one of the first steps of studying ore deposits is to identify the mineralsbelieved to have been cogenetic or in equilibrium (i.e. mineral assemblage or paragenesis)using appropriate compositional phase diagrams and projections, and to identify how theseassemblages have evolved through time (using paragenetic sequence diagrams; Figs. 1 &2). The third step will be to identify the spatial distribution of these assemblages (zoning).In a single ore-forming event, paragenetic sequences should be predictable, and are hencea means for prospecting for ore deposits. For example, the ideal mineralogy of acomposite vein crosscutting an I-type quartz monzonite should vary with decreasing P andT according to the predictions of Table 1 (# 10 à 9)."Zoning" in ore deposits is any regular pattern in the distribution of minerals or elementsin space which may be shown in an ore body, in a mineral district, or in a large region.Accordingly, zoning is of two broad types:a- Assemblage zoning: based on the spatial distribution of mineral assemblages, andb- Cryptic zoning: representing the spatial variations in chemical composition or in theconcentration of one or more elements. On the other hand, based on scale, three types of zoning are identified:a- Regional zoning: Is one that occurs on a regional scale. This type of zoning can beexplained to some extent by the concept of plate tectonics, and will be discussed inmore detail at the end of this course.b- District zoning: Is the zoning shown by closely grouped mines or occurrences.District zoning is one that is controlled mainly by changes in P-T-X relations of thechemical system considered (ore forming fluid + host rock). This type of zoning isgenerally predictable, and has led scientists to compile an idealized sequence as theone shown in Table 1. It should be noted that district zoning may be "telescoped"where two or more zones are found to overlap in space. This "telescoping" processtakes place at shallow depths where the ore minerals may be deposited rapidly.c- Ore body zoning: Is that type of zoning that occurs on the scale of an ore body, andwhich may determine variations in grade.2Geol 481: Earth ResourcesEl-Shazly, A. K., 2004II- Wall Rock Alteration· Introduction: The country rocks enclosing or surrounding ore deposits commonly show effects ofreaction with the ore-bearing fluids. Such reactions are manifested by "alterationhaloes" (also known as alteration zones, selvages or envelopes) which range in sizefrom a few centimeters to several kilometers. These alteration zones are characterizedby patterns that are quite predictable and explicable chemically, and are thereforeuseful for exploration and for understanding the processes of ore genesis. Alteration may result from:1. Diagenesis in sediments2. Regional metamorphism3. Post-magmatic cooling4. Direct mineralization · Factors controlling the size and nature of alteration zones:(1) Temperature differences between the ore bearing fluid (or chemical system) andthe surrounding country rock.(2) Chemical composition of the country rock(3) Chemical composition of the ore-bearing "system" or fluid.An additional less important factor is the pressure at which these reactions take place.Among these factors, the compositional gradients between the country rock and the orebearing fluid is the most important, particularly because temperature gradients aretransient, and tend to be erased over a relatively short time period. Accordingly, the largestand best developed alteration zones occur around hydrothermal deposits that occur atintermediate to shallow depths.Compositional gradients will therefore determine the kinds and sequence of chemicalreactions that will take place (where hydrolysis, hydration, dehydration, carbonation,decarbonation, silication, silicification and oxidation - reduction are the most common andmost important reaction types). The extent to which these reactions take place to"completion" will control the "type" of wall rock alteration.· Types of wall rock alteration:1- Potassic alteration: Is one in which K+ is added to the country rock, producing mainlyK-feldspar ± sericite ± biotite. In general, this type of alteration occurs at relativelyhigh temperatures and high K+/H+ ratios (for a hydrothermal fluid).2- Sericitic (phyllic) alteration: Is a type characterized by the alteration of feldspars andmicas to sericite + Qz. Rutile may be transformed to leucoxene. Characterized by strongleaching of alkalis (except K+), which takes place at temperatures and K+/H+ ratios lowerthan those of potassic alteration (Fig. 3).3- Propylitic alteration: Is one characterized by the assemblage: chlorite + epidote +carbonates replacing plagioclase and/or chlorite + epidote + montmorillonite replacinghornblende and biotite. This type of alteration is clearly characterized by the addition ofH2O, H+, CO2, and S to the country rock.3Geol 481: Earth ResourcesEl-Shazly, A. K., 20044- Argillic alteration: Characterized by the formation of Kln after Plag, and/ormontmorillonite after Plag + amphibole. Occurs at relatively low T when the hydrothermalfluids are quite acidic (low K+/H+ ratios; Fig. 3) which results in significant leaching. 5- Advanced argillic alteration: Produced by solutions that are highly acidic with lowK+/H+ and Na+/H+ ratios, and at relatively low T. Characterized by the minerals: Kln,dickite, (or Prl at T > 300°C) + Qz + alunite (KAl3(SO4)2(OH)6)+ topaz + tourmaline.6- Greisenization: Is similar to the advanced argillic alteration but without the formationof Pyrophyllite. Involves the addition of such elements as F, B, Ti, Sn and W to thecountry rock, manifested by the minerals: muscovite, topaz, tourmaline, fluorite,cassiterite and wolframite. Greisens are