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The igneous rocks of the Ok Tedi Intrusive Complex have been tentatively classified in previous chapters using several different methods. These include: the modal QAP diagram(Figure 50), the total alkali-silica (TAS) method (Figure 73) of Le Bas and others (1986) , and the normative Q'-ANOR method (<"fig070.htm">Figure 70) of Streckeisen and LeMaitre (1979) for rock names and the alkali-lime index (Figure 75) after Peacock (1931) for igneous rock series. Particular care has been given to the classification of the least-altered rocks because the nomenclature used in descriptions of the igneous host rocks form an important part of the criteria for the subdivision of porphyry copper deposits.
The least-altered phaneritic igneous rocks from the Sydney Intrusion are plotted on four different classification diagrams in Figure 107. Two of the three samples plot as quartz monzodiorite (field 9*) on the modal QAP plot. All three plot as quartz monzodiorite on the normative Q'A'P' and Q'-ANOR diagrams. In contrast, they are monzonite based on the TAS diagram of Middlemost (1994). The IUGS recommends that classification be done, wherever possible, from modes and on this rationale the term quartz monzodiorite is considered most appropriate for these samples.
The least-altered igneous rock with porphyritic texture is plotted on three classification diagrams in Figure 108. The sample is andesite on the Q'A'P' and Q'-ANOR whereas it is latite on the TAS diagram (Figure 73) of Le Bas and others (1986). The IUGS recommends that fine-grained igneous rocks be classified using the TAS diagram and on this basis the term latite is considered most appropriate for this sample.
The major oxides, trace elements, and modal and normative mineralogies of samples 331-218.5 (a phanerite) and 340-166.1 (a porphyry) shown in Figures 107 and 108 are compared in Table 26. The two samples exhibit nearly identical chemistry and mostly similar mineralogy. Nonetheless, and in accordance with preferred classification schemes for igneous rocks, the term quartz monzodiorite is appropriate for the phanerite and latite for the porphyry. Thus, a problem is obvious because neither of these two terms occupy the same field on any of the common classification diagrams.
In order to illustrate the how igneous rock names pertinent to the Ok Tedi intrusions are actually used in the geologic literature and how the term "latite" can be equivalent to "quartz monzodiorite" I examined two data bases of chemical analyses taken the geologic publications. One of these, the 19,516 sample IGBA database, is described by Brändle and Nagy (1995); it includes analyses of 2023 samples of andesite and 83 of (quartz) latite. I constructed the second database, that consists of analyses of 509 samples of (quartz) monzodiorites and other igneous rocks that are geographically and genetically associated with (quartz) monzodiorite. The second database was constructed because the IGBA database has few analyses of (quartz) monzodiorites..
The names applied to the IGBA samples by the original authors of source references were retained by the researchers who compiled the database in order to avoid biasing the data. The spread of the data for each rock type on the classification diagrams can, therefore, be considered to represent the amount of variation in chemical composition of rocks bearing that name. Analytical data for andesites and (quartz) latites from the IGBA database are plotted on the Q'A'P', Q'-ANOR, and TAS (Le Bas and others, 1986) classification diagrams in Figure 109.
Points representing andesites form a cluster in the left-hand portion of field 10* and right-hand part of field 9* in the normative Q'A'P' diagram (Figure 109A). Samples with greater amounts of normative quartz plot in the lower parts of fields 4 and 5. These same samples form a cluster extending from fields 5b* and 10b to the middle of fields 4 and 9* on the Q'-ANOR diagram (Figure 109B). The majority of andesites plot in the fields of quartz over-saturated rocks (basaltic andesite, and andesite) on the TAS diagram (Figure 109C) although many other samples plot in adjacent fields.
Points representing samples of latite plot in the right one-third of field 8* and the left two-thirds of field 9* on the Q'A'P' diagram (Figure 109A). Distributions of these same samples extend from the left side of field 10a* to the left side of field 8* in the Q'-ANOR diagram (Figure 109B). The majority of latites plot in the fields of quartz-saturated rocks (basaltic trachyandesite and trachyandesite) on TAS diagram (Figure 109C) although other samples plot nearby in adjacent fields.
The data presented in these diagrams illustrate the fact that andesites, as the term is used in the geological literature, are mainly restricted to the right-hand section of field 9* and to field 10* of the normative Q'A'P' and Q'-ANOR diagrams and that (quartz) latites are nearly as likely to plot in the left-hand portion of field 9 and 9* as they are to plot in fields 8 and 8*. The distribution of the two rock types on these diagrams suggest that the boundary between fields of andesite and latite, as they are drawn, may represent an artificial rather than a natural break.
Analytical data for 25 samples of (quartz) monzodiorite that have SiO2 values less than 65 weight percent are presented in Figure 110. All but two of the samples plot as monzodiorite or quartz-monzodiorite on the Q'A'P' diagram (Figure 110A). The samples are about evenly distributed between (quartz) monzodiorite and (quartz) monzonite on the Q'-ANOR diagram (Figure 110B). They plot in the fields monzodiorite, monzonite, and quartz monzonite on the TAS diagram (Figure 110C) of Middlemost (1994). It can be seen from Figure 110 that approximately half of the samples that have been named (quartz) monzodiorite by various authors plot as (quartz) monzonite on the Q'-ANOR diagram and that nearly all plot in the fields of quartz-saturated rocks and not in the fields of quartz-oversaturated rocks. Just as in the case of the samples from Ok Tedi, interpretation of the data from these samples implies that rocks with identical chemistry but different texture can be classified as (quartz) monzodiorite or latite. The fact that samples of quartz monzodiorite from the Ok Tedi deposit, and from other locations throughout the world, show similar distributions on the common classification diagrams suggests that the problems that I experienced in applying the petrographic nomenclature to the least-altered rocks at Ok Tedi are not unique to this investigation.
In summary, the least-altered rocks with phaneritic texture from the Sydney Intrusion are quartz monzodiorite and those with porphyritic texture are quartz latite in spite of the fact that they are chemically equivalent. The problem in nomenclature is caused by differences in current chemical and modal classification schemes. I suggest that criteria for the classification of rocks intermediate between andesite and latite may require clarification.
The altered intrusive rocks of the Mt. Fubilan, Kalgoorlie, and Ningi Intrusions have been modified from their original pristine igneous compositions. They are metasomatic rocks consisting of mineral assemblages formed by the hydrothermal replacement of the original igneous minerals. Therefore, the IUGS terminology is not appropriate for classifying them and they are best described as altered-quartz monzodiorites (those with phaneritic texture) and altered-latites (those with porphyritic texture). The IUGS names and ternary QAP diagrams such as Figure 51 or binary Q'-ANOR diagrams such as Figure 70, although inappropriate for classification purposes, can be useful in illustrating the direction, gross chemistry, and extent of hydrothermal alteration. It is important to note, however, that these rocks demonstrate the problems involved in classifying rocks strictly on their chemical composition. It is unlikely that the hydrothermal nature of these rocks would be recognized from chemical criteria alone without microscopic examination and that an automated geochemical sampling program that was not accompanied by petrographic studies would result in incorrect classification.
