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THE STRATIGRAPHY and GEOCHEMISTRY of the GRANITE GNEISSES, BROKEN HILL, N.S.W. by IAN D. BLUCHER Department of Mining and Minera

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THE STRATIGRAPHY and GEOCHEMISTRY of the GRANITE GNEISSES, BROKEN HILL, N.S.W. by IAN D. BLUCHER Department of Mining and Minera THE STRATIGRAPHY AND GEOCHEMISTRY OF THE . GRANITE GNEISSES, BROKEN HILL, N.S.W. by IAN D. BLUCHER Department of Mining and Mineral Sciences, w.s. & L.B. Robinson University College, University of New South Wales. MARCH, 1983. This thesis contains no material which has been accepted for the award of any other degree or diploma in any Tertiary Institution; nor does it contain any material previously published or written by any other person except where due reference and acknowledgement is made in the text. I.D. BLUCHER. ACKNOWLEDGEMENTS I would like to thank my supervisors Ors. K.D. Tuckwell and P.C. Rickwood for their advice and help during the preparation of this thesis. The analytical expertise of both Dr. T. Hughes of Melbourne University and The Zinc Corporation, Limited assay laboratory is greatly appreciated. The financial support provided by the Broken Hill Mining Managers' Association, the geological staff of both The Zinc Corporation, Limited and North Broken Hill Limited for specimens, maps and helpful discussions and the technical expertise of Mr. J. Vaughan, Mrs. K. Goldie, Ms. J. Gray and Mrs. J. Day all contributed to the successful completion of this study. Abstract The Granite gneisses located at Broken Hill have been examined in order to establish their internal stratigraphy, the significance of any chemical trends present and also an origin for these gneisses. Mineralogically these gneisses,which are chemically indistinguish­ able from one another can be divided into garnet-bearing or garnet-absent, quartz-feldspar-biotite gneisses, and an aplitic-textured quartz-feldspar­ rich fels. The first two gneiss types may in places be rich in feldspar augen and grade vertically into augen-poor or layered gneisses. Augen are interpreted as premetamorphic crystal relicts and the presence of garnet in stratigraphically distinct zones is believed to be due to the local, premetamorphic, mobilization of Na and K from original feldspars and clays. These alkali-deficient sites could then generate garnet when metamorphosed. The aplitic fels lllarks the stratigraphic top of the whole Granite Gneiss sequence. It is present within the lllain granite gneiss body and, although texturally and chemically distinct from the other two gneiss types exhibits similar chemical trends and ratios which indicate its close affinities with these gneisses. The origin andlllOde of deposition proposed for these gneisses is that they were felsic, calcalkaline, crystal-rich volcanic material of dacitic composition, deposited in an aqueous environment as a series of debris or turbidite flows. Physical conditions prevailing during the deposition of this -volcanic sedilnent developedgrainsize abundance, density and total grainsize grading as well as layering features resembling those of Bouma sequences. The forward motion of these debris flows was sufficient to cause ablation ,t.o ..take place of the finer grained material from the outer surface, this material settled out of suspension and eventually formed what is now the aplitic quartz-feldspar fels. The development of these features has led directly to a chemical signature which closely resembles an igneous fractionation pattern even though it is the sole result of sedimentological processes. Table of Contents Page No. 1. Introduction 1 1.1 Terminology 1 2. Granite gneiss stratigraphy 2.1 Introduction 4 2.2 Methodology 4 2.2.1 Stratigraphic Synthesis 7 2.3 Stratigraphic reconstruction 2.3.1 Upper Granite Gneiss 9 2.3.2 Lower Granite Gneiss 14 2.4 Layering 15 2.5 Description of Gneiss types 2.5.1 Aplitic textured quartz-feldspar fels (Ap) 15 2.5.2 Feldspar biotite garnet gneiss (Fbg) 18 2.5.3 Feldspar biotite gneiss (Fb) 18 2.5.4 Green-grey aplite (Gap) 19 2.5.5 Quartz augen rocks 19 2.5.6 Amphibolite 21 3. Mineralogy 24 3.1 Metamorphic conditions 24 3.2 Aplitic textured quartz-feldspar fels (Ap) 25 3.3 Feldspar biotite garnet gneiss (Fbg) 26 3.4 Feldspar biotite gneiss (Fb) 29 3.5 Green-grey aplite (Gap) 29 3.6 Quartz augen rocks 30 4. Granite gneiss chemistry 31 4.1 Sample analysis 31 4.2 Analytical results 33 4.2.1 Harker-style variation diagrams 33 4.2.2 Trace- and interelement variation 41 (cont.) Table of Contents (cont.) Page No. 4.3 Multivariate data analysis 52 4.3.1 Principal components Analysis 53 4.3.2 Cluster Analysis 55 4.3.3 Results of Multivariate data analysis 56 4.3.4 Kolmogorov-Smirnov two sample test 62 5. Petrology 65 5.1 Alteration 66 5.2 Petrological character 71 5.2.1 Alkaline vs. subalkaline composition 72 5.2.2 AFM variation 72 5.2.3 An-Ah-Or projection 75 6. Discussion 77 6.1 Origins of Granite gneiss 77 6.2 Depositional mechanisms for the Granite gneisses 6.2.1 Air fall tuffs 79 6.2.2 Mass flow 80 6.3 Feldspar augen and megacryst formation 84 6.4 Garnet formation in the Granite gneisses 86 6.5 Geochemical trends in relation to a mass flow model 87 6.6 The significance of non-Granite gneiss lithologies 89 7. Conclusions 93 References 96 Appendices 101 List of Figures Figure Description Page No. 1 Locality plan of study area and major structural Appended elements. 2a-d Unfolded diamond drill sections showing the Appended relative position of stratigraphic and mineralogical variations in the Upper and Lower Granite Gneisses from the Northern Leases and Southern Extensions. 3 & 4 Small scale mapping depicting the relationships 5, 7, present between Quartz augen rocks, amphibolites and Granite 9neiss in the Lower Granite Gneiss. 5 Schematic, cutaway block reconstruction of the Appended Upper and Lower Granite Gneisses. 6a-i Harker style plots of sio2 vs. Tio2 , A1 203 , Fe2o3 , 42-45 MnO, MgO, CaO, K20, Na2o and P 205 . 7a,b,c Plots of K20 vs. Rb, Ba and Na20. 46 8a,b Plots of Fe2o3 vs. Ti02 and MgO. 47 9a,b,c Plots of Ti02 vs. Zr, y and er. 48 lOa,b,c Plots of Ni vs. Cr, V and MgO. 49 lla,b Plots of Zr vs. Y and Ce. 50 12a,b,c Plots of niggli mg vs. niggli si, niggli er and 51 niggli ni. 13 Principal Component analysis results for Fbg, Fb 58 and Ap Granite gneiss types. 14 Cluster analysis dendrograph of Northern Leases 60 Granite .gneiss, based on chemistry. 15 Cluster analysis dendrograph of Northern Leases 61 Granite gneiss, based on mineralogy. 16 Plot of K2o_+ Na2o vs. Si02 for Northern Leases 73 Granite gneisses. 17 AFM diagram for Northern Leases Granite gneisses. 74 18 An-Ab-Or projection for Northern Leases Granite 76 gneisses. 19 Postulated method of deposition for the Granite 83 gneisses. List of Tables Table Description Page No. 1 Average volume fraction analyses for Granite gneisses 27 and associated rock types. 2 Analytical results for USGS G-2, AGV-1. 32 3a Aplitic textured quartz-feldspar fels chemistry. 34 3b Feldspar biotite garnet gneiss chemistry. 35 3c Feldspar biotite gneiss chemistry. 36 3d Quartz augen and related rock types chemistry. 37 3e Amphibolite, Biotite selvage, and Green-grey aplite 38 chemistry. 4 Principal components for all Fbg, Fb and Ap gneiss 57 types. 5 Kolmogorov-Smirnov two sample comparison between Fbg 63 and Fb gneisses. 6 Average composition of rock types used in Barth Standard 68 Cell calculations. 7 Changes in Barth Standard Cell needed to convert Arkose, 69 Sillimanite Gneiss, Dacite and Rhyolite to Granite gneiss type lithologies. 8 Chemical characteristics of Granite gneiss types with 70 respect to all possible parents. List of Plates Plate Description Page No. 1 Feldspar-biotite gneiss clasts in Feldspar-biotite, 11 and Feldspar-biotite-garnet gneiss. 2 Feldspar augen grain size and abundance decreasing 12 stratigraphically upwards. 3 Aplitic textured fels and feldspar augen-bearing 13 Feldspar-biotite gneiss. 4 Amphibolite and biotite-selvage layers within 16 Feldspar-biotite gneiss. 5 Fine grained layering developed in Feldspar-biotite 17 and Feldspar-biotite-garnet gneisses. 6 Mottled and fragmental, layered Green/grey aplite and 20 layered quartz augen rock. 7 Amphibolite textures representing possible premetamorphic 23 flow top breccias and infilled vesicles. 1 CHAPTER 1 1. Introduction The Willyama Complex of New South Wales contains a number of coarse grained, quartzo-feldspathic gneisses which are locally termed granite or granitic gneisses. At Broken Hill two bodies of gneiss crop out sporadically in two parallel zones on the eastern and western sides of the Broken Hill ore bodies from Kellys Creek, 10 kms to the south­ west of Broken Hill, to beyond Piesses Nob, 15 kms to the northeast (Fig. 1). It is this close proximity to the ore zone which has caused the origins of these bodies of gneiss in particular to be subject of considerable contro­ versy and debate. The aim of this study is to resolve if possible, through an examination of the spatial variations of geochemistry and mineralogy, a likely mode of origin for these gneisses and the method by which they arrived at the stratigraphic position they now occupy. Willyama Complex stratigraphy has been firmly established by Stevens et al. (1979). These workers place the granite gneiss at the top of Suite 3~ a metamorphic/stratigraphic group of rocks which consists largely of abundant feldspathic psai:nopelitic metasediments, amphibolites and bodies of granite gneiss. Suite 4, which overlies Suite 3 and contains all of the major lead-zinc mineralization in the Willyama Complex, consists predominant­ ly of interlayered metapsammites and pelites, psammopelites and amphibolites. No granitic gneisses are present in Suite 4, although a fine to medium grained garnet-rich quartz-feldspar-biotite gneiss locally known as "Potosi Gneiss" occurs in the vicinity of mineralization. Thus on a local scale the granite gneisses under study here are a "basement" to Suite 4 lithologies.
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