Spotted Structures in Gneiss and Veins from Broken Hill, New South Wales, Australia
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MINERALOGICAL MAGAZINE, JUNE I980, VOL. 43, PP- 779-87 Spotted structures in gneiss and veins from Broken Hill, New South Wales, Australia BRYAN E, CHENHALL AND EVAN R. PHILLIPS* Department of Geology, University of Wollongong, Wollongong, NSW, 2500, Australia AND R. GRADWELL Deceased. Formerly Department of Geology and Mineralogy, University of Queensland, St. Lucia, Queensland, 4o67, Australia SYNOPSIS SPOTTED eye-like structures, made up of densely eyes. Moreover, the compositions of corresponding packed biotite 'pupils' mantled by quartz and feld- minerals in these phases are very similar. For spar, constitute about Io % by volume of a high- example, plagioclase is consistently about An 35-40, grade quartzofeldspathic gneiss located on the road the biotites are all very iron-rich (Fe/(Fe + Mg) to Purnamoota Homestead about IO km north of 0.87) with compositions near siderophyllite, and K- Broken Hill. These spotted structures have an feldspar has the composition Or94Ab 6. uneven distribution within rocks exposed at this It has been suggested that the biotite spots locality. In some parts of the outcrop they tend to formed by alteration of garnet probably during a be closely spaced (fig. I), whereas elsewhere they are retrograde metamorphic event. However, studies of less abundant and the spotted gneiss merges into the microstructure of the biotite and garnet in the an essentially homogeneous gneiss composed of spotted gneiss show that biotite of the spots does quartz (34~. by volume), plagioclase (24%), K- not replace garnet. Furthermore, the chemical simi- feldspar (i 7 ~), biotite (20 %), and garnet, muscov- larity between the spots and the matrix is best ite, and opaques (5 %). This homogeneous gneiss is explained by an isochemical rearrangement of believed to have developed into a 'matrix' gneiss components of the matrix phase to form the eyes. phase holding the 'eyes' which have a felsic mantle Thus, a more likely origin is best related to the dis- to mafic clot ratio of 4: I. placement of matter along chemical potential The whole eye-like structure appears to be a gradients possibly induced by deformation in the distorted prolate spheroid with its axis set close to rock system--a process described by the term the foliation plane of the gneiss. The internal biotite metamorphic differentiation. clot forms another prolate spheroid with its axis Veins in the Purnamoota Road gneiss are of two inclined at approximately 70 ~ to that of the main contrasting types--regularly-disposed vein- enveloping quartz-feldspar mantle. Although most lets which are composed almost entirely of K- of the eyes are isolated within the matrix, some are feldspar and quartz, and irregularly shaped dis- linked to hold three or four biotite aggregates continuous trondhjemitoid variants rich in biotite (fig. S I) and rarely a vein-like patch which contains spots and carrying rare garnet porphyroblasts. The some ten or more partly linked spots of biotite may field relationship between these two main types of be found. In addition to the biotite, xenoblasts of veins is difficult to discern. In some parts of the out- almandine occur within some of the eye-like crop the veins occur adjacent to one another and structures. locally appear to merge. Veins with modal (and Whole-rock chemical analyses (including both chemical) compositions intermediate between the major oxides and trace elements) show that the spotted and the K-feldspar-rich veins have also homogeneous gneiss and the spotted gneiss are been recorded from the outcrop. very similar. The most notable feature of the geo- Despite a wide whole-rock chemical variation, chemistry of the spotted rocks is the close chemical the minerals in all the veins are similar chemically correspondence between the matrix and the whole and compare closely with corresponding minerals * Professor E. R. Phillips died on iIth May i98o. 780 B.E. CHENHALL ET AL. FIG. SI. A general view of the spotted Purnamoota Road gneiss. The eye-like form of the spots and mantles is evident. The white quartzofeldspathic portions are elongate parallel to the main foliation in the gneiss, whereas the elongate biotite 'pupils' tend to be arranged at angles of about 5o 7o~ to the foliation. Most of the eyes contain one mafic spot but some have up to four closely associated biotite clots. in the host gneiss. One exception may be the plagio- places some doubt on the suggestion that partial clase in veins rich in K-feldspar where normative melting has played a signficant role in the genesis calculations indicate a composition near An14. of the veins. Interpretation of the trace element data counts Although mechanisms of vein formation such as against the veins having formed by partial melting igneous injection or metasomatism are considered and the variable K-feldspar contents place both as other alternatives for vein formation, we suggest main vein types well away from ternary minima in that metamorphic segregation (as proposed for the the system Q-Ab-Or. Calculations based on the origin of the spots in the gneiss) will also account compositions of co-existing biotite and garnet in for the development of the veins. the host quartzofeldspathic gneiss and in associated prograde pelitic schists indicate an equilibrium temperature of 65o~ 5 ~ ~ at pressures between [Manuscript received 20 June 1979, revised 19 November I979] 3 and 4 kilobars. This fact, together with trace ele- ment data and the calcic nature of the plagioclase in the homogeneous gneiss and the matrix phase, Copyright the Mineralogical Society SPOTTED STRUCTURES IN GNEISS 78x SPOTTED STRUCTURESIN GNEISS AND VEINS FROM BROKEN HILL~ NEW SOUTH WALESt AUSTRALIA Bryan E. Chenbe]l and Even R. Phillips Department of Geology, University of Wollongong, gollongong, R.S.W., 2500, Australia and R. Gradwell Deceased. Formerly Department of Geology and Mineralogy, University of queensIand, St. Lucia. QLD., 4067, Austra]ia. QUARTZOFELDSPATNICgneisses (or 'granite' gneisses) are abendant throughcsJt the Wil]yama Complex of western New South Wales (Binns. 1964; Vernon, Ig69; Chenhall eta]., 19771. In the vicinity of Mine Mile Creek. about g to 11km nortIT-oFBroken Hill. some of these exhibit an unusual, spotted, eye- like structure described by Andrews (1922), Browne (1922) aed Chenhall et al. (19781. One of the best examples of this structure, together wi~ ~lated spotted and felsic veins, is seen in rocks located just east of the road to Purnamoota Homestead 2km MNE of the Nine Mile Mine (fig. 1; approx- imately grid reference 1773 OOOE 21238 BOON on the GeologicaI Map of the Broken Hill District published by the Australasian Institute of Mining and Metallurgy in 19681. The rocks from this area are knowtl as the Purnamoota Road gneiss. The 'eyes' consist of '... a dark centre or nucleus [of biotite], generally with suboircular outline, surrounded by an elongated area of white minerals, the whole simulating with startling clearness the appear- ante of a rather slit-like eye, so that the name "augen" gneiss is sing- ular]y appropriate...' (Browne, 1922. p. 3311. Tbese eyes are dispersed through a biotite gneiss host crossed by veins which may be grouped into two types -- one containing biotite spots and tending to have a low K- feldspar content, the other usually being almost biotite-free but tending to be rich in K-feldspar. The distinctive spotted gneiss grades into a more even-grained, weIl-foliated but homogeneous 'platy' variant in which the spots are virtually absent. Both of these 9neissic varieties display, to the east. a sharp, irregular contact with a leucocratic (biotite-poor or biotite-free) quartzofeldspathic gneiss containing numerous, apparent]y isolated 'pegmatitic' elongate patches (fig. 2). Sillimanite schists with Fig. 1. Locality map shOWing major rock outcrops north of Broken Hill, incipient retrogression (MObbs eta|.. 1968 Vernon, 1969; ChenhalI. 1973) After the Geological Map of the Broken Rill District, Australasian Instit- occur along the western contac~--o~the quartzofeldspathic gneisslc vari- ute of Mining & Metallurgy. 1968. ants. aggregates range from 0.25 to 2ram in length. Many felsic crystals vary Browne (19221 suggested that the biotite spots within the eyes were from O.5 to 1.0ram across, indicating a slightly coarser grain-size than for derived by alteration of garnet. However, we note here the similarity of the homogeneous variety. the eyes to stictolithic, fleck or 'cigar' structures described elsewhere quartz sh~s undulatory extinction and cool,only occurs in aggregates (e.g. Loberg, 1963; Mehnert, 1968; Russell, 1969; Dahl. 1972; Fisher. 19731 of polygonal crystals. Most pIagioclase grains An~_~by opt cal deter- and to diffusion spheres (Atherton, 19761, and the hypothesis that the eyes mination; An39 by chemical analysis. Table 11, col~n~-6) are almost un- were derived by metamorphic differentiation is considered below. In add- altered and virtually unzoned with sharp, regular and pers steot albite ition, the association of such structures with quartzofeIdspathic veins twins. Apparently non-perthittc K-feldspar shows incipient cress-botched appears to be of fundamental importance (especially Loberg, 1963. p.102). twinning and contains lobate myrmekite. It occurs in lesser amount than in Since the genesis of sow~quartzofeldspathic veins and ]eucoso~s in mig- the homogeneous gneiss. Strongly pleochroic Biotite er straw ~t = ~= matites is again in dispute (Ashworth, 1977. 1979; Yardlny, 1977, 1979) or very dark brown; chum ca analysis, Table 1I. column 2) is identical in is the subject of detailed attention (e.g. OIsen, 1917; Yardlny, 1978; also colour and compesition to that forming spots within the eyes. Almandine Hedge. 1972; Fershtater. 19771. we be]ieve that the time is opportune to (Table [I. columns 13-15) occurs as sporadic, individually roughly spher- present an account of both the eye-like structures and the veins in the ical crystals or as aggregates of grains 3 to 5mm in diameter.