Appendix 3
Thin section descriptions of basement chips Mason Geoscience Pty Ltd Petrological Serv~cesfor the Minerals Exploration and Mining Industry ABN 64 140 231 481 ACN 063 539 686 Postal: PO Box 78 Glenside SA 5065 Australia Delivery: 141 Yarrabee Rd Greenhill SA 5140 Australia Ph: +61-8-8390-1507 Fax: +61-8-8390-1194 e-mail: [email protected]
Petrographic Descriptions for Six Aircore Rock Samples from the Andrew Young Project, Northern Territory
REPORT # 3076
CLIENT Mithril Resources Ltd
ORDER NO Memo, P. Hill, 7 April 2005
CONTACT Mr Peter Hill
REPORT BY Dr Douglas R Mason
SIGNED
for Mason Geoscience Pty Ltd
DATE 21 April 2005 Petrographic Descriptions for Six Aircore Rock Samples from the Andrew Young Project, Northern Territory
SUMMARY
1. Rock Samples
A suite of 6 aircore rock samples from the Andrew Young Project (Northern Territory) has been studied using routine optical petrographic methods.
2. Brief Results
A summary of rock names and mineralogy is provided in TABLE 1.
Primary rock types
- Intrusive igneous rocks are represented in all samples of the suite.
- Two-pyroxene-biotite micro-diorite (AYAC04) is composed of plagioclase, orthopyroxene, clinopyroxene, biotite, and minor opaques and apatite. It represents the least-fractionated magma in the igneous rocks of this suite. It formed from a somewhat fractionated, K-rich, andesitic magma that may have a tholeiitic magmatic association. The fine grain size suggests that this rock formed in a relatively small intrusive body, or possibly the quickly-cooled margin of a larger intrusion.
- Hornblende-biotite diorites (AYAC07, AYAC08) formed as coarse-grained rocks composed of plagioclase, pyroxene, hornblende, biotite, and minor quartz, K-feldspar, opaques and apatite. Early-formed pyroxene was completely replaced by deuteric actinolite. These rocks possibly represent cumulates from magma similar to that of AYAC04.
- Mesocratic to felsic granitoids are represented by (from least-fractionated to most-fractionated) hornblende-biotite-pyroxene quartz monzodiorite (AYAC02), hornblende-biotite granodiorite (AYACOI), biotite monzogranite (AYACOG), and syenogranite ?dykelet (AYAC08). Through this sequence of rock types, quartz and K-feldspar increase and the ferromagnesian minerals decrease in abundance with pyroxene and then amphibole leaving the assemblage.
- Genetic relationships are difficult to determine from a limited number of small aircore samples. However, the fine grain size, uniform mineral distribution and clean intergranular texture of the two- pyroxene-biotite micro-diorite is interpreted to indicate that this rock crystallised from a high-K andesitic magma of possible tholeiitic association. In the mesocratic to felsic granitoids, gradational mineralogical changes suggest that they are genetically related, but some petrographic observations suggest that these rocks may have formed by disequilibrium magma mingling processes: grain shapes tend to be subhedral to anhedral, inequigranular textures are common, large quartz and plagioclase grains are present in AYACOI, strong and complex compositional zoning is observed in large plagioclase prisms where present, poikilitic textures are commonly developed between K-feldspar and quartz, and a coarse felsic granitoid xenolith is present in AYACO6. The varied rock types and their textures, together with the indistinctly zoned aeromagnetic anomalies within ovoid intrusive bodies, allows the interpretation that these intrusions represent magma mingling zones that formed by uprise of variably mingled mafic and felsic magmas from deeper magma chambers. Examples of this phenomenon are discussed in the literature (eg MacColl 1964; Vernon et al. 1988; Collins 1998). Hoatson and Stewart (2001, p.16) suggest that magma mingling was important in the Andrew Young Hills intrusion of their study. The observed rock types in the present suite do not appear to represent a layered mafic- ultramafic intrusion.
Mason Geoscience Pty Ltd Report #3076 Mithril Resources Ltd Alteration
- Selective alteration of pyroxene by actinolite and plagioclase by trace epidote is observed in some samples. These modifications of the primary mineralogy are considered to have formed in response to deuteric alteration during final cooling.
- Lack of metamorphic effects suggests that these rocks have not been modified by a significant metamorphic event.
Mason Geoscience Pty Ltd Report #3076 Mithril Resources Ltd TABLE 1: SUMMARY OF ROCK NAMES AND MINERALOGY
SAMPLE ROCK NAME MINERALOGY*
Primary** Alteration Veins Weathering
AYACOI Weakly weathered, hornblende-biotite Pla, Kf, qtz, hbl, bio, Cla granodiorite opq, apa, zir
AYAC02 Hornblende-biotite-pyroxene quartz Pla, qtz, Kf, cpx, hbl, monzodiorite bio, opq, apa, zir AYAC04 Two-pyroxene-biotite micro-diorite Pla, opx, cpx, bio, opq, - qtz, spa AYACO6 Partly weathered, xenolithic biotite monzogranite:
Biotite monzogranite Kf, pla, qtz, bio, opq, - Cla apa, zir Felsic granitoid xenolith Pla, hbl, Kf, qtz Cla
AYAC07 Partly weathered, hornblende-biotite Pla, bio, hbl, qtz, opq, Act, epi Cla diorite a Pa AYAC08 Partly weathered, hornblende-biotite Pla, hbl, bio, qtz, Kf, Act, epi Cla quartz diorite OPq, aPa Partly weathered, micro-biotite Qtz, Kf, pla, bio, opq Epi Cla, goe monzogranite Syenogranite ?dykelet in micro-biotite Kf, qtz, pla, bio, opq, Epi Cla, goe monzogranite SPn
NOTES *: Minerals are listed in each paragenesis according to approximate decreasing abundance. **: Only primary minerals currently present in the rock are listed. Others may have been present, but are altered
Mineral abbreviations Act = actinolite; apa = apatite; bio = biotite; cla = undifferentiated clays (possibly ?illitic); cpx = clinopyroxene (augite); epi = epidote; goe = goethite; hbl = hornblende; Kf = K-feldspar (microcline); opq = undifferentiated opaques (mainly ?Fe-Ti oxides); opx = orthopyroxene (hypersthene); pla = plagioclase; qtz = quartz; spn = sphene; zir = zircon.
Mason Geoscience Pty Ltd Report #3076 Mithril Resources Ltd 1 INTRODUCTION
A suite of 6 aircore rock samples was provided by Mr Peter Hill (Mithril Resources Ltd, Dulwich, South Australia) during a visit to Mason Geoscience on 7 April 2005.
Background information was provided, indicating that the samples originate from the Andrew Young Project in the Northern Territory. An aircore drilling program had tested a large circular magnetic anomaly near the Andrew Young Hills. The body does not crop out and there are no other drill holes in the area. It is thought that the body might be a compositionally zoned gabbroic intrusion. The drill hole locations were indicated on a magnetic image of the area, and photocopied excerpts were provided from Hoatson and Stewart (2001).
Particular requests were to prepare a thin section and petrographic description for each sample, using brief descriptions where repetition of rock types occurs.
Excerpts from this report were provided by email to Mr Hill on 20 April 2005. This report contains the full results of this work.
2 METHODS
At Mason Geoscience Pty Ltd conventional transmitted polarised light microscopy was used to prepare the routine petrographic descriptions.
Paragenetic stages of development of each rock are indicated in the mineral modal list, where each mineral is assigned to a numerical paragenesis (paragenesis 1 is earliest; paragenesis 2 overprints 1; paragenesis 3 overprints both 2 and 1; etc). The paragenetic stages display relative timing within each sample, and are not meant to be directly equated between samples although this may be correct for some samples.
Igneous rock types were classified according to the IUGS scheme (Streckeisen 1976).
3 PETROGRAPHIC DESCRIPTIONS
The petrographic descriptions are provided in the following pages.
Mason Geoscience Pty Ltd Report #3076 Mithril Resources Ltd SAMPLE : AYACOI (Andrew Young Hills, Northern Territory)
SECTION NO : AYACOI
HAND SPECIMEN: The aircore rock fragment represents a massive, fine- to medium-grained crystalline rock of mesocratic appearance, with scattered large equant quartz grains.
ROCK NAME : Weakly weathered, hornblende-biotite granodiorite
PETROGRAPHY :
A visual estimate of the modal mineral abundances gives the following: I Vol % Origin I Plagioclase lgneous 1 K-feldspar (microcline) lgneous 1 Quartz lgneous 1 Hornblende lgneous 1 Biotite lgneous 1 Opaques (mainly ?Fe-Ti oxides) lgneous 1 Apatite lgneous 1 Zircon lgneous 1 Clays Weathering 2
In thin section, this sample displays a massive inequigranular granitoid texture, modified only by weak selective weathering.
Plagioclase is abundant, forming small randomly oriented subhedral twinned prisms -0.5 mm long, and uncommon larger blocky twinned prisms -1-2 mm in size. The plagioclase is quite fresh.
K-feldspar is moderately abundant, forming anhedral angular grains scattered through the rock, in granular relationship with similarly sized clear quartz grains. Well-developed 'tartan' twinning in the K-feldspar confirms it is microcline. Larger optically continuous pools of K-feldspar enclose small ovoid grains of optically continuous quartz. Large anhedral grains of polycrystalline quartz are sparsely scattered through the rock.
Hornblende is present in significant amount as anhedral pleochroic green grains mostly -0.4-0.6 mm in size. Biotite occurs in comparable amount, forming randomly oriented platy flakes pleochroic from reddish tan brown to very pale yellow (ie relatively reduced composition).
Opaques occur in minor amount as equant to anhedral or lobate grains. They most likely are Fe-Ti oxides (?magnetite, ?ilmenite).
Apatite forms minor small prisms in quartz and K-feldspar.
Zircon is present in trace amount as small terminated prisms in close association with hornblende, biotite, opaques and plagioclase.
Clays occur in minor amount as very fine-grained pale replacements around and within the ferromagnesian grains.
Mason Geoscience Pty Ltd Report #3076 Mithril Resources Ltd INTERPRETATION
This sample formed as an intrusive igneous rock, broadly of granodioritic composition. Early-formed plagioclase, hornblende and Fe-Ti oxides were followed in the crystallisation interval by K-feldspar, quartz, biotite and accessory apatite.
Particular textures suggest that the magma had an atypical crystallisation history. The presence of uncommon but very large quartz grains, and minor large plagioclase prisms, suggests that phenocrysts from another magma may have been incorporated into a more basic magma. Similarly, the presence of scattered poikilitic K-feldspar grains with small quartz inclusions suggests that the final stages of crystallisation were relatively rapid.
A small amount of clay formed in response to weathering processes.
Mason Geoscience Pty Ltd Report #3076 Mithril Resources Ltd SAMPLE : AYAC02 (Andrew Young Hills, Northern Territory)
SECTION NO : AYAC02
HAND SPECIMEN : The aircore rock sample represents a massive, medium-grained, mesocratic crystalline rock composed of waxy grey feldspar grains and dark green ferromagnesian grains.
ROCK NAME : Hornblende-biotite-pyroxene quartz monzodiorite
PETROGRAPHY :
A visual estimate of the modal mineral abundances gives the following:
Mineral Vol % Origin
Plagioclase lgneous Quartz lgneous K-feldspar (microcline) lgneous Clinopyroxene (augite) lgneous Hornblende lgneous Biotite lgneous Opaques (mainly ?Fe-Ti oxides) lgneous Apatite lgneous Zircon lgneous
In thin section, this sample displays a massive mesocratic igneous texture.
Plagioclase is abundant, forming twinned prismatic crystals mostly -0.4-0.6 mm long, and less common larger blocky prisms with strongly zoned cores mantled by thick weakly zoned rims. The plagioclase is quite fresh.
Quartz occurs in moderate amount as clear angular interstitial grains scattered uniformly through the rock. Local small ovoid quartz grains are enclosed in larger poikilitic K-feldspar grains with 'tartan' twinning (ie microcline). Anhedral interstitial discrete grains of K-feldspar also are present.
Colourless clinopyroxene (augite) formed blocky prisms -0.2-1.0 mm long. Most have been mantled by pleochroic green hornblende. Randomly oriented plates of biotite, pleochroic from reddish tan brown to straw yellow (ie relatively reduced composition), in turn overgrow the hornblende and pyroxene. Much of the biotite also lies in textural equilibrium with K-feldspar and quartz.
Opaques occur in moderate amount as blocky, anhedral and lobate grains that tend to occur as inclusions within biotite and also in hornblende.
Apatite occurs in significant amount for an accessory phase, forming acicular colourless prisms that lace the interstitial K-feldspar and quartz.
Zircon forms small squat prisms in quartz and K-feldspar.
INTERPRETATION :
This sample formed as a monzodioritic intrusive igneous rock. Plagioclase and clinopyroxene formed as early phases, and the clinopyroxene was quickly overgrown by later-formed hornblende, biotite and opaques (Fe-Ti oxides). Interstices were finally filled by quartz, K-feldspar, biotite, apatite and zircon. Some poikilitic patches of K-feldspar and quartz provide a textural link with AYACOI.
Mason Geoscience Pty Ltd Report #3076 M~thrilResources Ltd SAMPLE : AYAC04 (Andrew Young Hills, Northern Territory)
SECTION NO : AYAC04
HAND SPECIMEN: The aircore rock fragment represents a massive, fine-grained, dark grey crystalline rock.
The sample responds weakly to the hand magnet, suggesting minor magnetite is present.
ROCK NAME : Two-pyroxene-biotite microdiorite
PETROGRAPHY :
A visual estimate of the modal mineral abundances gives the following:
Mineral Vol % Origin
Plagioclase 56 Igneous Orthopyroxene (hypersthene) 20 Igneous Clinopyroxene (augite) 10 Igneous Biotite 10 Igneous Opaques (?Fe-Ti oxides, includes magnetite) 2 Igneous Quartz 2 Igneous Apatite Tr Igneous
In thin section, this sample displays a massive intergranular igneous texture.
Plagioclase is abundant, forming randomly oriented twinned prisms mostly -0.4-0.6 mm long.
Two pyroxenes are distinguished:
i) Orthopyroxene is moderately abundant, forming subhedral elongate prismatic crystals that tend to occupy interstices between the plagioclase prisms. Locally, orthopyroxene forms larger prisms. All display weak pleochroism from pale pink to colourless, consistent with a hypersthenic composition.
ii) Clinopyroxene forms small blocky to prismatic crystals, similar in size and shape to the intergranular orthopyroxene crystals. The clinopyroxene is distinguished by its lack of pleochroism, inclined extinction and higher birefringence compared with the orthopyroxene.
Biotite occurs in significant amount as pleochroic reddish brown to straw yellow flakes that tend to occur in interstices with minor interstitial clear quartz. Small opaque grains tend to occur within the biotite flakes, and most likely are Fe-Ti oxides including magnetite and probably ilmenite.
Apatite occurs in trace amount as tiny acicular prisms in the interstitial quartz.
INTERPRETATION :
This sample formed as a dioritic intrusive igneous rock. Early-formed plagioclase, orthopyroxene and clinopyroxene were followed by late-formed biotite, opaques (?magnetite, ?ilmenite), quartz and trace apatite. The relatively fine-grained texture compared with other rocks in this suite suggests that it may have formed as a more quickly cooled magma at the margins of a larger intrusive body, or within a discrete smaller intrusive body.
The high abundance of mafic minerals and the intergranular texture provide links to basic igneous magma. In particular, the high abundance of orthopyroxene provides a link to tholeiitic basaltic magma. The dioritic magma may have formed by a small degree of differentiation of a tholeiitic basaltic magma.
Mason Geoscience Ply Ltd Report #3076 -9- Mithril Resources Ltd SAMPLE : AYACOG (Andrew Young Hills, Northern Territory)
SECTION NO : AYACO6
HAND SPECIMEN : The aircore rock fragment represents a massive, fine- to medium-grained, mesocratic crystalline rock composed of waxy pale grey feldspar grains and lesser uniformly distributed black ferromagnesian grains. Captured in the rock is a single large ovoid felsic fragment (?xenolith) -1 cm in diameter composed mainly of cream feldspar.
ROCK NAME : Partly weathered, xenolithic biotite monzogranite
PETROGRAPHY :
A visual estimate of the modal mineral abundances gives the following:
Mineral Vol % Origin
Felsic granitoid xenolith Plagioclase (includes ?illitic clay) lgneous 1 (includes trace weathering 3) Hornblende lgneous 1 K-feldspar (microcline) lgneous 1 Quartz lgneous 1
Biotite monzogranite K-feldspar (microcline) lgneous 2 Plagioclase lgneous 2 Quartz lgneous 2 Biotite lgneous 2 Opaques (?Fe-Ti oxides) lgneous 2 Apatite lgneous 2 Zircon lgneous 2 Clays Weathering 3
In thin section, this sample displays different textures and mineralogies in the xenolith and host granitoid.
Felsic granitoid xenolith displays a heterogeneous coarse- to medium-grained granitoid texture. Plagioclase forms large crystals many millimetres in size, which in places have suffered partial replacement by ragged patches of K-feldspar (microcline). Pleochroic dark green to olive hornblende forms equant euhedral prisms at margins, or within embayments, of the large plagioclase crystals. Clear quartz forms optically continuous grains that occupy embayments or patches within the plagioclase crystals, and smaller anhedral grains form granular aggregates with microcline in interstices between plagioclase and hornblende crystals.
Biotite monzogranite displays a massive, fine- to medium-grained granitoid texture. Quartz and K-feldspar are abundant: quartz forms anhedral grains in equilibrium texture with adjacent K-feldspar grains ('tartan' twinned microcline), and small optically continuous ovoids enclosed within K-feldspar grains. Plagioclase occurs in lesser amount as small twinned prismatic crystals. Biotite occurs in moderate abundance as randomly oriented platy crystals: they were originally pleochroic in browns, but all have suffered the effects of weathering, causing drab greenish discolouration of all plates and also causing partial replacement by very fine-grained clays along cleavage traces and in larger patches.
Tiny equant to lath-like opaque crystals are sparsely and irregularly sprinkled through the quartz and K-feldspar: the lath-like shapes of many of these crystals suggest that they are mainly ?ilmenite.
Apatite occurs in trace amount as tiny colourless prisms in quartz and K-feldspar.
Zircon forms small stumpy prisms within, or at margins of, the biotite plates.
Mason Geoscience Pty Ltd Report #3076 -10- Mithril Resources Ltd INTERPRETATION
This sample formed as a mesocratic intrusive granitoid of monzogranitic composition. It crystallised to form the massive assemblage of early-formed plagioclase and biotite, followed by abundant K-feldspar, quartz, minor plagioclase and accessory apatite. A significant amount of the quartz and K-feldspar crystallised as intergrown spongy to poikilitic composite grains, suggesting that final stages of crystallisation were relatively rapid.
Prior to emplacement, the granitoid magma captured a xenolith of felsic granitoid origin. It originally formed as a coarse-grained assemblage of plagioclase + minor hornblende + quartz + K-feldspar. The origin of this xenolith is obscure, but it may have formed by crystallisation of late silicate residual liquid in a metaluminous (possibly basaltic to dioritic) magma. It appears to be genetically unrelated to the enclosing biotite quartz monzonite rock, an interpretation supported by the apparent partial reaction between the xenolith and enclosing magma which generated embayments in the coarse plagioclase and partial replacement by K-feldspar.
At a much later time, circulation of near-surface meteoric waters caused partial replacement of biotite by clays.
Mason Geoscience Pty Ltd Report #3076 Mithrrl Resources Ltd SAMPLE : AYAC07 (Andrew Young Hills, Northern Territory)
SECTION NO : AYAC07
HAND SPECIMEN : The aircore rock fragment represents a massive, coarse-grained, mafic crystalline rock that has suffered partial weathering to generate irregularly distributed yellow to brown ferruginous staining.
ROCK NAME : Partly weathered, hornblende-biotitediorite
PETROGRAPHY :
A visual estimate of the modal mineral abundances gives the following:
Mineral Vol % Origin
Plagioclase lgneous 1 Biotite (includes goethite staining) lgneous 1 (includes weathering 3) Hornblende lgneous 1 Quartz lgneous 1 Opaques (?Fe-Ti oxides) lgneous 1 Apatite lgneous 1 Actinolite Deuteric alteration 2 Epidote Deuteric alteration 2 Clay (illitic) Weathering 3 (after plagioclase)
In thin section, this sample displays a massive, coarse-grained crystalline texture modified by weak deuteric alteration and moderately strong selective pervasive weathering effects.
Plagioclase was abundant, forming randomly oriented stumpy prismatic crystals mostly -2 mm long. All have suffered partial to severe replacement by tiny colourless phyllosilicate flecks inferred to be illitic clay of weathering origin.
Amphiboles are moderately abundant, and two types are distinguished:
i) Most occurs as actinolite, which forms pleochroic pale green anhedral grains to subhedral stumpy prisms. These occur as dense replacement mosaics pseudomorphous after precursor subhedral blocky primary ferromagnesian crystals -2-3 mm in size. The actinolite is inferred to have replaced pyroxene, but none is preserved for confirmation.
ii) A smaller amount of amphibole occurs as pleochroic dark green mantles on the actinolite-altered ?pyroxene crystals, and also forms ragged replacements within the actinolite-altered crystal sites. This is inferred to represent primary igneous amphibole.
Biotite occurs in significant amount as randomly oriented platy flakes. Some form discrete crystals intergrown with minor interstitial clear quartz, but others form denser biotite-rich aggregates. All of the biotite has suffered modification in response to weathering, causing loss of primary pleochroism and partial to severe replacement by dark reddish brown goethite.
Opaques occur in minor amount as small equant to elongate subhedral grains, enclosed in hornblende and biotite. They most likely represent primary Fe-Ti oxide grains, but may have been modified by weathering effects.
Epidote occurs in trace amount as small pale yellow to colourless subhedral to anhedral grains within the plagioclase crystal sites.
Apatite is present in trace amount as acicular prisms in interstitial quartz.
Mason Geoscience Pty Ltd Report #3076 -12- Mithril Resources Ltd INTERPRETATION :
This sample formed as a moderately mafic intrusive igneous rock of dioritic composition. Early-formed plagioclase and ?pyroxene were the principal minerals, with minor late-formed biotite, Fe-Ti oxides, quartz and trace apatite. During cooling, actinolite formed by replacement of the primary ?pyroxene, and a trace amount of epidote formed by replacement of plagioclase.
After uplift and erosion, near-surface weathering generated ?illitic clay after plagioclase and goethite after biotite.
Mason Geoscience Pty Ltd Report #3076 Mithril Resources Ltd SAMPLE : AYAC08 (Andrew Young Hills, Northern Territory)
SECTION NO : AYAC08
HAND SPECIMEN: The aircore rock sample has captured two different rock fragments: one fragment represents a coarse-grained, massive, mafic crystalline rock comparable with sample AYAC07, and another fragment represents massive waxy pinkish cream felsic rock.
ROCK NAMES : Partly weathered, hornblende-biotite quartz diorite
Partly weathered, micro-biotite rnonzogranite with syenogranite ?dykelet
PETROGRAPHY :
A visual estimate of the modal mineral abundances gives the following:
Mineral Vol Oh Origin
Partly weathered, hornblende-biotite quartz diorite Plagioclase 17 lgneous 1 Hornblende 20 lgneous 1 Biotite (includes goethite) 10 lgneous 1 (includes weathering 3) Quartz 5 lgneous 1 K-feldspar 2 lgneous 1 Opaques (?Fe-Ti oxide) < 1 lgneous 1 Apatite Tr lgneous 1 Actinolite 15 Deuteric alteration 2 Epidote Tr Deuteric alteration 2 Clay (?illitic) 30 Weathering
Partly weathered, micro-biotite monzogranite with syenogranite ?dykelet Micro-biotite monzogranite K-feldspar (microcline) 32 Igneous 1 Quartz 40 Igneous 1 Plagioclase (includes ?illitic clay) 25 igneous 1 (includes weathering 3) Biotite (includes goethite) 2 Igneous 1 (includes weathering 3) Opaques (?Fe-Ti oxide)
Syenogranite ?dykelet K-feldspar (microcline) 58 Igneous 1 Quartz 30 Igneous 1 Plagioclase (includes ?illitic clay) 10 Igneous 1 (includes weathering 3) Biotite (includes goethite) Tr Igneous 1 (includes weathering 3) Opaques (?Fe-Ti oxide) Tr Igneous 1 Sphene Tr Igneous 1 Epidote Tr Deuteric alteration 2
In thin section, this sample displays different textures and mineralogies in the two rock fragments.
Partly weathered, hornblende-biotite quartz diorite displays a massive coarse-grained igneous texture. Plagioclase formed large randomly oriented prisms -2-3 mm long. All have suffered partial to severe replacement by minute colourless phyllosilicate flecks (?illitic clay), and uncommon small anhedral pale yellow to colourless epidote grains occur in some of the plagioclase crystals.
Pleochroic dark green hornblende is moderately abundant, forming subhedral prismatic crystals in interstitial areas, and also ragged mantles on paler green actinolite aggregates after precursor primary ?pyroxene.
Mason Geoscience Pty Ltd Report #3076 -14- Milhril Resources Ltd Biotite forms small flakes and aggregates in interstitial areas, but all have suffered some staining by goethite in response to weathering. In the interstices, the biotite is associated with clear anhedral quartz grains and less abundant K-feldspar which forms micrographic intergrowths with the quartz. Apatite forms acicular prismatic crystals that lace the quartz and K-feldspar. Opaques (?Fe-Ti oxides) occur in minor amount as subhedral grains within biotite.
Partly weathered, micro-biotite monzogranite with syenogranite ?dykelet displays varied textures within the one fragment. Most appears to represent a massive micro-biotite monzogranitic rock, composed of small equant blocky quartz crystals, anhedral K-feldspar grains, anhedral plagioclase grains partly flecked by minute ?illitic clays, small pleochroic dark brown to yellow biotite flakes, and scattered small opaques (?Fe-Ti oxide). Epidote occurs in trace amount as small ragged grains associated with plagioclase.
Coarser-grained felsic granitoid (syenogranite) displays a non-sharp contact with the micro-biotite monzogranite. It is dominated by large blocky to subhedral K-feldspar crystals (microcline), lesser clear anhedral interstitial quartz, and minor anhedral plagioclase grains. Biotite forms uncommon small scattered flakes, opaques occur as rare small grains, and sphene forms uncommon pleochroic ragged grains. Epidote occurs as rare small grains in plagioclase.
INTERPRETATION
This sample has captured different rock types of varied igneous origins.
Quartz diorite formed the coarse-grained assemblage of early-formed plagioclase and ?pyroxene, followed by interstitial hornblende, biotite, minor quartz, K-feldspar and trace opaques (?Fe-Ti oxide) and apatite. Deuteric alteration generated actinolite after ?pyroxene and trace epidote after plagioclase. This rock is closely similar to AYAC07, but is somewhat more fractionated in containing more quartz and interstitial micrographic intergrowths.
Fine-grained micro-biotite monzogranite formed the massive assemblage of K-feldspar, quartz, plagioclase, and minor biotite and opaques (?Fe-Ti oxide). Its fine grain size suggests it formed in a relatively small body (eg dyke). Also present in this rock is a band or ?dykelet of syenogranitic composition: it crystallised to form the coarser-grained leucocratic assemblage of K-feldspar, quartz, plagioclase, and trace biotite, opaques (?Fe-Ti oxide) and sphene. Trace epidote formed by replacement of plagioclase. It is likely that the syenogranitic ?dykelet formed from the parental monzogranitic host magma by late magmatic segregation.
Mason Geoscience Pty Ltd Report #3076 Mithril Resources Ltd 4 REFERENCES
Collins W.J. 1998. Evaluation of petrogenetic models for Lachlan Fold Belt granitoids: implications for crustal architecture and tectonic models. Australian Journal of Earth Sciences 45. 483-500.
Hoatson D. and Stewart A. 2001. Field investigations of Proterozoic mafic-ultramafic intrusions in the Arunta Province, central Australia. Geoscience Australia Record 2001139.
MacColl R.S. (1964). Geochemical and structural studies In batholithic rocks of Southern California: Part I Structural geology of Rattlesnake Mountain Pluton. Geological Society of America Bulletin 75, 805-822.
Streckeisen, A. 1976. To each plutonic rock its proper name. Earth-Science Reviews 12, 1-33.
Vernon R.H., Etheridge M.E. and Wall V.J. 1988. Shape and microstructure of rnicrogranitoid enclaves: indicators of magma mingling and flow. Lithos 22, 1-1 1.
Mason Geoscience Pty Ltd Report #3076 Mithril Resources Ltd Appendix 4
Drill hole Assays Drill sample assays (ascii dump) Drill sample assays (asci~dump) Dnll sample assays (asc~~dump) Dr~llsample assays (ascl~dump)