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Broken Hill — A Precambrian hot spot? — a comment
Precambrian Research, 4 (1977) 215--220
215
© Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands Discussion
B R O K E N HILL .... A P R E C A M B R I A N HOT SPOT? - A COMMENT
G.N. PHILLIPS Monash University, Department of Earth Sciences, Clayton, Vic. (Australia)
On the basis of metamorphic grade variations, Katz (1976) has concluded that the "Broken Hill Basin" (B.H.B.) may be a former hot spot; and that mineralization is closely related to high grade metamorphism. Necessarily these conclusions depend on an accurate metamorphic synthesis of the Willyama Complex. Attention to detail, however, reveals that most of the evidence does n o t support Katz's conclusions, because: (1) the B.H.B. of Katz is n o t significantly different lithologically, structurally or metamorphically from its surrounds; (2) incomplete sampling around the "B.H.B." by Katz erroneously suggests it is a local high grade area, when it is not; (3) metamorphic conditions (apart from temperature) and rock composition have n o t been adequately considered; (4) Katz uses ill-defined criteria for the recognition of granulite facies rocks. The area Katz designates as the B.H.B. differs little from adjacent parts of the Willyama Complex. Laing (pers. comm., 1975) has shown the presence of the Broken Hill Mine Sequence to the NW of the Globe Vauxhall Shear Zone -a supposed transform fault of Katz. With the exception of banded iron formations (B.I.F.), the set of lithologies considered peculiar to the B.H.B. by Katz, can be found at many localities (Allendale, Southern Cross, Parnell, Angus). At Maybell, 25 km north of Broken Hill, a similar group of lithologies with B.I.F., is found in Amphibolite Facies rocks (Bunting, 1975). Marjoribanks (personal communication, 1975) has shown the structure of the "B.H.B." to be a complex series of folds and retrograde zones with similar structural history to other parts of the Willyama Complex. However, the major objection to the h o t spot model is based on metamorphic criteria. Before discussing granulite h o t spots it is important to have a clear definition of the Granulite Facies. In the past, this has been done in several ways: (1) appearance of t w o pyroxene--plagioclase assemblages in mafic rocks (Turner, 1968); (2) a textural and mineralogical definition (Winkler, 1974); and (3) existence of hypersthene, or clinopyroxene--almandine--quartz (Winkler, 1974). Katz has presumably chosen this definition. As in many terrains, it is the basic rocks that most readily indicate former
216
granulite facies conditions around Broken Hill. Many of the basic rocks are particularly Fe-rich (up to 25 wt.% Fe as FeO -- Hobbs, 1975) and contain garnet + cummingtonite in the Amphibolite Facies. At higher grades, these Fe rich basic rocks with one pyroxene, hypersthene, are "granulites" according to definition (3) but not necessarily (1) or (2). Often nearby lithologies indicate Amphibolite Facies conditions. This compositional control over apparent metamorphic grade is well illustrated at Black Bluff (B.H.B.) where Mg-rich prograde hornblende-plagioclase assemblages are interlayered with Fe-rich two pyroxene rocks. For the purpose of the following discussion definition (1) will be adopted, keeping in mind that it may also have shortcomings. There is a close correlation between the distribution of Fe-rich basic rocks, the "granulites" of Katz and mineralization. This is interpreted by this author as primarily a function of premetamorphic events. The Fe rich basic metamorphics have no unaltered chemical counterpart among igneous rocks, but are equivalent to premetamorphically altered basalts (Hobbs, 1975). Field observations support this conclusion. Mineralization is associated with these areas of premetamorphic alteration (Fe enrichment) and thus with pyroxene assemblages.
WILLYAMA COMPLEX
31 ° 3,0'
0
I0
20
KM
J
rl
O
O
•
I
D
•
•
Q
O
B
O
Q
Q
O
Q
•
O
I
O
Q
O
•
6
•
°
•
•
•
~2
b
~
i
•
•
•
°
•
•
•
•
•
o
• • • " " " "1
I ANDALUST IE ~SILM LU IM AO NV IIT SC T EE- I rT1] GARNET~ KFELDSPAR 141 °
Dotted! GRANULI TE 141=30' |
Fig. 1. M e t a m o r p h i c z o n a l d i s t r i b u t i o n a r o u n d B r o k e n H i l l , W i l l y a m a C o m p l e x . H i l l B a s i n " o f K a t z ( 1 9 7 6 ) is s h o w n .
"Broken
217
In an area such as Broken Hill, where several deformations and periods of retrograde metamorphism have occurred (Rutland and Etheridge, 1975) the local paucity of two pyroxene rocks does n o t indicate an area has never attained granulite grade. Inspection of a regional map of the Willyama (AIMM, 1968) shows the " h o t spots" to be in exactly the areas where basic rocks are most prominent, namely L.B.H. (Little Broken Hill) B.H.B. and Centennial-Parnell--Champion area. Some of the surrounding areas that lack basic rocks (e.g. directly northwest of Broken Hill) may n o t show obvious signs of granulite facies metamorphism even though they have reached those grades (N.B., cordierite is not diagnostic of the granulite facies c.f. Katz). A necessary criterion for postulating hot spots is that areas of high grade metamorphism are surrounded by lower grade areas. Granulite facies rocks exist on three sides of the B.H.B. (Fig.1 and Phillips, G.N. in prep.). This metamorphic map is broadly similar to Binns' (1964) and indicates the maximum metamorphic grade attained at places throughout the Willyama. It is based on a more thorough sampling programme than fig.2 of Katz and recognises the overwhelming importance of regional and local retrogression. The L.B.H. " h o t s p o t " is also surrounded by granulite facies lithologies, even to the SE. In this direction outcrop is poor, b u t careful sampling (Arden, 1974) has shown that t w o pyroxene assemblages continue several kilometers past the boundary of Katz. To illustrate the importance of thorough sampling, one hundred and seven basic rocks from R o u n d Hill were examined (collected b y Clemens, 1974). This area has well preserved high grade basic and pelitic lithologies, yet thin sections revealed that only 22% have t w o pyroxenes, 27% have pseudomorphous textures that suggest former pyroxenes and 51% have retrograde Amphibolite Facies assemblages. Pyroxene assemblages are less c o m m o n still in some areas southeast of Broken Hill. When retrogression is taken into account and Fig.1 used, Katz's objections to progressive metamorphism dissolve. Naturally retrograde low grade rocks will occur in the granulite facies b u t no prograde low grade rocks have been found here. Apart from a gravity anomaly the B.H.B. seems to have no peculiarities that suggest its existence: certainly it is n o t "faulted into contact with lower grade rocks". The Willyama Complex is considered to be a Low-Intermediate Pressure terrain (Vernon, 1967) in which high geothermal gradients existed. Thus at moderate depths, extensive areas of granulite assemblages formed. Application of the Wood and Banno (1973) two-pyroxene geothermometer and other solid-solid reactionsindicates that marked temperature gradients were n o t developed in high grade areas, as would be required in Katz' model. Clearly any relationship between mineralization and metamorphic grade requires that the latter is understood. In this respect, fig.2 of Katz is misleading. Most areas of "Broken Hill" t y p e mineralization occur in granulite facies rocks b u t certainly n o t all (Allendale). The appearance of granulite facies assemblages is clearly temperature d e p e n d e n t b u t at Broken Hill, water activity and
218 c h e m i c a l c o m p o s i t i o n are at least as i m p o r t a n t . T h u s Katz " h o t s p o t s " are m p a r t just " d r y s p o t s " and " F e s p o t s " . Fig.1 shows these spots d o n o t exist at all. ACKNOWLEDGEMENT Mr. V.J. Wall has o f f e r e d m u c h h e l p f u l advice on this s u b j e c t and considera b l y i m p r o v e d the m a n u s c r i p t b y his c o n s t r u c t i v e criticisms.
REFERENCES
Arden, P.J., 1974. Structural, Petrological and Economic Geology Aspects of the Little Broken Hill Area, New South Wales. Thesis, Monash Univ., Clayton, Vic., unpubh Australian Institute Mining Metallurgy, 1968. Geological Map of the Broken Hill District. Binns, R.A., 1964. Zones of progressive regional metamorphism in the Willyama Complex, Broken Hill District, New South Wales. J. Geol. Soc. Aust., 11: 283--330. Bunting, W.F., 1975. Geology of the Maybell Group of Mines and Old Mt. Gipps Homestead Area, Broken Hill. Thesis, Monash Univ., Clayton, Vic., unpuhl. Clemens, J.C., 1974. The Round Hill Area, Broken Hill, N.S.W. Thesis, Monash Univ., Clayton, Vic., unpubl. Hobbs, B.E., 1975. The Broken Hill Lode Horizon Project. Unpubl. Rep. on second year of work to B.H. Mine Managers' Association. Katz, M.B., 1976. Broken Hill -- A Precambrian hot spot? Precambrian Res., 3: 91--106. Rutland, R.W.R. and Etheridge, M.A., 1975. Two high grade schistosities at Broken Hill and their relation to major and minor structures. J. Geol. Soc. Aust., 22: 259--274. Turner, F,J., 1968. Metamorphic Petrology: Mineralogical and Field Aspects. McGraw-Hill, New York, N.Y. Vernon, R.H., 1969. The Willyama Complex, Broken Hill area. J. Geol. Soc. Aust., 16: 20--55. Winkler, H.G.F., 1974. Petrogenesis of Metamorphic Rocks. Springer Verlag, Berlin, 3rd ed. Wood, B.J. and Banno, S., 1973. Garnet--orthopyroxene and orthopyroxene--clinopyroxene relationships in simple and complex systems. Contrib. Miner. Petrol., 42: 109--124.
B R O K E N H I L L - - A P R E C A M B R I A N H O T S P O T ? -" A R E P L Y
M.B. KATZ School of Applied Geology, University of New South Wales, Kensington, N.S.W. (Australia)
Phillips ( 1 9 7 6 ) q u e s t i o n s t h e c o n c e p t o f a h o t spot c e n t r e d a r o u n d the B r o k e n Hill Basin (B.H.B.) (Katz, 1 9 7 6 ) . His a r g u m e n t s suggest t h a t m o s t o f the Wfllyama C o m p l e x is o f granulite facies grade {Phillips, 1 9 7 6 , f i g . l ) , w h i c h is e n t i r e l y misleading, and t h a t the B . H . B , c a n n o t be distinguished as a high grade m e t a m o r p h i c spot. Most o f his c o n s i d e r a t i o n s derive f r o m u n p u b l i s h e d
215
© Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands Discussion
B R O K E N HILL .... A P R E C A M B R I A N HOT SPOT? - A COMMENT
G.N. PHILLIPS Monash University, Department of Earth Sciences, Clayton, Vic. (Australia)
On the basis of metamorphic grade variations, Katz (1976) has concluded that the "Broken Hill Basin" (B.H.B.) may be a former hot spot; and that mineralization is closely related to high grade metamorphism. Necessarily these conclusions depend on an accurate metamorphic synthesis of the Willyama Complex. Attention to detail, however, reveals that most of the evidence does n o t support Katz's conclusions, because: (1) the B.H.B. of Katz is n o t significantly different lithologically, structurally or metamorphically from its surrounds; (2) incomplete sampling around the "B.H.B." by Katz erroneously suggests it is a local high grade area, when it is not; (3) metamorphic conditions (apart from temperature) and rock composition have n o t been adequately considered; (4) Katz uses ill-defined criteria for the recognition of granulite facies rocks. The area Katz designates as the B.H.B. differs little from adjacent parts of the Willyama Complex. Laing (pers. comm., 1975) has shown the presence of the Broken Hill Mine Sequence to the NW of the Globe Vauxhall Shear Zone -a supposed transform fault of Katz. With the exception of banded iron formations (B.I.F.), the set of lithologies considered peculiar to the B.H.B. by Katz, can be found at many localities (Allendale, Southern Cross, Parnell, Angus). At Maybell, 25 km north of Broken Hill, a similar group of lithologies with B.I.F., is found in Amphibolite Facies rocks (Bunting, 1975). Marjoribanks (personal communication, 1975) has shown the structure of the "B.H.B." to be a complex series of folds and retrograde zones with similar structural history to other parts of the Willyama Complex. However, the major objection to the h o t spot model is based on metamorphic criteria. Before discussing granulite h o t spots it is important to have a clear definition of the Granulite Facies. In the past, this has been done in several ways: (1) appearance of t w o pyroxene--plagioclase assemblages in mafic rocks (Turner, 1968); (2) a textural and mineralogical definition (Winkler, 1974); and (3) existence of hypersthene, or clinopyroxene--almandine--quartz (Winkler, 1974). Katz has presumably chosen this definition. As in many terrains, it is the basic rocks that most readily indicate former
216
granulite facies conditions around Broken Hill. Many of the basic rocks are particularly Fe-rich (up to 25 wt.% Fe as FeO -- Hobbs, 1975) and contain garnet + cummingtonite in the Amphibolite Facies. At higher grades, these Fe rich basic rocks with one pyroxene, hypersthene, are "granulites" according to definition (3) but not necessarily (1) or (2). Often nearby lithologies indicate Amphibolite Facies conditions. This compositional control over apparent metamorphic grade is well illustrated at Black Bluff (B.H.B.) where Mg-rich prograde hornblende-plagioclase assemblages are interlayered with Fe-rich two pyroxene rocks. For the purpose of the following discussion definition (1) will be adopted, keeping in mind that it may also have shortcomings. There is a close correlation between the distribution of Fe-rich basic rocks, the "granulites" of Katz and mineralization. This is interpreted by this author as primarily a function of premetamorphic events. The Fe rich basic metamorphics have no unaltered chemical counterpart among igneous rocks, but are equivalent to premetamorphically altered basalts (Hobbs, 1975). Field observations support this conclusion. Mineralization is associated with these areas of premetamorphic alteration (Fe enrichment) and thus with pyroxene assemblages.
WILLYAMA COMPLEX
31 ° 3,0'
0
I0
20
KM
J
rl
O
O
•
I
D
•
•
Q
O
B
O
Q
Q
O
Q
•
O
I
O
Q
O
•
6
•
°
•
•
•
~2
b
~
i
•
•
•
°
•
•
•
•
•
o
• • • " " " "1
I ANDALUST IE ~SILM LU IM AO NV IIT SC T EE- I rT1] GARNET~ KFELDSPAR 141 °
Dotted! GRANULI TE 141=30' |
Fig. 1. M e t a m o r p h i c z o n a l d i s t r i b u t i o n a r o u n d B r o k e n H i l l , W i l l y a m a C o m p l e x . H i l l B a s i n " o f K a t z ( 1 9 7 6 ) is s h o w n .
"Broken
217
In an area such as Broken Hill, where several deformations and periods of retrograde metamorphism have occurred (Rutland and Etheridge, 1975) the local paucity of two pyroxene rocks does n o t indicate an area has never attained granulite grade. Inspection of a regional map of the Willyama (AIMM, 1968) shows the " h o t spots" to be in exactly the areas where basic rocks are most prominent, namely L.B.H. (Little Broken Hill) B.H.B. and Centennial-Parnell--Champion area. Some of the surrounding areas that lack basic rocks (e.g. directly northwest of Broken Hill) may n o t show obvious signs of granulite facies metamorphism even though they have reached those grades (N.B., cordierite is not diagnostic of the granulite facies c.f. Katz). A necessary criterion for postulating hot spots is that areas of high grade metamorphism are surrounded by lower grade areas. Granulite facies rocks exist on three sides of the B.H.B. (Fig.1 and Phillips, G.N. in prep.). This metamorphic map is broadly similar to Binns' (1964) and indicates the maximum metamorphic grade attained at places throughout the Willyama. It is based on a more thorough sampling programme than fig.2 of Katz and recognises the overwhelming importance of regional and local retrogression. The L.B.H. " h o t s p o t " is also surrounded by granulite facies lithologies, even to the SE. In this direction outcrop is poor, b u t careful sampling (Arden, 1974) has shown that t w o pyroxene assemblages continue several kilometers past the boundary of Katz. To illustrate the importance of thorough sampling, one hundred and seven basic rocks from R o u n d Hill were examined (collected b y Clemens, 1974). This area has well preserved high grade basic and pelitic lithologies, yet thin sections revealed that only 22% have t w o pyroxenes, 27% have pseudomorphous textures that suggest former pyroxenes and 51% have retrograde Amphibolite Facies assemblages. Pyroxene assemblages are less c o m m o n still in some areas southeast of Broken Hill. When retrogression is taken into account and Fig.1 used, Katz's objections to progressive metamorphism dissolve. Naturally retrograde low grade rocks will occur in the granulite facies b u t no prograde low grade rocks have been found here. Apart from a gravity anomaly the B.H.B. seems to have no peculiarities that suggest its existence: certainly it is n o t "faulted into contact with lower grade rocks". The Willyama Complex is considered to be a Low-Intermediate Pressure terrain (Vernon, 1967) in which high geothermal gradients existed. Thus at moderate depths, extensive areas of granulite assemblages formed. Application of the Wood and Banno (1973) two-pyroxene geothermometer and other solid-solid reactionsindicates that marked temperature gradients were n o t developed in high grade areas, as would be required in Katz' model. Clearly any relationship between mineralization and metamorphic grade requires that the latter is understood. In this respect, fig.2 of Katz is misleading. Most areas of "Broken Hill" t y p e mineralization occur in granulite facies rocks b u t certainly n o t all (Allendale). The appearance of granulite facies assemblages is clearly temperature d e p e n d e n t b u t at Broken Hill, water activity and
218 c h e m i c a l c o m p o s i t i o n are at least as i m p o r t a n t . T h u s Katz " h o t s p o t s " are m p a r t just " d r y s p o t s " and " F e s p o t s " . Fig.1 shows these spots d o n o t exist at all. ACKNOWLEDGEMENT Mr. V.J. Wall has o f f e r e d m u c h h e l p f u l advice on this s u b j e c t and considera b l y i m p r o v e d the m a n u s c r i p t b y his c o n s t r u c t i v e criticisms.
REFERENCES
Arden, P.J., 1974. Structural, Petrological and Economic Geology Aspects of the Little Broken Hill Area, New South Wales. Thesis, Monash Univ., Clayton, Vic., unpubh Australian Institute Mining Metallurgy, 1968. Geological Map of the Broken Hill District. Binns, R.A., 1964. Zones of progressive regional metamorphism in the Willyama Complex, Broken Hill District, New South Wales. J. Geol. Soc. Aust., 11: 283--330. Bunting, W.F., 1975. Geology of the Maybell Group of Mines and Old Mt. Gipps Homestead Area, Broken Hill. Thesis, Monash Univ., Clayton, Vic., unpuhl. Clemens, J.C., 1974. The Round Hill Area, Broken Hill, N.S.W. Thesis, Monash Univ., Clayton, Vic., unpubl. Hobbs, B.E., 1975. The Broken Hill Lode Horizon Project. Unpubl. Rep. on second year of work to B.H. Mine Managers' Association. Katz, M.B., 1976. Broken Hill -- A Precambrian hot spot? Precambrian Res., 3: 91--106. Rutland, R.W.R. and Etheridge, M.A., 1975. Two high grade schistosities at Broken Hill and their relation to major and minor structures. J. Geol. Soc. Aust., 22: 259--274. Turner, F,J., 1968. Metamorphic Petrology: Mineralogical and Field Aspects. McGraw-Hill, New York, N.Y. Vernon, R.H., 1969. The Willyama Complex, Broken Hill area. J. Geol. Soc. Aust., 16: 20--55. Winkler, H.G.F., 1974. Petrogenesis of Metamorphic Rocks. Springer Verlag, Berlin, 3rd ed. Wood, B.J. and Banno, S., 1973. Garnet--orthopyroxene and orthopyroxene--clinopyroxene relationships in simple and complex systems. Contrib. Miner. Petrol., 42: 109--124.
B R O K E N H I L L - - A P R E C A M B R I A N H O T S P O T ? -" A R E P L Y
M.B. KATZ School of Applied Geology, University of New South Wales, Kensington, N.S.W. (Australia)
Phillips ( 1 9 7 6 ) q u e s t i o n s t h e c o n c e p t o f a h o t spot c e n t r e d a r o u n d the B r o k e n Hill Basin (B.H.B.) (Katz, 1 9 7 6 ) . His a r g u m e n t s suggest t h a t m o s t o f the Wfllyama C o m p l e x is o f granulite facies grade {Phillips, 1 9 7 6 , f i g . l ) , w h i c h is e n t i r e l y misleading, and t h a t the B . H . B , c a n n o t be distinguished as a high grade m e t a m o r p h i c spot. Most o f his c o n s i d e r a t i o n s derive f r o m u n p u b l i s h e d