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Petrographic and geochemical characteristics of iron-rich rocks and their significance in exploration for massive sulfide deposits, Bathurst, New Brunswick, Canada
Journal o f Geochemical Exploration, 19 (1983) 705--721
705
Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
PETROGRAPHIC AND GEOCHEMICAL CHARACTERISTICS OF IRON-RICH ROCKS AND THEIR SIGNIFICANCE IN EXPLORATION FOR MASSIVE SULFIDE DEPOSITS, BATHURST, NEW BRUNSWICK, CANADA 1
SAIFUL-ISLAM SAIF*
National Science and Engineering Research Council, Geological Survey o f Canada, 601 Booth Street, Ottawa, Ont. K I A OE8 (Canada) (Received May 14, 1982; accepted for publication April 3, 1983)
ABSTRACT Saif, S.-I., 1983. Petrographic and geochemical characteristics of iron-rich rocks and their significance in exploration for massive sulfide deposits, Bathurst, New Brunswick, Canada. In: G.R. Parslow (Editor), Geochemical Exploration 1982. J. Geochem. Explor., 19: 705--721. About 25 economically significant, Kuroko-type massive sulfide bodies lie in a metamorphosed volcano-sedimentary complex (probable Middle Ordovician) known as Tetagouche Group, in Bathurst area, New Brunswick. Despite unresolved structural complexities, it does appear that they were deposited during a particular phase of volcanic activity and are, therefore, contemporaneous. Most of the sulfide bodies are closely associated with iron-rich rocks representing various facies of iron formation, and together with sulfides it constitutes the "ore h o r i z o n " which, therefore, is highly magnetic. Aero-magnetic and ground-magnetic techniques are useful to locate the ore horizon but problems are created because of the occurrence of iron-rich rocks with no sulfide, along another horizon in the Tetagouche Group. Petrographic and geochemical characteristics of various types of iron-rich rocks have been studied to see if the iron-rich rocks of the ore horizon can be distinguished from the iron-rich rocks of the other horizon. The iron-rich rocks found in the Tetagouche Group can be classified into five types: ( 1 ) c h e r t y magnetitic rocks; (2)iron-rich chloritic rocks; (3) sideritic rocks; (4) basic iron formation; and (5) maroon shale. The basic iron formation, which is quite magnetic, gives a false indication of the ore horizon wherein the presence of any of the first three types of rocks is expected. Moreover, the basic iron formation is generally similar in appearance and mineralogy to some of the cherty magnetitic and chloritic rocks. Regarding major element composition, TiO2, Na~O, A1203 and CaO are higher whereas Fe203, FeO and MnO are lower in the basic iron formation than in the other iron-rich rocks. These geochemical characteristics can help distinguish the barren rocks of the basic iron formation from those of the ore horizon during the exploration programs. 1This paper is a contribution to IGCP Project 60-Correlation of Caledonian stratabound sulphides *Present address: Department of Earth Sciences, University of Petroleum & Minerals, Airport Box No. 144, UPM BOX 1853, Dhahran, Saudi Arabia. 0375-6742/83/$03.00
© 1983 Elsevier Science Publishers B.V.
706 INTRODUCTION
AND PURPOSE
OF INVESTIGATION
The discovery and mining of several economically significant, Kuroko~ t y p e massive sulfide bodies near Bathurst, New Brunswick {Fig. 1) has turned the area into an important centre of exploration activity. The sulfide bodies lie in a m e t a m o r p h o s e d volcano-sedimentary complex of probable Middle Ordovician age, known as the Tetagouche Group. Most of the stra~iform sulfide bodies lie close to a felsic pyroclastic sequence considered to be ash-flows {Boyle and Davies, 1964), and generally known as " p o r p h y r y " The association of most of the sulfide bodies with p o r p h y r y indicates that they were deposited during a particular phase of volcanic activity and are, therefore, contemporaneous. The sulfide bodies are also closely associated with various types of iron-rich rocks generally known as iron formation. McAllister (1960) proposed that the sulfides and iron-rich rocks represent various facies of chemical origin and consist of iron in its various oxidation forms. As such, the sulfide bodies and the iron-rich rocks, despite unresolved structural complexities, lie along a specific stratigraphic horizon which is generally referred to as " o r e horizon", or sulfide horizon. Structural and stratigraphic complexities as well as poor exposure have combined to create problems in exploration for the massive sulfide deposits. 72 °
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707 Geophysical techniques are useful, particularly aero-magnetic and groundmagnetic surveys which are utilized to locate the highly magnetic ore horizon. Problems in the use of magnetic techniques are created by the fact that iron-rich rocks occur also along another horizon in the Tetagouche Group but with no sulfide deposits. Such magnetic rocks give false indications of the ore horizon during magnetic surveys. The present project was initiated to study and compare the petrographic and chemical characteristics of the various types of iron-rich rocks, and to see if these characteristics can be used to distinguish the rocks associated with the sulfide bodies from the other iron-rich rocks including maroon shale. FIELD AND LABORATORY METHODS The fieldwork was mainly aimed at collecting samples from the iron-rich rocks for petrographic and geochemical analyses. Due to extreme structural and stratigraphic complexities, the relative stratigraphic positions of some of the rocks are u n k n o w n . 167 samples were collected from outcrops, diamond drill cores and underground mines from the following areas (Fig. 2): (1) Key Anacon mine. (2) Brunswick No. 6 mine. (3) Brunswick No. 12 mine. (4) Heath Steele mine. (5) Austin Brook iron mine. (6) Sabena mining property. (7) The Narrows area. (8) Nepisiguit Falls. (9) A trench dug by Atlantic Coast Copper. (10) Area about 13 km west of Caribou mine along road. Eleven major elements were determined by XRF and H20, FeO and CO2 were determined by rapid chemical m e t h o d on 122 of the samples. REGIONAL GEOLOGY OF BATHURST MINING CAMP The Bathurst--Newcastle district was described by Smith and Skinner {1958) as geologically constituted of the following three regional units (Fig. 2): (3) The Pennsylvanian--Mississippian Cover. (2) The Silurian--Devonian Folded Belt. (1) The Ordovician Folded Belt. The Ordovician F o l d e d Belt
The Ordovician Folded Belt consists of highly deformed interlayered metavolcanic and metasedimentary rocks which underlie the central part of
708
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Argillaceous sedimen~aO rockS, gre} ~acke, quartzite, conglomerate: minor [erruginous, Irlanganiferous, and carbonaceons chert and argJllite; minor !imestone, tuff m d v o l c a n i c flows ~v ~ . 40
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Fig. 2. Geology of the Bathurst-Newcastle area, New Brunswick (after J.L. Davies 1972); sample localities 1 to 10.
the district. These volcano-sedimentary strata comprise the Tetagouche Group and are generally considered to be Middle Ordovician (Poole, 1963; Neuman, 1968) but its age is still debatable (Rast and Stringer, 1974). Several attempts have been made by various investigators t o subdivide the Tetagouche Group but no stratigraphic sequence has y e t been agreed upon for the entire Tetagouche Group. Sail (1977 and in prep.) and Sail et al. (1978) have proposed an informal stratigraphic nomenclature for the area between The Narrows and the Key Anacon mine (Fig. 2). The Tetagouche Group was subjected to several phases of deformation. The complicated structural features, absence of good marker horizons and poor exposure have so far prevented a coherent structural interpretation for the entire group, but it is generally regarded as part of a Taconic folded belt that has been refolded during the Acadian Orogeny (Smith and Skinner, 1958; Neale et al., 1961; Poole, 1967; G.H. Davis, 1972). The rocks are intruded by Ordovician, and possibly also Silurian and Devonian gabbro, diabase and diorite, and by Devonian granitic masses (Skinner, 1974). This volcano-sedimentary complex contains economically significant massive sulfide bodies, as well as the iron formation and other iron-rich rocks t h a t are the subject of this investigation. For the detailed geology of the other two regional units the reader is referred to Skinner (1974). GEOLOGY AND PETROLOGY OF THE IRON-RICH ROCKS Apart from sulfides which are n o t included in this study, the following five types of iron-rich rocks have been identified:
709 (1) Cherty magnetitic rocks (oxide facies). (2) Iron-rich chloritic rocks (silicate facies). (3) Sideritic rocks (carbonate facies). (4) Basic iron formation. (5) Iron-rich maroon shale. The first three types and the sulfides occur along a single and narrow stratigraphic zone of the ore horizon and constit~lte the four facies of iron formation k n o w n as the "Austin Brook iron formation". The basic iron formation is stratigraphically above the ore horizon b u t the stratigraphic position of the maroon shale is n o t certain. It is thought, due to the abundance of hematite, that the maroon shale may be a time equivalent of the ore horizon. The rocks of the Key Anacon mine area are unique in the sense that they physically resemble cherty magnetitic and iron-rich chloritic rocks on the one hand and basic iron formation on the other. They could not be clearly classified with any of these three types on the basis of field characteristics and association, and will be treated separately. (1) Cherty magnetitic rocks The rocks are black, dark grey, light and dark red, and brown. They are usually massive and extremely hard, b u t a flaggy variety, due to local concentration of specular hematite is present. Typically, they comprise bands of alternate magnetite-hematite and quartz- or jasper-rich layers with individual layers varying from thin laminae to a b o u t 2 cm thick (Fig. 3A). The layers represent primary bedding as they are conformable with the contacts. The rocks consist mostly of magnetite and quartz. Hematite, jasper, chlorite, carbonate (calcite and siderite) and biotite are other important constituents. The relative amounts of quartz and the iron oxide minerals vary considerably. Feldspar which is present in some samples only in minor amounts, is mostly potassic. Apatite occurs in most of these rocks; some samples contain as much as 10--12%. Other accessory minerals are muscovite, epidote, zircon, sphene, titanomagnetite ilmenite, rutile, pyrite, sphalerite, and galena. Grunerite (Fig. 3B) was identified in only one sample collected from an o u t c r o p close to a huge mass of rhyolite. It is considered to be due to the c o n t a c t metamorphic effect of the rhyolite. The occurrence of these rocks with several sulphide bodies has been clearly d o c u m e n t e d (Lea and Rancourt, 1958; McAllister, 1960, Boyle and Davies, 1964; Stockwell and Tupper, 1966; J.L. Davies, 1972; Pertold, 1972; Rutledge, 1972; Whitehead, 1973; Goodfellow, 1975; Luff, 1975; McBride, 1976). (2) Iron-rich chloritic rocks The rocks are dark green, schistose and usually friable b u t some are relatively hard. Texturally, the rocks are fine-grained with well to poorly devel-
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711 oped fine laminae of chlorite-magnetite and chert of a green, pink or grey colour. Mineralogically, the rocks consist mostly of chlorite, magnetite and quartz. Chlorite c o n t e n t is as high as 64% in some samples but on average it is approximately 30%. Magnetite and quartz contents are, on average, 15 and 20%, respectively but their relative amounts vary considerably. Other main constituents are biotite and carbonates. Sericite and apatite are present as accessory minerals. Minor amounts of K-feldspar have also been detected in a few samples. Other accessory minerals are zircon, ilmenite, pyrite, chalcopyrite, pyrrhotite, sphalerite, galena and monazite. The iron-rich chloritic rocks represent the silicate facies of iron formation. They are closely associated and often interbedded with the cherty magnetitic rocks in many places. (3) Sideritic rocks The rocks are light grey and buff~ show light and dark laminae of varying thickness (Fig. 3C). The rocks are massive, hard, and have a cherty appearance. They consist mainly of a cryptocrystalline mixture of carbonate and quartz; the carbonate is dominantly siderite with minor calcite. Chlorite ranges from a small a m o u n t to about 20% in some samples. Apatite is present in variable amounts and is abundant in some samples. Other accessory minerals include muscovite, K-feldspar, magnetite, pyrite, sphalerite and galena. The sideritic rocks have been found in the Brunswick No. 6 and No. 12 areas representing the carbonate facies of iron formation. Thin layers of siderite occur within the oxide and chlorite facies but in Brunswick No. 12 area about 5 m thick beds of sideritic rocks are found. A green rock which is present in the Brunswick No. 12 mine area and near the Nepisiguit Falls, locally referred to as ' s p o t t e d iron f o r m a t i o n " , is an iron-rich chloritic rock with light grey patches (Fig. 3D). The patches are either fine-grained aggregates or individual coarse crystals of carbonate (mostly siderite) which contain a little Mn, Ca and Mg. Some of the coarse carbonate crystals are zoned. Micro-probe investigation reveals t h a t the cores of the zoned crystals are rich in Fe and Mn but Fe and Mg increases and Mn decreases toward the margins; Ca is constant throughout.
Fig. 3.A. Laminae of jasper alternate with magnetite-rich laminae; cherty magnetitic rocks, Austin Brook iron mine. B. Photomicrograph showing grunerite and magnetite; cherty magnetitic rock, from the trench of Atlantic Coast Copper. C. Chert- and carbonate-rich laminae; sideritic rock, Brunswick No. 12 mine area. D. Photomicrograph showing patches and ooliths of Carbonate surrounded by groundmass rich in chlorite and biotite; "spotted iron formation", near Nepisiguit Falls.
712 (4) Basic iron f o r m a t i o n The term "basic iron f o r m a t i o n " is used locally for thin layers and lenses of magnetic rocks that occur within the mafic volcanic unit of the T e t a gouche Group in the Brunswick No. 12 mine area. The layers and lenses range in thickness from about 2 to 50 cm. The rocks are light and dark grey, very fine-grained, massive and hard. Most of them show primary laminae consisting of alternating magnetite-rich and chlorite-quartz- or carbonate-rich material (Fig. 4A). Mineralogically, the important constituents are magnetite, chlorite, carbonate (calcite and siderite), quartz and feldspar (mostly albite). Other important minerals are sphene, apatite, sericite, epidote and hematite. Sphene is found as round or oval ooliths most of which have a distinct core of chlorite (Fig. 4B), Another type of oolith is larger, more irregular in outline, less common, and consists of mixtures of rutile, quartz, calcite and chlorite (Fig. 4C). Other accessary minerals are titanomagnetite, ilmenite, pyrite and galena. The basic iron formation has not been found in genetic association with the massive sulfide deposits. Layers of similar iron formation have been also reported in greenstone along the east side of Pabineau River (Sidwell, 1952; Skinner, 1956) and in the Grand Brook spilite body (Skinner, 1974). According to Gross (1976), similar material is c o m m o n in belts of volcanic rocks, and represents an u n c o m m o n facies of some Algoma type iron formation. (5) Iron-rich maroon shale Red and maroon argillite, shale and slate occur in abundance in the Tetagouche Group. They contain pink nodules of rhodochrosite, and veinlets of manganite at Tetagouche Falls (Skinner, 1974). Their color is t h o u g h t to be due to the presence of hematite, but no detailed work has been done on these rocks. The rocks include varieties such as siliceous, hard and massive laminated, friable and fractured. In places t h e y have slatey cleavage. Their mineralogy is rather simple, most consist of very fine quartz and hematite with subordinate chlorite, sericite and carbonate. The quartz and hematite are uniformly distributed but in some cases t h e y are concentrated in distinct fine laminae. Fig. 4.A. Photomicrograph showing fine laminae o f magnetite alternate with laminae rich in carbonate, quartz and chlorite; basic iron formation, Brunswick No, 12 mine area. B. Photomicrograph showing ooliths o f sphene most o f which have a distinct core o f ironrich chlorite, the groundmass is carbonate, quartz, chlorite and magnetite; basic iron formation, Brunswick No. 12 mine area. C. Photomicrograph showing ooliths which consist of a mixture o f rutile, quartz, calcite and chlorite, the groundmass also consists of the latter three minerals; basic iron formation, Brunswick No. 12 mine area. D, Photomicrograph showing laths and fine grains of albite, the other minerals are magnetite, chlorite, quartz and carbonate ; Key Anacon Group B rocks.
¢.0
714
(6) Key An a co n iron-rwh rocks Two strong aero-magnetic anomalies in the Key Anacon mine area i n d i cated the presence of iron formation. Ground geophysical investigation over the stronger aero-magnetic anomaly led to the discovery of the Key Anacon massive sulfide ore deposit in 1954. The exposure of iron-rich rocks in the Key Anacon mine area is limited. An abundance of iron-rich rocks is noticed in the only available surface drill core of a single hole. None of the numerous available underground drill cores contain iron-rich rocks but have a b u n d a n t pyrite and some pyrrhotite. The iron-rich rocks are light to dark grey, greyish green and dark green; some are massive, homogeneous and thick bedded whereas others are schistose and thin bedded. In places, pink-grey cherty laminae alternate with dark-grey iron-rich laminae. Calcite veinlets and patches are quite abundant in some of the rocks. The rocks are mostly fine-grained but some mediumand coarse-grained magnetite layers are found. The rocks consist mostly of magnetite, quartz, chlorite and calcite. They also contain significant a m o u n t of feldspar which is mostly albite. The albite appears quite fresh and is f o u n d mostly as interlocked laths which are random ly oriented and cut across the laminae (Fig. 4D). The albite is also dispersed as fine grains in the matrix. The accessory minerals include muscovite, piedmontite, apatite, rutile, zircon, chalcopyrite, ilmenite, siderite and sphene. GEOCHEMISTRY OF IRON-RICH ROCKS Table I shows the means and standard deviations of the major oxides in the five types of iron-rich rocks. The Key Anacon rocks are shown separately and were divided into two groups on the basis of their chemical composition. All the five types of rocks show variation in their major oxide composition. Extreme ranges of SiO2 (4.3 to 80%) Fe203 (undetected to 77.0%) and FeO (undetected to 38.8%) are found in the cherty magnetitic rocks whereas the highest range of A1203 (4.8 to 16.6%) is found in the chloritic rocks. The other groups also show considerable ranges. The standard deviations reflect the ranges of various oxide constituents. In spite of the overlap of ranges, the five types can be compared on t h e basis of their mean oxide values. The comparison o f basic iron formation with the other four types show t h a t the mean values of A1203, TiO2, MgO, CaO and Na20 are higher, and those of Fe203 and MnO are quite lower in the basic iron formation than in the other types. The K20 mean values in the basic iron formation are also higher than those in the other types except in the maroon shale. On the other hand, the mean values o f FeO, CO2 and S in the basic iron formation are lower than ,ose in the cherty magnetitic, chloritic and sideritic rocks. In maroon shale, the mean values of FeO and S are lower than, and that of CO2 is equal to, t h a t of the basic iron formation. The rocks collected from the Key Anacon mine area fall into two distinct
17.90 2.60 0.14 19.60 8.50 2.60 0.90 2.13 0.80 0.61 1.12 1.78 3.60 1.88
S
63 63 63 62 59 63 63 62 8 46 63 63 61 54
N
S
46.50 9.30 9.30 3.10 0.41 0.27 12.20 6.40 15.70 4.90 1.37 1.62 4.50 3.20 1.27 1.22 . . 0.56 0.30 0.93 0.77 0.13 0.18 3.70 4.10 4.71 2.03
X 11 11 11 11 11 11 11 11 . 9 11 10 7 11
N 23.60 2.70 0.14 18.50 26.00 5.37 1.80 5.09 . 0.22 2.57 1.31 12.90 2.57
X 11 11 11 11 10 11 11 11 7 11 11 11 9
0.30 1.52 2.22 5.60 2.25
N
10.40 3.60 0.15 11.50 7.00 3.60 1.30 2.36
S
Sideritic r o c k s
43.30 13.60 4.16 7.00 10.10 0.31 5.60 5.87 3.40 0.99 1.24 0.05 0.60 4.25
X 11.20 2.10 2.26 4.00 4.00 0.12 3.00 2.57 2.50 1.06 0.79 0.03 0.30 2.45
S
Basic iron formation
8 8 8 8 8 8 8 8 7 8 8 8 8 8
N 62.90 7.30 0.40 19.10 1.50 2.13 1.80 0.59 1.10 1.69 0.44 0.03 0.60 2.00
X 14.90 2.50 0.16 12.40 0.90 3.21 1.00 0.62 1.10 0.82 0.39 -1.50 1.1
S
M a r o o n shale
10 10 10 10 3 10 10 10 5 10 10 5 10 10
N 40.60 13.80 2.58 6.70 5.90 0.27 2.30 11.81 3.70 1.90 0.44 0.04 7.50 2.44
X
N
3.60 14 2.20 14 0.46 14 3.50 14 1.80 14 0.19 14 1.00 14 4.74 14 1.70 14 1.43 14 0.26 14 0.02 9 3.70 14 0.99 14
S
Group A
43.60 11.20 0.45 17.60 12.30 2.25 2.10 3.56 2.80 0.05 0.45 0.08 1.20 2.42
X
N 9.60 5 2.80 5 0.17 5 5.90 5 3.60 5 0.73 5 1.10 5 3.18 5 0.50 5 0.06 5 0.13 5 0.10 4 1.60 5 1.14 5
S
Group B
Key A n a c o n r o c k s
X = m e a n ; S = s t a n d a r d deviation ; N = n u m b e r o f s a m p l e s w i t h d e t e c t a b l e a m o u n t o f o x i d e . Percentage error ( + / - ) for various o x i d e s is: SiO 2 a n d A1203 = 0.3;TIO2, MnO2, K~O, P205 a n d S = 0.02; Fe203, F e O and CO 2 = 0.1; MgO and Na20 = 0.2; CaO = 0.05; H20 ~ 0.08.
SiO 2 28.10 A1203 2.90 TiO 2 0.16 Fe~O 3 33.50 FeO 19.40 MnO 3.10 MgO 1.40 CaO 3.57 Na20 0.70 K20 0.54 P205 1.96 S 0.83 CO 2 4.70 H20 2.36
X
Oxides C h e r t y m a g n e t i t i c Iron-rich wt.% rocks chloritic r o c k s
Mean and s t a n d a r d deviation o f t h e m a j o r o x i d e c o m p o s i t i o n o f t h e iron-rich r o c k s
TABLE I
716 chemical groups- "high TiO2 rocks" (Group A) and ' low TiO2 rocks" (Group B). The comparison of the mean values of oxides in Group A rocks with those of the five types of iron-rich rocks, shows that they are remarkably similar to those of the basic iron formation except that the CaO and CO~ mean values of Group A rocks are higher than those of the basic iron formation. When compared with the other four types of iron-rich rocks, the mean values of oxides of Group A rocks follow almost exactly the pattern of basic iron formation except for Mg and K20. The mean MgO value in Group A rocks, unlike the basic iron formation, is lower than that of the chloritic rocks but when compared with maroon shale, cherty magnetitic and sideritic rocks, the MgO mean value of Group A rocks also follows the pattern of the basic iron formation. Similarly, the mean value of K20 of Group A rocks, unlike the basic iron formation, is higher than that of maroon shale, but when compared with cherty magnetitic, chloritic and sideritic rocks, the K20 mean value also follows the pattern of the basic iron formation. The comparison of Group B rocks of the Key Anacon mine area with the five types of rocks shows that except for four oxides (MgO, CaO, Na20, K20) the mean values of the major oxides are quite close to the corresponding values of the chloritic rocks. The mean values of MgO and CaO of the Group B rocks are close to the corresponding values of the cherty magnetic rocks. Na20 mean value is quite high and that of K20 is quite low in the Group B rocks as compared to cherty magnetitic and chloritic rocks. Figure 5 shows binary plots of TiO2 against MnO, MgO, P2Os, A1203 and Fe~O3+FeO for all the five types of iron-rich rocks and for the two groups of the Key Anacon rocks. Two distinct chemical fields are present in all these diagrams. One is a high TiO2 field constituted of almost all the basic iron formation and the Key Anacon Group A rocks. The second is a low TiO2 field which is constituted of the rest of the four types of rocks and t h e Key Anacon Group B rocks. These plots further show that the basic iron formation and the Key Anacon Group A rocks are also relatively higher in A1203 and MgO, and lower in MnO, P2Os and Fe203+FeO than the other four types and the Key Anacon Group B rocks. Figure 6 shows three ternary plots of TiO2-MnO-(Fe203+FeO), TiO2-MnOMgO and TiO2-MnO-Na20. In all these diagrams, the basic iron formation and the Key Anacon Group A rocks plot together in a field quite distinct from that of the other four types of iron-rich rocks. The Key Anacon Group B rocks also fall in the field of the other four types. In TiO2-MnO(Fe203+FeO) diagram, both the fields are narrow and most distinct. The basic iron formation and the Key Anacon Group A rocks show a clear trend towards TiO2 away from the MnO-(Fe203+FeO) trend of the other rocks. In the TiO2-MgO-MnO diagram, the field of basic iron formation and the Key
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Anacon G r o u p A rocks is close to TiO2-MgO axis whereas the o t h e r iron-rici~ rocks and the Key Anacon G r o u p B rocks are close to MgO-MnO axis, with the e x c e p t i o n o f one sample. In the TiO2-MnO-Na20 diagram, the basic iron f o r m a t i o n and the Key Anacon G r o u p A rocks plot close to TiO2-Na20 axis showing an e n r i c h m e n t of TiO2 on the one end and of Na20 on the other, Most o f the o t h e r iron-rich rocks plot along TiO2-MnO axis closer to the MnO apex b u t the K ey Anacon G r oup B rocks and some samples of the o t her rocks show an e n r i c h m e n t in Na20. MgO Na201 MnO --'-7 " T .... '
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Fig. 6. Ternary plots of some of the major oxides showing chemical differences between basic iron formation and the other types of rocks. Key Anacon Group A rocks fall in the field of basic iron formation but Group B rocks fall in the field of other rocks~
Interpretation of geochemical data Th e mineralogy o f the rocks investigated is clearly reflected in their major oxide composition. The e x t r e m e ranges of SiO2, Fe203 and FeO within each group, particularly in the cher t y magnetitic rocks, is due to the e x t r e m e variations in their mineralogical composition. T he chert y magnetitic rocks, for example, vary from highly magnetitic with a little quartz t o highly siliceous rocks with small a m o u n t of magnetite or hematite. The o t h e r oxide constituents vary accordingly. Th e chemical data suggest that the basic iron f o r m a t i o n is chemically distinct and can be distinguished f r o m t he o t h e r iron-rich rocks o f t he area on the basis of major el em e nt composition. The most i m p o r t a n t chemical character o f the basic iron f o r m a t i o n is the high TiO2 c o n t e n t , b u t high A1203, MgO, CaO and Na20 as well as low MnO and Fe203 are also distinguishing chemical criteria. Care must be observed in using CaO and Na~O because secondary calcite and albitization have enriched some of the o t h e r rocks with these t w o oxides. T he binary and t ernary diagrams o f Figs. 5 and 6 can be used t o distinguish t h e basic iron f o r m a t i o n from t he o t h e r iron-rich rocks.
719 In Key Anacon, t he data show t h a t the G r o u p A rocks are almost chemically identical to t he basic iron f o r m a t i o n e x c e p t t hat their CaO and CO2 contents are higher, which is due to secondary carbonate f o u n d in abundance in these rocks. T he K e y Anacon G r o u p A rocks are, therefore, considered to be the same as t he basic iron f o r m a t i o n . The G r o u p B rocks of the Key Anacon area have chemical features similar t o those of the chert y magnetitic as well as chloritic rocks and are, therefore, considered t o be mixed rocks representing t he oxide and silicate facies of iron form at i on. The higher Na20 in the G r o u p B rocks than in the chert y magnetitic and chloritic rocks is due to albitization. In spite o f high SiO2 and very low FeO mean values, t he o t h e r oxide mean values in the m a r o o n shale are generally comparable t o those of the chert y magnetitic, chloritic and sideritic rocks. The m a r o o n shale also falls in the fields o f these three types of iron-rich rocks in all the binary and t ernary diagrams. Th ey , therefore, appear to have chemical affinity to t he oxide, silicate and carbonate facies o f t he iron f orm at i on. This supports the general view th at th e m a r o o n shale m a y be stratigraphic equivalent of the iron-rich rocks occurring in the ore horizon. EXPLORATION SIGNIFICANCE The distinction of the basic iron f o r m a t i o n on the basis o f geochemistry f r o m the o t h e r iron-rich rocks in the structurally com pl ex Tetagouche G r o u p , is emphasized below f r om t he expl orat i on poi nt of view as its distinction in the field or u n d e r t he microscope m ay n o t be possible: (1) Since th e basic iron f o r m a t i o n indicates a barren horizon its identification can help prevent f u r t h e r futile e xpl orat i on work based on the geophysical results. (2) The identification of the cher t y magnetitic, chloritic or sideritic rocks will encourage the expl or a t i on work as these rocks do indicate the massive sulfides ore horizon. (3) Th e basic iron f o r m a t i o n lies stratigraphically above the ore horizon, its sure identification in drill holes or out crops will help lead the expl orat i on work in the correct stratigraphic direction towards the ore horizon. (4) In most places thick felsic volcanic rocks occur stratigraphically between the basic iron f o r m a t i o n hor i z on and the ore hori zon and the two horizons are spatially discrete. But in places, the felsic volcanic rocks are absent and th e ore horizon lies stratigraphically just below the basic iron formation horizon, and t he t w o are t h e r e f o r e juxtaposed. T he identification of the basic iron f o r m a t i o n m a y help locate t h e nearby ore horizon even if none o f the three types of iron-rich rocks, characteristic of the ore horizon, is present. (5) Th e occurrence o f t he basic iron f o r m a t i o n near the ore hori zon along which th e o t h e r three t ypes of iron-rich rocks are also present, make the situation quite complicated f or ground magnetic and stratigraphic interpre-
720 ration keeping in mind the complex structure. The Key Anacon area is a typical example where the closely located basic iron formation and the ironrich rocks of the ore horizon could n o t be distinguished in the field or even under the microscope, but were distinguished on basis of geochemistry. (6) The possible stratigraphic equivalence of the maroon shale, as suggested by its chemical affinity to the other three iron facies of the ore horizon, should be further investigated. ACKNOWLEDGEMENTS Most of the research was carried out when the author was a Visiting Research Fellow at the Natural Science and Engineering Research Council of Canada, but the work could not be completed during the fellowship period. After the author joined the University of Petroleum and Minerals, Dhahran, Saudi Arabia, the work was resumed and this manuscript was completed. Funds for the fieldwork and other logistic support were provided, as well as photographic work and chemical analyses were carried out by the Geological Survey of Canada. The author is t h a n k f u l to all the three organizations. Thanks are due to D.F. Sangster of the Geological Survey of Canada for his scientific discussions and valuable suggestions. The author is also thankful to A.L. McAllister of the University of New Brunswick, J.L. Davies of New Brunswick Department of Natural Resources, and the geologists of the Brunswick Mining and Smelting Corp. Ltd. (particularly D.W. Rutledge), and Heath Steele Mines Ltd. for their cooperation during the fieldwork. This is a contribution to IGCP Project 60 (Correlation of Caledonian Stratabound Sulfides). REFERENCES Boyle, R.W. and Davies, J.L., 1964. Geology of the Austin Brook and Brunswick No. 6 sulfide deposits, Gloucester County, New Brunswick. Geol. Surv. Can., Pap. 63-24. Davies, J.L., 1972. The geology and geochemistry of the Austin Brook area, Gloucester County, New Brunswick with special emphasis on the Austin Brook iron formation. Ph.D. Thesis, Carleton University, Ottawa; Ontario (unpubl.). Davis, G.H., 1972. Deformational history of the Caribou stratabound sulfide deposits, Bathurst, New Brunswick, Canada. Econ. Geol., 67: 634---655. Goodfellow, W.D., 1975. Rock geochemical exploration and ore genesis at Brunswick No. 12 deposit, New Brunswick. Ph.D. Thesis, University of New Brunswick, Frederiction, New Brunswick (unpubl.). Gross, G.A., 1976. Geology of iron deposits in Canada, Iron deposits in the Appalachian and Grenville Regions of Canada: Geol. Surv. Can., Econ. Geol. Rep. No. 22, v.2. Lea, E.R. and Rancourt, C., 1958. Geology of the Brunswick Mining and Smelting orebodies, Gloucester County, New Brunswick. Can. Inst. Min. Metall. Bull., 51: 167-177. Luff, W.M., 1975. Structural geology of the Brunswick No. 12 open pit. Can. Inst. Min. Metall. Bull., 68: 64--74. McAllister, A.L., 1960. Massive sulfide deposits in New Brunswick. Can. Inst. Min. Metall. Bull., 53: 88--98.
721 McBride, D., 1976. Geology of Heath Steele Mines, New Brunswick. Ph.D. Thesis, University of New Brunswick, Frederiction, New Brunswick (unpubl.). Neale, E.R.W., Beland, J., Potter, R.R. and Poole, W.H., 1961. A preliminary tectonic map of the Canadian Appalachian region based on age of folding. Can. Inst. Min. Metall. Bull., 54: 687--694. Neuman, R.B., 1968. Paleogeographic implications of Ordovician shelly fossils in the Magog Belt of the northern Appalachian Region. In: E-An. Zen et al. (Editors), Studies of Appalachian Geology - - Northern and Maritime, Wiley and Sons, New York, London. Pertold, Z., 1972. Surface plan and cross section, Brunswick No. 6 orebody. In: A.L. McAllister and R.Y. Lamarche (Editors), Mineral Deposits of Southern Quebec and New Brunswick. 24th Int. Geol. Congr., Field Excursion A58-C58 Guideb., pp. 60-61. Poole, W.H., 1963. Hayesville, New Brunswick. Geol. Surv. Can., Map 6-1963. Poole, W.H., 1967. Tectonic evolution of the Appalachian region of Canada. In: Geology of the Atlantic Region. E.R.W. Neale and Williams, H., Geol. Assoc. Can. Spec. Pap., 4: 9--51. Rast, N. and Stringer, P., 1974. Recent advances and the interpretation of geological structure of New Brunswick. Geosci. Can., 1 (4): 15--25. Rutledge, D.W., 1972. Brunswick Mining and Smelting Corporation, No. 6 and 12 Mines. In: A.L. McAllister and R.Y. Lamarche (Editors), Mineral deposits of Southern Quebec and New Brunswick. 24th Int. Geol. Congr. ; Field Excursion A58-C58 Guideb., pp. 58--67. Saif, S.I., 1977. Identification, correlation and origin of the Key Anacon-Brunswick mines ore horizon, Bathurst, New Brunswick. Ph.D. Thesis, University of New Brunswick, Frederiction New Brunswick (unpubl.). Saif, S.I., McAllister, A.L. and Murphy, W.L., 1978. Geology of the Key Anacon mine area, Bathurst, New Brunswick. Can. Inst. Min. Metall. Bull., 71: 161--168. Saif, S.I., in prep. Correlation and origin of the Key Anacon and Brunswick mines ore deposits, Bathurst, New Brunswick, Canada. Sidwell, K.O.J., 1952. Anomalies in the Bathurst Iron Mines area. New Brunswick Resources Development Board, Frederiction, New Brunswick. Skinner, R., 1956. Geology of the Tetagouche Group, Bathurst, New Brunswick. Ph.D. Thesis, McGill University, Montreal, Quebec (unpubl.). Skinner, R., 1974. Geology of Tetagouche Lakes, Bathurst, and Nepisiguit Falls map areas, New Brunswick. Geol. Surv. Can. Mem., 371. Smith, C.H. and Skinner, R., 1958. Geology of the Bathurst-New castle mineral district, New Brunswick. Can. Inst. Min. Metall. Bull., 51: 150--155. Stockwelt, C.H. and Tupper, W.M., 1966. Geology of Brunswick No. 6 and No. 12 mining areas, Gloucester County, New Brunswick. Geol. Surv. Can., Pap. 65-13. Whitehead, R.E., 1973. Application of rock geochemistry to problems of mineral exploration and ore genesis at Heath Steele Mines, New Brunswick. Ph.D. Thesis, University of New Brunswick, Frederiction, New Brunswick (unpubl.).
705
Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
PETROGRAPHIC AND GEOCHEMICAL CHARACTERISTICS OF IRON-RICH ROCKS AND THEIR SIGNIFICANCE IN EXPLORATION FOR MASSIVE SULFIDE DEPOSITS, BATHURST, NEW BRUNSWICK, CANADA 1
SAIFUL-ISLAM SAIF*
National Science and Engineering Research Council, Geological Survey o f Canada, 601 Booth Street, Ottawa, Ont. K I A OE8 (Canada) (Received May 14, 1982; accepted for publication April 3, 1983)
ABSTRACT Saif, S.-I., 1983. Petrographic and geochemical characteristics of iron-rich rocks and their significance in exploration for massive sulfide deposits, Bathurst, New Brunswick, Canada. In: G.R. Parslow (Editor), Geochemical Exploration 1982. J. Geochem. Explor., 19: 705--721. About 25 economically significant, Kuroko-type massive sulfide bodies lie in a metamorphosed volcano-sedimentary complex (probable Middle Ordovician) known as Tetagouche Group, in Bathurst area, New Brunswick. Despite unresolved structural complexities, it does appear that they were deposited during a particular phase of volcanic activity and are, therefore, contemporaneous. Most of the sulfide bodies are closely associated with iron-rich rocks representing various facies of iron formation, and together with sulfides it constitutes the "ore h o r i z o n " which, therefore, is highly magnetic. Aero-magnetic and ground-magnetic techniques are useful to locate the ore horizon but problems are created because of the occurrence of iron-rich rocks with no sulfide, along another horizon in the Tetagouche Group. Petrographic and geochemical characteristics of various types of iron-rich rocks have been studied to see if the iron-rich rocks of the ore horizon can be distinguished from the iron-rich rocks of the other horizon. The iron-rich rocks found in the Tetagouche Group can be classified into five types: ( 1 ) c h e r t y magnetitic rocks; (2)iron-rich chloritic rocks; (3) sideritic rocks; (4) basic iron formation; and (5) maroon shale. The basic iron formation, which is quite magnetic, gives a false indication of the ore horizon wherein the presence of any of the first three types of rocks is expected. Moreover, the basic iron formation is generally similar in appearance and mineralogy to some of the cherty magnetitic and chloritic rocks. Regarding major element composition, TiO2, Na~O, A1203 and CaO are higher whereas Fe203, FeO and MnO are lower in the basic iron formation than in the other iron-rich rocks. These geochemical characteristics can help distinguish the barren rocks of the basic iron formation from those of the ore horizon during the exploration programs. 1This paper is a contribution to IGCP Project 60-Correlation of Caledonian stratabound sulphides *Present address: Department of Earth Sciences, University of Petroleum & Minerals, Airport Box No. 144, UPM BOX 1853, Dhahran, Saudi Arabia. 0375-6742/83/$03.00
© 1983 Elsevier Science Publishers B.V.
706 INTRODUCTION
AND PURPOSE
OF INVESTIGATION
The discovery and mining of several economically significant, Kuroko~ t y p e massive sulfide bodies near Bathurst, New Brunswick {Fig. 1) has turned the area into an important centre of exploration activity. The sulfide bodies lie in a m e t a m o r p h o s e d volcano-sedimentary complex of probable Middle Ordovician age, known as the Tetagouche Group. Most of the stra~iform sulfide bodies lie close to a felsic pyroclastic sequence considered to be ash-flows {Boyle and Davies, 1964), and generally known as " p o r p h y r y " The association of most of the sulfide bodies with p o r p h y r y indicates that they were deposited during a particular phase of volcanic activity and are, therefore, contemporaneous. The sulfide bodies are also closely associated with various types of iron-rich rocks generally known as iron formation. McAllister (1960) proposed that the sulfides and iron-rich rocks represent various facies of chemical origin and consist of iron in its various oxidation forms. As such, the sulfide bodies and the iron-rich rocks, despite unresolved structural complexities, lie along a specific stratigraphic horizon which is generally referred to as " o r e horizon", or sulfide horizon. Structural and stratigraphic complexities as well as poor exposure have combined to create problems in exploration for the massive sulfide deposits. 72 °
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707 Geophysical techniques are useful, particularly aero-magnetic and groundmagnetic surveys which are utilized to locate the highly magnetic ore horizon. Problems in the use of magnetic techniques are created by the fact that iron-rich rocks occur also along another horizon in the Tetagouche Group but with no sulfide deposits. Such magnetic rocks give false indications of the ore horizon during magnetic surveys. The present project was initiated to study and compare the petrographic and chemical characteristics of the various types of iron-rich rocks, and to see if these characteristics can be used to distinguish the rocks associated with the sulfide bodies from the other iron-rich rocks including maroon shale. FIELD AND LABORATORY METHODS The fieldwork was mainly aimed at collecting samples from the iron-rich rocks for petrographic and geochemical analyses. Due to extreme structural and stratigraphic complexities, the relative stratigraphic positions of some of the rocks are u n k n o w n . 167 samples were collected from outcrops, diamond drill cores and underground mines from the following areas (Fig. 2): (1) Key Anacon mine. (2) Brunswick No. 6 mine. (3) Brunswick No. 12 mine. (4) Heath Steele mine. (5) Austin Brook iron mine. (6) Sabena mining property. (7) The Narrows area. (8) Nepisiguit Falls. (9) A trench dug by Atlantic Coast Copper. (10) Area about 13 km west of Caribou mine along road. Eleven major elements were determined by XRF and H20, FeO and CO2 were determined by rapid chemical m e t h o d on 122 of the samples. REGIONAL GEOLOGY OF BATHURST MINING CAMP The Bathurst--Newcastle district was described by Smith and Skinner {1958) as geologically constituted of the following three regional units (Fig. 2): (3) The Pennsylvanian--Mississippian Cover. (2) The Silurian--Devonian Folded Belt. (1) The Ordovician Folded Belt. The Ordovician F o l d e d Belt
The Ordovician Folded Belt consists of highly deformed interlayered metavolcanic and metasedimentary rocks which underlie the central part of
708
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Argillaceous sedimen~aO rockS, gre} ~acke, quartzite, conglomerate: minor [erruginous, Irlanganiferous, and carbonaceons chert and argJllite; minor !imestone, tuff m d v o l c a n i c flows ~v ~ . 40
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Fig. 2. Geology of the Bathurst-Newcastle area, New Brunswick (after J.L. Davies 1972); sample localities 1 to 10.
the district. These volcano-sedimentary strata comprise the Tetagouche Group and are generally considered to be Middle Ordovician (Poole, 1963; Neuman, 1968) but its age is still debatable (Rast and Stringer, 1974). Several attempts have been made by various investigators t o subdivide the Tetagouche Group but no stratigraphic sequence has y e t been agreed upon for the entire Tetagouche Group. Sail (1977 and in prep.) and Sail et al. (1978) have proposed an informal stratigraphic nomenclature for the area between The Narrows and the Key Anacon mine (Fig. 2). The Tetagouche Group was subjected to several phases of deformation. The complicated structural features, absence of good marker horizons and poor exposure have so far prevented a coherent structural interpretation for the entire group, but it is generally regarded as part of a Taconic folded belt that has been refolded during the Acadian Orogeny (Smith and Skinner, 1958; Neale et al., 1961; Poole, 1967; G.H. Davis, 1972). The rocks are intruded by Ordovician, and possibly also Silurian and Devonian gabbro, diabase and diorite, and by Devonian granitic masses (Skinner, 1974). This volcano-sedimentary complex contains economically significant massive sulfide bodies, as well as the iron formation and other iron-rich rocks t h a t are the subject of this investigation. For the detailed geology of the other two regional units the reader is referred to Skinner (1974). GEOLOGY AND PETROLOGY OF THE IRON-RICH ROCKS Apart from sulfides which are n o t included in this study, the following five types of iron-rich rocks have been identified:
709 (1) Cherty magnetitic rocks (oxide facies). (2) Iron-rich chloritic rocks (silicate facies). (3) Sideritic rocks (carbonate facies). (4) Basic iron formation. (5) Iron-rich maroon shale. The first three types and the sulfides occur along a single and narrow stratigraphic zone of the ore horizon and constit~lte the four facies of iron formation k n o w n as the "Austin Brook iron formation". The basic iron formation is stratigraphically above the ore horizon b u t the stratigraphic position of the maroon shale is n o t certain. It is thought, due to the abundance of hematite, that the maroon shale may be a time equivalent of the ore horizon. The rocks of the Key Anacon mine area are unique in the sense that they physically resemble cherty magnetitic and iron-rich chloritic rocks on the one hand and basic iron formation on the other. They could not be clearly classified with any of these three types on the basis of field characteristics and association, and will be treated separately. (1) Cherty magnetitic rocks The rocks are black, dark grey, light and dark red, and brown. They are usually massive and extremely hard, b u t a flaggy variety, due to local concentration of specular hematite is present. Typically, they comprise bands of alternate magnetite-hematite and quartz- or jasper-rich layers with individual layers varying from thin laminae to a b o u t 2 cm thick (Fig. 3A). The layers represent primary bedding as they are conformable with the contacts. The rocks consist mostly of magnetite and quartz. Hematite, jasper, chlorite, carbonate (calcite and siderite) and biotite are other important constituents. The relative amounts of quartz and the iron oxide minerals vary considerably. Feldspar which is present in some samples only in minor amounts, is mostly potassic. Apatite occurs in most of these rocks; some samples contain as much as 10--12%. Other accessory minerals are muscovite, epidote, zircon, sphene, titanomagnetite ilmenite, rutile, pyrite, sphalerite, and galena. Grunerite (Fig. 3B) was identified in only one sample collected from an o u t c r o p close to a huge mass of rhyolite. It is considered to be due to the c o n t a c t metamorphic effect of the rhyolite. The occurrence of these rocks with several sulphide bodies has been clearly d o c u m e n t e d (Lea and Rancourt, 1958; McAllister, 1960, Boyle and Davies, 1964; Stockwell and Tupper, 1966; J.L. Davies, 1972; Pertold, 1972; Rutledge, 1972; Whitehead, 1973; Goodfellow, 1975; Luff, 1975; McBride, 1976). (2) Iron-rich chloritic rocks The rocks are dark green, schistose and usually friable b u t some are relatively hard. Texturally, the rocks are fine-grained with well to poorly devel-
[]
711 oped fine laminae of chlorite-magnetite and chert of a green, pink or grey colour. Mineralogically, the rocks consist mostly of chlorite, magnetite and quartz. Chlorite c o n t e n t is as high as 64% in some samples but on average it is approximately 30%. Magnetite and quartz contents are, on average, 15 and 20%, respectively but their relative amounts vary considerably. Other main constituents are biotite and carbonates. Sericite and apatite are present as accessory minerals. Minor amounts of K-feldspar have also been detected in a few samples. Other accessory minerals are zircon, ilmenite, pyrite, chalcopyrite, pyrrhotite, sphalerite, galena and monazite. The iron-rich chloritic rocks represent the silicate facies of iron formation. They are closely associated and often interbedded with the cherty magnetitic rocks in many places. (3) Sideritic rocks The rocks are light grey and buff~ show light and dark laminae of varying thickness (Fig. 3C). The rocks are massive, hard, and have a cherty appearance. They consist mainly of a cryptocrystalline mixture of carbonate and quartz; the carbonate is dominantly siderite with minor calcite. Chlorite ranges from a small a m o u n t to about 20% in some samples. Apatite is present in variable amounts and is abundant in some samples. Other accessory minerals include muscovite, K-feldspar, magnetite, pyrite, sphalerite and galena. The sideritic rocks have been found in the Brunswick No. 6 and No. 12 areas representing the carbonate facies of iron formation. Thin layers of siderite occur within the oxide and chlorite facies but in Brunswick No. 12 area about 5 m thick beds of sideritic rocks are found. A green rock which is present in the Brunswick No. 12 mine area and near the Nepisiguit Falls, locally referred to as ' s p o t t e d iron f o r m a t i o n " , is an iron-rich chloritic rock with light grey patches (Fig. 3D). The patches are either fine-grained aggregates or individual coarse crystals of carbonate (mostly siderite) which contain a little Mn, Ca and Mg. Some of the coarse carbonate crystals are zoned. Micro-probe investigation reveals t h a t the cores of the zoned crystals are rich in Fe and Mn but Fe and Mg increases and Mn decreases toward the margins; Ca is constant throughout.
Fig. 3.A. Laminae of jasper alternate with magnetite-rich laminae; cherty magnetitic rocks, Austin Brook iron mine. B. Photomicrograph showing grunerite and magnetite; cherty magnetitic rock, from the trench of Atlantic Coast Copper. C. Chert- and carbonate-rich laminae; sideritic rock, Brunswick No. 12 mine area. D. Photomicrograph showing patches and ooliths of Carbonate surrounded by groundmass rich in chlorite and biotite; "spotted iron formation", near Nepisiguit Falls.
712 (4) Basic iron f o r m a t i o n The term "basic iron f o r m a t i o n " is used locally for thin layers and lenses of magnetic rocks that occur within the mafic volcanic unit of the T e t a gouche Group in the Brunswick No. 12 mine area. The layers and lenses range in thickness from about 2 to 50 cm. The rocks are light and dark grey, very fine-grained, massive and hard. Most of them show primary laminae consisting of alternating magnetite-rich and chlorite-quartz- or carbonate-rich material (Fig. 4A). Mineralogically, the important constituents are magnetite, chlorite, carbonate (calcite and siderite), quartz and feldspar (mostly albite). Other important minerals are sphene, apatite, sericite, epidote and hematite. Sphene is found as round or oval ooliths most of which have a distinct core of chlorite (Fig. 4B), Another type of oolith is larger, more irregular in outline, less common, and consists of mixtures of rutile, quartz, calcite and chlorite (Fig. 4C). Other accessary minerals are titanomagnetite, ilmenite, pyrite and galena. The basic iron formation has not been found in genetic association with the massive sulfide deposits. Layers of similar iron formation have been also reported in greenstone along the east side of Pabineau River (Sidwell, 1952; Skinner, 1956) and in the Grand Brook spilite body (Skinner, 1974). According to Gross (1976), similar material is c o m m o n in belts of volcanic rocks, and represents an u n c o m m o n facies of some Algoma type iron formation. (5) Iron-rich maroon shale Red and maroon argillite, shale and slate occur in abundance in the Tetagouche Group. They contain pink nodules of rhodochrosite, and veinlets of manganite at Tetagouche Falls (Skinner, 1974). Their color is t h o u g h t to be due to the presence of hematite, but no detailed work has been done on these rocks. The rocks include varieties such as siliceous, hard and massive laminated, friable and fractured. In places t h e y have slatey cleavage. Their mineralogy is rather simple, most consist of very fine quartz and hematite with subordinate chlorite, sericite and carbonate. The quartz and hematite are uniformly distributed but in some cases t h e y are concentrated in distinct fine laminae. Fig. 4.A. Photomicrograph showing fine laminae o f magnetite alternate with laminae rich in carbonate, quartz and chlorite; basic iron formation, Brunswick No, 12 mine area. B. Photomicrograph showing ooliths o f sphene most o f which have a distinct core o f ironrich chlorite, the groundmass is carbonate, quartz, chlorite and magnetite; basic iron formation, Brunswick No. 12 mine area. C. Photomicrograph showing ooliths which consist of a mixture o f rutile, quartz, calcite and chlorite, the groundmass also consists of the latter three minerals; basic iron formation, Brunswick No. 12 mine area. D, Photomicrograph showing laths and fine grains of albite, the other minerals are magnetite, chlorite, quartz and carbonate ; Key Anacon Group B rocks.
¢.0
714
(6) Key An a co n iron-rwh rocks Two strong aero-magnetic anomalies in the Key Anacon mine area i n d i cated the presence of iron formation. Ground geophysical investigation over the stronger aero-magnetic anomaly led to the discovery of the Key Anacon massive sulfide ore deposit in 1954. The exposure of iron-rich rocks in the Key Anacon mine area is limited. An abundance of iron-rich rocks is noticed in the only available surface drill core of a single hole. None of the numerous available underground drill cores contain iron-rich rocks but have a b u n d a n t pyrite and some pyrrhotite. The iron-rich rocks are light to dark grey, greyish green and dark green; some are massive, homogeneous and thick bedded whereas others are schistose and thin bedded. In places, pink-grey cherty laminae alternate with dark-grey iron-rich laminae. Calcite veinlets and patches are quite abundant in some of the rocks. The rocks are mostly fine-grained but some mediumand coarse-grained magnetite layers are found. The rocks consist mostly of magnetite, quartz, chlorite and calcite. They also contain significant a m o u n t of feldspar which is mostly albite. The albite appears quite fresh and is f o u n d mostly as interlocked laths which are random ly oriented and cut across the laminae (Fig. 4D). The albite is also dispersed as fine grains in the matrix. The accessory minerals include muscovite, piedmontite, apatite, rutile, zircon, chalcopyrite, ilmenite, siderite and sphene. GEOCHEMISTRY OF IRON-RICH ROCKS Table I shows the means and standard deviations of the major oxides in the five types of iron-rich rocks. The Key Anacon rocks are shown separately and were divided into two groups on the basis of their chemical composition. All the five types of rocks show variation in their major oxide composition. Extreme ranges of SiO2 (4.3 to 80%) Fe203 (undetected to 77.0%) and FeO (undetected to 38.8%) are found in the cherty magnetitic rocks whereas the highest range of A1203 (4.8 to 16.6%) is found in the chloritic rocks. The other groups also show considerable ranges. The standard deviations reflect the ranges of various oxide constituents. In spite of the overlap of ranges, the five types can be compared on t h e basis of their mean oxide values. The comparison o f basic iron formation with the other four types show t h a t the mean values of A1203, TiO2, MgO, CaO and Na20 are higher, and those of Fe203 and MnO are quite lower in the basic iron formation than in the other types. The K20 mean values in the basic iron formation are also higher than those in the other types except in the maroon shale. On the other hand, the mean values o f FeO, CO2 and S in the basic iron formation are lower than ,ose in the cherty magnetitic, chloritic and sideritic rocks. In maroon shale, the mean values of FeO and S are lower than, and that of CO2 is equal to, t h a t of the basic iron formation. The rocks collected from the Key Anacon mine area fall into two distinct
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46.50 9.30 9.30 3.10 0.41 0.27 12.20 6.40 15.70 4.90 1.37 1.62 4.50 3.20 1.27 1.22 . . 0.56 0.30 0.93 0.77 0.13 0.18 3.70 4.10 4.71 2.03
X 11 11 11 11 11 11 11 11 . 9 11 10 7 11
N 23.60 2.70 0.14 18.50 26.00 5.37 1.80 5.09 . 0.22 2.57 1.31 12.90 2.57
X 11 11 11 11 10 11 11 11 7 11 11 11 9
0.30 1.52 2.22 5.60 2.25
N
10.40 3.60 0.15 11.50 7.00 3.60 1.30 2.36
S
Sideritic r o c k s
43.30 13.60 4.16 7.00 10.10 0.31 5.60 5.87 3.40 0.99 1.24 0.05 0.60 4.25
X 11.20 2.10 2.26 4.00 4.00 0.12 3.00 2.57 2.50 1.06 0.79 0.03 0.30 2.45
S
Basic iron formation
8 8 8 8 8 8 8 8 7 8 8 8 8 8
N 62.90 7.30 0.40 19.10 1.50 2.13 1.80 0.59 1.10 1.69 0.44 0.03 0.60 2.00
X 14.90 2.50 0.16 12.40 0.90 3.21 1.00 0.62 1.10 0.82 0.39 -1.50 1.1
S
M a r o o n shale
10 10 10 10 3 10 10 10 5 10 10 5 10 10
N 40.60 13.80 2.58 6.70 5.90 0.27 2.30 11.81 3.70 1.90 0.44 0.04 7.50 2.44
X
N
3.60 14 2.20 14 0.46 14 3.50 14 1.80 14 0.19 14 1.00 14 4.74 14 1.70 14 1.43 14 0.26 14 0.02 9 3.70 14 0.99 14
S
Group A
43.60 11.20 0.45 17.60 12.30 2.25 2.10 3.56 2.80 0.05 0.45 0.08 1.20 2.42
X
N 9.60 5 2.80 5 0.17 5 5.90 5 3.60 5 0.73 5 1.10 5 3.18 5 0.50 5 0.06 5 0.13 5 0.10 4 1.60 5 1.14 5
S
Group B
Key A n a c o n r o c k s
X = m e a n ; S = s t a n d a r d deviation ; N = n u m b e r o f s a m p l e s w i t h d e t e c t a b l e a m o u n t o f o x i d e . Percentage error ( + / - ) for various o x i d e s is: SiO 2 a n d A1203 = 0.3;TIO2, MnO2, K~O, P205 a n d S = 0.02; Fe203, F e O and CO 2 = 0.1; MgO and Na20 = 0.2; CaO = 0.05; H20 ~ 0.08.
SiO 2 28.10 A1203 2.90 TiO 2 0.16 Fe~O 3 33.50 FeO 19.40 MnO 3.10 MgO 1.40 CaO 3.57 Na20 0.70 K20 0.54 P205 1.96 S 0.83 CO 2 4.70 H20 2.36
X
Oxides C h e r t y m a g n e t i t i c Iron-rich wt.% rocks chloritic r o c k s
Mean and s t a n d a r d deviation o f t h e m a j o r o x i d e c o m p o s i t i o n o f t h e iron-rich r o c k s
TABLE I
716 chemical groups- "high TiO2 rocks" (Group A) and ' low TiO2 rocks" (Group B). The comparison of the mean values of oxides in Group A rocks with those of the five types of iron-rich rocks, shows that they are remarkably similar to those of the basic iron formation except that the CaO and CO~ mean values of Group A rocks are higher than those of the basic iron formation. When compared with the other four types of iron-rich rocks, the mean values of oxides of Group A rocks follow almost exactly the pattern of basic iron formation except for Mg and K20. The mean MgO value in Group A rocks, unlike the basic iron formation, is lower than that of the chloritic rocks but when compared with maroon shale, cherty magnetitic and sideritic rocks, the MgO mean value of Group A rocks also follows the pattern of the basic iron formation. Similarly, the mean value of K20 of Group A rocks, unlike the basic iron formation, is higher than that of maroon shale, but when compared with cherty magnetitic, chloritic and sideritic rocks, the K20 mean value also follows the pattern of the basic iron formation. The comparison of Group B rocks of the Key Anacon mine area with the five types of rocks shows that except for four oxides (MgO, CaO, Na20, K20) the mean values of the major oxides are quite close to the corresponding values of the chloritic rocks. The mean values of MgO and CaO of the Group B rocks are close to the corresponding values of the cherty magnetic rocks. Na20 mean value is quite high and that of K20 is quite low in the Group B rocks as compared to cherty magnetitic and chloritic rocks. Figure 5 shows binary plots of TiO2 against MnO, MgO, P2Os, A1203 and Fe~O3+FeO for all the five types of iron-rich rocks and for the two groups of the Key Anacon rocks. Two distinct chemical fields are present in all these diagrams. One is a high TiO2 field constituted of almost all the basic iron formation and the Key Anacon Group A rocks. The second is a low TiO2 field which is constituted of the rest of the four types of rocks and t h e Key Anacon Group B rocks. These plots further show that the basic iron formation and the Key Anacon Group A rocks are also relatively higher in A1203 and MgO, and lower in MnO, P2Os and Fe203+FeO than the other four types and the Key Anacon Group B rocks. Figure 6 shows three ternary plots of TiO2-MnO-(Fe203+FeO), TiO2-MnOMgO and TiO2-MnO-Na20. In all these diagrams, the basic iron formation and the Key Anacon Group A rocks plot together in a field quite distinct from that of the other four types of iron-rich rocks. The Key Anacon Group B rocks also fall in the field of the other four types. In TiO2-MnO(Fe203+FeO) diagram, both the fields are narrow and most distinct. The basic iron formation and the Key Anacon Group A rocks show a clear trend towards TiO2 away from the MnO-(Fe203+FeO) trend of the other rocks. In the TiO2-MgO-MnO diagram, the field of basic iron formation and the Key
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Interpretation of geochemical data Th e mineralogy o f the rocks investigated is clearly reflected in their major oxide composition. The e x t r e m e ranges of SiO2, Fe203 and FeO within each group, particularly in the cher t y magnetitic rocks, is due to the e x t r e m e variations in their mineralogical composition. T he chert y magnetitic rocks, for example, vary from highly magnetitic with a little quartz t o highly siliceous rocks with small a m o u n t of magnetite or hematite. The o t h e r oxide constituents vary accordingly. Th e chemical data suggest that the basic iron f o r m a t i o n is chemically distinct and can be distinguished f r o m t he o t h e r iron-rich rocks o f t he area on the basis of major el em e nt composition. The most i m p o r t a n t chemical character o f the basic iron f o r m a t i o n is the high TiO2 c o n t e n t , b u t high A1203, MgO, CaO and Na20 as well as low MnO and Fe203 are also distinguishing chemical criteria. Care must be observed in using CaO and Na~O because secondary calcite and albitization have enriched some of the o t h e r rocks with these t w o oxides. T he binary and t ernary diagrams o f Figs. 5 and 6 can be used t o distinguish t h e basic iron f o r m a t i o n from t he o t h e r iron-rich rocks.
719 In Key Anacon, t he data show t h a t the G r o u p A rocks are almost chemically identical to t he basic iron f o r m a t i o n e x c e p t t hat their CaO and CO2 contents are higher, which is due to secondary carbonate f o u n d in abundance in these rocks. T he K e y Anacon G r o u p A rocks are, therefore, considered to be the same as t he basic iron f o r m a t i o n . The G r o u p B rocks of the Key Anacon area have chemical features similar t o those of the chert y magnetitic as well as chloritic rocks and are, therefore, considered t o be mixed rocks representing t he oxide and silicate facies of iron form at i on. The higher Na20 in the G r o u p B rocks than in the chert y magnetitic and chloritic rocks is due to albitization. In spite o f high SiO2 and very low FeO mean values, t he o t h e r oxide mean values in the m a r o o n shale are generally comparable t o those of the chert y magnetitic, chloritic and sideritic rocks. The m a r o o n shale also falls in the fields o f these three types of iron-rich rocks in all the binary and t ernary diagrams. Th ey , therefore, appear to have chemical affinity to t he oxide, silicate and carbonate facies o f t he iron f orm at i on. This supports the general view th at th e m a r o o n shale m a y be stratigraphic equivalent of the iron-rich rocks occurring in the ore horizon. EXPLORATION SIGNIFICANCE The distinction of the basic iron f o r m a t i o n on the basis o f geochemistry f r o m the o t h e r iron-rich rocks in the structurally com pl ex Tetagouche G r o u p , is emphasized below f r om t he expl orat i on poi nt of view as its distinction in the field or u n d e r t he microscope m ay n o t be possible: (1) Since th e basic iron f o r m a t i o n indicates a barren horizon its identification can help prevent f u r t h e r futile e xpl orat i on work based on the geophysical results. (2) The identification of the cher t y magnetitic, chloritic or sideritic rocks will encourage the expl or a t i on work as these rocks do indicate the massive sulfides ore horizon. (3) Th e basic iron f o r m a t i o n lies stratigraphically above the ore horizon, its sure identification in drill holes or out crops will help lead the expl orat i on work in the correct stratigraphic direction towards the ore horizon. (4) In most places thick felsic volcanic rocks occur stratigraphically between the basic iron f o r m a t i o n hor i z on and the ore hori zon and the two horizons are spatially discrete. But in places, the felsic volcanic rocks are absent and th e ore horizon lies stratigraphically just below the basic iron formation horizon, and t he t w o are t h e r e f o r e juxtaposed. T he identification of the basic iron f o r m a t i o n m a y help locate t h e nearby ore horizon even if none o f the three types of iron-rich rocks, characteristic of the ore horizon, is present. (5) Th e occurrence o f t he basic iron f o r m a t i o n near the ore hori zon along which th e o t h e r three t ypes of iron-rich rocks are also present, make the situation quite complicated f or ground magnetic and stratigraphic interpre-
720 ration keeping in mind the complex structure. The Key Anacon area is a typical example where the closely located basic iron formation and the ironrich rocks of the ore horizon could n o t be distinguished in the field or even under the microscope, but were distinguished on basis of geochemistry. (6) The possible stratigraphic equivalence of the maroon shale, as suggested by its chemical affinity to the other three iron facies of the ore horizon, should be further investigated. ACKNOWLEDGEMENTS Most of the research was carried out when the author was a Visiting Research Fellow at the Natural Science and Engineering Research Council of Canada, but the work could not be completed during the fellowship period. After the author joined the University of Petroleum and Minerals, Dhahran, Saudi Arabia, the work was resumed and this manuscript was completed. Funds for the fieldwork and other logistic support were provided, as well as photographic work and chemical analyses were carried out by the Geological Survey of Canada. The author is t h a n k f u l to all the three organizations. Thanks are due to D.F. Sangster of the Geological Survey of Canada for his scientific discussions and valuable suggestions. The author is also thankful to A.L. McAllister of the University of New Brunswick, J.L. Davies of New Brunswick Department of Natural Resources, and the geologists of the Brunswick Mining and Smelting Corp. Ltd. (particularly D.W. Rutledge), and Heath Steele Mines Ltd. for their cooperation during the fieldwork. This is a contribution to IGCP Project 60 (Correlation of Caledonian Stratabound Sulfides). REFERENCES Boyle, R.W. and Davies, J.L., 1964. Geology of the Austin Brook and Brunswick No. 6 sulfide deposits, Gloucester County, New Brunswick. Geol. Surv. Can., Pap. 63-24. Davies, J.L., 1972. The geology and geochemistry of the Austin Brook area, Gloucester County, New Brunswick with special emphasis on the Austin Brook iron formation. Ph.D. Thesis, Carleton University, Ottawa; Ontario (unpubl.). Davis, G.H., 1972. Deformational history of the Caribou stratabound sulfide deposits, Bathurst, New Brunswick, Canada. Econ. Geol., 67: 634---655. Goodfellow, W.D., 1975. Rock geochemical exploration and ore genesis at Brunswick No. 12 deposit, New Brunswick. Ph.D. Thesis, University of New Brunswick, Frederiction, New Brunswick (unpubl.). Gross, G.A., 1976. Geology of iron deposits in Canada, Iron deposits in the Appalachian and Grenville Regions of Canada: Geol. Surv. Can., Econ. Geol. Rep. No. 22, v.2. Lea, E.R. and Rancourt, C., 1958. Geology of the Brunswick Mining and Smelting orebodies, Gloucester County, New Brunswick. Can. Inst. Min. Metall. Bull., 51: 167-177. Luff, W.M., 1975. Structural geology of the Brunswick No. 12 open pit. Can. Inst. Min. Metall. Bull., 68: 64--74. McAllister, A.L., 1960. Massive sulfide deposits in New Brunswick. Can. Inst. Min. Metall. Bull., 53: 88--98.
721 McBride, D., 1976. Geology of Heath Steele Mines, New Brunswick. Ph.D. Thesis, University of New Brunswick, Frederiction, New Brunswick (unpubl.). Neale, E.R.W., Beland, J., Potter, R.R. and Poole, W.H., 1961. A preliminary tectonic map of the Canadian Appalachian region based on age of folding. Can. Inst. Min. Metall. Bull., 54: 687--694. Neuman, R.B., 1968. Paleogeographic implications of Ordovician shelly fossils in the Magog Belt of the northern Appalachian Region. In: E-An. Zen et al. (Editors), Studies of Appalachian Geology - - Northern and Maritime, Wiley and Sons, New York, London. Pertold, Z., 1972. Surface plan and cross section, Brunswick No. 6 orebody. In: A.L. McAllister and R.Y. Lamarche (Editors), Mineral Deposits of Southern Quebec and New Brunswick. 24th Int. Geol. Congr., Field Excursion A58-C58 Guideb., pp. 60-61. Poole, W.H., 1963. Hayesville, New Brunswick. Geol. Surv. Can., Map 6-1963. Poole, W.H., 1967. Tectonic evolution of the Appalachian region of Canada. In: Geology of the Atlantic Region. E.R.W. Neale and Williams, H., Geol. Assoc. Can. Spec. Pap., 4: 9--51. Rast, N. and Stringer, P., 1974. Recent advances and the interpretation of geological structure of New Brunswick. Geosci. Can., 1 (4): 15--25. Rutledge, D.W., 1972. Brunswick Mining and Smelting Corporation, No. 6 and 12 Mines. In: A.L. McAllister and R.Y. Lamarche (Editors), Mineral deposits of Southern Quebec and New Brunswick. 24th Int. Geol. Congr. ; Field Excursion A58-C58 Guideb., pp. 58--67. Saif, S.I., 1977. Identification, correlation and origin of the Key Anacon-Brunswick mines ore horizon, Bathurst, New Brunswick. Ph.D. Thesis, University of New Brunswick, Frederiction New Brunswick (unpubl.). Saif, S.I., McAllister, A.L. and Murphy, W.L., 1978. Geology of the Key Anacon mine area, Bathurst, New Brunswick. Can. Inst. Min. Metall. Bull., 71: 161--168. Saif, S.I., in prep. Correlation and origin of the Key Anacon and Brunswick mines ore deposits, Bathurst, New Brunswick, Canada. Sidwell, K.O.J., 1952. Anomalies in the Bathurst Iron Mines area. New Brunswick Resources Development Board, Frederiction, New Brunswick. Skinner, R., 1956. Geology of the Tetagouche Group, Bathurst, New Brunswick. Ph.D. Thesis, McGill University, Montreal, Quebec (unpubl.). Skinner, R., 1974. Geology of Tetagouche Lakes, Bathurst, and Nepisiguit Falls map areas, New Brunswick. Geol. Surv. Can. Mem., 371. Smith, C.H. and Skinner, R., 1958. Geology of the Bathurst-New castle mineral district, New Brunswick. Can. Inst. Min. Metall. Bull., 51: 150--155. Stockwelt, C.H. and Tupper, W.M., 1966. Geology of Brunswick No. 6 and No. 12 mining areas, Gloucester County, New Brunswick. Geol. Surv. Can., Pap. 65-13. Whitehead, R.E., 1973. Application of rock geochemistry to problems of mineral exploration and ore genesis at Heath Steele Mines, New Brunswick. Ph.D. Thesis, University of New Brunswick, Frederiction, New Brunswick (unpubl.).