Geofluids (2011) 11, 260–279 doi: 10.1111/j.1468-8123.2011.00336.x

Fluid dynamics and fluid-structural relationships in the Red Lake mine trend, Red Lake greenstone belt, Ontario, Canada

Y. LIU1 ,G.CHI1 ,K.M.BETHUNE1 AND B. DUBE´ 2 1Department of Geology, University of Regina, Regina, SK, Canada; 2Geological Survey of Canada, Quebec, QC, Canada

ABSTRACT

The Red Lake mine trend, a deformation zone in the Archean Red Lake greenstone belt that hosts the world-class Campbell-Red Lake gold deposit, is characterized by abundant foliation-parallel iron-carbonate ± quartz veins with banded colloform-crustiform structures and cockade breccias overprinted by silicification and gold mineraliza- tion. There is an apparent incompatibility between the cavity-fill structures of the veins and breccias (typically developed at shallow crustal depths) and the upper greenschist to lower amphibole facies metamorphic conditions recorded in the host rocks (indicating relatively deep environments). This, together with the development of veins along the foliation plane, represents an enigmatic problem that may be related to the interplay between fluid dynamics and stress field. We approach this problem through systematic study of fluid inclusion planes (FIPs) in the vein minerals, including the orientations of the FIPs and the pressure–temperature conditions inferred from fluid inclusion microthermometry. We find that fluid inclusions in the main stage vein minerals (pregold minerali- zation ankerite and quartz and syn-ore quartz) are predominantly carbonic without a visible aqueous phase, whereas many inclusions in the postore stage contain an aqueous phase. Most FIPs are subvertical, and many are subparallel to the foliation. High fluid pressure coupled with the high wetting angles of the water-poor, carbonic fluids may have been responsible for the abundance of brittle deformation features. The development of subverti-

cal FIPs is interpreted to indicate episodic switching of the maximum principal compressive stress (r1) from sub-

horizontal (perpendicular to the foliation) to subvertical (parallel to the foliation) orientation. The subvertical r1 is favorable for the formation of foliation-parallel veins, as fractures are preferentially opened along the foliation in such a stress regime, the origin of which may be linked to the fluid source.

Key words: carbonate-quartz veins, CO2 fluid, fluid inclusion planes, fluid inclusions, foliation-parallel, gold mineralization, Red Lake

Received 8 November 2010; accepted 5 April 2011

Corresponding author: Guoxiang Chi, Department of Geology, University of Regina, 3737 Wascana Parkway, Regina, SK, S4S 0A2, Canada. Email: [email protected]. Tel: +1 (306) 585 4583. Fax: +1 (306) 585 5433.

Geofluids (2011) 11, 260–279

well as a number of gold occurrences (Dube´ et al. 2004; INTRODUCTION Sanborn-Barrie et al. 2004; Chi et al. 2006, 2009; Lichtb- The Archean Red Lake greenstone belt in northwestern lau & Storey 2007). Ontario, Canada hosts a number of gold deposits mainly The gold deposits in the Red Lake mine trend consist of along deformation zones (Andrews et al. 1986; Dube´ et al. numerous barren to low-grade banded colloform-crusti- 2004; Sanborn-Barrie et al. 2004). One of the deformation form iron-carbonate ± quartz veins and cockade breccias zones is the SE-trending Red Lake mine trend in the east- overprinted by silicification and sulfidation accompanying ern part of the greenstone belt, which hosts the world-class gold mineralization (MacGeehan & Hodgson 1982; Mat- Campbell-Red Lake gold deposit (total production plus hieson & Hodgson 1984; Penczak & Mason 1997, 1999; remaining reserve = 840 t of gold) and the smaller Coche- Tarnocai 2000; Dube´ et al. 2001, 2002, 2003, 2004). nour-Willans deposit (total production = 39 t of gold), as The cavity-filling structures in the veins and breccias are in

2011 Blackwell Publishing Ltd Fluid dynamics and fluid-structural relationships in Red Lake 261 striking contrast with the development of penetrative folia- FIPs) (Lespinasse & Pecher 1986; Boullier & Robert tion and the upper greenschist-lower amphibole facies 1992; Lespinasse 1999). The study also aims to provide metamorphic conditions recorded in the host rocks new insights into the fluid dynamics and fluid-structural (Thompson 2003; Sanborn-Barrie et al. 2004). Further- relationships in shear zone-controlled hydrothermal sys- more, most of the carbonate ± quartz veins are parallel to tems in general. the foliation (Zhang et al. 1997; Dube´ et al. 2001, 2002), which is at odds with open-space filling style textures in GEOLOGICAL SETTING, SAMPLE the veins, as first noticed by MacGeehan & Hodgson LOCATIONS, AND METHODS OF STUDY (1982). This apparent contradiction between the vein-fill- ing styles and structural environments has led to different The Red Lake greenstone belt genetic models for gold mineralization. Penczak & Mason (1997, 1999) and Damer (1997) proposed that the Camp- The Red Lake greenstone belt is located in the Uchi Sub- bell-Red Lake deposit is an epithermal deposit that was province of the western Superior Province (Fig. 1). The later deformed and reworked during peak regional meta- greenstone belt is made up of mafic-ultramafic to felsic vol- morphism and deformation, whereas other studies suggest canic rocks with subordinate sedimentary rocks surrounded that gold mineralization is broadly contemporaneous with by granitoid batholiths. The supracrustal rocks have been peak regional metamorphism and protracted deformation divided into two packages: the 2.99–2.89 Ga Mesoarchean (Mathieson & Hodgson 1984; Andrews et al. 1986; and the 2.75–2.73 Ga Neoarchean packages, which are Zhang et al. 1997; Parker 2000; Tarnocai 2000; Dube´ separated by a regional unconformity (Fig. 1; Sanborn-Bar- et al. 2001, 2002, 2003, 2004; Chi et al. 2003, 2006, rie et al. 2001, 2004). 2009). The latter scenario is comparable to that invoked Two main episodes of penetrative deformation have been for orogenic type gold deposits (Groves et al. 1998), established in the Red Lake greenstone belt (Sanborn-Bar- although the unusual vein characteristics and abundance of rie et al. 2000, 2001, 2004). D1 deformation, which took carbonate veining make the Red Lake examples atypical place between 2742 and 2733 Ma, is interpreted to have (Dube´ & Gosselin 2007). occurred in response to W-E shortening, as reflected by a Two lines of evidence support the second model pervasive N-, NNW-, and NE-trending foliation (Sanborn- described previously. While most carbonate ± quartz veins Barrie et al. 2001, 2004). D2 deformation, bracketed with colloform-crustiform structures and cockade breccias between 2723 and 2712 Ma, is more intensely developed appear to predate gold mineralization, some carbonate and preserved, and is represented by a pervasive E-, NE-, breccia veins contain gold-mineralized rock fragments and SE-trending foliation and related F2 folds resulting (Dube´ et al. 2002), suggesting that the formation of at from an overall N-S shortening (Sanborn-Barrie et al. least some of these veins overlaps in time with the develop- 2001, 2004; Dube´ et al. 2004). According to Percival ment of ductile structures (foliation) during the main et al. (2006), D2 deformation is an expression of the phase of deformation, rather than during an early deforma- Uchian orogeny, which involved collision between the tion event as would be expected for the epithermal model North Caribou terrane to the north of the Uchi Subpro- (Dube´ et al. 2002). Fluid inclusion studies (Chi et al. vince and the Winnipeg River Subprovince to the south.

2002, 2003, 2006, 2009) yielded similar isochores for pre- Two post-tectonic deformation events (D3 and D4) are ore ankerite and quartz and syn-ore quartz, which indicate locally recorded. maximum fluid pressures in the range of 2.7–3.6 kbar for a The supracrustal rocks, which record greenschist- to temperature range of 350–550C, further supporting that amphibolite-facies metamorphism, were intruded by syn- both carbonate ± quartz veins and overprinting silicifica- to postvolcanic granitoid stocks and batholiths, as well as a tion and gold mineralization took place in fairly deep envi- series of mafic, lamprophyric, and feldspar porphyry dikes. ronments, likely during the main phase of deformation. Major granitoid intrusions were emplaced in three stages: It remains poorly understood why colloform-crustiform pretectonic (2.74–2.73 Ga), syn-tectonic (2.72–2.71 Ga) structures and cockade breccias, typically found in shallow and late- to post-tectonic (2.71–2.70 Ga) (Parker 2000; crustal levels, are so abundant in the Red Lake mine trend Sanborn-Barrie et al. 2004). Generally, the metamorphic if the veining events took place at great depths, and why grade increases from greenschist facies in the center of the most of the veins are parallel to the foliation. The purpose greenstone belt to amphibolite facies toward the margins, of this study is to tackle these problems through a system- suggesting that metamorphism was driven by heat supplied atic study of microfractures containing fluid inclusions, by surrounding batholiths (Andrews et al. 1986; Damer known as fluid inclusion planes (FIPs), in vein minerals, 1997; Menard & Pattison 1998). However, the timing of which can provide information on both the stress field metamorphism is still debatable. While some suggest that

(through measurement of orientation of the FIPs) and peak regional metamorphism coincided with D2 deforma- fluid pressure (through study of the fluid inclusions in the tion and was overprinted by amphibolite-facies contact

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Fig. 1. Geological map of the eastern part of the Red Lake greenstone belt, showing the location of the Red Lake mine trend and the study areas (sample locations) (After Dube´ et al. 2003). The inset shows the location of the Red Lake greenstone belt in the western Superior Province (ER = English River Sub- province; WR = Winnipeg River Subprovince; BR = Berens River Subprovince). metamorphism (Menard et al. 1999), others postulate that While the E-W- and NNW-trending subvertical defor- the amphibole facies metamorphism represents part of the mation zones have been interpreted as oblique- to strike- regional metamorphism (Thompson 2003). slip shear zones developed in response to NE-SW subhori- The rocks of the Red Lake greenstone belt were variably zontal shortening (Zhang et al. 1997; Dube´ et al. 2001, affected by different types of hydrothermal alteration, 2002), the mechanical nature of the SE-trending, generally including carbonate, potassic, and aluminous alteration as foliation-parallel high-strain zones is debatable, including well as silicification (Parker 2000 and reference therein). simple shear (normal, reverse-sinistral, and reverse-dextral) Among these, carbonatization is the most extensive and and pure shear. Based on the interpretation that some foli- consists of two types: calcite alteration and ferroan-dolo- ation-oblique veins are Riedel shear fractures (R and R¢) mite alteration, the latter being generally proximal to gold and the observation that folded and boudinaged veins are mineralization (Parker 2000). asymmetrical relative to the foliation, Andrews et al. (1986) interpreted the SE-trending deformation zones as shear zones with normal displacement (SW side down), The Red Lake mine trend which is supported by the sense of rotation of synkinemat- The Red Lake mine trend is a SE-trending protracted ic, metamorphic minerals documented in oriented thin sec- deformation-alteration-veining zone in the eastern part of tions from the Red Lake mine by Hugon & Schwerdtner the Red Lake greenstone belt (Fig. 1). The main structures (1984). However, based on mapping and apparent dis- were formed in D2 and consist of SE-trending high-strain placements of rock units, as well as observations of local zones, antiforms, synforms, and penetrative foliation C-S-type fabrics (C parallel to the shear zone and S obli- (Fig. 1), plus some associated E-W- and NNW-trending que to the shear zone), it has been shown that the deformation zones (Dube´ et al. 2001, 2002 and references SE-trending deformation zones in the Campbell-Red Lake therein). The shortening fabric (S2 foliation) is heteroge- deposit accommodated either reverse-sinistral movement neously developed, strikes SE (110–130), and dips steeply (the Campbell fault and the Red Lake or New Mine fault) SW (60–80). The attitude of the foliation is fairly consis- or reverse-dextral movement (the Dickenson fault) (Mac- tent across the deformation zones, regardless of the inten- Geehan & Hodgson 1982; Mathieson & Hodgson 1984; sity of deformation (Zhang et al. 1997). Rogers 1992; Penczak & Mason 1997; Dube´ et al. 2001,

2011 Blackwell Publishing Ltd, Geofluids, 11, 260–279 Fluid dynamics and fluid-structural relationships in Red Lake 263

2002). In contrast, Zhang et al. (1997), in studying the ate ± quartz veining mainly occurred before or in the early deformation zones in the Campbell mine, noted that the stages of D2, whereas silicification and associated gold min- fabrics within the SE-trending high-strain zones are gener- eralization mainly took place during the main phase of D2 ally symmetrical and lack shear-sense indicators and sug- (Dube´ et al. 2001, 2002). Importantly, it has been docu- gested that they resulted from pure shear deformation mented that some barren carbonate ± quartz veins post- related to NE-SW subhorizontal shortening. date gold mineralization (Dube´ et al. 2002), suggesting Regardless of the different kinematic interpretations of that carbonate ± quartz veining and gold mineralization the deformation zones, the SE-trending subvertical folia- took place in similar structural conditions during pro- tion indicates a dominant NE-SW subhorizontal maximum tracted hydrothermal activity, rather than as two separate principal stress (r1) during D2. However, the intermediate events in distinct environments.

(r2) and minimum (r3) principal stresses may have The main phase of gold mineralization was followed by switched their orientations one or more times. Many of emplacement of 2714–2712 Ma feldspar porphyry dikes the foliation-parallel carbonate ± quartz veins have been (Dube´ et al. 2004). These dikes, crosscutting the S2 folia- boudinaged, and there are two different orientations of the tion, trending SE and E-W and containing weak SE-trend- boudin axis: subhorizontal and subvertical. According to ing foliation, are considered to have intruded into the Red

Zhang et al. (1997) and Dube´ et al. (2002), the subhori- Lake mine trend in the late stages of D2 or early in D3 zontal boudinage indicates that r2 was subhorizontal and (Dube´ et al. 2004). Numerous SE-trending lamprophyre r3 subvertical, and the subvertical boudinage indicates that dikes, dated as 2702–2699 Ma, are postore and clearly cut r2 was subvertical and r3 subhorizontal. The E-W- and D2 structures (Fig. 2E). The weakly developed SE-trending NNW-trending shear zones, which form a conjugate shear foliation in these lamprophyre dikes is thought to have system approximately bisected by the SE-trending defor- been formed in D3 (Dube´ et al. 2004), when the stress mation zones, also indicate that r2 was subvertical (Zhang field is interpreted to have been similar to that in D2. et al. 1997; Tarnocai 2000). Zhang et al. (1997) docu- The latest stage deformation (D4) was characterized by mented that subhorizontal boudins are overprinted by sub- formation of the discrete brittle ‘black-line faults’. These vertical ones and suggested that the minimum principal faults, cutting and displacing auriferous veins and lamp- stress (r3) switched from subvertical to subhorizontal dur- rophyre dikes, are barren, discrete, 2- to 5-mm slip planes ing D2. composed of fine quartz, tourmaline, and dark chlorite Numerous centimeter- to meter-wide, banded collo- (Penczak & Mason 1997; Tarnocai 2000; Dube´ et al. form-crustiform carbonate (mainly ankerite) ± quartz veins 2003). Locally, conjugate NNW- and WNW-trending (Fig. 2) and cockade breccias, which are barren to low black-line faults show sinistral and dextral displacement, grade, occur in the Red Lake mine trend and elsewhere in respectively (Fig. 2F), suggesting a NW-SE-trending maxi- the district. These veins occur in varied orientations with mum principal stress. respect to the S2 foliation, but are mainly parallel to S2 (Fig. 2A, B; Zhang et al. 1997; Dube´ et al. 2001, 2002). Samples and methods of study Carbonate ± quartz veins of different orientations show complex crosscutting relationships and are variably A total of 32 oriented samples and 13 nonoriented samples deformed. The foliation-parallel veins were boudinaged, of vein material were collected from the Campbell mine whereas the foliation-perpendicular veins were symmetri- (underground), Red Lake mine (underground), Cochenour cally folded and the foliation-oblique veins were asym- mine (outcrop), Redcon prospect (outcrop), and Sandy metrically folded or boudinaged (Fig. 2B, C). Gold Bay–Woodland outcrops along Highway 125 (Fig. 1). Of mineralization is related to silicification and sulfidation of these samples, 14 were analyzed for microthermometry and the earlier barren to low-grade carbonate±quartz veins and 22 were measured for FIP orientations. The sample loca- breccias and replacement of adjacent wall rocks (Dube´ tions, lithologies, and relationship to structures are listed in et al. 2001, 2002 and references therein). It also occurs in Appendix S1. The first four localities lie within the ‘proxi- the form of veinlets within the carbonate ± quartz veins mal ferroan-dolomite alteration zone’ of Parker (2000), (Fig. 2D). Dube´ et al. (2001, 2002) presented evidence where the main carbonate mineral is ankerite, whereas the that the auriferous silicic replacement and associated arse- last locality is in the ‘distal calcite alteration zone’ (Parker nopyrite were, at least in part, syn- to postdevelopment of 2000). Most samples are from within the Red Lake mine asymmetrical boudinage of carbonate ± quartz veins. trend, except for those from Redcon (Fig. 1). The fact that most of the pre-ore carbonate ± quartz The samples from the Campbell, Red Lake, and Coche- veins were controlled by D2 structures suggests that the nour mines are ankerite ± quartz veins with or without emplacement of the veins likely took place during D2.On overprinting gold silicification. The samples from the the other hand, the observation that most of these veins Sandy Bay – Woodland outcrops are centimeter-scale cal- were deformed by D2 strain suggests that carbon- cite–quartz veins without gold mineralization. The Redcon

2011 Blackwell Publishing Ltd, Geofluids, 11, 260–279 264 Y. LIU et al.

(A) (B)

(C) (D)

(E) (F)

Fig. 2. (A) Closely spaced, foliation-parallel carbonate veins with colloform-crustiform structures, cut by subhorizontal extensional veins, in metavolcanic rocks; picture taken at 2651-3 Decline (4930N-4050E-6170Z), Campbell mine (oriented sample no. 20–4, foliation-parallel, boudinaged carbonate vein); (B) a large, foliation-parallel carbonate vein (left side) and two thinner subparallel carbonate veins with colloform-crustiform structures, in metavolcanic rocks; note the boudinage of the small veins; picture taken at Safety Bay 2651-2EDR (3950N-5125E-6320Z), Campbell mine (oriented sample no. 20–2, foliation- parallel, boudinaged carbonate vein) (pen at the top of photograph for scale); (C) a plan view photograph showing folding of foliation-perpendicular exten- sional veins in extensively carbonate-altered wall rock overprinted by intensive silicification and gold mineralization, picture taken at 33-806-1 Cut (119N-5724E-5232Z), Red Lake High-Grade zone, Red Lake mine (oriented samples no. 21–3, 21–4, 21–6, 21–7, all silicified carbonate veins) (pen is aligned with the strike of foliation); (D) quartz veinlets (ladder veins) overprinting a foliation-subparallel crustiform ankerite vein, Cochenour outcrop (oriented sample no. 23–10, carbonate vein with quartz veinlets); (E) a lamprophyre dike (marked by dotted lines) cutting the intensively carbonate-altered, foliated basalt, picture taken at 34-786-2 ⁄ 4 (172N-5671E-5174Z), Red Lake High-Grade zone, Red Lake mine (nonoriented sample no. 21–8, lamprophyre dike); (F) a plan view photograph showing conjugate black-line faults offsetting carbonate-quartz veins, indicative of NW-SE compression, Cochenour outcrop (pen for scale) (oriented samples no. 23–1, 23–2, black-line fault). Information about the samples is listed in the Appendix S1. prospect is situated about 4 km northeast of the Red Lake whereas the main vein mineral during the syn-ore stage is mine trend, in an area of relatively weak deformation, quartz. In the Sandy Bay–Woodland outcrop, the main where a 1–2-m wide ankerite vein cuts weak foliation and vein mineral is calcite, which is considered to be equivalent is in turn cut by quartz-actinolite stringers associated with to pre-ore ankerite in other localities, and quartz overprint- Au. The Redcon samples were studied to provide informa- ing calcite is considered to be equivalent to syn-ore quartz, tion about the fluid pressure and stress field outside the although there is no gold mineralization at this locality. main deformation zones, for comparison with those inside Postore quartz and calcite occur in the form of veins that the deformation zones. crosscut the postore feldspar porphyry dikes and lamp- Vein minerals are divided into three stages: pre-ore, syn- rophyre dikes, or fill fractures and residual pores in earlier ore, and postore, based on their relationships with gold carbonate veins. mineralization. The predominant mineral in the pre-ore Doubly polished thin sections were made from horizon- barren or low-grade carbonate ± quartz veins is ankerite, tally cut slabs of the oriented samples, and were used for

2011 Blackwell Publishing Ltd, Geofluids, 11, 260–279 Fluid dynamics and fluid-structural relationships in Red Lake 265 measurement of the orientation of FIPs, and for microth- fluid P-T-X conditions and stress regimes. A general prob- ermometric studies. The strike of an FIP is readily deter- lem with secondary fluid inclusions is that they may have mined by rotating the manual stage to find the angle been entrapped any time after the formation of the host between the strike of the FIP and the marked azimuth minerals. However, it is possible that secondary inclusions direction on the section. The dip angle of the FIP is esti- entrapped in one crystal represent the same fluids that were mated by changing the focus on different depths of the entrapped as primary inclusions in another crystal within FIP as follows. First, the stage is rotated so that the FIP is the same vein, because most veins experienced multiple parallel to the crosshair and focus is made on the upper deformation and incremental events. Two methods have part of the FIP. Then readings of the FIP on the horizon- been used in this study to evaluate whether or not the sec- tal crosshair (H1) and the focusing screw (V1) are ondary inclusions in a given mineral phase represent the recorded. The focus is then changed to a deeper part of fluids present during precipitation of that mineral. One is the FIP, and new readings of these parameters (H2,V2) to compare the microthermometric data between the FIP are taken. The horizontal displacement of the focus of the and non-FIP inclusions. The FIP inclusions are most likely

FIP (DH) is equal to (H2-H1) multiplied by the length per secondary (some may be pseudosecondary), whereas the unit for the objective used (e.g. for the ·50 objective the non-FIP ones (including isolated, random, and clustered length per unit is 2 microns). The vertical displacement of inclusions and those along growth zones) are possibly pri- the focus of the FIP (DV) is equal to (V2–V1) multiplied mary or pseudosecondary. A similarity in microthermomet- by the depth per unit reading, which can be obtained by ric attributes between the FIP and non-FIP inclusions is using a slide of known thickness. The dip angle (a) can taken to indicate that the secondary inclusions have then be calculated using the formula a = atan (DV ⁄ DH). entrapped the same fluids as primary and pseudosecondary The error of strike measurement is <1. The error of dip inclusions. The second method is to compare the fluid angle measurement depends on the vertical extent of the inclusion types between minerals of different stages; if a FIP and the accuracy of reading of focused objects, which combination of fluid inclusion types is present in a rela- is better than 1 micron. For an FIP with a dip angle of tively late mineral phase but absent in a relatively early 45 running through the whole thickness of the doubly mineral, it is inferred that the late fluids did not infiltrate polished section (100 microns), an error of 1 micron the early mineral and were not entrapped as secondary reading would introduce an error of dip angle estimation inclusions in that mineral. of 0.3, whereas if the vertical extent of the FIP is 10 microns, an error of 1 micron reading corresponds to FLUID INCLUSION TYPES, OCCURRENCES, an error of 3 in dip angle. The error for gently dipping AND MICROTHERMOMETRY fluid inclusion assemblages (FIAs) is generally larger than for steeper ones. For example, for an FIP with a dip Types of fluid inclusions angle of 10, the error of dip angle estimation introduced by an error of 1 micron reading is up to 6. For most Four types of fluid inclusions were recognized in the sam- of the FIPs studied, the error of dip angle is estimated to ples studied (Table 1): (i) CO2 ±CH4, (ii) H2O-NaCl, be <5. (iii) CO2-H2O-NaCl, and (iv) CH4-CO2. Type 1 Microthermometry was carried out with a Linkam inclusions are CO2-dominated and are generally mono- THMSG 600 heating ⁄ freezing stage. Fluid inclusions were phase at room temperature with no visible aqueous phase studied from seven oriented samples as well as seven non- (Fig. 3A, E). Microthermometry and laser Raman spectros- oriented samples. The stage was calibrated using synthetic copy indicate the presence of minor to trace amounts of

fluid inclusions of H2O (ice-melting temperature = 0C; CH4 and trace N2 in some cases (Chi et al. 2006). The critical temperature = 374.1C) and H2O-CO2 (CO2-melt- homogenization temperatures within individual FIAs are ing temperature = )56.6C). The accuracy of CO2-melting fairly consistent (Liu 2010), indicating that type 1 inclu- temperature (TmCO2), CO2-homogenization temperature sions did not result from H2O leakage from initially aque- (ThCO2), clathrate-melting temperature (Tmclath), and ice- ous-carbonic inclusions (Chi et al. 2006, 2009; Liu 2010). melting temperature (TmH2O) is about ±0.2C, whereas Type 2 inclusions are aqueous fluid inclusions with vari- that of the total homogenization temperature is around able amounts of salt (Fig. 3B). Trace amounts of gas are ±0.5C. The fluid inclusion assemblage (FIA) analysis present in some type 2 inclusions as indicated by clathrate method of Goldstein & Reynolds (1994) was employed to formation in freezing runs (Fig. 3B). These inclusions con- constrain the validity of the microthermometric data. sist of two phases at room temperature with vapor ⁄ total Unlike most fluid inclusion studies, which focus on pri- ratios generally <20%. mary or pseudosecondary inclusions, this study is mainly Type 3 inclusions are aqueous-carbonic inclusions, concerned with secondary or pseudosecondary fluid inclu- mostly consisting of two phases at room temperature: a sions along FIPs because they provide information on both liquid aqueous phase and a liquid CO2 phase (Fig. 3C),

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Table 1 Distribution of different types of fluid inclusions in different host minerals and study areas.

Sandy Campbell-Red Lake Cochenour Redcon Bay-Woodland

Fluid type Major components Pre-ore Syn-ore Postore Pre-ore Syn-ore Postore Pre-ore Syn-ore Syn-ore Postore

Type 1 CO2 ±CH4 +++ +++ ++ +++ +++ + +++ +++ ++ +

Type 2 H2O-NaCl2 –– +–– +–– ++

Type 3 H2O-CO2 -NaCl2 –– ––– ++–+ –

Type 4 CH4-CO2 ––+–

+++, predominant; ++, abundant; +, common; –, rare; blank, not observed.

(A) (B)

Fig. 3. Photomicrographs of four different types

of fluid inclusions. (A) Type 1, CO2-dominated inclusions, monophase at room temperature (syn-ore quartz, Redcon prospect, sample no.

GC–132); (B) type 2, H2O-NaCl fluid inclusions, photograph taken at )1C showing some gas (C) (D) hydrate crystals (postore calcite, Red Lake mine,

sample no. GC–116); (C) type 3, CO2-H2O-NaCl inclusions (postore quartz, Cochenour outcrop,

sample no. 23–8); (D) type 4, CH4-CO2 inclu- sions, photograph taken at )95C, showing three coexisting phases (postore quartz, Coche- nour outcrop, sample no. 23–8); (E) an isolated type 1 inclusion (syn-ore quartz, Red Lake mine, sample no. 21–6); (F) growth zones lined by type 1 fluid inclusions (pre-ore ankerite, Red (E) (F) Lake mine, sample no. KG 2000-50-5, from Chi et al. 2002).

with CO2 phase ⁄ total ratios generally larger than 90%. The in pre-ore and syn-ore minerals in all the localities studied, water phase is invisible in some but is indicated by the for- whereas the other three types mainly occur in the postore mation of clathrate during freezing. minerals. Type 4 inclusions are monophase at room temperature and appear similar to type 1, but they have significantly Occurrences of fluid inclusions lower melting temperatures and homogenization tempera- tures than type 1 (Fig. 3D). Raman spectroscopic analysis The four types of fluid inclusions occur both in FIPs and also indicates the presence of significant amounts of CH4. in various non-FIP modes. The non-FIP occurrences The types of fluid inclusions present in individual sam- include clusters (Fig. 3B, C, D), isolated inclusions ples are listed in Appendix S1, and their abundance in min- (Fig. 3E), randomly distributed inclusions (Fig. 3A), and erals of different stages at different localities are growth zones (Fig. 3F). FIPs appear as intracrystal or in- summarized in Table 1. Type 1 inclusions are predominant tercrystal arrays consisting of numerous tiny (generally

2011 Blackwell Publishing Ltd, Geofluids, 11, 260–279 Fluid dynamics and fluid-structural relationships in Red Lake 267

(A) (B)

Fig. 4. Photomicrographs showing characteristics of fluid inclusion planes (all photographs were taken in plane-polarized light): (A) Fluid inclusion planes (FIPs) in a quartz vein showing very uni- form orientation, independent of the host min- eral’s crystallographic properties (syn-ore quartz, Sandy Bay–Woodland outcrop, sample no. 24– 12); (B) a group of subparallel FIPs showing simi- lar (type 1) fluid inclusion characteristics (postore (C) (D) quartz, Cochenour outcrop, sample no. 23–8); (C) two sets of FIPs with different orientations; note set 2 FIP overprints but does not offsets set 1 FIP (postore quartz, Redcon outcrop, sample no. 24–10); (D) two sets of FIPs at right angle to each other; the broader FIPs are younger than the thinner one (postore quartz, Cochenour out- crop sample no. 23–8); (E) gold-bearing inclu- sions (black dots) along several closely spaced microfractures (syn-ore quartz, Red Lake Mine, sample no. 21–6); (F) gold-bearing inclusions and fluid inclusions along the same FIP (syn-ore (E) (F) quartz, Red Lake Mine, sample no. 21–6).

<10 lm) fluid inclusions (Fig. 4). In a given sample or measurements were taken in this study are listed in Appen- mineral, there is commonly a preferred orientation of FIPs, dix S1. which appears to be independent of the host minerals’ Pre-ore ankerite and quartz are dominated by type 1 crystallographic properties (Fig. 4A, B). Where FIPs of dif- fluid inclusions, with few type 2 and 3 inclusions. Microth- ferent orientations are present, their relative timing may be ermometry was only performed on relatively large non-FIP established if one set clearly overprints the other. However, inclusions, and no data were obtained from fluid inclusions no displacement of grain boundaries or intersecting FIPs in FIPs. The melting temperature of the carbonic phase has been found (Fig. 4C, D). FIPs of similar orientation (TmCO2) of type 1 inclusions ranges from )61.2 to generally show similar length and thickness, and fluid )56.6C (Table 2), consistent with CO2 being the domi- inclusions within them are also similar in size, shape, and nant composition with minor other gases such as CH4 and vapor ⁄ total ratios. Some gold grains are found in FIPs in N2. The temperatures of homogenization of the carbonic syn-ore quartz or in microfractures parallel to them phase (ThCO2), mostly to the liquid phase, cover a wide (Fig. 4E, F). range from )4.1 to 30.6C (Table 2 and Fig. 5A). The

TmCO2 values from Cochenour are generally lower than those from other localities (Table 2 and Fig. 5A). The few Fluid inclusion microthermometry type 2 inclusions studied by Chi et al. (2002, 2003) Microthermometric measurements were made for fluid yielded homogenization temperatures from 153 to 344C inclusions in pre-ore ankerite, syn-ore quartz, and postore (Table 2 and Fig. 5B) and ice-melting temperatures quartz, both in FIPs and in non-FIPs. The results are sum- (TmH2O) from )22.7 to )21.3C (Table 2). marized in Table 2 and Figs 5 and 6, which include data Syn-ore quartz is also dominated by type 1 inclusions, from Chi et al. (2002, 2003) in addition to those obtained with few type 2 and 3 inclusions. The type 2 and type 3 in this study. The samples in which microthermometric inclusions do not occur in the same FIP or cluster as type

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Table 2 Microthermometric data for different types of fluid inclusions in different stages and study areas*.

FI type Stage Data Campbell Red Lake Cochenour Redcon Sandy Bay-Woodland

Type 1 Pre-ore TmCO2 )57.5 to )56.6 )57.0 to )56.6 )58.5 to )57.1 )61.2 to )59.5 –

ThCO2 )2.4 to 28.1(L) 1.9 to 30.6(L) # )4.1 to 13.8(L) 2.0 to 26.5(L) –

Syn-ore TmCO2 )57.8 to )56.9 )60.8 to )57.5 )56.6 )61.2 to )56.5 )57.1 to )56.8 )59.1 to)56.9 FIP )61.7 to )56.8 FIP )58.3 to )56.7 FIP

ThCO2 )7.6 to 12.7(L) 10.1 to 26.3(L) )0.2 to 20.5 )26.0 to 31.0(L) )16.5 to )14.0(L) 4.2 to 24.2(L) FIP 3.2 to 26.6(L) FIP )0.9 to 30.5(L) FIP

Postore TmCO2 )57.9 to )57.7 )57.0 to )56.9 )60.9 to )56.9 – –

ThCO2 0.9 to 28.8(L) # 22.0 to 23.4(L) )15.3 to 22.7(L) – –

Type 2 Pre-ore TmH2O – )22.7 to )21.3 – – –

ThH2O 171(L) 192 to 344(L) 153 to 173(L) – –

Syn-ore TmH2O –– – )6.2 to )5.6 –

ThH2O – 252 to 380(L) – 162 to >250(L) –

Postore TmH2O – )25.6 to )6.4 – )0.6 to )0.4 )1.4 to )0.2 FIP

ThH2O 72 to 326(L) 75 to >200(L) 106 to 284(L) – 168 to 298(L) 104 to 388(L) # FIP

Type 3 Syn-ore TmCO2 –– – )57.0 to )56.6 –

Tmclath 8.2(L) – – 7.8 to 8.9(L) –

ThCO2 –– – )8.4 to 28.2 –

Thtotal >200 – – 235 to >255(V) –

Postore TmCO2 –– )61.2 to )56.9 –– )57.9 to )57.1 FIP

Tmclath – – 2.0 to 8.9 –– 7.5 to 7.9 FIP

ThCO2 –– )5.8 to 22.5(L) –– 2.8 to 9.2(L) FIP

Thtotal – – 116 to >310(L) –– 210 to >240(L) FIP

Type 4 Postore TmCO2 –– )71.8 to )67.2 – –

ThCO2 –– )69.8 to )48.2 – –

*Fluid inclusion planes (FIP) data are shaded and non-FIP data are not shaded. All FIP data and most non-FIP data are from this study; some of the non-FIP data (underlined) are from Chi et al. (2002, 2003); TmCO2,CO2-melting temperature; ThCO2,CO2 homogenization temperature; TmH2O, Ice-melting tem- perature; ThH2O, aqueous inclusion homogenization temperature; Tmclath, clathrate-melting temperature; Thtotal, total homogenization temperature of car- bonic-aqueous inclusions; L, homogenized to liquid; V, homogenized to vapor; #, some homogenized to vapor.

1 inclusions, and their relative timing cannot be deter- )5.8 to 22.5C, respectively (Table 2). In samples where mined. TmCO2 of type 1 inclusions ranges from )61.7 to both FIP and non-FIP inclusions were measured, their

)56.6C (Table 2), and ThCO2 (to liquid) ranges from ThCO2 values largely overlap (Fig. 6B). Type 2 inclusions )26.0 to 31.0C (Table 2 and Fig. 5A). Most of the low show homogenization temperatures from 72 to 388C

TmCO2 values (<0C) are from Redcon and Sandy Bay– (Table 2 and Fig. 5B), and type 3 inclusions have homoge- Woodland (Fig. 5A). In samples where both FIP and non- nization temperatures from 116 to >310C (Table 2 and

FIP inclusions were measured, their ThCO2 values largely Fig. 5C). TmH2O of type 2 inclusions ranges from )25.6 overlap, although those of non-FIP inclusions tend to to )0.2C, and Tmclath of type 3 inclusions ranges from extend to lower values (Fig. 6A). The few type 2 and 3 2.0 to 8.9C (Table 2). Type 4 fluid inclusions have inclusions studied show homogenization temperatures TmCO2 ranging from )71.8 to )67.2C and ThCO2 from from 162 to 380C and 235 to >255C, respectively )69.8 to )48.2C (Table 2), with estimated XCH4 between

(Table 2 and Fig. 5B, C). TmH2O of type 2 inclusions 0.6 and 0.8 based on the diagrams of Thiery et al. (1994). ranges from )6.2 to )5.6C, and clathrate-melting tem- peratures (Tm ) of type 3 inclusions range from 7.8 to clath ORIENTATION OF FIPS 8.9C (Table 2). Postore quartz contains all four types of inclusions. Dif- FIPs in pre-ore ankerite ferent types of fluid inclusions occur in separate FIPs or clusters. TmCO2 of type 1 and type 3 inclusions ranges The FIPs in pre-ore ankerite are generally intracrystal and from )60.9 to )56.9C and from 61.2 to )56.9C, short (<0.1 mm) and are typically lined by small (mostly respectively, and ThCO2 from )15.3 to 28.8C and from <2 lm), rhombic, elliptic, and irregularly shaped fluid

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(A) (B)

(C)

Fig. 5. (A) Histograms of CO2 homogenization temperatures of type 1 fluid inclusions; (B) histograms of homogenization temperatures of type 2 fluid inclu- sions; (C) histograms of homogenization temperatures of type 3 fluid inclusions. Part of the data are from Chi et al. (2002, 2003) (see also Table 2). inclusions. The attitudes of the FIPs are variable, but two 321⁄86, 330⁄78, 298⁄89, and 146⁄87 for Red preferred orientations are recognized: one strikes NNE- Lake mine, Campbell mine, Redcon outcrop, and Sandy SSW and dips steeply to WNW or ESE (group A1), and Bay–Woodland outcrop, respectively (Fig. 7, column B). the other strikes NW-SE and dips steeply to NE or SW The two sets of FIPs show mutual crosscutting relation- (group A2) (Fig. 7, column A). The strike and dip angle ships, suggesting that they are broadly contemporaneous. of the preferred orientations vary slightly for different localities, with A1 being 031⁄88, 193⁄76, 017⁄74, FIPs in postore quartz and 194⁄75 for Red Lake mine, Campbell mine, Coche- nour outcrop, and Redcon outcrop, respectively, and A2 The postore quartz is less deformed than the pre-ore being 330⁄75, 326⁄70, 311⁄75, and 136⁄84 for ankerite and syn-ore quartz, and the FIPs tend to display these localities (Fig. 7, column A). Most fluid inclusions simple distribution patterns. In contrast to the FIPs in ear- are CO2 dominated and are monophase at room tempera- lier-formed minerals, the FIPs in postore quartz commonly ture. Crosscutting relationships indicate that the NNE- consist of carbonic-aqueous or aqueous inclusions, which SSW-trending group is older than the NW-SE-trending are composed of two phases at room temperature. Four one. Locally, both sets are crosscut by some NE-striking groups of FIPs are recognized (Fig. 7, column C). The and SE-dipping FIPs. first and oldest group (C1), consisting of mainly mono-

phase CO2-dominated inclusions, strikes NW-SE and dips steeply to the NE or SW. The strike and dip angle of FIPs in syn-ore quartz group C1 are 318o ⁄ 83o for the Campbell mine, 114⁄76 The FIPs in the syn-ore quartz are generally longer and for the Cochenour outcrop, and 137⁄76 for the Sandy thicker, with fluid inclusions of larger size than those in Bay – Woodland outcrop (Fig. 7, column C). The second the pre-ore ankerite. The rounded, irregular, or negative- statistically more abundant group (C2) strikes WNW-ESE crystal-shape fluid inclusions that characterize these FIPs and dips steeply to the NNE or SSW, and is generally are mostly CO2 dominated and are monophase at room composed of two-phase aqueous or carbonic-aqueous fluid temperature. There are two preferred orientations of the inclusions. The strike and dip angle of group C2 are FIPs (Fig. 7, column B): one strikes NE-SW and dips stee- 280⁄76 for the Red Lake mine, 274⁄73 for the Camp- ply to NW or SE, mainly found in the Campbell mine bell mine, and 104⁄84 for the Sandy Bay–Woodland out- (group B1), and the other strikes NW-SE and dips steeply crop (Fig. 7, column C).The third group of FIPs (C3), to SW or NE (group B2). The strike and dip angle of also consisting of two-phase aqueous inclusions, strikes group B2 vary slightly for different localities, being NNE and dips steeply to the ESE, with the strike and dip

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(A)

(B)

Fig. 6. Comparison of CO2 homogenization temperatures between fluid inclusion planes (FIP) and non-FIP fluid inclusions in syn-ore quartz (A) and postore quartz (B). angle being 006⁄71 for the Campbell mine, 001⁄73 ies dealing with microcracks in relation to regional stress for the Cochenour outcrop, and 011⁄72 for the Redcon field (Tuttle 1949; Lespinasse & Pecher 1986) have outcrop (Fig. 7, column C). No mutual crosscutting rela- revealed that FIPs tend to develop in a preferred orienta- tionships have been observed between C2 and C3. In the tion: parallel to the maximum principal compressive stress

Redcon outcrop, group C3 FIPs are cut by FIPs that strike (r1) and perpendicular to the least principal stress (r3). NNW and dip ENE (C4, 341⁄71, Fig. 7). Thus, the FIPs are interpreted as mainly Mode I (exten- sional) cracks and can be used as structural markers of paleostress fields (Lespinasse & Pecher 1986; Boullier & Implications for the stress field Robert 1992; Lespinasse 1999), although it cannot be Both experimental tests (Brace & Bombolakis 1963; Tap- ruled out that some microcracks may have formed as shear ponnier & Brace 1976; Krantz 1979) and structural stud- fractures.

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(A) (B) (C)

Fig. 7. Equal-area, lower-hemisphere stereo- graphic projection of poles to fluid inclusion planes (FIPs) in pre-ore ankerite, syn-ore quartz, and postore quartz. A1–A2, B1–B2, and C1–C4 represent the preferential orientations of poles to FIPs at different stages – see text for detailed discussion.

Similarities and differences in FIP orientations between and in part on comparison of FIP orientations at individual different stages and between different localities can be per- localities. For example, at the Red Lake mine and Sandy ceived in Fig. 7. The similarities between different localities Bay–Woodland outcrop, C2 is well developed in the may reflect a common general stress field and similar struc- postore quartz, but FIPs of the same orientation are not tural environment along the Red Lake mine trend, and the found in the pre-ore ankerite and syn-ore quartz (Fig. 7, differences are likely caused by variation of local stress first and last rows). At the Redcon outcrop, C3 and C4 are fields. Although the similarities between different stages well developed in the postore quartz, but FIPs with similar may be interpreted simply as relatively late deformation orientations are absent in the syn-ore quartz (Fig. 7, fourth events overprinting relatively early mineral phases, it row). appears more likely that most of the FIPs were formed Based on the relative timing of the host minerals and during the increment-deformation history within individual the crosscutting relationships of the FIPs, the sequence of stages. This inference is based in part on the similarities in development of the major sets of FIPs is summarized in microthermometric attributes between FIP and non-FIP Fig. 8: from foliation-subperpendicular (A1) to foliation- inclusions from the same stage, in part on the observation subparallel (A2) in the pre-ore stage, through foliation-per- that aqueous and aqueous-carbonic inclusions abundant in pendicular (B1) and foliation-parallel (B2) in the syn-ore postore FIPs were not found in pre-ore and syn-ore FIPs, stage, to foliation-parallel (C1), then foliation-subparallel

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Fig. 8. Evolution of preferential fluid inclusion planes (FIP) orientations from pre-ore, through syn-ore to postore stage, shown in equal-area lower-hemi- sphere stereonets. The star on each stereonet represents the pole of the regional foliation plane. A1–A2, B1–B2, and C1–C4 represent the preferential orien- tations of poles to FIPs (and inferred orientation of the minimum principal stress, r3) at different stages.

(C2) and foliation-subperpendicular (C3), and finally folia- The abundant carbonic (type 1) fluid inclusions provide tion-oblique (C4) in the postore stage. The orientations of a good opportunity for fluid pressure estimation. This type FIPs from the Redcon outcrop, which is outside the Red of fluid inclusion has relatively gentle isochores in the tem- Lake mine trend, are similar to those within the Red Lake perature (horizontal axis)–pressure (vertical axis) space, and mine trend (Fig. 7). represents a better geobarometer than aqueous and aque- The observation that most FIPs are subvertical suggests ous-carbonic inclusions (Roedder & Bodnar 1980; Brown that r3 (represented by poles of the FIPs) was subhorizon- 1989). Calculation of fluid pressure from isochores tal during the formation of the FIPs, whereas the r1–r2 requires knowing fluid temperature, which can be approxi- plane was subvertical. This contradicts the inference from mated by the total homogenization temperature of aque- boudinage and other stretching structures that r3 switched ous (type 2) or aqueous-carbonic inclusions (type 3) if between subhorizontal and subvertical orientations (Zhang these inclusions were entrapped simultaneously from a fluid et al. 1997; Dube´ et al. 2002), and was not just subhori- system consisting of two immiscible phases (Roedder & zontal. This discrepancy, together with the orientation of Bodnar 1980). Unfortunately, such conditions do not r1 and r2 (i.e. whether r1 was subvertical and r2 was sub- appear to be satisfied in the Red Lake mine trend, as there horizontal, or vice versa), which cannot be determined is no petrographic evidence to indicate that the aqueous from the FIP orientation data alone, will be further dis- fluid was in equilibrium with a carbonic phase when it was cussed below. entrapped, even though both aqueous and carbonic inclu- sions occur in the same minerals, and phase modeling indi- cates that the carbonic inclusions may have resulted from DISCUSSION phase separation of a CO2-dominated carbonic-aqueous fluid (Chi et al. 2006, 2009). The wide range of homoge- Estimation of fluid pressure during veining and nization temperatures of type 2 fluid inclusions (153–344, microfracturing 162–380, and 72–388C for pre-ore, syn-ore and postore Fluid pressure plays an important role in fracturing and minerals, respectively; Table 2 and Fig. 5B) can be best vein formation (Hubbert & Willis 1957; Secor 1965, explained by fluid pressure fluctuation and physical separa- 1969; Phillips 1972; Etheridge 1983; Sibson et al. 1988; tion of aqueous and carbonic fluids during entrapment Hodgson 1989; Cox 1995, 2005). Fluid pressure can also (Robert & Kelly 1987; Chi et al. 2009), which also implies be used to estimate the depth of hydrothermal circulation, that the fluid temperatures were higher than the fluid and depth is one of the important factors controlling the inclusion homogenization temperatures. brittle or ductile behavior of rocks. The fluid pressures in Therefore, an independent estimate of temperature the Red lake mine trend during formation of the pre-ore conditions is required to calculate fluid pressures from the carbonate ± quartz veins, syn-ore silicification and quartz isochores of type 1 inclusions. As discussed earlier, various veins, and postore quartz-carbonate veins, as well as during lines of evidence from structural and petrographic studies microfracturing in the various stages, may be estimated indicate that gold mineralization (and syn-ore silicification) from fluid inclusion microthermometric data. took place during peak regional metamorphism (Dube´

2011 Blackwell Publishing Ltd, Geofluids, 11, 260–279 Fluid dynamics and fluid-structural relationships in Red Lake 273 et al. 2001, 2002), which is in the upper greenschist – tainties of temperatures; the actual scale of pressure fluctu- lower amphibole transition in the study areas (except ation within each stage may be comparable to the Sandy Bay – Woodland, which is in the greenschist facies). difference between the maximum and minimum pressures Therefore, the formation temperature of the syn-ore quartz for a given temperature, for example, 919–2384 bars at is inferred to be in the range of 400–550C. Although car- 300C for the pre-ore stage at the Campbell mine bonate ± quartz veining predates the main phase of gold (Table 3). mineralization, they likely took place in generally similar conditions rather than as two separate events in distinct Vein formation depths and brittle deformation environments (Dube´ et al. 2002). Therefore, the formation mechanisms temperature of the carbonate ± quartz veins may have been similar to, or slightly lower than, the 400–550C range. As discussed earlier, the abundance of colloform-crustiform A temperature range of 300–450C is tentatively adopted structures and cockade breccias in the carbonate ± quartz in this study. In the postore stage, the fluid temperatures veins from the Red Lake mine trend has led to debate cannot be determined with certainty, but it is likely that regarding whether the veins were formed in an epithermal they overlap with but are generally <400–550C peak environment or at great depths during regional metamor- metamorphism range, or are in the upper part of the 72– phism. Vein formation depths may be inferred from fluid 388C range of type 2 inclusion homogenization tempera- pressures if the pressure regime is known. Generally the tures. A temperature range of 250–400C is tentatively fluid pressure is near-hydrostatic at shallow crustal levels used in this study. and near-lithostatic at greater depths, with the transition Using the temperature ranges of 300–450, 400–550, zone being typically between 5 and 15 km (Cox 2005). and 250–400C for pre-ore, syn-ore, and postore respec- Fluid pressure fluctuation associated with rock rupturing is tively, and the isochores of type 1 fluid inclusions calcu- typically between hydrostatic and subhydrostatic in epither- lated from the equation of state for CO2 of Bottinga & mal environments, and between lithostatic and hydrostatic Richet (1981) as provided in Brown (1989), ranges of in mesothermal environments (Sibson et al. 1988). fluid pressures were calculated for each stage (Table 3). In the case of the Red Lake mine trend, the fluid pres- Fluid pressures calculated from FIP inclusions overlap with sure values estimated above are incompatible with an epi- those from non-FIP inclusions (Fig. 9). Fluid pressures in thermal environment for any of the three stages. For the pre-ore stage range from 919 to 3381 bars, 706 to example, for a pressure range of 919–3381 bars for the 3114 bars, 1606 to 3462 bars, and 1012 to 3107 bars for pre-ore stage in the Campbell mine, a hypothesis of a Campbell mine, Red Lake mine, Cochenour, and Redcon, hydrostatic-subhydrostatic system would mean a formation respectively. Fluid pressures in the syn-ore stage range depth of about 34 km. Therefore, it is more reasonable to from 1481 to 4322 bars, 1321 to 3610 bars, 1704 to assume hydrostatic-lithostatic fluid-pressure fluctuation and 3891 bars, 942 to 5379 bars, and 3563 to 4827 bars for that the minimum pressures represent hydrostatic pres- Campbell mine, Red Lake mine, Cochenour, Redcon, and sures. For the pre-ore stage, when most of the anker- Sandy Bay – Woodland, respectively. Fluid pressures in the ite ± quartz veins were formed, the smallest fluid pressure postore stage range from 733 to 2873 bars, 981 to 1615 found was 706 bars (at 300C) in the Red Lake mine bars, and 1009 to 3621 bars for Campbell mine, Red (Table 3), which corresponds to a depth of about 7 km Lake mine, and Cochenour, respectively. These pressure under a hydrostatic regime, still much larger than what is ranges may have been exaggerated because of the uncer- defined as an epithermal environment.

Table 3 Ranges of fluid pressures (bars) calculated from isochores of type 1 fluid inclusions for different temperature ranges.

Pre-ore Syn-ore Postore Fluid pressure Localities (bars) T = 300C T = 450C T = 400C T = 550C T = 250C T = 400C

Campbell mine Upper limit 2384 3381 3280 4322 1914 2873 Lower limit 919 1347 1481 1991 733 1146 Red Lake mine Upper limit 2186 3114 2305 3610 1037 1615 Lower limit 706 1030 1321 1777 981 1531 Cochenour Upper limit 2443 3462 2946 3891 2444 3621 Lower limit 1606 2326 1704 2287 1009 1573 Redcon Upper limit 2181 3107 4112 5379 Lower limit 1012 1483 942 1265 Sandy Bay–Woodland Upper limit 3675 4827 Lower limit 3563 4684

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(A) another way, brittle deformation may not develop at great depths unless high-pressure fluids are involved. High fluid pressures recorded by fluid inclusions in the various hydro- thermal stages (Table 3) are at least in part responsible for the brittle deformation observed in the Red Lake mine trend. The argument that high fluid pressure played a critical role in brittle deformation is also supported by our obser- vation that FIPs containing aqueous-bearing fluid inclu- sions (types 2 and 3), which are common in postore minerals, were rarely found in the pre-ore and syn-ore min- erals (Table 1). Given that type 2, 3, and 4 inclusions occur as primary and secondary inclusions in postore min- erals, i.e. representing fluids circulating after the formation (B) of the pre-ore and syn-ore minerals, it is remarkable that they rarely occur as secondary inclusions in FIPs in these minerals. The pre-ore and syn-ore minerals may have been cut off from the fluid conduits in the postore stage because of reduced porosity and permeability, so that there was insufficient fluid around the host minerals to trigger forma- tion of microfractures. Another factor that may have contributed to brittle deformation in the Red Lake mine trend is the ‘dry’ nature of the fluids in the pre- and syn-ore stage, as reflected by the predominance of CO2-dominated fluid inclusions. It is

known that CO2-dominated fluids have higher wetting angles than aqueous fluids (Watson & Brenan 1987). This may have prevented interconnection of the fluid phase in (C) the host rocks (leading to build-up of fluid pressure) and thus enhanced fluid pressure build-up and brittle failure.

Formation of foliation-parallel veins and stress analysis Most shear zone-hosted gold deposits are characterized by abundant veins emplaced at various angles to the main foli- ation (S fabric) and occupying structural elements such as extensional fractures and various shear fractures (R, R’, P, P’, and C) (Hodgson 1989). The central veins (fault-fill veins), which are located at the center of the shear zones and parallel to shear zone boundaries, are also commonly oblique – in plan or cross-section view depending on the sense of motion, to the main foliation (S fabric) – even Fig. 9. Isochores of type 1 fluid inclusions showing the ranges of fluid pres- though they are parallel to the foliation immediately sur- sures for temperature from 400 to 550C in the syn-ore stage from Camp- bell mine (A), Red Lake mine (B), and Redcon (C); note the overlap of fluid rounding them (the C fabric) (Hodgson 1989). However, pressures between fluid inclusion planes (FIP) and non-FIP fluid inclusions. in the Red Lake mine trend, most veins are parallel or sub- parallel to the foliation, and the mechanism remains poorly understood. While extensional fracturing caused by outer Fluid pressure can shift a Mohr circle from the region of arc stretching during folding may be invoked to explain stability in a Mohr diagram toward lower normal stress val- the occurrence of foliation-parallel carbonate ± quartz veins ues (the effective stress effect) so that the failure envelope near fold hinges (Dube´ et al. 2001, 2002), it is difficult to is intersected and fractures produced (Hubbert & Willis appeal to such a mechanism for veins in other structural 1957; Phillips 1972). Therefore, high fluid pressure tends locations. The formation of foliation-parallel veins is most to neutralize load pressure so that brittle deformation can likely related to the interplay between the stress field and occur at deep crustal levels (Hodgson 1989). Stated in fluid pressure fluctuation, as explained below.

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Vein formation requires that fractures be opened, and fracture plane and r1 (Fig. 10A). For 4 |T0|< r1–r3 < 5.7 fracture opening depends on a combination of differential |T0| (circle a2, Fig. 10A), extensional-shear (or trans-ten- stress, fluid pressure, and fracture orientation (Hubbert & sional) fractures are formed and they are able to open Willis 1957; Phillips 1972). Open space may be produced because the effective normal stress is negative; the fractures o along shear fractures through mechanisms such as uneven are at high angles to r3, as shown by 2h values of 0–50 rotation of the fractured rocks (Ramsay 1967), movement (Fig. 10A). on nonplanar shear surfaces (Guha et al. 1983), and inho- For the foliation-parallel veins, the creation and opening mogeneous deformation (Hodgson 1989). When the effect of fractures needs special conditions because this orienta- of fluid pressure is considered, open space may be created tion (perpendicular to r1) is generally unfavorable for frac- along a fracture if the effective normal stress perpendicular turing. The key factor is the high mechanical anisotropy of to the fracture (rn–Pfluid) is negative (tensile). Such condi- foliated rocks: the tensile strength in the direction of the tions may be met if the differential stress (r1–r3) is small foliation (labeled T? because the tensile fracture thus pro- and if the fluid pressure is high enough that the effective duced is perpendicular to the foliation) is much higher minimum principal stress (r3-Pfluid) is negative (Cox than that perpendicular to the foliation (labeled T ⁄⁄ 2005), as illustrated in Fig. 10A. When the differential because the tensile fracture thus produced is parallel to the stress is >5.7 | T0|, where T0 is the tensile strength, the foliation) (Kerrich 1989). Under these circumstances, two Mohr circle will intersect the Coulomb failure envelope at Coulomb-Griffith failure envelopes can be used to evaluate a position with positive normal stress, so shear fractures are failure criteria on the Mohr diagram, one for failure normal created; however, they cannot be opened because the effec- to foliation and the other for failure parallel to foliation tive normal stress is positive (compressive). For r1–r3 <4 (Fig. 10B). Foliation-parallel fractures will only occur when | T0| (circle a3, Fig. 10A), the Mohr circle intersects the the maximum effective stress (r1*) intersects the T ⁄⁄ ten-

Griffith failure envelope at the tensile strength (T0), and a sile failure envelope, while the minimum effective stress tensile fracture is formed; the fracture is perpendicular to (r3*) is not small enough to intersect T? envelope and r3, as shown by the 2h of 0, where h is the angle between cause failure in the direction normal to the foliation

(A) (B)

(C)

Fig. 10. (A) Mohr diagram showing the relationship between stress and fracturing in mechanically isotropic rocks. Circles a1, a2, and a3 depict shear fractur- ing, extensional-shear fracturing, and extensional fracturing, respectively (modified from Davis & Reynolds 1996). T0 is the tensile strength of the rock, and h is the angle between fracture plane and r1; (B) Mohr diagram showing the relationship between stress and fracturing in foliated rocks, which have different tensile strength along foliation (T ⁄⁄) and perpendicular to foliation (T?). Circle b1 indicates that foliation-parallel fracturing can take place when r1–r3 <|

T?– T ⁄⁄| and r1*=T ⁄⁄; (C) block diagrams showing three possible stress configurations, which may have been developed in the Red Lake mine trend and the attitudes of veins and fluid inclusion planes; for clarity, only foliation-parallel and foliation-perpendicular veins are shown.

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(Kerrich 1989) (circle b1 in Fig. 10B). The right side of In the first situation (Fig. 10c-1), the main foliation and the Mohr circle is in contact with the T ⁄⁄ tensile failure boudinage structures indicating subvertical stretching were envelope so that 2h is 180 (Fig. 10B), meaning that the developed, and minor subhorizontal veins or FIPs were angle between the fracture and r1 (h)is90, i.e. fracturing formed. In the second situation, the main foliation and along foliation. For this to occur, the differential stress boudinage structures indicating subhorizontal stretching must be smaller than the difference in tensile strength were developed, and either foliation-perpendicular veins (if between the foliation-parallel and foliation-perpendicular the Mohr circle was in contact with T? in Fig. 10b) or directions, i.e. (r1–r3)<|T?– T ⁄⁄|, otherwise tensile or foliation-parallel veins (if the Mohr circle was in contact extensional-shear fractures will be created at high angles to with T ⁄⁄ in Fig. 10b), as well as foliation-perpendicular the foliation. FIPs, were formed (Fig. 10c-2). In the third situation, foli- Although the mechanical anisotropy of foliated rocks, ation-parallel veins and FIPs were developed (Fig. 10c-3); combined with high fluid pressures and small differential the foliation represents a mechanical weakness in the rock stresses, may explain the development of foliation-parallel and fractures were preferentially opened along it even when fracturing and veining, it cannot explain the development r3 was not perpendicular to it. The alternating develop- of foliation-parallel microcracks (FIPs), because there is no ment of the three stress regimes implies that the differen- evidence to indicate that there is a foliation-related tial stress was probably relatively small and that the rapid mechanical anisotropy at the crystal scale. Therefore, the changes in stress orientations were a local rather than oro- development of foliation-parallel FIPs most likely indicates gen-scale phenomenon. a switch of r1 from an orientation perpendicular to the The subvertical-r1 scenario may seem unusual in a tec- foliation to one parallel to the foliation. This, in turn, sug- tonic environment such as the Red Lake greenstone belt gests that the development of foliation-parallel fracturing that was presumably dominated by horizontal shortening. and veining may have been related to changes of orienta- However, it has been inferred in other orogenic gold sys- tion of the principal stresses, in addition to mechanical tems elsewhere, such as the gold-quartz vein deposits in anisotropy. the Val d’Or area in the Abitibi greenstone belt (Boullier

The FIP analysis (Figs 7 and 8) indicates that r3 was & Robert 1992) and the Yellowknife greenstone belt mostly subhorizontal from pre-ore, through syn-ore to (Kerrich & Allison 1978). It is possible that such a stress postore stages, which appears inconsistent with megascopic regime is inherently associated with shear zone-controlled structural analysis indicating that r3 switched between sub- hydrothermal systems, and two mechanisms are proposed vertical and subhorizontal orientations (Zhang et al. 1997; here. One is the upward force associated with the Dube´ et al. 2002). This discrepancy may be interpreted to overpressured fluid reservoir below the shear zones, com- indicate that most microfractures were formed when r3 monly assumed to be present underneath shear zone-con- was subhorizontal and fluid flow was active, whereas few trolled vein systems (e.g. Sibson et al. 1988), which may microfractures were formed when r3 was subvertical and have been episodically larger than the horizontal compres- fluid flow was inactive. Another factor that may have con- sive principal stress. The other is that, as they were tributed to the scarcity of subhorizontal FIPs is that the formed, the veins may have been physically detached oriented thin sections were horizontally cut, so that sub- from the surrounding rock mass, resulting in vertical horizontal FIPs were less discernible than steep ones and principal stress being locally larger than the horizontal were underestimated. ones. A third possible mechanism is the upward force The subvertical FIPs have two preferred orientations: exerted by magmatic intrusions. However, the mutual one is subparallel to the foliation, and the other is subper- crosscutting relationships of FIPs suggest that the stress pendicular or at high angles to the foliation (Figs 7 and 8). field changed more frequently than can be accounted for The mutual crosscutting relationship of FIPs suggests mul- by the limited number of episodes of magmatic activity. tiple, episodic changes in the direction of r3. Combining Based on fluid composition (CO2-dominated) and noble the FIP analysis of this study with megascopic structural gas isotope analysis, it has been proposed that the fluids analysis carried out previously (Andrews et al. 1986; Zhang responsible for carbonate veining and gold mineralization et al. 1997; Dube´ et al. 2002), three stress configurations in the Red Lake mine trend were derived from devolatil- can be envisaged for the Red Lake mine trend (Fig. 10c): ization in granulite facies conditions at deep crustal levels

(i) r1 = NE-SW subhorizontal, r2 = NW-SE horizontal, (Chi et al. 2006, 2009). It is postulated that the upward r3 = subvertical; (ii) r1 = NE-SW subhorizontal, r2 = sub- force associated with the development of this deep fluid vertical; r3 = NW-SE horizontal; and (iii) r1 = subvertical, reservoir is responsible for the subvertical r1 inferred r2 = NW-SE horizontal; r3 = NE-SW horizontal. These from the FIP data. The coupling of a subvertical maxi- configurations appear to have alternated multiple times mum principal stress with the fluid source represents an during the evolution of the structural-hydrothermal sys- ideal condition for the formation of foliation-parallel tem. veins.

2011 Blackwell Publishing Ltd, Geofluids, 11, 260–279 Fluid dynamics and fluid-structural relationships in Red Lake 277

Williamson and Paul Barc of Goldcorp, Farhad Bouzari of CONCLUSIONS the University of British Columbia, and Mary Louise Hill (1) Fluids circulating in the Red Lake mine trend during of Lakehead University for their help in fieldwork and for carbonate veining and subsequent silicification and ensuing discussion. Critical reviews by Drs. Yanhua Zhang,

gold mineralization were dominated by CO2, whereas Gema Olivo, and Ole Kaven significantly improved the aqueous-bearing fluids were active after mineralization. quality of the paper, for which we are grateful. The high fluid pressure and ‘dry’ nature (high wetting angles) of the CO -dominated fluids may have been 2 REFERENCES responsible for the brittle deformation and formation of veins with colloform-crustiform structures and cock- Andrews AJ, Hugon H, Durocher M, Corfu F, Lavigne M (1986) ade breccias at depths around 7 km or more, under The anatomy of a gold-bearing greenstone belt: Red Lake, northwestern Ontario, Canada. In: Proceedings of Gold’86 upper greenschist- to lower amphibole-facies conditions. Symposium (ed. MacDonald AJ), pp. 3–22. 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Thiery R, Kerkhof AM, Dubessy J (1994) VX properties of CH4- CO2 and CO2-N2 fluid inclusions; modeling for T < 31ºC and SUPPORTING INFORMATION P < 400 bars. European Journal of Mineralogy, 6, 753–71. Thompson PH (2003) Toward a new metamorphic framework for Additional Supporting Information may be found in the gold exploration in the Red Lake greenstone belt. Ontario Geo- online version of this article: logical Survey, Open File report 6122, 51. Appendix S1. Information of individual samples: loca- Tuttle OF (1949) Structural petrology of planes of liquid inclu- tion, lithology, relationship to structures, and fluid inclu- sions. Journal of Geology, 57, 331–56. sion studies*. Watson EB, Brenan JM (1987) Fluids in the lithosphere, 1. Exper- imentally determined wetting characteristics of CO2-H2O fluids Please note: Wiley- Blackwell are not responsible for the and their implications for fluid transport, host-rock properties, content or functionality of any supporting materials sup- and fluid inclusion formation. Earth and Planetary Science Let- plied by the authors. Any queries (other than missing ters, 85, 497–515. materials) should be directed to the corresponding author Zhang G, Hattori K, Cruden A (1997) Structural evolution of for this article. auriferous deformation zones at the Campbell mine, Red Lake

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