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October 2005 Issue 87

Classifying, Distinguishing and Exploring for Intrusion-Related Gold Systems Craig J.R. Hart Yukon Geological Survey P.O. Box 2703 (K-102) Whitehorse, Yukon, Y1A 2C6 Canada E-mail: [email protected] Introduction Intrusion-related gold systems (IRGS) are a newly-defined (1999) deposit classification (based mainly on well-studied deposits in Alaska and Yukon) that is already mired in confusion, nomenclature uncertainty and misapplication. Increasingly, gold deposits are mis-assigned an IRGS classification because: 1) the nomenclature of intrusion-related gold models has been rapidly evolving; 2) the characteristics of the classification are broadly defined to include a wide range of deposit types that overlap with other gold deposit types; and 3) granitoid intrusions are common features of orogenic belts and are an obvious fluid source for any proximal gold occurrence. Ambiguous, poorly understood or poorly-defined deposit model classifications are an economic geologist’s greatest weakness as they result in ineffective exploration strategies. Deposit classifications can be better differentiated from others by using a set of distinguishing features and not characteristics that may be common to many hydrothermal deposit types. Herein, the historical evolution of intrusion-related gold deposit concepts is briefly presented, followed by a statement of the current state of intrusion-related gold models, and concluded with a set of characteristics by which IRGS can be

distinguished from other gold deposit types in similar geological settings. Evolution of Intrusion-Related Gold Classifications Intrusion-related gold classifications imply a genetic connection between gold ores and granitic plutons. These associations rocks have been long recognized (e.g., Agricola, 1556; DeLaunay, 1900; Lindgren, 1933, Spurr, 1923; Niggli, 1929; Emmons, 1926, 1933), but the contemporary geological literature prior to 1990 was essentially devoid of intrusion-related gold models. For example, there is no mention of such milestone contributions on mineral deposit types in models such as the 75th Anniversary Volume of Economic Geology (Skinner, 1981), the U.S. Geological Survey’s Ore Deposit Models (Cox & Singer, 1986), GAC Ore Deposit Models volumes (Roberts and Sheahan, 1988, 1994) or the British Columbia Geological Survey’s original Mineral Deposit Profiles (http://

www.em.gov.bc.ca/mining/GeolSurv/ MetallicMinerals/MineralDepositProfiles/ default.htm). The lack of progress resulted, in part, from the dearth of gold deposit research prior to its price increase in 1980. There were of course gold-rich skarn deposits, which, by their very nature are intrusion-related, and of course, gold was recognized at Bingham porphyry since mining started in 1906, but Sillitoe (1979) was among the first to emphasize a gold-rich porphyry classification. Early intrusion-related gold classifications were based on a porphyry copper classification. Gold-rich and gold-only porphyry copper systems had been long recognized and were identified and documented by Sillitoe (1979, 1995). Sillitoe (1995) recognized that they represented gold-rich end members and were not a new classification. The “porphyry” nomenclature was exported to refer to intrusion-hosted, low-grade, gold-only deposits at Fort Knox (Hollister, 1992; Bakke,

Inside this issue: 2006 Derry & Gross Winners

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Editorial - D. Duff

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Book Review - Inco comes to

Labrador Calendar of Events

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Figure 1. Broad distribution of the Tintina Gold Province (TGP) across Alaska and Yukon showing individual gold deposits (stars) and notable occurrences mentioned in the text that are considered to be of intrusion-related origin. However, many deposits are not intrusion-related or are at least controversial. The TGP is composed of numerous different gold districts with varying forms and ages of mineralization. (Continued on page 4)

2005 2005--2006 GAC MINERAL DEPOSITS DIVISION EXECUTIVE LIST Chairperson: Jan Peter Geological Survey of Canada, 601 Booth Street, Ottawa, ON K1A 0E8; Tel: (613) 992-2376; Fax: (613) 996-3726 Email: [email protected]

Past Chairperson: Hendrik Falck C.S. Lord Northern Geoscience Centre, Box 1500, 4601-B, 52 Avenue, Yellowknife, NT X1A 2R3; Tel: (867) 669-2636; Fax: (867) 669-2725 Email: [email protected]

Vice Chairperson: Suzanne Paradis Natural Resources Canada, 9860 West Saanich Road, Room 4718, Sidney, BC V8L 4B2; Tel: (250) 363-6732; Fax: (250) 363-6565 Email: [email protected]

Secretary: ‘Lyn Anglin

2005 2005--2006 GAC MINERAL DEPOSITS DIVISION DIRECTORS • Ross Sherlock (2003-2006) ESS/GSC-MRGB/CNGO, Natural Resources Canada, 626 Tumiit Building, P.O. Box 2319, Iqaluit, NU; Tel: (867) 9793539; Fax: (867) 979-0708 Email: [email protected]

• Steve McCutcheon (2003-2006) New Brunswick Department of Natural Resources, P.O. Box 50, 495 Riverside Drive, Bathurst, NB; Tel: (506) 547-2070; Fax (506) 547-7694 Email: [email protected]

• Gema Olivo (2003-2006) Geological Sciences, Queens University, Tel: (613) 533-6998; Fax: (613) 533-6592 Email: [email protected]

Kingston,

ON;

• Bob Cathro (2004-2007)

Geoscience BC, 410-890 W. Pender St., Vancouver, BC V6C 1J9; Tel: (604) 662-4147; Fax: (604) 662-4107 Email: [email protected]

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Treasurer: Jason Dunning

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• Rebecca Sproule (2004-2007) Department of Earth Sciences, Laurentian University, Sudbury, ON, P3E 2C6; Tel: (705) 675-1151, ext. 1325; Fax: (705) 675-4898 Email: [email protected]

• Craig Hart (2004-2007) Yukon Geological Survey, Box 2703 (K-10), Whitehorse, YK, X1A 2C6; Tel: (867) 667-8508; Fax: (867) 393-6232 Email: [email protected]

Short Course Coordinator: Steve Piercey

• Robert Carpenter (2005-2008)

Mineral Exploration Research Centre, Dept. of Earth Sciences, Laurentian University, Ramsey Lake Road, Sudbury, ON P3E 2C6; Tel: (705) 675-1151 ext. 2364; Fax: (705) 675-4898 Email: [email protected]

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October 2005– Gangue No. 87

• Moira Smith (2005-2008)

2

2005 2005--2006 CIM GEOLOGICAL SOCIETY EXECUTIVE LIST

2005 2005--2006 CIM GEOLOGICAL SOCIETY REPRESENTATIVES

President: Damien Duff

• Reg Olson (Awards)

Ontario Ministry of Northern Development and Mines Willett Green Miller Centre, 933 Ramsey Lake Road Sudbury, ON, P3E 6B5; Tel: (705) 670-5876; Fax: (705) 670-581 Email: [email protected]

Alberta Geological Survey, 4th Floor, Twin Atria 4999-98 Avenue, Edmonton, AB, T6B 2X3; Tel: (780) 427-1741; Fax (780) 422-1459 Email: [email protected]

Past President: Reg Olson

Nova Scotia Department of Natural Resources, P.O. Box 698, Halifax, NS, B3J 2T9; Tel: (902) 424-2517; Fax: (902) 424-0527 Email: [email protected]

Alberta Geological Survey, 4th Floor, Twin Atria 4999-98 Avenue, Edmonton, AB, T6B 2X3; Tel: (780) 427-1741; Fax: (780) 422-1459 Email: [email protected]

Vice President: Steve McRoberts Teck-Cominco Ltd., PO Box 938, Stn. Main, Kamloops, BC, V2C 5N4; Tel: (250) 372-0032; Fax: (250) 372-1285 Email: [email protected]

Vice President Elect: Chris Davis Inco Technical Services Ltd., P.O. Box 1516, Capreol, ON, P0M 1H0; Tel: (705) 858-0386 Email: [email protected]

Secretary/Treasurer: Laurie Gaborit High River Gold Mines Ltd., Suite 1700, 155 University Avenue, Toronto, ON, M5H 3B7; Tel: (416) 947-1440; Fax: (416) 360-0010 Email: [email protected]

Publications: David Sinclair Geological Survey of Canada, 601 Booth Street, 6th Floor, Room 675, Ottawa, ON, K1A 0E8; Tel: (613) 992-9810; Fax: (613) 996-3726 Email: [email protected]

Field Conference Coordinator: Position Vacant

• George O’Reilly (Bulletin Associate Editor)

• Jeremy Richards (EMG Editor) Department of Earth and Atmospheric Sciences, University of Alberta, Earth Sciences Building Room 3-02, Edmonton, AB, T6G 2E3; Tel: (780) 492-3430; Fax: (780) 492-2030 Email: [email protected]

• Andrew

Conly (Mineral Deposits search/University Visiting Lecturer)

Re-

Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, ON, P7B 5E1; Tel: (807) 343-8463; Fax: (807) 346-7853 Email: [email protected]

• Tom Schroeter (Special Volumes Editor) British Columbia Geological Survey, Vancouver Mineral Development Office, Mining and Minerals Division, Suite 300-865 Hornby Street, Vancouver, BC, V6Z 2G3; Tel: (604) 660-2812 Email: Tom. [email protected]

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• Frank Santaguida (Liaison) Falconbridge, Exploration Office, Kidd Creek Minesite, PO Box 1140, Timmins, ON, P4N 7B5; Tel: (705) 264-5200, ext. 8231; Fax: (705) 267-8874 Email: [email protected]

• Phil Thurston (Sudbury Section Representative) Mineral Exploration Research Centre, Laurentian University, Sudbury, ON, P3E 2C6; Tel: (705) 675-1151, ext. 2372 Email: [email protected]

• Phil Olson (Saskatoon Section Representative) Claude Resources Inc., 214 Coben Cr., Saskatoon, SK, S7S 1B3; Tel: (306) 244-6916; Email: [email protected]

October 2005– Gangue No. 87

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1995) and Dublin Gulch (Hitchins and Ossrich, 1995). These examples however, lack numerous defining features of porphyry deposits, such as multidirectional stockworks, disseminations, hydrothermal breccias, and widespread and concentric alteration shells, but served to emphasize the differences in the physical and chemical processes involved in their formation compared to typical porphyry systems. Most of these distinctions result from their deeper level of intrusion compared to porphyry systems. Both Lang et al. (2000) and Rombach and Newberry (2001) recognized that shallower IRGS, such as Shotgun, had characteristics more similar to typical porphyry systems, and therefore emphasized the depth control on intrusion-related characteristics. Sillitoe (1991) first established an intrusion-related classification by defining a “broad spectrum of gold mineralization styles” within epizonal environments. He showed clear evidence of deposit variability and interrelationships within an intrusion-related environment and identified six classes: porphyry, skarn, intrusion-hosted, carbonate-replacement, breccia and vein. Most examples were associated with porphyry copper-style mineralization in magnetite-series (oxidized), I-type intrusions in circum-Pacific island arc settings. As such, these deposits largely represent an “oxidized” intrusion-related gold classification. Newberry et al. (1995) introduced plutonic-hosted gold deposits emphasizing Bi and Te enrichments and correlations with “intrinsic” or genetically-associated gold deposits. Similarly, McCoy et al. (1997) used the term plutonic-related gold deposits to classify a wide range of gold deposit types from several districts through Alaska, and emphasized magmatic fluid evolution of, and differences from, porphyry systems. Their efforts assembled a broad array of characteristics into a single model over a range of depths. Wall and Taylor (1990) and Wall (1999) created a Thermal Aureole Gold model that emphasized proximity to plutons as favourable mineralizing sites due to structures, steep fluid, temperature and chemical gradients and other features. The model indicated numerous possible fluid sources for the deposits, but didn’t specifically imply a direct genetic association with the pluton. An intrusion-related gold vein classification indicated by Sillitoe (1991) was further amplified by Sillitoe and Thompson (1998). The examples used are wide-ranging, and thought to reflect differences in magma chemistry and oxidation state. The broad variations in associated characteristics partly overlap those of orogenic gold veins (Groves et al., 1998) and further emphasize the need for distinguishing characteristics between these two classifications, as was discussed by Sillitoe and Thompson (1998) and Groves et al. (2003). A new intrusion-related class introduced by Thompson et al. (1999) emphasized intrusion-related gold mineralization in regions lacking copper, but known for W and Sn deposits, and having an associated Bi-Te-As-Mo-Sb metal tenor. The deposits are associated with intrusions that are more felsic and more reduced than those highlighted by Sillitoe (1991, 1995) and are partly of Stype character. These systems are clearly distinct from intrusion-related gold deposits associated with chalcophile oxidized magmas. In order to differentiate between the two types, Lang et al. (2000) introduced the term intrusion-related gold “systems” (IRGS), and provided perhaps the clearest refinement of defining characteristics. “System” was emphasized to highlight the wide range of associated gold deposit styles within this scheme. The contribution by Thompson and Newberry (2000) further emphasized the difference between the two classifications by calling them “reduced” IRGS to emphasize the importance of the reduced oxidation state of the associated granitoids and exsolved fluids. Some examples of reduced IRGS granitoids may not be strongly reduced, but are significantly less oxidized than typical granitoids associated with porphyry occurrences. Although gold is traditionally considered to be associated with oxidized intrusions and chalcophile enrichments (e.g., Ishihara, 1981), the reduced character of some gold-related intrusions had been previously recognized for Alaskan examples by Leveille et al. (1988) who also indicated an alkalic association, and by Keith and Swan (1987) for intrusion-related gold deposits in the southwestern USA. A reduced intrusion-related mineral class was also recognized by Rowins (2000) who introduced “reduced porphyry Cu-Au deposits”, using Fort Knox, Shotgun and Telfer as examples, emphasizing them as a base-metal depleted variation of the traditional porphyry theme. Robert (2001) reintroduced syenite-associated gold deposits as Archean examples, but these appear unrelated to other intrusion-related classifications. The reduced IRGS model was mainly developed in response to observations, exploration, discoveries, and research on gold systems across central Alaska and Yukon where they are part of the Tintina Gold Province (TGP) (Figs. 1 & 2), and includes wellstudied examples such as Fort Knox (Bakke, 1995), Dublin Gulch (Maloof et al., 2001), Clear Creek (Marsh et al., 2003), and Scheelite Dome (Mair, 2004). The association of the Tintina Gold Province with intrusion-related gold systems was crystallized by Tucker and Smith (2000) such that TGP gold deposits are broadly considered be entirely intrusion-related. This led Hart et al. (2002) to emphasize that TGP deposits comprise parts of numerous gold districts that formed at several times during the Mesozoic in response to different geological events. They further emphasized that only some of the mineralization is unequivocally intrusion-related, and that many deposits are better characterized as epizonal or shear-hosted because they lack most intrusionrelated characteristics and a causal pluton is not apparent. Hart et al. (2004a) further showed that the TGP is underlain by several different plutonic suites with different metallogenic expressions with the best intrusion-related gold systems (e.g., Fort Knox, Dublin Gulch) being related to the once continuous Fairbanks-Tombstone-Mayo plutonic suite. Misclassified Deposits? Despite the confusion, or perhaps because of it, this recently-developed intrusion-related gold classification is extremely appealing and has been broadly adopted in exploration and research circles. It is the topic of special volumes (Tucker and Smith, 2000; Lang and Baker, 2001), and intrusion-related models have been called upon to describe the genesis of an increasing number of gold deposits and districts throughout the world. The proliferation of the classification has resulted firstly because of the ease of associating a gold deposit with any proximal intrusion, but secondly because the classification is commonly confused and mixed with characteristics of the oxidized intrusion-related gold deposits classification of Sillitoe (1991). The resultant large range of intrusion-related characteristics span a wide range of hydrothermal mineralization characteristics and have resulted in

October 2005– Gangue No. 87

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Table 1. Historical Development of Intrusion-related Nomenclature and concepts

Name

Main Features

Notable Examples

Reference

Gold-rich porphyry copper deposits

• Recognized gold enrichments as part of porphyry spectrum

Bingham, Marte, Lepanto, Skouries

Sillitoe, 1993

Intrusion-related gold deposits

• Wide variety of epizonal mineralization in the porphyry to epithermal transition

Kidston, Boddington, Salave, Kori Kollo, Muruntau

Sillitoe, 1991

• Six types - skarn, porphyry, replacement, breccia, vein • Chalcophile metal association • Associated with oxidized, I-type intrusions Gold porphyry

• Included as porphyry due to the intrusionhosted, bulk tonnage, low grade nature

Fort Knox, Dublin Gulch,

Bakke, 1995; Hitchins and Orrsich, 1995; Schroeter, 1995

Thermal Aureole Gold

• Proximity to plutons provides structurally favourable sites for mineralization due to thermal and chemical fluxes and multiple fluid sources

Fort Knox, Donlin Creek, Muruntau, Callie, Telfer, Sukhoi Log, Kumtor

Wall and Taylor, 1990; Wall,1999

Plutonic hosted gold, intrinsic

• Noted Bi and Te enrichments and correlations with intrusion-hosted gold deposits

Fort Knox, Ryan Lode, Circle, Vinasale

Newberry et al., 1995

Plutonic related gold

• Emphasized fluid differences from porphyry deposits

Fort Knox, Ryan Lode, Pogo, Cleary Hill, Nixon Fork, Shotgun, True North, Donlin Creek

McCoy, 1997

Intrusion-related gold veins

• Broad spectrum of characteristics reflect the nature of associated magma

Linglong, Ryan Lode, Dongping, Snip

Sillitoe and Thompson, 1998

Syenite-associated

• Archean examples in greenstone belt

Malartic, Beattie, HoltMcDermott

Robert, 2001

Fort Knox, Kidston, Timbarra, Kori Kollo, Mokrsko, Salave

Thompson et al., 1999

• Cu-rich sulphide disseminations Intrusion-related gold deposits in Sn-W provinces

• Included a wide range of deposit styles

Tintina Gold Belt

• Selected papers on a wide range of gold deposits in Alaska and Yukon included as intrusion-related

Fort Knox, Shotgun, Scheelite Dome, Longline, Pogo, Donlin Creek

Tucker and Smith, 2000

Intrusion-related gold systems

• Emphasis on “Systems”, and differences from porphyry deposits

Fort Knox, Pogo, Brewery Creek, Mokrsko, Salave, Kidston, Timbarra, Vasilkovskoe

Lang et al., 2000

Reduced Cu-Au porphyry

• Copper depleted Cu-Au porphyry

Fort Knox, Shotgun

Rowins, 2000

Reduced intrusion-related gold systems

• Reduced primary oxidation state of intrusion

Fort Knox, Pogo, Donlin Creek, Dublin Gulch

Thompson and Newberry, 2000

Tintina Gold Province

• Excluded gold districts and deposits from intrusion-related classification

Fort Knox, Dublin Gulch, Clear Creek, Scheelite Dome

Hart et al., 2002

• Gold associated with lithophile metal signature

• Divided Tintina Gold Province mineralization into intrusion-related, epizonal and shear hosted (Continued on page 6)

October 2005– Gangue No. 87

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(Continued from page 5)

numerous vein deposits being assigned an intrusion-related classification. For example, intrusion-related gold mineralization is interpreted within turbidite-hosted orogenic gold systems in the western Lachlan fold belt of Victoria (Miller and Wilson, 2004; Bierlein and McKnight, 2005), in New Zealand’s Otago region (de Ronde et al., 2000), and in the Meguma terrane of Nova Scotia (Kontak et al., 2004). Similarly, intrusion-related gold deposit models have been attributed to deposits in Archean orogenic gold camps including Wallaby (Hall et al., 2001), and some of the Golden Mile orebody (Walshe et al., 2005), both in the Yilgarn craton. Other Phanerozoic examples of intrusion-hosted gold have been stated, such as vein deposits in China’s Shandong Penninsula (e.g., Linglong; Wang et al., 1998) and many deposits (e.g., Donping) along the northern margin of the North China craton (Nie et al., 2004). Even major deposits within the world’s largest orogenic gold province of central Asia, such as Jilau (Cole et al., 2000) and Muruntau (Wall et al., 2004), have been re-interpreted as having intrusion-related origins. These controversial associations of classification are problematic, but are not surprising, as even key deposits within the TGP are controversial. Hart et al. (2002) suggested that this problem resulted from the inclusion of too many deposit types within a single, complex model and suggested a more-refined intrusion-related gold model that excluded controversial epizonal (e.g., Donlin Creek) and shear-hosted (e.g., Pogo) deposits. Both Pogo and Donlin Creek have been variably considered to be orogenic (Goldfarb et al., 2000, 2004, 2005; Groves et al., 2003), and intrusion-related deposits (Newberry et al., 1995; McCoy et al., 1997; Ebert et al., 2000; Smith et al., 1999; Thompson and Newberry, 2000; Rhys et al., 2003) and as such highlight the classification conundrum. The Current Situation Intrusion-related gold deposits, as currently utilized in the geological literature, refer to an incoherent group of deposits with wide-ranging characteristics, granitoid associations and tectonic settings. Beyond those deposits associated with oxidized porphyry-copper systems, the most coherent classification is for reduced IRGS. The characteristics of reduced IRGS deposits, as compiled from Lang and Baker (2001), with contributions from Lang et al. (2000) and Thompson and Newberry (2000) are listed below. Emphasis is on well-studied examples from Alaska and Yukon. A plan perspective of the model presented in Figure 2. 1. 2. 3.

metaluminous, subalkalic intrusion of intermediate to felsic compositions that lie near the boundary between ilmenite and magnetite series; carbonic hydrothermal fluids; a metal assemblage that variably combines gold with elevated Bi, W, As, Mo, Te, and/or Sb and low concentrations of base metals;

Figure 2. General plan model of intrusion-related gold systems from the Tintina Gold Province. Note the wide range of mineralization styles and geochemical variations that vary predictably outward from a central pluton (modified from Hart et al., 2002).

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A

B

Figure 3. Outcrop scale exposure of A) an array of auriferous sheeted quartz veins in the apex of the pluton at Clear Creek, Yukon. Alteration is limited to narrow selvages adjacent to the vein. Marker for scale. B) Sheeted veins at the Fort Knox gold deposit in Alaska, here shown offset slightly by a fracture, are less parallel and slightly more irregular but still do not form multi-directional stockworks, thus distinguishing this deposit type from porphyry deposits. Alteration is limited to thin selvages adjacent to the veins.

4. 5. 6. 7.

a low sulphide mineral content, mostly