Discovery of New Deposits at Depth Examples of a Flexible Approach to Exploration

John F.H. Thompson

Teck Corporation, 200 Burrard Street, Vancouver, B.C. V6C 3L9 Canada

 

Key Words: VMS, Mexico, high-grade gold, intrusion-related, Alaska

Abstract

Although outcropping mineralization continues to be discovered in some parts of the world, it is widely accepted that new discoveries in many areas will require exploration for deep targets.  Deep mineralization may be concealed by unmineralized bedrock and/or post-mineral cover.  Two examples of recent discoveries in which Teck Corporation was involved are described, the large San Nicolas polymetallic volcanogenic massive sulfide deposit in central Mexico and the high-grade gold deposit at Pogo in east-central Alaska.  Neither discovery involved the simple application of a conventional ore deposit model in well-understood areas.  Both required a combination of careful geological observations and interpretation, pragmatic exploration techniques, and a flexible approach.

Introduction

In mature areas, usually where major deposits are known and exploration has been active for more than a decade, the emphasis of exploration commonly shifts towards exploring at depth below post-mineral cover.  Prominent recent discoveries resulting from this approach include the Pipeline sediment-hosted gold deposit in Nevada, the Spence porphyry copper deposit in Chile, and the Ernest Henry copper-gold and Cannington lead-zinc-silver deposits in northeastern Australia.  Exploration is guided by extending known mineralized trends and geology under adjacent shallow cover sequences, the hope being that major deposits will be discovered at depths suitable for bulk mining.  The characteristics of the target are generally well understood, based largely on known deposits in adjacent areas of exposed bedrock. Geophysics has played an important role in some areas (e.g. Cloncurry) and while this may help to locate mineralization of the expected type, it occasionally results in unexpected discoveries. 

 

In this paper, two recent discoveries of mineralization at depth are described.  These differ from the general theme of exploration in covered areas described above for three reasons: 1) mineralization is concealed below largely unmineralized bedrock plus, in one case, post-mineral cover; 2) they occur in areas with no known deposits of the type that was discovered; and, 3) the geological model and the exploration approach had to be modified during the exploration program.  These examples illustrate the importance of flexible exploration programs.

San Nicolás, Central Mexico

The San Nicolás volcanogenic massive sulfide (VMS) deposit is located in central Mexico approximately 60 km east-southeast of the city of Zacatecas.  The deposit consists of an upper Main zone of massive sulfide with estimated reserves of 72 Mt at 1.35% Cu, 2.27% Zn, 0.53 g/t Au and 30 g/t Ag, and a copper-rich Lower zone containing a resource of 11.4 Mt grading 1.62% Cu and 0.48% Zn (Johnson et al., 1999). 

 

Central Mexico is known for significant silver ± lead, zinc and gold deposits.  These occur as epithermal veins, locally with bonanza grades, and related manto replacement bodies.  Major vein deposits in the area include those at Fresnillo and Zacatecas.  Much of the epithermal mineralization is hosted by Lower Tertiary volcanic rocks and is mid-Tertiary in age (Chavez et al., 1999).  Some deposits are hosted by Upper Jurassic-Lower Cretaceous volcanic rocks (e.g., veins around Zacatecas) and Lower to Upper Cretaceous carbonates and siliciclastic sedimentary rocks (e.g, Real de Angeles), although the mineralization is still interpreted to be Tertiary.  Cretaceous sedimentary rocks also host the Francisco I. Madero zinc-lead-silver deposit west of Zacatecas.  Initially discovered in 1974, Francisco I. Madero has been postulated to be sedex-type mineralization, although a high temperature replacement origin has also been proposed (Chavez et al., 1999).  The possible sedex origin of Franciso I. Madero was the first indication that syngenetic sulfide mineralization might exist in the Zacatecas area.  Volcanic rocks in the lower part of the stratigraphy, the Upper Jurassic to Lower Cretaceous Chilitos Formation, were recognized as submarine sequences containing extensive pillow basalts (de Cserna, 1976), but VMS deposits were not considered a serious exploration target.

 

Exploration in the San Nicolás area from 1982 until 1995 was directed towards epithermal silver-gold targets.  In 1996, Minera Teck S.A. de C.V. (the Mexican subsidiary of Teck Corporation) and Western Copper Holding Ltd. formed a joint venture to explored the El Salvador property.  The initial target was copper oxide showings, particularly those exposed in a small pit at El Salvador.  Early inspection of the pit suggested that mineralization might be stratiform and possibly of VMS origin.  This suggestion was confirmed during diamond drilling when drillhole SAL-5 intersected 2.1 m of massive sulfide grading 2.07%Cu, 1.53% Pb, 16.57% Zn, 3.68 g/t Au and 213 g/t Ag.  Further drilling outlined a small zone of massive sulfide mineralization hosted by siliceous sedimentary rocks at the contact between underlying felsic tuffs and breccias and overlying porphyrtic andesite.

 

Following the discovery of VMS mineralization at El Salvador, geological mapping and related sampling, and airborne geophysical surveys (magnetics, EM, radiometrics) were carried out.  These assisted with the interpretation of the geology on the property, identified prospective volcanic sequences and other copper showings, but failed to define specific drill targets. During the summer of 1997, Quantec Geofísica de México, S.A. de C.V was contracted to carry out an induced polarization (IP) survey over El Salvador.  The known mineralization at El Salvador was detected using Quantec’s gradient array IP approach (“Realsection IP”), however property-wide use of this technique was prohibitively expensive.  Consequently, broad (AB spacing of 900 m) time domain gradient array IP was selected for further work on the basis that this would detect mineralization similar to that at El Salvador within 200 m of surface.  Twenty-two line km were completed at 100 m line spacing in the El Salvador area, resulting in the definition of a large (500 x 500 m) zone of high chargeability and low resitivity centred approximately 2 km west of El Savador.  Diamond drilling commenced in November 1997, resulting in the intersection of 179.7 m of massive sulfide in drill hole SAL-25.  By mid-1998, the San Nicolás deposit had been delineated by 58 diamond drill holes and preliminary reserve figures were announced later that year.

 

The Main zone of massive sulfide at San Nicolás forms a keel-shaped lens up to 280 m thick, 900 m long and 200-400 m wide.  A thick pile of felsic volcanic rocks on the southwest side of the massive sulfide body is interpreted to be a felsic dome (Johnson et al., 1999).  The upper part of the massive sulfide body, 2-35 m in thickness, consists of laminated to brecciated polymetallic sulfide with minor barite that is overlain by black mudstones.  The bulk of the massive sulfide is composed of fine-grained pyrite, which is locally brecciated and contains minor chalcopyrite and sphalerite. The basal contact of the zone grades into stringer mineralization.  The Lower zone is a tabular chalcopyrite-rich southwest dipping body, which appears to join the southeast part of the Main zone at a depth of approximately 400-450 m below surface.  The Lower zone consists largely of stringer and replacement mineralization and is interpreted to be a feeder to the Main zone (Johnson et al., 1999).

 

The top of the massive sulfide lies 150-220 m below the surface and is covered by up to 100 m of unmineralized mafic volcanic flows, fragmental rocks, and volcaniclastic and argillaceous sedimentary rocks of presumed similar age to the underlying volcanic sequence and massive sulfides (Upper Jurassic-Lower Cretaceous). These rocks are in turn covered by 50-150 m of Tertiary volcaniclastic breccias, which locally contain saline ground waters.

 

Following the discovery of the deposit, geological, geochemical and geophysical work has continued with the aim of improving the understanding of the deposit and assisting exploration elsewhere on the property. The presence of conductive saline water in the Tertiary cover sequence causes problems for most electrical geophysical techniques.  As a result, a combination of gravity and IP appear to best define the deposit although similar responses are generated by a combination of dense lithologies and sulfidic sediments.  The depth of mineralization below unmineralized bedrock and post-mineral cover precludes the use of conventional geochemistry.  The use of selective leach geochemistry has been investigated but results are ambiguous.

 

The discovery of the San Nicolás VMS deposit resulted from two main factors:

1. The early realization that mineralization in the El Salvador area might have a VMS origin and refusal to accept the local dogma – that VMS mineralization did not exist in the area.

2. The adaptation of exploration techniques to suite the local environment and provide effective coverage of the property.

Pogo, Alaska

High-grade gold mineralization was discovered recently in the Liese Zone on the Pogo Claims, located in east-central Alaska approximately 145 km east of Fairbanks.  The Liese Zone consists of at least three tabular gently dipping quartz bodies designated L1 (uppermost), L2 and L3 (lowermost). A conservative geological resource for the L1 and L2 bodies is 9 Mt at an average grade of 17.7 g/t gold (Smith et al., 1999). 

 

 

Central Alaska has been an area of active gold exploration since the late-1980s following the discovery of the Fort Knox gold deposit (Bakke, 1995).  Mineralization at Fort Knox occurs in pegmatite, sheeted quartz ± K-feldspar, and quartz shear veins that cut a small stock of porphyritic granite.  The association of gold mineralization with similar mid-Cretaceous intrusions was subsequently recognized throughout the Fairbanks area and in an extensive belt extending for approximately 700 km through the central part of the Yukon (McCoy et al., 1997; Thompson et al., 1999).  Most of the mineralization of interest occurs as sheeted quartz veins in the intrusions, but mineralization in veins, structures and replacement zones also occurs outside intrusions (e.g., Ryan Lode, True North, Brewery Creek; Sillitoe and Thompson, 1998; Thompson et al., 1999).  Mineralization is typically characterized by an association with As, Sb, Bi, W and Te.

 

Most of the exploration in Alaska for intrusion-related mineralization focused in and around the intrusions of known mid-Cretaceous age in the Fairbanks region, particularly in areas with historical or active placer operations.  By the early 1990s, however, exploration began to spread to other areas of central Alaska.  In 1981, prior to this phase of gold exploration, WGM Inc. had conducted stream sediment and pan concentrate geochemical sampling in the Goodpastor River area and had identified Au, As and W anomalies in Pogo and Liese Creeks, creeks with no history of serious placer mining.  In 1991, these data were used as the basis for staking claims in the Pogo Creek area, subsequently packaged as the Stone Boy joint venture between Sumitomo Metal Mining Arizona Inc. and three other international companies.  The other companies withdrew in 1993 and WGM operated the project on behalf of Sumitomo until mid-1997 when Teck became operator through an agreement with subsidiaries of Sumitomo Metal Mining and Sumitomo Corporation whereby Teck may earn a 40% interest.

 

Following staking in 1991, surface exploration around the Pogo and Liese Creeks led to the definition of a 2 km² area of >100 ppb Au in soils.  Diamond drilling of this anomaly in 1994 intersected gold mineralization in quartz veins in what became the Liese Zone. Drilling continued in 1995 and 1996, returning additional mineralized intercepts, mostly at depths greater than 100-150 m below the surface, which were interpreted to be steeply dipping parallel veins.  In 1996, it was suggested that the intercepts might be part of several relatively flat zones, a model that was confirmed by drilling in 1997.  Further drilling in 1998 led to the preliminary resource estimate mentioned previously, and on-going drilling is aimed at further definition of this resource.  An adit is being driven to permit in-fill drilling and testing of mineralization at depth in the L3 zone.

 

The Liese Zone, currently comprising the L1 and L2 zones, is hosted by gneissic rocks of probable mid-Paleozoic age, approximately 1.5 km south of the mid-Cretaceous Goodpaster Batholith.  The L1 and L2 quartz bodies are discordant to gneissic fabrics, are 1-20 m thick and dip 25-30^o to the north or northwest.  The quartz, which appears to be both vein-fill and replacement in origin, contains approximately 3% ore minerals, including pyrite, pyrrhotite, loellingite, arsenopyrite, chalcopyrite, various bismuth minerals and gold (Smith et al., 1999).  The mineralized intervals contain elevated Ag, Te, Bi, As, Sb, Cu, Pb, Mo and/or Co and exhibit a strong correlation between Au and Bi.  This geochemical signature is similar to other intrusion-related gold systems in the Fairbanks district and central Yukon, but the L1 and L2 zones differ in the morphology and geometry of the mineralized bodies.  Based on preliminary data, the Liese Zone is interpreted to be a deep-seated manifestation of an intrusion-related gold deposit, although the progenitor and precise age are uncertain (Smith et al., 1999).

 

Potentially economic mineralization discovered to date on the Pogo claims occurs at depths >100 m in a relatively flat lying geometry that conforms to topography.  This geometry hinders discovery.  Relatively flat-lying auriferous quartz vein deposits are rare and are poorly understood.  Early recognition of this geometry was important to the evaluation at Pogo and is likely to be critical to exploration in other areas.  The nature and geometry of the L1 and L2 zones provide no useful geophysical expression and these were not the source of the anomalous stream and soil geochemistry.  The surface geochemical signature is interpreted to reflect minor hanging wall mineralization probably subcropping in Liese Creek.  A halo of weak mineralization is perhaps the best indicator of mineralization at depth, particularly if the geological environment is permissive for intrusion-related mineralization and subhorizontal vein geometry. 

 

The discovery of the Pogo deposit resulted from pragmatic exploration in a region that was permissive for intrusion-related mineralization, but was outside the area that was regarded as most prospective and was not associated with significant placer gold.  The unusual geometry suggests that this type of deposit is unlikely to be exposed and its lack of geophysical expression requires a combination of practical exploration – drilling – and careful geological interpretation.  The discovery at Pogo represents a different but intriguing example of exploration at depth.

Summary

Both of the discoveries described above illustrate the flexible and pragmatic approach to exploration pursued by Teck, an approach that is particularly important when exploring for deep or covered targets.  Exploration utilizes current geological concepts and exploration methodology that is effective in the area of interest. Exploration is not however driven by highly specialized models or reliance on individual techniques.  Follow-up exploration by Teck Corporation and partners in both areas is continuing to investigate appropriate exploration techniques and adapt them as necessary for efficient and effective exploration.

Acknowledgements

Many geologists working for Teck Exploration and Teck’s partners were involved in the success of programs described herein and are acknowledged for their contributions. This paper was improved by comments from Fred Daley, Roger Scammell and Moira Smith.

References

Bakke, A.A., 1995, The Fort Knox ‘porphyry’ gold deposit – Struturally controlled stockwork and shear quartz vein, sulphide-poor mineralization hosted by a Late Cretaceous pluton, east-central Alaska.  In: Schroeter, T.G. (ed) Porphyry deposits of the northwestern Cordillera of North America.  Can. Inst. Mining Metall., Spec. Vol., 46, p. 795-802.

Chavez Martinez, L, Ubaldo Alarcon L, F. and Parga P, J de J, 1999, Mining exploration potential of VMS and carbonate-hosted polymetallic deposits in cetral Mexico, In: Jambor, J.J. (ed), VMS and Carbonate-Hosted Polymetallic Deposits of Central Mexico, B.C. and Yukon Chamber of Mines Cordilleran Roundup, p. 1-13.

de Cserna, Z., 1976, Geology of the Fresnillo area, Zacatecas, Mexico, Geol. Soc. America Bull., v. 87, p. 1191-1199.

Johnson, J., Montante, A., Kearvell, G., Janzen, J. and Scammell, R., 1999, Geology and exploration of the San Nicolas polymetallic (Zn-Cu-Au-Ag) volcanogenic massive sulfide deposit, In: Jambor, J.J. (ed), VMS and Carbonate-Hosted Polymetallic Deposits of Central Mexico, B.C. and Yukon Chamber of Mines Cordilleran Roundup, p. 45-54.

McCoy, D., Newberry, R.J., Layer, P., DiMarchi, J.J., Bakke, A.A., Materman, S., and Minehane, D.L., 1997, Plutonic-related gold deposits of Interior Alaska. In: Goldfarb, R.J. and Miller, L.D. (eds), Mineral Deposits of Alaska, Econ. Geol., Mon. 9, p. 191-241.

Sillitoe, R.H. and Thompson, J.F.H., 1998, Intrusion-related vein gold deposits: types, tectono-magmatic settings and difficulties of distinction from orogenic gold deposits.  Resource Geol., v. 48, p. 237-250.

Smith, M., Thompson, J.F.H., Bressler, J., Layer, P., Mortensen, J.K., Abe, I. And Takaoka, H., 1999, Geology of the Liese zone, Pogo property east-central Alaska, Soc. Econ. Geol., Newsletter Number 38, p. 1-21.

Thompson, J.F.H., Sillitoe, R.H., Baker, T., Lang, J.R. and Mortensen, J.K., 1999, Intrusion-related gold deposits associated with tungsten-tun provinces, Mineral. Deposita, v. 34, p. 323-334.