NOVA SCOTIA GOLD

Within the mining industry, there are varying opinions on the potential for an economic gold deposit in Nova Scotia. For this reason, we will attempt to give some insights on the past as well as current knowledge of the potential of this province’s gold industry, based upon new research by the authors. These studies focused on surface and underground exploration conducted during the past several years.

Recent Developments

During the recent exploration boom in Nova Scotia (1983 to 1987), exploration programs were effective in delineating numerous intersections of gold mineralization in old mining districts. Subsequently, a variety of bulk-sampling techniques were used to evaluate the economic potential of the gold deposits. These included:

* small open pit operations on narrow vein systems (for example, Harrigan Cove, Cochrane Hill, Mile Lake, Beaver Dam, Goldenville and Fifteen Mile Stream);

* selective, underground, narrow-vein mining (for example, Forest Hill, Beaver Dam, Tangier and Moose River);

* bulk underground mining of quartz vein systems, both bedding- parallel type veins and stockwork veins with or without mineralized host rocks (for example, Cochrane Hill and Caribou); and

* bulk open pit mining of disseminated gold in argillites (as at the Touquoy zone at Moose River).

At present, underground feasibility studies and geological investigations on two types of quartz vein systems (bedding-parallel and stockwork) are being carried out at the Caribou gold district. In addition, a large, saddle-reef type en echelon vein system at Upper Seal Harbour (Goldboro) is in the initial stages of a large bulk-sampling program.

A brief geological overview is given below to highlight the many new observations and resulting changes in interpretation affecting the metallogenic history of gold mineralization in southern Nova Scotia.

Stratigraphy

Most known gold mineralization in the province is confined to the Lower Paleozoic Meguma group (about 15 km thick) of southern Nova Scotia. This turbidite sequence is composed of a thick, lower greywacke-dominant sequence (Goldenville formation) conformably overlain by a slate-dominant sequence (Halifax Formation). Although stratigraphic subdivision of the Halifax formation has been attempted locally (O’Brien, 1988), the overall lack of detailed geological mapping remains a major hinderance to mineral exploration. Stratigraphic correlation across the province does not exist, but on a mine scale individual marker beds or sequences of beds are continuous for more than 1 km along strike. Although gold mineralization occurs throughout the Meguma group stratigraphy, the majority of historical gold production came from a 2-km thick sequence in the Goldenville formation immediately below the Goldenville/Halifax contact or transition zone.

Structure

Recent structural studies of the Meguma group in southeastern Nova Scotia (Smith, 1983) have revealed five deformation events (Figure 2). One of the new revelations toward understanding the metallogenic history of the Meguma group is the temporal association of newly recognized late deformational events, hydrothermal alteration, anomalous metamorphism, plutonism, voluminous quartz veining and gold mineralization (Smith and Kontak, 1988a, 1988b; Kontak and Smith, 1987; Kontak et al., 1989). Brittle/ductile shear structures (D4; for example, Forest Hill and Beaver Dam) and brittle crosscutting structures (D5; for example, flexure zone at Caribou) superimposed on earlier regional structures (D2) are fundamental factors controlling mineralization at all Meguma gold districts.

Metamorphism

The role of regional greenschist facies metamorphism (M1) as a mechanism for generating gold mineralization has long been debated for Meguma gold deposits. Recent studies (Kontak and Smith, 1987; Smith and Kontak, 1988a) indicate a stage of anomalous, hydrothermal metamorphism (M2) postdates regional metamorphism, and this M2 metamorphism is spatially associated with the Meguma gold districts and temporally associated with pervasive, hydrothermal alteration and some quartz veining. The associated M2 mineral phases occur in both wall rock and veins and include chlorite, muscovite, biotite, plagioclase, amphibole, garnet and andalusite. Although gold mineralization generally crosscuts the aforementioned mineral phases, minor deformation structures in gold at some districts (Tangier, for example) suggest that M2 metamorphism, D4 shear deformation and gold deposition may be contemporaneous.

Alteration

Lack of alteration has historically been regarded as one of the characteristic features of turbidite-hosted gold deposits, including those of the Meguma group (Boyle, 1979). However, Kontak and Smith (1987) and Smith and Kontak (1988a,b) have described an early stage of pervasive alteration (A1) and a more restricted alteration (A2) within gold districts. A1 alteration consists of silicification, carbonitization, sericitization and sulphidization whereas carbonate and chlorite (plus or minus arsenopyrite) characterize less pervasive A2 alteration. In all observed cases, the alteration is superimposed on M2 metamorphic minerals. Although A1 alteration may extend for several kilometres along strike on either side of individual gold deposits, it is variably developed at all districts.

From an exploration standpoint, the most interesting consequence of the alteration is the broad depression of the magnetic characteristics of the host rocks. Some of the gold districts (Goldenville, Moose River and Mooseland, for example) stand out as small gradiometric highs (caused by high sulphide content) enclosed within large gradiometric lows. Similar patterns may be observed elsewhere in apparently barren parts of the Meguma Terrane where there is thick glacial till cover and no bedrock exposure.

Deposit Mineralogy

Gold is the main ore of the Meguma deposits although minor tungsten (scheelite from Moose River) and antimony (stibnite from West Gore) have also been produced. Associated mineralization includes arsenopyrite, base metal sulphides, pyrrhotite, pyrite, oxides and the newly recognized bismuth-tellurium-silver-sulphur complexes (including native bismuth and several bismuth-tellurium-silver alloys) and well as greenockite (cadmium sulphide). High cobalt and nickel concentrations at some deposits (such as Tangier) are associated with abundant magnetic pyrrhotite.

Gangue mineralogy of the veins includes, in addition to variable quartz content, plagioclase (An0-70), muscovite, biotite, chlorite, tourmaline, apatite, amphibole, epidote, garnet, andalusite and minor amounts of staurolite (as at Cochrane Hill). Modal mineralogy of particular veins is dependant on the vein type and the associated M2 metamorphism.

Absolute Age

Fundamental in understanding the genesis of any mineral deposit is the determination of the absolute age of mineralization. Gold deposits in the Meguma group have generally been regarded as “old” (that is, synchronous with regional D2 deformation). However, 40Ar/39Ar age determinations on vein biotite, muscovite and amphibole from five gold deposits in the eastern Meguma Terrane (Kontak et al, 1988, 1989) indicate that the vein-forming event and attendant mineralization are much younger. In all 12 samples analyzed internally, concordant age spectra were obtained with apparent age plateaus of ca. 370 Ma. The ages are consistent with the relative geologic age relationships established by field observations. Although these absolute ages suggest a temporal association with peraluminous granites throughout the Meguma Terrane, no direct genetic relationship is inferred (i.e., fluid or gold reservoir).

The recent exploration and development boom in Nova Scotia has provided the best opportunity in more than 50 years to investigate the styles and controls of gold mineralization for both quartz vein and disseminated-type deposits. As a result, a new appreciation of gold ore control, not witnessed since E.R. Faribault’s work in the early 1900s, has been acquired. Finally, reliable predictions base
d on a minimum amount of underground development or diamond drilling can be made regarding location, size and orientation of high-grade gold ore shoots in narrow quartz veins. This has the potential of resulting in tremendous monetary savings by permitting one to target downdip extensions during diamond drill programs and of reducing expensive underground exploration development.

Ore Controls

Perhaps the most fundamental geologic problem concerning the economic viability of gold mineralization in the Meguma group is ore grade and continuity. If the geometry of high-grade ore shoots in vein deposits is not established, chances of a successful operation are minimal. This problem is less critical in auriferous argillites. Equally important is the question of gold recovery due to the relatively high content (10% to 30%) of micron-sized gold, some of which is trapped as disseminations within sulphides.

Several structural factors that characterize these high-grade ore shoots include:

— intra-vein slickenside lineations;

— intersections of angular and bedding parallel vein types;

— rapid increase in size of individual bedding parallel veins, both along strike and down dip; and

— spatial association with sulphide alteration.

One or more of these features is associated with high-grade ore shoots in bedding-parallel veins at all Meguma group gold districts. Orientations of ore shoots may vary dramatically between different parallel veins. Wall rocks (that is, altered argillites) adjacent to these vein-type ore shoots generally carry minimal ore grade.

In stockwork vein systems, high-grade gold mineralization is restricted to the core of the stockwork or to sulphide-rich wall rocks intersected by the stockwork (as at Caribou). The stockwork and the high-grade ore are restricted to zones of brittle deformation in competent lithologies (i.e. greywacke) related to late-stage crosscutting flexures (Figure 3). In all cases, individual parts of the stockwork occur as en echelon, centipede-like, cylindrical features (Hodgson, 1989) confined to individual beds (ore zones have plunging tube-like geometry along bedding planes) which together define the axial plane of the flexure. Relative to ore mineralization, there is no apparent stratigraphic control of these zones other than being a competent lithology which yielded to brittle deformation.

At the present time, Caribou is the only Meguma gold district known to have such a flexure-controlled stockwork system. However, similar kink-type structures at Goldenville define high-grade ore in bedding-parallel veins (Hedley, 1941). Detailed examination of a single zone of the stockwork mineralization (specifically, the Main zone at Caribou) has allowed for geological prediction of the unexplored remainder of the flexure zone (and presumably gold mineralization) with great success.

Disseminated gold mineralization in altered argillites void of quartz veining has only recently received attention in Nova Scotia. Open pit bulk sampling of a thick sequence of altered argillite known as the Touquoy zone (in the Moose River gold district) has indicated encouraging results (Figure 4). Average ore reserves from the deposit are 1.05 million tonnes of 2.63 grams per tonne (1.5 grams per tonne cutoff), with the higher-grade part of the body lying immediately below the open pit. The ore zone is confined to the north limb of the Beaver Dam anticline, sandwiched between two perpendicular faults. Carbonate alteration is pervasive throughout the argillites, which host fine-grained (1 to 10 sm) as well as rare coarse (2 mm) disseminated gold mineralization. Gravity separation has been effective in extracting more than 90% of the gold from these rocks.

Gold mineralization associated with sulphide alteration in argillites and greywackes is well known throughout southern Nova Scotia gold districts (Smith and Kontak, 1988a,b). Disseminated, fine-grained and coarse-grained gold in wall rocks adjacent to quartz vein margins has been documented at Cochrane Hill (Smith, 1983) and other deposits. However, it has generally been common practice to ignore all areas within the Meguma group that do not have an abundance of quartz veining and associated sulphides (that is, conventional vein-type deposits). Hence, interest is growing in the potential for high-tonnage, low-grade gold mineralization throughout the Meguma group. At present, there are no clearly defined characteristics for predicting the auriferous nature of a particular argillite unit. However, unusually thick (more than 30 metres) argillites with intense carbonate alteration and superimposed secondary structures appear to provide a favorable host rock. Mineralized argillites are also present at the Fifteen Mile Stream deposit and similar lithologies (with no known mineralization) have been observed by the authors elsewhere throughout southeastern Nova Scotia.

In the recent past, several attempts have been made to mine gold by open pit methods (at Cochrane Hill, Fifteen Mile Stream, Goldenville and Beaver Dam). All have focused on dilution of vein-hosted gold with adjacent wall rock. Although each of these development efforts was unsuccessful in realizing economic, ore-grade gold, none of the development work focused on high-grade oreshoots.

Gold Potential

Opinions vary as to the gold potential of the Meguma group. Past mining history indicates that numerous high-grade oreshoots yielded average ore grades between 0.2 and 0.5 oz. gold per ton, but that deposit size is small and, hence, marginally economic at today’s standards. Nothing was known in the past of the potential for large-tonnage, low-grade disseminated mineralization.

Recent exploration and development work on narrow vein-type deposits in the eastern Meguma Terrane suggests that the “unpredictable nature” of the ore zones, their relatively low tonnage and high development costs render the deposits uneconomic. However, the numerous gold deposits and prospects across the Meguma Terrane suggest that significant gold deposits may be present. Gold is easily extracted from the ore and there appears to be no limit to its distribution, both along strike and downdip (for example, Upper Seal Harbour, Caribou and Tangier). Furthermore, disseminated gold in argillites (as at the Touquoy zone at Moose River) yields reliable assay information and excellent gold recovery as opposed to the inherent problems of assaying coarser-grained, vein-type gold mineralization. In addition, all geological constraints, including field observations and geochemical data, indicate that many Meguma group gold deposits have similar characteristics to Archean mesothermal lodegold deposits, albeit, different host rock environments.

The new wealth of knowledge regarding controls for gold mineralization gained during the recent exploration and development boom may serve to alleviate some of the anxiety experienced by many. Some of that information for both vein and disseminated deposits is summarized below in point form.

Vein Deposits

* The location, size and orientation of high-grade oreshoots along individual quartz veins may be predicted with great accuracy, based on a minimum amount of selective diamond drilling or underground development.

* Stratigraphic continuity within a deposit enhances the ability to project ore zones.

* Large, alteration haloes allow targeting of potential mineralization in till-covered areas.

* Gold disseminations in host rocks adjacent to vein margins enhance the overall ore grade.

* Gold disseminations associated with sulphides (especially arsenopyrite) in wall rock can significantly increase ore potential.

* The identification of superimposed secondary structures (i.e. flexures) are frequently coincident with high-grade oreshoot geometry.

* Excellent gold recovery (both fine-grained and coarse-grained) may be realized with careful gravity separation.

Disseminated Deposits

— Fine-grained gold is amenable to gravity extraction and micron-sized gold may represent further potential.

— Thick argillite units are easily recognized during d
etailed geologic mapping; recognition of secondary structures may indicate areas of potential ore. It is interesting to note that there has been no systematic attempt (with several local exceptions) to define the stratigraphy of the Goldenville formation, yet thick argillite sequences are numerous and are not confined to known districts.

— Intense alteration characterizes mineralized argillite stratigraphy. In addition, geophysics (particularly gradiometer) defines areas with similar responses to known deposits (i.e. Goldenville) and may represent zones of potential new mineralization.

Conclusions

Question: Is it possible to have a profitable gold mine in the Meguma group of southern Nova Scotia?

Answer: a qualified yes. Gold mineralization could be present in sufficient quantities and concentrations to qualify as “good deposits,” and the geological knowledge exists to understand ore zone geometry in vein-type mineralization and, to some extent, in disseminated deposits. Experience and dedication, as in other areas, may be the missing factors critical for a prosperous gold mining industry in Nova Scotia. Hence, favorable political climate, established logistical and infrastructural support, a willing and able labor force, as well as a suitable geological environment, collectively welcome those willing to explore with a clear understanding of the risks involved (as pertains to exploration in similar geological environments).

REFERENCES:

Boyle, R.W. (1979): The geochemistry of gold and its deposits (together with a chapter on geochemical prospecting for the element); Geological Survey of Canada, Bulletin 280, p. 584.

Hedley, P.M. (1941): Geology and structure as related to mining, Guysborough Mines, Goldenville, N.S.; Mining Society of Nova Scotia, Transactions, Vol. XLIV, pp. 251-258.

Hodgson, C.J. (1980): The structure of shear-reladed, vein-type gold deposits: a review; Ore Geology Review, Vol. 4, pp. 231-273.

Kontak, D.J. and Smith, P.K. (1987): Alteration haloes and their implications for gold mineralization in the Meguma Group of Nova Scotia; in Mines and Minerals Branch, Report of Activities 1986, eds. D.R. MacDonald and K.A. Mills; Nova Scotia Dept. of Mines and Energy, Report 87-1, pp. 65-74.

Kontak, D.J., Smith, P.K. and Reynolds, P.H. (1988): Meguma Gold Studies III: Timing of mineralization at the Beaver Dam gold deposit: implications for Meguma gold deposits; in Mines and Minerals Branch, Report of Activities 1987, Part B, eds. D.R. MacDonald and Y. Brown; Nova Scotia Department of Mines and Energy, Report 88-1, pp. 77-83.

Kontak, D.J., Smith, P.K., Reynolds, P.H. and Taylor, K. (1989): 40Ar/39Ar geochronological studies of Meguma gold deposits, Meguma Terrane, Nova Scotia; in Mines and Minerals Branch, Report of Activities 1989, Part A, eds. J.L. Bates and D.R. MacDonald; Nova Scotia Dept. of Mines and Energy, Report 89-3, pp. 49-54.

Kontak, D.J., Kerrich, R. and Williams, P.F. (1990): Integrated model for Meguma Group lode gold deposits, Nova Scotia, Canada; Geology, in press.

O’Brien, B.H. (1988): A study of the Meguma Terrane in Lunenburg County, Nova Scotia; Geological Survey of Canada, Open File 1823, 80 pp.

Smith, P.K. (1983): Geology of the Cochrane Hill gold deposit, Guysborough County, Nova Scotia; in Mines and Minerals Branch, Report of Activities, 1982, ed. K.A. Mills; Nova Scotia Dept. of Mines and Energy, Report 83-1, pp. 225-256.

Smith, P.K. and Kontak, D.J. (1988a): Meguma Gold Studies I: generalized geological aspects of the Beaver Dam gold deposit, eastern Meguma zone, Nova Scotia; in Mines and Minerals Branch, Report of Activities 1987, Part B, eds. D.R. MacDonald and Y. Brown; Nova Scotia Dept. of Mines and Energy, Report 88-1, pp. 45-59.

Smith, P.K. and Kontak, D.J. (1988b): Meguma Gold Studies II: vein morphology, classification and information, a new interpretation of “crack-seal” quartz veins; in Mines and Minerals Branch, Report of Activities 1987, Part B, eds. D.R. MacDonald and Y. Brown; Nova Scotia Dept. of Mines and Energy, Report 88-1, pp. 61-76.