Spar Lake, Troy

Montana, USA

Main commodities: Cu Ag
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The Spar Lake sediment hosted copper-silver deposit is located some 27 km south of the town of Troy in Lincoln County, north-western Montana, USA (#Location: 48° 13' 46"N, 115° 15' 35"W).


The earliest exploragtion in the area located several small mineral occurrences in the 1920s and 1930s. Between the 1960s and early 1980s, three significant Ag-Cu deposits were outlineded by Bear Creek Mining Co (Kennecott Utah Copper), namely Spar Lake (Troy), Rock Creek and Rock Lake (Montanore), as well as numerous smaller deposits. All are hosted within the Revett Formation, definig a narrow corridor of mineralisation extending from the Coeur d'Alene mining district in the south to the Kootenai River in the north.
  Drilling from 1964 to 1967 delineated the Troy deposit, optioned to ASARCO in 1973 to develop and operate a mine. Room-and-pillar underground mining of the 18 m thick middle quartzite ore zone was commenced in 1981, and lasted for 12 years. During that period, the mine produced around 8500tpd of ore, which yielded a total of 1380 tonnes (44.4 Moz) of silver and 0.1768 Mt of copper.
  The operation was placed on a care and maintenance in 1993, following a fall in metal prices. Revett Silver purchased the property in 1999, and as metals prices began to improve, brought the mine back into production, with the first concentrate produced in the December 2004. To the end of 2010, the mine has produced 199 t (6.4 Moz) of silver and 0.57 Mt of copper.


The Spar Lake orebody is hosted by the 0 to 600 m thick Revett Formation of the Ravalli Group, which is in turn a member of the up to 20 000 m thick Mesoproterozoic Belt Supergroup sediments (predominantly clastics with only minor carbonates) which were deposited within an intracratonic basin on the western margin of the Canadian Shield, and are now widespread in north-western USA and south-western Canada.

Click here for a regional setting image.

The Revett Formation is composed of a series of coarse and fine lithofacies.   The coarse facies are very variable, but characteristically comprise thick sets of crosslaminated, well sorted, fine to medium grained quartzites, generally accompanied by minor siltite and argillite.   These sandy sets range from 5 to 200 m in thickness.   Thin lensoid intraformational conglomerate beds with pebble to cobble sized clasts are present in the coarser facies at Spar Lake, and as such according to Hayes (1986) are the only sedimentary features unique to the mineralised areas.

The fine lithofacies are from a few to 100 m thick, made up of thin and very thin beds of poorly sorted quartzite, siltite and argillite in graded beds.   The graded beds have ripple cross laminated siltite or quartzite at their base grading upwards to wavy or flat laminated argillite.   The quartzite-argillite cycles are 5 to 45 cm thick while the siltite-argillite cycles are a few mm' to a few cm's thick.   Argillite laminae in the upper sections of the cycles are commonly disrupted by dessication cracks, soft sediment intrusions, scour surfaces and/or water escape structures.

In the Spar Lake area the Revett Formation has been divided into Lower, Middle and Upper Members on the basis of the relative abundance of these two lithofacies.   The Lower Member comprises alternating quartzite and siltite beds, the Middle Member is dominantly siltite and the upper bed is composed of alternating quartzites and siltite with the quartzite predominating.

Ore grade mineralisation is restricted to the Upper Member only, although mineralisation is found in all lithologies of the Member, with higher Cu-Ag grades restricted to the coarse facies rocks, although non-grade accumulations of copper mineralisation are also found in the Lower Member of the Revett Formation.

At Spar Lake the Upper Member is subdivided into five beds, three quartzite beds (the lower, middle and upper), separated by two silty beds (the lower and upper). The ore is predominantly restricted to the Middle Quartzite Beds, with lesser zones in the other two quartzite beds of the Upper Member.

The Spar Lake deposit is influenced by a number of faults, all of which are steep and normal. The two most significant are the East and Cross Faults. The East Fault trends in a NNW-SSE direction and largely limits the eastern margin of the orebody. Movement is east side down. Mineralisation and the lateral mineralogical zonation occurs on both sides of the fault, although to the east the ore grade is in the Upper and Middle Quartzite Beds where it represents the southeast trending continuation of the main orebody, while to the west where the major portion of the ore is found, it is in the Middle and Lower Beds. The ore is generally at an RL that is 45m lower to the east, which is less than the total displacement on the fault. This can be taken to imply that the fault is pre ore, but has undergone further movement after ore emplacement.


The main Spar Lake orebody is flat lying to gently dipping with an average thickness of 21 m and lateral dimensions of 2300 x 500 m.   Its margins and mineralogical zone boundaries (ore, sub ore and gangue) are elongated parallel to the East Fault. Underground the East Fault is up to 10 m thick and variably mineralised with anastomising 25 to 50 cm thick zones of gouge containing high grade veins (chalcocite-clay, quartz-Cu sulphide veins) in places while at others it is barren.

There is a mineralogical zonation including and surrounding the main ore deposit. Both vertically and laterally outwards from the core of the orebody the zonation is as follows:

Chalcocite-chlorite -to- bornite-calcite -to- chalcopyrite-calcite -to- galena-(chalcopyrite)-calcite -to- pyrite-calcite.

On the southeastern margin of the orebody each of these zones grades into a chalcopyrite-ankerite zone, while on the distant margins both the pyrite-calcite and chalcopyrite-ankerite zones grade into what is known as the 'lavender zone' which is characterised by hematite rather than a sulphide.

Each of these zones is characterised by the minerals present, irrespective of their abundance. While the orebody is exclusively within the chalcocite-chlorite and bornite-calcite zones, there are sections of each of these zones that do not achieve ore grade. At Spar Lake ore grade are only developed where these two zones are present within the quartzites, never in the fine facies (according to Haynes 1989). The East Fault closely limits the bornite-calcite zone to the east, except for the south-eastern extension of the orebody. The other zones however do continue across the fault. Vertically these zones are influenced by the upper and lower silty bed. Where the bornite-calcite zone come into contact with these beds some zones are telescoped or absent.

Silver occurs in solid solution in chalcocite, bornite and digenite and as native silver in the chalcocite-chlorite zone. There is 0.8 to 1.5% Pb in the galena-calcite zone which apparently is detrimental to the metallurgical circuit.

Sulphur isotope studies of chalcopyrite-galena pairs in the galena-calcite zone indicate a gradient outwards from the chalcopyrite-calcite zone of temperatures of formation of >120 to 50°C near the pyrite-calcite zone. Similarly fluid inclusion studies indicate a temperature of formation for the bornite-calcite and chalcocite-chlorite zone of between 150 and 170°C, while temperatures in the chalcopyrite-ankerite zone are interpreted as being comparable to those in the galena-calcite zone (Hayes 1990). This implies that the ore zone bornite-calcite and chalcocite-chlorite zones were formed at a higher temperature than the enveloping zones, and hence the ore bearing fluids did not travel into the ore zone laterally along the 'ore bed', but outwards from the chalcocite-chlorite core. In the light of the temperature data, the coincidence of the East Fault and the eastern margin of the chalcocite-chlorite zone and the elongation of the ore along this fault has more significance. The chalcopyrite-ankerite zone is taken to be a lower grade zone developed from the overprint of Cu mineralisation onto the oxidised hematitic lavender zone, in contrast to the ore zone which appears to have been introduced to a reduced sulphidic zone, probably from the same source zone which borders both zones, ie., the East Fault. The temperature studies imply a reduction in temperature away from the East Fault due to mixing with lower temperature fluids in the host sandstone and hence this boundary is temperature gradient controlled. The porosity does appear important, in a gross sense, with all of the ore being present in the three quartzite beds of the Revett Formation, Upper Member. In addition sub ore grade mineralisation (a few hundred ppm and higher) Cu is also found in the Lower Member in the coarser sections of Beds A, C and I.
Clot and disseminated ore
Upper. Typical ore at Spar Lake within weakly bedded quartzite. The reddish line near the bottom of the image is bedding, with other faint planes being evident towards the centre of the image. Bornite and chalcocite occurs as fine disseminations and as clots (arrow hear on upper image).
Lower. Close up of clot from upper image. Arrow head indicates a disseminated bornite grain. There is a weak trend for disseminations to follow bedding as seen to the left of the clot. Note the bright specks are due to reflectance from dark crystal faces, rather than light minerals. Scale graduations are in millimetres.

The ore bearing quartzite is a variably laminated clean looking pale grey rock with a generally vitreous appearance. It is fine grained with bedding made obvious by fine sporadic millimetric laminae that are marginally darker than the groundmass and by light (almost white) leucoxene laminae. Copper mineralisation occurs as disseminations of miniscule flyspecks of bornite and chalcocite (with a grain size determined by the pore space in the host quartzite) that are grossly distributed along particular laminae and are barely discernable without a hand lens. Such bands of disseminated sulphides follow both planar and cross bedding foresets as well as bands that are not apparently related to bedding or fractures. Mineralogical zone boundaries have been observed to cross bedding and bands of disseminated sulphides. Within the ore zone the Cu minerals are the only sulphides present.

Bornite and chalcocite are also present as coarser clots, again sometimes preferentially developed along particular laminae, although seldom in contact. These clots appear as dark circular to irregular spots from 1mm to 1cm in diameter and are very sporadically distributed, not being evident over significant areas, and then being common in others. In some clots iron oxide minerals are present at the core. The clots are not solid sulphide, but usually contain 20 to 70% detrital silica within their confines and in some cases have a dusting of pink hematite on their outer margins.

Thin hair line fractures also carry some bornite and chalcocite within the host quartzites, accounting for less than 10% of the ore. The bulk of the copper mineralisation is present as the disseminated chalcocite and bornite 'flyspecks' described above, with a lower proportion as clots. Thin lensoid siltite bands a few cm's thick or less are found within the ore bearing quartzite.
Fracture ore
Upper. Spar Lake clot and disseminated ore in a more siliceous, indurated quartzite.
Lower. Remnants of bornite and chalcocite that were developed on a thin fracture in a sample that has broken along the fracture. Scale graduations are in millimetres.

The pre-mining geological reserve has been variously quoted as:

58 Mt @ 0.76% Cu, 54 g/t Ag, to 64 Mt @ 0.74% Cu, 54 g/t Ag.

NI 43-101 compliant Ore Reserves and Mineral Resources at the end of December 2013 were (Revett Mining website, 2014):
    Proved reserves - 1.40 Mt @ 48.7 g/t Ag, 0.70% Cu;
    Probable reserves - 13.63 Mt @ 34.3 g/t Ag, 0.33% Cu;
    TOTAL reserves - 15.03 Mt @ 35.7 g/t Ag, 0.36% Cu

    Measured resources - 45.99 Mt @ 45.6 g/t Ag, 0.65% Cu;
    Indicated resources - 22.75 Mt @ 35.0 g/t Ag, 0.34% Cu;
    TOTAL measured + indicated resources - 68.74 Mt @ 42.2 g/t Ag, 0.55% Cu.
    Inferred resources - 1.4 Mt @ 24.3 g/t Ag, 0.30% Cu.

Pillars included in the Measured and Indicated resources contain 45.28 Mt @ 45.6 g/t Ag, 0.65% Cu.

The nearby Rock Creek deposit has resourecs as follows at the end of December 2013 were (Revett Mining website, 2014):
    Inferred resources, Chicago Block - 70.8 Mt @ 49.7 g/t Ag, 0.65% Cu;
    Inferred resources, St. Paul Block - 43.5 Mt @ 72.0 g/t Ag, 0.92% Cu;
    Inferred resources, North Basin Block - 9.1 Mt @ 51.4 g/t Ag, 0.57% Cu;
    TOTAL Inferred resources - 123.4 Mt @ 57.2 g/t Ag, 0.77% Cu.

Note: tonnages are quoted as 'Mst' (million short tons ?) and silver grades as opt. These have been converted to the internationally accepted units million tonnes (1000 kg) and g/t.

For detail consult the reference(s) listed below.

The most recent source geological information used to prepare this summary was dated: 1992.    
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd.   Unauthorised copying, reproduction, storage or dissemination prohibited.

  References & Additional Information
   Selected References:
Aleinikoff J N, Hayes T S, Evans K V, Mazdab F K, Pillers R M and Fanning C M,  2012 - SHRIMP U-Pb Ages of Xenotime and Monazite from the Spar Lake Red Bed-Associated Cu-Ag Deposit, Western Montana: Implications for Ore Genesis : in    Econ. Geol.   v.107 pp. 1251-1274
Hayes T S, Einaudi M T  1986 - Genesis of the Spar Lake strata-bound Copper-Silver deposit, Montana: Part I. Controls inherited from sedimentation and preore diagenesis: in    Econ. Geol.   v81 pp 1899-1931
Hayes T S, Landis G P, Whelan J F, Rye R O and Moscati R J,  2012 - The Spar Lake Strata-Bound Cu-Ag Deposit Formed Across a Mixing Zone Between Trapped Natural Gas and Metals-Bearing Brine : in    Econ. Geol.   v.107 pp. 1223-1249

Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge.   It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published.   While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo takes no responsibility what-so-ever for inaccurate or out of date data, information or interpretations.

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