Prosperity, Fish Lake

British Columbia, Canada

Main commodities: Au Cu
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The Prosperity or Fish Lake copper-gold deposit is located 130 km SW of Williams Lake and 10 km north of Taseko Lakes in south-central British Columbia, Canada.

Published reserve and resource figures include:

    544Mt @ 0.32% Cu, 0.55 g/t Au (Resource, 1991),
    633 Mt @ 0.25% Cu, 0.466 g/t Au, 0.5 g/t Ag, (Mineable mineral reserve, 1997),
    491 Mt @ 0.22% Cu, 0.43 g/t Au (Measured + indicated resource, 2005).

The ore deposit lies within an embayment in the north contact of a northwest elongated, fine-grained, Late Cretaceous porphyritic quartz diorite stock and Lower Cretaceous marine shale and greywacke and marine to non-marine Lower to Upper Cretaceous calc-alkaline andesitic to dacitic ash-tuffs and pyroclastic rocks with intercalated massive to porphyritic flows. East trending feldspar porphyry dyke swarms to the north of the stock, and north to NW trending faults north and south of it, disrupt the Cretaceous succession. The deposit occurs where biotite hornfelsed sedimentary, volcanic and pyroclastic country rocks along the contact are intruded by a complex of post-diorite dykes and small stocks. Much of the area was subsequently covered by Miocene lavas and Pleistocene to Recent alluvial deposits. The deposit is exposed an erosional windows through the Miocene lavas.

The country rocks are a mixed assemblage of massive to porphyritic volcanic and volcaniclastic rocks, greywackes and shales which dip about 20 degrees. They are pervasively converted to biotite hornfels, with the distribution of biotite being complicated both by superimposed hydrothermal biotite alteration and by later overprinting of argillic-propylitic alteration assemblages. Porphyries are grouped into i). pre-ore plagioclase- and quartz-plagioclase porphyry and ii). post-ore mafic plagioclase porphyry. Two pre-mineral and one post-mineral phase of porphyries have also been differentiated. These are texturally, similar, with the distinctions being based of alteration and relative percentages of quartz.

Mineralisation is contained within a plagioclase porphyry diorite of the intrusive complex and within hornfels altered Cretaceous andesitic ash tuffs and flows adjacent to and below the intrusive. Mineralisation terminates out at a depth of 600 m where a thrust fault cuts the volcanic unit. Mineralisation within the intrusive from the surface down has a grade of 0.24% Cu, 0.47 g/t Au, while that within the volcanics at a depth of around 350 m and greater, carries between 0.3 and 0.6% Cu and 0.5 to 0.7 g/t Au. A later cross-cutting phase of the intrusive complex, a plagioclase quartz porphyry diorite carries zones of around 0.15% Cu.

The volcanic to sedimentary country rock appear to have been subjected to at least two pulses of pervasive biotite alteration: i). during intrusion of the large porphyritic quartz diorite stock, and ii). accompanying intrusion of the 'younger' plagioclase porphyry and quartz-plagioclase porphyry bodies.

Alteration is distributed coaxially in a north-east direction across the deposit. A core of K-silicate alteration (biotite, magnetite, chlorite and minor anhydrite) grades to an outer phyllic zone with quartz, sericite and pyrite coincident with quartz veins containing tetrahedrite, sphalerite and galena. The inner, first stage biotite (K-silicate) alteration was partially, or in places, totally destroyed by the overprinting, inner biotite and outer phyllic-argillic alteration characterised by formation of sericite and quartz, accompanied by carbonate, with lesser clay, hydromica, and some gypsum and actinolite. This younger alteration has pervasively altered large areas of hornfelsed rocks outside the outer 0.25% Cu contour. At depth, alteration in the volcanics is characterised by chlorite-magnetite alteration with no biotite, and with no phyllic overprint. Outside of the phyllic envelope higher in the deposit, there is a zone of propylitic alteration consisting of epidote-chlorite-calcite with superimposed phyllic quartz veins carrying Au. A late epithermal over-print of quartz veins with tetrahedrite and sphalerite has also affected the deposit.

Despite the differing ages of alteration, there are coherent grade, alteration, and metal zonation patterns apparently controlled by the younger intrusives, particularly the quartz-plagioclase porphyries with the best copper grades in zones of biotite alteration commonly adjacent to these porphyries which cut older porphyritic intrusions and hornfelsed volcanic and sedimentary rock. The hydrothermal system was maintained for some time after the younger porphyries and somewhat beyond the time when post-ore hornblende plagioclase porphyry dykes were emplaced.

The earliest veining post-dates pervasive biotite alteration and comprises quartz, magnetite, hematite, sulphides and chlorite. Carbonate was subsequently added to the assemblage. The main stage mineralisation resulted in the deposition of sulphides, along with quartz, biotite, chlorite and sericite. The second phase biotite and the accompanying outer phyllic-argillic alteration associated with the porphyry system was apparently also formed at this time. Late main stage veining comprises barren quartz, quartz with sulphides, carbonate and hematite, and gypsum with chlorite or pyrite. Gypsum with minor amounts of anhydrite followed by carbonate veining marked the collapse of the hydrothermal system.

Mineralisation represents multiple overprinting phases of similar vein styles. Vein styles also vary with the alteration type. In the K-silicate zone, which also has a high magnetite content, mineralisation is generally disseminated fracture fill of chalcopyrite and bornite. The phyllic zone contains disseminated and vein hosted gold, while the outer propylitic zone contains larger Au mineralised structures. Elevated mercury characterises the upper 200 m of the deposit with values of up to 30 ppm Hg encountered, commonly associated with tetrahedrite-sphalerite veins.

The earliest vein and fracture fill assemblage comprised magnetite with quartz, hematite, some pyrite and chalcopyrite, and lesser chlorite which cut hornfelsed rocks but were probably deposited very shortly after the hornfels alteration. As the hydrothermal system evolved, the mineralogy of fracture and vein fillings changed with carbonates appearing. During main stage mineralisation chalcopyrite, pyrite and molybdenite accompanied by quartz, biotite, chlorite and sericite were deposited, along with pervasive country rock alteration of varying intensity. During this pervasive alteration, particularly as mafic minerals were altered, disseminated magnetite, pyrite, chalcopyrite and minor amounts of bornite were deposited. Sphalerite has been observed in quartz veins and in quartz-specularite veins with pyrite halos, while arsenopyrite and tetrahedrite occurs locally with pyrite and chalcopyrite.

Sulphide deposition decreased gradually with time, such that during the waning stages, barren quartz veins and quartz+sulphide±carbonate±hematite veins and fractures dominated. Minor amounts of gypsum+chlorite and gypsum+pyrite were also deposited. Formation of gypsum and anhydrite±carbonate and lesser carbonate-hematite veins was followed by deposition of carbonate and finally graphitic carbonate in veins and fractures marking the collapse of the hydrothermal system.

Typical vein and fracture assemblages include: i). quartz+pyrite±quartz+flaky sericite envelopes; ii). quartz±pyrite±chalcopyrite±molybdenite with mainly quartz+flaky sericite envelopes); iii). chalcopyrite±pyrite; iv). pyrite±chalcopyrite; v). biotite+chlorite±pyrite±chalcopyrite; vi). sericite±chlorite±biotite±pyrite±chalcopyrite vii). sericite+quartz±pyrite±chalcopyrite±carbonate.

Mineralisation took place over a significant time span which saw many episodes of fracturing, healing, and refracturing, with several ages of a given vein assemblage overprinting at a single location.

Fracture orientations varied with time. The oldest, magnetite-rich veins are predominantly subvertical, while pyrite and chalcopyrite-bearing veins are also commonly steeply inclined. Late-stage gypsum veinlets dip shallowly, up to 25 to 45°, carbonate, which is calcite in part, occurs in veins with minor chlorite that have dip at between 10 and 75°.

The current dimensions of the deposit based on the >0.40% Cu equivalent cutoff grade are 853 x 1310 m, elongated in an east-west direction and extending to 820 m depth. Two higher grade zones (>0.60% Cu equivalent) have been defined within the overall deposit, namely, the larger Main zone which is 500 x 600 m, again aligned east-west and to 823 metres deep. The West zone is 250 x 183 m, elongated north-south and also extends to a180 maximum depth.

Much of the information in this summary is derived from the British Columbia Geological Survey online MINFILE record.

The most recent source geological information used to prepare this summary was dated: 2006.    
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:
Wolfhard M R,  1976 - Fish Lake: in Sutherland Brown A (Ed.), 1976 Porphyry Deposits of the Canadian Cordillera, Canadian Institute of Mining and Metallurgy,   Special Volume 15, pp 317-322

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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