The Prosperity (formerly Fish Lake) deposit lies within an embayment in the north contact of a northwest elongated, fine-grained, porphyritic quartz diorite stock. The Late Cretaceous stock was emplaced into marine Lower Cretaceous shale and greywacke and marine to non-marine Lower to Upper Cretaceous andesitic pyroclastic rocks with intercalated massive to porphyritic flows (possibly correlative with the informally-named Powell Creek and Silverquick formations), which occupy the Tyaughton Trough. The Tyaughton Trough is a successor basin infilled by both marine and non-marine sedimentary and volcanic rocks. Swarms of east trending feldspar porphyry dykes north of the stock, and north to northwest trending faults north and south of it, disrupt the Cretaceous succession. One of these, the Yalakom fault, had significant transgressive movement during Late Cretaceous time accompanied by volcanism and continental sedimentation. Mineralization at Fish Lake may be genetically linked to later stages of this fault movement and volcanism. In the embayment of the contact of the quartz diorite stock where the Prosperity deposit lies, hornfelsed sedimentary, volcanic and pyroclastic country rocks are intruded by a complex of post-diorite dykes and small stocks. Subsequently, much of the area was covered by Miocene lavas or Pleistocene to Recent alluvial deposits. Erosion has opened windows in the Miocene lavas and the deposit is exposed in one of these.
The Prosperity porphyry system occurs in an area of volcanic, sedimentary and porphyritic intrusive rocks within a plication in the contact of the fine-grained porphyritic quartz diorite stock. As a result of contact metamorphism associated with the various intrusions, the country rocks are pervasively converted to biotite hornfels. The distribution of biotite is complicated both by a superimposed wave of hydrothermal biotite alteration and by later overprinting of argillic-propylitic alteration assemblages. The country rock is a mixed assemblage of massive to porphyritic volcanic and volcaniclastic rocks, greywackes and shales which dip about 20 degrees. Porphyries are grouped into pre-ore plagioclase porphyry and quartz-plagioclase porphyry and post-ore mafic plagioclase porphyry. Wolfhard (Canadian Institute of Mining and Metallurgy Special Volume 15) also describes two younger pre-mineral porphyry phases and one post-mineral phase. Texturally, the porphyries are all similar. Distinctions between them are made on the basis of alteration and relative percentages of quartz. Post-ore dykes appear to strike northeast and dip moderately northwest.
Rocks of the Prosperity property have a complex history of alteration. The volcanic to sedimentary country rock appear to have been subjected to at least two periods of pervasive biotite alteration; one resulted from intrusion of the large porphyritic quartz diorite stock, the other from intrusion of the "younger" plagioclase porphyry and quartz-plagioclase porphyry bodies. Matrices of "older" plagioclase porphyries were flooded with biotite during the latter event or events. Vein associations, formation in earlier intrusions, grade distribution, and the alteration zoning patterns indicate that some of the biotite alteration was of hydrothermal origin and was associated with metallization. The zone of best mineralization in the deposit has a core area of pervasive biotite alteration.
From thin section work it is evident that early biotite alteration was later partially, or in places, totally destroyed by younger alteration characterized by formation of sericite, quartz, carbonate, with lesser clay, hydromica, and some gypsum and actinolite. The distribution and relative intensity of the younger alteration have produced a zone of propy-argillic alteration which is apparently fringed by a propylitic zone. The younger alteration laps onto the biotite core and has pervasively altered large ares of hornfelsed rocks outside the 0.25 per cent copper contour of the deposit. Despite the differing ages of alteration, there are coherent grade, alteration, and metal zoning patterns which appear to be controlled by the younger porphyries, particularly the quartz- plagioclase porphyries. It is in, and more commonly adjacent to, these porphyries that the best copper grades occur.
The earliest veining at Prosperity post-dates pervasive biotite alteration and consists of quartz, magnetite, hematite, sulphides and chlorite. With time, carbonate was added to the assemblage. During main stage mineralization, sulphides were deposited along with quartz, biotite, chlorite and sericite. The biotite and propy-argillic alteration associated with the porphyry system apparently also formed at this time. Late main stage veining comprise 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.
Mineralization occurred in an area of ongoing igneous activity and this is reflected in the orientation of veins and mineralized fractures. Through time, dips of dominant fracture orientations varied from steep to moderate or low and back to steep. In part they probably reflect regional stresses but in part they are related to intrusive activity, evolution of magmatic fluids, and pressure generated by the hydrothermal system (Geology in British Columbia 1976).
Emplacement of porphyritic intrusive bodies at Prosperity was accompanied by extensive biotite hornfelsing of the volcanic country rocks. The earliest vein and fracture-filling minerals cut hornfelsed rocks but were probably deposited very shortly after their formation. These veins carry magnetite with quartz, hematite, some pyrite and chalcopyrite, and lesser chlorite. As the hydrothermal system became established and evolved, the mineralogy of fracture and vein fillings changed. For example, carbonate joined magnetite and its associated minerals prior to main stage mineralization. During main stage mineralization chalcopyrite, pyrite, and molybdenite along with quartz, biotite, chlorite and sericite were deposited. Pervasive country rock alteration of varying intensity also occurred during main stage mineralization. During this pervasive alteration, particularly in altered mafic minerals, disseminated magnetite, pyrite, chalcopyrite, and minor amounts of bornite were deposited. Magnetite in altered mafic minerals may be a simple byproduct of alteration but sulphides required addition of sulphur to the system. Sphalerite was observed in quartz veins and in quartz-specularite veins with pyrite halos in drill core. Arsenopyrite and tetrahedrite occurs with pyrite and chalcopyrite in another drill intersection. Typical vein and fracture assemblages include: 1) quartz + pyrite (often with quartz + flaky sericite envelopes) 2) quartz +/- pyrite +/- chalcopyrite +/- molybdenite (mainly with quartz + flaky sericite envelopes) 3) chalcopyrite +/- pyrite 4) pyrite +/- chalcopyrite 5) biotite + chlorite +/- pyrite +/- chalcopyrite 6) sericite +/- chlorite +/- biotite +/- pyrite +/- chalcopyrite 7) sericite + quartz +/- pyrite +/- chalcopyrite +/- carbonate Sulphide deposition decreased gradually. During the waning period barren quartz veins and quartz + sulphide +/- carbonate +/- hematite veins and fractures predominated. 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. These mark the collapse of the hydrothermal system (Geology in British Columbia 1976).
A sample of hornfels containing 40 per cent secondary biotite was obtained from drill core for an age determination by J.E. Harakal at the University of British Columbia. A whole rock age of 77.2 Ma +/- 2.8 Ma was obtained. As there are biotite-sulphide veinlets present, as well as matrix biotite coexisting with quartz-sulphide veinlets, the radiometric age is considered to be the date of mineralization (Canadian Institute of Mining and Metallurgy Special Volume 15).
Orientations of mineralized fractures and veins were measured from vertical drill holes in the deposit. Most dip between 70 and 90 degrees and a less well-developed set of structures dips about 45 degrees. Fracture orientations changed slightly with time. Most of the oldest, magnetite-rich veins are subvertical, although there are lesser concentrations with dips near 65 and 15 degrees. Similarly, pyrite and chalcopyrite-bearing veins are also dominantly steeply inclined. Late-stage gypsum veinlets have dip maxima of 45 and 25 degrees. Carbonate, which is calcite in part, occurs in veins with minor amounts of chlorite. These have dip maxima of 75 and 45 degrees but a significant number also dip between 10 and 40 degrees. Younger carbonate + graphite(?) veins have steep orientations again (Geology in British Columbia 1976).
In summary, alteration at Prosperity is related to younger porphyritic intrusions which cut a country rock comprised of older porphyritic intrusions and hornfelsed volcanic and sedimentary rocks. The best mineralization is in zones of biotite alteration adjacent to, and in, bodies of this "young" quartz plagioclase porphyry. 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. All have been subjected to carbonate-sericite dominated propy-argillic alteration.
While main phase veins and fractures are grouped and treated as a unit, in fact several ages of, for example, quartz + pyrite + chalcopyrite veins occurs in a single piece of drill core. Mineralization obviously took place over a significant time span which saw many episodes of fracturing, healing, and refracturing. The porphyry intrusions may have acted as a heat "engine" to drive a convective cell of metal-bearing hydrothermal fluids. Whether the metals in the system were scavenged from the country rock or supplied by the porphyries is open to speculation (Geology in British Columbia
Current dimensions of the deposit above a 0.40 per cent copper equivalent cutoff grade are 853 metres north-south, 1310 metres east-west and extending to 823 metres deep. The deposit remains open to extension north, west and southwest (George Cross News Letter No.180 (September 17, 1992).
Two higher grade zones (higher than 0.60 per cent copper equivalent) have been defined within the overall deposit. The large Main zone measures 503 metres north-south, 609 metres east-west and to 823 metres deep. A West zone measures 250 metres north-south, 183 metres east-west and to 183 metres deep (George Cross News Letter
Reserves in the combined drill indicated and inferred category as of July 1991 were as follows (George Cross News Letter No.142, July 1991):
| TONNES | COPPER (%) | GOLD (g/t) | STRIP RATIO |
| 526,429,000 | 0.20 | 0.38 | 0.7:1 |
| 449,232,000 | 0.21 | 0.41 | 1.0:1 |
| 361,770,000 | 0.23 | 0.45 | 1.5:1 |
By the end of October 1991, drilling had confirmed a block of 544,200,000 tonnes grading 0.32 per cent copper and 0.55 grams per tonne gold (0.86 per cent copper equivalent) (George Cross News Letter No.209, 1991). The grade continuity is reported to be good and reserves are open in all directions.
Taseko Mines Ltd. has reported a preliminary reserve estimate of 1,080,356,100 tonnes grading 0.23 per cent copper and 0.41 gram per tonne gold for a copper equivalent of 0.52 per cent (George Cross News Letter No. 197 (October 13), 1992.
Mineral Resources Development Inc. has calculated preliminary mineable reserves for the Prosperity deposit. Preliminary mineable reserves are reported for several progressively deeper pit designs (George Cross News Letter No. 50 (March 12), 1993).
Strip Ratio Reserve Grade
Waste:Ore Million Tonnes Cu % Au g/t
0.80:1 148 0.24 0.51
1.16:1 404 0.24 0.47
1.49:1 505 0.24 0.47
1.84:1 640 0.24 0.44
2.11:1 811 0.24 0.44
Note: In situ reserves are within preliminary 42 degree pit designs, no dilution, if any, included; block gold grades cut to 1.0 gram per tonne gold.
The property was first discovered by Phelps Dodge Corporation in 1963. In 1966, their claims lapsed and the area was restaked by Taseko Mines Limited. During the period from 1969 to 1990 a number of mining companies, including Bethlehem Copper and Cominco, completed exploration programs on the property. Early exploration drilling totalled 27,005 metres in 176 shallow holes which determined the potential for a large-scale disseminated gold-copper porphyry deposit. Many of these early drillholes bottomed in significant gold-copper mineralization.
A major multi-rig drilling program conducted from late 1991 to 1993 by Taseko resulted in the completion of 139 large diameter vertical core holes for a total of 77,392 metres. Throughout 1993, Taseko further advanced the project by undertaking a wide spectrum of detailed mine planning and environmental baseline studies, and in August of that year, Taseko commenced the mine permitting process with the province of British Columbia.
A detailed pre-feasibility study completed in 1995 was based on an open-pit reserve of 675 million tonnes grading 0.236 per cent copper and 0.435 gram per tonne gold at a stripping ratio of 1.57 to 1. The company (Taseko Mines Ltd.) has submitted an application for a Mine Development Certificate. The project was renamed Prosperity in 1995 (from Fish Lake) and was under review with respect to the Environmental Assessment Act (Information Circular 1996-1, page 10; 1997-1, page 18). In June 1997, Taseko completed a large diameter core drilling program totalling 49,462 metres in 107 holes.
During 1997, Taseko Mines Ltd. completed pilot plant metallurgical and process testing programs, including bulk sample testing of the Prosperity gold-copper deposit. All pilot plant program results were reported to compare favourably with the previously announced pre-feasibility metallurgical results. Results from a 50,000 kilogram test program averaged 90 per cent copper recovery and 75.6 per cent gold recovery producing a 25 per cent copper concentrate containing 38.9 grams per tonne gold. A detailed geological and gold-copper grade computer model of the deposit, based on 123,414 metres of drilling in 248 holes, is nearing completion and will lead to a new mineable reserve estimate and open pit design and production schedules. In addition, comprehensive environmental and socio-economic studies for presentation to the government Project Review Committee are continuing. The previously established mineable reserve for the deposit was 675,000,000 tonnes grading 0.236 per cent and 0.435 gram per tonne gold. At a projected 90,000-tonnes per day milling rate, the company forecasted annual production of 11,688 kilograms of gold, 16,485 kilograms of silver and 69,460 tonnes of copper, over a mine life of 21 years. The capital costs were estimated at US $430 million. Between 1996 and 1997, Taseko completed a large (13.5 million), in-fill drilling program; this was the largest exploration program ($5 million) in the province in 1997.
Based on 143,945 metres of drilling in 326 holes (including 92 angle drill holes completed during 1996 and 1997), Independent Mining Consultants calculated a new mineable mineral reserve of 633 million tonnes at an average grade of 0.253 per cent copper, 0.466 gram per tonne gold and 0.5 gram per tonne silver (silver grade is based on planned production levels). The geometry and continuity of the mineable mineral reserve provides for efficient open pit mining with an overall life of mine waste to ore stripping ratio of 1.89 to 1. The copper grade has increased by 7.2 per cent and the gold grade has increased by 7.4 per cent from the previously announced mineable mineral reserve which was based on 76,134 metres of drilling in 147 holes. The mineral reserve includes 65 per cent measured, 30 per cent indicated and 5 per cent inferred. (Taseko Mines Limited, Press Release, March 16, 1998). At a projected 90,000 tonnes per day milling rate, the company forecasts annual production of 12,940 kilograms of gold and 95,795 tonnes of copper, over the 25.3-year mine life.
In 2005, Taseko Mines Ltd, applied and was granted an extension of the Environmental Assessment Act period to April 30, 2007. The company continues to communicate and consult with area First Nations, and is reviewing and optimizing previous project economic studies. The most recent information from the company gives an estimated measured and indicated resource of 491 million tonnes grading 0.43 grams per tonne gold and 0.22 per cent copper (Exploration and Mining in BC 2005, page 59).
In September 2007 Taseko Mines Ltd. announced the results of a feasibility study. The Proven reserves reported were 286 million tonnes grading 0.25 per cent Cu and 0.47 grams per tonne Au. The Probable reserves reported were 201 million tonnes grading 0.18 per cent Cu and 0.37 grams per tonne Au. The project is currently in the Environmental Assessment Process (Taseko Mines Ltd. 2007 Annual Report).
In November 2009 Taseko Mines announced an increase in reserves at Prosperity. Reporting Proven reserves of 481 million tonnes 0.46 gram per tonne Au and 0.26 per cent Cu and Probable reserves of 350 million tonnes grading 0.35 gram per tonne Au and 0.18 per cent Cu. This was based on a $5.50 net smelter return cut-off. (Stockwatch News Release November 2, 2009).