The Heff magnetite skarn deposit is located on the north side of Heffley Lake, 27 kilometres northeast of Kamloops. Previously, the Heff mineralization has been considered to be either a skarn or to have resulted from syngenetic submarine fumerolic activity. Recent mapping by the Geological Survey Branch (Ray, G.E. and Webster, I.C.L.) proposes a skarn that formed by the infiltration of hydrothermal fluids from the mafic-ultramafic Heffley Creek pluton, which was discovered in the 1999 mapping program. The pluton probably represents an Alaskan-type intrusion, similar to the Tulameen body and other Late Triassic to Jurassic mafic-ultramafic complexes that intrude rocks of the Quesnel and Stikine terranes elsewhere in British Columbia.
The Heff skarn represents an unusual Cu +/- Au +/- REE +/- P-bearing magnetite skarn whose location and distinctive chemistry suggests it differs from the typical iron skarns occurring along the west coast of British Columbia. It possibly resulted from a Fe-oxide +/- Cu +/- Au +/- REE +/- P-bearing hydrothermal system similar to those responsible for deposits in the Ernest Henry (Australia)- Candelaria (Chile)-Wernecke Breccias (Canada) spectrum. The Heff skarn lacks the extensive brecciation and widespread Na +/- K metasomatism that characterizes many deposits of the Ernest Henry-Candelaria-Wernecke Breccias spectrum (Fieldwork 1999). The property area is extensively covered with superficial glacio-fluvial deposits. The stratified rocks were originally mapped as Cache Creek Group but more recently they are considered to belong, in part, to the Paleozoic Harper Ranch Group and/or Upper Triassic Nicola Group. They mainly comprise steeply dipping, northwest striking argillites and calcareous siltstones with lesser andesitic ash and lapilli tuff and some limestone. These rocks were intruded by the possible Late Triassic to Early Jurassic Heffley Creek pluton and then folded and overprinted by lower to sub-greenschist metamorphism producing slaty and phyllitic fabrics. Bleached marbles and calcsilicate-rich metasediments are developed where hydrothermal or thermal alteration has occurred. The magnetite-bearing garnet-pyroxene skarn alteration comprising the Heff property is hosted by limestone and a suite of dioritic minor intrusions; it is best exposed in a number of overgrown trenches that lie 200 to 300 metres north of the Heffley Creek-Sun Peaks Ski Resort road.
Units of blue-grey crinoidal limestone and black argillites occur south of Heffley Lake while coarsely clastic to conglomeratic limestones in the vicinity of the Heff skarn, north of Heffley Lake, lack crinoids and the argillites are less organic-rich. This and other lithological differences suggest that the supracrustal rocks may be separated into northern and southern packages; these are tentatively believed to represent the Nicola and Harper Ranch groups respectively. The northwest trending contact between these packages is thought to pass under the Heffley lakes and continue southeastwards along Armour Creek. This original stratigraphic contact has been intruded by the mafic-ultramafic Heffley Creek pluton and has subsequently been the locus of brittle movement along the Armour Creek fault.
Most of the Heffley Lake pluton lies south and southeast of Heffley Lake and probably forms an elongate body that intruded Nicola and Harper Ranch groups. It contains a variety of rock types, including early ultramafics (pyroxenites and hornblendites), younger mafic and felsic gabbros, diorites and quartz diorites, and minor amounts of late leucocratic monzodiorite. In addition, a strongly altered swarm of andesite sills and dikes on the Heff property is probably related to both the pluton and the magnetite skarns. Disseminated cumulate magnetite is common throughout the pluton.
At least two types of copper-bearing mineralization are recognized in the area. These are: (1) magnetite-rich chalcopyrite +/- Au +/- REE garnet-pyroxene skarns which occur on the Heff property north of Heffley Lake, and (2) disseminations and veins of chalcopyrite +/- magnetite-pyrite mineralization in the Heffley Creek pluton south of Heffley Lake (see Andy Lake, 092INE068; Hawk, 092INE093; and Shaw Hill, 082LNW089).
The Heff skarns have had a long history of exploration. Mineralization is hosted in massive to well bedded limestone within a sequence of massive to poorly bedded calcareous tuffs and green andesitic to dacitic tuffs, grey to black argillite and bedded siltstone proximal to the mafic-ultramafic Heffley Lake pluton. The more extensive pyroxene +/- garnet skarn horizons reach up to 50 metres thick but most are generally less than 2 metres wide. Surface mineralization in the skarn consists of pods and massive lenses of magnetite with lesser amounts of pyrite and pyrrhotite and traces of chalcopyrite. The sulphides occur as disseminations and veinlets, and pyrite is generally dominant to pyrrhotite. Magnetite lenses up to 1 metre wide crop out on surface but massive magnetite zones over 10 metres thick have been intersected by drilling. One of the better drill intersections was 13.7 metres assaying 13 per cent iron and 0.11 per cent copper (Assessment Report 8211). Mineralization occurs locally in the intrusions and in what is believed to be the adjacent exoskarn. On a local scale, many of the calcsilicate layers are mineralogically zoned with a central core of coarser grained garnet-dominant skarn and a wider outer halo of finer grained pyroxene-dominant skarn. Magnetite-sulphide mineralization tends to be better developed in the garnet-dominant skarn. Other minerals identified in thin section in the skarns include epidote, chlorite, quartz, albitic plagioclase, zoisite and ilmenite.
Many grab samples of mineralized skarn contain greater than 25 per cent iron which reflects the abundant magnetite, and some are anomalous in gold (up to 0.44 gram per tonne) and copper (up to 0.11 per cent) as well as being sporadically enriched in REE's (up to 490 ppm cerium and 570 ppm lanthanum).
Copper-iron mineralization north of Heffley Lake was first investigated by surface cuts prior to 1915. The discovery of mineralization led to the staking of a number of claims, including the Monarch, owned by H. McLeod and associates of Vancouver, and about 800 metres in an easterly direction, the Lake View owned by A. MacDonald of Heffley Lake. Work reported included trenching and sampling. One of the claims was reportedly under option to The Granby Consolidated Mining, Smelting and Power Company, Limited. In the early 1940s, the property was held as the Iron Range group, owned by H. Stephens of Kamloops and workings at that time comprised a number of opencuts. Madison Oils Limited staked claims in the area in late 1964 and 1965 after a reconnaissance magnetometer survey extending out from old pits indicated the presence of several strong magnetic anomalies. The property was held as the Hal group of 36 claims, owned by L.C. Hunt for Madison Oils Limited. Geological mapping, sampling of surface pits and 254 metres of diamond drilling in 4 holes were completed in late 1964; two other holes were started but failed to penetrate overburden and were abandoned. A ground magnetometer survey was conducted in 1966. The company carried out a geochemical survey in 1967, which indicated several areas anomalous in copper. Subsequent diamond drilling indicated reserves of 63,497,000 tonnes of 30 per cent iron (Northern Miner, July 27, 1967). In 1967, J.M. Black conducted a magnetometer survey on the Eve 1,2 claims which were located almost 10 years prior on copper mineralization exposed in old trenches. In 1969-70, J.M. Black conducted a magnetometer and scintillometer survey on the Eve 3,9 claims and a magnetometer survey on the Nan 1,2 claims. In 1970-71, J.M. Black on behalf of Western Canada Steel Limited conducted a magnetometer survey on the Eve 5,6 claims. In 1973, line cutting was completed on the Eve 3,5,6,9 and Nan 1,2 claims on behalf of Western Canada Steel Limited followed by magnetometer and EM-16 surveys. In 1976, the property was held as the Freda 1 claim by H. Allen of Merritt who completed one diamond-drill hole totaling 92 metres (deepening of old hole). In 1980, the Heff Lake claims were staked and Cominco Ltd. conducted geological mapping, soil sampling and a magnetometer survey. In 1985, one diamond-drill hole totalling 57.9 metres was completed by H. Allen. The Mesabi claims were staked in 1993-94 on behalf of the Mesabi Syndicate and VLF-EM and magnetometer surveys, and rock sampling was conducted. In 1994, the property was sampled by the Geological Survey Branch as part of a project to examine skarn occurrences throughout British Columbia. This work revealed that the Heff magnetite skarn, unlike the large iron skarn deposits on the west coast of British Columbia, contained anomalous quantities of rare earth elements (up to 570 ppm lanthanum and 490 ppm cerium). In 1994, the property was optioned by Coronation Mines Ltd. and in 1995, soil sampling, VLF-EM surveying and geological mapping was conducted. In 1997, Echo Bay Mines optioned the Mesabi property, staked additional claims and established a ground control grid and performed geological mapping, rock and soil sampling and a ground magnetometer survey. In 1999, the Geological Survey Branch conducted geological mapping in the property area to investigate the potential for Fe-oxide-Cu-Au-REE mineralization of the Candelaria- Ernest Henry types. While this mapping was in progress, a magnetometer and VLF-EM survey was being completed by one of the present property owners. In 2000, a ground magnetometer survey (27.7 kilometres) was completed over the Mesabi claims by R. McMillan.