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Ministry of Energy Mines and Responsible for Core Review

Geology of the Heffley Lake Area, Southcentral British Columbia
(NTS 92I/16)

BCMEMPR Open File 2000-10

by G.E. Ray and I.C.L. Webster

Digital File (PDF, 17.0 MB)

Open File 2000-10 covers the geology of the Heffley Lake area (92I/16) in south central British Columbia. The 1:20 000 scale coloured geological map includes a legend, an inset map of the Heff skarn occurrence at 1:5 000 scale, a description of the geology and mineralization, diagrams of structural measurements, and tables with major and trace element analyses for the Heffley Creek pluton, assay data from the skarn occurrence, and assay results from mineralized intrusive rocks.

The Heffley Lake area lies approximately 26 kilometres northeast of Kamloops and is accessible via the road between the township of Heffley Creek and the Sun Peaks ski resort. It lies within the Intermontane Belt and is underlain by metasediments of the Quesnel Terrane and younger mafic intrusive rocks.

Past exploration and drilling have been concentrated on the Heff property, which lies at the base of a series of northwest trending limestone cliffs, immediately north of Heffley Lake. In 1915, Cu and Fe mineralization were first noted in the area (MMAR. 1916), and in 1966 drilling intersected over 10 metres of massive magnetite and another 4.5 metres thick zone of semi-massive magnetite with chalcopyrite; the latter zone graded 1.67 percent Cu and 0.48 g/t Au.

In 1980, Cominco identified a broad, 600 metres-long Cu soil anomaly; later, a drill-hole intersected 16 metres of semi-massive magnetite, a 6 metre interval of which assayed 25 percent Fe and 0.5 g/t Au.

Later sampling showed that the Heff magnetite skarn contained moderate anomalous quantities of REE's (up to 570 ppm La and 490 ppm Ce). In 1997, Echo Bay Mines outlined soil anomalies adjacent to the magnetite skarn with values exceeding 1200 ppm Cu and 1000 ppb Au.

The area is extensively covered with superficial glaciofluvial deposits. The stratified rocks were originally mapped as Cache Creek Group but more recently they are considered to belong, in part, to the Harper Ranch 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 folded and overprinted by lower to sub-greenschist metamorphism producing slaty and phyllitic planar fabrics. The magnetite-bearing garnet-pyroxene skarn on the Heff property is hosted by calcareous metasediments and dioritic minor intrusions; it is best exposed in a number of overgrown trenches that are 200 to 300 metres north of the Heffley Creek-Sun Peaks road.

The area southwest of Heffley Lake includes units of blue-grey crinoidal limestone and black argillites whilst the 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 believed to represent the Nicola and Harper Ranch groups respectively. Limestone samples from these packages are currently being processed for microfossils that may verify this, suggestion. The contact between these packages is thought to pass under the Heffley lakes and continue southeastward along Armour Creek. The Heffley Creek Pluton has intruded this presumed stratigraphic contact and has undergone later brittle movement along the Armour Creek Fault.

Two intrusive phases are recognized. The oldest and most economically important of these is the Heffley Creek Pluton and its marginal dike-sill swarm. This intrusion is probably related to the Heff skarn and was coeval with the district-wide folding. A younger generation, which post-dates the folding, resulted in minor bodies of megacrystic, feldspar-porphyry syenite.

The main Heffley Creek pluton is exposed south and southwest of Heffley Lake, where small, scattered outcrops are traceable over a 10 km2. It contains pyroxene-amphibole-bearing ultramafic rocks as well as some gabbros, diorites, quartz diorites and monzodiorite. The ultramafics contain up to 10 percent disseminated magnetite, which results in an elongate aeromagnetic anomaly that extends southeast of Little Heffley Lake.

Major and trace element analytical data for the various rock types in the Heffley Creek Pluton are presented in Table 1 on the map. The total alkali content of the gabbroic and dioritic rocks indicates a weak alkalic affinity; however, chemical plots suggest these rocks were originally calc-alkaline and that their higher alkali content is due to hydrothermal alteration.

The Nicola Group limestones along the northeast margin of the pluton are cut by a swarm of strongly altered, rusty-weathering dioritic sills and dikes that are probably related to the Heffley Creek Pluton. Individual bodies are usually less than 10 metres wide; they mostly strike northeast, but some intrusions are irregular and bifurcating.

A younger phase of porphyritic syenite is best seen south of Heffley Lake, although the Nicola limestones northeast of Heffley Lake are cut by a few dikes up to 3 metres thick. These leucocratic rocks contain up to 7 percent chloritized amphibole and biotite, as well as some minor quartz. They are characterized by abundant elongate feldspar crystals up to 15 cm in length.

Only one episode of major folding is recognized (FI); this resulted in the moderately to steeply dipping beds and a southeast trending axial planar slaty cleavage (Sl). No fold axial linear features (e.g. mineral lineations) were seen and very few minor Fl folds were identified. However, bedding-cleavage intersections in the Nicola Group limestones north of Heffley Lake reveal the presence of several tight synforms and antiforms.

During the Fl episode, the limestones throughout the area underwent ductile deformation while most of the more competent dikes were disrupted by boudinage and brittle extension. Stereo plots of bedding and Sl cleavage measurements are presented on the map. These suggest that the tight F1 folds in the northern part of the area have sub-horizontal to moderately southeasterly plunging axes and that their axial planes are southeast striking and steeply northeast dipping. Plots of the southern structural data also indicate the presence of southeast striking isoclinal folds but their axial planes are more vertical. The large arcuate unit of strongly cleaved limestone exposed southwest of Heffley Lake may represent a major isoclinal Fl structure.

Most of the pyritic sills and dikes on the Heff property strike northeasterly and were structurally controlled by a-c fractures developed during the Fl folding. However, some thin sills in the limestones have undergone open to tight Fl folding, and these folded intrusions are invariably overprinted by the S1 slaty cleavage. The folding of some sills together with the joint control of the dikes is supportive evidence that both the intrusions and the skarns were coeval with the FI deformation.

The area contains two types of chalcopyrite-bearing mineralization: magnetite-rich garnet-pyroxene skarns, like the Heff occurrence, and pyrite with or without magnetite disseminations and veins in the Heffley Creek pluton. The Heff skarn is an unusual copper with or without gold, REEs and phosphorous. The skarn is related to northeast striking sills and dikes that have potential for more occurrences. The Heffley Creek pluton probably represents an Alaskan-type body. It has the potential to host copper, gold, chrome and PGE mineralization.

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