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File Created: 24-Jul-1985 by BC Geological Survey (BCGS)
Last Edit:  30-Nov-2013 by Karl A. Flower (KAF)

Summary Help Help

NMI 092F11 Cu1
Name GEM LAKE, MEG Mining Division Nanaimo
BCGS Map 092F063
Status Prospect NTS Map 092F11W
Latitude 049º 41' 04'' UTM 10 (NAD 83)
Longitude 125º 24' 39'' Northing 5506337
Easting 326103
Commodities Copper, Gold, Silver, Molybdenum Deposit Types L04 : Porphyry Cu +/- Mo +/- Au
Tectonic Belt Insular Terrane Plutonic Rocks, Wrangell
Capsule Geology

The Gem Lake occurrence is located in the headwaters of Gem Creek, approximately 1.3 kilometres north west of Moat Lake.

The area is underlain by basaltic and andesitic flows of the Upper Triassic Karmutsen Formation, Vancouver Group. These volcanics were intruded by stocks of quartz diorite of the Late Eocene to Early Oligocene Mount Washington Intrusive Suite (formerly Catface Intrusions), as well as several felsite dikes. The rock units were faulted and at about the same time mineralized with several generations of quartz veins. A polymictic intrusive breccia occurs consisting of 30 to 70 per cent clasts of volcanic material, quartz diorite and felsite. The matrix consists of hornblende, feldspar and quartz.

The mineralization at Gem Lake can be divided into 5 types (Assessment Report 17002):

(1) Dilational quartz veins 0.1 to 10 centimetres in width with varying amounts of magnetite are common close to the intrusive breccia. The highest gold and silver assays obtained were 0.5 and 8 grams per tonne respectively.

(2) Disseminations, filled amygdules and fracture coatings of pyrrhotite and chalcopyrite are common. Typical gold and silver values are 0.1 and 4 grams per tonne respectively.

(3) Massive sulphide pods (several) with up to 5 per cent chalcopyrite are found. The largest of these pods outcrops over an area of 2 by 4 metres. Except for one sample assaying 1.1 grams per tonne gold most samples contained less than 0.2 grams per tonne. Silver was generally below 5 grams per tonne and always below 10 grams per tonne.

(4) Quartz veins with 1 to 20 per cent pyrrhotite and chalcopyrite occur throughout the property. The veins are from 0.5 to 15 centimetres in width. The percentage of chalcopyrite is usually greater than that of pyrrhotite. The veins typically contain 0.3 and 10 grams per tonne gold and silver respectively. High values were 7.8 and 40 grams per tonne gold and silver respectively.

(5) A tectonic breccia mineralized with chalcopyrite is exposed over and area of about 15 by 30 metres (Main showing). This breccia is associated with several parallel, steeply dipping, east trending faults. The host rock is mainly the intrusive breccia but a portion of the mineralization also occurs in basalt flows. The chalcopyrite is both disseminated throughout the tectonic breccia and occupies the open spaces between the fragments that resulted from faulting. The host rocks are not pervasively altered except in the areas of intense shearing where fault gouge has formed. Four samples were collected and assays showed that values range from 0.64 to 3.0 grams per tonne gold and from 9.8 to 49 grams per tonne silver.

A 1961 drill hole encountered 1 per cent copper over an interval of 18 metres (McDougall, 1964). Minor molybdenite (0.02 per cent) was found in the deepest intrusive body intersected in 1963. One report also describes pyrite and molybdenite as occurring in fractures and veins. Carson describes the deposit as a porphyry copper type related to forcible intrusion of Tertiary stocks (Geolical Survey of Canada Paper 68-50, page 45).

In 1987, Falconbridge completed a program of geological mapping and rock sampling on the area as the Meg 1-8 claims.

EMPR EXPL 1988-C91
EMPR FIELDWORK 1988, pp. 61-74
EMPR PF (McDougall, J.J. (1961): Report on Gem Lake (Meg Group)
Copper Prospect-1961; McDougall, J.J. (1961): Preliminary Report
on Gem Lake (Meg Group) Copper Prospect; *McDougall, J.J. (1964):
Summary Report on Gem Lake Copper-1963; Geology map 1:600 scale,
1961; Contour Map, undated, 1:5 000 scale)
GSC MAP 2-1965; 17-1968; 1386A
GSC OF 463
GSC P 68-50, pp. 39,45; 72-44
*Carson, D.J.T. (1968): Metallogenic Study of Vancouver Island With
Emphasis on the Relationships of Mineral Deposits to Plutonic
Rocks, Unpublished Ph.D. Thesis, Carleton University
Falconbridge File