CH12 Exploration for Copper-Gold Porphyry – Mount Milligan/Phillips Lakes Area, BC, Canada


(A new “blind” discovery)GamX CG12 Mount Milligan Photo MBX Zone Looking West

 

An airborne gamma ray spectrometric/magnetic survey was flown over the Mount Milligan Mine area, British Columbia in 1991 using the GSC Skyvan fixed wing aircraft. Flight line spacing was 500m. The survey delineated potassium anomalies over Mount Milligan, Taylor, Wit, Chuchi and several other prospects. The photo (right) shows drill pattern (circa 1993) on extensive overburden cover at the Mount Milligan MBX deposit, looking west towards North Slope zone.

GamX CH12 Mount Milligan Area, BC Airborne MAG map

 

 

 

 

 

Figure 1 shows airborne Mag and K images from part of the survey, overlain by regional geological contacts (black lines) and drainage (white). White circles locate 3 zones at Mount Milligan Mine: MBX, North Slope and Southern Star. Two white “X’s” mark K anomalies located 11 km E and SE of Mount Milligan Mine called K5 and K6, respectively. Upper Triassic Takla Group volcanic rocks include: Rainbow Creek formation (RC) slate, siltstone; Witch Lake formation (WL) augite porphyry tuffs, flows; Eocene sediments and basalts (E) and rocks of the Wolverine metamorphic complex (WMC). Outcrop exposures are rare, as thick till and glacial-fluvial deposits blanket much of the area.

 

 

Mag patterns (Figure 1A) are dominated by a large mag high associated with monzonitic intrusions extending NNW of Mount Milligan Cu-Au deposits where they outcrop at Mount Milligan (the mountain).

 

GamX CH12 Mount Milligan Area, BC Airborne K map

 

 

 

Airborne K concentrations (Figure 2A) are highest (>2 %K) over the exposed intrusion at Mount Milligan mountain (labeled). Recognizable anomalies (1.5-2 %K) occur over the deposits at Mount Milligan Mine, but similar amplitude patterns also occur over glacial fluvial, well drained sandy surficial deposits. Till-covered areas have consistently lower K (<1.5 %K). Thus, moderate K anomalies known to occur in areas mapped as till, where K is typically low, provide a first order exploration targeting strategy. This was tested during field follow-up (by GSC staff: Shives, Ballantyne, Harris, Plouffe, Dunn2) resulting in the discovery of previously unmapped, exposed outcrop of  potassically altered Witch Lake formation volcanic rock at K5 site, and a new, completely blind discovery of porphyry copper-gold at site K6.

 

GSC field studies at site K6 included ground spectrometry, overburden mapping, boulder sampling and analyses, till sampling and analyses and biogeochemical surveying. The area was correctly mapped as till, thus the K anomaly was NOT due to the presence of unmapped glacial-fluvial material. Rather, the till was a “granite” till containing many large angular boulders of potassically altered, mineralized (chalcopyrite, pyrite) monzonitic intrusive cut by quartz stockwork veins. Bulk till sampling produced several sites with gold grains (6 to 24 Au grains/10 kg sample), and anomalous Cu (up to 737 ppm) in the clay fraction.

 

 

 

 

Based on the very encouraging results, site K6 was staked by a local prospector as the Christina-Jean prospect, and subsequently optioned by a junior exploration company. In 1994, three holes were drilled to test the core of the airborne K anomaly. This drilling intersected Cu-Au mineralization within altered intrusion and host Takla volcanic rocks. Soil sampling and induced polarizations surveys were conducted and in 1996 three additional holes were drilled. Results intersected low copper (0.04% Cu) but interesting gold (0.70 gpt Au) over 24 meters. IP chargeability values exceeded 17mV/V. The property was dropped then optioned again by Terrane Metals Corp, who drilled another three holes in 2007. Terrane also dropped the option but stated clearly that exploration was still warranted.

 

Whether significant economic mineralization exists at the K6 site (Christina-Jean Prospect) remains to be seen. Regardless, the discovery of sub-cropping, porphyry-hosted, Cu-Au mineralization based entirely on follow-up to the airborne K anomaly represents a significant application, demonstrating that careful evaluation of the airborne gamma ray patterns, in a geological context, can lead to new discovery.

 

 

 

1. Airborne geophysical survey of the Mount Milligan Area, NTS 93O/4W, 94N/1, 93N/2E, British Columbia. Geological Survey of Canada Open File 2535.

2. Shives, R.B.K., Ballantyne, S.B. and Harris, D.C.; Gamma ray spectrometry: Applications to the search for ore. POSTER presented at Cordilleran Roundup; part of promotional display of Geological Survey of Canada Open File 2535