CH14 Exploration for Copper Porphyry – Casino Deposit, Yukon Territory, Canada


 

GamX CH14 Pattison Creek Pluton - Hart-Selby 1998

 

This case history describes both geological mapping and mineral exploration applications. In 1993 the GSC contracted Sander Geophysics Limited to complete a 500 m line-spaced survey1 of the Selwyn River area, Yukon. The region is unglaciated and deeply weathered, presenting mapping and exploration challenges, but is well suited to the “geochemical mapping” ability of gamma ray spectrometry. In concert with field follow-up, results were shown to aid bedrock geological mapping over the mid-Cretaceous Dawson Range2 (see GamX Case History CH2) and to expand the known extent of the Pattison Creek pluton3, by about three times. The Pattison Creek pluton is a “Casino-type Cu-Mo” highly fractionated intrusion containing granite, peraluminous muscovite-bearing alaskite and aplite. These phases generate some of the highest airborne values of eU (>3 ppm) and eTh (>15 ppm) within the entire survey3 and were used in combination with a magnetic low signature to re-map boundaries (Fig 1).

GamX CH14 Casino Deposit YT Canada Photo

Located about 27 km north of the Pattison Creek pluton, the Casino Cu-Au-Mo deposit (photo on left) occurs in a similar, relatively small intrusion cutting the Dawson Range Batholith. Casino is Yukon’s largest copper deposit, containing proven and probable mill ore reserves of 914 million tonnes grading 0.2% Cu, 0.24 g/t Au and 0.024% Mo4.  Mine development is underway to produce 8.9 million ounces Au, 65 million ounces Ag, 4.5 billion pounds Cu and 463 million pounds Mo, over a projected 22 year mine life4.

GamX CH14 Casino Airborne Gamma Ray Survey K and eTh-K

 

 

 

 

 

A strong K anomaly (Figure 3) over Casino coincides with a prominent low eTh/K ratio (Figure 4) indicating that the potassium is indeed anomalous relative to thorium, and we know there is significant K alteration associated with the mineralizing system. But there are other K “anomalies” to NW and SE of the deposit (Fig 3). Are they also worthy targets? NO, they are not. The “normal”  eTh/K ratio tells us that although the K may be moderately high, the proportion of K is merely normal relative to eTh. Field investigations (Shives et al) in the area shown by the white box revealed that the higher K (and eTh) relates only to barren felsic rocks within the Paleozoic Yukon Metamorphic Complex (YMC).

 

GamX CH14 Casino Area YT ground spectrometry summary Th-K chart

 

 

 

 

 

Generalized results of eTh and K concentrations determined from hundreds of in-situ ground spectrometric readings on outcrops are shown in Figure 5. The major rock units in the area each span ranges in eTh and K, but can generally be distinguished radiometrically. Potassically altered mineralized rocks at Casino are clearly the most K-rich, providing the strong airborne K anomaly observed.

 

GamX CH14 Casino YT Airborne Survey Stacked Profile

 

 

 

 

 

 

At the scale of an airborne survey, magnetic signatures associated with porphyry deposits may vary, appearing as lows, highs or as in this case, “flanking” magnetic anomalies as shown by the stacked profile in Figure 6. When used in combination with the gamma ray patterns, exploration vectoring is improved.

 

 

 

 

 

 

 

 

For interest: To view a very interesting video of the Casino Mine design, visit http://www.casinomining.com/

 

  1. GSC Open File 2816, 1994. Airborne geophysical Survey of the Selwyn River Area, Yukon Territory, NTS 115J/10, 11, 14, 15, colour maps and stacked profile booklet. Geological Survey of Canada. Scale 1:150,000

 

  1. Johnston, S.T., and Shives, R.B.K., 1993. Interpretation of an airborne multiparameter geophysical survey of the northern Dawson Range, central Yukon: A Progress Report. In: Yukon Exploration and Geology, 1994. Exploration and Geological Services Division, Yukon, Indian and Northern Affairs Canada, p. 105-111.

 

  1. Hart, C.J.R. and Selby, D., 1998. The Pattison Creek Pluton – a mineralized Casino intrusion made bigger with gamma rays. In: Yukon Exploration and Geology, 1997; Exploration and Geological Services Division, Indian and Northern Affairs Canada, p 89-96.

 

  1. http://www.casinomining.com/project/proposed_mine/