The Castle South occurrence is reported to be mainly underlain by weakly phyllic-altered Stuhini Group sedimentary rocks, and volcanic rocks of the Hazelton Group.

Castle South is located 1 kilometre south of the Castle Alteration Zone (see Castle (104G 076)) and is defined by a very strong, linear IP chargeability anomaly (25 to over 40 millivolts/volt) 2100 metres in length, 580 metres wide, and 300 metres deep (the limit of the survey), and which is still open to the west and east.

Soil geochemical results are subdued directly over the chargeability feature, but are weakly to moderately anomalous in copper, gold, molybdenum, arsenic and antimony to the west and east. Limited prospecting in the past has identified quartz-carbonate veins with high copper-gold values on the periphery of the anomaly with values of 0.366 to 2.16 per cent copper to the west, and up to 2.5 per cent copper and 11.9 grams per tonne gold to the east (Press Release, Colorado Resources Ltd., December 9, 2019).