Geology, alteration, mineralization and metal zonation of the Mt. Milligan porphyry copper-gold deposits

• Main Author: Delong, R. Campbell
• Language: English
• Published: 2009
• Online Access: http://hdl.handle.net/2429/4504

The Mt. Milligan porphyry copper-gold deposits are in central British Columbia, 155 km northwest of Prince George. They were discovered in 1987 after extensive soil and lithogeochemical sampling, geological mapping, ground and airborne geophysical surveys, trenching and diamond drilling. An indicated resource of 299 million tonnes grading 0.45 g/t Au and 0.22% Cu has been delineated in two deposits: Mt. Milligan Main, which includes the 66 zone, and the Southern Star. The Mt. Milligan deposits are hosted by porphyritic monzonite stocks and adjacent volcanic rocks of the Witch Lake formation of the Takla Group. All are within the Early Mesozoic Quesnel Terrane. Potassic and propylitic alteration characterize the Mt. Milligan deposits. Potassic alteration is most intensely developed around the contacts of the MBX and Southern Star stocks. An inner biotite subzone is spatially related to the core of copper-gold mineralization. Propylitic alteration occurs peripheral to, and locally superimposed on, the outer borders of the potassic alteration. Central parts of most of the deposits are: (i) gold and copper rich, (ii) mineralized with chalcopyrite, and minor bornite and pyrite, and (iii) associated with potassic alteration. Peripheral mineralization, including the gold rich but copper poor 66 zone, is marked by minor lead-zinc-silver mineralization (mainly in sparse veins) associated with propylitic alteration. Higher gold concentrations in the 66 zone can be accounted for by overprinting of two periods of gold deposition from AuCl⁻¹ and Au(HS)₂⁻ bearing solutions. Other characteristics consistent with this two stage model of gold deposition are: (1) two populations of gold, one correlating with copper (gold transport as AuCl⁻¹ similar to the higher temperature copper chloride complexes), and one independent of copper (transport as Au(HS) ₂⁻), (2) two periods of alteration: (i) potassic (consistent with gold being transported as AuCl⁻¹), and (ii) a propylitic retrograde overprint of an assemblage of epidote-pyrite associated with alteration of biotite to chlorite, and (3) elevated zinc, lead and silver concentrations, indicative of lower temperature solutions that would favour gold transport as Au(HS) ₂⁻.