The Mt. Milligan (Southern Star) occurrence is located 9.25 kilometres south-southeast of Mount Milligan peak about 158 kilometres north-northwest of Prince George (Open File 1991-3).

The area of the Mt. Milligan porphyry copper deposits is underlain by the Upper Triassic Takla Group consisting of north-northwest trending volcanic rocks of the Witch Lake Formation and minor sedimentary rocks of the Rainbow Creek Formation. Early Tertiary volcanic and sedimentary rocks are also present. The Witch Lake rocks are intruded by coeval Takla Group and post-Takla Group intrusions related to the informally named Early Jurassic Mount Milligan Intrusive Complex centred around Mount Milligan, about 9 kilometres to the northwest. The monzonitic MBX, Southern Star, Goldmark and North Slope stocks host mineralization on the property. The Mt. Milligan property is made up of the Main and Southern Star deposits. Refer to Mt. Milligan (093N 194) for a description of the Main deposit and for further details of the Southern Star deposit.

The Southern Star deposit comprises the Southern Star zone, which occurs in the hangingwall and footwall of the Southern Star stock. Approximately 70 per cent of the ore in the Southern Star deposit is in brecciated monzonite and only about 30 per cent in volcanic rocks of the Witch Lake Formation. The volcanic rocks consist mainly of monolithic fragmental andesitic varieties which include actinolite-altered augite porphyritic lapilli tuff with minor augite crystal and lithic tuff. Minor augite porphyritic flows and heterolithic debris flows are interbedded with the fragmental rocks. Plagioclase and/or hornblende phenocrysts occur locally within flows and within lapilli or crystal tuffs. Latitic volcanic rocks (potassically altered andesites) underlie much of the area around the MBX stock of the Main zone to the north and less commonly, areas adjacent to the Southern Star stock.

The Southern Star stock is a moderately west dipping, northwest striking, tabular body of monzonite, which forks at its northern end. The stock is approximately 800 by 300 metres in area and is composed mainly of plagioclase porphyritic monzonite and, along some of the stock margins, plagioclase hornblende porphyritic monzonite. Hydrothermal breccias occur throughout the Southern Star stock, and less commonly in adjacent volcanic rocks. These areas are characterized by potassium feldspar flooding.

Potassic and propylitic alteration assemblages are present throughout the Mt. Milligan deposits with gold and copper mineralization mainly associated with the potassic assemblage. Minor post-mineral carbonate alteration is also present. The potassic alteration is best developed around the contacts of stocks, decreasing in intensity towards the core of the stock and away from the contacts for several hundred metres into fractured country rocks. The alteration is characterized by secondary potassium feldspar and, in areas of intense potassic alteration, fine grained secondary biotite (up to 40 per cent), chalcopyrite, lesser magnetite and minor bornite. Pyroxene phenocrysts, where present, are replaced by actinolite. The propylitic alteration is widespread and generally pervasive. It is developed best outside the zone of potassic alteration, outward from the stocks for up to 2500 metres. The assemblage consists of epidote with varying amounts of calcite, chlorite, albite and pyrite.

Widespread disseminated mineralization accompanied by lesser veinlet and fracture-filling mineralization occurs. Mineralization consists mostly of chalcopyrite, lesser magnetite and minor bornite in areas of potassic alteration, and pyrite in areas of propylitic alteration. Sporadic supergene enrichment also occurs in the Southern Star deposit. Secondary copper minerals identified in these areas consist of the sulphides, covellite, chalcocite and djurleite; the oxides, cuprite and tenorite; the carbonates, malachite and azurite and; native copper. The sulphides occur as rims around chalcopyrite; and the oxides, in particular cuprite, occur as surface coatings on native copper. Secondary copper minerals commonly occur with goethite, magnetite, hematite and siderite. Limonite, which includes goethite, commonly replaces sulphide minerals or occurs as coatings on fracture surfaces and hairline cracks.

The area was prospected as early as 1937, but no serious work was done until 1972 when Pechiney Development Ltd. conducted induced polarization (IP), geochemistry and a five-hole drill program. The property was dormant until 1983 when Selco Inc. did extensive geochemical work which identified a gold-arsenic anomaly east of Heidi Lake. Selco amalgamated with BP Resources Canada Limited in 1984. In that same year, R. Haslinger staked claims on adjacent ground, which BP then optioned. Extensive geological, geochemical, lithogeochemical, magnetic, IP and trenching work was done by BP until 1986, when Lincoln Resources Inc. optioned the property. Lincoln’s twelfth drillhole was the first to intersect the MBX zone in 1987. Lincoln later reorganized to become United Lincoln Resources Inc., and then amalgamated with Continental Gold Corp in 1989. Drillhole 199 was the first to intersect the Southern Star zone later that year.

Placer Dome Inc. purchased the property in 1990, did extensive infill and step-out drilling, and completed a pre-feasibility study in 1991. There has been little work done on the property since that time. A 1997 geological work program consisted of examination of 25 intervals of varying lengths in eleven separate drillholes in the MBX zone. Twenty core specimens were collected for polished thin section examination and 26 samples were analyzed. Placer Dome Inc. re-visited the property in 2004, drilling a dozen HQ bore holes totalling about 2200 metres, in order to provide core for metallurgical testing. Placer Dome plans to complete a full feasibility study on the project in 2005.

In 2005, Placer Dome Limited completed bulk leach extractable gold, standard silt and water geochemistry sampling to characterize the stream geochemical signature of the Mt. Milligan deposit; to determine if there are any anomalous drainages that have not been adequately explored; to establish how far down the drainage system Mt. Milligan can be tracked; to establish background geochemical levels; and to determine best sampling and analytical procedures to detect buried copper-gold porphyry deposits in the Quesnel Trough. There were 38 sample sites planned within the Main deposit area and along King Richard and Rainbow creeks, but due to difficulties with access and availability of suitable sampling material, only 31 sites were sampled.

In 2018 and 2019, programs of ground induced polarization surveys, airborne magnetic surveys and 183 diamond drill holes, totalling 73 763 metres, were completed on the Mount Milligan property.

In December 2019, an updated mineral resource was reported at 125 370 000 tonnes combined and indicated grading 0.19 per cent copper and 0.35 gram per tonne gold with an additional inferred resource of 3 736 000 tonnes grading 0.12 per cent copper and 0.46 gram per tonne gold using a resource pit shell of 0.2 per cent copper equivalent cut-off grade (Fitzgerald, J., Jago, C.P. (2019-12-31): Technical Report on the Mount Milligan Mine, North-Central British Columbia). Also at this time, a proven and probable mineral reserve of 191 028 000 tonnes grading 0.23 per cent copper and 0.39 gram per tonne gold was reported (Fitzgerald, J., Jago, C.P. [2019-12-31]: Technical Report on the Mount Milligan Mine, North-Central British Columbia).

Separate reserve figures are available for the Southern Star deposit and refer to Mt. Milligan (093N 194), the reserve figure of which includes the Southern Star orebody.