Summary:
The Black Butte Copper Project includes the Johnny Lee and Lowry Deposits, about 2km apart. The deposits blend features of SEDEX and sediment-hosted strata-bound copper types. The Johnny Lee Deposit has two lenses: the Upper Copper Zone (UCZ) and the Lower Copper Zone (LCZ), extending at least 1km along strike and up to 300m down dip. The recent drilling program tested potential lateral extensions.
The Black Butte Cu deposits are hosted by Paleoproterozoic sedimentary rocks of the Newland Formation of the Belt Supergroup that were deposited in a portion of the Belt-Purcell basin referred to as the Helena Embayment. The Newland Formation comprises shale, carbonaceous shale, calcareous shale, dolomite, conglomerate, sedimentary breccia and bedded-pyrite massive sulphide (massive sulphide) that was deposited along the fault controlled, northern margin of the Helena Embayment.
The deposits are considered hybrid SEDEX (Sedimentary Exhalative Sulphide)–SSC (Sediment-hosted Stratabound Copper) deposits.
At the Johnny Lee deposit, Cu mineralization is hosted by two massive sulphide units at different stratigraphic levels, referred to as the Upper Sulphide Zone (USZ) and Lower Sulphide Zone (LSZ). Economically significant Cu mineralization in both zones is developed sub-parallel to bedding and is interpreted to have formed after the massive sulphide host units. The zone of greater than 0.25% Cu mineralization in the USZ is termed the Johnny Lee Upper Copper Zone (JL UCZ), while the mineralized zone within the LSZ is termed the Johnny Lee Lower Copper Zone (JL LCZ).
At the Lowry deposit, economically significant Cu mineralization is hosted by a succession of sub-parallel ferruginous sediment with interbedded shale and conglomerate lithofacies. Two distinct mineralized zones are recognized: the Lowry Middle Copper Zone (LMCZ) and the Lowry Lower Copper Zone (LLCZ). Mineralization at Lowry occurs in both massive sulphide and interlayered clastic sediment and is predominantly chalcopyrite, with minor tennantite.
Local and Property Geology
The majority of the Black Butte Copper Project (BBCP) area is underlain by gentle to moderately southwarddipping rocks of the Belt Supergroup. The Belt Supergroup lithofacies are juxtaposed against gently south-dipping Cambrian sedimentary rocks along the VVFZ. To the north of the Project area, Belt Supergroup rocks are also exposed below the basal Cambrian unconformity. Intermediate to mafic, Early Tertiary intrusive dykes and sills occur within the Belt Supergroup. Late Tertiary sedimentary lithofacies unconformably overlie all other units in the area.
Johnny Lee Lower Copper Zone
The LCZ occurs at depths of 340 to 520 m below surface, strikes approximately EW and dips at 15° to 30° to the south. Mineralization in the LCZ is primarily hosted by the LSZ located in the FW of the VVFZ and HW of the Buttress Fault. The LSZ is overlain by a unit comprising interlayered shale and conglomerate and is underlain by a conglomerate unit. The LCZ deposit comprises three lenses of mineralization termed the East, Central, and West Lenses. These lenses are defined by the outer limit of >2.0% Cu mineralization which extend outside of the LSZ into the HW intercalated conglomerate and shale unit. Minor Cu mineralization also occurs in the conglomerate below the LSZ but does not exceed 2.0% Cu.
Johnny Lee Upper Copper Zone
The north-eastern corner of the UCZ is exposed on surface and the top of the mineralized zone extends to a depth of 210 m below surface. The high-grade portion of the UCZ (>1.2% Cu) is entirely encapsulated by the USZ although >0.25% Cu Halo mineralization extends into the shale that is located in the HW and FW of the USZ. The UCZ is gently folded by a W/NW plunging syncline-anticline pair such that strike is variable and dip ranges between 0° and 20°. With the exception of its extreme north-eastern corner, the UCZ is situated below the level of surficial oxidation. Acidic groundwater, preferentially focused along a layer parallel, brittle-ductile shear zone, resulted in localized supergene alteration of copper sulphides below the base of oxidation. The volume of supergene altered copper sulphide minerals along the shear zone is generally low, except at the junction of the shear zone with Fault 1.
Lowry Middle Copper Zone
The LMCZ is hosted by a succession of ferruginous sediment (massive sulphide and ferruginous shales) with interbedded conglomerate, carbonaceous shale and shale lithofacies. Three zones of >1.2% Cu mineralization occur in the LMCZ, termed LMCZ Vein 1 – LMCZ Vein 3. These mineralized zones dip to the south at 25 - 30°. The >1.2% Cu zones are surrounded by a zone of >0.25% Halo mineralization. The northern tip of the LMCZ occurs at 245 m below surface and the southern tip occurs at 755 m below surface. The LMCZ is situated below the depth of surficial oxidation. The LMCZ has plan view dimensions of 830 m (NS) by 280 m (EW). The maximum true thickness of the LMCZ is 45 m and it progressively reduces in width and pinches out to the north, south, east, and west.
Mineralization in the LMCZ occurs in both massive sulphide and the interlayered clastic sediment. Pyrite + Marcasite concentrations of typical mineralized intersections range from 9.1 – 45.2% and gangue mineral contact ranges from 38.7 – 76.0% (McArthur, 2019). Copper sulphide mineralization is predominantly chalcopyrite (8.3 – 17.0% in typical mineralized intersections) with minor tennantite (0.01 – 0.23%) observed in 30% of mineralogical samples studied to date (McArthur, 2019). The amount of primitive pyrite in the LMCZ ranges from 10.4 – 60.0% (average 49.1%) which is lower than that of the northern part of the UCZ (average 62.7%) indicating that the amount of recrystallization in the LMCZ is higher than that of the northern part of the UCZ.
Lowry Lower Copper Zone
The LLCZ occurs in the FW of the VVFZ and both the host and mineralization is truncated in the SE by the Volcano Valley FW Fault. The LLCZ is hosted by a succession of ferruginous sediment (massive sulphide and ferruginous shales) with interbedded shale and conglomerate lithofacies. Two bedding-sub-parallel zones of >1.2% Cu mineralization occur within the LLCZ: LLCZ Vein 1 and LLCZ Vein 2.
The Black Butte Copper Project deposits exhibit attributes of both Sedimentary Exhalative sulphide deposits (SEDEX deposits, e.g. Emsbo et al, 2016) and Sediment-hosted Stratabound Copper (SSC) deposits (SSC deposits, e.g., Hayes et al, 2015).