Blue Hill Deposit
The Blue Hill deposit is located approximately 1.5 kilometres to the northwest of the Ann Mason deposit. Two main styles of porphyry mineralization occur at Blue Hill: near surface oxide and mixed oxide-sulphide copper mineralization, and underlying copper-molybdenum sulphide mineralization.
A pit-constrained resource estimate, completed by AGP Mining Consultants Inc., is based on Entrée’s drilling of 30 RC and core holes totalling approximately 6,822 metres. In addition, the estimate incorporates approximately 2,381 metres of RC drilling (7 holes) and 1,057 metres of core drilling (2 holes) completed by PacMag Metals Limited, and 10 historic Anaconda RC and core holes totalling approximately 2,927 metres. The resource estimate was prepared as a first step in determining if Blue Hill could serve to generate early cash flow for the Ann Mason Project, should the Ann Mason deposit advance to production.
Preliminary metallurgy (column leach tests at ¾ inch crush size) suggests the oxide and mixed copper mineralization is amenable to low-cost, heap leach and SX/EW processing. Average copper recovery in the oxide mineralization in column leach testing is 86%, while the mixed material returned 83% recovery.
The 2013 drilling at Blue Hill successfully tested for westward extensions of the current deposit and also highlighted the structural complexity of the Blue Hill area. To the east, oxide and mixed mineralization is truncated by the low angle Blue Hill Fault, however, underlying sulphide mineralization continues in this direction below the fault. Drilling of the underlying sulphide target remains very widely-spaced, but has identified a target area more than one kilometre in width, which remains open in most directions. Significant molybdenum mineralization was also intersected in two of the drill holes targeting the sulphide mineralization. Most recent drill holes were targeted to test oxide mineralization; however, two diamond holes (EG-BH-11-019 and -021) were drilled east of the oxide copper zone to test deeper sulphide copper potential. In addition, hole EG-BH-11-031, located approximately one kilometre east of Blue Hill, intersected a near-surface zone of copper-oxide mineralization assaying an average of 0.28% copper over 13.8 metres from a depth of 22.2 metres. Further drilling will be required in this area and if successful could provide additional feed for a potential SX/EW operation.
The Blue Hill deposit occurs in a geologic environment very similar to the Ann Mason deposit, but in a separate fault block, below the Blue Hill Fault. Two main styles of porphyry mineralization occur at Blue Hill: near surface oxide and mixed oxide-sulphide copper mineralization, and underlying copper-molybdenum sulphide mineralization.
Both styles of mineralization are hosted by quartz monzonite with lesser amounts of porphyritic quartz monzonite and quartz monzonite porphyry. The low-angle, southeast dipping Blue Hill Fault strikes northeast through the middle of the target, cutting off a portion of the near-surface oxide mineralization. However, sulphides continue below the fault to the southeast.
The oxide zone is exposed on surface and has been traced by drilling as a relatively flat-lying zone covering an area of about 900 metres by 450 metres, and continuing for several hundred metres further to the west as a thinner zone. Significant copper oxides, encountered in both RC and core drill holes extend from surface to an average depth of 124 metres. Oxide copper mineralization consists of malachite, chrysocolla, rare azurite, black copper-manganese oxides, copper sulphates, and copper-bearing limonites. Mineralization occurs primarily on fracture surfaces and in oxidized veins or veinlets. A zone of mixed oxide-sulphide mineralization with minor chalcocite is present below the oxide mineralization to depths of up to 185 metres. The copper oxide zone remains open to the northwest and southeast.
Oxide copper mineralization at Blue Hill is interpreted to be the result of in-place oxidation of copper sulphides with only minor transport of copper into vugs, fractures, and faults or shear zones. No significant zones of secondary enrichment have been observed.
The copper sulphide zone underlies the southern half of the oxide mineralization and continues to depth towards the southeast, below the Blue Hill Fault. Mineralization consists of varying quantities of pyrite, chalcopyrite, and molybdenite. Local, higher-grade sulphide mineralization commonly occurs within zones of sheeted veins containing chalcopyrite, magnetite and secondary biotite. Significant amounts of disseminated molybdenum mineralization have been observed locally, often in contact with dykes. To the northwest, only a few holes have tested the sulphide potential below the oxides; however, in this direction the sulphides appear to be increasingly pyritic with only minor amounts of copper.
Alteration assemblages are similar to Ann Mason except that original zoning is difficult to discern in areas of pervasive oxidation. Within zones of sulphide mineralization, propylitic alteration is more widespread and potassic alteration is more restricted to quartz monzonite porphyry dykes and immediately adjacent rocks of the Yerington batholith. Late stage sodic alteration locally reduces copper grades, similar to what has been observed at Ann Mason.
The sulphide mineralization remains open is several directions, most importantly, to the southeast, towards Ann Mason.