Ann Mason Deposit
Since 2010, exploration and development activities on the Ann Mason Project have included:
- Geophysics: induced polarization and detailed NSAMT surveys
- Core and RC drilling: approximately 58,279 metres in 82 holes at Ann Mason, 11,505 metres in 46 holes at Blue Hill, 871 metres in 2 holes at Blackjack IP and 2,308 metres in 7 holes at Roulette
- Historical core re-assaying: 13,750 metres of Anaconda core from 44 holes (6,142 samples) at Ann Mason
- Metallurgical testwork at Ann Mason and Blue Hill
- Baseline environmental studies: biology (vegetation and wildlife), archaeology/cultural, and Waters of the U.S./Wetlands (“WOUS/Wetlands”) surveys completed on approximately 4,063 hectares (10,040 acres) of the project area
On April 1, 2016, an approved WOUS/Wetlands jurisdictional determination was received from the U.S. Army Corps of Engineers (“USACE”). According to USACE, the water drainages on the Ann Mason Project are considered “isolated waters with no apparent interstate or foreign commerce connection” and as a result, no permit under Section 404 of the Clean Water Act is required for Ann Mason.
No significant obstacles to the development of Ann Mason were identified in any of the other baseline environmental studies completed to date.
Induced polarization (“IP”) geophysical surveys and drilling suggest the Ann Mason sulphide system extends for at least 1.5 kilometres northwest to the Blue Hill deposit. The area between the Ann Mason and Blue Hill deposits has seen only wide-spaced, mostly shallow drilling to date and remains a high priority target for future exploration for both sulphide and oxide mineralization.
The Ann Mason copper-molybdenum deposit is the main mineralized zone within the Ann Mason Project and one of several porphyry and skarn deposits that form the Yerington copper district. In the range of 2 billion pounds of copper were produced historically in the district, mainly from the Yerington open pit mine (1953-1978), but also from several smaller, but higher grade skarn-related deposits south of the Ann Mason deposit (late 1880s to early 1900s). Two of these historic skarn mines are located in the project area: the McConnell and Western Nevada mines.
Ann Mason deposit
- The Anaconda Company discovered the Ann Mason deposit in the 1960s and conducted exploration and resource drilling through the early 1980s with over 100 holes drilled. The deposit was further explored by the Arizona Metals Company (1990), Mount Isa Mines (2002), and PacMag Metals Limited (2006-2008), before Mason Resources’ predecessor company, Entrée Resources Ltd. (“Entrée”), acquired the mineral rights to the project in 2010.
- Exploration by Entrée at the Ann Mason deposit focused on expanding and upgrading the mineral resource, leading to a first Preliminary Economic Assessment in 2012 (“2012 PEA”). An in-fill drill program designed to upgrade the mineral resources contained in the 2012 PEA Phase 5 pit was carried out from August 2014 to January 2015. It comprised 40 holes and a total of approximately 19,265 metres of core drilling with many of the holes pre-collared using reverse circulation (“RC”) drilling.
- In September 2015, the results of an updated Preliminary Economic Assessment (“2015 PEA”) for the Ann Mason deposit were released, incorporating the results of the in-fill drill program and a new resource estimate, as well as the results of preliminary metallurgical testing completed at SGS and finalized in 2016. The resource estimate showed that approximately 95% of the mineralization constrained within the PEA Phase 5 pit is classified as either Measured or Indicated resources, with the remaining 5% classified as Inferred resources.
The Ann Mason deposit is the 4th largest copper porphyry resource in Canada/U.S.A. and has the characteristics of a typical, large copper-molybdenum porphyry system. Projected to the surface, the 0.15% copper envelope covers an area approximately 2.8 kilometres northwest and up to 1.3 kilometres northeast. At depth, this envelope extends more than 1.2 kilometres below surface. Mineralization remains open in most directions.
The deposit is hosted by several phases of the Jurassic Yerington batholith, including: granodiorite (Jgd), porphyritic quartz monzonite (Jpqm), quartz monzonite (Jqm) and younger quartz monzonite porphyry dykes (Jqmp-a, Jqmp-b and Jqmp-c). Within the Yerington district, Tertiary volcanic rocks, Mesozoic host rocks and copper-molybdenum porphyry deposits have been rotated 60-90º westward by Miocene normal faulting and extension. As a result, mineralized intercepts in vertical drill holes through Ann Mason represent approximately horizontal intervals across the original pre-tilt geometry of the deposit.
Within the 0.15% copper envelope the highest grades occur within a 200 metre to 800 metre thick, west-plunging zone that surrounds the intrusive contact between granodiorite (Jgd) and porphyritic quartz monzonite (Jpqm). Within this zone, copper grade is dependent on vein density, sulphide species, frequency and relative age of quartz monzonite porphyry dykes and the mafic content of the granodiorite. Mineralization is closely associated with quartz monzonite porphyry dykes (Jqmp-a, -b and -c).
Sulphide zoning is that of a typical porphyry copper deposit with an outer pyritic shell, and concentric zones of increasing chalcopyrite and decreasing pyrite progressing inward to a central zone of chalcopyrite-bornite. Within the northeast, southeast, and southwest quadrants of the deposit chalcopyrite and chalcopyrite-bornite are the primary sulphide domains and are the most dominant in terms of overall deposit tonnage. Little or no overlap occurs between pyrite and bornite or between pyrite and molybdenite. In the northwest quadrant the primary sulphide domain is chalcopyrite ≥ pyrite; a domain that forms thick intervals of >0.3% copper, with only minor bornite present at depth, near the granodiorite-porphyritic quartz monzonite contact.
Chalcopyrite occurs as individual grains in veins and disseminated in rock, as fillings in brecciated pyrite grains, attached to or included in pyrite grains, and attached to or included in bornite. Bornite occurs as separate grains in veins, and disseminated in rock and attached to chalcopyrite. Sparse chalcocite occurs as replacement rims on chalcopyrite, but more commonly as replacement rims or exsolution replacement of bornite.
Molybdenum occurs as molybdenite in quartz and quartz-chalcopyrite veins and on fracture or shear surfaces as molybdenum paint. Within quartz veins, molybdenite occurs as disseminations, centerline segregations and discontinuous selvages. Molybdenum within a 0.005% molybdenum grade shell occurs largely within the 0.15% copper grade shell. Where late albite alteration has reduced copper grade, molybdenum mineralization is mobilized into fractures and shear zones and extends to greater depth than copper.
Silver ≥0.6 grams per tonne (“g/t”) and gold ≥0.06 g/t are closely associated with the occurrence of bornite within the chalcopyrite-bornite sulphide domain.
Alteration assemblages include an outer propylitic zone (chlorite±epidote±pyrite), widespread potassic alteration (secondary biotite, secondary biotite+K-feldspar or K-feldspar) associated with main-stage copper-molybdenum mineralization, and more restricted late-stage zones of chlorite±epidote±albite, sodic (albite±chlorite), and sericitic alteration. Molybdenum mineralization is not significantly affected by the late sodic alteration, beyond partial remobilization from veins into nearby fractures and shears.
Two prominent structures form boundaries to the Ann Mason mineral resource:
- The relatively flat Singatse Fault truncates the upper surface of the 0.15% copper envelope over a portion of the deposit and juxtaposes unmineralized Tertiary volcanic rocks on top of the mineralized intrusives.
- The high-angle, northwest-trending, southwest dipping Fault 1A marks the current southwest margin of >0.15% copper mineralization in the deposit, juxtaposing propylitically altered rocks with pyrite mineralization in the hanging wall against potassically-altered rocks with copper-molybdenum mineralization in the footwall. Fault 1A and other northwest-trending structures offset the intrusive contact between granodiorite (Jgd) and porphyritic quartz monzonite (Jpqm) to successively deeper levels towards the west and southwest. Copper-molybdenum mineralization in the footwall of the fault remains open at depth along the entire strike length of the fault.