Ann Mason Deposit (Cu-Mo)

Project Summary

The Ann Mason deposit is a porphyry copper-molybdenum deposit located in west-central Nevada, approximately 75 km southeast of Reno and 7 km west of Yerington.. It is one deposit of a cluster that forms the Yerington copper district.  It is estimated that approximately 1.8 billion pounds of copper were produced historically in the district. The Ann Mason deposit is the main mineralized zone within the Ann Mason Project. For general information on access and ownership, click here for the Ann Mason Project page.

The results of a positive Preliminary Economic Assessment (“PEA”) for the Ann Mason Project were released October 24, 2012. The Project is expected to yield a base case (“Base Case”), pre-tax, 7.5% net present value ("NPV7.5") of $1.11 billion and an internal rate of return ("IRR") of 14.8%, using assumed copper, molybdenum, gold and silver prices of $3.00/lb, $13.50/lb, $1,200/oz and $22/oz, respectively.  Using October 15, 2012 spot commodity prices of $3.71/lb copper, $10.43/lb molybdenum, $1,736/oz gold and $33.22/oz silver (“Spot Case”) the pre-tax NPV7.5 and IRR increase to $2.54 billion and 22.9%, respectively.

The PEA envisions an open pit and conventional sulphide flotation milling operation with an initial 24 year mine life.  Over the life of mine (“LOM”), the project is estimated to produce an annual average of 214 million pounds of copper at total cash costs per pound sold, net of by-product sales, of $1.46 per pound copper.

The PEA was completed by AGP Mining Consultants (“AGP”), an independent Canadian-based engineering firm.  The following table summarizes the main economic outputs of the discounted cash flow.  Currency is in US dollars.

The current resource estimate is contained within a constraining Lerchs-Grossmann (“LG”) pit shell, generated by AGP, and is based on approximately 33,000 metres of recent drilling in 30 holes and approximately 49,000 metres of historic drilling in 116 holes. The resource database also includes re-assaying of 6,142 samples from 44 historical Anaconda core holes, to allow molybdenum, gold and silver values to be estimated.

Metallurgical testing has been completed on samples of drill core at Metcon Research in Tucson, Arizona. The work was done on four separate composites representing the two main mineralogical domains, and consisted of mineralogical characterisation, grindability testing, and batch and locked cycle flotation testing. Preliminary grindability work has established that the feed material is of moderate hardness, with a Bond Ball Work Index of 15.7 kWh/t and an Abrasion Index of 0.283. Locked cycle flotation testing has demonstrated that a simple flotation flow sheet with moderate grinds, two stages of cleaning, and low reagent additions is able to generate a saleable copper concentrate, with no penalty elements identified. Payable by-product levels of gold and silver are present in the copper concentrates.

Metallurgical predictions of 93.5% copper recovery to a concentrate grading 30% copper are based on average values (last four cycles) from locked cycle test data on the main zone composites.

An induced polarization (IP) geophysical survey completed in 2010 suggests that the Ann Mason sulphide system may extend for at least 2 km northwest, to the Blue Hill target. Mineralization at Ann Mason remains open in most directions.

 Plan Maps and Sections

Section 304300E   Section 4317700N

 Exploration and Development Activities

• Preliminary Economic Assessment (2012)
• Core drilling (30 holes completed, Click here for results)
• Metallurgical testwork (2011, 2012)
• Historical core re-sampling (2011, 2012)
• Detailed NSAMT geophysical survey (2011) 
• Induced polarization geophysical surveys (2010)

Links to Technical Reports

October 2012 - Preliminary Economic Assessment on the Ann Mason Project Nevada, U.S.A
March 2012 - Technical Report and Updated Mineral Resource Estimate on the Ann Mason Project
March 2011 - Ann Mason Property Technical Report
January 2010 Ann Mason Project Resource Estimate

Geology and Mineralization

The Ann Mason deposit has the characteristics of a typical, large copper-molybdenum porphyry system.  Projected to the surface, the 0.15% copper envelope covers an area approximately 2.3 kilometres northwest and up to 1.3 kilometres northeast.  The known vertical extent of the envelope is more than a kilometre below the surface.  The mineralization remains open in most directions.

The deposit is hosted by lithologies from several phases of the Jurassic Yerington batholith, including, granodiorite (Jgd), porphyritic quartz monzonite (Jpqm), quartz monzonite (Jqm) and younger quartz monzonite porphyry dykes (Qmp-a, Qmp-b and Qmp-c). Within the Yerington district, Mesozoic host rocks and copper-molybdenum porphyry deposits have been rotated 60-90º westward by Miocene age 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.

Copper mineralization occurs within two primary mineral assemblages: a broad zone of main-stage potassic alteration containing chalcopyrite and bornite; and an overprinted assemblage of chalcopyrite-epidote or chalcopyrite-epidote-quartz. Within the 0.15% copper envelope the highest grades occur within a 200 to 800 m thick, west-plunging zone that surrounds the intrusive contact between granodiorite and porphyritic quartz monzonite.  Within this zone, higher grades are 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 (Qmp-a and Qmp-b).

Sulphide domains within the deposit include:

• an outer pyrite-dominated shell (<0.10% Cu),
• varying ratios of pyrite > chalcopyrite (0.10 to 0.2% Cu),
• chalcopyrite ≥ pyrite (0.2 to 0.5% Cu),chalcopyrite (0.2 to 0.5% Cu),
• chalcopyrite-bornite (0.3 to 0.6% Cu).

Of these domains, the chalcopyrite and chalcopyrite-bornite are the most dominant in terms of overall deposit tonnage; however, the chalcopyrite ≥ pyrite domain provides significant tonnage in the northwest portion of the deposit, where recent interpretations show this domain to be more than 600 m thick, grading above 0.30% Cu.

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.  Sparce chalcocite occurs as replacement rims on chalcopyrite, but more commonly as replacement rims or exsolution replacement of bornite.

Molybdenum occurs as molybdenite in quartz veins and on fracture or shear surfaces as molybdenum paint in several of the copper domains.  In the current resource model molybdenum is constrained within a >0.005% Mo grade envelope that occurs almost entirely within the 0.15% Cu envelope and extending further below, where sodic (albite) alteration has removed copper mineralization, leaving molybdenum largely in place.  The molybdenum mineralization also remains open towards the north.

Silver ≥0.6 g/t and gold ≥0.06 g/t are closely associated with the occurrence of bornite within the chalcopyrite-bornite sulphide domain.

Alteration types include a broad, main-stage zone of potassic alteration (secondary biotite, K-feldspar), an outer propylitic zone (chlorite and epidote occurring with pyrite) and restricted late-stage overprints of chlorite, sodic (albite or albite + chlorite), sericite, zeolite and gypsum. Late-stage sodic and sericite alteration occur along late, high-angle faults and as local, pervasive alteration of rocks.  In areas of strong (>15%) albite or sericite alteration, the copper grades can locally be reduced, resulting in copper grades < 0.2% and in places, < 0.05%.  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 structural boundaries to the Ann Mason resource:

• The relatively flat Singatse Fault truncates the upper surface of the 0.15% copper envelope over a portion of the deposit and juxtaposes sterile, Tertiary volcanic rocks on top of the mineralized intrusives.  
• A high-angle, northwest-trending, southwest dipping fault located along the southwest margin of the resource juxtaposes chlorite-altered rocks with pyrite mineralization in the hanging wall against potassically-altered rocks with copper-molybdenum mineralization in the footwall. Copper-molybdenum mineralization in the footwall remains open at depth along the entire strike length of the fault.

Other late, high-angle faults, either with or without sodic or sericite alteration cross the deposit in various orientations.

Ann Mason Deposit 3D View

Last Updated: December 2012