The Assessment of the ARD Potential for a 'Climax' Type Porphyry Molybdenum Deposit in a High Arctic Environment

Author(s): 
Thomas Barnes, Rob Bowell, Devin Sapsford, J. Kofoed, Matthew Dey, Keith Williams, R. Annells, J. Grogan
Date: 
Monday, June 22, 2009
First presented: 
Securing the Future and 8th ICARD 2009
Type: 
Published paper
Category: 
Project Evaluation

The Malmbjerg (a porphyry granitic ridge) in eastern Greenland hosts a significant world class molybdenum deposit of the ‘Climax’ (high-F low base metals) type. The detailed characterization of the potential of the Malmbjerg deposits to release Acid Rock Drainage (ARD) and Potentially Problematic Elements (PPE) was undertaken as part of the feasibility study. To do this a comprehensive assessment has been undertaken of the deposit’s potential waste-rock and low grade ore environmental geochemistry using static (acid base accounting, whole rock assays, modified EPA 1312 contact test) and kinetic (humidity cell) testwork. The pyritic mineralisation of the Malmbjerg occurs as a shell around the molybdenite zone. However, in general, sulfide content of the rocks is low, being less than 0.03wt% in much of the intrusive complex, the mafic and trachyte dykes and in the unmineralized sandstone unit. The majority of rocks that are likely to acid generating are the greisen zones, the pyritized sandstone, the pyritized granite and the mineralized porphyry units. Although sulfide sandstone, the pyritized granite and the mineralized porphyry units. Although sulfide content does not exceed 2wt%, inorganic carbon or carbonate minerals are not abundant and are dominated by siderite which has a low capacity for acid buffering. Consequently the actual potential to neutralize acid is low in the system so any acid generation is noticeable. The majority of lithologies in the Malmbjerg deposit show low levels of reactivity and as such it is difficult by static testing to confirm the potential for acid generation. However, humidity cell testwork (run for up to 95 weeks) has demonstrated that leachate pH has in many samples demonstrated a gradual decrease in pH conditions over an extended period of time but at released rates significantly less than indicated by static testing alone.     

 

SRK North America