The Howard's Pass deposits contain large tonnages of moderate and high grade zinc-lead mineralization. This mineralization consists of high grade bands of zinc and lead minerals separated by barren or weakly mineralized bands of mudstone. Selwyn undertook preliminary heavy media separation (HMS) testwork during fall of 2004 to confirm preliminary testwork completed by US Steel Corporation in 1970's. The testwork by US Steel found that HMS (utilizing heavy liquids to make the gravity separation) was quite effective in separating the less dense waste mudstone material from more dense mineralized material and indicates that gravity techniques may provide an effective means of pre-concentration of run-of-mine open pit mineralization.
Additional HMS testwork was conducted in the fall of 2005 on drill core from several of the zones and on a bulk sample of dump materials from the previous test mining. This more detailed test work has confirmed the previous favourable results indicating that HMS may be an effective pre-concentration step in the processing of ores.
Placer and Cygnus undertook extensive metallurgical testwork including flotation and metal leaching studies in the 1970's and concluded that with fine grinding they could achieve acceptable recoveries of zinc and lead in the low to mid 80 percent range using standard flotation methods. Selwyn undertook additional flotation testwork in 2006 and 2007 that suggests 83% zinc and 75% lead recovery. More recent testwork is continuing to determine optimum grind size and the application of pre-float of carbon before undertaking differential flotation to recover separate zinc and lead concentrates.
Typically SEDEX deposits achieve average recoveries of about 85% for zinc and 75% for lead. Concentrate quality from Selwyn Chihong's testwork has generally been good with zinc grade of 55 to 57% for the zinc concentrate and 60% for the lead concentrate. Zinc recovery is estimated at approximately 85%. Lead recovery is highly variable and more testwork is required to better estimate lead recovery to lead concentrate. The concentrates are low in iron and have very low levels of deleterious materials and should be attractive feed for most zinc smelters. Silver grades in the concentrates are low and not likely to be at levels for which payment will be received.
The lack of pyrite in the ores results in better recoveries than what might be otherwise expected given the grain size of the average run-of-mine materials; higher grade ores tend to be coarser grained and may provide generally higher recoveries. Testwork is continuing towards increasing recoveries and evaluating ore variability effects.
A metallurgical drilling program was completed in 2011 in the XY Central and Don deposits to acquire additional samples for bench scale testing. Although the primary goal of this work is to collect metallurgical samples, each drill hole also provided additional detailed grade and rock mechanics information.
Results of this testwork will provide additional information for detailed design and costing and finalization of expected recoveries and concentrate grades for zinc and lead.