It's a race to the bottom between the USD Zinc price and the Aussie dollar - the only factor that is fairly steady is the Australian dollar zinc price at around $1.40 to $1.42/lb and that is the main figure here.
The main negative being the cost of TC's in $A terms and the additional cost of paying back the Varde debt that is expressed in USD.
It would really be hurting American producers such as Red Dog in Alaska owned by Teck Resources.