I think, it is important to define a low-grade ore. Polymetallic operations are used to adopt a different approach to consider an economic low-grade material. Probably, there are two key factors to evaluate, the first is the silver and/or gold grade, and the second one is the zinc grade. Obviously, the mineralogy has an impact on the flotation process, the presence oxides and interlocks are usually a serious problem. The silver grade is important to consider because the operating cost could be high to produce a saleable copper and/or lead concentrate. The zinc grade is other important point because the zinc/copper and zinc/lead ratio could be less than 1, therefore, the production of zinc concentrate would not be possible.

For example, 60 g/t Ag, 0.05% Cu, 0.8% Pb, and 0.7% Zn. In this case, it would be possible to produce a lead concentrate and most of the silver and copper should be reported in the lead concentrate. The production of a copper concentrate depends of the copper minerals, the presence of chalcopyrite helps to produce a copper and lead concentrate; the presence of argentiferous copper sulphides could be a restriction to produce a copper concentrate. Try to produce a zinc concentrate would not be possible because part of zinc (sphalerite) is lost in the lead-copper flotation circuit, consequently, the zinc grade would be much lower than 7%.  Other factor to keep in mind is the iron content. The presence of pyrite is usually a problem in the zinc circuit. This is just an example, there other variable to consider. If the zinc grade is for example 2%, it is possible to produce a zinc concentrate.