TECHNOLOGY FOCUS - METALLURGY - Preferred Method: The ore contains at least one of nickel, cobalt, copper or zinc, present in MRL, and optionally at least one (B), present in the residue. MRL is:(a) applied to an ion-exchange resin and then eluted with an inorganic acid; or(b) treated with a microorganism that reduces sulfates to sulfides (which are separated).Where MRL contains zinc, it may be extracted with organic solvent and the extract stripped with either nitric acid (the zinc nitrate formed is heated to form zinc oxide, with recovery of nitric acid) or sulfuric acid (the zinc sulfate is electrolyzed to deposit zinc at the cathode). The recovered nitric acid, or spent electrolyte, is used for stripping. Optionally the ore is formed into a heap before treatment with (A), which is applied as an (A)-containing solution.In method (c), the ore (or concentrate) is formed into a heap and treated with (A) to form MRL which is separated. The residue is ground to a size suitable for flotation and floated to give a concentrate rich in (B). This is upgraded to form a product suitable for further refining. Alternatively, the ore is first separated into major and minor fractions and the former floated to produce a low-grade base and concentrate rich in (B). The concentrate is mixed with the minor fraction, formed into a heap and treated with (A) and (I) to produce MRL, containing the non-precious metals.The residue is floated to give a (B)-rich concentrate for upgrading as above. The pH during leaching is 1-5, preferably 1.5-3.5.    TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Materials: (I) is elemental sulfur and/or a sulfide.    TECHNOLOGY FOCUS - BIOLOGY - Preferred Microorganisms: Typical (A) are Thiobacillus thiooxidans, T. ferroxidans, Leptospirillium species and Sulfolobus brierleyi. Suitable sulfate-reducing bacteria are Desulfovibrio, Desulfatomaculum and/or Desulfomonas species. EXAMPLE - Saprolite ore (5 g), sulfur (2 g) and MKM (90 ml; not defined) were mixed with 10 ml inoculum that had been grown on elemental sulfur, and the mixture incubated in an orbital shaker at 35degreesC. Samples were periodically analyzed for nickel; about 75% was released after 15 days and 100% after 30 days. A similar experiment using only 0.5 g sulfur did not result in significant release of nickel.