Scott reported in 1990 that the origin of alunite-jarosite family minerals, found in surface samples from the Mt. Leyshon Au deposit, northeast Queensland, has been determined using electron microprobe and textural data. The base-metal contents of the alunite-group minerals provide significant additional information to help evaluate whether they formed during hypogene or weathering processes.
K- and Na-rich members of the alunite group (K Na)Al3(SO4)2(OH)6 generally form as hydrothermal minerals in the acid-sulfate epithermal system at Mt. Leyshon. Because of their high-level formation they may be affected by subsequent weathering, during which they may incorporate Ag, Bi, Cu, and Pb derived from sulfides. Pb-rich members of the alunite group [plumbogummite, PbAI3H(PO4)2(OH)6, and hinsdalite, PbAI3(PO4(SO4)-(OH6)] that form during weathering derive components from both sulfides and apatite.
Jarosite-group minerals, (K, Na)Fe3 (SO4)2 (OH)6, that replace alunite-group minerals during weathering retain the Na-K ratio of the parent; substantial Al is also retained by the resultant jarosite. Jarosites that directly replace pyrite have low Al contents and Na-K ratios that differ from those of any alunite-group minerals present within the sample.
K- and Na-rich alunite-group minerals originally formed as hydrothermal alteration minerals at Mt. Leyshon. Because of their formation close to the original surface, i.e., above the water table(?) now, they were susceptible to modification during subsequent weathering. During such weathering they may have incorporated Ag, Bi, Cu, and Pb derived from the breakdown of sulfides. Pb- and P-rich alunite-group mineral formation might have required the destruction of both apatite and Pb-rich sulfide. Such specialized conditions may be reflected by the restricted distribution of plumbogummite and hinsdalite at Mt. Leyshon.
The author concluded that Jarosite-group minerals at Mt. Leyshon formed either by replacement of alunites, retaining the Na-K ratio of the parental alunite and generally some Al, or directly from pyrite, resulting in only a low Al content.
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