Chlorites, phengites, and siderites from the Prince Lyell ore deposit, Tasmania, and the origin of the deposit

The Prince Lyell ore deposit consists of disseminated lenses of pyrite and chalcopyrite that are broadly concordant with a sequence of altered felsic-intermediate volcanic rocks. Detailed mapping has indicated that the main mineralized horizon is zoned and the key alteration assemblages from north to south are: (i) pyrite + Mg-chlorites, (ii) pyrite + (Mg-Fe) chlorites, (iii) pyrite + Fe-chlorite + or - minor magnetite, (iv) (Mg-Fe) chlorite + magnetite, and (v) magnetite + barite + Mg-chlorite. In general the pyrite content decreases and magnetite increases from assemblages (i) to (v). The Mg/(Mg + Sigma Fe) ionic ratio in the chlorite varies from 20 in the deposit center to 50 at the margins. The composition of siderite varies sympathetically with that of chlorite.A unit rich in monazite and apatite occurs stratigraphically above this zoned alteration envelope and is followed by a monotonous sequence of fragmental lavas and pyroclastics. The Fe content of phengites decreases from the stratigraphic base of the deposit toward the lower boundary of the phosphatic unit. This gradational sequence from Fe-rich to Fe-poor phengites is repeated from the phosphatic unit into the fragmental volcanics. The Mg/(Mg + Sigma Fe) ionic ratio for phengites is positively correlated with that for chlorite.The alteration assemblages are considered to represent facies of early diagenetic/syngenetic alteration that occurred on or close to the sea floor. Primary compositional variations may have played a vital part in the final alteration assemblage produced. The distribution of high-grade Cu lenses is locally discordant with those alteration assemblages, and temporal relations between the two are unclear.