Mineralogical zonation and metasomatism of the alteration pipes of Cyprus sulfide deposits

Univ. Newcastle upon Tyne, Dep. Geol., Newcastle upon Tyne, United Kingdom

The stockwork mineralized lavas associated with the cupriferous pyrite deposits of Cyprus occur in the cores of concentrically zoned alteration pipes. These represent the channels of ascending hydrothermal solutions. This paper reports a detailed mineralogical and geochemical study of the Pitharokhoma alteration pipe and reconnaissance studies of other pipes. Together with complementary work on the Agrokipia B pipe (Robinson et al., 1987) and the Mathiati pipe (Lydon and Galley, 1986), these studies allow three types of pipe to be distinguished, namely, P (Pitharokhoma), K (Kokkinopezoula), and M (Mathiati) types.All the pipes have similar outer alteration zones, the outermost being characterized by smectite-rich chlorite-smectite mixed layer minerals. Farther in, the layer silicates are increasingly dominated by chlorite and plagioclase is altered to albite, giving rise to a chlorite + albite + quartz + sphene assemblage (chlorite-albite facies). The major differences between pipe types are in the intensely mineralized and metasomatized centers. In the P-type pipes, most of the mineralized zone is composed of lavas altered to an Mg chlorite + illite + quartz + pyrite + anatase assemblage (chlorite-illite facies), whereas in K-type pipes, the assemblage is Mg chlorite + rectorite + quartz + pyrite + anatase (chlorite-rectorite facies). In both types, chlorite abundance decreases as mineralization increases toward the pipe center. At Pitharokhoma, the pipe center is exposed and consists of lavas altered to illite + quartz + pyrite + anatase. The M-type pipes have mineralized zones in which the outer parts are altered to chlorite-rectorite facies, but the central parts are altered to Fe-rich chlorite + quartz + pyrite + anatase. All types of pipe show Mg enrichment in the peripheral zones of chlorite-smectite and chlorite-albite facies alteration and also in the outer parts of the mineralized zones. The most intensely leached lavas at Pitharokhoma are enriched in K, Rb, and Ba, and are depleted in Mg, Ca, Sr, and Na, and Al has been significantly mobile. In the K-type pipes, the metasomatism is in many respects similar to the P type, but the composition of rectoritc results in there being no enrichment in K, Rb, and Ba. In M-type pipes, Mg is not depleted in the pipe center, whereas Ca, Sr, Na, K, Rb, and Ba are all strongly depleted.The alteration pipes have all been affected to some degree by postmineralization low-temperature alteration. Effects of this alteration include formation of smectite, K-feldspar, and calcite and possibly, alteration of original hydrothermal sericite to illite (in P-type pipes) and illite to rectoritc (in K-type pipes). If all the rectorite in K-type pipes has been derived in this way, then K-type alteration would represent P-type alteration overprinted by low-temperature alteration, and so only P- and M-type pipes would be the result of differing conditions during sulfide mineralization. One of the chief differences was probably in the S/Fe ratios in the hydrothermal fluids. In P-type pipes, the calculated ratios of S addition to Fe addition in the mineralized lavas are greater than the ratio for pyrite addition, whereas the reverse is true at Mathiati. This difference is paralleled in the modern black smoker fluids, of which some have an excess of dissolved sulfide over dissolved Fe, whereas some have an excess of Fe over sulfide.

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