156 NEW YORK STATE MUSEUM 



A .thin section of a rather lean ore from the Stella shaft, showing 

 to the naked eye pyrite, graphite, quartz, feldspar and the usual soft 

 gray material, under the microscope shows much nearly isotropic 

 chloritic alteration product mingled with bleached mica and sericite. 

 There is also considerable quartz and some untwinned feldspar. 

 Graphite is abundant, much of it being inclosed in the pyrite though 

 in one or two instances the latter mineral has grown between and 

 forced apart plates of graphite. The pyrite is often in distinct 

 crystals which apparently have been the last development in the rock 

 unless it Jbe the alteration products. As to the latter there is some 

 uncertainty, for while the chlorite sometimes forms borders around 

 or veins in pyrite there is a tendency, noted elsewhere, for the pyrite 

 to have more perfect crystal form when in contact with chlorite, as 

 though the latter were more yielding than the other minerals. With- 

 out doubt some of the chlorite, and probably most of it, antedates the 

 pyrite, while the very pronounced formation of chlorite in the 

 pyritiferous rocks, as compared with the average gneisses, strongly 

 indicates a direct connection between chloritization and deposition of 

 pyrite, as products of closely related and associated processes and 

 conditions. So far as the evidence goes this rock seems to be a 

 highly altered gneiss carrying unusual quantities of graphite and 

 pyrite, the latter mineral having evidently formed very late, replacing 

 the older minerals. 



A section of richer ore differs greatly from the preceding, having 

 even more graphite, and the pyrite developed almost to the complete 

 exclusion of other minerals. The section is quite striking, showing 

 the pyrite in distinct though interlocking crystals including abundant t 

 graphite, the interspaces being filled with sericite, secondary quartz 

 and chlorite (see figures 18 and 19). Though as a whole so 

 unlike the preceding section, this one shows remnants of greatly 

 altered minerals of the gneiss and there can be no doubt that this 

 rich ore is a part of the gneiss in which the processes of alteration 

 and replacement have been carried to an extreme degree. 



The immediate hanging wall of the Stella ore body is a fine- 

 grained dark gray schist, with much graphite, shot through with tiny 

 veins and bunches of pyrite (see figure 20). A thin section shows 

 it to be a graphite-mica schist, the graphite giving the dark color. 

 The mica is largely altered to a weak doubly refracting chlorite 

 similar to that in the ores, and sometimes to a fibrous mass of yel- 

 lowish brown color. Areas of this latter sometimes have a narrow 

 border of sericite. Quartz is abundant, but feldspar is present only 

 in very small quantity, and none with the twinning of plagioclase. 



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