620 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.45. 



levels. The pyrite mass, however, is not homogeneous throughout 

 m values. Its footwall portion is richer in copper, gold, and silver 

 than its hanging-wall portion, and several places about its periphery, 

 but especially on the footwall side, are characterized by bonanzas 

 formed of important admixtures of copper or silver sulphides, or 

 both. Such areas of higher values are principally border phenomena; 

 in the heart of the ore body only one such occurrence has been noted. 

 This was a small, pipelike zone running from 3 to 6 per cent copper, 

 due to chalcopyrite, and extended vertically from about the 400 to 

 500 foot level. 



In the ore sections studied evidences of secondary enrichment were 

 largely lacking. In certain specimens, however, chalcocite of prob- 

 able secondary deposition was recognized as different from other and 

 predominant chalcocite, which was in part of contemporaneous forma- 

 tion with the bornite and ever in sequential genetic relation with the 

 other primary sulpirides. Certain areas of tetrahedrite and chalcopy- 

 rite also showed relations suggestive of secondary origin. 



GENESIS OF DEPOSITS. 



To recapitulate, the ore deposits are of two kinds: (1) Great lens- 

 shaped masses of nearly pure sulphide ores, the Mount Lyell type, 

 and (2) mineralized bands of schist (fahlbands), the North Mount 

 Lyell type. Microscopic study of the ores shows that this difference 

 is one of degree and not of kind, and indicates that the same set of 

 ore-bearing solutions gave rise to both kinds of deposition. Gregory ^ 

 has discussed at length the ore genesis, and his conclusions may be 

 briefly summarized as follows: Alkaline ore-bearing waters, rising 

 along fault planes during the period of faulting, absorbed heat gen- 

 erated by these earth movements. With decreased pressure and 

 lowered temperature incident upon approach to the surface, they de- 

 posited their content, forming f alilbands in the less shattered portions 

 of the schists and producing replacement masses in the highly shattered 

 and extremely permeable areas adjacent to fault loci. The deposi- 

 tion, therefore, is regarded as "due to tectonic and not to igneous 

 action." Gregory does not explain the ultimate origin of the solu- 

 tions nor of their metallic contents. 



Consideration of the microscopic features of the ores has led the 

 writers to believe that Gregory's explanation is not entirely adequate, 

 and that the ore-bearing solutions were a deep-seated development 

 from a differentiating mass of igneous rocks ^ and that these solutions 

 rose along structurally developed channels, changing gradually in 

 composition from the beginning to the end of the depositional epoch. 



' Australian Inst. Min. Eng., vol. 10, 1905, pp. 145- 15C. 



2 A similar conception was developed by Spurr in 1907 (A theory of ore deposition, Econ. Geol., vol. 2, 

 1907, pp. 781-795) and later further elaborated by him (Econ. Geol., vol. 7, 1912, pp. 485-492). Finlayson 

 applies a somewhat similar explanation to the origin of the Huelva pyrite deposits. 



