Mineralisation, paragenesis and geochemistry of the Mount Wellington mine, Cornwall

Kettaneh, Yawooz Abdullah Hamdi (1978) Mineralisation, paragenesis and geochemistry of the Mount Wellington mine, Cornwall. University of Southampton, Doctoral Thesis.

Abstract

The Mount Wellington Mine lies on the southwestern extension of the Wheal Jane Mine lodes. The mine, which recently closed, produced tin, copper and zinc from the shallow dipping No.1 Lode underneath the Wellington Elvan and from the steeper dipping No.2 Lode which joins the No.1 Lode in the upper levels of the mine. The uneconomic No.3 Lode is similar to the No.2 Lode. The emplacements of the elvan and the main lodes are controlled by two sets of faults which were probably formed as a response to fluid over pressures around the Carnmenellis granite cupola. Four mineralisation phases have been recognised in the lodes, each separated by a period of faulting. The first two (dominant) phases cemented brecciated fragments in the fault zones and the later phases filled dilatancies in the earlier phases. The lodes were formed coevally and do not crosscut each other, but simply join under the elvan, which had an important control on their configuration. The Hot Lode has many similarities to phase C of the main lodes with which it is probably coeval and cogenetic, it is noticably rich in copper and silver. The lodes have a complex mineralogy of mixed oxides-sulphides-sulphosalts and native metals. The bulk paragenesis in the lodes is from cassiterite through base metal sulphides to native metals and sulphosalts. Many previously unreported minerals including bismuth, silver, gold, stannite, galena, covellite, neodigenite, siderite, malachite and azurite have been identified. In addition a number of unknown sulphosalts occurring as small inclusions in the sulphides have been located. Their small size and complexity have prevented their full identification, but the existence of a number of new phases is suspected. The telescoped polyphase mineralisation has resulted in the absence of distinct mineral zonation, but a crude, modified metallogenetic zonation has been detected over part of the No.1 Lode with zinc increasing upward and to the east. The wall-rocks to mineralisation (elvan and killas) were affected by kaolinisation, sericitisation, silicification, tourmalinisation, chloritisation, haematitisation, fhuoritisation and mineralisation. Complex and overlapped dispersion haloes around the lodes have been recognised. In general K, Zn, Rh, Pb, Sn, As, Cu and S are dispersed from the lodes into the wall-rocks; and Sr, Zr, Ni, Ti and Mg show opposite trends. So and Rb appear to be the only potentially useful "pathfinder" elements.The main lodes, the Hot Lode and their host rocks were cut by caunter lodes and crosscourses. The caunter lodes, which strike parallel to the main lode, but have a complimentary dip, contain sulphides- predominantly pyrite- in a quartz-chlorite gangue, but are of no economic significance. The cross courses contain an epithermal mineral assemblage (mostly marcasite) which is much younger than all other mineralisation. Nevertheless fluctuations in the style of mineralisation in the main lodes around the cross courses may suggest an earlier origin of these structures. However it is argued here that original irregularities in the elvan footwall controlled both the style of main lode mineralisation and the location of later faulting. Supergene weathering has resulted in redistributions in the upper levels of the mine and locally around open fissures at deeper levels.

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