Paragenesis of Hydrothermal Mineralization in Amphibolites and Granulites Around Porthkerris Point, the Lizard, Cornwall

A. F. Seager
Department of Geology, Birkbeck College, University of London

Synopses: A study of the post-metamorphic hydrothermal mineralization in the gabbros of the Lizard has already been published (Seager, 1971). The present paper extends the investigation of mineralization in the peninsula to an adjacent area, on the NE. coast around Porthkerris Point. Table 1 gives the more important events of the paragenesis. Calcite and adularia are the only common minerals; all others are scarce or rare.

The calcite that precedes and succeeds adularia is called early calcite and late calcite, respectively. Both early and late calcite occur in several generations differing in habit, fluorescence, and phosphorescence. Ortho-serpentine is present as inclusions along a growth horizon approximately 0·5 mm thick in some specimens of early calcite. After the growth of natrolite a phase of dissolution Table I. Paragenesis of the hydrothermal mineralization at Porthkerris* Green and white botryoidal prehnite Early calcite Analcime Natrolite Dissolution and removal of some analcime Adularia Quartz Dissolution of some early calcite Late calcite Stilbite Dissolution channels in calcite formed during growth of stilbite *This is an abbreviated version of the table of paragenesis in Miniprint section. occurred, which particularly affected analcime (Seager, 1978). Subsequently there was a wide-spread deposition of adularia, which shows varied replacement relationships with analcime. Adularia also formed encrustations upon early calcite and replaced it superficially and along grain boundaries. During the second phase of dissolution early calcite was strongly attacked, converting the encrustations of adularia on calcite to epimorphs. In the subsequent period of mineral formation, several generations of late calcite were precipitated. These crystals developed varied habits, on which many forms have been recorded. Stilbite is a relatively scarce mineral that occurs as small scattered bladed crystals, or as encrustations upon late calcite. The solutions associated with the deposition of stilbite formed solution channels in calcite, and appear to have caused some metasomatism in that mineral.

Discussion. The character and mineral content of the veins indicate their hydrothermal origin, and the cross-cutting relationship to the foliation proves that mineralization post-dates metamorphism. Several aspects of the paragenesis have significance for the interpretation of the geological history of the area. There is a noteworthy alternation of periods of mineral formation and dissolution in amphibolites and granulites at Porthkerris. This phenomenon, the sequence of mineral formation and the modes of crystal growth of several minerals show a striking similarity to the paragenesis in the gabbros of Dean quarry (Seager, 1971). The most satisfactory explanation is that the mineralization in both areas took place simultaneously, by precipitation from the same series of hydrothermal solutions. These two parageneses were based on studies in limited areas, because suitable specimens were obtainable there. However, there is ample evidence that hydrothermal mineralization is wide-spread in the Lizard complex. Many occurrences of minerals of the prehnite-zeolite-calcite suite have been recorded from metabasic rocks (Holyer, 1972). Prehnite, analcime, natrolite, and calcite occur in the Traboe hornblende-schists on the west coast, approximately 16 km from Porthkerris, on the opposite side of the peninsula. These minerals also occur in a gabbro quarry on the east coast at Porthoustock, and natrolite has been found at the contact of an epidiorite dyke with serpentinite. Adularia occurs at many localities around the coast of the Lizard peninsula, in the Traboe and Lande-wednack hornblende-schists, gabbros, and rocks of the Old Lizard Head Series. The serpentinites have been examined by the present author for evidence of hydrothermal alteration, which appears to be widespread; it is present in many outcrops around the coast, and at quarries inland. The veins consist mainly of hydrous magnesium minerals and calcite, rarely containing hematite or silica minerals. No trace of prehnite, zeolites, or adularia has been found. Thus post-metamorphic hydrothermal mineralization is present in the major rock suites on a scale that appears to justify the description of regional mineralization in the Lizard complex.

Some evidence is available of the date of hydrothermal mineralization in the Lizard peninsula. Adularia occurs within the parageneses of the mineralization in the gabbro at Dean quarry and in the granulites and amphibolites around Porthkerris. Close minimum ages of crystallization for adularia from these localities have been determined as 201 ± 4 Ma and 211 ± 2 Ma respectively (Seager et al., 1975, 1978). Halliday and Mitchell (1976a) dated specimens of adularia from these and other localities in the Lizard complex, and suggested that its crystallization took place during a hydrothermal event at 210–20 Ma. The adularia, which is common to the two parageneses quoted above, thus appears to have been formed during the same event. Adularia from Holseer Cove yields evidence of another hydrothermal event at 160–70 Ma (Halliday and Mitchell, 1976a). A specimen of adularia from Dean quarry, which appeared to have suffered hydrothermal alteration, gave an apparent age of 147 ± 3 Ma. If partial overprinting is present, it would have taken place in Cretaceous or Tertiary times (Seager et al., 1975, 1978). Adularia is the only mineral from the hydrothermal suite that can be used for dating in the Lizard. Since adularia occurs within the paragenesis, no hydrothermal events that preceded its crystallization have been dated, and there is little evidence of the date of subsequent events. It is tentatively suggested that the parageneses established at Porthkerris and at Dean quarry may indicate that hydrothermal activity extended over a distinctly longer period than the formation of adularia. It is most desirable that means be sought to date the several phases of hydrothermal activity, in order to place events in the Lizard in their regional setting.

Many aspects of primary mineralization in south-west England have been reviewed by Hosking (1964). It was noted that mesothermal veins (with which he included some epithermal veins) are widespread, but are usually comparatively small and uneconomic, mineralization having occurred during two or three periods separated by large intervals. Most activity was related to the Late Jurassic and/or Tertiary igneous phase. The Jurassic and Tertiary activity wrought mineralogical changes largely by remobilization of earlier components, rather than by introducing large quantities of ‘new’ ore components. The cross-courses may contain a mesothermal suite of minerals, barren quartz, or fault clay. The clay-filled crosscourses were the last to be formed.

A study of uranium mineralization in south-west England led Darnley et al.. (1965) to conclude that there were periods of uranium mineralization at c. 290 Ma, c. 225 Ma, and c. 50 Ma and probably at c. 165 Ma. The younger ages were derived from lower-temperature minerals in NS veins. There are several different vein fillings at Geevor mine, west Cornwall, and the time interval between their formation may be of the order of tens or one or two hundreds of millions of years. 40Ar-39Ar step hearing studies suggest that a hydrothermal event occurred at Geevor mine close to 210 Ma, and a K-Ar age determination of a quartz-tourmaline-cassiterite stringer gave an age about 165 Ma (Halliday and Mitchell, 1976b).

Several aspects of the hydrothermal activity in the Lizard appear analogous to that in adjacent areas. The alternating periods of mineral formation and dissolution suggest that mineralization occurred in pulses. The periodic access of solutions to veins has been demonstrated elsewhere in Comwall. The growth of adularia in the Lizard at 210–20 Ma virtually coincides with a period of uranium mineralization and another hydrothermal event at Geevor mine in Late Jurassic times. Adularia was preceded by the formation of prehnite, analcime, natrolite, and some calcite, as well as a phase of dissolution, which may be Late Jurassic or earlier in age. There were two periods of dissolution and one of mineralization after the formation of adularia, the overprinting of which may indicate a Cretaceous or Tertiary event.

Hydrothermal solutions, which introduced little or no new material, but altered and redistributed existing minerals, could account very satisfactorily for the type of mineralization found in the Lizard, in which the mineral composition of veins is closely related to the chemistry of the country rock. If some means can be found of dating other stages of mineralization in the Lizard complex, it may be possible to use the detailed parageneses already established for the mineralization at Dean quarry and at Porthkerris in a comparative study of phases of hydrothermal activity throughout south-west England.

Mineralogical Magazine; December 1978 v. 42; no. 324; p. 509-511; DOI: 10.1180/minmag.1978.042.324.15
© 1978, The Mineralogical Society
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