Exploration for porphyry-style copper mineralisation near Llandeloy
Share Embed Flag
Download ePaper

Exploration for porphyry-style copper mineralisation near Llandeloy

Exploration for porphyry-style copper mineralisation near Llandeloy Exploration for porphyry-style copper mineralisation near Llandeloy

nora.nerc.ac.uk
from nora.nerc.ac.uk More from this publisher
04.08.2013 Views

the Brunel Beds or the Tetragraptus Shales. The intrusive episode can be dated, therefore, as being pre-Arenig and later than middle Cambrian. The form of both the individual intrusions and complexes is difficult to resolve. Porphyritic microdiorite intersected in borehole 8 shows markedly discordant relationships with the Solva Group and in other boreholes the porphyritic microdiorite appears to form veins intersecting other intrusive rocks. Porphyritic quartz-microdiorite and microtonalite in borehole 8, however, form concordant sheets. There is evidence of repeated injection of sheet-like bodies of the same rock; in borehole 7, for example, four sheets of quartz- microdiorite were intersected showing intrusive contacts against each other. It is not uncommon to find cognate xenoliths in quartz-microdiorite and porphyritic microtonalite. Where both the top and bottom of a sheet were located none exceeded 16.5 m in thickness. The intrusive complexes are unlikely to be stocklike. It is most probable that they are concordant or semi- concordant laccolithic bodies, the results of repeated injection of thin sheets of magma, with, perhaps the porphyritic microdiorite forming a late dyke phase. Relative orders of intrusion among some of the rock types can be established by the examination of contacts and identification of xenoliths in some boreholes. Extrapolation between them, however, is not easy and no order of intrusion applicable to all boreholes can be devised for all the rock types. This particular problem is compounded by the considerable amount of variation within some rock types and among transitional types: thus, though distinct rock names are given it is conceivable that differently named rocks could belong to the same intrusive phase and vice versa. Petrography of the intrusive rocks The seven distinct rock types, each of which is described below, can be grouped into three broader divisions which may be more meaningful when considering magma genesis. These are: a) Porphyritic microdiorite b) Microdiorite Quartz-microdiorite Porphyritic quartz-microdiorite Quartz diorite c) Tonalite Porphyritic microtonalite The porphyritic microdiorite, though displaying some textural variation is outstanding in being quartz-free or containing a very low content of it. The second group, with the exception of the porphyritic quartz-microdiorite, which is of restricted occurrence, is texturally homogeneous. The principal variations are in the quartz content and grain size. The quartz, however, is unevenly distributed even within a single slide and the microdiorite and quartz-microdiorite may comprise a single series. The third group is consistently quartz-rich and texturally distinct. All the microtonalites are strongly porphyritic and the tonalites, though differently named, may be regarded as densely porphyritic microtonalites. Among the rocks sampled at surface no equivalent was found of the porphyritic microdiorite or of the microdiorite to quartz-microdiorite series. The Hollybush quartz diorite compares with the quartz diorite in borehole 3A. The porphyritic quartz- microdiorites found at surface are similar to and as uncommon as among the borehole rocks. Most of the surface rocks are porphyritic microtonalite and, though no textural equivalents of the tonalites were found, they may be compared directly with the tonalite-porphyritic microtonalite group in the boreholes. Microdiorite This rock (E 53249-53) is sparsely porphyritic with phenocrysts of plagioclase and amphibole up to 5 mm long. The bulk of the rock is composed of stubby plagioclase crystals with a median size of 0.5 mm patchily altered, mostly at the crystal centres, to sericite, epidote and minor chlorite. Small amounts of (?)exsolved K-feldspar occurs within the plagioclase. Relict zoning shows through the alteration. Hornblende, when fresh is pale green and forms poikilitic crystals, but mostly it is replaced by chlorite and calcite. Quartz is interstitial, locally recrystallised and comprises no more then 4% of the rock, though it is irregularly distributed. Accessory minerals include magnetite and sphene. Three sheets of this rock occur in borehole 4 (Fig. 24). In all cases, there is a fine-grained porphyritic margin, containing euhedral amphibole and plagioclase phenocrysts, which passes into coarse-grained only sparsely porphyritic rock. In the upper sheet this rock shows little variation through over 14 m of core. In the middle sheet, only 0.58 m thick, the porphyritic variety reappears in the middle; but in the lower sheet, 4.44 m thick, there is complete mixing of the two types with irregular transitions between them. Patches of medium- grained uniform feldspar and amphibole rock merge into rock containing feldspar crystals from 0.25-2.5 mm long dispersed in irregular concentrations through fine- grained feldspar, hornblende and quartz. Porphyritic microdiorite In borehole 4 a sheet of porphyritic microdiorite beneath the lower microdiorite sheet passes inperceptibly into it and, like it, it displays some textural heterogeneity. Plagioclase phenocrysts (E 53254) up to 2 mm long, showing sutured margins, are altered to sericite and epidote. Some phenocrysts of greenish brown, poikilitic, euhedral hornblende are slightly smaller. The groundmass comprises plagioclase, hornblende, minor quartz with abundant chlorite and epidote, but is partly recrystallised. In boreholes 1 and 5 porphyritic microdiorite forms irregular, discordant veins and minor intrusions no more than 1.48 m thick. They display sharp, chilled contacts with flames and lobes penetrating the wall rock. The rock (E 53169, 53244-5, 53248) is texturally quite different from the type in borehole 4. It is stongly porphyritic with euhedral and subhedral amphibole phenocrysts commonly up to 5 mm long, rarely up to 1 cm lpng and abundant somewhat smaller plagioclase phenocrysts. The pseudomorphs after amphibole are composed of chlorite and ?magnetite; in places containing plagioclase inclusions. The feldspar phenocrysts are altered to sericite and albite, but display relict twinning and zoning. Typically, the groundmass consists of feldspar laths about 0.05 mm long with variable amounts of chlorite and opaque dust. The texture, however, is obscured by alteration. One rock only has a fine-grained recrystallised matrix. Quartz is very rare. Accessory minerals include apatite, sphene and hematite. The two-metre thick discordant intrusion intersected in borehole 8 (E 53846) differs from the others in being fine-grained and having closely packed phenocrysts. The rock is chlorite rich and it is veined by slightly more acid, uniformly fine-grained feldspar-quartz rock. Quartz-microdiorite Two large complexes of this rock were intersected; one in borehole 2, the other in borehole 7. The rock bears a strong resemblance texturally to the non-porphyritic microdiorite in borehole 4, but the quartz content is higher, ranging from 6.5 to over 10%. Quartz is irregularly distributed through this rock giving concentrations as high as 12.5% in parts, but the overall content is below 10%. Four units of this rock were drilled in borehole 2. They ranged from 0.91 to 8.32 m thick. Each of the top three units has a richly xenolithic basal zone, the inclusions all being of quartz-microdiorite. The rock in borehole 2

0 0 Dril? deposits l2-l Interbedded feldspar sand and clay v v v v Volcanic rocks : : Tonalite :I Porphyritic rnicrotonalite Depth of weathering including effects noted only along joints BH3A 1 16.2111 X X X X X X X X X X X X X X X X X X Porphyritic micmdiorite Fig.29. SIMPLIFIED GRAPHIC LOGS OF BOREHOLES 1 -7,PLOTTED AGAINST 0.0. SIMPLIAED ALTERATKIN ZONES Feldspar and amphibole stable with limited overprint of lete-stage ptvp ylitic Biotitic with limited overprint of late-stage propylitic ~at*-stapepmpvlitic overprint dominant ~imary~lpylitic

0 0 Dril? deposits l2-l<br />

Interbedded feldspar<br />

sand and clay<br />

v v v v Volcanic rocks<br />

: :<br />

Tonalite<br />

:I Porphyritic rnicrotonalite<br />

Depth of weathering including<br />

effects noted only along joints<br />

BH3A 1 16.2111<br />

X X X<br />

X X<br />

X X X<br />

X X<br />

X X X<br />

X X X X X<br />

Porphyritic micmdiorite<br />

Fig.29. SIMPLIFIED GRAPHIC LOGS OF BOREHOLES 1 -7,PLOTTED AGAINST 0.0.<br />

SIMPLIAED ALTERATKIN ZONES<br />

Feldspar and amphibole stable with<br />

limited overprint of lete-stage ptvp ylitic<br />

Biotitic with limited overprint of<br />

late-stage propylitic<br />

~at*-stapepmpvlitic overprint dominant<br />

~imary~lpylitic

logo

products

Resources

Company

Terms of service
Privacy policy
Cookie policy
Cookie settings
Imprint
Change language
Made with love in Switzerland
© 2024 Yumpu.com all rights reserved

Hooray! Your file is uploaded and ready to be published.

Saved successfully!

Ooh no, something went wrong!