Page:Transactions and proceedings of the New Zealand Institute (IA transactionsproc41newz).pdf/110

88 Dykes. SThe granitic rocks are penetrated here, as elsewhere on the Coast, by numerous basic and semibasie dykes, including varieties of diorite, por- phyrite, camptonite, and similar types,

The slates are also intrusled by a series of dykes which, so far as yet known, are exclusively of diabase. The outcrops are diffieult to locate. owing to the extensive alteration which the dykes have undergone, some of them being slightly schistose. They have generally been exposed in mine-workings only, and, as many of the mines are now closed down, obscrvations cannot readily be made. They have been found in the Specimen Hill, Inglewood, and keep It Dark Vines, but the only one that I saw was in the last-named and its occurrence is described inader the description of that mine.

It is notable that these dykes seein confined to the gold-bearing belt, but their intrusion seems to antedate the veiu-formation. Their intense alteration is probably due to solutions acting at this period, and their occasional schistosity indicates that they have been subjected to considerable pressure, probably during the period of folling of the slates. There is thus evidence for asemming that they are the oldest known intrusives in the district.

Petrography of Diabase.

Megascopically the diabase is a rather soft dark-green rock, full of black cleavage-laces of small augite crystals.

Cnder the microscope the rock, when fresh, has the typical ophitic structure of a diabase. It is composed of twinued laths of andesine-feldspar containing some secondary scricite and calcite arranged evenly along the centre of the erystals. Augite is fairly fresh and pale in colour, the crystals filling up interspaces between the feldspars, or occurring as small cletached crystals. The individuals are occasionally twinned. The relative propor- tions of augite and feldspar vary somewhat: at times augite predomi- nates, with the local lisappearance of other constituents. It is then coarse and in better-outlined crystal. Olivine is oecasionally seen in some quantity, but very largely replaced by a nesh of serpentine, without the separation of mueh iron-oxide. It is probably not a highly ferriferous varielt. Of other constitucuts, a good deal of magnetite (both primary and secondary), a little basaltic bornblende (often chloritized), and needles of apatite are present.

The alteration of this rock close to the rein is well shown under the microscope. In the first stage the foldspars become more and more replaced by carbonates till the iwinning is indistinguishable, and they practically consist of scricite-calcite pseudomorple. Olivine is replaced by chlorite and serpentine, angite by magnetite (or ilmonite) and a Jerk-green serpen- tine. The ophitic structure can, however, still be traced.

Tith further alteration all strueture is lost, even the outlines of crystals being obliterated. The rock becomes an aggregate of carbonates and scricite, with a good deal of opaque iron-oxide, and a little quartz, probably introdneed. Strings and patelies of pale-green chlorite are present at first. but these are eventually altered to sericitie matter. Pyrite crystals are also frequently present.

Thus we are able to trace the progressive steps of alteration. The lime- soda feldspar is changed to carbonates itu sericite; the augite and olivine first to transitional chlorite and serpentine, with separation of icon-oxides,