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Art. XLI.—Notes on a Hypersthene Andesite from Waihi Mine, Waihi.
[Read before the Auckland Institute, 15th October, 1900.]
This rock was cut through in the south-east crosscut from No. 1 shaft, at the 300 ft. level of the mine, on the hanging-wall of the Martha lode, near the Grand Junction boundary-line. It approaches close to the surface of the old valley which existed to the east of the Martha Hill before the eruption of the rhyolites which cover the plains and wrap-round the Rosemont, Amaranth, and Black Hills, and surround the Martha Hill on all sides, excepting the narrow neck of andesite on the north side, which connects it with the great andesitic area of the Hauraki Goldfields. It occurs as an undecomposed bar or core passing insensibly into the soft decomposed andesite or propylite enclosing the Martha and Empire lodes.
The colour of the rock is greyish-black; when wet, dense black. In hand specimens phenocrysts of feldspar and pyroxene are prominent. Fracture splintery, subconchoidal. Under the microscope the essential minerals are seen to be plagioclase and pyroxene, with numerous square and rod-shaped microliths set in a glassy base. The base is not abundant, is mostly clear, and dusted evenly with a black-brown or black dust, apparently magnetite.
The feldspars are fairly fresh and glassy, and occur both as short tabular phenocrysts, up to 5 mm. long, and as narrow laths. The former are well developed, and show very marked polysynthetic twinning, and sometimes zonal structure. The smaller feldspars are often irregular in form and not much twinned. The effects of corrosion by the cooling magma are clearly seen in the rounded angles and indentations of some of the feldspar plates. Binary twin crystals occur in each slide. Their straight extinction indicates orthoclase.
The indentations and cracks of the larger feldspar plates are filled with calcite and fine magnetite dust, which often shows oxidation to hæmatite. In some of the phenocrysts enclosures of colourless square-shaped microlites are common, in many cases zonally arranged.
Hypersthene is abundant and generally much decomposed, showing alteration into serpentinous products, quartz, and magnetite. In some cases the crystal is almost replaced by magnetite; in others—perhaps the more common
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—a skeleton of magnetite outlines the original form and cleavage-cracks. The magnetite always shows alteration to hæmatite. A little enstatite is present in each slide.
There may be a little augite present but it could not be distinctly identified, as the ferro-magnesian minerals are more or less altered into serpentinous matter.
Hornblende, much altered, occurs sparingly in well developed crystals, showing the characteristic prismatic cleavage. In each slide there are several long prismatic and lozenge-shaped bodies, rendered almost opaque with magnetite dust. They may originally have been hornblende. In one slide a plate of one of the dark bodies, besides the magnetite dust which renders it almost opaque, contains enclosures of large grains of magnetite and a small idiomorphic crystal of plagioclase feldspar, and, embayed on the edge, a small grain of quartz.
Quartz occurs as hexagonal plates and interstitial. The former are rare and may be a primary generation.
There is much calcite present in the rock, and a little carbonate of iron and magnesia.
From the crosscut at the 300 ft. level, at a point immediately below No. 5 shaft, occurred a few small kernels of black greenish-grey rock enclosed in the decomposed andesite. Microscopic examination proved it to be the same rock as that just described from the south-east crosscut from No. 1 shaft, some 1,000 ft. distant but more altered, with the result that calcite, hæmatite, and quartz are more abundant.