Art. XLVIII.—On the Geology of the Rock-phosphate Deposits of Clarendon, Otago.

By Professor James Park, F.G.S., Director, Otago University School of Mines.

is now a portion of the Horseshoe Estate situated near Clarendon Railway-station, a place thirty miles south of Dunedin. Subsequent search has shown that phosphatic rock occurs in many places on the estate and adjoining properties, including, among other places of interest, the wellknown lime-quarries at Millburn.

In June of this year I was enabled through the courtesy of the owners to make an examination of the geological conditions under which the phosphate occurred. The result of my investigation was contained in an article contributed to the Otago Witness in July^* of this year. After an interval of five months I again visited the locality, with the view of collecting any fresh facts disclosed by the extensive mining operations undertaken during that period. The result of my observations on both occasions is contained in the present paper.

Physical Features.

From the low-lying valley at the south end of Waihola Lake, along which the Dunedin-Invercargill Railway runs to Milton, the land rises to the westward by a succession of low, gentle, undulating hills, from which, by a long easy slope, is reached the summit of the semicircular ridge whose contours probably suggested the present name of the estate. The point of the horseshoe is directed toward Milton, with the Millburn quarries on the outer rim and Horseshoe Bush on the inner. The summit of the ridge is fairly flat. The descent into the bend is long and easy, but on the Milton side it is generally abrupt, and in many places quite precipitous.

The surface is open agricultural land, much of which is at present under cultivation.

General Geological Structure.

The general geological structure of the district is very well seen in the section running from Clarendon westward across the Horseshoe Estate, for a distance of perhaps a mile and a half from the railway-line. After leaving the flat the low hills first crossed are composed of mica-schist of probably Silurian age, lying nearly horizontal. Proceeding westward, the schist is overlain by Upper Eocene quartz grits and conglomerates which usually form the lowest member of the coal-measures in southern Otago. The grits are in turn followed conformably by glauconitic greensands, limestones, often glauconitic, and a soft brown sandstone. The latter is overlain by a flow of basalt which caps the horseshoe ridge referred to above. The series of beds associated with the limestone lies nearly horizontal in the Horseshoe Bend,

but rises gently to the south-west, the inclination for the most part being so gradual as to be perceptible only by comparing the altitudes of the limestone outcrops over wide intervals. The phosphate rock, to be more particularly described hereafter, occurs in pockets in the limestone, and is covered in most places with an overburden of brown-coloured sands and clays.

A study of the topographical features of this area, when considered in connection with the disposition of the rock formations, shows that the present contours were determined by denudation long after the eruption of the basalt cap.

Classification of Rock Formations.

in many other localities which need not be specified it has been designated by a local name.

This limestone is very variable in physical character and composition. Even in the same horizontal plane it may be seen to pass gradually and insensibly from a compact limestone into a calcareous sandstone, often within a distance of half a mile or less.

(a.) Brown Sandstone.—From the upper surface of the limestone to the basalt cap there is an interval of 120 ft. to 150 ft. in vertical height, apparently occupied by a yellowish-brown sandstone, the character and disposition of which could not be ascertained on account of its outcrop being obscured by a heavy slope deposit of black earth mixed with sand.

In the Oamaru and Weka Pass districts, where the sequence of Lower Tertiary strata is very complete and characteristic, the Oamaru and Weka Pass calcareous sandstones, which, as we have seen, are the time-equivalents of the Millburn limestone, are followed quite conformably by the Hutchison quarry, or Mount Brown beds, which consist of yellowish-brown calcareous sandstones containing a rich assemblage of marine forms. This overlying series is so closely associated with the Oamaru series that it cannot be regarded as a separate formation, but only as the closing horizon of the Oamaru series itself. Until something more definite is ascertained about the sandstone lying above the limestone on the Horseshoe Estate, it may be correlated with the Hutchison quarry horizon of the Oamaru formation.

Basalt.

This occurs as a true flow. It rests on the upper surface of the brown sandstone and caps all the higher hills. As its junction with the underlying rock is everywhere obscured by slope deposit, its thickness cannot be determined, but at the old cemetery quarry the depth of the flow is not less than 100 ft.

This basalt is excessively fine in texture, at most places possessing a clean, splintery fracture. In the face of the cemetery quarry it exhibits a rudely columnar structure. Here, also, its weathered surfaces possess a deeply corroded appearance, and the usual splintery character is absent except in one narrow band near the centre of the higher part of the quarry-face.

In polarised light thin slices of this rock show an abundant dull-grey, or semi-opaque feldspathic base, with augite and olivine, the latter often serpentinised. Idiomorphs of feldspar are absent. The base, however, is crowded with acicular microlites, some of which appear to exhibit polysynthetic twinning. Magnetite is very abundant.

Rock-Phosphate.

This was first found at Discovery Point, at the head of the bend, where it rests on the upper surface of the limestone. Here it forms a massive outcrop from 12 ft. to 18 ft. high and from 4 to 5 chains long. It consists of a very dense grey or yellowish-grey rock-phosphate, very rich in calcium phosphates. In places it is nearly pure phosphorite, occurring in narrow-banded pale-yellow and grey concretionary masses, possessing a tendency to exfoliate in layers when struck with a hammer. Cavities in this rock were found to be incrusted with apatite possessing a mammillary structure. The extent of the deposit at this place has not yet been determined.

Another outcrop of rock-phosphate crops out on the side of the valley opposite Discovery Point, and near it several large masses of this mineral occur in a small depression in the hill about 20 ft. above this outcrop. Recent excavations show that the phosphate-deposit here is of considerable extent. It has been exposed by open trenches for a distance of 4 chains along the side of the hill, and is found to rest on an eroded surface of the greensands. Loose masses of rock-phosphate, lying on the slopes to the north of this, point to the presence of another deposit in that direction.

At Kiln Point a considerable amount of stripping and trenching has been effected, and here much interesting information was obtained concerning the mode of occurrence of the rock-phosphate. At this place the outcrop has been stripped for a distance of nearly 2 chains, exposing a very clear vertical section of the phosphate - deposit and underlying limestone. The phosphate varies from 3 ft. to 12 ft. in thickness, and rests in a series of pockets in a deeply eroded surface of the limestone. In the face behind the old lime-kiln there are three shallow pockets, the most southerly being

45 ft. wide; the second 18 ft., and the third about 22 ft., varying in depth from 2 ft. to 6 ft. The pockets are separated from each other by ridges of limestone averaging 3 ft. or 4 ft. wide, as shown in the following diagram:—

Section I.

Kiln Point: Face exposed behind old kiln. A. Phosphate rock. B. Limestone.

The phosphate fills the pockets and rises above the level of the dividing-ridge of limestone to a height varying from 3 ft. to 10 ft., the greatest depth occurring at the north-east end of the section.

The phosphate rock exposed in the face of the open cutting is much broken and crushed, and sometimes shows slickenside surfaces. It is yellowish-brown in colour, with irregular seams and patches of whitish-grey. The presence of sand renders it soft and friable, and of lower grade than that exposed at Discovery Point.

At the most easterly point of the open cut masses of fairly pure phosphate rock contain inclusions of basalt, occurring in small angular or nodular fragments which are seldom over 4 in. in diameter. In a deep narrow trench above the open cutting the phosphate is mixed with glauconitic greensands which are said to be highly phosphatic.

Section II.

Kiln Point. A. Phosphatic greensands. B. Phosphate rock. C. Limestone. D. Glauconitic greensands.

The rock-phosphate has been exposed by a long trench some 12 chains north of Kiln Point, but no feature of special interest is disclosed in this direction.

At Millburn lime-quarry the surface of the Eocene lime-

stone presents the most marked irregularity. Under the influence of both chemical and physical erosion it has been formed into wide basins and deep well-like holes, surmounted by overhanging knobs and spires of limestone. The basins are filled with yellowish-brown sands, as shown in Section III. below.

Section III.

Millburn Quarry, showing surface of limestone in present working-face. A. Brown sands. B. Limestone.

On the right side of the present quarry-face the upper horizon of limestone has been eroded down to the lower more sandy and glauconitic horizon, on the irregular surface of which there rest two small patches of rock-phosphate, as shown in Section IV.

Section IV.

Millburn Quarry, showing two patches of phosphate rock resting on lower horizon of limestone.

On the right bank of a small stream near Sutherland's limestone quarry there is a high face of rock-phosphate resting in a basin in the lower horizon of limestone, and a few hundred yards south-west of Millburn quarry there is a similar but smaller outcrop, which also appears to lie on the higher part of the lower horizon. The surface contours and the presence of basalt fragments in the phosphates at Kiln Point tend to show that the formation of the deposits took place in comparatively recent time—probably in the Post-Pliocene period—and, obviously, since the present contours of the district were determined. Hence it seems probable that the phosphate-deposits will be marginal, and follow the line of limestone outcrop, contouring around the slopes of the hills bounding the valleys and shallow basins.

Origin of Rock-Phosphate.

Rock-phosphate consists of tricalcium phosphate, which has the formula Ca₃P₂O₅. It is often called “bone-phosphate,” because it is the substance of which bone is composed.

The bones of all vertebrate animals contain about 60 per cent. of tricalcium phosphate, while the excrement of some is also rich in the same substance.

Although invertebrates rarely contain phosphate of lime, there are some notable exceptions—namely, the Brachiopods Lingula and Orbicula, also Conularia, Serpulites, and some recent and fossil crustaceans. Hence deposits rich in phosphoric anhydride (P₂O₅) are found in rocks of all ages, from the Laurentian up to nearly the Recent period.

The formation of phosphate-deposits is generally believed to have been due to the leaching or lixiviation of phosphate-bearing rocks by waters containing carbonic and other organic acids, followed by the subsequent concentration of the phosphate under favourable conditions. In some cases they deposited their calcium phosphate in caverns formed in limestone or calcareous sandstone, and the subsequent removal by solution of the walls of the caverns, either wholly or partially, left the phosphate in the remaining sands.

It may be of some interest to note that the apatite beds and veins of Ottawa, in Canada, occur in rocks of Laurentian age. The brown rock-phosphate of Tennessee is believed to have been derived from the weathering of certain phosphatic layers in the Lower Silurian limestone which forms the basin of middle Tennessee. These layers do not occupy an unvarying stratigraphical position, but occur in various horizons in the Lower Silurian formation.^*

The phosphate-deposits in the South of England, in France, and Belgium occur associated with Cretaceous chalk. Those of Algeria and Tunis are of Eocene age, the phosphates occurring in nodules in marl or as phosphatic limestone. In Algeria, which has been estimated by M. Chateau, a French mining engineer, to contain from 150,000,000 to 300,000,000 tons of phosphate rock, it is considered risky to mine rock under 60 per cent. of the tricalcic phosphate.^†

The celebrated phosphate-deposits in Peninsular Florida occur in detached pockets in the uneven surfaces of an Eocene limestone, and in Western Florida on Miocene limestone, under geological conditions which seem almost the same as those existing on the Horseshoe Estate at Clarendon.