VEIN FILLING AT BENDIGO, VICTORIA.

WALDEMAR LINDGREN.

The mode of formation of the gold quartz deposits occurring as saddle reefs at Bendigo has recently been discussed in the issues of ECONOMIC GEOLOGY.1 Mr. Stillwell has shown that to some extent quartz may replace the black shale especially in thin wedges and sheets of the latter, but it may well be doubted whether this process accounts for the predominating vein filling which consists of pure milky white quartz in large, optically undisturbed individuals. On the whole the metasomatic action of the vein-forming solutions on the black slate is very slight, often entirely imperceptible. In places a little pyrite has developed in the country rock and some sections show a little carbonatization of the slate.

It is very common in gold-bearing quartz deposits to find veins or bodies of pure milky quartz lined along their contacts by ankerite. Mr. Stillwell refers to this frequent crustification at Bendigo2 and my own collection contains several good instances of this structure. One of these specimens is described in the following paragraphs and a photograph of it is reproduced as Pl. X.

In the same volume of ECONOMIC GEOLOGY Professor Stephen Taber discusses the mode of formation of the Bendigo veins and states his belief that Mr. E. J. Dunn is correct in ascribing the opening of the veins to the pressure of the crystallizing quartz. Mr. Taber admits that the vein forming solutions were of mag1 F. L. Stillwell, "Replacement in the Bendigo Quartz Veins and its Relation to Gold Deposition," ECON. GEOL., Vol. 13, 1918, pp. 100–111.

Alan M. Bateman, "Replacement in the Bendigo Quartz Veins," EcoN. GEOL., Vol. 13, 1918, pp. 222-223.

Stephen Taber, "Origin of the Bendigo Quartz Veins," EcoN. GEOL., Vol. 13, 1918, pp. 538-546.

2 Op. cit., p. 102.

matic origin and that they were forced into the strata under heavy pressure. The real opening of the veins was, however, according to him caused by the "force of crystallization" of the quartz which thus must have been of a magnitude very much. greater than the pressure of the solutions which carried the quartz.

I can not share this opinion and as evidence for my point of view simply present the description referred to. It does not seem to me that the phenomena shown can be accounted for on the theory of any force of crystallization." However, I may be mistaken and if so should appreciate any better explanation.

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The specimen was taken from one of the dumps of the New Chum reef line. The country rock shown dark on the photograph consists of black slate, in places bent and foliated. There is no perceptible alteration or replacement except that it contains a few minute crystals of pyrite which are a little more abundant around the two larger inclusions which appear in the ankerite. The specimen shows an irregular opening in the slate filled with ankerite and quartz. The reverse side of the specimen shows only black slate with a narrow branching vein of the same minerals. The large opening is, therefore, simply an enlargement by the bulging slate lamellæ. The opening is coated by an ankerite crust averaging 5 millimeters in thickness, made up of individuals, 2 to 3 millimeters thick and separated from the quartz by a contact line which in detail is quite irregular. The ankerite, which weathers brown, not only coats the walls but also surrounds several slate inclusions and a long thin sliver of slate. These, however, are not unsupported but connect with the main mass of slate on the reverse side of the specimen. There are also at several places thin slivers of slate separated from the main. mass by a thickness of about 1 millimeter of ankerite.

The central mass is milky white quartz penetrated by a few branching cracks. On close examination it is observed that the quartz has corroded the ankerite and that the latter contains a network of veinlets of quartz. A thin section demonstrates at once that the quartz is later than the ankerite, replacing it along

the contact and along the numerous veinlets. Many of these veinlets consist of granular quartz with abundant albite. The main mass of the quartz filling consists of large individuals 10 to 20 millimeters in diameter, showing no crystal outlines and being absolutely free from optical disturbances. The same quartz individual may continue deep into the embayments of the ankerite.

It seems difficult to explain these relations by any other theory than that the cavity in the slate was first formed and that it remained open until the coating of ankerite had been deposited. At that time the character of the solutions changed and quartz was deposited. The large size of the quartz individuals suggests high temperature. Rapid filling is suggested by the only partial replacement of the ankerite. To the action of the "force of crystallization" may fairly be ascribed the splitting off of thin slate slivers from the wall and perhaps also some movement of the inclusions in an open space filled with solution. Such movements may be easily effected by the growing force exerted by the ankerite crystals. My main proposition is that the cavity was there before the quartz was deposited. This, it will be observed, is not admitted on the basis of the theory advanced by Mr. Taber. MASSACHUSETTS INSTITUTE OF TECHNOLOGY,

CAMBRIDGE, MASS.