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have been metamorphosed more or less profoundly into hornfels, schist, marble, and other metamorphic types. In some of these metamorphosed rocks ore deposits containing copper, bismuth, and gold have been found. The granite core forms the highest part of the concession, and hence the drainage is roughly radial from the center of the massif. Streams crossing the contact between the granite and the sedimentary rocks, where gold values are present at the contact, carry the metal downstream across the upturned edges of the sedimentary rocks which form excellent natural riffles. The Koreans have worked these placers for the old Korean Household Department for centuries.
Sedimentary Rocks.-There are at least two sedimentary series on the Suan Concession. If the gneiss of the western part of the concession be of sedimentary origin, there are three series. This Siroo Pong San gneiss (the name being derived from the mountain where it is extensively developed) is the oldest rock on the concession; and over it and complexly folded into it are certain schists and a thin quartzite. These latter rocks are the oldest definitely sedimentary series of the area.
The second series, which detailed study would doubtless show to comprise two series, is composed of limestones, slates, and quartzites. The intricacy of the structure in the immediate neighborhood of the Collbran contact is such that no definite
Length of section approx i mine
FIG. 3. Showing the contact north of FIG. 4. Diagrammatic section through the Tul Mi Chung. a part of the Tul Mi Chung mine.
sequence can be decided upon except from general field relations. Fossils are lacking; the batholith cuts across the sediments irregularly (see Fig. 2), and the Hol Kol limestone invariably underlies the Suan slates. This is shown in section in Figs. 3 and 4.
In general the field relations show the Hol Kol limestone to be closer to the batholith than the Suan slates.
Assuming then that the oldest sediments are the nearest to the intrusion, all being upturned about its edges, we have in order from oldest to youngest: Hol Kol limestones, Suan slates, quartzite (with a little conglomerate), Tul Mi Chung limestone, and Tong Am limestone. The Suan mine and the Tul Mi Chung mine are in the Hol Kol limestone. This is the only limestone surficially in contact with the granite. The Hol Kol limestone is in part. pure calcite, in part magnesian (but this point needs to be definitely established by analyses), and in no place is it very siliceous or argillaceous, though "blue limestone" is abundant. the contact, the limestone is usually a marble, characteristically saccharoidal.
Igneous Rocks.-In order of age the igneous rocks are as follows: the dolerite from its metamorphosed appearance in the field and under the microscope is considered to be older than the main granite intrusion; originally it was a gabbro composed almost entirely of augite, plagioclase at least as calcic as oligoclase, and ilmenite. Almost all of the augite has been altered to hornblende (uralite), and the plagioclase to chlorite, quartz, and other alteration products. The ilmenite has been partially altered to leucoxene.
Near the western part of the contact are certain dikes and stocks. The Tong Am quartz-diorite dikes, closely related to the "Weigall granite" referred to later, are doubtless one of the last phases of the intrusion of the Suan granite. These dikes, strangely enough, have exercised the strongest metamorphic influences observed on the concession. They carried large amounts of mineralizers and metallic sulphides including molybdenite in considerable amount. Molybdenite was not seen elsewhere except in very small amount at Peh Wha.
Certain other granites and diorites of this western part seem to be older than the Suan granite, which is the main intrusive. Weigall Granite.-The "Weigall granite," the so-called “sye
2 So named after Mr. A. R. Weigall, who first (in 1913) identified the rock as being closely associated with the ores in the lower level of the No. 2 crosscut tunnel at Tul Mi Chung.
nite," occurs in large and small dikes in the workings of the Tul Mi Chung mine (see Fig. 4). There it is without doubt closely connected with the origin of the ores. Microscopic study shows the rock to vary from a quartz monzonite through a quartz diorite to a mica diorite. The average composition is probably that of a quartz-biotite diorite. It is probably an offshoot of the parent magma of the Suan granite intruded shortly after the main mass, but before any great cooling had taken place. Locally through alteration the rock has developed a dirty white or buff color. The biotite was the last of the minerals to succumb to this alteration, and in some places a limey-looking rock with numerous black or brown mica flakes in it marks this stage of the alteration. The alteration has occurred along and out from cracks and small faults in the formation (Fig. 5); and it has
FIG. 5. Vein of calcite in quartz-diorite in the Tul Mi Chung mine.
proceeded so far in many places that the product cannot be recognized as having been derived from the diorite without tracing the intermediate stages. At present the writer can only advance the hypothesis, which seems to be supported by all the evidence available, that the alteration of this igneous rock has been caused by the circulation of hot water through fissures, and presumably given off by the main Suan granite, or possibly, in part, by the Weigall granite" itself during cooling. These waters were of first importance in the deposition of the Tul Mi Chung ores. In fact, it seems not improbable that a part of the ore has been deposited in a highly altered phase of the diorite.
Suan and Other Granites.-The Hol Kol granite and the Tul Mi Chung quartz-porphyry and tourmaline granite are stocks
which were intruded soon after the main granite. Tourmaline indicates the presence of mineralizers. They are in all probability differentiation products of the parent magma, occurring significantly close to ore deposits. The Chil Sing Dei granite (later granite, Fig. 2) is of the same type as regards origin, but is entirely surrounded by the Suan granite. The possibility must not be overlooked, however, that the Chil Sing Dei granite may have been the primary stock subsequent to the solidification of which the Suan granite may have been intruded. Certain aplite dikes were both precursors and followers of the great intrusion. They do not appear in connection with ores except at Sang Dai, the eastern prospect of the contact (see Fig. 2). The Suan granite itself appears in two principal masses, one being the Unjinsan massif batholith which occupies an area of about thirty square miles; and the other the granite boss of An Ka Roo Kai (Fig. 2) occupying an area of about 14 square mile. The general shape of the intrusion (Fig. 2) is elliptical, five by eight miles. The long axis extends in a west-north-west, east-southeast direction, which seems to be the general orogenic axial direction of the region. At no place has the writer observed the contact to dip toward the granite, but a local bulge may have caused such a condition to develop in some restricted localities. The average dip of the contact varies from 30° to vertical. About 60° is most common, but great variations occur (see Fig. 6).
The Suan granite is porphyritic in texture, about 5 per cent. being phenocrysts of orthoclase from 1/4 inch to 3 inches in size. and the remainder a rather coarsely granitoid ground-mass. The phenocrysts are very commonly in the form of Carlsbad twins which in places weather out so as to show the characteristic form of this combination. The phenocrysts are never free from small inclusions of biotite, quartz, and hornblende.
The ground-mass of the Suan granite is made up essentially of plagioclase, orthoclase, quartz, hornblende and biotite. The comparatively large amount of plagioclase and of ferro-magnesian minerals indicate that this is not a true granite, but that it has dioritic affinities. Its true name should be a tonalite
porphyry; but the term granite has here been used. The plagioclase is oligoclase with an average composition close to Abg2An18 as determined microscopically; but the composition varies slightly in different parts of the intrusion. The accessory minerals noted were titanite, apatite, magnetite, and zircon.
About the periphery of the granite, dikes of basalt varying from a few inches to seven feet in width, and from a few hun
FIG. 6. Diagram showing present and hypothetical former section at the Suan mine. The actual structure is more complicated than shown in figure.
dred feet to half a mile in length, are of frequent occurrence. There is also a small area of basalt of slightly more than an acre in extent in the Morachi Valley southwest of Hol Kol. This basalt is the youngest igneous rock of the contact area. It is doubtless older, however, than the basalt of the lava plains of Korea, one of which lies a number of miles to the southeast of the concession. The basalt cuts both granite and sediments, and seems to have been intruded shortly after the solidification of the granite.
Metamorphic Rocks.-The metamorphic rocks are the granite gneiss of the Siroo Pong San massif with its associated schists and quartzite, all products of regional metamorphism; the hornfels, schist and marble, and garnet-epidote-actinolite, diopsidephlogopite-garnet, diopside-vesuvianite, wollastonite-tremolite