Notes on

The Geology of

THE RAINBOW MINE

Alleghany,

California.

(Note: For an account of the geology of the region reference should be made to Bulletin 580-I, U.S. Geological Survey, "Lode Deposits of the Alleghany District, California", by Henry G. Ferguson, 1914)

The Rainbow properties at Alleghany are situated on the south side of Kanaka Creek, about 3000 feet down stream from the Tightner Tunnel. The old workings reached by the Groves Tunnel were not accessible at the time of my visit, but practically all of the stopes are open for inspection from the Main tunnel level 350 feet below.

Like so many of the other mines of the Alleghany District, the main shoot of ore mined appears to have been related, genetically, to the intersection of two veins. The Rainbow vein, to which the ore seems to have been confined, strikes N 70° W and dips to the north at angles of from 30° to 70°. The second vein, which may be called the North Vein, strikes about N 15° W; the prevailing dip is 60° to the West. This vein is characterized by mashed amphibolite "formation" and rather variable quartz. The Rainbow appears to pass through the North vein but it has not been followed west of the intersection. The southern segment of the North vein

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has the appearance of having been faulted to the east by the Rainbow, the movement having taken place both before and after mineralization. The total horizontal displacement is less than ten feet.

After passing through the North vein, the Main drift follows a steep, barren vein for some 300 feet to the east. The results of this work were evidently so discouraging that a 200-foot crosscut was driven in the footwall and the North vein/prospected north and south of the intersection. This work clearly shows that the Rainbow vein is split up into a number of weak and ill-defined fissures in the vicinity of the North vein. One of these fissures intersects the North vein as described above; it is not known that the others do so.

As will be seen from the map, a well defined shoot of ore was encountered on the Rainbow vein above the Main level and east of the No.2 Crosscut. The long axis of this shoot is nearly parallel to the line of intersection of the Rainbow with the North vein; the shoot as developed, however, is confined to the Rainbow and is not at the actual intersection. The western limit of the stope is about 150 feet east of the probable line of intersection of the two veins. It is believed that the fissure of the North vein is older than that of the Rainbow, but that lateral movement subsequently took place on the plane of the Rainbow fissure faulting the North vein. It is, perhaps, this later, pre-mineral movement on the Rainbow that is responsible for the present location of the ore shoot some distance to the east of the intersection.

The work now under way has been planned with the idea

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of exploring the Rainbow in depth and to the east of the inter-section. The shaft was started on what is believed to be the principal Rainbow fissure lying in the footwall of the vein followed by the main drift. When the proper depth has been reached the Rainbow will be prospected to the west as well as to the east and ultimately the second vein will be explored.

When convenient to do so, the Rainbow vein should be sought to the west of the North vein. At the junction of the North and Main drifts the Rainbow fissure seems to cross the North vein, but it has never been followed. It should be traced westward far enough to determine whether it merely dies out or increases in strength west of the intersection. The reason for the need of this exploration is that the apparent shift eastward of the North vein south of the Main drift suggests faulting by the Rainbow. This faulting is believed to be in part, at least, pre-mineral, and evidence of the fault should be found west of the North vein in the form of a fissure. The structural relations at the junction of the two veins are not altogether clear and it is, of course, possible that the supposed fault is more apparent than real.

The North vein also should be prospected farther to the north. It is a strong fissure and might develop ore. The possibility of an intersection with another vein parallel to the Rainbow and belonging to that family should also be kept in mind. Such relationships characterize the Tightner-Sixteen to One area immediately to the north and are unquestionably responsible for the occurrence of the rich ore shoots. The Rainbow appears to

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lie near the southern end of this zone of intersecting veins.

The interpretation of the geology of the veins is
based on good evidence and justifies the proposed exploration.

Ernest Howe.

January 13, 1922.