greenstone are most numerous but as yet have resulted in no productive mines. The limestone is the best rock of the only productive bodies and most of them are in the dolomitic phase. The only commercial limestone ores so far known occur at Kennecott, but the greenstone mineralization is widespread, being found over hundreds of square miles. Although this paper is concerned with the ore deposits in limestone it must treat briefly of the mineralization in the greenstone because of its possible bearing on the Kennecott deposits.

Copper in Greenstone.-Copper deposits are found in the greenstone in the immediate vicinity of Kennecott and also scattered over the surrounding districts. All of them show strikingly similar characteristics. The greenstone itself, remote from ore bodies, invariably yields small amounts of copper by assay. J.D. Irving learned in 1907 from examinations of many greenstone localities that hardly a greenstone specimen could be found which did not show appreciable copper, and numerous assays of greenstone from unmineralized areas showed .II to .60 per cent. copper.16 The form in which this copper occurs was not determined. The deposits of copper in the greenstone occur as (1) veins. (2) disseminated replacements and impregnation, (3) amygdule fillings.

The veins are the most important and the only type that gives promise of becoming productive. Considerable work has been done on them in the districts surrounding Kennecott. They are mostly short and shallow, terminating abruptly at some cross fracture or bedding plane in the greenstone. Their width is usually measured in inches rather than feet. The minerals are extremely irregularly distributed in the veins. Bunches of ore may be found of such size that their economic possibilities seem realized, only to disappear into a narrow stringer of no commercial value. Most of the veins in greenstone contain chiefly bornite and chalcopyrite with minor amounts of chalcocite. Quartz, calcite and lesser amounts of epidote are the chief gangue minerals. Native copper frequently occurs in the veins, and flat 18 Statement contained in a private report by J. D. Irving.

slabs of it are often found in joint planes unconnected with veins. Some veins exhibit comb structure with native copper lying between the projecting quartz crystals. Most of the veins appear to have been formed chiefly by a filling of preexisting cavities, with minor replacement of the walls.

The disseminated replacement deposits are usually narrow and elongated zones of mineralization formed by a partial replacement in and along sheeted zones and are very spotty. They consist of bornite, chalcocite, chalcopyrite, and native copper. The greenstone is usually considerably altered, and chlorite, epidote, albite, calcite, and zeolites occur. A disseminated deposit is reported by Irving29 on the upper Kotsina River in which native copper averaging 0.65 per cent. occurs in flakes and particles up to inch in diameter, associated with specks of quartz and minute grains of epidote. The augite of the greenstone has been altered to hornblende indicating deep-seated alteration by heated waters, but the rock has not been altered by surface agencies.

The amygdule fillings are of scientific rather than commercial interest. They occur in widely scattered localities of greenstone, but are not numerous in any one locality. They contain native copper, bornite, chlorites, epidote, serpentine, quartz, calcite, and zeolites. The rock adjoining the amygdules in places contains fresh olivine and iddingsite.

Native copper has been reported from a number of localities throughout the belt of greenstone rocks and is known to occur as placer copper in all of the gulches and streams which cut greenstones. Nuggets weighing up to a pound are common and some have been found weighing several hundred pounds.

Mineralization in Limestone.-The valuable ore deposits at Kennecott occur as fissure veins, irregular massive replacements, and stockworks in limestone. Between the first two classes there are all gradations, the end phases being distinctly veins or massive replacements and the intermediate phases being replacement veins. The third group consists of stockworks, irregular in outline, formed by the filling and adjacent replacement of small 19 Private Report, 1907.

ramifying fractures and joints. The veinlets are rarely more than a few inches in length, less than an inch in width, and a few inches apart. These constitute the lower grade milling ores. In all of the groups of deposits the amount of ore formed by filling of cavities is relatively small; by far the greater part has been formed by replacement of the country rock. The fissures have acted as channels for directing the circulation of the mineralizing solutions, thereby giving the solutions access to the rock to attack and replace it.

The veins exhibit little evidence of banding, crustification, or other structures characteristic of fissure veins, because their fillings consist chiefly of only the one mineral. In a few places a prominent banding of chalcocite and covellite occurs, and occasional specimens show microscopic banding with other minerals (see page 36). Most of the ore in the veins has been formed by replacement of the limestone walls of individual fractures. The ore may be in the form of one continuous band of solid sulphide or of several bands with braided structure. At one point there may be three or four bands of sulphide separated by narrow bands of limestone; a little further along these may coalesce into two bands or even one, by replacement of the intervening country rock. The individual bands vary greatly in width within short horizontal distances, in places pinching to mere seams, and in others swelling out to a width of many feet. The veins may be accompanied by disseminated sulphides in the walls.

The replacement bodies are of three classes: (1) irregular massive replacement, (2) replacement veins, and (3) disseminated replacement, with all gradations between them. The massive replacements have been localized by fractures or bedding, to be described later, and their shapes are partly controlled by these features. In this class are many examples of replacement so complete that not a vestige of the original rock is to be seen in the ore. In the replacement veins the ore bodies have been localized by zones of fracture in which the fractured rock has been wholly or partially replaced by ore, and the unfractured walls also replaced to varying widths, thus giving the ore body the vein

like appearance. In the disseminated replacements, grains and veinlets of ore are scattered through massive or fractured limestone. They customarily form a marginal zone to other ore bodies or occupy the area between two adjacent veins or massive bodies. In places, veins of massive ore may pass along their strike into disseminated ores.

In general the replacement was singularly complete where once started, and yet with a few exceptions the solutions which caused the replacement appear to have been passive. They completely replaced the rock with which they came readily in contact, but lacked the vigor to penetrate far into massive limestone away from the fractures, and were terminated abruptly by insignificant slips and bedding planes. It is not uncommon to see a large body of solid chalcocite abruptly ended by a pre-mineral slip containing only one quarter inch of gouge.

The completeness of the replacement and resulting purity of the ore may best be understood by the classes of ore mined. The "first class" or "high grade" consists of practically pure copper minerals obtained from the massive replacements, and massive parts of the replacement veins. Large stopes have been opened in which practically nothing else but pure chalcocite could be seen, so that the stope resembled a black coal mine and the ore as drawn from the chutes has run as high as 70 per cent. to 76 per cent. Cu.29 Examination of car load lots failed to disclose a speck of limestone. The "second class" ore consists of bunches or bands of copper minerals, mostly chalcocite, either attached to limestone or diluted with it in the process of mining. This class comes from the margins of the massive replacements or from the less massive parts of the veins, and is shipped directly to the smelter. It contains from 20 to 50 per cent. Cu. The milling ore consists of disseminated grains of copper minerals or larger pieces which cannot be mined without much admixed limestone. The individual grains or pieces of copper minerals are themselves practically free from included gangue matter, but are contained in, or attached to, fragments of country rock. This low-grade 20 Pure chalcocite contains 79.8 per cent. of Cu.

class comes from the margins of other ore bodies, from the disseminated replacements and stockworks, or from those parts of the veins where the mineralization is spotted or where the bands of copper minerals are too small to constitute any considerable proportion of the broken ore.

Talus Deposits or Slide Ore.-A unique deposit of copper ore occurs alongside the Bonanza Mine, mined by nature and gathered on the hillside as a talus slope. The Bonanza vein outcropped as a great mass of rich ore, mostly solid chalcocite, along the top of a knife edge ridge projecting out from Kennecott Spur. (See Fig. 5.) The rapid and continued erosion of the

FIG. 5. Diagrammatic cross-section at Bonanza Mine to show glacier ore body and talus slope ore.

ridge and the vein forming its backbone resulted in débris, composed of ore, falling down the side of this steep ridge. Its accumulation on the S.E. side formed the talus slope ore known as the "Slide ore body." It lies at the angle of repose and forms a mass, irregular in outline, extending a few hundred feet up and down the hillside. The thickness is several feet and the width several tens of feet. More than 90,000 tons of high-grade milling ore were thus formed, most of which has now been mined by surface scrapers. The rock fragments vary from small grains up to the size of one's head, and scattered through it are more finely comminuted pieces of copper minerals, chiefly chalcocite free from limestone, with a minor amount of copper carbonate. Glacier Copper Deposit.-An ice glacier seems an impossible 'country rock" for an ore body, yet such is the case on the side