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therms due to deep burial and to the post-Carboniferous intrusive period when a general rise in temperature could effect changes in the more susceptible impure sediments.

Whether this tentative explanation is correct or not, the importance of recognizing the general distribution in the Abrigo of minerals which may be mistaken for indications of contact metamorphism cannot be overemphasized. This formation is of considerable economic importance and the underground search for ores cannot be aided by the same indications as in the higher limestones.

CONTACT AND HYDROTHERMAL METAMORPHISM

That the granite-porphyry intrusion was the supreme factor in the ore deposition of the district cannot be doubted in view of the intimate association of the ores with the porphyry and the intense changes that have taken place in the rocks around the main intrusions.

It has been customary to separate the metamorphism of the rocks around a contact into contact metamorphism, caused by the igneous rock itself, and hydrothermal metamorphism, caused by the heated solutions emanating from the cooling magma. This distinction will not be made in the present article, because there has been nothing found by the authors to indicate that there is any genetic difference in the causes for changes in the rocks, outside of differences in intensity or quantity. The differences in effects are not to be confused with the causes, as the first are subject to the many variations of the rocks encountered, their physical and chemical differences, and their previous alteration. The metamorphosing agencies vary also with the distance from their source in the effect they may have on the same rock, but all changes observed so far have usually been graded and not susceptible to sharp definition.

In general, it may be stated that the two main centers of porphyritic intrusion are also the centers of metamorphism. Around Sacramento Hill the effects of metamorphism can be seen on the surface, but around the Lowell center the alteration did not extend through the covering of upper Carboniferous limestone. The effects around Sacramento Hill are more pronounced on the side of the calcareous sediments, which have been so much more altered than the schist of the north side. The porphyry on the south, as well as the covering remnants of contact breccia, are highly silicified and stand out as the crest of Sacramento Hill. The topographical depression around this hill is due to less silicified porphyry breaking through the contact breccia where this last turns from a more or less vertical to a horizontal body. Outside of this moat-like depression is a ring formed by the highly silicified contact breccia which grades into silicified metamorphic limestone, and this into limestone with a decreasing amount of metamorphic minerals until a belt of marbleized rock is

reached, from 200 to 1,000 ft. away from the porphyry. The zone of recrystallization also fades outward irregularly to the unaltered sediments.

On the north, or schist side of the contact, there is some silicification of the older rock, accompanied by considerable pyritization, but this very soon fades out into the unaltered schist.

Underground, in tracing out the zones of rock alteration due to the intrusion and its accompanying solutions, it has been found that they roughly correspond to the ones just mentioned as appearing on surface, with the following general modifications: The zones extend farther than the general surface contours along extensions of the porphyry mass, especially the zone in Lowell ground, and along lines of fracturing which may or may not also be lines of faulting. The extent of the zones is remarkably influenced by the formation they are traced in, the changes going farthest at the top of the Devonian and the top of the Cambrian. Local variations are found to break the arrangement of the zones of alteration where there are minor centers of strong action, such as close to the Shattuck dike in the Uncle Sam and Shattuck ground, in the Southwest mine, and in several other places.

The Effect of Contact and Hydrothermal Metamorphism on the Formations Before taking up the effects of contact and hydrothermal alteration on the rocks of the district it is well to state that hydrous metamorphism or oxidation has obscured or complicated in a great measure the results of contact metamorphism, and in many cases made the exact separation of the process involved too difficult and useless to be undertaken in the course of economic work.

The changes due to circulation of meteoric waters will be taken up later. The mine workings have now gone down far enough in all zones to disclose the fact that there is a depth below which the rocks as well as the ores are different from those closer to the surface. Here there are none of the minerals evidently due to the process of oxidation, and mineralogical associations continue the same indefinitely downward, without the vertical variations of the upper portions. This is the criterion that has been used in separating the effects of primary and secondary processes in the rocks and in the ores. Though not found in this order in the mines, primary effects will be considered first.

As a whole the impression given by the rock alterations in the Warren district is one of abundance and persistence rather than great intensity. There are no high temperature minerals developed in any great amount, or if they were developed at some stage, they have now been replaced. Garnet, diopside, wollastonite, scapolite, and vesuvianite have been observed, but in very small amount, and never forming an important part of a formation. Tremolite, actinolite, and edenite are far more common.

The distinguishing minerals of alteration zones are quartz, sericite, chlorites, especially penninite, and the oxides of iron, magnetite and specularite, as well as the metallic sulphides of iron, copper, zinc and lead.

Metamorphism in Contact Breccia

Around any of the granite porphyry that has been accompanied by mineralizing emanations, such as that of Sacramento Hill, the contact breccia is marked generally by an extreme amount of silica which replaces all other gangue and rock-forming minerals. Pyrite and sericite are next in abundance, with chloritic minerals in variable amounts and calcite practically unknown. Schist and quartzite fragments have naturally remained the least altered in this mass and are consequently recognized most easily. In this contact breccia, or at the edges, there are bodies of intergrown magnetite, hematite and pyrite, with associations of the best-formed garnet, wollastonite, and other contact minerals. Silicification usually decreases toward the outer edge of the breccia, sericite and chlorites increasing. In many thin sections studied, the prevailing impression gathered is that in most of the breccia silicification is the last process involved, following sericitization and chloritization.

There are, however, distinctly later porphyry dikes cutting the breccia, as has already been stated, which have also been accompanied by metamorphosing emanations, the result being a complication and confusion of the processes involved. Certain highly chloritized portions of the breccia, where penninite replaces quartz in a noticeable way, are the result of these conditions, which are economically important, since copper minerals are concentrated in very pure form by the superimposed alteration. There are also other observed forms of this repeated alteration depending on the variations of either the first or the last. But it is generally true that the second process never reaches the point of adding silica or even adding sericite to the previously changed rocks.

Metamorphism in the Contact Limestones

The transition from breccia to limestone that can be recognized is apt to be very sudden in the Carboniferous horizons, and graded in those below. This is because the alteration of the lower limestones is more pronounced. The impure, and at the same time easily crushed, Devonian and Cambrian formations become a mass of sericite, penninite, calcite, and quartz, with some tremolite, garnet, diopside, wollastonite, and epidote, while the Escabrosa limestone usually has just a narrow fringe or a few bands of metamorphic minerals and the mass of the rock is simply recrystallized to marble. As extremes of alteration in this contact zone there are places where the limestone, especially the Devonian, is converted into a highly sericitic or chloritic mass, or into almost pure quartz.

There is some evidence in this zone also that silicification is the last process affecting the rocks under ordinary conditions.

Farther from the contacts or the centers of mineralization the changes visible in the limestone are a decrease in silica and sericite in all the rock, while these minerals may persist in breaks and joints. Recrystallization of calcite, but not of the impurities, is very common.

In this contact zone the accumulation of sulphides is marked by the increase in either sericite, silica, or chloritic minerals around the sulphides.

Metamorphism Outside of Contact Zones

Sometimes rocks with considerable alteration are encountered far from any known porphyry, in which case it may be that solutions have traveled along fractures, or that they may have come with undiscovered porphyry. Usually, however, the copper ores have been found replacing the limestone very much farther away than any general alteration of the formations, in which case the transition from fairly unaltered rock to sulphides is sudden, but bears some relation to the local structure.

Metamorphism of the Porphyry

The primary solutions emanating from the granite porphyry affect that porphyry in about the same way as they do the adjacent rocks. If the nearby rocks, whatever they may be, are silicified, the porphyry will be also. If sericitized or chloritized, the same processes will have affected the igneous rock. The main difference comes in the relative amount of sulphides which have formed during these changes. Of course, the same minerals will not form in limestone as in igneous rock, but an increase or decrease in silica or magnesia or iron will be parallel in both cases.

The main mass of Sacramento Hill is so highly altered that a thin section of this rock shows only a mat of sericite plates, with variable amounts of quartz, pyrite, and chlorite, without much being left to distinguish between quartz, feldspar and mica phenocrysts, and the base. Close to the breccia, silicification is common, accompanied by disseminated copper deposits.

In the dikes radiating from the hill, the changes in alteration of the porphyry are very clearly marked, as well as the relative ages of succession of the processes. Receding from the silicified portions, sericite increases and the outlines of phenocrysts, first of quartz, then of feldspar, begin to appear. Lastly appear the outlines of biotite, marked generally by successive lamina of sericite and pyrite. Farther away penninite is found in increasing amount, the quartz phenocrysts being unattacked and some of the plagioclase feldspars scarcely altered. The chloritic minerals appear as remnants replaced by sericite.

When sericite has almost all disappeared, and there is no silicification, epidote, zoisite and calcite are sometimes found as alteration or meta

morphic minerals in the porphyry. At this zone there are also found portions of the rock converted into almost pure penninite and edenite, with some serpentine, as if the ferromagnesian minerals lacking in the more sericitic rock had been driven to accumulate and replace other portions. Only the presence of some unaltered quartz phenocrysts and very resistant apatite crystals helps to determine the original rock, although transitions have been exposed in some of the workings.

Sulphides in some amount invariably accompany the contacts of porphyry that is altered to this degree.

Farther away from the centers of action slight chloritization of the base, of some feldspars and of micas, and eventually of the micas alone, is the only alteration noticeable. This fades out in a few observed cases to fresh rock, or rock affected only by meteoric waters. The contacts of this less altered porphyry have either just a little pyrite and sphalerite, or no sulphides at all, and even have had no appreciable effect on the immediately adjacent limestone when seen in thin section.

The alteration of the more basic dikes which cut the contact breccia is also similar to the one induced in the breccia, except that the earlier changes are not observed in these dikes.

SUMMARY

To summarize, it may be said that the weakest as well as the farthest reaching alteration in the porphyry is chloritization. Next comes epidotization, which is of minor importance. Sericite and finally quartz are the additional minerals formed with the increase in the strength as well, probably, as the temperature of the emanations. The minerals formed by the stronger solutions are formed by replacement of the earlier ones, in the ordinary case when metamorphism is advancing in intensity from the center. Superimposed alteration may be caused by later intrusives, or in certain instances by the decrease in intensity in the dyingout period of alteration.

Some unaltered dikes in Holbrook and Czar ground are found in zones where the rest of the rocks show intense sericitization, including some masses of porphyry. These dikes are associated also with some sulphide ores, the contacts being far more distinct than ordinarily. If it were not for the slight sericitization of these dikes close to the ore, while away from it they show no hydrothermal action, it would be possible to believe that they are the result of post-mineralization intrusions.

The Shattuck dike shows similar phenomena, as it is very fresh for long stretches, and then, close to the ore zones in Uncle Sam and Shattuck ground is almost as altered as any rock around Sacramento Hill, showing that the altering solutions did not accompany all the porphyry but found their way through special channels.

The following partial analyses of altered porphyries are typical:

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