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In the East Tintic district lenticular beds of tuff and agglomerate are intercalated in the lower part of an Oligocene volcanic series, made up chiefly of porphyritic quartz latite lavas, which was deposited on strongly folded, faulted, and deeply dissected Paleozoic rocks, comprising a thick basement of quartzite and 5,500 feet of dominantly carbonate sediments above. The volcanic series and the Paleozoic formations are cut by intrusive quartz monzonite and porphyritic latites and by pebble dikes —dike-like bodies comprised chiefly of subangular or rounded fragments of quartzite. Hydrothermal alteration has affected all these rocks locally, but ore deposits, though later than the intrusives, have been found only in the rocks below the volcanic series. Most of the ore found in the calcareous sediments is in lead-zinc-silver replacement bodies and most of that found in the quartzite is in pyritic copper-gold veins.

The replacement ores commonly have a casing of pyritic jasperoid which in turn is surrounded by an envelope of hydrothermal dolomite. Sericite is abundant close. to the ore and clay-mineral alteration is common locally at the outer edge of the jasperoid. This zonal arrangement is probably the result of different solutions which followed the same path at different times.

The stages of alteration include an early, questionably hydrothermal intra-volcanic near-surface alteration, and five subsequent stages of undoubted hydrothermal alteration: (1) the early barren stage; (2) the mid-barren stage; (3) the late barren stage; (4) the early productive stage; and (5) the productive (ore) stage. In the early barren stage, limestone was altered to dolomite and the basal part of the volcanic series was chloritized. The solutions of the early barren stage followed more routes and penetrated farther than those of the later alteration stages, but probably nine times out of ten the ore solutions followed a conduit previously used by the early dolomitizing solutions. Valuable information on the hydrothermal plumbing system was gained through mapping the early barren stage alteration.

The mid-barren stage is characterized by the development of clay minerals (argillic alteration) which is most extensive in the volcanic rocks but is also common in the calcareous rocks. Argillization is concentrated around intrusive centers, but the alteration of the intrusives themselves ranges from intense to negligible. A series of alteration zones in the lava may surround partly argillized monzonite bodies; the zones are believed to reflect leaching of the warm intrusive and transfer of material to the cooler country rock during the latter part of the mid-barren stage.

Argillic alteration was accompanied by substantial leaching of nearly all the constituents of the rocks and by a corresponding increase in porosity. The solution of the rocks and enlargement of openings constituted an important factor in preparing ground for ore. Later solutions commonly changed the clay minerals into other alteration products along ore channels.

The late barren stage alteration is characterized chiefly by jasperoid, barite, pyrite, and sparse chlorite in the sediments and by allophane-quartz, barite, cubic pyrite, calcite, and minor delessite in the overlying volcanic rocks. This alteration is localized near major channels of mineralization and is more closely related to ore than to visible intrusives. Late barren stage silicification seldom reached far beyond the levels of ore deposition; it is meager in the lavas and most extensive in the hydro-thermal dolomite, but pyrite and calcite of this stage traveled much farther. Pyrite tends to concentrate in the volcanics directly above mineralized fissures, but the calcitic replacement is more pervasive and tends to spread out hundreds of feet beyond the pyritic zones. The close association of most of the known ore shoots with late barren stage alteration makes it a favorable indication of proximity to ore.

The early productive stage alteration is inconspicuous but almost everywhere present near ore bodies; it is marked by the introduction of potash and minor clear quartz and pyritohedral pyrite. The sericite-hydromica alteration characteristic of this stage seldom extended more than a few feet into the wallrocks of the ore shoots but followed suitable conduits hundreds of feet beyond the upper limits of ore bodies.

The productive stage was distinguished by the abundant precipitation of sulfides, sulfantimonides and sulfarsenides, some tellurides and gold, and very little gangue.

Weathering of the sulfide ores has given rise to the usual suite of metallic and nonmetallic supergene minerals. Kaolinite, halloysite, jarosite, alunite, and allophane are-prominent where acid sulfate waters have been active, but elsewhere the kaolin minerals and the sulfates are rare. Mont-morillonite, beidellite, endellite, allophane, gibbsite, and calcite are the common products of weathering under the slightly alkaline conditions that exist away from the sulfides. The limonites derived from pyrite, biotite, chlorite, epidote, and magnetite differ in appearance, and this difference greatly aids in the recognition of different types of altered rock. Oxidation of pyrite to limonite is attended by the generation of substantial amounts of heat and an increase in temperature gradient.

Both megascopic and microscopic studies of age relations lead to the same conclusions regarding the alteration stages. The mineralogy and paragenesis of the alteration zones change vertically and horizontally away from centers of hydrothermal activity; if arranged according to increasing distance. from these centers, the minerals are also arranged according to increasing age.

The genesis and chemistry of an alteration stage is inferred from the paragenesis and the chemical changes that took place. The losses and gains during alteration are summarized. in a series of diagrams, Figures 7,9,10, and 11. During the dolomitic and chloritic alteration of the early barren stage, MgO and FeO increased and CaO, SiO2, and A12O3 diminished. The alteration is assigned to hot chloride solutions containing minor bicarbonates.

All constituents of the rocks except water were leached to some extent during the early argillic alteration. These changes are ascribed to halogen acid emanations. In the zones bordering argillized monzonites— which themselves show the normal comprehensive leaching—potash and silica increased. The mineralogy of the surrounding argillic alteration zones is believed to reflect the transfer and redeposition of constituents gained from the hotter monzonite by solutions soaking out into coolor zones. Alunite was probably formed from essentially neutral solutions and the silica and koalin minerals from slightly acid ones.

The late barren stage jasperoid replacement bodies represent chiefly deposition of silica and solution of CaO, MgO, and CO2 from the carbonate rocks and substantial amounts of A12O3 and K2O from the shale. The silica, and accompanying minor barite, and pyrite are ascribed to neutral liquid magmatic bicarbonate solutions carrying an appreciable amount of sulfate and hydrogen sulfide. The presence of manganese on the outer fringe of jasperoidization is probably due to solution and reprecipitation of manganese first introduced during dolomitization. The calcium taken into solution from the dolomites was largely precipitated in the overlying volcanics but magnesium was carried away. The pyritic halos in otherwise unaltered quartz latite, lying over jasperoid, represent no appreciable change in the iron content of the lava and the addition of the sulfur may be due to gas phase transfer of H2S.

The solutions of the early productive stage added potash to the rocks but did very little else. Minor quartz and pyrite were deposited in open spaces. There is no indication of acid alteration and the abundant sericite formed indicates that the solutions were not strongly alkaline. The ore stage was probably not accompanied by other than sulfide alteration.

No alteration thus far recognized is a sure indication of ore. Detailed study of alteration and pebble dikes in the blanket of barren lava has yielded much new information on the structure, alteration, and character of the rocks below. Knowledge of the space relations of altered zones and the stages represented in an area of favorable structure, and the position of the various stages in the sequence of hydrothermal events that culminated in ore deposition, gives a reasonably secure basis for estimating the chance of finding an ore body.

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