Tectonic Analysis as an Exploration Tool

Three tectonic provinces within the Cordilleran System, are analyzed in terms of tectonic framework. (1) Precambrian terrain of North-Central Colorado; (2) The Pacific Northwest; and (3) The San Andreas-Garlock fault system and associated features. In each case a close relationship is seen to exist between: (1) Arcuate mountain development; (2) Precambrian basement fabric; (3) Major fault patterns; and Major mineral occurrences. The arcuate trends developed when the basement was still mobile. Later competent deformation rejuvenated pre-existing lines of weakness.

Tectonic Analysis of the Precambrian Terrain of North-Central Colorado

The Precambrian terrain of North-Central Colorado  is dominated by several north-eastward directed convex mountain arcs. The present discussion will concentrate on the arc whose axial trace trends northeastward from the northern Sawatch range through the Gore and Williams ranges. This axial trend continues on through the batholith south of Granby reservoir, and meets the Great Plains near Fort Collins, Colorado. The arcuate nature of this trend is outlined by the courses of the Eagle, Arkansas, Blue, Fraser, Arapahoe, Buckhorn and Cache la Poudre. (South Fork) rivers.

The following discussion will indicate that there is an intimate relationship between the major fault patterns of North Central Colorado and the overall arcuate framework.

The smooth arcuate

Geologic Mapping

The titaniferous magnetite ore body at Sanford Hill is associated with gabbro occurring in anorthosite. The contrast in the magnetic susceptibilities of these rock types, reflected in the magnetic measurements, allows geologic mapping showing rock types and inferred structural features. The anomalous magnetic high of the airborne magnetometer survey indicated a southern extension of the Sanford Hill ore body.

The rock type in the Lake Sanford area is almost entirely anorthosite. Here, as elsewhere, it is mostly a porphyritic, very coarse-grained rock composed mainly of andesine and labradorite feldspar thus having a very low magnetic susceptibility. Numerous magnetic susceptibility measurements made on this rock type show a range from .1 x 10-³ cgs units to 10.0 x 10-³ cgs units and ground magnetic traverses observing the vertical component of the earth’s magnetfc field run over known anorthosite masses gave readings in the range of ±500 gammas relative to a base station having an assumed value of zero over a known anorthosite mass.

The Sanford Hill ore body is composed of a mixture of ilmenite and magnetite in anorthosite and gabbro. The ore body exposed in the open pit occurs, in two parallel zones separated by a waste zone of anorthosite and/or gabbro

Expansible Shale Prospecting

Field criteria are needed to help the prospector recognize a potentially expansible shale, clay, or slate. Certain criteria have proven useful in California even though the physical-chemical mechanisms of expansion are not. thoroughly understood.

Three conditions must be satisfied before expansion can take place. The raw material must partially fuse at a low temperature (1800-2000°F.). Concurrently a gas must form. The raw material must be sufficiently impermeable to trap the gas within the particle. Low temperature melting, which is controlled by bulk chemical composition, is difficult to assess in the field but most common shale, clay, and slate is suitable in this respect. The presence of organic carbon, the most common source of an expanding gas, is indicated by dark color of the rock. A large proportion of clay-sized particles promotes impermeability; fine grain-size is indicated by conchoidal fracture.

Expanded shale is a commodity which mist be produced in high volume at a low cost in order for it to be competitive with normal sand and gravel aggregates. The initial cost of establishing an expanding plant is high, usually in the multimillion dollar bracket.

A most important early step is to learn where the major sales area will be and to determine the

Seismic Refraction Equipment

Within recent years portable seismic refraction units have found their place in the industry for shallow investigations. Completely portable, relatively inexpensive and of proven accuracy, the portable refraction seismograph is an electronic tool in an electronic age.

The seismic refraction theory is based on the ability of various materials to transfer shock waves at different velocities, depending upon their elastic properties and relative densities. If the velocity of a material can be determined and the area is geologically familiar, the material can be classified as to type and relative density.

The instrument detects the first and subsequent shock waves which reach the geophone from the point of shock wave initiation. The fastest possible travel path for the shock wave is indicated by the heavy-black arrowed lines. As indicated in the diagram the first wave to reach the geophone will naturally travel the fastest possible path which is dependent on the relative densities of the sub-surface materials. At successive intervals between the geophone and shock wave initiation, a time reading is taken for the arrival of the first wave. This time reading is then plotted on a time-distance graph. The time is measured in milliseconds (1/1000 of a second) while the distance is

Thrust Faulting

Limestone is rare in the eugeosynclinal Franciscan assemblage, constituting less than 0.1 percent of the total volume. On the northwest part of the San Francisco peninsula Calera Limestone is present as isolated masses from the type section near Rockaway Beach southeastward for ten miles, where the belt is truncated by the San Andreas fault.

Numerous faults, some intrusive rocks, gouge zones, and the varying quality of the limestone itself require that mining be based on a complete knowledge of the geology, and that a continuous development program be followed for future mine planning.

The section at Permanente contains limestone with and without interbedded chert, intrusive andesite with possibly some subaqueous extrusive pillow layers, and thin silt layers. The absence of terrestrial sediment, the sparse planktonic fauna, thin-bedded dark limestone with sulfide of probable organic origin, and many contorted slump structures indicate a deep water origin. Surrounding sedimentary rocks are typical of the eugeosynclinal Franciscan.

Chert beds are a few inches to a foot thick and persistent along bedding, although there is a great deal of pinch and swell, probably due to the distortion and dynamic metamorphism to which the rocks have been subjected. Cherty zones can be recognized everywhere and are valid stratigraphic

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Exploration of Alluvial/Eluvial Gold Deposits

The topic of this short information is the exploration of deposits of not consolidated (loose) gold bearing material with free

Consulting Geologist

One possible reason why some companies lack the wisdom to hire geologists is that they are dominated by engineers who still subscribe to the old idea that engineers are self-sufficient in all fields. They could have learned this from their professors, I once knew a Dean of Engineering who got purple in the face when a faculty committee was discussing the financing of a broad gauged engineering project and suggestions were made that assistance be obtained from the Schools of Business and Law and from some of the science departments. He pounded the table and shouted “Engineers are trained to handle all this.”

In addition to commodity knowledge a company may employ an outside consultant because of his prior field work in an area where the geology is strange to members of the geology staff. If the area is in a foreign country, a consultant with experience in that country could be a big help. He not only may know something about the local geology, but he also can help with previously acquired knowledge of how to obtain exploration concessions, the legal statue of aliens, the names and locations of pertinent bureaus, surveys, and other government agencies, the names and addresses

The Syngenetic Massive Sulphide Deposits

The large massive sulphide deposits present a formidable challenge to the economic geologist. They represent such huge concentrations of iron, sulphur, base metals and precious metals that it is difficult to conceive how they were derived from the crust or mantle of the Earth. The problem of the origin of massive sulphide deposits has generated a geological controversy which has lasted two hundred years and shows no sign of abating.

Field Evidence of the Origin of Massive Sulphide Deposits

Although much experimental and theoretical work has been done towards elucidating the origin of the massive sulphide deposit, the most significant advances have come from field observations, particularly those made by geologists whose activities have permitted them to see many deposits.

It is beyond the scope of this paper to describe all of the characteristics of the massive sulphide deposits found in volcanic and sedimentary rocks. The four characteristics described in the following pages have been selected as having a particular bearing on the origin of the deposits.

Lithologic and Stratigraphic Associations

Massive sulphide deposits show certain marked regularities in their stratigraphic and lithologic associations. These relationships appear to be consistent, regardless of the age of the deposit. Geologists whose experience has

Ore Fanglomerates as Exploration Guides for Porphyry Copper Deposits

Three porphyry copper districts in southern Arizona are partly overlain by post-ore fanglomerates which share certain physical and petrologic characteristics and which may comprise a distinct and significant class of rock. These units are the Locomotive fanglomerate of Ajo, the Helmet fanglomerate of Twin Buttes-Mission-Pima, and the Cloudburst formation of San Manuel-Kalamazoo, termed “Locomotive-type fanglomerates” herein. Post-ore units at Poston Butte and Lakeshore, Arizona, and Sar Chesmeh, Iran, also appear to fit this class.

“Locomotive-type fanglomerates” are composed of poorly sorted conglomerates, lenticular sandstone beds, andesitic to rhyolitic flows and tuffs, and unique monothologic landslide breccias. They contain fragments of holyokeite – a rare oligoclase lava of porcellaneous appearance. Their basal contacts are considered to be depositional and overlapping, sometimes complicated by later low-angle faulting.

Locomotive Fanglomerate of AJO

The fanglomerate is presently exposed over about 11 square miles, but had an apparent original areal extent of approximately 25 square miles. The fanglomerate unconformably overlies Precambrian gneiss, Cretaceous volcanics, and Laramide quartz monzonite. The fanglomerate at one time covered the entire New Cornelia orebody, but now only the southern and southeastern parts lie beneath it. Unpublished drill hole information indicates that the base of the fanglomerate has overlapping relations on the pre-fanglomerate

Practical Mining Geology

Operating geology may not seem as exciting and glamourous as exploration, especially to recent graduates, but it has its advantages as listed below:

  1. Stability: Exploration is generally the first activity curtailed in recessive periods.
  2. Satisfaction: The results of good work are visible sooner and more often.
  3. Family life is relatively normal.

Geological work at an operating mine deals with basic data and may be divided into the following four categories:

  1. Recording (mapping, logging, sampling).
  2. Posting (plans, sections).
  3. Interpretation (projection, descriptive geometry, computer modeling).
  4. Effectively “selling” your interpretations to management (development headings and/ or drilling, avoidance of incompetent ground, locating ore, etc.).

Procedure for Mapping

The following data are recorded by noting rib location normal to the tape (the Brunton should never be used to determine strike in a mine).
a) Faulting – properly weighted to indicate magnitude of movement (the amount and type of gouge is strongly influenced by rock type and/or pre-fault alteration).
b) Mineralization
c) Rock type (employ a continuous, rather than spot, description).
d) All notes are horizontal – light guide lines should be drawn and erased later.
e) Dip symbols should employ arrowheads three times as long as they are wide at the base and closed. All dip symbols should be of

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