Geology

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 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

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

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

The Louvicourt deposit is proving to be a very large one. Reserves now stand at approximately 30 million tons and, more importantly, the deposit contains some impressive widths of exceptionally high grade Cu-Zn-Ag-Au mineralization that should ensure its profitability in virtually any future metal price scenario.

Regional Setting

The Louvicourt deposit is located approximately 15 miles east of Val d’Or, and just one-half mile north of provincial highway 117 that links Val d’Or with Montreal. The deposit lies within a one-mile wide unit of felsic volcanic rocks that strikes east-west over a distance of at least 25 miles. This unit hosts numerous important copper-zinc massive sulphide occurrences, including four past-producing mines; the East Sullivan, Golden Manitou, Dunraine and Louvem mines. These four mines collectively produced over 30 million tons of Cu-Zn massive sulphide ore at the average grades listed in Table 1. However, despite these significant production figures, Val d’Or has generally not been recognized as an important base metal camp, and the announcement of the new discovery at Louvicourt came as a surprise to many industry observers who regarded Val d’Or strictly as a gold producing camp. (Of course, this was not without good reason as Val d’Or has

Carbonates are a very rare and volumetrically insignificant family of igneous rocks composed mainly of carbonate minerals, and normally are described in terms of their habits (intrusive) and composition (mineralogy). No other association of igneous rocks exhibit as great a diversity of rock types and intrusive habit. They vary in shape and size with depth as well as the completeness of differentiation, so that differences in the internal structure, habit and compositions of associated (co-genetic) rocks, as well as the type of metasomatic alteration (fenitization) with the country rock should be exposed with the depth of erosion level since emplacement.

Morphology of Carbonatite Complexes

1. Core carbonatite, in which (a) a carbonate-rich core is surrounded, in part or completely by, (b) cone-sheets and/or ring- dikes in the intermediate, outer, and fenite rings (aureole) of the complex, and (c) radial, tangential and irregularly distributed dikes within the complex, in the fenite aureole or at some distance into the country rocks. A variant of the above is the carbonatite that occurs only as arcuate dikes (ring-dikes and/or cone-sheets), and breccia zones in alkali ring complexes. These probably represent an unroofed core carbonatite.
2. Some carbonatites occur as a thick sheet and tabular to
   

The Grasberg copper-gold orebody is contained with a young igneous complex of dioritic composition which intrudes limestones of the Tertiary New Guinea Group. Alteration is typical of porphyry copper deposits worldwide except that higher grade primary chalcopyrite with significant gold values is associated with a centrally located zone of strong quartz stockwork.

The Grasberg was generally regarded as a 0.7% porphyry copper target with little or no chance of having an enriched chalcocite blanket. The leached capping appears weak and the general area has been glaciated within recent times.

Regional Geology

The island of New Guinea has long been recognized as the product of collision between the north-moving Australian plate and the southwest-migrating Pacific plate. Convergence and deformation of the northern edge of the Australian plate probably began during Eocene time. The resulting geologic and physiographic features of New Guinea, and especially Irian Jaya, can be divided into three distinct geotectonic provinces: (1) southern coastal plains, (2) central mobile fold belt, and (3) northern Pacific plate margin.

Undeformed

Rare earth element (REE)- and gold-bearing breccia pipes are a potentially significant economic target in the Pea Ridge iron ore mine, Washington County, Missouri. The Pea Ridge deposit is one of eight known volcanic-hosted iron ore deposits in the Middle Proterozoic St. Francois terrane, which are similar to the Olympic Dam-type deposits of Australia. Total REE oxide content of samples of the groundmass material, which are not diluted with lithic fragments, average about 20 weight percent. Grades from working faces in the mine are lower and average about 12 weight percent. Gold distribution is erratic, but concentrations are as high as 371 parts per million.

Geologic Setting

Middle Proterozoic rocks of the St. Francois terrane, which includes rhyolitic ash-flow tuffs, lava flows, and coeval granitic plutons, host Missouri’s Precambrian iron ore deposits. Zircons from the granites have yielded U-Pb isotopic ages of 1,480 to 1,450 Ma. The central plutons have distinct accessory minerals, such as fluorite, topaz, allanite, monazite, garnet, and cassiterite, and a characteristic trace element suite that includes elevated abundances of Sn, W, Nb, Y, Be, Li, Rb, Ba, and F. The central plutons have a unique negative magnetic anomaly signature.

Concentrations of minerals between 3.5 and 5 specific gravity such as ilmenite, rutile and zircon in large accumulations of sand constitute “heavy mineral” placers. Two fundamentally different modes of formation characterize “trap” and “bed” placer variants. Smaller and generally higher grade trap placers form during sediment erosion or reworking. Because of their smaller size, they rarely yield economic heavy mineral deposits.

Trap and Bed Placers

Fundamentally, a trap placer is one where the higher specific gravity grains have become caught in a manner similar to gold in a riffle box. Once deposited, the grains of greater specific gravity fail to become entrained and to move away with subsequent fluid flows as do the other grains. A critical point to make here is that erosion and failure to entrain occur sometime after the grains first were deposited. Trap placers, unlike the sluice box, are characterized by later reworking of the original sediment and passive collection of grains that are essentially not displaced by the later fluid flow.

Trap placers quite often exhibit high grades. They also often exhibit wide particle size discrepancies between the matrix and the heavy minerals. Great care should be taken when identifying by using such internal evidence, however,