CHAPTER 8 MINERAL EXPLORATION PHILOSOPHY

• Jurisdiction: United States

Mining Exploration Technology for Lawyers and Landmen
(Apr 1980)
CHAPTER 8
MINERAL EXPLORATION PHILOSOPHY
Leo J. Miller
Texasgulf Inc.
Golden, Colorado

Introduction

Economic geologists explore for ore deposits by outlining geological mineral environments. Great ore deposits are tied to specific environments while the ore minerals which constitute the deposit may occur through a wide range of geological environments. For example, lead and zinc minerals exist within a wide range of rocks. However, 95 percent of the world's production comes from two specific environments, shale hosted and carbonate shelf (to be defined later). It is a common mistake in exploration strategy to conduct lead-zinc exploration in vein zones within continental volcanic and granitic rocks, such as, the San Juan Mountains and Rocky Mountains.

From a broad point of view, there are four global geological environments that contain mineral deposits:

1. Eugeosynclines

2. Back Arc shelves

3. Cratonal shelves

4. Cratons

Eugeosynclines

Eugeosynclines consist of volcanic arcs and deep sea trenches. Modern eugeosynclines border the Pacific Ocean from Antarctica north to Alaska and South to New Zealand. A classic example of a modern eugeosyncline is the Aleutian volcanic arc and its adjacent deep sea trench.

The volcanic arc segment of a eugeosyncline contains the ore deposits of this environment. The volcanic arc consists of long linear belts of 90 percent andesitic rocks and 10 percent rhyolitic rocks with mudstones deposited within intermontane lakes. The volcanic rocks are derived directly from the mantle of the Earth via a fracture system known as the Benioff zone. Therefore, the metals of the volcanic arc class originate within the Earth's mantle of ultramafic rocks.

Volcanic arcs represent the oldest geological event recorded on Earth. Arcs are the only geological system that have existed from crustal consolidation 3,500 million years ago to the present. As a result, volcanic arcs have contributed to various classes of ore deposits from the oldest recorded rocks to the present.

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Classes of ore deposits within volcanic arcs are:

1. Massive pyritic copper-zinc with lead, silver and gold, Jerome, Arizona is an example.

2. Massive pyrrhotitic nickel with copper. No. U. S. examples but numerous deposits in Canada, Australia, and Finland.

3. Gold — Rare small deposits within the U. S. Many deposits in Canada and Australia.

4. Magnetite iron formation — No U. S. examples but numerous deposits in Canada, Australia, and Finland.

5. Chrome — Low grade chrome belts of California and Appalachia. Best deposits in Rhodesia.

6. Porphyry copper — gold — No. U. S. examples. Best deposits in British Columbia and volcanic arcs of Western Pacific.

The distribution of volcanic arc deposits with time is as follows:

Eugeosynclines	Most Prominent Ore Deposits	Age in Millions of Years before Present
Southern African Zone,; South Africa, Rhodesia	Selukwe, Chrome	3,400
Pilbara Zone-Australia	Mons Cupri-Salt Creek Cu-Zn-Ag	3,100
Yilgarn Zone, Australia	Kambalda, Ni	2,700
Slave Zone, Canada	Izok Lake, Cu-Zn-Ag	2,600
Superior Zone, Canada	Kidd Creek-Mattagami Cu-Zn-Ag-Au	2,500
Karelian Zone, Finland	Outokumpu, Cu-Zn, Ni	2,200
Skellefte Zone, Sweden	Boliden, Cu-Zn	2,000
Churchill Zone, Canada	Flin Flon, Cu-Zn	1,800
Jerome Zone, United States	United Verde, Cu-Zn	1,800
Prieska Zone, South Africa	Prieska, Cu-Zn	1,300
Grenville Zone, United States	Ducktown-Gossan Lead Cu-Zn, Cu	1,000
Mt. Lyell Zone, Australia	Mt. Lyell-Roseberry Cu-Zn-Pb	550
Caledonian Zone, U. S.-Canada-Norway	Bathurst-Tronjheim Cu-Zn-Pb	450
Gouldburn Zone, Australia	Woodlawn-Captains Flat Cu-Zn	400
Circum Pacific Devonian Zone	West Shasta-Arctic-Mt.

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