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Quartz-pebble-conglomerate gold deposits: Chapter P in Mineral deposit models for resource assessment

Scientific Investigations Report 2010-5070-P

By:
ORCID iD and ORCID iD
https://doi.org/10.3133/sir20105070P

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Abstract

Quartz-pebble-conglomerate gold deposits represent the largest repository of gold on Earth, largely due to the deposits of the Witwatersrand Basin, which account for nearly 40 percent of the total gold produced throughout Earth’s history. This deposit type has had a controversial history in regards to genetic models. However, most researchers conclude that they are paleoplacer deposits that have been modified by metamorphism and hydrothermal fluid flow subsequent to initial sedimentation.

The deposits are found exclusively within fault-bounded depositional basins. The periphery of these basins commonly consists of granite-greenstone terranes, classic hosts for lode gold that source the detrital material infilling the basin. The gold reefs are typically located along unconformities or, less commonly, at the top of sedimentary beds. Large quartz pebbles and heavy-mineral concentrates are found associated with the gold. Deposits that formed prior to the Great Oxidation Event (circa 2.4 giga-annum [Ga]) contain pyrite, whereas younger deposits contain iron oxides. Uranium minerals and hydrocarbons are also notable features of some deposits.

Much of the gold in these types of deposits forms crystalline features that are the product of local remobilization. However, some gold grains preserve textures that are undoubtedly of detrital origin. Other heavy minerals, such as pyrite, contain growth banding that is truncated along broken margins, which indicates that they were transported into place as opposed to forming by in situ growth in a hydrothermal setting.

The ore tailings associated with these deposits commonly contain uranium-rich minerals and sulfides. Oxidation of the sulfides releases sulfuric acid and mobilizes various metals into the environment. The neutralizing potential of the tailings is minimal, since carbonate minerals are rare. The continuity of the tabular ore bodies, such as those of the Witwatersrand Basin, has allowed these mines to be the deepest in the world. The extreme depths create engineering complications and safety issues for the miners, such as rock bursts as a result of pressure release.

The richness of these deposits makes them a desirable exploration target. However, the likelihood of future discoveries is minimal. Small deposits found in the United States include those found at Nemo in the Black Hills of South Dakota and Deep Lake in the Sierra Madre of Wyoming.

Suggested Citation

Taylor, R.D., and Anderson, E.D., 2018, Quartz-pebble-conglomerate gold deposits: U.S. Geological Survey Scientific Investigations Report 2010–5070–P, 34 p., https://doi.org/10.3133/sir20105070P

ISSN: 2328-0328 (online)

Table of Contents

  • Abstract
  • Introduction
  • Deposit Type and Associated Commodities
  • Historical Evolution of Descriptive and Genetic Knowledge and Concepts
  • Regional Environment
  • Physical Description of Deposit
  • Geophysical Characteristics
  • Ore Characteristics
  • Hydrothermal Alteration
  • Weathering and Supergene Processes
  • Geochemical Characteristics
  • Petrology of Associated Sedimentary Rocks
  • Theory of Deposit Formation
  • Exploration and Resource Assessment Guides
  • Geoenvironmental Features and Anthropogenic Mining Effects
  • References

Additional publication details

Publication type:
Report
Publication Subtype:
USGS Numbered Series
Title:
Quartz-pebble-conglomerate gold deposits: Chapter P in Mineral deposit models for resource assessment
Series title:
Scientific Investigations Report
Series number:
2010-5070
Chapter:
P
DOI:
10.3133/sir20105070P
Year Published:
2018
Language:
English
Publisher:
U.S. Geological Survey
Publisher location:
Reston, VA
Contributing office(s):
Central Mineral and Environmental Resources Science Center
Description:
v, 34 p.
Online Only (Y/N):
Y