XXVI. TEXTURES OF HYDROTHERMAL ORE DEPOSITS AND SKARNS

Question 1

Open-Space: Rounded concentric growths around clasts formed in open cavities

Answer 1

Cockade

Question 2

Colloidal Deposition: Curved or banded texture from precipitation of colloidal particles

Answer 2

Colloform

Question 3

Open-Space: Elongated crystals growing perpendicularly from cavity walls toward the center

Answer 3

Comb

Question 4

Open-Space: Layered crusts formed by successive mineral deposition in open spaces

Answer 4

Crustification

Question 5

Open-Space: Crystals with terminations on both ends indicating free growth in voids

Answer 5

Doubly-terminated crystals

Question 6

Pseudomorphs

Answer 6

Replacement: Minerals replacing others while retaining the original crystal shape

Question 7

Matching Walls

Answer 7

Open-Space: Opposing vein walls with mirror-image features from tension and mineral infill

Question 8

Five Factors Affecting the Commercial Viability of a Mineral Deposit

Answer 8

Grade of Ore – Higher concentrations of valuable minerals are more economically viable.

Size and Tonnage – Larger deposits are typically more favorable due to economies of scale.

Market Demand and Price – Fluctuating prices of metals or minerals impact profitability.

Location and Accessibility – Remote or difficult-to-access areas increase operating costs.

Mining and Processing Costs – Includes labor, energy, equipment, and ore treatment.

Question 9

Four Ways of Classifying Ore Deposits

Answer 9

By Origin (Genetic Classification) – e.g., hydrothermal, magmatic, sedimentary.

By Host Rock – e.g., deposits in ultramafic, felsic, or carbonate rocks.

By Metal Content – e.g., gold deposits, base metal deposits (Cu, Pb, Zn), or rare earths.

By Morphology – e.g., veins, stratiform, stockworks, or disseminated types.

Question 10

Four Evidences Regarding the Existence of Hydrothermal Fluids

Answer 10

Vein and breccia textures – Suggest fluid movement and mineral precipitation in fractures.

Fluid inclusions – Tiny trapped pockets of fluid in minerals analyzed microscopically.

Wallrock alteration zones – Systematic changes in mineralogy around ore zones.

Stable isotope signatures – Isotopic compositions indicating hydrothermal origin.

Question 11

Wallrock alteration refers to the chemical and mineralogical changes in the host rock due to interaction with hydrothermal fluids. These alterations are crucial for understanding ore-forming processes.

Common alteration types include:

Answer 11

Silicification – Addition of silica.

Sericitization – Formation of fine white mica (sericite).

Chloritization – Formation of chlorite minerals.

Argillic alteration – Formation of clay minerals.

Propylitic alteration – Formation of epidote, chlorite, and calcite (outer halo of alteration zones).