Deposits formed during Differentiation of a Silicate Melt

Question 1

Chromium is a lithophile element compatible in both ______ and ______ relative to ultramafic and mafic melts in the mantle; Significantly lower concentrations in average crust (~ 100 ppm) than in mantle (concentrations up to 1%).

Answer 1

spinel and clinopyroxene

Question 2

The spinel mineral chromite (FeCr2O4) is the only ore mineral of Cr. It contains up to approx. ___% Cr2O3, and is an accessory or minor mineral in most ultramafic rocks.

Answer 2

approx. 55%

Question 3

Chromites in ultramafic rocks are solid solutions with varying amounts of Al3+and Fe3+ replacing ____, and Mg2+ replacing ____.

Answer 3

Cr3+; Fe2+

Question 4

The chromite required has a Cr:Fe ratio of about _____.

Answer 4

2.8:1

Question 5

Two common types of chromite deposit are recognized on the basis of ore-body form and their geological environment:

Answer 5

1. Stratiform chromite deposits in large, layered ultramafic–mafic intrusions.
2. Podiform chromite ores in ophiolites or ‘Alpine peridotites’.

Question 6

The host intrusions are all abnormally large, sheet-like to upward-flaring intrusions of Archaean or Palaeoproterozoic age that intruded into crystalline rocks of cratons. Intrusion sizes based on outcrop area range from about 30 km2 at Kemi to 65,000 km2 at the Bushveld Complex.

Answer 6

Stratiform Chromite Deposits

Question 7

Host intrusions are characteristically layered with overall ultramafic lower cumulate units that grade up to mafic units:
Cumulate sequences onlap onto the base of the intrusion in such a way that the intrusion extends over a greater area at the level of the mafic layers than at the level of the lowest ultramafic layers.

Answer 7

Stratiform Chromite Deposits

Question 8

Ophiolite sequences and their processes of formation are similar for crust formed in slightly differing tectonic environments in the oceans, including mid-ocean ridges, backarc basins and primitive oceanic arcs.

Answer 8

Podiform chromite ores in ophiolites or ‘Alpine peridotites’

Question 9

It have been dredged from mature mid-ocean ridges
and we can thus infer similar processes of formation in this environment.

Answer 9

Podiform chromite ores in ophiolites or ‘Alpine peridotites’

Question 10

Masinloc Mines of Zambales Province, Northern Luzon

Answer 10

Refractory types of chromite

Question 11

Acoje Mines of Zambales Province, Northern Luzon

Answer 11

Metallurgical types of chromite