Chromium and PGM (Platinum Group Metals) Mantle like material intrudes into continental crust –Reacts with sulfides and metals settle out at bottom –Common way to make Nickel Sulfide Deposits Chromium and PGMs tend to settle out in specific layers Get a little from the Sudberry meteor impact too.
Settled Chromium Deposits
The Ocean Floor Mechanism Ocean floor comes from mantle like material forming new crust at a spreading ocean plate boundary Sometimes when continents collide or run into things just right a chunk of sea floor gets dragged up into a continent –Called an Ophiolite sequence –Bottom part of the ocean floor can contains pods of Chromium rich ore
Chromium Pods in an Ocean Floor Sequence
Where Chromium and PGMs are Found Green colored areas At plate edges are Scooped up ocean Floor with Chromium pods Black areas show Where chromium And PGM layers Settled from mantle Intrusions into Continental crust You can get a few placers from these primary deposits too.
Main Ore Mineral of Chromium is Chromite Chromite (black) interlayered with Sepentine (greenish) Fresh Chromite Surface Weathered Chromite Surface Fe 2+ Cr 2 O 4 46% Chromium S.G. 4.5 – 5.1 Hardness 5.5 Color Black to Brownish black Tends to have granular texture
Platinum, Osmium, Rhodium, Ruthenium, Palladium = PGMs Native Platinum Native Palladium PGMs can be native metals or substituted into other minerals
Sperrylite PtAs 2 57% Platinum S.G Hardness 6-7 Metal to white and brittle
Podform Chromite Deposits are Smaller 22,000 tons of ore is a median – big ones may be 250,000 tons Grade of the ore is around 46% Chromic Oxide
PGMs may be in the parts per billion range
Little Chromite Pods May be Only 143 tons Ore grades are again around 46% Chromic oxide
Chromium Processing Chromite tends to be lumpy big chunks –Allows some separation in mining –Rest if needed can usually be done with gravity separation Jigs Heavy media separators Tables Lump Ore is Then Sent to Smelter Furnaces –Often melted together with iron –Some very intensive oxygen and ammonia processes may kill off residual carbon.
The Gravity Concentrators Use the fact that something is heavy Heavies have more momentum – Shaking Tables Heavies sink faster – Jig Light stuff floats in a heavy fluid – Heavy Media Separator
PGMs – A Tough Act to Process PGMs in a good case are associated with nickel and copper sulfides –In nasty cases some of them are trapped in gangue minerals Try to Float out the PGMs Then send them to the smelter
What Do We Use Them For Chromium is mostly as a stainless steel alloy with some chemical and plating uses PGMs are used mostly for catalysts for air pollution control systems, with some laboratory applications, and some jewlery
What is Chromium Worth? About $0.40 to $0.60/lb (prices based on chrome content of ferro-chrome
What Are PGMs Worth About $5,000 to $10,000 per lb but varies with metal Platinum around $12,000, Palladium around $4,500 Rhodium around $12,000 to $48,000