"A half-mile diameter object of this type would contain an amount of gold equivalent to about three percent of all the refined gold ever mined on Earth. The amount of nickel and cobalt in that asteroid exceeded the 1985 world known reserves by four or five times."
"In 2001, it was estimated that all the gold ever mined totaled 145,000 tonnes."
http://en.wikipedia.org/wiki/Official_gold_reserves
So, about 4350 tonnes of gold?
Or roughly 1/2 the amount held as US gold reserves:
8,133.5 tonnes.
A sphere 1 km in diameter with density of 4 has volume
of about 1/2 billion cubic meters and mass of about 2 billion tonnes. 2 billion divided by 4350 is about 480,000 tonnes.
One would mine 480,000 tonnes of stuff to get 1 tonne of gold. Or roughly 2 to 4 ppm [and perhaps much better, allowing that one can be selective in the ore you process for the gold].
"A gold ore containing less than one gram of gold per ton can, in some cases (and depending on the gold price), be profitably treated."
http://www.miningbasics.com/html/gold_ore_processing_method___t.php
So 1 gram per tonne is 1 ppm. [Though ppm refers to volume and since gold is very dense it's actually less than 1 ppm]
So, such gold ore would probably be profitable to mine on earth [minable], though you probably make as much or more money from nickel and cobalt.
If one has 1% of "ore" which is water which can be extracted, it's 10,000 ppm.
Space/Science » in reply to A couple of answers
re: A couple of answers