What ore grade makes a mine?

Thank you D. D. Bryce for this information

This topic is so darned variable it will make your head spin. There are all sorts of variables that will spell success or failure for a mine, hard rock or placer. That is why there are plenty of places where there is for example hardrock ore 2 ounces per ton, and unmined; and where they mine the heck out of 1/100 or even less ounces per ton. THat is blasting, trucking, crushing, leaching, and precipitating 100 tons of living rock to get one ounce of gold.
Let's start a discussion that will apply to everyone from panners to barrick on this subject, generalities, concepts, and some specifics, about what it takes to really truly turn the dream into a mine, and a living.

Here are some of those factors that you meed to consider when looking to go mining for money:
-How easy it is to get to large quantities of ore. Open pits allow for mining hugely greater volumes of gold (or other metal) bearing rock over tunnel mines. Tunnel mines have to be hugely higher grade in order to make the same profit. Open pits require vast capital expenditures, tons of $$ and lots of experienced guys up front. Tunnel mines can be done with even one guy, but dern it better be rich stuff.
-How hard the host rock is (Harder = more blasting, more crushing expenses, so grade has to be higher to make a profit)
-How deep the ore is. (Blasting and moving non-ore rock out of the way costs a BUNCH of money).
-How much of the gold in the ore is readily recoverable. Any proper analysis of a hardrock prospect will include an evaluation of the expected % of recovery of gold, silver, copper, etc from the ore, and it is NEVER 100%, and often not in the 90s%. So that expensive assay that showed 0.2 ounces per ton might only really amount to 0.09 at the end of the day.
-How much water is available. You can have too much (can't get deep to the good stuff because too much flooding in the pit for the excavator or too deep for the dredge) or too little (many excellent fine gold properties in the desert left untouched because dry washing doesnt recover well enough).
-How much the ore is petrified - either with clay or with age/compaction. Too much stiffness/clay / calcification to the ore might require that it be treated more - trommel costs more than shaker for example -, or even that it cannot be profitably run.
-Size of gold; sort of the focus of this forum, at least in the old days, how to make equipment to recover gold so small that old timers couldnt recover it profitably.
-Fineness of the gold - this one will eat your lunch unexpectedly. You evaluate a placer and figure it produces say 0.1 ounce per ton, and plan a mine accordingly. Sure enough, when you weigh it, it is 0.1 ounces of placer "gold" per cubic yard recovered. Then you send it off to the refinery, and then they announce that it only assays "720 fine", or 72% gold and the rest silver, copper, etc. That means for example if you had say a 20% profit margin built in to your calculations originally (using erroneous figure of 100% gold) guess what, you are actually now LOSING 8% per ounce recovered. Welcome to bankruptcy.

Gotcha factors in common with both placer and hard rock:
-permitting: will the gubbernmint allow you to go after the gold the way you want to? Will their restrictions make it unprofitable to run the way they require you to run? Can you make a profit if you can't hydraulic or run amalgam plates or whatever?
-Location: Will it cost too much to get equipment, fuel, employees, to the mine? Is the governmental structure so weak that bandits will be the de facto government? What will it cost to pay them off? Can you afford to pay for the down time for climate (eg: Alaska = 3 months a year for placer aboveground, 8 months of digging if you are tunneling in the permafrost)? Will transport costs eat your bankroll and profits?
-Equipment: THe difference between a successful mine and a failure, when someone has made a good discovery, is almost always because they do not run enough ore, fast enough. After all, there was a discovery, to begin with. Otherwise who would pour money and time into the project? A location that will make a 400 cubic yard per hour plant's owner a multimillionaire will bankrupt a 20 yph owner. "Economy of scale" they call it. Bigger op = less profit per ton required to get rich.
Also on the subject of equipment, old tired cruddy stuff will close your project faster than a non-bribed MSHA examiner. Spending 3 weeks waiting for the part for your ancient Bucyrus dragline or cable rig dozer to arrive from the Amazon is the difference between success and failure, profit and loss.

PS Just wanted to add this - these factors apply whether you are
mining with a pan or with a company. For example if you are using a 10
inch pan to try and make a living, you will most likely starve; and if
your old broke down 30 year old Keene 4 incher is constantly broken
you will not be sending any gold to the refiner either. Just as using
a single sluice dredge in flood gold will starve ya, or using an old
beeper in hot ground will drive you to maniacal rage.