Exploration has to come before mining because they share a number of mechanisms, but exploration is simpler (and necessary for finding new places to mine):
On the face of it, in-system exploration is similar to mining: You approach a body, point a device at it, and collect some good that is liberated. But
in-system exploration according to Newsletter #39 (leaving aside
the plotting of hyperspace jumps into new systems as described in the DDF) should just be about pointing active scanners at a planetary body, perhaps from a few different angles to cover the whole surface area (Hey, when are we going to be able to go into non-geostationary orbits?). For this, the game needs to procedurally generate a set of data about each body, and track the segments of the body, perhaps for different sensor types, that the explorer player has scanned. The body data can be quite simple; I can't imagine that the scan models the imaging of the generated planet surface or that the procedural generation will model the chemical composition of individual sectors of the planet's crust.
Mining,
http://forums.frontier.co.uk/showthread.php?t=7196, will be a more complex activity minute to minute, and requires a larger set of data generated by the game. Asteroid, comet and gas clouds have a non-uniform distribution of some chemical elements and complex minerals (to make mining rare resources challenging). Presumably some asteroid belts have elements Fe, Si and Tn whereas others are good for Au, Pt and Th. Then within each rock/comet/cloud, there will be different concentrations of elements, so there's a skill-based game of finding the motherlode and accurately zapping it., and there are negative drops of contaminants and space critters to avoid to add to the body. That's 2 further dimensions of data to generate. These are just random numbers, no different to a visual surface texture to come up with, but it has to be done when a player enters a resource zone and again when they scan or approach a mineable body. Also, the UI and ship modules needed for the gameplay around them will be more complex than scanners. Lastly, the game will need to track the consumption of un-extracted resources per body. Resource locations will be randomly generated on island creation, not persistent, but any given rock should not be infinitely exploitable, or players will all call their friends and keep mining the Giant Floating Goldmine 24 hours a day.
And then there are resource refining and NPC claims to consider...