The oldest date we found in a lead mine when exploring with "Op Mole", was 1666 (the date of the Fire of London), which was carved into a rock face alongside lots of tally mark loads (four vertical lines with a slanting line crossed though them). This date is in the upper workings of the Oxclose Mine, Snitterton (to which there is restricted entry), and can be viewed in Op Mole's "Underground Journey" film, when it was wrongly attributed to being found in their exploration of the Meerbrook Sough, Wirksworth.
Henry Fearne of Bonsall whose descendants later lived at Snitterton Hall in the 1600's, in his Will of 1546 (which can be found amongst Wills held in the Court of Canterbury) owned at this date the lead rights of Upper OxClose, and other mines in the "King's Field" that lay within the Snitterton Township of Matlock Mining Liberty of the Great Barmote Court of the Soke and Wapentake of Wirksworth, including Holmes Vein, Liberty Rushens Vein and Nether OxClose. The Will date predates the carved date of 1666 by over 120 years. Other papers (researched by an American friend) shews an Edmund Fearne Jr., owning the OxClose lead rights in 1666.
p.s. I've just seen Peter's posting regarding the 1538 date. Oxclose mine is very close to the Brightgate workings, and with the documentation that my American friend (originally born a Fearne in Wirksworth!) has found and had copied, although I have not seen this date underground, it would appear that this small area was being actively mined for lead in the early 1500's which can be proved by documentation. Oxclose mine and the other mine names which I have mentioned come under Matlock Mining Liberty - Snitterton Township (although in the then parish of Darley Dale - normally mining liberty boundaries follow parish boundaries, but this is an exception to the rule), and the Brightgate mines were within Wensley Mining Liberty, Northern Dale being the boundary between the two Mining Liberties - gets confusing doesn't it!