London's benchmark index is posting deep losses as miners weigh on the market on worries that the recent gains may have been overdone.Concerns grow about the health of the US economy following Friday's dismal consumer sentiment data. Asian markets were down overnight, while US futures also point lower.If the US economy's revival is going to take longer to get into its stride then mining companies are figured to be among those most likely to be affected, and it is mining stocks leading the retreat in London today. Xstrata is the worst performer, closely followed by Rio Tinto, Kazakhmys, Antofagasta and Anglo American.Fresnillo is also down after the silver miner saw revenue slide in the first half of 2009 by 10.7% from a year earlier to $378.9m, as the realised silver price over the period dropped 21.9% to $13.92. Earnings before interest, tax, depreciation and amortisation eased 4.1% to $203.4m.Shares in Johnson Matthey were down after UBS cuts its rating for the specialty chemicals firm to "sell" from "neutral" as it downgrades all UK specialty chemicals stocks by one notch on valuation and share price performance grounds.Pre-tax profit halved at Michael Page in the first half, but the recruitment firm said the rate of decline in the UK and the Asia Pacific region has slowed. The group also said that it will continue to 'adjust' its headcount in response to market conditions. In the last twelve months Michael Page cut staff by a third, largely through natural attrition.Shaftesbury warned that demand for offices continues to fall, though the London's West End-focused property firm said demand for retail, restaurant and residential accommodation continues to be healthy.Investment trust Witan narrowed pre-tax losses to £8.77m in the six month ended 30 June compared with a loss of £139.32m last time.Oil and gas exploration and production firm Hardy Oil & Gas reported a first half pre-tax loss of $5.4m compared with a profit of $9.7m the year before.Aberdeen-based Venture Production today announced the discovery of gas on trend with the producing Bains field and 10 kilometres north east of the Morecambe South production complex in the East Irish Sea. Oil explorer Heritage Oil said it has completed the testing of the Miran West-1 discovery in the Kurdistan region of Iraq. The company said it achieved a flow rate of 3,640 barrels of oil per day (bopd) from a single reservoir interval, though the company estimates that a flow rate of between 8,000 and 10,000 bopd will be achievable when the well goes into production. Attempts to test deeper formations were made but the initial drilling operations had damaged the formations too badly for this to be possible.Speciality pharmaceuticals company BTG is to receive a $7m milestone payment from one of its partners.Construction and regeneration group Morgan Sindall said its affordable housing division has been confirmed as a preferred partners for a £35m regeneration scheme in Skipton, North Yorkshire.Shares in mining exploration and mineral investment company Red Rock shot up after the company said it may acquire a controlling interest in the Migori Gold Project in Kenya.