The Market Vectors Gold Miners ETF (NYSEArca: GDX), the largest and most heavily traded gold mining ETF, will look somewhat different next month when the NYSE Arca Gold Miners Index is reconfigured.

On September 20, the NYSE Arca Gold Miners Index will be opened to companies that trade outside of the U.S., but the index will also begin excluding miners with market values below $750 million, reports Hung Tran for Index Universe. “If a company’s market capitalization falls within a 2.5 percent range on either side of the upper or lower boundary of the index at the time of the reconstitution, it will remain in the index,” reported Index Universe, citing NYSE Euronext.

The NYSE Arca Gold Miners Index “is a modified market capitalization-weighted index…representing a diversified blend of small-, mid- and large- capitalization stocks. As such, the Fund is subject to the risks of investing in this sector,” according to Market Vectors.

Market Vectors has previously shifted some of its ETFs to in-house indexes, but it appears the changes to GDX’s underlying index come at the behest of NYSE Euronext. The $6.4 billion GDX is home to 30 stocks. Top holdings include Goldcorp (NYSE: GG), Barrick Gold (NYSE: ABX) and Newmont Mining (NYSE: NEM). Those stocks combine for over a third of the ETF’s weight. [Have Gold Mining ETFs Finally Hit Bottom?]

The move to the $750 million market cap requirement for the NYSE Arca Gold Miners Index could be a way of avoiding duplicate holdings across multiple indexes. GDX and the Market Vectors Junior Gold Miners ETF (NYSEArca: GDXJ) do not currently have an intense amount of duplication, but there are a few stocks that appear in both ETFs such as Golden Star Resources (NYSE: GSS) and Coeur d’Alene Mining (NYSE: CDE). [Silver Mining ETFs Test Short-Term Trend Lines]

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